B1

Insulated cables

Sheath bonding systems of ac

transmission cables - design,
testing, and maintenance
Reference: 797
March 2020
Sheath bonding systems of
AC transmission cables -
Design, testing, and
maintenance
WG B1.50

Members

T. ZHAO, Convenor US R. BASCOM, Secretary US

C. GRODZINSKI CA H. NYFFENEGGER CH
W. WANG CN R.A. OLSEN DK
G. DENCHE ES M. NGUYEN-TUAN FR
N. COUTURIER FR J. PILGRIM GB
L. COLLA IT S. MASHIO JP
P. VAN VELZEN NL O. CAKMAK TR
T. DU PLESSIS ZA

Corresponding Members
G. BUCEA AU N.H. RIBEIRO DE LOUREDO BR

Invited Contributor
T. WUNDERLIN CH

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Executive summary
This Technical Brochure contains the study outcome for sheath bonding of AC transmission cable
systems. The Technical Brochure (TB) incorporates design, testing (including after installation testing)
and maintenance of these sheath bonding systems. Sheath bonding is required for all cable systems
to ensure an effective bond to earth of the cable system metal sheath, armouring, and semi-
conductive outer sheath covering. Incorrectly performed sheath bonding systems may lead to cable
system failures or pose a human safety risk. Sheath bonding systems for the purpose of this TB
include related cable system components and equipment that are connected to form the required AC
cable system sheath, armouring and semi-conductive outer sheath bond and connection to earth.
The basic information and requirements needed to design such sheath bonding systems are included
in several documents such as Electra 128-1990, TB 189-2001, TB 268-2005, TB 283-2005, TB 347-
2008, TB 403-2010, TB 556-2013, and TB 680-2017. Due to recent developments, trends and industry
best practices in sheath bonding system designs, component standards, and related national
regulations, CIGRE Work Group (WG) B1.50 was established to develop this TB.
Chapter 1 of this document provides an overview of the sheath bonding system functions and
requirements through review of existing documents and other engineering information related to
sheath bonding systems. It furthermore includes the service experience feedback received in terms of
sheath bonding system configurations, schematics, standing voltages, and voltage withstand
requirements.
Chapter 2 lists and discusses the most commonly used sheath bonding system methods used for the
design of AC cable systems (i.e., single point, multiple point (solid), and cross-bonding) and the
challenges regarding the protection (insulation) of cable system sheaths. The TB provides basic
knowledge on voltage withstand requirements, current rating, and energy absorption for the selection
and implementation of bonding leads, link boxes and sheath voltage limiters, depending on the cable
system parameters, sheath bonding methods, earthing connection and insulation co-ordination
technical study results. The TB also provides guidelines for performing insulation co-ordination
technical studies related to the design and voltage withstand requirements of sheath bonding systems,
and the cable system models that can be used for computer software overvoltage calculations and
simulations.
Testing of sheath bonding systems is discussed in Chapter 3 to provide guidance on type testing of
bonding system components and equipment, and on performing after installation tests.
Maintenance of the sheath bonding systems is addressed in Chapter 4 by providing recommendations
on industry best practices for maintenance of sheath bonding systems (including sheath voltage
limiters and testing criteria). Condition monitoring options for the sheath bonding systems are also
discussed.
Appendices of this document include lists of Abbreviations, Term Definitions, Symbols, References,
Review of Service Experiences, and Review of Maintenance Practice.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Contents
Executive summary ............................................................................................................. 3

1. Basic information ...................................................................................................... 6

1.1 Overview of bonding systems and sheath voltage limiters ................................................................... 6
1.1.1 Cable metal screen design and screen bonding .................................................................................. 6
1.1.2 Sheath insulation ................................................................................................................................. 7
1.1.3 Sectionalized joints .............................................................................................................................. 7
1.1.4 Sheath voltage limiters ........................................................................................................................ 8
1.1.5 Link boxes ........................................................................................................................................... 9
1.1.6 Bonding and grounding leads ............................................................................................................ 10
1.1.7 Safety considerations ........................................................................................................................ 12
1.2 Review of related literature ..................................................................................................................... 12
1.2.1 Existing CIGRE publications .............................................................................................................. 12
1.2.2 Technical standards and guides ........................................................................................................ 14
1.2.3 Relevant national standards .............................................................................................................. 14
1.2.4 IEC standards .................................................................................................................................... 15
1.2.5 Cross references – existing standards............................................................................................... 15
1.2.6 Published papers ............................................................................................................................... 17
1.3 Review of service experience ................................................................................................................. 18
1.3.1 Bonding schematics ........................................................................................................................... 19
1.3.2 Withstand voltage level of bonding components ................................................................................ 19
1.3.3 SVLs .................................................................................................................................................. 20
1.3.4 Bonding lead cables .......................................................................................................................... 20
1.3.5 Link boxes ......................................................................................................................................... 20
1.3.6 Calculation criteria ............................................................................................................................. 20
1.3.7 Tests during installation ..................................................................................................................... 20
1.3.8 Maintenance test ............................................................................................................................... 20

2. Bonding system design and protection ................................................................. 21

2.1 Bonding designs...................................................................................................................................... 21
2.1.1 Solid or multi-point bonding ............................................................................................................... 21
2.1.2 Single point bonding .......................................................................................................................... 22
2.1.3 Mid-point bonding .............................................................................................................................. 24
2.1.4 Cross-bonding ................................................................................................................................... 25
2.1.5 Cross-bonding in tunnel installations ................................................................................................. 28
2.1.6 Impedance bonding ........................................................................................................................... 29
2.1.7 Siphon lines ....................................................................................................................................... 29
2.1.8 Bonding of special cable system designs .......................................................................................... 29
2.1.9 Example of induced voltage calculations of a single point bonded system ........................................ 30
2.1.10 Example of circulating current calculations for a solid bonded system .............................................. 32
2.1.11 Example of circulating current calculations for a cross-bonded system with two minor sections ....... 34
2.2 Sheath voltage limiter selection and application .................................................................................. 34
2.2.1 Sheath voltage limiters ...................................................................................................................... 34
2.2.2 Selection of sheath voltage limiters ................................................................................................... 35
2.2.3 SVL connection configurations: ......................................................................................................... 37
2.2.4 SVL installations ................................................................................................................................ 37
2.3 Cable system models for overvoltage calculations .............................................................................. 37
2.3.1 Cable impedances and admittances .................................................................................................. 38
2.3.2 Power frequency studies ................................................................................................................... 39
2.3.3 Transient studies ............................................................................................................................... 41
2.3.4 Modelling of other components .......................................................................................................... 42
2.4 Insulation coordination of bonding systems ........................................................................................ 44
2.4.1 Sheath bonding system insulation ..................................................................................................... 44
2.4.2 Sheath bonding system and component requirements ...................................................................... 45
2.5 special protection on GIS cable terminations against high frequency transient overvoltage .......... 45

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

3. Testing of bonding systems ................................................................................... 49

3.1 Introduction and section scope.............................................................................................................. 49
3.2 Testing of system components .............................................................................................................. 49
3.3 System/commissioning tests ................................................................................................................. 53

4. Maintenance of bonding systems........................................................................... 55

4.1 Maintenance of bonding systems .......................................................................................................... 55
4.2 Common failure modes ........................................................................................................................... 55
4.3 Corrective maintenance of bonding systems ....................................................................................... 56
4.4 Preventative maintenance of bonding systems .................................................................................... 57
4.4.1 Online maintenance ........................................................................................................................... 57
4.4.2 Offline maintenance ........................................................................................................................... 58
4.5 Maintenance schedule of bonding systems .......................................................................................... 59
4.5.1 Safety considerations during maintenance ........................................................................................ 59
4.5.2 Parameters to consider for maintenance planning............................................................................. 60
4.5.3 Recommendations for maintenance schedule for cable bonding systems ........................................ 60

5. Conclusions ............................................................................................................. 63

APPENDIX A. Abreviations, definitions, and symbols .................................................... 65

A.1. Abbreviations ........................................................................................................................................... 65
A.2. Specific terms .......................................................................................................................................... 65
A.3. Symbols .................................................................................................................................................... 66

APPENDIX B. Bibliography/References ........................................................................... 69

APPENDIX C. Review of service experience – Survey details (1) .................................. 71

APPENDIX D. Review of service experience – Survey details (2) ................................ 101

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

1. Basic information
1.1 Overview of bonding systems and sheath voltage limiters
1.1.1 Cable metal screen design and screen bonding
Single-core high voltage transmission (66 kV and above) cables are normally provided with an outer
concentric conductor, generally referred to as the metal screen which surrounds the current carrying
conductor and insulation. The metal screen can be in the form of a sheath (welded or extruded), wires,
tapes, or a combination thereof. Metal tubes enclosing fibre optics may also be part of the metal
screen. Metal sheaths also have the added benefit of providing a radial water barrier. Many different
designs for metal screens are used worldwide, of which two examples are shown in Figure 1.1.

1
2

Figure 1.1 Examples of metal screens of high voltage transmission land cables - Left: 150-kV EPR
insulated with copper wire screen (1); Right: 400-kV XLPE insulated with longitudinally welded aluminium
sheath (2)
In this document, metal screens are called metal sheaths or sheaths to align with traditionally used
terminology. Where more than a general reference to metal screens is required, the specific metal
screen type is further described to prevent confusion on the use of the words metal sheaths or
sheaths. The sheath also includes any armouring layer of the cable. When single core cables carry AC
currents, voltages are induced in the metal sheaths and currents flow along the metal sheaths if they
are connected so as to form a closed circuit, for example, by earthing the metal sheaths at both ends
of the cable. These sheath currents cause additional power losses in the cable that reduce the cable
current rating. Metal sheath bonding methods have therefore been developed to ensure the cable
sheaths are bonded and earthed in such a way to eliminate or reduce these longitudinal sheath
currents. Sheath bonding methods to eliminate or reduce sheath current are economically desirable,
as the reduction in sheath current losses for cable circuits allows an appreciably smaller conductor
size to be used (or conversely an increase in the current rating of the same cable) and lower energy
losses to the cable system operator.
A sheath bonding system is a system to protect the insulation of the following components against
normal operating voltages and transient overvoltages from lightning, switching, and fault surges:
▪ Cables and accessories
▪ Cable sheath insulating jacket or outer (over) sheath
▪ Joint casing outer protection
▪ Sheath interruption insulators or gaps between interrupted semi-conductive shield
▪ Stand-off insulators for outdoor terminations
▪ Insulating flanges for GIS
▪ Bonding lead insulation
▪ Link box internal components
▪ Fluid-filled hydraulic insulation

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Connecting Components of the bonding system include:

▪ Bonding lead cables

▪ Bonding lead connectors
▪ Link boxes or link enclosures
▪ Sheath voltage limiters
▪ Earth continuity conductors
▪ Metal sheaths
▪ Earthing

The bonding system connections shall be rated for fault current and circulating current where
applicable.
The cable metal sheath is designed to:

▪ Provide cable capacitive return path

▪ Provide fault current return
▪ Provide earth potential for human safety
▪ Provide moisture barriers to cable insulation, where applicable

For single-core cable circuits carrying currents in excess of 500 A, special sheath bonding
considerations in accordance with TB 283 is economically desirable as the reduction in sheath current
losses allows an appreciably smaller conductor size to be used. Special sheath bonding systems are
therefore single point or cross-bonded systems used to eliminate or reduce sheath currents. There is
however no clear-cut load level for which sheath bonding methods to eliminate or reduce sheath
currents should be introduced. The extra cost of the energy losses and larger conductor cable and/or
multiple cables per phase system required for solidly bonded systems must be weighed against the
cost of the additional equipment and the maintenance cost arising from the greater complexity of a
special bonded system to eliminate or reduce sheath currents. The choice shall be based on the cable
system design, case by case. Bonding options are discussed in Chapter 2.
1.1.2 Sheath insulation
Sheath insulation is necessary to electrically isolate the cable metal sheath from earth and to prevent
metal sheath corrosion. The sheath insulation is subjected to voltages induced by the power frequency
cable conductor current and fault current, and by the transient voltages imposed to the cable
conductor by lightning or switching surges.
Sheath insulation should have appropriate dimensional and physical characteristics for the intended
application. Sheath insulation can be coated with a semi-conducting layer of graphite or can be
covered with an extruded semi-conducting layer in order to facilitate on-site testing of the sheath
insulation after installation and periodically thereafter. Special caution shall be taken when any semi-
conductive layers are applied on cable system installations. The semi-conductive layers are usually
applied to the bonding and earthing design to mitigate the risk of any capacitive coupled steady state
or transient overvoltages. When the user requires an extruded outer semiconducting layer over the
insulating sheath for tunnel installations, it is required to solidly bond the semi-conductive outer sheath
layers to the metal structures along the entire tunnel length in order to prevent any voltage difference
between the semi-conductive outer sheath and the tunnel metal structure. The voltage difference
could lead to sparking. It is advisable that any personnel contact with the cable system semi-
conductive outer sheath and racking system in the event of switching operations should be prevented.
Insulation coordination technical studies in accordance with TB 189, TB 268, TB 283, TB347, and TB
556 are recommended to be performed for each AC cable system sheath bonding system design, and
shall include considerations for all transients as defined by IEC 60071 [37]. These insulation co-
ordination technical studies shall also consider the impact of the connecting power system and various
network operating condition that may exist.
1.1.3 Sectionalized joints
A sectionalized joint is particularly suited for bonded systems. For this joint, the metal screen,
semiconducting screen, and metal casing (if present) are electrically interrupted. The metal screen is
interrupted by an insulating ring or other similar designs, while the semiconducting screen is
interrupted by means of a gap in the semi-conductive layer in the pre-moulded joint or paper lapping,
which is located in an area of relatively low or no electric field in order not to impair the performance of

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

the joint. IEC 60840 and IEC 62067 provide testing procedures for sectionalized joints. Figure 1.2
shows a design example of a sectionalized joint.

Figure 1.2: Example of high voltage land cable sectionalized joint/casing

1.1.4 Sheath voltage limiters

Sheath voltage limiters (SVLs) are surge arrester devices that are connected to the cable metal
sheaths in the bonding systems that eliminate or reduce sheath currents. SVLs are required in these
bonding systems to protect the sheath insulation, sectionalizing interruption at joints, GIS insulation
flanges, and other accessories against overvoltages on the metal sheaths during system transients.
System transients may be lightning, switching, or fast transient associated with the initial part of a
short circuit event. SVLs should withstand the power frequency overvoltage associated with short
circuit events.
Three main types of SVLs are typically used:
a) Nonlinear resistances, such as metal-oxide surge arresters without spark gaps
b) Nonlinear resistances, such as silicon carbide (SiC) blocks, in series with spark gaps
c) Spark gaps
Currently, metal-oxide surge arresters are most widely used due to their fast response to transients,
compact design, and AC voltage withstand recovery following a transient.
SVLs must be selected in relation to power frequency and transient voltages to which SVLs may be
subjected. The energy absorption capability of SVLs must be considered in the designs. SVLs should
limit slow and fast transient overvoltages while they should not intervene to limit power frequency
overvoltages. Commercially available SVLs have generally either porcelain or polymer housing. SVLs
are often housed in link boxes. The link boxes represent an accessible place to allow for inspection
and maintenance while ensuring proper operation under local environmental conditions. Figures 1.3 to
1.5 show examples of installation connection configurations of SVLs.

Sheath Voltage
Limiter

Parallel earth
continuity
conductor

Concentric Bonding leads

bonding lead for other phases

Figure 1.3: Sheath voltage limiters used at termination of single point bonded cables

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Concentric Bonding Lead

Optional Earth Link

Sheath Voltage Limiters (may also be connected in delta)

Figure 1.4: Sheath voltage limiters placed in a link box remote from buried joints

Sheath Voltage Limiter Bonding Leads

Figure 1.5: Sheath voltage limiters used close to joint sleeve sectionalizing insulators

1.1.5 Link boxes

Link boxes provide housing for bonding and earthing connections generally made of removable or
disconnecting links. Link boxes may also contain SVLs when necessary according to the bonding
system design. Link boxes are expected to be corrosion resistant and match the specified installation
requirements, such as being water tight when installed below the earth surface. Some users require
explosion proof link boxes. Different housing materials are used, such as, stainless steel, fiberglass, or
cast iron. Different mounting designs are used for underground vault, transition pole or substation,
kiosk, pit, and direct buried installations. Chapter 3 discusses testing requirements of link boxes and
other bonding system components. Figure 1.6 shows an example of a link box. The sheath bonding
system design and connection to earth shall ensure safe touch and step potentials for any part of the
bonding system, including the link box housing. Due to safety reasons, the link boxes should not be
opened while the cable circuits are energized or while any adjacent circuits are energized that may
lead to induced voltages at the link box location.

Figure 1.6: Example of a cross bonding link box with SVLs and single-core bonding leads

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

1.1.6 Bonding and grounding leads

Bonding leads are insulated conductors connecting between the cable metal sheath and the bonding
connections within link boxes. Grounding leads are also insulated conductors connecting between link
boxes and earth or termination ground. In the following document, bonding leads are used with the
inclusion of the grounding leads, but the requirements for bonding leads, grounding leads or earth
continuity conductors can be different.
Figure 1.7 and 1.8 show examples of sheath bonding configurations for the identification of bonding
leads, grounding leads and earth continuity conductors.

Figure 1.7: Example of a single-point bonding system where bonding and grounding leads are identified

Figure 1.8: Example of a cross-bonding system where bonding and grounding leads are identified
Connection between SVLs and the metal sheath of a power cable requires proper insulation
coordination, taking into account of insulation withstand of bonding leads, metal sheaths, insulators,
and the protective level of the SVLs. In general, it is desirable to keep bonding lead lengths as short
as possible to provide proper protection against transient overvoltages. Bonding leads could be made
of single-core cables or concentric cables. Bonding leads must be adequate to carry the expected
short circuit currents and to withstand the expected overvoltages. Further consideration for the type of
bonding leads selected and bonding lead length shall be considered in the insulation co-ordination
study performed.
Bonding leads are normally insulated with extruded dielectrics. The most widely used are PVC, XLPE,
and EPR. The higher permittivity of PVC results in a lower surge impedance and a lower propagation
velocity. The lower permittivity of PE increases the propagation velocity (which means also a shorter
“electrical length” is required) and a higher surge impedance. EPR offers intermediate conditions. The

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

choice of the most appropriate insulating material must be made within a specific cable system design.
Some users require bonding leads with outer semiconducting layer for testing purposes. Special
caution shall be applied for any outer semiconducting layer bonding for capacitive coupled
overvoltages during transients and steady state normal or fault conditions.
Bonding leads when installed below earth surface should use some form of longitudinal water blocking
to prevent water ingress into high voltage cable joints in case of bonding lead damages or water
ingress into link boxes.
The insulation requirements of the bonding leads selected shall be equivalent to that of the joints and
cables used.
The general construction of a single-core bonding lead is as follows. Figure 1.9 shows an example of
a single-core bonding lead.
▪ Conductor (copper or aluminium)
▪ Water blocking inside conductor (if specified)
▪ Conductor semi-conductive layer (if specified)
▪ Insulation layer (XLPE – current technology, PE – previous technology, EPR) (if specified)
▪ Insulation semi-conductive layer (if specified)
▪ Oversheath layer (PE), fire protection, if required
▪ Outer semi-conductive layer (when field testing is needed)

Figure 1.9: Example of a single-core bonding lead

The general construction of a co-axial (concentric) bonding lead is as follows. Figure 1.10 shows an
example of a co-axial (concentric) bonding lead.
▪ Inner conductor (copper or aluminium)
▪ Water blocking within conductor (if specified)
▪ Inner conductor semi-conductive layer (if specified)
▪ Inner insulation layer (XLPE – current technology, PE – previous technology, EPR)
▪ Inner Insulation semi-conductive layer, if specified
▪ Outer conductor (copper or aluminium) with water blocking (if specified)
▪ Outer conductor semi-conductive layer, if specified
▪ Outer insulation layer (XLPE – current technology, PE – previous technology, EPR) (if
specified)
▪ Outer Insulation semi-conductive layer, if specified
▪ Oversheath layer (PE), fire protection if required
▪ Outer semi-conductive layer (when field testing is needed)

Figure 1.10: Example of a co-axial (concentric) bonding lead

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Electrical insulation is considered by the test requirements described in Chapter 3. Design of the
bonding leads to prevent radial moisture ingress (metal foil, welded screen barrier, or other possible
solutions), in addition to longitudinal moisture migration, may also be considered. Where jointing of the
bonding leads is required, the joint assembly shall also meet the requirements for the bonding leads.
1.1.7 Safety considerations
The following two paragraphs are from IEEE 575 – Chapter 6.2. They are included here for completion
of this document.
Potentially hazardous voltages can be present on the exposed portions of the metal shields/sheaths of
high-voltage cables, the outer surface of conducting cable jackets, the conductor of bonding cables,
the conductor of grounding leads, across exposed shield/sheath interrupts, the SVLs, and various
hardware connections within the link boxes and other equipment connected to or associated with
bonded cable systems. Appropriate precautions must be taken to provide access control to these
areas to ensure that safety procedures are followed in order to protect both personnel and equipment.
Exposed portions of the metal shield, sheath, bond cable, or other conductive connections in electrical
contact with the cable’s shield/sheath, or bond cable of a bonded cable system, should never be
assumed to be at ground potential. The allowable shield/sheath voltage at full load varies considerably
among utilities and among countries. The shield/sheath voltage will be significantly higher during
system transients and short circuit conditions. As a consequence, appropriate protection and
precautions must be taken to ensure that personnel who may come into contact with any of the above
conductive components are familiar with the design, take adequate protection against potentially
related hazards, and follow proper safety procedures.
Additionally, safe touch potential and step potential studies shall also be performed for the selected
earthing and bonding system. Any unsafe condition shall be mitigated and prevented by the design
methodology.

1.2 Review of related literature

This section provides a review of the existing literature within the technical area covered by this
Technical Brochure. Existing Cigré publications are highlighted, along with relevant national and
international standards. Finally, a review of the Technical Literature is provided, in support of the
technical scope of the remainder of this document.
1.2.1 Existing CIGRE publications
This section summarises the contents of existing Cigré Technical Brochures of relevance to Sheath
Voltage Limiters and Bonding Systems. It should be noted that it is not the intention of the Working
Group to reproduce the content of these earlier documents, hence they remain a valuable source of
reference in their respective topics.
Electra 28 and Electra 47
Electra 28 [1] and Electra 47 [2] comprise two parts of one document entitled “The design of specially
bonded cable systems”. They represent the earliest widely published review of the design
considerations associated with these systems, having been released in 1973. Although many
advancements have been made in the field of bonding since this time, much of the introductory
material in these papers remains relevant.
Electra 28 presents the general principles behind bonding, starting from the calculation of voltage
gradients. Circuit arrangements are shown for both single point and cross bonded options, with a
discussion on the considerations which need to be made when choosing a bonding system. An outline
discussion is presented on how the bonded circuits respond to power frequency overvoltages
associated with system faults. This remains a useful introduction to the key concepts.
Electra 47 (published in 1976) follows directly on from Electra 28 and deals primarily with transient
overvoltages. It includes an overview of sheath voltages limiters and their application, although it
should be noted that some of this information is now outdated as zinc-oxide based systems were still
in their infancy at the time. Some outline guidance is provided on commissioning and maintenance of
bonded cable systems; however, the relevant section is brief. The guidance supplied in this document
is much more comprehensive. As service experience with bonded systems began to grow, it became
apparent that some revisions were necessary regarding the protection of the sheath against
overvoltages. This led to the publication of Electra 128 in 1990.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Electra 128
Electra 128 “Guide to the protection of bonded cable systems against sheath overvoltages” reports
upon the work done by Working Group 21-07 and was published in 1990 [3]. This document was
intended to replace Electra 47 by providing a guide to the selection and application of SVLs and to the
general insulation coordination of the system, informed by the available service experience since the
publication of the prior documents. On this basis, Electra 47 should no longer be considered a
definitive reference.
The main body of the Electra 128 report consists of a review of the use of SVLs (including spark gaps,
silicon carbide and zinc oxide devices). Qualification testing for SVLs is also reviewed, although this
primarily refers to the then effective IEC 99, which has now been replaced by the IEC 60099 series of
documents. The important topic of insulation coordination is reviewed, including that of the link
box/pillar, bonding leads, joint sectionalising insulation and cable jacket. Two appendices are
provided, the first of which covers recommendations for testing of SVLs and the second covers the
calculation of sheath overvoltages.
TB 283 Special Bonding of High Voltage Cables
Produced by Working Group B1.18, Technical Brochure 283 was published in October 2005 [4]. The
document presents a review of calculation methods appropriate to both single point bonded and cross
bonded systems under both power frequency conditions and during transient overvoltage events.
Recommendations are made for the appropriate ways of undertaking the calculations for practical
systems, including circuit design considerations. A short discussion of insulation coordination is also
presented in the context of the calculations required. In practical terms, the publication of TB 283
effectively supersedes the following previous articles:
▪ Electra 28, “The design of specially bonded cable systems”, Section 7
▪ Electra 47, “The design of specially bonded cable systems: Part II”
▪ Electra 128, “Guide to the protection of specially bonded cable systems against sheath
overvoltages”

However, it must be noted that some parts of these earlier documents do remain relevant, and that
TB283 does not fully replicate all of the earlier material. For power frequency conditions, equations are
provided which permit the analysis of all common fault conditions. Given the importance of
understanding the limitations of each calculation method, guidance is provided on the assumptions
inherent within the equations, along with factors that might influence the performance of the bonding
system.
The review of transient overvoltage applications provides a summary of the different calculation
methods which can be used, from relatively simple formulae through to the use of EMTP (Electro-
Magnetic Transient Programme) software. Two worked examples are provided which take the user
through the different steps of the calculations for voltage across sectionalising joints, exploring the
influence of the bonding lead design and the effects of the sheath voltage limiter.
The Technical Brochure concludes by inspecting three particular ‘special considerations’, namely
cross bonding without the use of SVLs, the effect of the DC component of the SVL voltage and the
impact of bonding lead configurations through CTs.
TB 347 Earth Potential Rises in Specially Bonded Screen Systems
One of the conclusions arising from the work presented in TB 283 regarding Earth Potential Rise
(EPR) associated with bonded cable systems was:
Where a link connecting two substations with low earth resistances is considered, EPR at the ends
and at the cross-bonding locations may generally be disregarded. Conversely, for siphon systems
EPR has to be taken into account since sheath to earth voltage may exceed the nominal withstand
level and the SVL energy handling capability if they are star-connected with earthed neutral point.

Following on from the publication of TB 283 in 2005, questions remained regarding EPR within urban
underground systems, with some field experience contradicting the conclusions of TB 283. As a result,
Cigré Task Force B1.26 was set up to develop TB 347 [5], with the specific remit to improve the
design of bonded systems with regard to EPR, by providing:
▪ More information on EPR which may occur during single phase to earth faults
▪ Details of a calculation method based on the Complex Impedance Model (CIM).

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

▪ Calculation examples, especially for typical situations.

The consideration of EPR within TB 347 is different to that seen in many other reports, as instead of
considering EPR from the perspective of safety (for example step and touch voltages), the focus is on
the integrity of the cable sheath earthing system (including the SVLs). TB 347 provides a review of
both simplified and detailed calculation methods for assessing the likely EPR. Worked examples are
provided for systems of varying complexity to provide a practical reference for users.
One of the most important issues highlighted by TB 347 concerns the modelling of circuits with both
underground and overhead sections, where a simplified model will often not provide a good
representation of the behaviour of the system. This is compounded by the fact that the information
needed to perform detailed calculations needs to cross the interface between the cable supplier and
the utility, or other parties responsible for the overhead line sections. Siphon systems in particular can
present issues here, meaning that a degree of collaboration between the parties will become
necessary.
1.2.2 Technical standards and guides
This section briefly reviews Technical Standards and Technical Guides which cover the design of, or
calculations associated with, sheath bonding systems. Where there are similarities between different
national specifications, these are discussed further in the chapter on Service Experience.
IEEE 575
IEEE 575 [6] describes the methods of calculating sheath voltages and currents for common types of
sheath bonding systems for three phase, single conductor cable systems. It focuses on higher
voltages, with most of the information presented being related to circuits operating at 60kV or above,
but the fundamental principles could be applied to single conductor circuits at lower voltages. It should
be noted that the assumed definition of sheath in this case refers to ‘non-magnetic metal shielding’.
The bulk of the document examines either single point bonded or cross bonded systems, including a
discussion of the relative merits of each. The majority of the information provided relates to power
frequency conditions, including the calculation of sheath standing voltages, however some notes are
also provided in relation to transient calculations. Sections of particular relevance to the scope of this
report include the ‘informative Annex’ sections Annex D (Calculation of Induced Voltages) and Annex
F (Current and Voltage Distribution on cable sheaths with multiple cables per phase).
Engineering Recommendation C55/5
C55/5 is a technical recommendation originating in the United Kingdom, although it has also been
used in other countries. The latest revision (bringing the document up to Issue 5) was published in late
2014 [7]. The scope of the document is the bonding and earthing of three phase systems for operation
at 33kV and above. It should be noted that the document covers both single core and three core cable
systems, meaning that a brief discussion is provided in relation to solidly bonded systems, although
the majority of the document considers specially bonded cable systems.
Guidance is given for both sectionalised and continuous cross bonding, along with single point
bonding. Section 5 of C55 covers the design and technical requirements of the various components in
the bonding system, including SVLs, bonding leads and link boxes/pillars. C55 is a valuable source of
information regarding the design and application of link boxes and pillars, which is a topic which has
not been covered in as much detail by many of the other guidance documents available to the
industry. This includes a review of the standard ranges of link box designs approved for the UK
market, with cross referencing to diagrams of the respective bonding system. A wide range of
permitted standard bonding options are presented, which although intended for the UK market can still
serve as a valuable reference for all users. C55 does not provide details of calculation methods,
instead documenting appropriate test procedures and design requirements.
1.2.3 Relevant national standards
As part of the scope of work of this Working Group, a review of service experience in different
countries has been undertaken. A number of countries have National Standards/Specifications which
may be a useful source of information generally. It should be noted that the list below is not
considered to be exhaustive and the inclusion of a document in this list does not necessary mean that
it is widely available in the public domain.
United Kingdom: In the UK, the transmission utility National Grid maintains a separate TS (technical
specification) to the widely used UK document ENA C55. National Grid TS 3.05.04 “Sheath bonding

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

and earthing for insulated sheath power cable systems” covers requirements for single core cables at
voltages above 33kV [8]. Of primary interest to non-UK readers are test requirements for bonding
system components. Sheath voltage limiters are dealt with in a separate document, TS 3.05.03
“Sheath Voltage Limiters”, covering performance and testing requirements [9].
Denmark: The Danish TSO Energinet maintains a specification bonding systems and accessories,
ETS-0054 and ETS-0067. The standard is for all 132 – 400 kV cable projects using link boxes with
direct grounding, single point bonding or cross bonding. Functional, technical and design requirements
are given.
France: The French standard concerning cable bonding systems is NF C 33-254.
United States: AEIC CS9-15 – Specification for Extruded Insulation Power Cables and Their
Accessories Rated above 46 kV through 345 kV, is widely used [10]. Section 5.0 of this standard
addresses Sheath Bonding/Grounding Systems. IEEE 575 is referred for general considerations.
Overvoltage protection between cable sheath and disclosure is required for GIS terminations.
Requirements for bonding cables, link boxes, sheath voltage limiters, and ground conductors are
provided. Appendix C of CS9 – Electrical Withstand and Insulation Coordination Requirements for
Bonded, Insulated Metal Shield/Sheath Systems, describes bonding system insulation coordination.
1.2.4 IEC standards
Although the IEC does not maintain a separate standard regarding bonded cable systems, there are
some IEC documents which have relevance. The primary documents of interest form part of the IEC
60099 series. IEC 60099-4 “Surge Arresters: Part 4: Metal Oxide surge arresters without gaps for AC
systems” provides a comprehensive set of requirements and test criteria for surge arresters in general,
including those used on overhead line networks. It is not specific to SVLs used for cable bonding
purposes, but elements of it have been widely adopted by different national standards, particularly for
testing purposes.
Many other IEC standards on the topic of either cable testing or current rating also have relevance to
the design and analysis of sheath bonding systems. The following cable testing standards are of
particular relevance:
▪ IEC 60840 – Power cables with extruded insulation and their accessories for rated voltages
above 150 kV (Um=170 kV) up to 500 kV (Um=550 kV) – Test methods and requirements
▪ IEC 62067 – Power cables with extruded insulation and their accessories for rated voltages
above 30 kV (Um=36 kV) up to 150 kV (Um=170 kV) – Test methods and requirements
▪ IEC 62895 – High Voltage Direct Current (HVDC) power cables with extruded insulation and
their accessories for rated voltages up to 320 kV for land applications - Test methods and
requirements

Those interested in calculating the losses associated with circulating and eddy currents in cable
sheaths should refer to:
▪ IEC 60287-1-1 – Electric cables – Calculation of the current rating – Part 1-1: Current rating
equations (100% load factor) and calculation of losses - General

1.2.5 Cross references – existing standards

In order to assist readers in determining which of the existing documents might provide useful
background information on a particular topic, Table 1.1 draws a comparison of the main documents
listed in this section. For each technical area, a reference is given to the relevant section in the
published documents. In an effort to integrate this with the structure of this Technical Brochure, the
topics have been grouped under the same headings.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Table 1.1: Evolution Cross reference of Existing Published Guidance (numbers refer to section
numbering in the respective document)
Topic Electra Electra Electra TB TB C55/5 IEEE 575
28 47 128 283 347
General Intro (Section 1)
Permissible standing voltages Annex C
Guide to bonding system terminology App A 4 3
Service Experience 1.2
Bonding Designs (Section 2.1)
Choice of bonding system 5 6.1 6.7
Arrangements for single point bonding 3 4.4 6.3
Application of earth continuity conductor 3.2 6.3.3
Arrangements for sectionalised cross 4.2.1 4.5, 6.5.3,
bonding 4.5.3 6.5.4
Arrangements for continuous cross 4.2.2 4.5, 6.5.5
bonding 4.5.2
Choice of cross bonding system 5.3
Cross bonding without SVLs 5.1,
6.4
Multiple cables per phase 6.3 Annex F
Screen Protection (Section 2.2)
Power frequency overvoltages 7 Annex E
Description of effect of transient 8,9,10 4.0
overvoltages
Types of SVLs in use 11 2 7
Application of SVLs 11.2 3.1,3.2 4.3.2 5.1 7.6
Earthing of SVLs 11.2.4 3.3 4.3.3 5.1.3
Selection of SVLs 12 5 Anne 7.5
xA
Bonding lead designs 12.4 4.3.5, 5.2 7.5.2
4.3.6
Link box, link pillar designs 5.3 7.5.1
Insulation coordination 13 6, App 4.6
1
Jacket withstand requirements Annex E
Cable System Models (Section 2.3)
Calculations of sheath standing 6 Annex D
voltages
Calculations of power frequency 7.3 App 2, 3.4 6.1 Annex E
voltages due to faults (single point) Part 3
Calculations of power frequency 7.4 App 2, 3.2, 6.2 Annex E
voltages due to faults (cross bonded) Part 4 3.3
Review of calculation methods (power 3.1 4
frequency)
Earth potential rise calculations 5
Effect of unequal section lengths 4.2.4 3.2.6
Testing of Bonding Systems (Section 3)
Commissioning tests 14.1
Testing of bonding leads 7.2
Testing of link boxes 7.3
Testing on complete circuits 7.5
Qualification testing of SVLs 4 7.1
Maintenance (Section 4)
Maintenance of SVLs 14.2

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

1.2.6 Published papers

In addition to the standards, guides and specifications listed thus far in this section, a number of
pertinent articles are listed below.
Sheath Voltage Calculations
The following paper specifically addresses the issue of conductor transposition, and the effect that this
will have on losses.
[11] Mighe, P. and de Leon, F. “Parametric study of losses in cross-bonded cables: conductors
transposed versus conductors non-transposed”, IEEE Transactions on Power Delivery, 28 (4), 2013.
Pp2273 – 2281.
Sheath Voltage Limiters
The following papers explore specific topics relating to SVLs themselves, including issues concerning
design specifications and unexpected failures.
[12] Parmigiani, B., Quaggia, D., Elli, E. and Franchina, S. “Zinc-oxide sheath voltage limiter for HV
and EHV power cable: field experience and laboratory tests”, IEEE Transactions on Power Delivery, 1
(1), 1986. Pp164- 170.
[13] Nichols, P. and Yarnold, J. “A sensitivity analysis of cable parameters and their influence on
design choices for minimum sheath voltage limiter specification in underground cable systems”,
Australasian Universities Power Engineering Conference, Adelaide, Australia, September 2009.
[14] Ghassemi, F. “Effect of trapped charges on cable SVL failure”, Electric Power Systems Research,
115, 2014, pp18-25.
[15] Nichols, P. “Minimum Voltage Rating of Sheath Voltage Limiters in Underground Cable Systems:
The Influence of Corrugated Cable Sheaths.” 47th International Universities Power Engineering
Conference, September 2012, London
Field Measurements
While a significant number of technical papers have been written on the subject of calculating the
sheath voltages and currents seen in cable circuits, relatively little published information is available
from field measurements. Gustavsen [16] formed a comparison between simulations and
measurements for the case of two sections of single core cable. The results obtained demonstrated
the significance of the proximity effect on the transient sheath voltage profile. Kaloudas et al [17]
examined the power frequency response of long medium voltage cable systems connecting wind
farms, comparing the results of simulations to field data. Calculated sheath voltages were found to be
within 10% of the measured value in most cases.
Subsequent work by Gudmundsdottir et al [18] reports on a validation test which compared the
modelled response of a cross bonded cable circuit to its physical behaviour. The circuit in question
has a number of cross bonding points and three grounding points. This was achieved by injecting a
conventional 1.2/50 μs impulse, at a magnitude of 4.08kV, into the cable circuit and measuring the
sending end voltage and current response. The results gained suggest that the model performs well
up until the first inter-sheath reflections are measured, at which point the level of agreement between
the model and the experimental data decreases. The differences are attributed to the representation of
proximity effects within the model, underlining the importance of fully validating simulation work with
physical measurements wherever this is possible.
[16] Gustavsen, B., Sletbak, J. and Henriksen, T. “Simulation of transient sheath overvoltages in the
presence of proximity effects,” IEEE Transactions on Power Delivery, vol. 10, no. 2, pp. 1066–1075,
Apr. 1995.
[18] Gudmundsdottir, U., Gustavsen, B., Bak, C., Wiechowski, W. “Field test and simulation of a 400-
kV cross bonded cable system”, IEEE Transactions on Power Delivery, 26 (3), 2011, pp1403 – 1410.
[17] Kaloudas, C., Papadopoulos, T., Gouramanis, K., Stasinos, K., Papagiannis, G. “Methodology for
the selection of long medium-voltage power cable configurations”, IET Generation, Transmission &
Distribution, 7 (5), pp526-536, 2013.
State of the Art: Modern Bonding methods
The following papers deal with new challenges in bonding and earthing, particularly with very long
cable circuits.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

[19] CIRED - 21st International Conference on Electricity Distribution –Frankfurt- June 2011- Paper 0499
[20] Chang, M., Shao, S., Ros, H. In Land Long Distance HVAC Cables, Innovative Examples at 225kV,
Application to 500kV.” B1- 1019 - AORC Technical Meeting 2014.
[21] Lesur, F., Mirebeau, P., Mammeri, M. and Santana, J. “Innovative insertion of very long AC cable
links into the transmission network” B1-301 CIGRE Session 2014.
[22] Khamlichi, A., Denche, G., Garnacho, F., Donoso, G and Valero, A. “Location of sheath voltage
limiters (SVLs) used for accessory protection to assure the insulation coordination of cable outer
sheath, sectionalising joints and terminations of high voltage cable systems”, B1 – 108 CIGRE
Session 2016.
Safe touch and step potential design requirements for cable system bonding and earthing designs:
This paper provides touch and step potential case study simulations for the bonding system earth
continuity conductor, joint bay earthing and link box designs, and make recommendations for safe
bonding designs.
[23] Du Plessis, T., Jagau, H. and Visagie, D-L. “Evaluating step and touch potential risks on earthing
systems of high voltage cable systems”, 8th CIGRE Southern Africa Regional Conference.

This paper explicitly deals with the theoretical simulation of switching transients on 400 kV cable
systems and the associated effect on semi-conductive outer sheaths.

[24] Schutte, P.J., van der Merwe, W.C. and van Coller, J.M., “Induced Voltage Behaviour Analysis of
an Un-Grounded Outer Layer Semi-Conductive Coating of A 400 kV Power Cable System”, 20th
International Symposium on High Voltage Engineering, Buenos Aires, August 2017.

1.3 Review of service experience

The following questionnaire as shown in Table 1.2 was sent to all Working Group members. A
summary of the received information is given below and details are included in Appendix C.
Table 1.2: Service Experience Review Questionnaire

BONDING SCHEMATICS
Solid bonding
Single-point bonding
Sectionalized cross-bonding
Continuous cross-bonding
Hybrid bonding
Direct cross-bonding without link box
WITHSTAND VOLTAGE LEVEL BONDING COMPONENTS
Screen to ground impulse withstand level for joints (kVp)
Screen interruption impulse withstand level for joints (kVp)
Screen to ground DC withstand level for joints (kV)
Screen interruption DC withstand level for joints (kV)
Screen to ground AC withstand level for joints (kV)
Screen interruption AC withstand level for joints (kV)
Impulse withstand level for outersheath (kVp)
DC withstand level for outersheath (kV)
AC withstand level for outersheath (kV)
Impulse withstand level for bonding cables (kVp)
DC withstand level for bonding cables (kV)
AC withstand level for bonding cables (kV)
DC withstand level between metal screen cable and metal enclosed GIS (kVp)
AC withstand level between metal screen cable and metal enclosed GIS (kVp)
Impulse withstand level for post insulator (kVp)
DC withstand level for post insulator (kV)
AC withstand level for post insulator (kV)
SVL
Type of resistor (Silicon Carbide, Zinc Oxide, other)
Typical rated voltage (Ur) installed

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Type of connection in sectionalized joints (delta connection, star connection with neutral
grounded, star connection without neutral grounded)
Nominal discharge current (kA) (wave 8/20µs)
Line discharge according to 60099-4
Are SVLs installed inside link boxes?
BONDING LEAD CABLES
Type of bonding cable: where single-core or concentric bonding leads are used?
Maximum length criteria
Type of insulation (XLPE, PVC, PE)
Outersheath with semi-conductive layer or graphite layer
Watertight
LINK BOXES
Location where link boxes are installed
Are the link boxes accessible?
Waterproof test
Internal arc test
CALCULATIONS CRITERIA
Method used for calculating induced voltage on the sheath (i.e. CIGRE formulation,
EMTP/ATP, etc.)
Sheath voltage limits during normal operation
SVL selection criteria during fault conditions
SVL selection criteria during transient overvoltage conditions (lightning and switching)
Are you considering internal cable fault conditions into selection criteria of SVL?
TESTS DURING INSTALLATION
Outersheath voltage tests
Bonding system connection
MAINTENANCE TEST
Outersheath voltage tests (line off)
Bonding system connection (line off)
Current by metal screens (line on)
SVL tests (line off)
1.3.1 Bonding schematics
Solid bonding design is typically used on Medium Voltage (MV) systems (up to 66 kV). On High
Voltage (HV) systems and Extra High Voltage (EHV) systems solid bonding design exists for some
cable systems but is not commonly used except for a few countries and except on submarine cables
where no other alternatives exist.
Sectionalized Cross-Bonding design is the most used for transmission lines (HV and EHV) except on
short lines where Single-Point bonding design is mostly used. The other type of cross-bonding design,
e.g., continuous cross-bonding design, was rarely used.
Hybrid bonding design (a mixture of 2 or more types of bonding designs) is common for longer circuits
where there were a number of sections that cannot evenly be divided into major cross-bonding
sections.
Other uncommon bonding designs, such as, direct cross-bonding without link box were beginning to
be used in some countries as a method to optimize installation and maintenance cost.
1.3.2 Withstand voltage level of bonding components
The following withstand voltage levels were considered for the cable circuits in service:
▪ Impulse withstand levels and DC withstand levels for joints according to IEC Standards 60840
and 62067 for HV and EHV lines.
▪ Impulse withstand levels and DC withstand levels for outer sheath according to IEC Standard
60229.

Ac withstand voltage levels for bonding components are not required except by some countries (e.g.,
France).
Withstand voltage level for metal enclosed GIS and post insulators were not required for the cable
circuits in service.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

1.3.3 SVLs
Sheath Voltage Limiters (SVLs) were usually installed inside link boxes with a very few exceptions
where the SVLs were installed in air (for example, between baseplate of terminations and steel
structures and between each side of sectionalized joints).
The most common type of SVLs used were zinc oxide. Their rated voltage was in the range from 3 to
15 kV depending on specified short circuit currents and length of cable minor sections. Nominal
discharge current was 10 kA and line discharge class was 1 or 2 in accordance with IEC Standard
60099-4.
The sectionalized cross-bonding bonding scheme is used in the majority of the cable circuits in
service. The type of connection in sectionalized joints was star connection with neutral grounded.
1.3.4 Bonding lead cables
Concentric or coaxial bonding lead cables were usually used on joints and single-core bonding lead
cables on terminations.
Bonding cables generally did not have an outer semi-conductive layer or graphite coating. Watertight
properties were not generally required, although it should be noted that water blocking can be important
in cases where the link box may be submerged in ground water.
All countries considered the criterion of 10 m as the maximum bonding lead cable length for connections
between SVLs and accessories.
1.3.5 Link boxes
Link boxes were mainly located in dedicated pits close to joint manholes. In the case of outdoor
terminations, link boxes were installed on structures above ground.
All link boxes were accessible to permit maintenance activities.
The latest generation of link boxes were waterproofed according to IEC Standard 60259 or NEMA
(Protection Grade IP 68 or equivalent NEMA is recommended).
1.3.6 Calculation criteria
Methods used for calculating induced sheath voltages were mostly based on CIGRE documents.
EMTP/ATP was used in some cases.
In some countries, the sheath induced voltage during normal operations was limited. The maximum
value depended on utility company specific criteria although it did not usually exceed 600 V.
During fault conditions, SVLs should not be activated by induced power frequency voltages.
Therefore, the SVLs should withstand the temporary overvoltage resulting from the system faults.
There were not clear criteria for SVL selection during transient overvoltage conditions (lightning and
switching). Traditionally, the maximum value of discharge voltages was limited by utility and cable
manufacture experience.
In any case internal cable fault conditions have not been considered to specify SVLs.
1.3.7 Tests during installation
The following after installation tests were carried out on the majority of lines in service:
▪ Outer sheath voltage tests according to IEC60229 (maximum 10 kV dc)
▪ Visual inspection of bonding system connections.
▪ Measurements of contact resistance (only in some countries).

1.3.8 Maintenance test

Off-line maintenance tests were mostly carried out. In recent years, some countries began to perform
on-line measurements, such as, measurements of metal sheath currents.
The following maintenance tests were carried out by a majority of the countries:
▪ Outersheath voltage tests according to IEC60229 but at a reduced (typical 50 %) voltage.
▪ Visual inspection of bonding system connections

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

2. Bonding system design and protection

2.1 Bonding designs
This section discusses the sheath bonding designs which are commonly used in transmission
systems. The following clauses cover the practical and theoretical aspects of different bonding
configurations. It is the intention that the reader will take common practical issues into account when
designing the sheath bonding system.
In the design of a sheath bonding arrangement, consideration must be given to:
a) Choice of sheath bonding systems.
b) Cable sheaths are usually expected to be nominally at earth potential. In some schemes, the
sheaths may reach an appreciable voltage to earth along the cable system. Metal sheaths must
therefore be provided with adequate insulation.
c) Complete suppression of circulating currents may not always be possible in practice. The
residual sheath currents should be calculated to assess their effect on cable rating.
d) Sheath overvoltages during system transients and faults. Sheath voltage limiting devices may
be needed with technical features to be coordinated with sheath insulation level and expected
overvoltages.
e) Failure of a part of the sheath insulation or of a Sheath Voltage Limiter (SVL) may result in
higher sheath currents which may overheat the cables. A prudent circuit design requires that
consideration be given to the duty imposed on the SVL device and to periodic monitoring and
maintenance of the complete system during operation.

Some bonding options include:

 Solid or multi-point bonding, see Section 2.1.1
 Single point bonding, see Section 2.1.2
 Mid-point bonding, see Section 2.1.3
 Cross-bonding, see Section 2.1.4
 Cross-bonding in tunnel installation, see Section 2.1.5
 Impedance bonding, see Section 2.1.6
 Siphon lines, see Section 2.1.7
 Bonding for special cable systems, see Section 2.1.8

Any variations from the bonding configurations discussed in the document should be studied
individually to ensure they meet performance requirements.
2.1.1 Solid or multi-point bonding
Metal sheaths are directly grounded at both ends (substation, tower) of the underground link (See
Figure 2.1) and, sometimes, at defined intermediate points, as shown in Figure 2.2.
Cable end

Metallic sheath
Core conductor

A B

Figure 2.1: Solid Bonding System without Intermediate Grounding Points

The solid bonding system uses bonding leads at both ends and at intermediate points of a cable
circuit. It is a simple and low cost option with minimum maintenance requirements.
Cable conductor current will induce circulating currents on the metal sheaths. The magnitude of the
induced currents can be high, e.g., up to 80 % of the conductor current for a 225 kV cable circuit (See

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Section 2.1.9). The induced currents produce Joule losses, whose influences on cable circuit current
rating can be significant. As a result, the cross-section of the cable conductor needs to be increased to
maintain the circuit ratings due to the circulating current losses of the sheath. It is noted that the
magnitude of the induced current is independent of the length of the underground cable line.
The voltage induced on the metal sheaths by phase conductor current is proportional to the length of
the underground cable line. In the case of a solid bonding, metal sheaths are considered at earth
potential at every point of the link. Solid bonding is commonly used for low and medium voltage
systems. Actual sheath voltage profile of the link depends on grounding resistances at both ends.
If a fault occurs on the cable, solidly bonded metal sheaths are useful to evacuate zero-sequence
current. Bonding leads should be designed to withstand this current.
Intermediate grounding points can be inserted on the cable, commonly at jointing locations, to prevent
any damage of the outer sheath in case of disconnection of a solid grounding point at the end of the
cable (Figure 2.2). When the link is too long, such a disconnection can generate induced voltage on
the metal sheath greater than its maximum allowable sheath voltage. This design is commonly used
with up to 2 intermediate grounding points, considering that intermediate points cannot be accidentally
disconnected. The distance between an intermediate grounding point and the end of the cable must
be considered to control the induced voltage within the design limit. Grounding every joint may reduce
the induced voltage to a minimum.
With solid or multi-point bonding, the magnetic field external to the cable is relatively low due to the
opposing sheath current to the cable phase conductor current.

Joint with Joint with

grounding grounding

A D
Groun Groud
ding ing
point point
B C

Figure 2.2: Solid bonding system with intermediate grounding points

This bonding configuration was widely used over the previous decades, and tends to be abandoned
for new installations at transmission voltages, due to the advantages of other bonding configurations.
It remains a solution for short links with low current rating requirements, low or medium voltage level
systems, and submarine cables.
For effective solid bonding, it may be a good practice to use two independent, parallel earth bonding
leads at each connection to minimize the effect of a disconnection or bad connection of one of the two
leads.
2.1.2 Single point bonding
For single point bonding configuration, only one end of the cable metal sheath is directly grounded to
the earth link at, e.g., substation or tower. The other end of cable metal sheath is open with a sheath
voltage limiter (SVL) typically connected between the open end and the local earth link. The SVL is to
protect the cable outer protection at the open end from electromagnetic transient. An example of such
a configuration is shown in Figure 2.3.
An earth continuity conductor (ECC) is installed in parallel with the power cable for single point bonded
systems except when the cable terminals share the common earthing system.
As shown in Figure 2.3, the ECC should be transposed in the middle of cable section in order to
minimize the circulating current induced in the ECC by the three phase currents whilst an effective
earth return path is provided in the proximity of power cables. Multiple transposing locations may be
used. Relative locations of the ECC to the phase cables are also a factor that can be evaluated using
available engineering tools.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Cable end

Metal sheath Sheath voltage

limiter

Earth continuity conductor

Figure 2.3: Single point bonding system

Single point bonding is considered less complicated for managing losses for rating purposes. It
provides improved cable current carrying capacity by eliminating the circuiting current losses in the
metal sheath.
Single point bonding can be applied to a single section of cable between substations and/or overhead
towers. It can also be applied at the ends or in the middle of an underground cable circuit if one or two
extra cable section(s) exist in addition to major cross-bonding sections. Single point bonding is
insensitive to balanced adjacent section lengths compared to cross-bonding configurations.
Single point bonding can also be used in a sectionalised scheme where long cable systems consists
of multiple individual single point bonding sections (sectionalised), Figure 2.4. A sectionalised single
point bonding system is a way to increase the maximum cable system length while maintaining low
losses (cross-bonding also fulfils these requirements). All minor cable sections between joints have
one end connected solidly to ground, and one end connected across SVLs. Similar to single point
bonding, in addition, sectionalised single point bonded systems allows for longer cable system lengths
with reduced screen voltages, while maintaining low screen losses. Similar to single point bonding, it is
especially noted that the zero sequence impedance of the sectionalized single point bonded systems
may be high and that the induced screen voltages may be higher than for cross bonded systems.
Sectionalised single point bonded systems are simple by design, compared to cross-bonding, but may
experience increased losses due to circulating currents in the ECC.
Single point bonding in general is insensitive to balanced adjacent section lengths compared to cross-
bonding configurations.

SVLs

Joint Joint

ECC SVLs SVLs

Figure 2.4: Sectionalised single point bonding system

The voltage induced on the metal sheaths by phase conductor currents is proportional to the length of
the underground cable section. Therefore, depending on the limitation of standing sheath voltages, the
applicable cable length can be limited. Standing voltage limits are typically 50 to 400 V, but may be
higher depending on requirements from different countries. For some countries, if the voltage is higher

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

than 50 V, the live component should be shielded for safety reasons. If a fault occurs on the circuit, the
sheath induced voltage could be the highest among all bonding configurations due to the high zero
sequence impedance. Generally, the bonding system insulation must withstand the induced AC
voltage for the entire lifetime, even under conditions of chemical, water, or other possible environment
stresses.
Using an additional earth return path such as the earth continuity conductor (ECC) or local common
earth is necessary to reduce the sheath induced voltage and the interference to other electrical
equipment (e.g., communication cables). To ensure the efficiency of the ECC in the fault condition, it is
desirable to install ECC close enough to the three power cables, but yet the ECC should not be
derating the power cable as a result of the circulating current in ECC in the operating condition. ECC
is required to withstand the earth fault current and withstand attendant voltage rise from the earth.
It is worth mentioning that to ensure the efficiency of SVLs in transient conditions, lower rated voltage
SVL is desirable in terms of low residual voltage but SVL should not be damaged by power frequency
induced voltages in fault conditions.
In case that both single point bonding and cross-bonding are applicable, and if cross-bonding is more
economical than single point bonding, cross-bonding is recommended due to its known advantages:
generally, no ECC required, lower zero sequence impedance, lower sheath induced voltage, and
lower earth potential rise.
2.1.3 Mid-point bonding
Mid-point bonding system consists of two single point bonding systems. Both ends of the cable metal
sheath are open and the mid-point is grounded through the parallel earth continuity conductor. SVLs
are typically connected between metal sheath and the local earth at the open ends (e.g., in
substations, or at transition towers). Figure 2.5 is a typical example of such bonding configuration.

Cable end

Metal sheath Sheath voltage limiter

Earth continuity conductor

Figure 2.5: Mid-Point bonding system

Mid-point bonding is used to reduce the sheath induced voltage to about half of the single point
bonding of the entire cable length.
One configuration of mid-point bonding is to use two minor sections of the cable sections by adding
one earth joint in the middle of the cable route. This configuration bonding can be applied either by a
sectionalized joint with both sides of shielding interrupts grounded or by connecting three cable metal
sheaths together (star connection) at the midpoint to the earth without a sectionalized joint if it is
applicable to the cable design.
Another configuration of the mid-point bonding is to have the open ends and SVLs in the middle and
connect the cable ends in substations or at transition poles to earth. This configuration may result in
doubling the voltage at the joint interrupt in the middle, but improving safety concerns at the cable
ends in substations or at transition poles.
The same considerations for the single point bonding can be taken to ensure efficiency of ECC and
SVLs. Especially, if either end of the mid-point bonding system is not at the termination of the cable
route, but connected to the sectionalized cable joint. Special care must be taken to protect screen
interrupts from electromagnetic transients.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

2.1.4 Cross-bonding
Figure 2.6 shows a configuration of a cross-bonding system. The sheath continuity is interrupted at
regular minor section length by cross-bonding joints. Connections are made between the sheaths so
that each sheath circuit connects the three phase conductors successively. In this way, induced
voltages in the screen circuits are reduced (in the ideal case, three voltages offset by 120° are added
up), and thus the sheath circulating currents are reduced.
(induced voltage in the screens for a trefoil laying)

Screen voltage

Distance

Figure 2.6: Typical cross-bonding system

2.1.4.1 Continuous cross-bonding

In order to protect screen interruptions from electromagnetic transients, sheath voltage limiters (SVL)
are installed at cross-bonding points, see Figure 2.7. Compared to single-point bonding, this
configuration has the advantage of not limiting the length of the cable system.

Terminations

Cross-bonding joints

Figure 2.7: Continuous cross-bonding system

The sheaths constitute a return path for zero-sequence fault currents. The screening effect reduces
the induced voltages in parallel conductors (such as telecommunication cables) more efficiently than
the screening offered by the earth continuity conductor in case of single-point bonding.
Continuous cross-bonding has the major advantage that dissimilar minor section lengths are evened
out over a larger number of minor sections than sectionalised cross-bonding. Besides, if a residual
voltage is allowed, the number of minor sections does not need to be a multiple of three. Generally,
sheath induced voltages and circulating currents decrease when the number of sections increases. On
the other hand, a complete compensation may never be achieved. It is essential to consider this
possibility when determining the current rating of the circuit, rather than simply assuming that the
sheath circulating current will be zero.
2.1.4.2 Sectionalised cross-bonding
The sectionalized cross-bonding configuration consists of multiple major sections along a cable circuit.
A major section consists of three minor sections. Figure 2.8 is a typical example of such configuration.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Terminations

Cross-bonding joints Earthing joints

Minor section
Major section

Figure 2.8: Sectionalised cross-bonding system

Compared to the continuous cross-bonding, the sheaths of the sectionalised cross-bonding are
connected to the ground at the ends of the major sections to complete one section (sectionalised).
Dissimilar minor section lengths may cause sheath circulating currents. The circulating current may
reduce the circuit rating. For example, an imbalance of about 30% in minor section length for a
touching trefoil laying or an imbalance of about 25% in minor section length for a trefoil laying in ducts
may result in a reduction of 1% in circuit ampacity. For a flat formation with 40 cm spacing between
phase cables, the same 1% reduction is obtained with an imbalance of about 15% in minor section
length. The imbalance here refers to the length difference between two minor sections and the third
one is the average of the other two.
Compared to continuous cross-bonding, sectionalised cross-bonding has two advantages: it requires a
smaller number of SVLs, and the more frequent grounding leads to the reduction of transient
overvoltages travelling along the circuit.
The drawback, compared with continuous cross bonding, is that it may be more difficult to perform
sheath jacket integrity tests. For continuous cross-bonding, the jacket test can be performed from one
termination only after disconnecting the grounding connections at both terminations.
For circuits without a multiple of three minor sections, a combination of bonding configurations may be
applied, e.g., cross-bonding and single point bonding.
2.1.4.3 Cross-bonding and transposition
In order to reduce the induced voltages and sheath circulating currents by cable phase conductors,
cables can be transposed, e.g., at each joint pit/chamber. This way, the mean geometric distances
between parallel cables are equal. The induced voltage in the sheath is then near zero when the
circuit configuration is unchanged. It is worth mentioning that, even for a trefoil laying, transposition of
the cables is recommended to limit the induced voltages by nearby conductors. Figures 2.9 and 2.10
show the cable transposition concept.
Parallel conductor Transposed cables

Apparent distance between

cables and parallel conductor:

Figure 2.9: Cable transposition

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Terminations

Cross-bonding joints Earthing joints

Minor section
Major section

Figure 2.10: Sectionalised cross-bonding system with cable transposition typically near joints
It must be acknowledged that perfect cross-bonding, and thus total cancellation of circulating currents,
is only achieved when the cable mutual impedance is equal between different minor lengths. This is
especially important when one minor section has a different laying configuration than the other two of
that major section, which may be the case for long horizontal directional drillings (HDDs), etc. For such
installations special attention must be given to ensuring equal mutual impedance between the cables.
2.1.4.4 Direct cross-bonding
Direct cross bonding is a method where the screens are transposed directly (without the need for link
boxes). Direct cross bonding is a simple scheme that allows for fewer accessories (link boxes and
SVLs) than for other cross bonding methods, Figure 2.11.

Figure 2.11: Direct cross bonding – Middle major section is directly cross bonded
In direct cross bonding, shorter single core bonding leads can be used, as the leads do not need go to
a link box, but may go directly from one joint to the next. The highest transient overvoltages occur in
sections closest to the terminations. Therefore, it is conceivable to limit the protection of screen
interruptions to major sections located at both ends of the circuit. At the other cross-bonding points,
cross-bonding of the screens is then performed “directly” by jointing single-core bonding leads without
SVLs. In some cases, when the overvoltages which are likely to stress the materials are deemed
acceptable, it is possible to design circuits without any SVLs.
The simplicity of direct cross bonding reduces the number of accessories to install and therefore
reduces possibility of failures and maintenance requirements. As no link boxes are used,
disconnection of the cable screens is not straight-forward. For jacket testing and fault location, it may
therefore be necessary to cut the bonding cables in order to perform some bonding system
measurements.
2.1.4.4.1 Direct sectionalised cross-bonding
The highest transient overvoltages appear in the sections closest to the terminations, if the transient
voltages are caused by lightning or switching surges originated from the substations or transition poles
where the terminations are located. Based on this assumption, transient voltage protection for the
sheath insulation jacket and sheath joint interrupts is only located at the termination ends of the circuit.
At the other cross-bonding points of the circuit, cross-bonding of the screens is then performed
“directly” by jointing single-core bonding leads, without SVLs. In some cases, when the overvoltages

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

which are likely to stress the materials are deemed acceptable, it is even possible to do without any
SVL on the whole link (Figure 2.12).

Terminations

Cross-bonding joints Earthing joints

Minor section
Major section

Figure 2.12: Direct sectionalised cross-bonding system without SVLs

2.1.4.5 Cross-bonding of short lines

Some cable circuits may be too long for single point bonding due to standing voltage limits, and
practical issues may make it difficult for cross-bonding with three minor sections. For this case, cross-
bonding with only two minor sections may be applied. This solution is not optimal, but the circulating
currents can be still reduced, compared to solid bonding. It can be shown that the circulating currents
for cross-bonding with two minor sections are half of the circulating currents for solid bonding, Figure
2.13. (Also see Appendices 2.1.B and 2.1.C).

Terminations

Cross-bonding joints

Figure 2.13: Cross-bonding system of short lines with two minor sections

2.1.5 Cross-bonding in tunnel installations

Special attention must be given to cross bonded systems installed in tunnels. Generally, in tunnels, all
joints and the bonding system are accessible for maintenance, while local grounding locations are
often limited or prohibited inside the tunnel due to constraints by the tunnel construction (concern of
water seal or cracking, etc.). In order to take advantage of the accessibility to the joints in tunnels,
SVLs are generally connected with the shortest possible leads across the sheath interruptions. A
direct cross bonded system is normally used in tunnels as shown in Figure 2.14. With this
arrangement, the cross bonding leads connecting the sheaths of different phases are not required to
carry surge current and hence their length and inductance are not of importance. However, the leads
must be of adequate cross section to carry system short circuit currents.

Fig 2.14: Direct cross bonding with SVLs in tunnels (direct grounding at both ends not shown)

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

This cross bonding scheme does not require link boxes. Grounding point is also not required at the
sheath sectionalised joints. Sheath interruption of joints are protected by SVLs installed with a delta-
connection between three phases of bonding leads, comparing with regular cross bonding link box
that uses a star-connection with neutral point grounded. The effectiveness of delta connection of SVLs
is described in CIGRE Electra 128 and the shortest bonding lead connection of SVLs can provide
more effective protection for the sheath interrupter of joints.
SVLs and their bonding leads to joints shall be protected from moisture ingress by suitable insulation
thus there is no easy way for temporary disconnection of SVLs from the particular joints. Higher rated
voltage (usually two times) is required for SVLs than for star-connected SVLs in regular cross bonding
link box because of the higher induced voltage appearing across sheath interruptions under normal
and transient conditions.
2.1.6 Impedance bonding
Impedance bonding is a form of solid bonding but with three-phase sheath bonding transformers. The
system configuration is described in IEEE 575. The primary advantage of the shield/sheath bonding
transformer scheme is that it is effective in limiting induced shield/sheath currents regardless of
whether or not the distances between cable joint bays are equal or unequal. It is noted that the use of
the impedance bonding system is limited. The primary disadvantage of the shield/sheath bonding
transformer scheme is that additional space is required in the joint vaults to accommodate the
additional components (compared to other screen bonding methods). The cost of the equipment for
implementing transformer bonding is also generally higher than that for single-point or cross bonding
schemes. Impedance bonding is not further discussed in the present document.
2.1.7 Siphon lines
Circuits connecting overhead lines and underground cables (so-called siphon lines) require additional
analyses in order to establish a safe and optimum cable sheath bonding system.
At substations, the sheath can be grounded through a well-established earthing method. However,
outside of substations, ideal grounding may not always be available. Due to the exposure of the
sheath when going from overhead line to underground cable, high voltages on the sheath and on the
sheath interruption at cross-bonded joints may be experienced during faults or lightning strikes.
The approach to designing the bonding system of a siphon line is similar to a regular cable system.
However, the designer must be especially aware that the grounding resistance at the transition pole
may be larger than in a substation. Therefore, the overvoltages during faults and lightning strikes may
be significantly higher than in a substation. The design requirement of SVLs, cable jacket, and sheath
interruption for cross-bonding joints must consider these factors.
2.1.8 Bonding of special cable system designs
2.1.8.1 Parallel cable systems
Special attention should be given to situations where two cable systems are installed in parallel or in
close proximity of each other. In relation to the bonding schemes, it must be noted that the mutual
coupling between the cable phase conductors of the first circuit to the screens of the second circuit
must be properly considered because the otherwise balanced bonding system may become
unbalanced due to the effect of the adjacent circuit. In general, it may be recommended to transpose
the phase conductors in order to obtain maximum balance of the system. The losses in parallel cable
systems must also be considered very carefully. Some additional guidance is available for parallel
solidly bonded cable systems in, for example, IEC 60287-1-2 for the eddy loss factors of parallel
circuits in flat formation [25].
2.1.8.2 Multiple cables per phase
The ampacity requirement for some cable projects may lead to the use of more than one cable
conductor per phase. In general, this system design should be handled in a way similar to cable
systems with circuits in parallel, and thus the special considerations are also similar.
2.1.8.3 Cable systems with a fourth conductor
For some cable systems, it may be beneficial to install four cables per circuit instead of three. The
“fourth phase” can be used as a spare in case of a fault on one of the three operating phases to
minimize repair time. For all bonding schemes, it is noted that the fourth phase can cause unbalance
to the system due to the interphase distance change. The analysis of this unbalance should be made
for each cable project in order to optimize the design of the cable and bonding system.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

2.1.9 Example of induced voltage calculations of a single point bonded system

E1

E2
Sheath Voltage Limiter
E3

1
𝑗
2𝜋 2𝑒 2
𝛼=𝑒 3 𝐷= 𝛾 = 𝑒 0,577
𝜔𝜇
𝛾√
𝜌𝑠𝑜𝑖𝑙

The following notations are used.

 𝑅𝑠 : Metal sheath resistance
 𝑟𝑠 : Metal sheath mean radius
 𝑑𝑖𝑗 : Distance between phases

Describe or define all symbols (e.g., R1, R2, D, etc.)

The metal sheath self-impedance is:
𝜔𝜇 𝜔𝜇 𝐷
𝑍𝑠 = + 𝑅𝑠 + 𝑗 ln
8 2𝜋 𝑟𝑠
The mutual impedance between core conductor and metal sheath is:
𝜔𝜇 𝜔𝜇 𝐷
𝑍𝑚 = +𝑗 ln
8 2𝜋 𝑟𝑠
The mutual impedance between phases is:
𝜔𝜇 𝜔𝜇 𝐷
𝑍𝑖𝑗 = +𝑗 ln
8 2𝜋 𝑑𝑖𝑗
(1) Normal operating condition and three phase symmetrical fault

Assuming that the current flowing in the earth continuity conductor are negligible, the sheath
voltage at open end may be derived:
𝐸1 = 𝑍𝑚 𝐼1 + 𝑍12 𝐼2 + 𝑍13 𝐼3
Assuming three phase symmetrical currents,
1 √3 1 √3
𝐼1 = (𝐼, 0), 𝐼2 = 𝐼 (− , − ) , 𝐼3 = 𝐼 (− , )
2 2 2 2
Real part of the mutual impedance is cancelled, thus
𝜔𝜇 𝐷 √𝑑12 √𝑑13 √3 𝑑12 𝐷 𝜔𝜇 1 𝑑12 ∙ 𝑑13 √3 𝑑12
𝐸1 = 𝑗 ∙ 𝐼 ∙ [ln ( ∙ ∙ )+𝑗 ln ( ∙ )] = 𝑗 ∙ 𝐼 ∙ [ ln ( 2
)+𝑗 ln ( )]
2𝜋 𝑟𝑠 √𝐷 √𝐷 2 𝐷 𝑑13 2𝜋 2 𝑟𝑠 2 𝑑13
For a trefoil formation, s= d12= d13= d23, where s = phase spacing
𝜔𝜇 𝑠
𝐸1 = 𝑗 ∙ 𝐼 ∙ 𝑙𝑛
2𝜋 𝑟𝑠

(2) Single phase earth fault external to cables (solidly earthed neutral)

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

It is assumed the single phase short circuit return current flows entirely in the earth continuity
conductor and that the phase currents in the non-faulted phases are negligible. The sheath voltage to
local earth may be derived:
𝐸𝐹 = 𝑍𝑖𝑓 𝐼𝑓 + 𝑍𝑖𝑐 𝐼𝑐
where,
If = Earth fault current in conductor
Ic= Earth fault return current in ECC
The mutual impedance between fault conductor and metal sheath i is
𝜔𝜇 𝜔𝜇 𝐷
𝑍𝑖𝑓 = +𝑗 ln
8 2𝜋 𝑠𝑖𝑓
The mutual impedance between ECC and metal sheath i is:
𝜔𝜇 𝜔𝜇 𝐷
𝑍𝑖𝑐 = +𝑗 ln
8 2𝜋 𝑠𝑖𝑐
The self-impedance of ECC is:
𝜔𝜇 𝐷
𝑍𝑐 = 𝑅𝑐 + 𝑗 ln
2𝜋 𝛾𝑐
Assuming If=-Ic, the real part of impedance is cancelled, thus the sheath induced voltage is expressed
by:
𝜔𝜇 𝑠𝑖𝑐
𝐸𝑓 = 𝑗 ∙ 𝐼 ∙ 𝑙𝑛
2𝜋 𝑓 𝑠𝑖𝑓
where,
scf: spacing between the ECC and the faulty cable
sif: spacing between screen of cable I and the fault cable
sic: spacing between screen of cable and the ECC
Note that Ef is the voltage to the earth at the current injection point, not to the local earth at remote
end. As the biggest concern for the cable outer protection is normally the voltage between the cable
metal sheath and the local earth, Earth Potential Rise (EPR) is taken into account which is expressed
by:
𝐸𝐸𝑃𝑅 = 𝑍𝑐 ∙ 𝐼𝑐 + 𝑍𝑐𝑓 ∙ 𝐼𝑓
Therefore, the voltage from the screen to the local earth is expressed by
𝜔𝜇 𝑠𝑐𝑓 𝜔𝜇 𝑠𝑖𝑐
𝐸𝑓 − 𝐸𝐸𝑃𝑅 = 𝑍𝑖𝑓 𝐼𝑓 + 𝑍𝑖𝑐 𝐼𝑐 − 𝑍𝑐 𝐼𝑐 − 𝑍𝑐𝑓 𝐼𝑓 = 𝑗 ∙ 𝐼 ∙ 𝑙𝑛 − 𝐼𝑐 [𝑅𝑐 + 𝑗 𝑙𝑛 ]
2𝜋 𝑓 𝑠𝑖𝑓 2𝜋 𝛾𝑐
Assuming Ic=-If, the screen to local earth voltage of the faulty cable is derived:
2
𝜔𝜇 𝑆𝑐𝑓
𝐸 = [𝑅𝑐 + 𝑗 ∙ 𝑙𝑛 ] ∙ 𝐼𝑓 [V/m]
2𝜋 𝑟𝑠 ∙𝛾𝑐

According to Electra 128 and CIGRE TB 283, this assumption is normally true and leads to sheath
overvoltages which are slightly higher than those observed in practice.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

2.1.10 Example of circulating current calculations for a solid bonded system

1
𝑗
2𝜋 𝑅1 + 𝑅2 2𝑒 2
𝛼=𝑒 3 𝑅= 𝐷= 𝛾 = 𝑒 0,577
𝐿 𝜔𝜇
𝛾√
𝜌𝑠𝑜𝑖𝑙

The following notations are used:

▪ 𝑅𝑠 :Metal sheath resistance
▪ 𝑟𝑠 : Metal sheath mean radius
▪ 𝑑𝑖𝑗 : Distance between phases

The metal sheath self-impedance is:

𝜔𝜇 𝜔𝜇 𝐷
𝑍𝑠 = + 𝑅𝑠 + 𝑗 ln
8 2𝜋 𝑟𝑠
The mutual impedance between core conductor and metal sheath is:
𝜔𝜇 𝜔𝜇 𝐷
𝑍𝑚 = +𝑗 ln
8 2𝜋 𝑟𝑠
The mutual impedance between phases is:
𝜔𝜇 𝜔𝜇 𝐷
𝑍𝑖𝑗 = +𝑗 ln
8 2𝜋 𝑑𝑖𝑗
Assuming a trefoil laying configuration with a spacing between phase conductors 𝑆 and assuming:
𝑍12 = 𝑍13 = 𝑍23 = 𝑍𝑐
Through Kirchhoff’s laws, the following equation is derived:
𝑅1 𝑅2
𝑍𝑠 𝐼𝑠1 + 𝑍𝑚 𝐼𝑐1 + 𝑍𝑐 (𝐼𝑐2 + 𝐼𝑠2 + 𝐼𝑐3 + 𝐼𝑠3 ) + (𝐼 + 𝐼𝑠2 + 𝐼𝑠3 ) + (𝐼𝑠1 + 𝐼𝑠2 + 𝐼𝑠3 ) = 0
𝐿 𝑠1 𝐿
𝑅1 𝑅2
𝑍𝑠 𝐼𝑠2 + 𝑍𝑚 𝐼𝑐2 + 𝑍𝑐 (𝐼𝑐1 + 𝐼𝑠1 + 𝐼𝑐3 + 𝐼𝑠3 ) + (𝐼𝑠1 + 𝐼𝑠2 + 𝐼𝑠3 ) + (𝐼𝑠1 + 𝐼𝑠2 + 𝐼𝑠3 ) = 0
𝐿 𝐿
𝑅1 𝑅2
{𝑍𝑠 𝐼𝑠3 + 𝑍𝑚 𝐼𝑐3 + 𝑍𝑐 (𝐼𝑐1 + 𝐼𝑠1 + 𝐼𝑐2 + 𝐼𝑠2 ) + 𝐿 (𝐼𝑠1 + 𝐼𝑠2 + 𝐼𝑠3 ) + 𝐿 (𝐼𝑠1 + 𝐼𝑠2 + 𝐼𝑠3 ) = 0
Thus:
(𝑍𝑠 + 𝑅)𝐼𝑠1 + (𝑍𝑐 + 𝑅)𝐼𝑠2 + (𝑍𝑐 + 𝑅)𝐼𝑠3 + (𝑍𝑚 − 𝑍𝑐 )𝐼 = 0
{(𝑍𝑠 + 𝑅)𝐼𝑠2 + (𝑍𝑐 + 𝑅)𝐼𝑠1 + (𝑍𝑐 + 𝑅)𝐼𝑠3 + (𝑍𝑚 − 𝑍𝑐 )𝛼²𝐼 = 0
(𝑍𝑠 + 𝑅)𝐼𝑠3 + (𝑍𝑐 + 𝑅)𝐼𝑠1 + (𝑍𝑐 + 𝑅)𝐼𝑠2 + (𝑍𝑚 − 𝑍𝑐 )𝛼𝐼 = 0
With 𝑎 = 𝑍𝑠 + 𝑅 and 𝑏 = 𝑍𝑐 + 𝑅, then:
𝑎 𝑏 𝑏 𝐼𝑠1 1
(𝑏 𝑎 𝑏 ) (𝐼𝑠2 ) = − (𝛼²) (𝑍𝑚 − 𝑍𝑐 )𝐼
𝑏 𝑏 𝑎 𝐼𝑠3 𝛼
𝑎 𝑏 𝑏
1
with 𝐴 = (𝑏 𝑎 𝑏) and 𝐴−1 = det 𝐴 𝑡com𝐴:
𝑏 𝑏 𝑎

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

det 𝐴 = 𝑎3 − 3𝑎𝑏 2 + 2𝑏 3
𝑎2 − 𝑏² 𝑏 2 − 𝑎𝑏 𝑏 2 − 𝑎𝑏
com𝐴 = (𝑏 2 − 𝑎𝑏 𝑎2 − 𝑏² 𝑏 2 − 𝑎𝑏)
𝑏 2 − 𝑎𝑏 𝑏 2 − 𝑎𝑏 𝑎2 − 𝑏²
Hence:
𝐼𝑠1 1 𝑎2 − 𝑏 2 𝑏 2 − 𝑎𝑏 𝑏 2 − 𝑎𝑏 1
(𝐼𝑠2 ) = − 3 ( 𝑏 2 − 𝑎𝑏 𝑎2 − 𝑏 2 𝑏 2 − 𝑎𝑏) (𝛼 2) (𝑍𝑚 − 𝑍𝑐 )𝐼
𝐼𝑠3 𝑎 − 3𝑎𝑏 2 + 2𝑏 3 2
𝑏 − 𝑎𝑏 𝑏 2 − 𝑎𝑏 𝑎2 − 𝑏 2 𝛼
𝐼𝑠1 1 𝑎2 − 𝑏 2 − 𝑏 2 + 𝑎𝑏 𝐼
(𝐼𝑠2 ) = − 2 2
( 𝑎2 − 𝑏 2 − 𝑏 2 + 𝑎𝑏) (𝑍𝑚 − 𝑍𝑐 ) (𝛼 2 𝐼 )
(𝑎 − 𝑏)(𝑎 + 𝑎𝑏 − 2𝑏 ) 2
𝐼𝑠3 𝑎 − 𝑏 2 − 𝑏 2 + 𝑎𝑏 𝛼𝐼
𝐼𝑠1 1 𝐼
(𝐼𝑠2 ) = − (𝑍𝑚 − 𝑍𝑐 ) (𝛼 2𝐼 )
𝐼𝑠3 𝑎−𝑏
𝛼𝐼

𝑍𝑚 − 𝑍𝑐 𝑍𝑚 − 𝑍𝑐 𝑍𝑚 − 𝑍𝑐
𝐼𝑠1 = − 𝐼; 𝐼𝑠2 = − 𝛼²𝐼; 𝐼𝑠3 = − 𝛼𝐼;
𝑍𝑠 − 𝑍𝑐 𝑍𝑠 − 𝑍𝑐 𝑍𝑠 − 𝑍𝑐
Thus, the metal sheath current to core conductor current ratio is:
𝜔𝜇 𝜔𝜇 𝐷 𝜔𝜇 𝜔𝜇 𝐷
𝐼𝑠𝑖 𝑍𝑚 − 𝑍𝑐 + 𝑗 ln − − 𝑗 ln
8 2𝜋 𝑟𝑠 8 2𝜋 𝑆
𝜂=| |=| | = |𝜔𝜇 𝜔𝜇 𝐷 𝜔𝜇 𝜔𝜇 𝐷|
𝐼 𝑍𝑠 − 𝑍𝑐 + 𝑅𝑠 + 𝑗 ln − − 𝑗 ln
8 2𝜋 𝑟𝑠 8 2𝜋 𝑆
𝜔𝜇 𝑆
𝑗 ln
2𝜋 𝑟𝑠
𝜂=| 𝜔𝜇 𝑆|
𝑅𝑠 + 𝑗 ln
2𝜋 𝑟𝑠

𝜔𝜇 𝑆
ln
2𝜋 𝑟𝑠
𝜂=
𝜔𝜇 𝑆 2
√𝑅𝑠2 + ( ln )
2𝜋 𝑟𝑠

A numerical application with a 225 kV cable commonly used in France gives the following table:
630 Al 1200 Al 1600 Al 2000 Al 2500 Al 1200 Cu 1600 Cu 2000 Cu 2500 Cu
2𝑟𝑠
82.6 95.2 102.4 109 117 96.3 103.3 109 117
(mm)
𝑅𝑠 9.12E- 9.50E- 1.10E- 1.04E- 9.66E- 9.39E- 1.09E- 1.04E- 9.66E-
(Ω/km) 02 02 01 01 02 02 01 01 02
𝑆
200 240 240 240 290 240 240 240 290
(mm)
𝜂 74 % 73 % 66 % 67 % 72 % 73 % 66 % 67 % 72 %

33
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

2.1.11 Example of circulating current calculations for a cross-bonded system with

two minor sections

Following the same approach as for solid bonding (see Section 2.1.9), Kirchhoff’s laws yield:
𝑍𝑠 + 𝑅 𝑍𝑐 + 𝑅 𝑍𝑐 + 𝑅 𝐼𝑠1 𝑍𝑚 − 𝑍𝑐 𝛼
( 𝑐+𝑅
𝑍 𝑍𝑠 + 𝑅 𝑍𝑐 + 𝑅) (𝐼𝑠2 ) = 𝐼( 1 )
𝑍𝑐 + 𝑅 𝑍𝑐 + 𝑅 𝑍𝑠 + 𝑅 𝐼𝑠3 2 𝛼2
where,
𝑅1 + 𝑅2
𝑅=
2𝐿
After inversion of the matrix, the following equation is derived:
𝐼𝑠1 1 𝑍𝑚 − 𝑍𝑐 𝛼
𝐼
( 𝑠2 ) = 𝐼( 1 )
𝐼𝑠3 2 𝑍𝑠 − 𝑍𝑐 𝛼2
The equation shows that circulating currents are half of the circulating currents of the solid bonded
system.

2.2 Sheath voltage limiter selection and application

2.2.1 Sheath voltage limiters
Standard surge arresters at power system voltage levels are generally used at the cable terminals
(e.g., substations or transition poles) to protect the primary insulation of the cable systems. Sheath
voltage limiters are used to protect the sheath bonding system insulation from transient overvoltage
events due to lightning strikes and switching surges. The designed insulation level of the sheath
insulation and bonding system should also consider overvoltages in the event of cable faults or fault
currents running in the cable system. The sheath voltage limiter is usually rated for distribution voltage
levels.
The selection of a sheath voltage limiter (SVL) is important. Without some means to limit the transient
overvoltages, the excessive voltage may cause an electrical breakdown of the sheath insulating
jacket, sheath interrupts in sectionalizing joints, bonding cables, the termination mounting insulation at
the cable terminals, or other components as part of the sheath bonding systems. For example, the
damage to the insulating jacket may make the metal sheath susceptible to corrosion and can result in
unanticipated sheath circulating currents that may increase losses of the cable systems and cause hot
spots along the cable route.
To protect the cable insulating jacket and other components of the bonding systems from transient
overvoltages, sheath voltage limiters must be applied to limit the overvoltage across the insulation to
prevent insulation breakdown. Industry standards (e.g., IEC 60229 as referred by both IEC 60840 and
IEC 62067) require that the insulating jackets withstand the nominal AC voltage and impulse voltages.
The magnitudes of power frequency overvoltages depend on rated system voltages, cable section
lengths and the magnitude of cable fault currents. Both longer cable section lengths and greater fault
currents contribute to greater magnitude of power frequency overvoltages occurring between the
metal sheath and the earth.
Sheath voltage limiters (SVLs) are generally defined as surge arresters with metal oxide varistors
(MOV) in the protective housing but without spark gaps. SVLs are generally designed and tested in

34
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

accordance with IEC 60099-4, “Part 4: Metal-oxide surge arresters without gaps for AC systems” [26].
Their characteristics are selected for operation in the event of transient overvoltage that will exceed
the insulation withstand range for the cable metal sheath of the extruded dielectric or low-pressure oil-
filled cables.
Some utilities still use spark gaps, including the specialized “ring gaps”, independently or in series with
SVLs. A few utilities retain these devices in their specifications. The spark gaps are not subjected to
deterioration in the manner of non-linear devices, and they can be left in the system during
maintenance testing and fault location without adverse effects. However, this brochure is focused on
the application of metal oxide varistor surge arrestors only.
The selection of appropriate SVLs must be determined based on the anticipated AC voltage under
normal and fault conditions and the required discharge voltages to protect the bonding system
insulation from transient overvoltages due to lightning strikes, switching surges or cable system faults.
The SVL is designed to withstand the power frequency voltage appearing during normal system
conditions and during system faults and to protect the bonding system insulation from transient
overvoltages, considering the effect of voltage drop of the bonding leads and connection
configurations. The SVL is not designed to mitigate the power frequency voltages due to system
faults.
The maximum magnitude of fault currents is usually determined by system planners based upon
power system studies at various possible system faults, e.g., three-phase-to-earth fault, phase-to-
phase fault, and single-phase-to-earth fault. This fault current magnitude combined with knowledge of
the cable construction and trench geometry can determine the extent of induced voltages on the cable
metal sheath layer during the fault events. Electra 28 [1] Electra 128 [3], TB 283 [4], and IEEE 575 [6]
provide guidance for the calculations. Internal cable faults create the highest induced sheath voltage,
and the SVLs are not designed to protect such overvoltages, and will fail at such conditions if not
correctly designed and selected. Usually, the highest induced sheath voltage occurs during a single-
phase-to-ground fault for a single-point bonded system, and three-phase fault for a cross-bonded
system. Earth potential rises should be considered, especially if the SVLs are star-connected to earth
(see TB 347 [5]). These parameters, along with the insulation protection levels for transient
overvoltages, provide guidance to selecting the protective properties of the sheath voltage limiters.
A general practice of many countries is that the physical selection of the SVL is done by the cable
material supplier in response to the user’s specifications. Users specify the SVL requirements by
providing fault current levels of the cable systems, the associated maximum induced voltages, and
parameters for the transient overvoltage calculations and simulations. The supplier then selects
particular models of SVLs and a link box enclosure.
2.2.2 Selection of sheath voltage limiters
Typical characteristics of SVLs include:
▪ Rated voltage Ur
▪ Maximum continuous operating voltage Uc
▪ Nominal discharge current
▪ High current impulse withstand
▪ Long duration current impulse withstand
▪ Short circuit withstand
▪ Maximum residual voltage, Ures
▪ Nominal creepage distance
▪ Typical temporary overvoltage curve
▪ Energy rating

The general procedure to select sheath voltage limiters includes the following. C55 [7], Annex A, and
Electra 128 [3] describe the Design Guide for SVL selection and provide application examples. The
procedure is further refined as below:
1. Calculate the sheath power frequency overvoltage during a fault at a specific fault duration, based
on provided cable system information.
2. Compare the calculated power frequency overvoltage with the temporary overvoltage-versus-time
(TOV) characteristics of a selected SVL. The calculated overvoltage shall be less than the
temporary overvoltage value at a specific duration with a typical 5% to 25% protection margin.

35
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

This is the minimum performance criteria for SVL selection. This selection approach requires that
the SVL power frequency temporary overvoltage-versus-time characteristics be given.
3. If the TOV characteristics of the SVL are not available, the SVL can be selected based on its
Maximum Continuous Operating Voltage, Uc. Uc should be higher than the calculated power
frequency overvoltage. An inherent safety margin is included by the difference between the
Maximum Continuous Operating Voltage, Uc, and the Rated Voltage, Ur.
4. Select the Rated Voltage, Ur, from the manufacturer’s SVL data sheet based on TOV comparison
or selected Uc. The typical value of Ur is between 3 and 12.5 kV. A value of 3 kV is usually
considered minimum. Note that Ur is 15-25% greater than Uc on published manufacturer’s data
sheets.
5. Consider the residual Voltage, Ures, of the selected SVL. The voltage is the maximum arrester
voltage tested at, e.g., 8/20 μs and 10 kA current impulse. This value should be less than the
transient overvoltage withstand level of the insulating components (cable jacket, sheath/screen
interrupts in joint bodies, etc.). This evaluation of the residual voltage should also consider the
length and type of bonding cables that can result in a greater voltage appearing at the
components (see TB 283 [4] and Paper B1-108 [22]. In comparing the residual voltage to the BIL,
the reader should be aware that the wave shapes are different (BIL is 1.2/50 μs). The margin of
protection for the residual voltage over the component capability should be selected. A typical
safety margin of 15 to 25% is used, with consideration of deterioration of the component
respective insulating strength over the life of the components. The selection of SVLs is often done
based on a 10 kA impulse current characteristic as this is often the nominal discharge current for a
given surge pulse.
6. Verify the energy absorption capability of the SVL to withstand the maximum energy by the
dissipated transient surge currents. The energy absorption capacity is determined as a function of
the current duration and magnitude dissipated through the SVL. The amount of energy from the
surge current is determined from a transient analysis study and compared to the manufacturer’s
SVL characteristics. In most cases, this is not a determining factor in the selection of the SVL as
the capability often exceeds the surge energy. Internal cable faults cause significant overvoltages
at power frequency. If the SVL temporary overvoltage capability cannot withstand the overvoltage
caused by the cable internal fault, the SVL often cannot withstand this energy level and may fail,
which leads to the necessity to inspect at least the SVLs adjacent to the fault location after the
fault event and to ensure designs with a correctly rated SVL selected for the bonding system.

Applications of the primary arresters at the terminals of the cable circuits significantly limit the energy
absorption requirements of the SVL. The SVL should not absorb energy at power frequency voltage
during a fault. However, it will absorb energy during lightning and switching surges. The energy
absorption can be evaluated at 10 kA with a standard current wave form for lightning impulse current,
although the energy from the lightning impulses may not be the critical determining factor. Longer term
events and associated conductive period such as switching surges and power frequency follow-up
currents should also be considered. An EMTP study may be needed for this evaluation (see Section
2.3).
The arrester devices are available in different classes based on different energy absorption capacity.
IEC 60099 provides additional details on selecting the class of SVLs if this is a factor that requires
further evaluation. Generally, the class designates the extent of the durability of the respective
arrester. In lightning protection applications, a Class 1 surge protection device is used with a Class 2
for switching surge protection. Calculation procedures as described in CIGRE TB 283 [4] and EMTP
can be used for the evaluation. (Note: Classes 1 to 5 are replaced in IEC 60099-4:2014 using arrester
classes of Station and Distribution. For each class, duties of Low, Medium and High are used. As
such, the classes are designated as SH, SM, SL and DH, DM, DL.)
In general, the SVLs are always designed to withstand (not to protect) temporary AC overvoltages
induced for the maximum external cable fault durations. The SVLs may however not be able to
withstand the temporary AC overvoltages caused by cable internal faults. If a cable system
experiences a cable internal fault, the SVLs should be inspected before continuing use (see Section
3).
The type and length of bonding lead between the cable sheath connections to the link box connection
or earth connection affects the protection level of the sheath voltage limiter. Two types of bonding
leads are used: single conductor or concentric construction. Selection of the type depends on the

36
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

characteristic impedance (inductance) of the bonding leads; the link box must be configured for the
selected bonding cable type. Single phase link boxes on some riser structures are required to closely
position the bonding leads to the individual termination supports (usually within 10m).
The induced AC overvoltages caused by a fault can be a function of the type of cable sheath, such as
helically or annularly corrugated sheath [15].
2.2.3 SVL connection configurations:
The voltage across the SVLs is a function of connection configurations and mounting schemes. The
following connection configurations are used:
▪ Star formation with the star point connected to earth continuity conductors
▪ Star formation without the star point connected to earth
▪ Delta connection
▪ Direct placement crossing sheath interrupt – using one SVL – higher voltage rating
▪ Direct placement crossing sheath interrupt – using two SVLs and earth connection between
the two SVLs

Both single core bonding leads (with sheath grounded) and coaxial cable leads are used.
2.2.4 SVL installations
SVLs can be installed in a specially designed enclosure or link box. They can also be covered by other
protection materials, such as, heat shrinkable insulation tubes.
The number of SVLs used along the cable circuit and locations for SVL installation may be optimized
through system study. In this optimized design, energy sharing among SVLs should be considered to
reduce SVL failures due to excessive energy from internal cable faults.
A general guide for the locations and number of sheath voltage limiter usages is listed below. An
optimized decision must be made if some criteria are in conflict.
▪ Installed in a protected location, such as, a substation, to prevent from explosive events or
other safety concerns
▪ Installed at the open end of single point bonding systems, with the grounded end of the single
point bonding preferably at the end more subjected to lightning or switching impulses, or the
end with the lower grounding resistance
▪ Installed at locations more accessible for maintenance

For single-point bonded cable systems, factors to consider in selecting the location of sheath voltage
limiters include:
▪ Accessibility of link boxes for inspection of SVLs following fault events.
▪ The SVL should be located at the terminal end of the circuit that will experience the highest
incoming transient voltage. An evaluation of switching surges and lightning impulse would
both need to be considered. A GIS substation may have additional considerations where SVL
shunts may be applied between the GIS flange and ground in the station.
▪ The earthing point should ideally be selected at the terminal with the lowest earthing
impedance.
▪ Cable connected to certain equipment (e.g., metal clad switchgear) may see a steeper
voltage rise, so this would be a preferable location for grounding the sheath.

2.3 Cable system models for overvoltage calculations

The objective of this section is to provide guidance to assess the overvoltages which are likely to
stress an underground cable circuit. Recommendations are given on modelling and calculation
methods. Figure 2.15 shows an overview of the modelling and calculations.

37
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

CABLE
TRANSIENTS
CP, FDQ, SVL
IMPEDANCES
Wideband
Bonding lead
Schelkunoff Grounding OHL
implemented in
Wedepohl Wilcox EMT software POWER
Simplified formulae at FREQUENCY
power frequency CIM, NV, symmetrical
components, formulae in
standards

Figure 2.15: Modelling for overvoltage calculations

Cable impedances have been established through solving Maxwell’s equations of cable circuits for a
defined installation environment. These impedances appear in the Telegrapher’s equations, which are
the basis of cable modelling for transient studies. These models, which are implemented in some
transient analysis software, are used to compute overvoltages stressing the underground cables
subjected to lightning or switching impulses.
Simplifications are obtained at power frequency. Easy to handle formulae can then be used to assess
voltages and currents in both normal operations and faults. These studies do not require transient
analysis software. Ready-made formulae are available in various documents and standards. When
precision is needed, or to deal with particular configurations, methods exist which allow calculations
almost by hand, for example: CIM (Complex Impedance Matrix), NV (Node Voltage), symmetrical
component analysis.
Guidance is also given on the modelling of other components which have a significant impact on
sheath bonding studies: SVL, bonding lead, grounding and overhead lines. Some case studies are
given as illustrations.
2.3.1 Cable impedances and admittances
One modelling method is based on works by Schelkunoff [27] for the internal impedances of single-
core cables, and on works by Pollaczek [28] for the mutual impedances between cables and for the
ground. The Pollaczek formulae are quite complex, as these models involve Bessel functions with
frequency-dependent argument. The integral for finding an analytical approximation is difficult, which
makes it difficult to compute numerically because its integrand is highly oscillatory.
On the basis of these above works, Wedepohl and Wilcox [29] developed a model using numerical
computation by replacing the Bessel functions using hyperbolic functions, with accuracy in a broad
frequency range (up to about 100 kHz), for common cable technologies.
In 1979, Ametani [30] synthetized all the works previously mentioned, leading to a general formulation
of the impedances and admittances of single-core and three-core cables. The work was implemented
into EMT software. At power frequency, simple formulae can be derived, as shown in Figure 2.16.
TB 531 [31] provides discussion on the current modelling and limitations. Besides analytical formulae,
cable impedances can also be calculated through numerical computational methods such as Finite
Element Analysis or Boundary Element Analysis. These methods are generally used for evaluation of
cable systems with complicated cable geometry or arrangement that is not covered by existing
analytical formulae.

38
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

   1  D 
Conductor self impedance
za   Ra .1  Ys  Y p   j .  ln 
conductor 8 2  4  r1 
r2 r1 insulation

r3
metal screen    D 
ze   Re  j . ln 
2  re 
outersheath Screen self impedance
r4 8
Iei
Vei
spacing Vai Mutual impedances    D     D 
S Iai zm  j . ln  z ij  j . ln
2  re  2  d ij 
conductor-screen
and between cables 8 8
cable i

kc 1
 Vai   z a z m   I ia   z ij z ij   I aj 
  z
Ra  Re 
 
D.C. resistances of the conductor and the screen
   
z Vei   z m z e   I ei  z ij   I ej 
. π.g1.r12 π.g 3 . r32  r22
i j ij
1
Depth of earth return path 2.e 2
 : Bessel’s constant (1.7811) D
 I   y  y1  Vai   g s .
  ai    1 
z  I ei   y1 y1  y 2  Vei 

Skin effect factor Ys

x s4  .
Admittances of the insulation and the outersheath
Proximity effect factor Yp Ys  xs  .k s
(three-phase link) 192  0,8.x s4  Ra

2 .g 2  j 2  2 .g 4  j 4   

y1  y2  2 
 r'  r  x 4p  2.r    2.r 
2
1,18  .
ln 2'  ln 4  Yp  . 1  0,312. 1    xp  .k p
 r1   r3  192  0,8.x p  S  
4
 S 
4
xp 
 .Ra
 0, 27
 192  0,8.x p
4 
 

Figure 2.16: Simple formulae for overvoltage calculations (TB 531)

2.3.2 Power frequency studies

CIM method
The Complex Impedance Matrix (CIM) method is illustrated in Figure 2.17 (see Cigré TB 283).
Capacitance is neglected for the calculations of 50/60 Hz currents and voltages. The CIM method
consists of using Kirchhoff’s laws for the system composed by all the conductors (cores, screens,
earth, etc.), including all the (linear) equations under the form of:
𝐴𝑋 = 𝐵
where 𝐴 is a matrix containing impedances and boundary conditions (e.g., grounding of the screens),
𝑋 is a vector containing unknown voltages and currents and 𝐵 is a vector containing known voltages
and currents. Solving is simply done by inverting the matrix 𝐴.

V1
I1 R1 L1
Conductor 1
V2
I2 R2 L2
Conductor 2
Zij
…

Vn
In Rn Ln
Conductor n

Figure 2.17: CIM method for overvoltage calculations (TB 283)

NV method
The Node Voltage (NV) method is the similar to the CIM method, but it uses admittances instead of
impedances for the calculations. The CIM method may be preferable to the NV method, because
inverting empty matrices may lead to numerical errors for some cases. TB 347 discusses both
methods and indicates that “The NV method is common for calculation of voltages in electrical
networks by means of computer programs because the rules to establish the equation system can be
formalized quite well.” “The NV method may be more attractive than CIM method, if multiple earthing

39
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

points are to be considered (i.e., intermediate earthing in cross-bonded circuits)”. Also, the NV method
is more convenient for considering several links in parallel.
Symmetrical component analysis
Symmetrical component analysis is discussed in many documents and is largely used for power
frequency concerns, such as load flow and short-circuit calculations. The method can be used to
calculate voltages and currents in phase conductors by assuming that the metal screen is at zero
voltage. It is a helpful way to provide quick answers and get a good understanding of the system
behaviour. Figure 2.18 includes a summary of this method. A more general approach can be done
with modal analysis. This cannot be used to evaluate metal screen voltages since the voltages are
assumed at zero potential.

FORTESCUE SEQUENCES
Positive sequence
Positive sequence I3
I1  I I 2   2 .I I 3   .I
The phase conductor currents I1
are equal in magnitude and 120° 
 Vi   z  m .I i  z d .I i
out of phase. z

Representative of normal zd is the positive sequence impedance I2

operation conditions.
Negative sequence
I2
Negative sequence I1  I I 2   .I I 3   2 .I I1

As the positive sequence, except


that the phase sequence is  Vi   z  m .I i  z i .I i
z
reversed. I3
zi is the negative sequence impedance

Zero sequence

Zero sequence I1  I I2  I I3  I
I2
The phase conductor currents I1

are equal in magnitude and  Vi   z  2.m .I i  z h .I i I3
z
phase.
zh is the zero sequence impedance.

SYMMETRICAL COMPONENTS BACKGROUND

 z m m
Diagonalization of the
impedance matrix
Z  m z m
m m z 

Eigenvalues: zd  z  m zi  z  m z h  z  2.m

Possible eigenvectors:
1  1  1
Fd   2  Fi     Fh  1
1 3 1 3
    j.  2    j.
2 2 2 2
    2  1

Fortescue matrix 1 1 1 1   2 
1 
and its inverse: F   2  1 F 1
 .1  2  
3
   2 1 1 1 1 


Figure 2.18: Symmetrical component analysis (TB 531)

A more general approach can be done with modal analysis, even though this approach is used mostly
for transient studies (Figure 2.19). Single-phase to earth fault is the superposition of positive, negative
and zero sequences. In each of these sequences, the current is 1/3 of the single-phase short-circuit
current. When earth resistances are nil at both ends, zero sequence screen potentials are nil, because
the return current in the screens generates a voltage equal to the opposite of the voltage induced by
the cores. Therefore, screen potential rises are only due to the positive and negative sequences, and
is given by:
2 𝐼𝑐𝑐,𝑠𝑖𝑛𝑔𝑙𝑒
𝑉𝑠𝑖𝑛𝑔𝑙𝑒 = 𝑉𝑡ℎ𝑟𝑒𝑒
3 𝐼𝑐𝑐,𝑡ℎ𝑟𝑒𝑒

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Positive sequence
Icc/3

a².Icc/3

a.Icc/3

Negative sequence
Icc/3

a.Icc/3

a².Icc/3

Zero sequence
Icc/3

Icc/3

Icc/3

Screens potential
Icc

Figure 2.19: General approach with modal analysis

2.3.3 Transient studies

Extensive works exist on the topic of cable modelling for transient studies. The starting point is the
Telegrapher’s equations:
𝜕𝑉 𝜕𝐼
− = 𝑍𝐼 − = 𝑌𝑉
𝜕𝑧 𝜕𝑧
where 𝑍 is the matrix of series impedances and 𝑌 the matrix of shunt impedance, or admittance.
The usual approach to solve these equations is through modal analysis: transformation
(diagonalization) matrices are used to change voltages and currents into independent variables, called
modal voltages and modal currents. More details about the phase domain/modal domain is given in
TB 531 [31]. The CP (Constant Parameters) model assumes that cable parameters and
transformation matrices are constant. The FD (Frequency Dependent) model takes the variation of
cable parameters with frequency into account. It should not be used for power frequency simulations.
The FDQ model also takes the variation of transformation matrices (Q) with frequency into account.
Following a different approach, another model, the wideband model, at least as accurate as the FDQ
model, has been developed. However, some problems have been discovered with this model. Both
the FDQ and wideband models are specific to EMTP and are implemented in EMT software programs.
It was noted that sheath overvoltage calculations may be less accurate than those for core
overvoltages. In EMTP’s CP model, the losses are included through lumped resistances. The more
sophisticated models, i.e., FDQ and Wideband, use a different representation which takes into
account the distributed nature of all parameters [32]. In the CP model, all conductors (core, metal
screen, pipe) are modelled the same way. The lumped resistances are used for the core and screen
calculations as well.
Table 2.1 lists the models actually implemented in EMT software programs. In EMTP CP model,
losses are included through lumped resistances. But the models, i.e., FDQ and Wideband, use a
different representation that takes into account the distributed nature of the parameters. In the CP

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

model, all conductors (cores, metal screen, pipes…) are modelled as metal conductors. The models
also use modelling of earth or cable environment mostly through estimation.
Table 2.1: List of models implemented in EMT software programs

Model Characteristics Frequen Advantages Drawbacks

cy
CP Parameters of the π circuit Fixed Simple Frequency to choose
calculated at a given depending on the study
Short computation time
frequency
Underestimates damping at
Allows verification of
Real and constant HF
the data
transformation matrix
Stability
FD Parameters of the π circuit HF More precise than CP Not recommended for
are frequency dependent beyond computation of outer-sheath
kHz overvoltages (which involve
Real and constant
low velocity modes, requiring
transformation matrix
accurate LF modelling)
FDQ Parameters of the π circuit LF and Accuracy Longer computation time
and transformation matrix HF
WB
are frequency dependent

2.3.4 Modelling of other components

References: Cigré TB 283 and IEC TR 60071-4
SVLs
If the study does not include overvoltages exceeding the rated voltage of SVLs, the SVLs can be
modelled as open circuits. Otherwise, SVLs are modelled as non-linear resistances. A typical curve is
shown in Figure 2.20. The characteristics of the SVLs depends on the wave shape (e.g., 8/20 μs) of
the tested voltage.

Figure 2.20: Example SVL model as non-linear resistance

Bonding leads
Bonding leads are used to connect cable sheaths to the ground or to SVLs and the bonding leads are
generally not included in power frequency studies. For transient studies, impedance of the bonding
leads should be included. A voltage drop appears along the leads due to the inductance of the
bonding leads at high frequency.
𝑑𝑖
𝑣=𝐿
𝑑𝑡
Both single-phase and coaxial bonding leads can be represented by inductance. The value of the
inductance is given per unit length:

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

𝜇0 𝑙 𝑙 2 𝑟 𝑟 2
𝐿′ = ln ( + √1 + ( ) + − √1 + ( ) )
2𝜋 𝑟 𝑟 𝑙 𝑙

Grounding
Grounding is modelled by a resistance as shown in Figure 2.21. Typical values of the resistance are
0.1 to 1 Ω in a substation; 8 to 10 Ω on a transition tower between an underground cable and an
overhead line, 10 to 20 Ω on a regular tower, and 5 to 10 Ω at a joint pit. Different resistance values
may be observed from different regions. Measurements are recommended to determine the actual
values.
Usually, the ECC is insulated as recommended. If the ECC is not insulated, the ECC is included as an
earth electrode.
When the current flows through this resistance, an earth potential rise (EPR) is observed. The earth
potential decreases when the distance from the grounding increases. EPR may damage cable
oversheaths, joint coverings and SVLs. Therefore, EPR should be taken into account in the design of
the link.

Single-point bonding
Ifault

VSVL
Ifault ecc

EPR

Earth potential

EPR

Figure 2.21: Modelling of Grounding

Overhead lines
Power frequency studies:
If overhead lines (OHL) are included in the studies, the fault current return path involves the
skywire(s), towers, and earth. In every span of the OHL, an induced voltage is generated due to the
phase conductor/skywire coupling. This effect can be modelled as a voltage source resulting in a
current flowing in the skywire, returning to the source, with magnitude 𝜇 ∙ 𝐼𝑠𝑐 , where 𝜇 is the coupling
factor and 𝐼𝑠𝑐 the short-circuit current.
𝑍𝑚𝑠𝑤
𝜇=
𝑍𝑠𝑤
𝑍𝑠𝑤 is the self-impedance of the skywire and 𝑍𝑚𝑠𝑤 is the mutual impedance between the faulted phase
and the skywire.
This flowing current is considered constant along the skywire. There is no current flowing through the
towers. The current flowing to the earth through the skywire and the towers is (1 − 𝜇) ∙ 𝐼𝑠𝑐 . The
impedance of the return path looking from the fault terminals is the impedance of a ladder network,
involving the self-impedance of the skywire and the resistances of the tower footings. This impedance

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

may be estimated for long lines, with constant tower footing resistances as shown in Figure 2.22. IEC
60909-3:2010 (part 3.11) provides more information on this topic.

Figure 2.22: Impedance of long overhead lines

𝑅𝑍𝑛
𝑍𝑛+1 = 𝑍𝑠𝑤 𝐿 +
𝑅+𝑍𝑛
where 𝑅 is the tower footing impedance and 𝐿 is the span length. For an infinite number of spans:
𝑅𝑍∞
𝑍∞ = 𝑍𝑠𝑤 𝐿 +
𝑅+𝑍∞
𝑍𝑠𝑤 𝐿 + √(𝑍𝑠𝑤 𝐿)2 + 4𝑅𝑍𝑠𝑤 𝐿
𝑍∞ = ≈ √𝑅𝑍𝑠𝑤 𝐿
2
Transient studies:
For transient studies, the overhead conductors and skywires can be modelled by the FD model and
the towers by the CP model with footing resistance and spark-gap. Special care should be given to the
modelling of the transition tower since the surge arresters and bonding leads should be taken into
account.

2.4 Insulation coordination of bonding systems

2.4.1 Sheath bonding system insulation
A cable sheath bonding system consists of components as listed:
▪ Insulating cable jacket for metal sheaths
▪ Sheath sectionalising insulators and outer covering with sectionalized joints
▪ Bonding cables from sectionalized joints to link boxes
▪ Internal insulating components of link boxes
▪ Earth connection conductor
▪ Mounting insulators for terminations

Insulation coordination addresses assessment of overvoltages subjected to the system, insulation

requirements of the components as a system, and effective application of protective devices.
The overvoltages subjected to the bonding system include overvoltages caused by lightning strikes
traveling mostly from the cable circuit terminals (substations and transition poles), switching surges
traveling from the cable terminals, and external or internal faults. The overvoltages caused by the
faults include power frequency temporary overvoltages with limited magnitude and duration and high
frequency overvoltage especially at the front of the fault duration.
For insulation coordination of a cable system, instead of considering the overvoltages and insulation
strength in a statistical nature as for outdoor in-air insulation, the insulation of cable is not self-
restoring and is assumed to fail if the overvoltage exceeds the insulation level.
Insulation and protection levels of the bonding system shall be designed as a system. The insulation
requirements of the system include:
▪ Power frequency currents and voltages under normal operations due to phase conductor
currents (sheath standing voltage) or external or internal cable faults (AC sheath temporary
overvoltage)
▪ Impulse voltages or transient overvoltage due to disconnector operations in GIS or lightning
strikes and switching surges at cable terminals.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

2.4.2 Sheath bonding system and component requirements

▪ Insulating jacket for metal sheath. Examples of requirements include the ones by IEC 60840,
IEC 62067, and French Standard NF C 33-254.
▪ Sheath sectionalizing insulators and outer covering with sectionalized joints with the same
requirements as for the insulating jacket for metal sheath.
▪ Bonding leads from sectionalized joints to link boxes with the similar requirements to for the
insulating jacket for metal sheath.
▪ Internal insulation of link boxes with specified insulation withstand capability of dc, ac, impulse
voltages, fault current, and emergency loading requirements.
▪ Earth connection conductor with fault current withstand capability
▪ Mounting insulators for terminations with additional requirements for the specific installation.

Details of the testing requirements are discussed in Section 3, Testing of Bonding Systems, of this
brochure.

2.5 special protection on GIS cable terminations against high frequency

transient overvoltage
According with CIGRE WG 23-10: ELECTRA 151, 1993, high frequency transient enclosure voltage
(TEV) is caused by lightning surges, operations of lightning arresters, phase-to-earth fault currents, or
switching surge discharges between contacts. The high frequency transient currents cause localized
transient overvoltages because of the relatively high reactance of earth connections.
The high frequency transient overvoltages are generally confined to the inside of the screening provided
by GIS enclosures. The GIS enclosures are designed to withstand such electrical stresses. The GIS
enclosures also include discontinuities. The discontinuities may allow the high frequency effects
transferred to the exterior of the GIS enclosures as it is the case for HV cable terminations according to
IEEE Std. 1300-2011 [39]. The GIS cable terminations should be mounted with sheath sectionalizing
insulators between the components electrically connected to the GIS enclosure and the cable sheath.
Two main options are used to protect the system from the high frequency transient overvoltages at the
discontinuities between the GIS enclosures and cable terminations.
1. Install a metallic earth connection between the GIS enclosure flange and cable sheath (see Figure
2.23).

Figure 2.23: Example of earth connection between GIS enclosure and cable sheath

This connection has the advantage of avoiding overvoltage between the GIS enclosure and the
cable sheath. The disadvantage is that it causes permanent circulation of current in a closed loop
formed by the multiple connections to ground (see Figure 2.24).

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Figure 2.24: Scheme of typical earth connection on GIS cable termination:

(A) Cable sheath connected to local earth by single-core bonded lead
(B) Earth connection between GIS enclosure and local earth
(C) Earth connection between GIS enclosure and cable sheath

This solution would not be convenient as the effects derived from the circulating currents between
the grounding system of both cable and GIS may not be considered in design, especially if the GIS
and the cable termination are supplied by different suppliers.

2. Install a non-linear resistor (or bypass SVLs) across the sheath sectionalizing insulator to limit the
overvoltage under transient conditions between the components electrically connected to the GIS
enclosure and the cable sheath. The SVLs need to be mounted close to the gap to be protected
and connected by short low-impedance leads (see Figure 2.25).

Figure 2.25: Typical physical location of bypass SVLs: 1 - Non-linear resistor 2 - GIS enclosure 3 – Cable
sheath 4 – Sheath sectionalizing insulator
IEEE Standard 1300-2011 [39] and IEC 62271-209 [38] indicated that the number of non-linear
resistors (bypass SVLs) to use and their characteristics shall be determined by the cable termination
manufacturer, taking into consideration of user and the switchgear manufacturer requirements.
Further details as guidance to install properly bypass SVLs can be found in [40].

In the case of single-point bonding connection where there are sheath voltage limiters (SVLs)
installed on the GIS side (see Figure 2.26), the sheath temporary overvoltage that appears between
cable sheath and earth when a phase-to-ground fault occurs in the power grid is seen by both
limiters: the sheath voltage limiter which protects the cable sheath, and the bypass sheath voltage

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

limiter located between the cable sheath and the GIS enclosure. Therefore, the bypass SVL’s rated
voltage, Ur, should be equal or higher than the sheath voltage limiter rated voltage, Ur sheath , 𝑜𝑟 𝑈𝑟 ≥
𝑈𝑟𝑠ℎ𝑒𝑎𝑡ℎ , in order to ensure the integrity of the bypass SVLs in case of phase-to-ground fault.

Figure 2.26: Single-point bonding connection where there are sheath voltage limiters installed on the GIS
side

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

3. Testing of bonding systems

3.1 Introduction and section scope
Cable sheath bonding systems consist of components that are subjected to overvoltages and
excessive currents. The systems and their components shall be properly designed and tested. There
is no internationally recognized standard that targets specifically the testing of the sheath bonding
system components or the bonding system as a whole after installation, for system commissioning or
for maintenance inspection.
The main components of the cable sheath bonding systems are defined in Section 2.4.1 and are also
listed below.
▪ Insulating cable jacket for metal sheaths
▪ Sheath sectionalising insulators and outer covering with sectionalized joints
▪ Bonding cables from sectionalized joints to link boxes
▪ Internal insulating components of link boxes
▪ Earth connection conductor
▪ Mounting insulators for terminations

Some medium voltage cable systems utilize unjacketed concentric neutral type cables. The exposed
concentric neutral conductor may become an extension of the substation grounding system and as
such it is not included as part of the bonding systems addressed in this document. It is noted that the
installation of such unjacketed concentric neutral type cables in the insulated ducts does not prevent
corrosion.
It is not the intent of this document to replace any existing applicable standards, regulations or country
specific legislations. This document does not intend to provide guidance for testing safety. Engineering
knowledge and judgment should therefore always be employed to achieve satisfactory results.

3.2 Testing of system components

Cable Sheath Insulating Jacket:
The cable sheath insulating jacket is used to insulate the cable metal sheath from earth for sheath
bonding and to improve protection of the metal sheath from corrosion. Testing requirements of the
jacket are described in cable Standards IEC 60840, IEC 62067, and IEC 60229, as shown in Tables
3.1 and 3.2. The impulse voltage test level of these components depends on the impulse voltage of
the main insulation of the power cables as well as the length of the bonding leads.
Table 3.1: Metal shield/sheath insulating covering impulse withstand voltage versus BIL (IEC 60840, IEC
62067 and AEIC CS9-2015 Table 4.2-1)

Impulse Test Level (1.2 x 50 μs)

Between Parts Each Part to Ground
Bonding Cable Bonding Cable Bonding Cable Bonding Cable
Rated BIL for Main Length Length Length Length
Insulation ≤ 3m 3m ≤ 10m ≤ 3m 3m ≤ 10m
kV kV kV kV kV
250 to 325 60 60 30 30
550 to 750 60 75 30 37.5
1050 60 95 30 47.5
1175 to 1425 75 125 37.5 62.5
1550 75 145 37.5 72.5

Routine electrical tests on non-metal sheath or jacket are a conditional test in accordance with IEC
60840 and IEC 62067 upon specifications in a contract or order. In most cases, only withstand voltage
tests are performed.
IEC 60229 provides details of the factory routine tests, product type tests, and after installation tests
from the insulating material perspective. As part of the routine tests, a DC voltage test is conducted

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

when the jacket is covered by a conductive layer. In this test, a DC voltage of 8 kV/mm of insulating
jacket thickness is applied for 1 minute, with a maximum of 25 kV. The type tests include abrasion
test, corrosion spread for aluminium sheath, and impulse voltage test. The impulse type test voltage
values specified by IEC 60229 are shown in Table 3.2.
Table 3.2: Impulse type test voltage values as specified by IEC 60229 for cable sheath insulating jacket

Rated lightning impulse withstand voltage of Impulse test voltage of cable sheath
main insulation voltage (kV peak) insulating jacket (kV peak)
V < 325 30
325 < V ≤ 750 37.5
750 < V < 1 175 47.5
1 175 ≤ V < 1 550 62.5
V ≥ 1 550 72.5

In addition to impulse voltage values, AC voltage withstand levels are also required by French
Standard NF C 33-254 as shown in Table 3.3.
Table 3.3: Metal shield/sheath insulating covering withstand voltage (NF C 33-254)

Nominal Voltage (kV) 36/63 52/90 130/225 230/400

(72.5) (100) (245) (420)
Lightning Impulse Voltage for Main 325 450 1050 1425
Insulation (kVc)
Screen to Ground Impulse Withstand 50 50 50 62.5
Level (kVc)
Screen Interruption Impulse Withstand 80 80 100 125
Level (kVc)
Screen to Ground AC Withstand Level 20 20 20 20
(kV)
Screen Interruption AC Withstand Level 25 25 25 35
(kV)

Sheath interruption insulators and joint casings:

The sheath interruption insulators are parts of the cable sectionalizing joints. In the metal sheath
bonding systems, the insulators separate two adjacent sections of the cable sheath. During system
disturbances, the insulators are subjected to AC and transient overvoltages. IEC 62067 and IEC
60840 provide the test requirements for equipment designed in accordance with these standards, the
same as for cable sheath insulating jacket (see Tables 3.1 and 3.3).
Bonding Leads:
The cable leads or connections between the cable sheath and link boxes should use proper insulation
requirements as for the cable sheath insulation jacket. Generally, the bonding cable insulation test
level is also related to the impulse test level of the power cable main insulation. It is noted, however,
that commodity cables are usually applied for the bonding leads and are usually not manufactured to a
specific system. The voltage peak value should be in accordance with IEC 60229 as for cable sheath
insulating jacket (Table 3.2) as a minimum. The suggested BIL level for bonding leads can also be
found in Electra 128 and IEEE 575. The impulse test procedure should follow IEC 60502-2 clause
18.1.7. In the case of the concentric neutral cables, the impulse value for the type test should apply to
the test between internal and external conductor as well as between external conductor and earth.
This is also in agreement with requirements for the accessory testing specified in the IEC 60840 and
62067, Annex G.
In addition to the standard tests, the DC voltage withstand type test for the outer covering and
sheath/screen sectionalizing insulation of 25 kVdc, as specified by IEC 60229, should also be applied.
The leads should also be tested for short circuit conditions. The cable leads are installed in free air or
submerged. Their external surface may be covered with either graphite or extruded semi-conductive
layer to facilitate field testing. If the semi-conductive layer is applied, proper earthing of the layer must
be ensured.
Bonding leads that are intended to be used as a link to earth connection can be tested as per IEC
60502-1.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

For leads without outer semi-conductive jacket covering, the spark test method as described in the
IEC 62230 – Electric cables - Spark-test method, could be used in determining if the outer jacket of a
coaxial cable withstands a specified voltage. A spark tester includes a voltage source capable of
maintaining the test voltage and a means of grounding of the conductor and sheath. The external
electrode (bead or link) should be capable of making a good contact with the external part of the
cable. The centre and outer conductors should be securely connected to the ground, making a low
impedance path for the current. The jacket should pass through the external electrode at a rate
calculated as per relevant standards.
If the concentric bonding leads used in cross-bonding systems are equipped with swelling tapes or
other means that restricts the moisture migration, the cable should be tested as per relevant cable
standards to ensure that the water will not migrate to the inside of a joint casing or under other
coverings. If the cables used for bonding purposes are regular single conductor cables, the ingress of
water under insulation should be restricted.
The limit of the bonding lead length of 10 m cannot always be met. In case that the terminations are
installed on a tower, which is the case for siphon systems, the bonding leads can be in the range of 15
to 20 m. In this case, a larger conductor cable can be used to reduce the system impedance. Also in
some countries, the maximum length of the bonding connections employing 1000-V cable is
established as 5 m. In any installation, the bonding leads should be installed as close to each other as
possible. A trefoil arrangement is preferable.
During a ground fault of the electrical systems, zero sequence current returns to the source through
any available path. The current returning by the earth path can cause the ground potential rise to be
hazardous to the personnel and equipment. In this case, the ECC should be insulated. In case of
single-point bonding (and sometimes cross-bonding), the earth continuity conductor installed parallel
to the power cables is used to provide low impedance path for the fault current. Because of the above
and to protect the ECC cable, the cable should be insulated to the 1000 V minimum level. Some
utilities require the minimum thickness level of insulation to be 3.3 mm. In certain systems, it may be
advisable to calculate induced voltage in the ECC and adjust the insulation requirements.
Sheath voltage limiters:
Sheath voltage limiters (SVLs) are critical components of the cable bonding system to protect the
system insulation from transient overvoltages. Selection of SVLs is an important part of the entire
bonding scheme and grounding coordination. It is noted that the sheath voltage limiters are not
designed to carry excessive current that may appear during a short circuit event.
Non-linear metal-oxide varistor type surge arresters are the most widely used type of SVLs. These
arresters are tested in accordance with IEC 60099-4 and other applicable standards. The arresters are
also generally factory tested for the purpose of the surge energy dissipation requirements. The SVLs
are also tested as part of routine factory tests. The zinc oxide units can be tested by applying a DC
test voltage to achieve reference current flow. The test is performed in forward and reversed directions
and an average value is calculated for future reference. As the temperature is an important factor, the
ambient test temperature is recorded. It is recommended that the voltage be corrected (decreased or
increased) to a common temperature value.
Link Box or Enclosures:
Link boxes or enclosures are used to contain SVLs, link connections, earth cable terminals, sockets
for bonding lead entry. Stainless steel, fiberglass, or cast iron is used for the link box housing. All
designs of the link boxes must take potential corrosion into account. Requirements for water tightness
and extent of sealing must be specified, considering interface transitions, such as, connecting cable
lead entries, link box opening seals, and grounding terminals.
Link boxes should withstand the mechanical forces and electrical currents expected during short
circuit event, transient overvoltages during lightning, switching and fault events, and water ingress
especially when installed in submerging environments.
a) Electrical tests

Electrical tests include impulse voltage withstand tests, fault current withstand tests, and AC and DC
voltage withstand tests.
Impulse withstand voltage should not be less than that specified for the maximum system voltage
given in Table 3.1. The fault current withstand test is recommended to prove that the enclosure will
survive the flow of the short circuit current of specified magnitude and duration. In addition, DC

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

withstand tests of 25 kVDC with 5 min duration should be performed between links and grounded
metal parts of the enclosures.
b) Environmental tests

From the installation point of view, the enclosures can be mounted above or below ground surface.
The link boxes or enclosures shall be tested for water tightness, otherwise known as moisture ingress
protection (IP). The ingress protection degree and testing requirements are included in IEC Standard
60529 and NEMA 250. The ratings are not directly equivalent between these two standards.
In the case that the link boxes are installed in a hazardous, otherwise known as classified, location,
the construction of the link boxes should adhere to the regulations required for those locations. In
many cases, such devices are called “intrinsically safe” products. The regulations are different from
country to country for the intrinsically safe or explosion proof devices. They can be found in the
standards, such as:
▪ Europe: EN60079 (IEC 60079), EN 61241 (IEC61241), Directive 79/196/EEC, and Directive
2006/95/EC.
▪ United States: Factory Mutual - FM3610, NEC/CEC, and NEMA ICS6
▪ Canada: CSA C22.2 NO 157.92-CAN/CSA

Some utilities require that the enclosure be arc-resistant. In this case, the enclosure should be tested
by initiating an internal arc by passing a maximum rated short circuit current for a period of twice the
expected circuit breaker arc clearance duration, i.e., 10 cycles. To pass the test, the enclosure should
not be ruptured or burst open and no debris could be ejected.

c) Endurance tests

The enclosures used in the bonding systems are intended to work for many years under exposure to
the environmental conditions. Their water tightness depends mostly on the seal. Seals comprised of
elastomeric material should be tested for decay of elasticity over years of compression. The
endurance test can be conducted based on standards ASTM – D6147, ISO 3384, ISO 188, and ISO
132.
d) Explosion requirements

The link boxes must be explosion proof. Explosion within the link boxes may occur due to ignition of
combustible inner component parts, if any, or due to earth gas entered the link boxes. The explosion
could be triggered by a failure of SVLs, or by flashover or sparking of high contact resistance at the
interface of links with studs or any other connections. All component parts of link boxes should be
made of non-flammable materials. SVLs should also be encased in non-flammable casings/envelopes.
Some users specify that all types of link boxes should withstand an inner explosion pressure of 250
kPa. Generally, the test is not specifically included in any particular standard. The testing protocols
have been developed and included in some technical specifications for related cable projects.
It is noted that the static test does not replicate the possible event as the explosion inside the
enclosure is dynamic. It is advisable that the enclosure test includes an internal arc resistance test.
Such a test should prove that in case of an internal fault the enclosure remains intact. The test
magnitude and duration should cover the expected short circuit current at operational frequency and
circuit breaker clearing time.
Mounting Insulators (standoff insulators) and GIS insulation flange for terminations:
Insulation coordination requires that the components insulating the current carrying parts from the
ground be subjected to electrical, mechanical, and environmental tests. The electrical test values
should take into consideration all other parts of the bonding system. The mechanical test values
depend on the weight and installation position of the terminations. The GIS insulation flanges should
be tested to at least the same values as the standoff insulators. Additional tests should be carried out
to prove the seal and gas leakage.
The standoff insulators used for the bonding systems should be tested to the following standards:
▪ IEC 60168 - Tests on indoor and outdoor post insulators of ceramic material or glass for
systems with nominal voltages greater than 1000 V

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

▪ IEC 60273:1990 - Characteristic of indoor and outdoor post insulators for systems with
nominal voltages greater than 1000 V

Many utilities standardize on using 7.5-kV standoff insulators in supporting the cable terminations. In
some cases, especially in single-point bonding systems, these insulators may not be sufficient to
protect the bonding system from unexpected transient voltages. In any case, their testing values
should be comparable to those specified for the bonding leads. This requirement applies to standoff
insulators as well as to GIS insulating flanges.
It is noted that the termination standoff insulators should not fail during the following occurrences:
▪ Maximum expected travelling wave voltage
▪ Voltage induced during short circuit current flow
▪ Jacket voltage test

The values in Table 3.1 should be used as minimum electrical requirements. Compression and
seismic characteristics should be considered for mechanical requirements. Contamination
performance should be considered for outdoor environment applications.

3.3 System/commissioning tests

To ensure performance of the bonding system insulation, several commissioning tests are applied.
There are no commonly recognized sheath bonding system commissioning tests. In the recent years,
there are attempts by some users to specify a set of after installation tests to prove that the system is
designed and built to the requirements. There are a few tests that may assist in proving the system
installation.
Induced Voltage and Bonding Test:
The test is described in Electra 47. It requires applying a low voltage to the main conductors star-
connected at the remote end and regulating the current to a predefined value, i.e., 100 A. A portable
three phase generator with voltage regulation is necessary. Measuring the voltage magnitude at the
bonding points and comparing results with calculated values may assist in proving correctness of
system designs and connections. However, the test results may be influenced by the induction coming
from energized nearby lines or equipment.
Sheath jacket integrity test:
This test is a commonly used DC test on the oversheath of the power cables. The test can also be a
part of test before installation, installation check for each installed cable section or joint, system
commissioning, and maintenance. If necessary, leakage current can be used as a benchmark for
future tests comparison. It is noted that the leakage current depends also on factors external to the
system, such as, temperature, air humidity and cleanliness of the connections. The test includes a DC
voltage of 4 kV/mm of jacket thickness with a maximum of 10 kV DC for 1 minute, with all metal layers
connecting with the sheath connected together. This test requires a conductive layer outside the
insulating jacket or using moist backfill as the conductive layer. (See IEC 60229)
Contact resistance test:
Since the system contains a significant number of the connection points, their resistance could be
detrimental to the bonding performance. The contact resistance test assures that the connections do
not create a bottleneck for the passage of short circuit current. The approval value of such can be
arbitrarily assigned or a laboratory test conducted. In practice, a value of 20 µΩ is used and accepted
by some users.
Others:
Other tests such as, ECC continuity test, SVL integrity tests, and visual verification of connections are
also carried out by some users.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

4. Maintenance of bonding systems

This section discusses the issues related to maintenance of bonding systems. It is the intention of the
following clauses to suggest good practice maintenance schedules and list areas where the user
should pay special attention. This section is based on the responses to a questionnaire circulated
amongst TSOs, cable manufacturers, consultants and utilities.

4.1 Maintenance of bonding systems

Whereas other HV equipment is subjected to regular maintenance, it has been considered that
extruded dielectric underground transmission cables are almost maintenance free. However, recent
service experience showed that inexpedient operation may arise if the cable systems are not properly
maintained. It is suggested that maintenance of bonding systems follow a regular schedule in order to
ensure safe and optimal operation of the cable system under all operational modes (normal,
emergency, or fault).
Planning of maintenance should be an optimisation between what is technically possible, the cost
associated, and the benefits thereof.
As for all other assets, two different methodologies can be applied to maintenance of bonding
systems:
▪ Corrective maintenance
 Corrective maintenance is done once the asset shows proof of errors in the operation,
either due to total breakdown or partial malfunction.

▪ Preventative maintenance
 Preventative maintenance is inspection of the asset and repair of operational
equipment which may fail before next scheduled preventative maintenance.
The two different methodologies can be combined into one maintenance program. Possible
Maintenance Actions are discussed below.

4.2 Common failure modes

In order to create a maintenance program, it is necessary to understand issues associated with the
bonding system. The following lists some commonly experienced issues with the bonding systems and
bonding system components.
Jacket damage:
The sheath insulating jacket may be damaged during handling and installation, overvoltage caused by
lightning or switching surges (if the protection by the SVLs are not effective), objects with cable
construction surroundings, or cable movement. In addition to the possibility of water ingress, the
puncture may result in a malfunction of the bonding scheme, as the damaged areas may appear as a
grounding point. For single-point bonded and cross-bonded systems, an extra ground point may lead
to unexpected circulating currents which may create localized heating and damage the power cables.
It is therefore important to prevent jacket damages and detect jacket damages on a regular basis and
perform corrective maintenance when a damage is found.
SVL damage:
SVLs are an important part of the bonding system to prevent the damage of the sheath insulating
jacket and other insulating components in the bonding system from damages caused by transient
overvoltages. In general, there are two possible failure modes for SVLs. Firstly, the SVL may fail so
that it does not conduct during overvoltages, i.e. it appears as an open circuit. Secondly, the SVL may
fail so that it appears as a short circuit, and thus conduct current at normal operating screen voltage;
this failure mode is considered more common than the first one. The first failure mode results in
possible high screen voltages during transients (e.g., lightning or switching) and leads to possible
jacket damage. The second failure mode results in circulating sheath currents that may create over
heating of the power cable.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Loose connections (bonding leads, SVLs):

A common failure mode with the bonding systems is loose connections to, or disconnected, SVLs.
Due to erroneous/insufficient installation or movements, etc., the connections may not be sufficiently
tight which may lead to insufficient overvoltage protection of the bonding system insulation.
Damaged bonding leads:
Damaged bonding leads share the same failure modes described under “Jacket damage”.
Furthermore broken bonding lead conductors may result in open circuits where there should be short
circuits, e.g. between joints and link boxes. This may lead to imbalance and/or hazardously high
voltage during normal operation, cable faults, and transient overvoltages.
Link box failure:
The link box may fail due to exposure to the harsh environment, including moisture, heat, UV,
manufacturing defect, or mechanical impact. Common failures that can be inspected externally include
corrosion, physical damages, and moisture ingress. Opening the link box may assist in inspecting
components within the link box for signs of moisture ingress, corrosion, loose connections,
disconnections, incorrect connection, ineffective sealing gaskets, and failed SVLs. However, it should
be noted that opening link boxes may increase the risk for future water ingress due to aging of the
water seal. All of these different failure modes may result in losing a grounding point, losing jacket
protection by the SVLs, or getting a new unintended grounding point leading to circulating currents.
Stand-off insulator (termination support) failure:
The standoff insulators as termination support may fail due to mechanical stress, material aging, or
electrical stress. The insulator should be free from scratches and not show any arcing traces.
Other failure modes:
An additional list of performance issues and failures found during maintenance inspections and
system operations is included as below.
▪ Cracking of terminal support post insulators
▪ Insufficient clearance within link boxes resulting in internal flashovers
▪ Poor mounting insulator at terminations
▪ Poor insulators within link boxes
▪ Punched link holes at link boxes that reduce contact surface area
▪ Poor SVL connections where small SVL studs pass through slotted holes
▪ Insufficient rigidity in lids and poor gasket retention
▪ Pits often contain water and water sealing at penetrations is an issue
▪ Generally, the local earth mat should have reasonable earthing resistance of, say, 10 Ohms
▪ In addition to the inner earthing connection, there should be an independent external
connection to the local earth mat of the LB metal casing/body. The external earthing
connection is for personnel safety while the inner connection is the integrated part of cable
bonding system.

4.3 Corrective maintenance of bonding systems

As corrective maintenance by definition is the repair/change of defected equipment, it will not be
further discussed here. However, it should be acknowledged that the main part of the costs for
corrective maintenance can be related to the time it takes to find the defect and possible associated
outage time, rather than the costs of the replacement components themselves. A major point in
corrective maintenance of the bonding system may therefore be to find a proper way of rapidly
location defective equipment. Such failure location strategies are outside the scope of the present
report.
It should be noted that a failure of a HV cable may lead to high overvoltages which may harm the
SVLs and other components of the bonding systems, especially when the SVLs are not chosen
correctly or when the SVLs have experienced excessive aging. As part of the inspection after a fault, it
is therefore advisable to perform testing of the SVLs and other components at least nearest to the fault
location for integrity. In addition to a visual inspection of the SVLs, electrical tests, as presented in
Section 3 of this document, should be performed.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Based on the above, it is determined advisable to perform preventative maintenance, wherever

possible, to avoid costs of repair and outage time. It should be recognised that while preventative
maintenance time is limited to performing inspection of the system, corrective maintenance is much
more time consuming as it includes fault detection, mobilisation, visual inspection, fault location,
repair, testing, etc. and it is thus much more time economic to perform preventative maintenance than
corrective.

4.4 Preventative maintenance of bonding systems

As described, it may be more cost efficient to prevent failures with regular maintenance, than to
perform corrective maintenance after a failure has occurred. Preventative maintenance can be divided
into two sub-categories, online and offline. Online maintenance is the work that can be performed
while the cable system is operating, whereas the cable system has to be taken out of service (and
grounded) during offline maintenance work. The following presents the identified possibilities for
maintenance of bonding systems for HV transmission cables.
4.4.1 Online maintenance
Maintenance inspections performed when the cable systems are still in service are described in this
section. Test criteria for these inspections must consider electromagnetic interference with applied
inspection equipment and induced voltages and currents from adjacent circuits. The inspection or
monitoring must also consider the number of measuring or monitoring points across the entire circuit
length, especially with multiple minor cross-bonding sections. Device used for partial discharge
measurements for cable and accessory insulation diagnostics, such as, high frequency current
transformers, can also be used as an inspection tool for the bonding systems.

Patrolling:
In order to prevent damages from construction and excavation work near the cable route, it is
recommended to perform regular patrolling along the cable route. A visual inspection of the bonding
system is preferable on externally exposed components as this may be the easiest way to find aged
equipment or equipment which may soon fail. However as bonding systems are often buried,
measurements from the vaults or open sections must be considered.
Sheath current:
Measuring current magnitudes and phase angles in the sheath in operation can assist in determining if
the current meets the design specifications. If a cross-bonded or single-point bonded system shows
large sheath currents, it may indicate that an abnormal condition may have happened to the bonding
system, such as, a sheath insulation failure. If a multi-point bonded system shows no sheath currents,
it may also show the bonding system is not connected properly.
For cross-bonded systems, online measurements may require measurement devices installed at each
minor section, whereas for multi-point bonded or single point bonded systems, the measurement
devices may be required only at each grounding point. The device may be connected directly to a
central data acquisition centre or be manually read by dispatching maintenance personnel on site.
SVL integrity:
Some SVL manufacturers embed a sensor (optical fibre) within the SVLs. The sensor can be used to
detect the condition of the SVLs. The sensor may also be connected directly to a central data
acquisition centre or be manually read by dispatching maintenance personnel on site.
In addition to the possibility of integrating fibre sensors in the SVLs, it is possible to measure the
current in the SVLs. If the current is larger than the anticipated value, the operator may know that
something is wrong with the bonding system (including the SVL).
Distributed Temperature Sensing (DTS):
With a DTS system, it is possible to measure the temperature along the cable system. Besides an
overall evaluation of the performance of the cable system, the DTS measurements may provide an
indication of the performance/status of the bonding system.
For cross-bonded systems, a high temperature along one major section may indicate large currents
running in the sheath, meaning that the bonding system is faulted. Similarly, if higher temperatures
than expected arise in a single-point bonded system, it may indicate that currents are flowing in the
sheath, and that the bonding system therefore is faulted. Moreover, if lower temperatures than

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

expected are experienced in multi-point bonded systems, it may be an indication of a low sheath
current, meaning that the bonding system could be faulted.
It should be noted that a faulted cross-bonded or single point bonded system, as described above, is
not necessarily problematic as the grounded bonding system ensures a return path for fault currents.
The “only” problem is that a faulted bonding system may reduce ampacity of the cable system. A
faulted solid bonded system, as described above, is problematic as it does not ensure a return path for
fault currents. Immediate action should therefore be taken. It is noted that the metal tubes containing
the fibre cables must be connected to earth potential.
Visual or Thermal Images:
Visual or thermal images can be used to show general or mechanical conditions of exposed
components, such as, standoff insulators, SVLs, and bonding leads, of the bonding system.
4.4.2 Offline maintenance
Visual inspection:
A maintenance action preferred by many TSOs and utilities is the visual inspection of the bonding
system. A visual inspection of the following components may show the condition of the bonding
system.
▪ Link box
 Outside: damages, corrosion.
 Inside: moisture, corrosion, connections.
▪ Bonding lead: jacket conditions, connections
▪ SVL: conditions, free from scratches and arcing traces.
▪ Standoff insulator (termination support): Insulator condition, free from scratches and arcing
traces, etc.

Measurement of resistance and contact resistance:

In addition to the visual inspection, it may be preferable to perform measurements of contact
resistances inside the link box. The measurement may disclose loose connections. It may also be
considered to conduct a measurement of the grounding resistance on a regular basis to ensure a fully
functional bonding system.
SVL integrity:
The electrical characteristics of SVLs can be checked. The test includes a voltage/current profile of the
SVLs. A test as presented in Section 3 of this document is possible to perform on site, such that the
SVL can be put directly back into service, whereas a full voltage/current profile may be conducted in a
laboratory. In the latter case the SVLs may be exchanged with the ones that have been fully tested.
The SVLs tested at testing laboratories may be reused. It is possible to perform a voltage test for the
integrity of the SVLs. Special testers may be used to measure DC leakage current or 3rd harmonic
component of the leakage current to assess the conditions of SVLs. Dissipation factor tests may also
be used in combination of other diagnostic methods. Environmental conditions (temperature, humidity)
under which the tests are performed should be recorded for the evaluation. Sometimes, the SVLs may
not be faulty but the test results may show a trending of accelerated aging which can also call for
replacement of the SVLs.
Sheath voltage test:
By performing a screen voltage test, a DC voltage is applied between the metal screen and ground. In
this way the operator is able to see if the screen is unintentionally grounded at any point, e.g., a
defected bonding system. IEC 60229 includes a sheath integrity test after installation. The after
installation test described in IEC 60229 includes a DC voltage of 4 kV/mm with a maximum of 10 kV
for 1 minute, with all metal layers connecting with the sheath connected together. For maintenance
tests, the test voltage is typically reduced by 50%. This test requires a conductive layer outside the
insulating jacket or using moist backfill as the conductive layer.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

4.5 Maintenance schedule of bonding systems

A survey was carried out among the Working Group members representing different countries on
maintenance practices. Table 4.1 shows the list of the questionnaire. Details of the survey are
included in Appendix D.

Table 4.1: Questionnaire for maintenance practice survey

Maintenance of cable bonding systems Your company:

This spreadsheet is intended for collecting data from different cable suppliers and operators among
working member countries. The questionnaire is related only to the maintenance of cable bonding
systems. If some of the fields in the spreadsheet seems superfluous, please leave it blank or copy
the answer from one cell into another. The answers will be part of a Cigré Technical Brochure
delivered at a later stage by Working Group B1.50. Answers will be anonymized if requested.
Solid Single Cross Other
point schemes?
Bonding methods
Please give a brief description of how the different
bonding methods are made
Scheduled maintenance on the bonding systems
What kind of scheduled maintenance is performed
- What tasks are performed and with what
interval?
- What equipment is being maintained?
(e.g., link boxes, earthing boxes, bonding leads,
SVLs, mounting insulators, grounding points, earth
continuity conductors, etc.)
- What tests are performed and with what
interval?
- Are there different maintenance requirements
seen from different manufacturers?
Unscheduled maintenance (after cable fault)
What kind of maintenance is performed after a cable
fault?
- What tasks are performed?
- What equipment is being maintained?
(e.g., link boxes, earthing boxes, bonding leads,
SVLs, mounting insulators, grounding points, earth
continuity conductors, etc.)
- What tests are performed?
- Are there different maintenance requirements
seen from different manufacturers?
Other comments:

Based on the survey and discussions above, an offline maintenance schedule for cable bonding
systems is outlined here for reference only. Users should follow the maintenance schedule as defined
by their own practices, and with due consideration to the actual conditions experienced by their cable
circuits.
4.5.1 Safety considerations during maintenance
Before going into the technical aspects of a maintenance schedule, it is important to ensure that all
maintenance must be performed while taking the highest possible safety precautions. A maintenance
schedule should therefore be created with Health, Safety and Environment (HSE) in mind. This means
that all maintenance tasks must be performed while ensuring good HSE practices.
The following list is in no way exhaustive and all maintenance tasks must be thought over with HSE in
mind. However, hereunder is a list of some of the most common HSE issues that must be considered
while maintaining a bonding system:

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

▪ A strong grounding point must be established at all places of work with maximum grounding
resistance of 10 Ω.
▪ In order to prevent induced voltages build-up, all metal parts must be grounded at the place of
work.
▪ Workers should never work alone.
▪ Escape routes must be available and known to workers before maintenance work begins.
▪ Link and grounding boxes shall not be opened when the cable system is in operation.

Additional details on safety considerations are addressed by Working Group B1.44 – Guidelines for
Safe Work on Cable Systems under Induced Voltages or Currents.
4.5.2 Parameters to consider for maintenance planning
There are many different parameters to include in the analysis of optimising the maintenance schedule
for bonding systems of HV transmission cable systems. The following list includes (in no particular
order) the parameters which, as a minimum, should be considered when creating a maintenance
schedule.
▪ Joint/link box accessibility
 Directly buried cable systems are generally a low cost installation option, but access
to such systems for maintenance is mostly difficult as the link box and many
components of the systems may have to be excavated to access.
 Installation in tunnels, ductbanks and manholes is considered high cost installation
methods, but the access to link boxes, joints and other components is relatively
easier.
▪ Criticality/importance of the line
 Higher maintenance costs and more frequent maintenance may be justified for more
critical/important lines.
 Age of line
 Older cables may require more frequent maintenance than newer lines.
 Experience with components
 Components with more performance issues may be maintained more
frequently.

All of these parameters add to the determination of maintenance frequency. Maintenance work itself
may require a relatively low effort. For example, a visual inspection of the link box is usually straight
forward without the need for designated equipment. However, the logistics for creating an overall
maintenance schedule of all cable lines may set limits to how often the individual line can be subjected
to maintenance.
Furthermore, the accessibility of the bonding components could pose an obstacle to performing
frequent maintenance as excavating directly buried cable systems is inconvenient, costly, time
consuming, and it may need permits from the land owners. This factor is therefore especially important
when deciding on the maintenance strategy for cable bonding systems.
4.5.3 Recommendations for maintenance schedule for cable bonding systems
For easily accessible equipment (including equipment on substations), it is recommended to perform
the following maintenance work on a yearly basis:
▪ Visual inspection and thermal imaging inspection:
 Link boxes and their covers
 SVLs
 Link boxes: bars and connections
 Bonding and grounding leads
 Terminal base connectors

Furthermore, for easily accessible equipment (including equipment on substations), it is recommended

to perform the following maintenance work on a five-year basis:
▪ Tests:
 Test of SVL voltage/current characteristics
 Jacket test - 5 kV for 1 minute (voltage magnitude depending on SVL rating)

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

 Connection resistance in link box

For equipment which is difficult to access, the following maintenance work is recommended on a five-
year basis:
▪ Visual inspection of:
 Link boxes and their covers
 SVLs
 Link boxes: bars and connections
 Bonding and grounding leads
 Terminal base connectors
▪ Tests:
 Test of SVL voltage/current characteristics
 Jacket test - 5 kV for 1 minute (voltage magnitude depending on SVL rating)
 Connection resistances in link box

Any equipment found faulted during any of the above tests should be repaired or replaced.
In addition to the above offline measurements, it should be considered to implement one or more of
the online solutions mentioned in Section 4.3.1.
IEEE Standard 400 discusses testing for jacket fault, jacket fault location and pin-pointing, and after
repair.

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5. Conclusions
The CIGRE Working Group B1.50 performed and documented the study committee B1 approved
Working Group Terms of Reference for design, testing and maintenance of sheath bonding systems of
AC transmission cables. All achieved deliverables are documented within this Technical Brochure.
The Working Group deliverables achieved are listed as follows:
1) A general overview and detailed definition for insulated cable system sheath bonding systems.
2) The functionality, detailed description, and listing of available standards for insulated cable system
sheath bonding systems and for other cable system components that form part of the sheath
bonding systems.
3) A literature review of available published international standards, guidelines, CIGRE Technical
Brochures, and papers to summarise available information on sheath bonding systems. This
includes a quick reference table for key design, testing and maintenance aspects required for
sheath bonding systems.
4) A survey review to obtain service experience on sheath bonding system design, testing and
maintenance. That includes a summary of responses from member countries for all key design,
testing and maintenance aspects required for sheath bonding systems.
5) A detailed chapter on the design and protection of sheath bonding systems, that describes: the
standard type of known sheath bonding system designs; calculation formulas, calculation formula
references and examples for sheath induced voltages and circulating currents on sheath bonding
systems; sheath voltage limiter selection and application guidelines; sheath bonding system
overvoltage calculation models and software references for power frequency and transient
network conditions; and insulation co-ordination study requirements for sheath bonding systems.
6) A detailed chapter on the testing requirements of sheath bonding systems. That includes both type
testing and commissioning testing tables and references for known testing standards or CIGRE
Technical Brochures. A critical finding by the WG B1.50 is that an insulation co-ordination study
shall be performed for each specific project performed to establish the sheath bonding system
voltage withstand ratings required for both overvoltage and transient conditions.
7) A detailed chapter on sheath bonding system maintenance requirements, with a further
consideration for online and offline maintenance activities and scheduling thereof.

This Technical Brochure is to inform and guide insulated cable system design engineers, operating
units, and maintenance engineers on the international best practices to be considered for the design,
testing and maintenance of AC transmission cable sheath bonding systems.

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APPENDIX A. Abreviations, definitions, and symbols

A.1. Abbreviations
Basic Impulse Level (BIL)
Complex Impedance Matrix (CIM)
Constant Parameter (CP)
Earth Continuity Conductor (ECC)
Earth Potential Rise (EPR)
Frequency Dependent (FD)
Link Box (LB)
Metal oxide varistor (MOV)
Metal Sheath Also referred as Metal screens or Shield throughout this document.
Overhead Line (OHL)
Oversheath Also referred as Outer Sheath or Jacket throughout this document.
Sheath Voltage Limiter (SVL)
Transmission System Operator (TSO)

A.2. Specific terms

Sheath insulation electrically isolates the cable metal sheath from earth and protects the metal
sheath from corrosion.
Sheath voltage limiters are devices connected to the sheaths of bonded cables, with the purpose of
protecting sheath insulation, sectionalizing interruption at joints and other accessories, insulation
flange (at GIS) during system transients.
System transients may be lightning, switching, or fast transient associated with the initial part of a
short circuit event.
Link boxes provide housing for bonding and/or earthing connections to contain SVLs, link
connections, earth cable terminals, sockets for bonding lead entry generally made of removable links.
Bonding leads are insulated conductors, single core or co-axial, for bonding connections.
Multi-point bonding uses bonding leads at both ends and at intermediate points of a cable circuit. It
is a simple and a low cost option with minimum maintenance requirements. It is commonly used for
low and medium voltage systems.
Single-point bonding: Only one end of the cable metal sheath is directly grounded to the
underground earth link.
Mid-point bonding consists of two single point bonding systems. Both ends of cable metal sheath are
open and the mid-point is grounded through the parallel earth continuity conductor.
Cross-bonding interrupts sheath continuity at regular minor section length by cross-bonding joints.
Connections are made between the sheaths so that each sheath circuit surrounds the three phase
conductors successively.
Continuous cross-bonding installs sheath voltage limiters at the cross-bonding points to protect
screen interruptions from electromagnetic transients. The complete compensation of induced voltages
requires that the minor sections are of the same length and that the spacing between cables is
constant.
Sectionalized cross-bonding consists of multiple major sections along a cable circuit. A major
section consists of three minor sections.
Cross-bonding and transposition transposed cables at each joint chamber to reduce the induced
voltages and sheath circulating currents by cable phase conductors. The induced voltage is then near
zero. When cables are laid in flat formation, the compensation of the induced voltages can be

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

achieved if cables are transposed at cross-bonding points. It is worth mentioning that, even for a trefoil
laying, transposition of the cables is recommended to limit the induced voltages by nearby conductors.
Direct sectionalized cross-bonding consists of multiple major sections along a cable circuit. A major
section consists of three minor sections. The highest transient overvoltage appears in the sections
closest to the terminations. At the other cross-bonding points of the circuit, cross-bonding of the
screens is then performed “directly” by jointing single-core bonding leads, without SVLs.
Siphon lines connect overhead and underground cables.
Continuous operating voltage, Uc, designated permissible rms value of power-frequency voltage
that may be applied continuously between the sheath voltage limiter terminals – See IEC 60099.4.
Rated voltage, Ur, maximum permissible rms value of power-frequency voltage between sheath
voltage limiter terminals at which it is designed to operate correctly under temporary overvoltage
conditions. Notes: The rated voltage is used as a reference parameter for the specification of
operating characteristics. The rated voltage as defined is the 10 s power-frequency voltage used in the
operating duty test after high-current or long-duration impulses. – See IEC 60099.4.
Residual voltage, Ures, peak value of voltage that appears between the sheath voltage limiter
terminals during the passage of discharge current. – See IEC 60099.4.

A.3. Symbols
Symbol Definition First Occurrence
E or 𝐸𝑘 Induced voltage in the screen k (𝑘 = 1. .3) 2.1.3
j Complex parameter (schematic)
ω Angular frequency
µ Permeability
s Spacing between conductors
𝑟𝑒 Metal sheath mean radius
𝐼𝑘 Phase currents (𝑘 = 1. .3)
L Length of the cable
α 2𝜋
Complex rotation
3
𝑑1𝑘 Distance between cable k and parallel conductor (𝑘 = 1. .3) 2.1.4.3 (Figure
2.9)
𝜌𝑠𝑜𝑖𝑙 Soil electric resistivity 2.1.9
𝛾 Euler’s constant
D Short-circuit fictitious return path distance
𝑅𝑘 Earthing resistance at remote end k (𝑘 = 1. .2)
𝑅𝑠 Metal sheath resistance
𝑅𝑐 ECC resistance
𝑟𝑠 Metal sheath mean radius
𝑑𝑖𝑗 Distance between phases i and j (𝑖 = 1. .3) (𝑗 = 1. .3)
𝑍𝑠 Metal sheath self-impedance
𝑍𝑚 Mutual impedance between core conductor and metal sheath
𝑍𝑐 ECC self-impedance
𝑍𝑖𝑗 Mutual impedance between phases i and j (𝑖 = 1. .3) (𝑗 =
1. .3)
𝑍𝑖𝑓 Mutual impedance between phase i and faulted phase (𝑖 =. .3)
𝑍𝑖𝑐 Mutual impedance between phase i and ECC (𝑖 = 1. .3)
𝑍𝑐𝑓 Mutual impedance between faulted phase and ECC
𝐸𝑓 Sheath voltage to local earth during fault conditions
𝐼𝑓 Earth fault current in conductor
𝐼𝑐 Earth fault return current in ECC

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

𝑠𝑖𝑓 Spacing between metal sheath i and faulted phase (𝑖 = 1. .3)

𝑠𝑖𝑐 Spacing between metal sheath i and ECC (𝑖 = 1. .3)
𝑠𝑐𝑓 Spacing between faulted phase and ECC
𝐸𝐸𝑃𝑅 Earth Potential Rise
𝛾𝑐 ECC radius
R Equivalent earth resistance over length 2.1.10
η Metal sheath current to core conductor current ratio (for solid
bonding)
A Impedances and boundary conditions matrix 2.3.2
B Known voltages and currents matrix
X Unknown voltages matrix
𝑉𝑠𝑖𝑛𝑔𝑙𝑒 Screen potential rise for single-phase short-circuit
𝑉𝑡ℎ𝑟𝑒𝑒 Screen potential rise for Three-phase short-circuit
𝐼𝑐𝑐𝑠𝑖𝑛𝑔𝑙𝑒 Single-phase short-circuit current
𝐼𝑐𝑐𝑡ℎ𝑟𝑒𝑒 Three-phase short-circuit current
Z Impedance matrix 2.3.3
Y Admittance matrix
L’ Single-phase or coaxial bonding leads inductance 2.3.4
µ Coupling factor
𝑍𝑠𝑤 Skywire self-impedance
𝑍𝑚𝑠𝑤 Mutual impedance between faulted phase and skywire

Parameters from Section 2.3.1 are not included.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

APPENDIX B. Bibliography/References
[1] CIGRE WG 21.07, “The design of specially bonded cable systems,” Electra, vol. 28, pp. 55-81, 1973.
[2] CIGRE WG 21.07, “The design of specially bonded cable systems,” Electra, vol. 47, pp. 61-86, 1976.
[3] CIGRE WG 21.07, “Guide to the protection of bonded cable systems against sheath overvoltages,”
Electra, vol. 128, pp. 47-82, 1990.
[4] CIGRE WG B1.18, “TB283: Special bonding of high voltage power cables,” CIGRE, Paris, 2006.
[5] CIGRE WG B1.26, “TB347: Earth potential rises in specially bonded screen systems,” CIGRE, Paris,
2008.
[6] IEEE PES, “IEEE 575: Guide for Bonding Shields and Sheaths of single conductor power cables rated
5kV through 500kV,” IEEE, Piscataway, 2014.
[7] Electricity Networks Association, “Insulated Sheath Power Cable Systems,” ENA, 2014.
[8] National Grid plc, “TS 3.05.04: Sheath bonding and earthing for insulated sheath power cable systems,”
National Grid.
[9] National Grid plc, “TS 3.05.03: Sheath Voltage Limiters,” National Grid.
[10] AEIC, “CS9-15: Specification for Extruded Insulation Power Cables and Their Accessories Rated above
46 kV through 345 kV,” AEIC, 2015.
[11] P. Mighe and F. de Leon, “Parametric study of losses in cross-bonded cables: conductors transposed
versus conductors non-transposed,” IEEE Transactions on Power Delivery, vol. 28, no. 4, pp. 2273-2281, 2013.
[12] B. Parmigiani, D. Quaggia, E. Elli and S. Franchina, “Zinc-oxide sheath voltage limiter for HV and EHV
power cable: field experience and laboratory tests,” IEEE Transactions on Power Delivery, vol. 1, no. 1, pp. 164-
170, 1986.
[13] P. Nichols and J. Yarnold, “A sensitivity analysis of cable parameters and their influence on design
choices for minimum sheath voltage limiter specification in underground cable systems,” in Australasian
Universities Power Engineering Conference, Adelaide, 2009.
[14] F. Ghassemi, “Effect of trapped charges on cable SVL failure,” Electric Power Systems Research, vol.
115, pp. 18-25, 2014.
[15] P. Nichols, “Minimum Voltage Rating of Sheath Voltage Limiters in Underground Cable Systems: The
Influence of Corrugated Cable Sheaths.,” in 47th International Universities Power Engineering Conference,
London, 2012.
[16] B. Gustavsen, J. Sletbak and T. Henriksen, “Simulation of transient sheath overvoltages in the presence
of proximity effects,” IEEE Transactions on Power Delivery, vol. 10, no. 2, pp. 1066-1075, 1995.
[17] C. Kaloudas, T. Papadopoulos, K. Gouramanis, K. Stasinos and G. Papagiannis, “Methodology for the
selection of long medium-voltage power cable configurations,” IET Generation, Transmission and Distribution, vol.
7, no. 5, pp. 526-536, 2013.
[18] U. Gudmundsdottir, B. Gustavsen, C. Bak and W. Wiechowski, “Field test and simulation of a 400kV
cross bonded cable systems,” IEEE Transactions on Power Delivery, vol. 26, no. 3, pp. 1403-1410, 2011.
[19] A. Sobral, A. Moura and M. Carvalho, “Technical implementation of cross bonding on underground high
voltage lines projects,” in CIRED 2011, Frankfurt, 2011.
[20] M. Chang, X. Shao and H. Ros, “B1-1019: In Land Long Distance HVAC Cables, Innovative Examples at
225kV, Application to 500kV,” in AORC Technical Meeting 2014, 2014.
[21] F. Lesur, P. Mirebeau, M. Mammeri and J. Santana, “Innovative insertion of very long AC cable links into
the transmission network,” in CIGRE Session, Paris, 2014.
[22] A. Khamlichi, G. Denche, F. Garnacho, G. Donoso and A. Valero, “B1-108: Location of sheath voltage
limiters (SVLs) used for accessory protection to assure the insulation coordination of cable outer sheath,
sectionalising joints and terminations of high voltage cable systems,” in CIGRE Session, Paris, 2016.
[23] T. Du Plessis, H. Jagau and D. Visagie, “Evaluating Step and Touch Potential Risks on Earthing
Systems of High Voltage Cable Systems,” in 8th CIGRE Southern Africa regional conference.
[24] P. Schutte, W. van der Merwe and J. van Coller, “Induced Voltage Behaviour Analysis Of An Un-
Grounded Outer Layer Semi-Conductive Coating Of A 400 kV Power Cable System,” in International Symosium
on High Voltage Engineering, Buenos Aires, 2017.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

[25] International Electrotechnical Commission, “IEC 60287-1-2: Electric cables - Calculation of the current
rating - Part 1: Current rating equations (100 % load factor) and calculations of losses - Section 2: Sheath eddy
current loss factors for two circuits in flat formation,” IEC, 1993.
[26] International Electrotechnical Commission, “IEC 60099-4, Part 4: Metal-oxide surge arresters without
gaps for AC systems,” IEC, 2014.
[27] S. Schelkunoff, “The electromagnetic theory of coaxial transmission lines and cylindrical shields,” Bell
System Technical Journal, 1934.
[28] F. Pollaczek, “Sur le champ produit par un conducteur simple infiniment long parcouru par un courant
alternatif,” Revue Gén, Elec., vol. 29, pp. 851-867, 1931.
[29] L. Wedepohl and D. Wilcox, “Transient analysis of underground power transmission systems,”
Proceedings of the IEE, vol. 120, no. 2, pp. 253-260, 1973.
[30] A. Ametani, “A General Formulation of Impedance and Admittance of Cables,” IEEE Transactions on
Power Apparatus, vol. 99, no. 3, pp. 902-910, 1980.
[31] CIGRE WG B1.30, “TB531: Cable systems electrical characteristics,” CIGRE, Paris, 2013.
[32] EMTP, “EMTP Theory Book,” EMTP, 1995.
[33] International Electrotechnical Commission, “IEC 60840: Power cables with extruded insulation and their
accessories for rated voltages above 150 kV (Um=170 kV) up to 500 kV (Um=550 kV) – Test methods and
requirements,” IEC, 2011.
[34] International Electrotechnical Commission, “IEC 62067: Power cables with extruded insulation and their
accessories for rated voltages above 30 kV (Um=36 kV) up to 150 kV (Um=170 kV) – Test methods and
requirements,” IEC, 2011.
[35] International Electrotechnical Commission, “IEC 60287-1-1 – Electric cables – Calculation of the current
rating – Part 1-1: Current rating equations (100% load factor) and calculation of losses - General,” IEC, 2014.
[36] International Electrotechnical Commission, “IEC 62895 – High Voltage Direct Current (HVDC) power
cables with extruded insulation and their accessories for rated voltages up to 320 kV for land applications - Test
methods and requirements,” IEC, 2017.
[37] International Electrotechnical Commission, "IEC 60071-1- Insulation co-ordination - Part 1: Definitions,
principles and rules”, IEC, 2006; "IEC 60071.4 – Computational Guide to Insulation Co-ordination and Modelling
of Electrical Networks”, IEC, 2004.
[38] International Electrotechnical Commission, "IEC 62271-209 High-voltage switchgear and controlgear – Part
209: Cable connections for gas-insulated metal-enclosed switchgear for rated voltages above 52 kV – Fluid-filled
and extruded insulation cables – Fluid-filled and dry-type cable-terminations, IEC, 2007
[39] IEEE Standard 1300-2011, “Guide for Cable Connections for Gas-Insulated Substations”, IEEE, Jan. 2012.
[40] A. Khamlichi, G. Donoso, F. Garnacho, G. Denche, A. Valero, and F. Álvarez, “Improved Cable Connection to
Mitigate Transient Enclosure Voltages in 220-kV Gas-Insulated Substations,” IEEE Transactions on Industry
Applications, vol. 52, no. 1, 2016
[41] A. Khamlichi, G. Donoso, F. Garnacho, G. Denche, and A. Valero, “B3-308: Removing risk of eventual
discharges between GIS grounding parts and cable sheath connected to the substation earth through a separate
grounding lead,” CIGRE Session, Paris, 2016.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

APPENDIX C. Review of service experience –

Survey details (1)
A questionnaire was sent to the members of this Working Group. A summary of the survey was
included in Section 1.3, Review of Service Experience. This appendix includes details of the
responses to the questionnaire as received. A total of thirteen responses were received as listed in the
tables below, from:
▪ AUSTRALIA (AU)
▪ BRAZIL (BR)
▪ CANADA (CA)
▪ CHINA (CN)
▪ DENMARK (DK)
▪ FRANCE (FR)
▪ JAPAN (JP)
▪ NETHERLANDS (NL)
▪ SPAIN (ES)
▪ SOUTH AFRICA (SA)
▪ SWITZERLAND (CH)
▪ UNITED KINGDOM (UK)
▪ UNITED STATES (US)

C.1. Response 1
ITEMS
BONDING SCHEMATICS
Solid bonding Yes, it's used.
Single-point bonding Yes, it's used.
Sectionalized cross-bonding Yes, it's used.
Continuous cross-bonding Yes, it's used.
Hybrid bonding Yes, it's used.
Direct cross-bonding without link Not used.
box
WITHSTAND VOLTAGE LEVEL OF BONDING COMPONENTS
Screen to ground impulse As per IEC62067 for Extra High Voltage cable systems
withstand level for joints (kVp)
Screen interruption impulse In line with the provisions of the IEC62067 /Table D1 for Extra
withstand level for joints (kVp) High Voltage cable systems and insulation coordination
principles of cable bonding system. Usually this parameter is
verified by an EMTP analysis for EHV cable systems. The
value is, in general, in excess of 95kVp for EHV cable circuits.
Screen to ground DC withstand Minimum 40kV or as agreed between the client and cable
level for joints (kV) supplier.
Screen interruption DC withstand As above
level for joints (kV)
Screen to ground AC withstand A value of 20 kV was indicated. Generally, the D.C. withstand
level for joints (kV rms) (minimum 40kV) is quoted.
Screen interruption AC withstand The inner joint screen interrupter should have the highest
level for joints (kV) insulation withstand level when compared with the other
elements of the cable bonding system. A value of 20 kV AC
was quoted.
Impulse withstand level for Generally, the impulse testing of outersheath should comply
outersheath (kVp) with the provisions of IEC62067 and with the IEC60229 in
respect to DC and spark tests.
DC withstand level for The routine testing of cable outersheath is carried out in
outersheath (kV) accordance with the IEC60229 in respect to DC and spark
tests. However, for EHV cable systems the outersheath is

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

routinely tested with 40 kV DC and during installation the test is

carried out with 20kVD.C. and at commissioning with 10kV
AC withstand level for Minimum 20kV AC. All tests are carried out only with D.C.
outersheath (kV) voltage.
Impulse withstand level for The bonding leads are tested in the same manner as the cable
bonding cables (kVp) outersheath (see IEC62067 & IEC60229).
DC withstand level for bonding The testing of cable outersheath is carried out in accordance
cables (kV) with the IEC60229 in respect to DC and spark tests. However,
for EHV cable systems the outersheath is routinely tested with
40kV D.C.
AC withstand level for bonding As for cable outersheath.
cables (kV)
DC withstand level between
metal screen cable and metal
enclosed GIS (kVp)
AC withstand level between
metal screen cable and metal
enclosed GIS (kVp)
Impulse withstand level for post
insulator (kVp)
DC withstand level for post
insulator (kV)
AC withstand level for post
insulator (kV)
Sheath Voltage Limiters (SVLs)
Type of resistor (Silicon Carbide, ZnO
Zinc Oxide, others)
Typical rated voltage (Ur) Up to and including 12kV depending on the cable voltage,
installed length and the conclusion of the sheath voltages and EMTP (or
equivalent) analysis. For cables of lower voltages up to 4.5kV.
Type of connection in Star connection with neutral grounded
sectionalized joints (delta
connection, star connection with
neutral grounded, star connection
without neutral grounded)
Nominal discharge current (kA) Determine by an EMTP analysis for EHV cable systems.
(wave 8/20µs)
Line discharge according to SVLs are selected to withstand repeated discharges based on
60099-4 EMPT analysis by which are determined the PRV, heat
dissipation and the other parameters.
Are SVLs installed inside link Yes.
boxes?
BONDING LEAD CABLES
Type of bonding cable: where single-core and concentric construction
single-core or concentric bonding
leads are used?
Maximum length criteria < 10m
Type of insulation (XLPE, PVC, PVC older types, HDPE new cables or a combination of MDPE
PE) (XLPE) insulation protected with an anti-termite outersheath
made of HDPE
Outersheath with semi- Colloidal Graphite layer
conductive layer or graphite layer
Watertight Compacted conductor for single-core bonding leads and for the
inner conductor of concentric bonding leads. Application of
water swellable tapes or powder at concentric conductor.
LINK BOXES
Location where link boxes are Mainly in reinforced concrete underground pits. The pits are
installed provided with heavy duty protective covers/lids designed to

72
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

withstand heavy traffic loads. In some cases, the LBS are

installed on above ground structures.
Are the link boxes accessible? All link boxes are (must be) accessible. For underground pits
the access is from the top by lifting/removal of the protective
covers. These covers are installed flat (same level) with the
road or pedestrian walkway or the ground surface
Waterproof test The latest generation of Link Boxes are waterproofed to
withstand 7m water head and explosion proofed to withstand in
external pressure for up to 250 kPa.
Internal arc test Simulate an internal explosion.
CALCULATION CRITERIA
Method used for calculating CIGRE formulation. The calculations are part of cable supplier
induce voltage sheath (i.e. responsibilities. The client verifies for compliance.
CIGRE Documents, EMTP/ATP,
etc.)
Sheath voltage limits during 120 to 150V for cable systems up to and including 132kV and
normal operation 250V for EHV cable systems
SVL selection criteria during fault For EHV cable systems the selection of SVL's parameters
conditions (rated voltage, heat dissipation capabilities/discharge currents,
etc.) is carried out in conjunction with the overall study of
insulation levels of link boxes, link box leads, cable
outersheath, and joint sleeve sectionalizing barriers and inner
joint screen interruptions by a detailed insulation coordination
analysis/simulation. Similar principles are applied for cables of
lower voltages by considering the withstand voltage level of
each element of the cable bonding system.
SVL selection criteria during This is part of the EMTP analysis for EHV cable systems.
transient overvoltage conditions
(lightning and switching)
Are you considering internal This is part of the EMTP analysis for EHV cable systems.
cable fault conditions into
selection criteria of SVLs?
AFTER INSTALLATION TEST (BEFORE COMMISSIONING)
Outersheath voltage tests As per IEC60229 - Minimum 10kV at commissioning
Bonding system connections As per IEC60229 - Minimum 10kV & insulation resistance tests
and contact resistance test.
MAINTENANCE TEST
Outersheath voltage tests (off 2.5 to 5kV.
line)
Bonding system connection (off Visual inspection and the measurement of contact resistance.
line) Carryout HV test with 2.5 to 5kV in conjunction with the
outersheath testing. In addition, the SVLs are teste for I/V
characteristic and insulation condition (5kV D C and Insulation
resistance tests).
Current by metal screens (on This test is performed only at commissioning stage to
line) determine the Load Los Factor. The test is performed by
current injection in cable conductors.
SVL tests (off line)

C.2. Response 2
ITEMS
BONDING SCHEMATICS
Solid bonding It is usually used in short links and lower currents.
Single-point bonding Yes, it is used in cable systems with lower lengths (till 500 m long).
Middle point bonding it is also used

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Sectionalized cross-bonding It is used in long links.

Continuous cross-bonding It’s not used
Hybrid bonding Hybrid bonding (single-point bonding in combination with cross bonded
systems) is used at locations where the cable route is not suitable for
division into 3, 6, and 9 sections.
Direct cross-bonding without link box It's not used
WITHSTAND VOLTAGE LEVEL OF BONDING COMPONENTS
Screen to ground impulse withstand IEC60840 and IEC 62067.
level for joints (kVp)
Screen interruption impulse withstand According to IEC60840 and IEC 62067.
level for joints (kVp)
Screen to ground DC withstand level According to IEC60840 and IEC 62067.
for joints (kV)
Screen interruption DC withstand According to IEC60840 and IEC 62067.
level for joints (kV)
Screen to ground AC withstand level Not required values.
for joints (kV rms)
Screen interruption AC withstand Not required values.
level for joints (kV)
Impulse withstand level for According to IEC60229.
outersheath (kVp)
DC withstand level for outersheath According to IEC60229.
(kV)
AC withstand level for outersheath Not specified /No test
(kV)
Impulse withstand level for bonding Not required values by national standards, but should be in accordance
cables (kVp) with cable oversheath.
DC withstand level for bonding cables Not required values by national standards, but should be in accordance
(kV) with cable oversheath.
AC withstand level for bonding cables Not required values by national standards, but should be in accordance
(kV) with cable oversheath.
DC withstand level between metal
screen cable and metal enclosed GIS
(kVp)
AC withstand level between metal
screen cable and metal enclosed GIS
(kVp)
Impulse withstand level for post
insulator (kVp)
DC withstand level for post insulator
(kV)
AC withstand level for post insulator
(kV)
Sheath Voltage Limiters (SVLs)
Type of resistor (Silicon Carbide, Zinc ZnO
Oxide, others)
Typical rated voltage (Ur) installed Cross bonding and Single point bonding
• Older power systems: 4,5 kV (before 2012)
• Currently power systems: 9 kV

Type of connection in sectionalized Star connection with neutral grounded

joints (delta connection, star
connection with neutral grounded,
star connection without neutral
grounded)
Nominal discharge current (kA) (wave 10 kA
8/20µs)
Line discharge according to 60099-4

74
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Are SVLs installed inside link boxes? Yes

BONDING LEAD CABLES
Type of bonding cable: where single- Concentric or coaxial construction for joints. Usually 240 x 240 mm2.
core or concentric bonding leads are Single-core cable for terminations.
used?
Maximum length criteria It's limited in 10 m long
Type of insulation (XLPE, PVC, PE) EPR
Outersheath with semi-conductive Graphite
layer or graphite layer
Watertight Yes
LINK BOXES
Location where link boxes are Link boxes are installed in a dedicated manhole
installed
Are the link boxes accessible? Yes
Waterproof test Yes - Protection Grade IP 68
Internal arc test
CALCULATION CRITERIA
Method used for calculating induce SVL are dimensioned by the furnisher - CIGRE formulation - ELECTRA
voltage sheath (i.e. CIGRE
Documents, EMTP/ATP, etc.)
Sheath voltage limits during normal Sheath standing voltages levels used lies around 80 to 120 V. A
operation standing voltage of 120 V in polyethylene cable jacket (thickness of 3 to
5 mm) it is considered safety.
The limiter should be suitable for continuous operation with an applied
voltage equal to the sheath standing voltage under either normal or
emergency load.
SVL selection criteria during fault Induced Voltage limited to SVL
conditions
SVL selection criteria during transient
overvoltage conditions (lightning and
switching)
Are you considering internal cable No according to ANSI/ IEEE std. 575 -1988.
fault conditions into selection criteria
of SVLs?
AFTER INSTALLATION TEST (BEFORE COMMISSIONING)
Outersheath voltage tests 10 kV during 1 minute
Bonding system connections Visual inspection
MAINTENANCE TEST
Outersheath voltage tests (off line) 5 kV during 1 minute
Bonding system connection (off line) Visual inspection
Current by metal screens (on line) YES
SVL tests (off line) YES

C.3. Response 3
ITEMS
BONDING SCHEMATICS
Solid bonding Solid bonding is used in MV distribution circuits
Single-point bonding Single ended bonding is used mostly in transmission lines (HV
or EHV) where the cable length or number of sections does
not allow for sheath cross-bonding
Sectionalized cross-bonding This is the most used system for transmission lines
Continuous cross-bonding Not used

75
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Hybrid bonding It is used where the number of sections does not allow for full
cross-bonding
Direct cross-bonding without link Not used
box
WITHSTAND VOLTAGE LEVEL OF BONDING COMPONENTS
Screen to ground impulse There is no uniformed standard used by every utility. They
withstand level for joints (kVp) follow mostly IEC 62067, 60840 and Electra
recommendations.
Screen interruption impulse There is no uniformed standard used by every utility. They
withstand level for joints (kVp) follow mostly IEC 62067 and Electra recommendations for
new installations.
Screen to ground DC withstand The same as specified in IEC and IEEE standards as specified
level for joints (kV) for the oversheath.
Screen interruption DC withstand Same as above
level for joints (kV)
Screen to ground AC withstand The AC withstand not specified.
level for joints (kV rms)
Screen interruption AC withstand The AC withstand not specified.
level for joints (kV)
Impulse withstand level for Not used for cables in service.
outersheath (kVp)
DC withstand level for As per relevant IEC and IEEE standards
outersheath (kV)
AC withstand level for outersheath Not specified.
(kV)
Impulse withstand level for Not specified.
bonding cables (kVp)
DC withstand level for bonding Same as oversheath. 20kV after installation and 5-10kV in
cables (kV) service.
AC withstand level for bonding
cables (kV)
DC withstand level between metal
screen cable and metal enclosed
GIS (kVp)
AC withstand level between metal
screen cable and metal enclosed
GIS (kVp)
Impulse withstand level for post
insulator (kVp)
DC withstand level for post
insulator (kV)
AC withstand level for post
insulator (kV)
Sheath Voltage Limiters (SVLs)
Type of resistor (Silicon Carbide, ZnO
Zinc Oxide, others)
Typical rated voltage (Ur) installed There is no typical voltage. Each section is calculated.
Type of connection in Star connection with neutral grounded.
sectionalized joints (delta
connection, star connection with
neutral grounded, star connection
without neutral grounded)
Nominal discharge current (kA) 10kA
(wave 8/20µs)
Line discharge according to
60099-4
Are SVLs installed inside link Yes
boxes?
BONDING LEAD CABLES

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Type of bonding cable: where Single core and concentric neutral.

single-core or concentric bonding
leads are used?
Maximum length criteria Max 10m
Type of insulation (XLPE, PVC, There is not standard used for type of cable. Most is 1kV
PE) (RWU) cable or XLPE.
Outersheath with semi-conductive
layer or graphite layer
Watertight
LINK BOXES
Location where link boxes are The joint SVL or link boxes are installed in underground vaults.
installed The termination boxes are on relevant structures close to the
terminations.
Are the link boxes accessible? Yes, they are accessible to an appropriate person. In some
cases they are buried.
Waterproof test Link boxes can be watertight as per IEC60259 or NEMA rated
4X
Internal arc test
CALCULATION CRITERIA
Method used for calculating Mostly CIGRE formulation, however EMTP is used in some
induce voltage sheath (i.e. CIGRE cases.
Documents, EMTP/ATP, etc.)
Sheath voltage limits during Depends on utility. In some utilities induced voltage of 600V is
normal operation allowed during emergency operation for transmission systems.
SVL selection criteria during fault The SVL should be able to withstand the overvoltage resulting
conditions from the system faults. The maximum expected overvoltages
are calculated based on ELECTRA or IEEE std. 575
SVL selection criteria during SVL is selected based on maximum expected fault level.
transient overvoltage conditions
(lightning and switching)
Are you considering internal cable No
fault conditions into selection
criteria of SVLs?
AFTER INSTALLATION TEST (BEFORE COMMISSIONING)
Outersheath voltage tests 10kV DC
Bonding system connections 1. Contact resistance tests.
2. Some utilities require current injection and measure voltage
at each joint bay to prove the calculations.
MAINTENANCE TEST
Outersheath voltage tests (off Mostly 5kV
line)
Bonding system connection (off Visual inspection
line)
Current by metal screens (on line) not performed
SVL tests (off line) not performed

C.4. Response 4
ITEMS
BONDING SCHEMATICS
Solid bonding It's not used.
Single-point bonding It's used.
Sectionalized cross-bonding It's used.
Continuous cross-bonding It's not used.
Hybrid bonding It's used.
Direct cross-bonding without link It's not used.
box
WITHSTAND VOLTAGE LEVEL OF BONDING COMPONENTS

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Screen to ground impulse

withstand level for joints (kVp)
Screen interruption impulse
withstand level for joints (kVp)
Screen to ground DC withstand
level for joints (kV)
Screen interruption DC withstand
level for joints (kV)
Screen to ground AC withstand
level for joints (kV rms)
Screen interruption AC withstand
level for joints (kV)
Impulse withstand level for • 20 kVp up to 35 kV rated system voltage.
outersheath (kVp) • 37.5 kVp for 110 kV rated system voltage.
• 62.5 kVp for 330 kV rated system voltage.
• 74.5 kVp for 500 kV rated system voltage.
DC withstand level for outersheath
(kV)
AC withstand level for outersheath
(kV)
Impulse withstand level for
bonding cables (kVp)
DC withstand level for bonding
cables (kV)
AC withstand level for bonding
cables (kV)
DC withstand level between metal
screen cable and metal enclosed
GIS (kVp)
AC withstand level between metal
screen cable and metal enclosed
GIS (kVp)
Impulse withstand level for post
insulator (kVp)
DC withstand level for post
insulator (kV)
AC withstand level for post
insulator (kV)
Sheath Voltage Limiters (SVLs)
Type of resistor (Silicon Carbide,
Zinc Oxide, others)
Typical rated voltage (Ur) installed 1.8 and 3.6 kV
Type of connection in sectionalized
joints (delta connection, star
connection with neutral grounded,
star connection without neutral
grounded)
Nominal discharge current (kA) 10 kA
(wave 8/20µs)
Line discharge according to
60099-4
Are SVLs installed inside link
boxes?
BONDING LEAD CABLES
Type of bonding cable: where
single-core or concentric bonding
leads are used?
Maximum length criteria

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Type of insulation (XLPE, PVC,

PE)
Outersheath with semi-conductive
layer or graphite layer
Watertight
LINK BOXES
Location where link boxes are
installed
Are the link boxes accessible?
Waterproof test YES.
Because of humid air, higher underground water in the area,
the grounding boxes standard with strict requirement of
waterproof or
damp proof is needed
Internal arc test
CALCULATION CRITERIA
Method used for calculating induce
voltage sheath (i.e. CIGRE
Documents, EMTP/ATP, etc.)
Sheath voltage limits during At present, design standard of cable line induced voltage level
normal operation has changed from 50 V to 300 V.
SVL selection criteria during fault
conditions
SVL selection criteria during SVL is selected based on the impulse withstand voltage level
transient overvoltage conditions of outersheath. The residual voltage of SVL should be less
(lightning and switching) than 0.7 times of the withstand voltage level of outersheath.
Are you considering internal cable
fault conditions into selection
criteria of SVLs?
AFTER INSTALLATION TEST (BEFORE COMMISSIONING)
Outersheath voltage tests
Bonding system connections
MAINTENANCE TEST
Outersheath voltage tests (off line)
Bonding system connection (off
line)
Current by metal screens (on line)
SVL tests (off line)

C.5. Response 5
ITEMS
BONDING SCHEMATICS
Solid bonding Both end bonding is normally not used for underground
cables (though it of course is inevitable for submarine cables),
as capitalized losses are evaluated as part of the tender.
However, in principle the method is allowed, and especially
for short cables it may be the most optimal approach. Screens
are traditionally bonded, at the terminations, through three (3)
phase earthing boxes.
Single-point bonding Single point bonding is especially used in Hybrid bonding
systems.
The single point bonded sections must include at least one
earth continuity conductor (ECC) which is, if possible,
transposed at the center of the minor section.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Sectionalized cross-bonding It's used in long links.

In order to reduce probability of failure, the utility wants to
minimize the number of link boxes. The length of minor
sections can be maximized with straight joints, according to
the expected maximum short circuit current during the life
time of the cable and withstand level of the jacket.
Continuous cross-bonding
Hybrid bonding Hybrid bonding (single-point bonding in combination with
cross bonded systems) is used at locations where the cable
route is not suitable for division into 3, 6, 9 sections.

Direct cross-bonding without link It's used as possibility to minimize the number of link boxes.
box These cross bonding methods do not include link boxes and
cannot be used on the first major section from each cable
end, due to switching and lightning overvoltages.
Only every sixth cross bonding point is required to be
grounded through a link box.
Generally manufacturers prefers directly cross bonded
systems as the number of accessories can be limited (fewer
link boxes) and the cost therefore optimized.
However, some manufacturers are not yet comfortable with
this solution and the utility therefore still installs cables with
the traditional cross bonding methods.
WITHSTAND VOLTAGE LEVEL OF BONDING COMPONENTS
Screen to ground impulse
withstand level for joints (kVp)
Screen interruption impulse
withstand level for joints (kVp)
Screen to ground DC withstand
level for joints (kV)
Screen interruption DC withstand
level for joints (kV)
Screen to ground AC withstand
level for joints (kV rms)
Screen interruption AC withstand
level for joints (kV)
Impulse withstand level for
outersheath (kVp)
DC withstand level for outersheath
(kV)
AC withstand level for outersheath
(kV)
Impulse withstand level for
bonding cables (kVp)
DC withstand level for bonding
cables (kV)
AC withstand level for bonding
cables (kV)
DC withstand level between metal
screen cable and metal enclosed
GIS (kVp)
AC withstand level between metal
screen cable and metal enclosed
GIS (kVp)
Impulse withstand level for post
insulator (kVp)
DC withstand level for post
insulator (kV)
AC withstand level for post
insulator (kV)

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Sheath Voltage Limiters (SVLs)

Type of resistor (Silicon Carbide,
Zinc Oxide, others)
Typical rated voltage (Ur) installed Typically, the SVL size is in the range from 5 kV to 12 kV.
Type of connection in sectionalized
joints (delta connection, star
connection with neutral grounded,
star connection without neutral
grounded)
Nominal discharge current (kA)
(wave 8/20µs)
Line discharge according to
60099-4
Are SVLs installed inside link
boxes?
BONDING LEAD CABLES
Type of bonding cable: where
single-core or concentric bonding
leads are used?
Maximum length criteria Link boxes places in maximum distance of 10 m from the
cable joint
Type of insulation (XLPE, PVC,
PE)
Outersheath with semi-conductive
layer or graphite layer
Watertight Yes
LINK BOXES
Location where link boxes are
installed
Are the link boxes accessible? In urban areas, the link boxes shall be placed easily
accessible in the surface or elevated where possible,
depending on the local conditions.
Outside of urban areas, the cross bonding link boxes may be
buried, to avoid interference with the farmer’s normal work in
the field.
Grounding boxes, every third or every sixth link box, shall be
located so that they can be accessed easily. These must be
accessible for fault locating activities and for periodical jacket
testing.
Waterproof test Minimum an ingress protection rating IP68 according to
IEC60529 (meter water depth).
A link box shall be watertight, and filled with a compound that
covers the link cables to seal the cable glands and the
exposed ends of the link cable. This compound may not
overflow the SVLs or coupling bars
Internal arc test
CALCULATION CRITERIA
Method used for calculating induce SVLs are dimensioned by the cable system manufacturer.
voltage sheath (i.e. CIGRE
Documents, EMTP/ATP, etc.)
Sheath voltage limits during
normal operation

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

SVL selection criteria during fault The Transient Overvoltage Capability (TOC, 1s) must be high
conditions enough to allow for the 50 Hz voltage induced from the
maximum fault current in the system.
The minimum size of the SVL shall be able to withstand the
induced AC voltage on the metal sheath, during steady state
and short circuit plus a margin of 20% to 25%.
Furthermore, the voltage rating of the SVL shall be high
enough to allow periodic testing of the cable jacket, normally 5
kV DC or as specified.

SVL selection criteria during The SVL shall connect the metal sheath to ground in case of
transient overvoltage conditions overvoltages due to switching and lightning transients.
(lightning and switching) The maximum size of the SVL is limited to the residual
voltage [Ures] due to a traveling wave from a lightning. The
most harmful transient shall be used for maximum size
design, which normally is due to a lightning to the entrance of
the cable line.
Are you considering internal cable
fault conditions into selection
criteria of SVLs?
AFTER INSTALLATION TEST (BEFORE COMMISSIONING)
Outersheath voltage tests
Bonding system connections
MAINTENANCE TEST
Outersheath voltage tests (off line)
Bonding system connection (off
line)
Current by metal screens (on line)
SVL tests (off line)

C.6. Response 6
ITEMS
BONDING SCHEMATICS
Solid bonding It's used in short links, with current rating below 600 A
Up to 2 intermediate earthing can also be done for long
lengths.
Single-point bonding It's used.
Intermediate earthing can be done for longer links.
However, continuous single point bonding is not used.

Sectionalized cross-bonding It's frequently used: a 20% imbalance is allowed on induced

voltages between two minor sections.
For long lengths (more than 2 major sections): No SVL
beyond major sections close to the ends.
For 63 kV and 90 kV links between substations, no SVL at all
Continuous cross-bonding Only used in very specific cases.
Hybrid bonding A mix between single-point and sectionalized cross bonding is
carried out in some projects.
Single-point bonding can be performed on either one or both
sides of the link.
Direct cross-bonding without link Done for long lengths (more than 2 major sections) except for
box the first and last major sections (close to the ends).
For 63 kV and 90 kV links between substations, always direct
cross-bonding (no SVL at all).
WITHSTAND VOLTAGE LEVEL OF BONDING COMPONENTS

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Screen to ground impulse Different values than IEC 62067 annex G and IEC 60840
withstand level for joints (kVp) annex G standard.
Values are specified in national standards:
• 50 kVp for 63 kV, 90 kV and 225 kV rated system
voltage
• 62.5 kVp for 400 kV rated system voltage
Screen interruption impulse Different values than IEC 62067 annex G and IEC 60840
withstand level for joints (kVp) annex G standard.
Values are specified in specification:
• 80 kVp for 63 kV and 90 kV rated system voltage
• 100 kVp for 225 kV rated system voltage
• 125 kVp for 400 kV rated system voltage
Screen to ground DC withstand Values are specified in specification:
level for joints (kV) • 20 kV for all rated system voltage
Screen interruption DC withstand Not applicable
level for joints (kV)
Screen to ground AC withstand Values are specified in specification:
level for joints (kV rms) • 20 kV for all rated system voltage
Screen interruption AC withstand Values are specified in specification:
level for joints (kV) • 25 kV up to 225 kV rated system voltage.
• 35 kV for 400 kV rated system voltage.
Impulse withstand level for Same tests for outersheath and joints
outersheath (kVp)
DC withstand level for outersheath Same tests for outersheath and joints
(kV)
AC withstand level for outersheath Same tests for outersheath and joints
(kV)
Impulse withstand level for Coaxial bonding cables (between "core" and "screen"):
bonding cables (kVp) • 80 kV for 63 and 90 kV rated system voltage.
• 100 kV for 225 kV rated system voltage.
• 125 kV for 400 kV rated system voltage.
Coaxial bonding cables (between "screen" and ground) or
unipolar cables:
• 50 kV for 63, 90 and 225 kV rated system voltage.
• 62.5 kV for 400 kV rated system voltage.
DC withstand level for bonding • 20 kV for all rated system voltage.
cables (kV)
AC withstand level for bonding Coaxial bonding cables (between "core" and "screen"):
cables (kV) • 25 kV for 63, 90 and 225 kV rated system voltage.
• 35 kV for 400 kV rated system voltage.
Coaxial bonding cables (between "screen" and ground) or
unipolar cables:
• 20 kV for all rated system voltage.
DC withstand level between metal Not applicable
screen cable and metal enclosed
GIS (kVp)
AC withstand level between metal 20 kV for all rated system voltage.
screen cable and metal enclosed
GIS (kVp)
Impulse withstand level for post 50/62,5
insulator (kVp)
DC withstand level for post Not applicable
insulator (kV)
AC withstand level for post 20 kV for all rated system voltage.
insulator (kV)
Sheath Voltage Limiters (SVLs)
Type of resistor (Silicon Carbide, ZnOX
Zinc Oxide, others)

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Typical rated voltage (Ur) installed 15kV for single-point bonding (at substations or tower)
12 kV for cross-bonding
6 kV for cross-bonding on old links
Type of connection in star connection with neutral grounded
sectionalized joints (delta
connection, star connection with
neutral grounded, star connection
without neutral grounded)
Nominal discharge current (kA) 10 kA
(wave 8/20µs)
Line discharge according to
60099-4
Are SVLs installed inside link No
boxes?
BONDING LEAD CABLES
Type of bonding cable: where Concentric bonding cable (coaxial) in joints when SVL
single-core or concentric bonding involved.
leads are used? Single-core cable in direct cross bonding or outdoor and GIS
terminations
Maximum length criteria Must not exceed 10 m long in joints and 8 m in terminations.
Type of insulation (XLPE, PVC, PE in old lines.
PE) XLPE in new lines.
Outersheath with semi-conductive Yes
layer or graphite layer
Watertight Yes
(immersion test for 24 hours)
LINK BOXES
Location where link boxes are Link boxes are not used.
installed SVL are installed inside a dedicated manhole or pit close to
the joint bay.
Are the link boxes accessible?
Waterproof test
Internal arc test
CALCULATION CRITERIA
Method used for calculating induce Homemade tools based on classical CIGRE formulae.
voltage sheath (i.e. CIGRE When complex calculation is needed (to analyze fault
Documents, EMTP/ATP, etc.) circumstances for example), EMTP is required.
Sheath voltage limits during In normal operation induced voltage on metal sheath cannot
normal operation exceed 400 V.
SVL selection criteria during fault During fault conditions, SVL should not be activated
conditions
SVL selection criteria during 12 kV is always used in any case.
transient overvoltage conditions
(lightning and switching)
Are you considering internal cable No
fault conditions into selection
criteria of SVLs?
AFTER INSTALLATION TEST (BEFORE COMMISSIONING)
Outersheath voltage tests Before commissioning, once the cables laid, a 20 kV DC test
is performed during 15 minutes on each sections individually.
Once all the joints are finished, the same test is used on the
total line. This final test allows to also test bonding leads.
Bonding system connections Cross-bonding cables are connected during the final test (20
kV during 15 minutes on the total line), but grounding cables
must be disconnected: they are not tested.
MAINTENANCE TEST
Outersheath voltage tests (off line) No
Bonding system connection (off No
line)

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Current by metal screens (on line) No

SVL tests (off line) Every 5 years approx. and after every fault on the line.
Tested by an independent lab

C.7. Response 7
ITEMS
BONDING SCHEMATICS
Solid bonding It is used only for 66-77kV Triplex cable in which three cores
are twisted
Single-point bonding It is used in short links
Sectionalized cross-bonding It is used in long length
Continuous cross-bonding It is specially used in long bridge section where local earth
inside bridge is not allowed
Hybrid bonding It is used in case full cross-bond
system is not achieved.
Direct cross-bonding without link
It is commonly used where lightning strike is not considered
box (i.e. GIS - Trans or GIS - GIS) up to 154kV.
Where applicable, first major section from the termination will
have SVL but 2nd or later major sections will not have SVL
WITHSTAND VOLTAGE LEVEL OF BONDING COMPONENTS
Screen to ground impulse 154kV - 50kVp
withstand level for joints (kVp) 275kV - 55kVp
500kV- 65kVp
Screen interruption impulse 154kV - 50kVp
withstand level for joints (kVp) 275kV - 55kVp
500kV- 65kVp
Screen to ground DC withstand Not specified
level for joints (kV)
Screen interruption DC withstand Not specified
level for joints (kV)
Screen to ground AC withstand Not specified
level for joints (kV rms)
Screen interruption AC withstand Not specified
level for joints (kV)
Impulse withstand level for 154kV -65kVp
outersheath (kVp) 275kV -75kVp
500kV -90kVp
DC withstand level for outersheath 500kV- 25kVDC
(kV)
AC withstand level for outersheath Not specified
(kV)
Impulse withstand level for Same as joint
bonding cables (kVp)
DC withstand level for bonding Not specified
cables (kV)
AC withstand level for bonding Not specified
cables (kV)
DC withstand level between metal Not specified
screen cable and metal enclosed
GIS (kVp)
AC withstand level between metal Not specified
screen cable and metal enclosed
GIS (kVp)
Impulse withstand level for post Not specified
insulator (kVp)
DC withstand level for post Not specified
insulator (kV)
AC withstand level for post Not specified
insulator (kV)

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Sheath Voltage Limiters (SVLs)

Type of resistor (Silicon Carbide, ZnO
Zinc Oxide, others)
Typical rated voltage (Ur) installed 3kV
Type of connection in Delta connected without neutral grounded
sectionalized joints (delta
connection, star connection with
neutral grounded, star connection
without neutral grounded)
Nominal discharge current (kA) 10kA
(wave 8/20µs)
Line discharge according to Not specified
60099-4
Are SVLs installed inside link Not used
boxes?
BONDING LEAD CABLES
Type of bonding cable: where Single core lead only
single-core or concentric bonding
leads are used?
Maximum length criteria Typically, 1-2m
Type of insulation (XLPE, PVC, PVC
PE)
Outersheath with semi-conductive No semi-conductive layer
layer or graphite layer
Watertight No water tight conductor
LINK BOXES
Location where link boxes are NA -Link Boxes have never used
installed
Are the link boxes accessible? NA -Link Boxes have never used
Waterproof test NA -Link Boxes have never used
Internal arc test NA -Link Boxes have never used
CALCULATION CRITERIA
Method used for calculating CIGRE formula
induce voltage sheath (i.e. CIGRE
Documents, EMTP/ATP, etc.)
Sheath voltage limits during Maximum 600V is allowed where bonding lead is not
normal operation accessible in public.
SVL selection criteria during fault SVL is selected low enough to protect cable system against
conditions Lightning/Switching Impulse only, but selection of SVL is not
considered for Fault Conditions.
Once system fault happens, all bonding system and SVLs are
tested or replaced as "fuses."
SVL selection criteria during Follow previous studies and guideline based on EMTP
transient overvoltage conditions
(lightning and switching)
Are you considering internal cable No
fault conditions into selection
criteria of SVLs?
AFTER INSTALLATION TEST (BEFORE COMMISSIONING)
Outersheath voltage tests 1kV megger
Bonding system connections 1kV megger
MAINTENANCE TEST
Outersheath voltage tests (off line) No
Bonding system connection (off No
line)
Current by metal screens (on line) No

86
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

SVL tests (off line) Some utility uses a special SVL with "color mark" which
change the color to red by overheating during breakdown of
SVL

C.8. Response 8
ITEMS
BONDING SCHEMATICS
Solid bonding Yes, it’s frequently used medium voltage lines (up to 66 kV).
Above 66kV is used in short lines.
Single-point bonding It's used.
Sectionalized cross-bonding It's used in long length.
Continuous cross-bonding
Hybrid bonding It's used.
Direct cross-bonding without link It's used.
box
WITHSTAND VOLTAGE LEVEL OF BONDING COMPONENTS
Screen to ground impulse
withstand level for joints (kVp)
Screen interruption impulse
withstand level for joints (kVp)
Screen to ground DC withstand
level for joints (kV)
Screen interruption DC withstand
level for joints (kV)
Screen to ground AC withstand
level for joints (kV rms)
Screen interruption AC withstand
level for joints (kV)
Impulse withstand level for
outersheath (kVp)
DC withstand level for outersheath
(kV)
AC withstand level for outersheath
(kV)
Impulse withstand level for
bonding cables (kVp)
DC withstand level for bonding
cables (kV)
AC withstand level for bonding
cables (kV)
DC withstand level between metal
screen cable and metal enclosed
GIS (kVp)
AC withstand level between metal
screen cable and metal enclosed
GIS (kVp)
Impulse withstand level for post
insulator (kVp)
DC withstand level for post
insulator (kV)
AC withstand level for post
insulator (kV)
Sheath Voltage Limiters (SVLs)
Type of resistor (Silicon Carbide, ZnO
Zinc Oxide, others)
Typical rated voltage (Ur) installed 5 kV for cross-bonding
5kV or 10 kV for single-point bonding

87
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Type of connection in sectionalized Star connection with neutral grounded

joints (delta connection, star
connection with neutral grounded,
star connection without neutral
grounded)
Nominal discharge current (kA)
(wave 8/20µs)
Line discharge according to
60099-4
Are SVLs installed inside link Yes
boxes?
BONDING LEAD CABLES
Type of bonding cable: where single-core and concentric construction
single-core or concentric bonding
leads are used?
Maximum length criteria Yes, the length is limited to 10 m nearby cable joints.
Type of insulation (XLPE, PVC, PE in old links and XLPE in new links.
PE)
Outersheath with semi-conductive Both types are used.
layer or graphite layer
Watertight Yes
LINK BOXES
Location where link boxes are The boxes for the minor section link boxes are direct buried
installed close to the cable joint. The link boxes for the major sections
are installed inside a dedicated manhole or pit close to the
cable joint. In some cases, the link box is installed in above
ground structures.
Are the link boxes accessible? Yes, all boxes must be accessible.
Waterproof test
Internal arc test
CALCULATION CRITERIA
Method used for calculating induce
voltage sheath (i.e. CIGRE
Documents, EMTP/ATP, etc.)
Sheath voltage limits during
normal operation
SVL selection criteria during fault
conditions
SVL selection criteria during
transient overvoltage conditions
(lightning and switching)
Are you considering internal cable No
fault conditions into selection
criteria of SVLs?
AFTER INSTALLATION TEST (BEFORE COMMISSIONING)
Outersheath voltage tests
Bonding system connections
MAINTENANCE TEST
Outersheath voltage tests (off line)
Bonding system connection (off
line)
Current by metal screens (on line)
SVL tests (off line)

C.9. Response 9
ITEMS

88
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

BONDING SCHEMATICS
Solid bonding It's frequently used medium voltage links (up to 45 kV).
In 66 kV links and above is used in short links (i.e. transformer
links inside power station...)

Single-point bonding It's used in short links.

Sectionalized cross-bonding It's the most common bonding schematic used in long links.
The screens are usually only crossing and not transposing the
cables.
Continuous cross-bonding It's very rarely used
Hybrid bonding A mix between single-point and sectionalized cross bonding is
carried out in some projects. Single-point bonding can be
performed on either one or both sides of the link.
Direct cross-bonding without link It's not used.
box
WITHSTAND VOLTAGE LEVEL OF BONDING COMPONENTS
Screen to ground impulse Values according to IEC 62067 annex G and IEC 60840
withstand level for joints (kVp) annex G standard.
Some utilities in 66 kV rated system voltage request the same
values as 132 kV rated system voltage.
Screen interruption impulse Values according to IEC 62067 annex G and IEC 60840
withstand level for joints (kVp) annex G standard.
Some utilities in 66 kV rated system voltage request the same
values as 132 kV rated system voltage.
Screen to ground DC withstand Values according to IEC 62067 annex G and IEC 60840
level for joints (kV) annex G standard.
Screen interruption DC withstand Values according to IEC 62067 annex G and IEC 60840
level for joints (kV) annex G standard.
Screen to ground AC withstand Not required values by standard.
level for joints (kV rms) Since 2012 some utilities specification request 20 kV.
Screen interruption AC withstand Not required values by standard.
level for joints (kV) Since 2012 some utilities specification request 25 kV from 66
kV rated system voltage.
Impulse withstand level for Values according to IEC 60229
outersheath (kVp)
DC withstand level for outersheath Values according to IEC 60229
(kV)
AC withstand level for outersheath The same values as screen to ground AC withstand level in
(kV) joints
Impulse withstand level for Not specified
bonding cables (kVp)
DC withstand level for bonding Not specified
cables (kV)
AC withstand level for bonding Not specified
cables (kV)
DC withstand level between metal Not specified
screen cable and metal enclosed
GIS (kVp)
AC withstand level between metal Not specified
screen cable and metal enclosed
GIS (kVp)
Impulse withstand level for post Not specified
insulator (kVp)
DC withstand level for post Not specified
insulator (kV)
AC withstand level for post Not specified
insulator (kV)
Sheath Voltage Limiters (SVLs)

89
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Type of resistor (Silicon Carbide, ZnO

Zinc Oxide, others)
Typical rated voltage (Ur) installed 5 kV for cross-bonding
5kV or 10 kV for single-point bonding
Type of connection in sectionalized Star connection with neutral grounded
joints (delta connection, star
connection with neutral grounded,
star connection without neutral
grounded)
Nominal discharge current (kA) 10 kA
(wave 8/20µs)
Line discharge according to 1 or 2 depends on cable manufacturer.
60099-4
Are SVLs installed inside link Yes
boxes?
BONDING LEAD CABLES
Type of bonding cable: where Coaxial cable in joints.
single-core or concentric bonding Single-phase cable in outdoor terminations and GIS
leads are used? terminations
Maximum length criteria Yes. Must not exceed 10 m long in joints and terminations.
Type of insulation (XLPE, PVC, Depends on cable system manufacturer
PE)
Outersheath with semi-conductive Not required
layer or graphite layer
Watertight Not required
LINK BOXES
Location where link boxes are Link boxes are installed as close as possible to joints.
installed When joint bays are accessible link boxes are installed inside
the same joint bay where joints are installed.
When joint bay isn´t accessible link boxes are installed inside
a dedicated manhole or pit close to the joint bay.
Are the link boxes accessible? Yes, due to maintenance reason.
Waterproof test There is no standard. Some utilities stablish the following
requirements:
In link boxes installed below ground level is required an
ingress protection rating IP68 according to IEC60529 (1-meter
water depth).
In link boxes installed above ground level is required an
ingress protection rating IP65 according to IEC60529.
Internal arc test YES, in link boxes with SLV
CALCULATION CRITERIA
Method used for calculating induce CIGRE formulation.
voltage sheath (i.e. CIGRE From 2014 some utilities make EMTP/ATP studies.
Documents, EMTP/ATP, etc.)
Sheath voltage limits during Not required values by standard.
normal operation Some electricity utilities stablish that induced voltage on metal
sheath cannot exceed 150 V in normal operation.
SVL selection criteria during fault Maximum induced voltage on metal sheath during steady
conditions state and short circuit < Rated voltage SVL (Ur).
Some electric utility considers a safety margin of 10%.
SVL selection criteria during Peak residual voltage (Ures) is defined by cable manufacturer.
transient overvoltage conditions Traditionally, the maximum value of Ures has been limited to
(lightning and switching) 20 kV
Are you considering internal cable NO
fault conditions into selection
criteria of SVLs?
AFTER INSTALLATION TEST (BEFORE COMMISSIONING)
Outersheath voltage tests 10 kV DC during 1 min
Bonding system connections Visual inspection of bonding system connections.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

MAINTENANCE TEST
Outersheath voltage tests (off line) 5 kV DC during 1 minute at least each 5 years.
Bonding system connection (off Visual inspection of bonding system connections at least each
line) 5 years.
Current by metal screens (on line) No
SVL tests (off line) Some utilities check SVL integrity at least each 5 years

C.10. Response 10
ITEMS
BONDING SCHEMATICS
Solid bonding Not recommended for use due to circulating current derating
and additional losses
Single-point bonding Used
Sectionalized cross-bonding Used
Continuous cross-bonding Newly introduced
Hybrid bonding Used
Direct cross-bonding without link Not used at present
box
WITHSTAND VOLTAGE LEVEL OF BONDING COMPONENTS
Screen to ground impulse As per IEC
withstand level for joints (kVp)
Screen interruption impulse As per IEC
withstand level for joints (kVp)
Screen to ground DC withstand As per IEC
level for joints (kV)
Screen interruption DC withstand As per IEC
level for joints (kV)
Screen to ground AC withstand As per IEC
level for joints (kV rms)
Screen interruption AC withstand As per IEC
level for joints (kV)
Impulse withstand level for As per IEC
outersheath (kVp)
DC withstand level for outersheath As per IEC
(kV)
AC withstand level for outersheath TBA
(kV)
Impulse withstand level for As per IEC
bonding cables (kVp)
DC withstand level for bonding As per IEC
cables (kV)
AC withstand level for bonding TBA
cables (kV)
DC withstand level between metal TBA
screen cable and metal enclosed
GIS (kVp)
AC withstand level between metal TBA
screen cable and metal enclosed
GIS (kVp)
Impulse withstand level for post TBA
insulator (kVp)
DC withstand level for post TBA
insulator (kV)
AC withstand level for post TBA
insulator (kV)
Sheath Voltage Limiters (SVLs)
Type of resistor (Silicon Carbide, Zinc oxide
Zinc Oxide, others)
Typical rated voltage (Ur) installed 3kV, 6kV and 10kV

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Type of connection in sectionalized Star connection with neutral grounded

joints (delta connection, star
connection with neutral grounded,
star connection without neutral
grounded)
Nominal discharge current (kA) 10kA
(wave 8/20µs)
Line discharge according to TBA
60099-4
Are SVLs installed inside link Yes
boxes?
BONDING LEAD CABLES
Type of bonding cable: where Both
single-core or concentric bonding
leads are used?
Maximum length criteria TBA
Type of insulation (XLPE, PVC, XLPE
PE)
Outersheath with semi-conductive Both, but mostly graphite
layer or graphite layer
Watertight TBA
LINK BOXES
Location where link boxes are Above ground level and underground in public areas and
installed substations
Are the link boxes accessible? Yes
Waterproof test TBA
Internal arc test No
CALCULATION CRITERIA
Method used for calculating induce CIGRE, EPRI
voltage sheath (i.e. CIGRE
Documents, EMTP/ATP, etc.)
Sheath voltage limits during 65V
normal operation
SVL selection criteria during fault maximum 3 phase and 1phase 50 Hz calculated induced
conditions voltage
SVL selection criteria during TBA
transient overvoltage conditions
(lightning and switching)
Are you considering internal cable No
fault conditions into selection
criteria of SVLs?
AFTER INSTALLATION TEST (BEFORE COMMISSIONING)
Outersheath voltage tests 10kV/1min
Bonding system connections zero sequence, visual and contact resistance
MAINTENANCE TEST
Outersheath voltage tests (off line) 5kV/1 min yearly
Bonding system connection (off yearly or 3 years
line)
Current by metal screens (on line) yearly or 3 years
SVL tests (off line) 3 years

C.11. Response 11
ITEMS
BONDING SCHEMATICS
Solid bonding Substations, short connections, hardly applied for HV cables,
rather for MV cables

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Single-point bonding Most HV-cable systems have metal sheaths/screens single-

side bonded (with SVL at the open end).
The HV-underground cable lines are usually short (in the
average 3 km route length).
In special cases a “Midpoint earthing” is installed.
In most cases cables are laid trefoil formation and in ducts. An
earth continuity conductor (ECC) is laid parallel to circuit.
Sectionalized cross-bonding The metal sheath/screens of cables are sequentially cross-
bonded only.
The Cables (Phases) are not sequentially transposed!
Minor sections of a CB-System may consist of several lengths
of cables connected by normal joints. In this way Minor
section may become up to max. 3 km and therefore total
length of one Major section may become up to 9 km. Equal
length of the minor sections in a cross bonded system is
normally achieved.
Continuous cross-bonding Not applied
Hybrid bonding In special situations a combination of single point-bonding and
cross-bonding is used to limit the number of major sections
and have more flexibility with the installation of link boxes.
Single-point bonding is in general installed on one side or both
sides of the link and not in between.
Direct cross-bonding without link Not applied
box
WITHSTAND VOLTAGE LEVEL OF BONDING COMPONENTS
Screen to ground impulse Values as per IEC 62067 Table G.1 and IEC 60840 Table
withstand level for joints (kVp) G.1 Bonding leads ≤3m
• 30 kVp for 110 kV, 150 kV, 220 kV and 400 kV rated system
voltage
Screen interruption impulse Values as per IEC 62067 Table G.1 and IEC 60840 Table G.1
withstand level for joints (kVp) Bonding leads ≤3m
• 60 kVp for 110 kV, 150 kV, 220 kV and 400 kV rated system
voltage
Screen to ground DC withstand Values as per IEC 62067 annex G.1 and IEC 60840 G.4.2
level for joints (kV)
• 25 kV 1 min
Screen interruption DC withstand Values as per IEC 62067 annex G.1 and IEC 60840 G.4.2
level for joints (kV) • 25 kV 1 min
Screen to ground AC withstand • 10 kV 1 min for all rated system voltages
level for joints (kV rms)
Screen interruption AC withstand • 10 kV 1 min for all rated system voltages
level for joints (kV)
Impulse withstand level for Not tested
outersheath (kVp)
DC withstand level for outersheath 10 kV DC 1 min as per IEC 60229
(kV)
AC withstand level for outersheath Not tested
(kV)
Impulse withstand level for Same levels as for the accessories and the bonding devices
bonding cables (kVp) Values as per IEC 62067 Table G.1 and IEC 60840 Table G.1
Bonding leads ≤3m
DC withstand level for bonding New installations: 10 kV DC 1 min // Aged installations: 5kV
cables (kV) DC 1 min, leakage current may be measured
Single-core as well as coaxial bonding cables are designed to
withstand 20 kV AC (Type Test)

AC withstand level for bonding Not tested

cables (kV) Single-core as well as coaxial bonding cables are designed to
withstand 20 kV AC (Type Test)

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

DC withstand level between metal

screen cable and metal enclosed
GIS (kVp)
AC withstand level between metal
screen cable and metal enclosed
GIS (kVp)
Impulse withstand level for post
insulator (kVp)
DC withstand level for post
insulator (kV)
AC withstand level for post
insulator (kV)
Sheath Voltage Limiters (SVLs)
Type of resistor (Silicon Carbide, ZnO
Zinc Oxide, others)
Typical rated voltage (Ur) installed 3,5kV 6.0 kV (depending on specified short circuit current and
length of minor section)
Type of connection in Star connection with neutral grounded
sectionalized joints (delta
connection, star connection with
neutral grounded, star connection
without neutral grounded)
Nominal discharge current (kA) 10 kA
(wave 8/20µs)
Line discharge according to YES
60099-4 SVLs are selected to withstand repeated discharges based on
EMPT calculations
Are SVLs installed inside link Yes (Very few exceptions where SVL installed between
boxes? Baseplate of Termination and Steel structure)
BONDING LEAD CABLES
Type of bonding cable: where Coaxial-cables for cross-bonding between CB-joints and CB-
single-core or concentric bonding link boxes.
leads are used? Cable sizes: 95/95mm2 Cu, 120/120mm2 Cu, 150/150 mm2
Cu or 240/240mm2 Cu
Single-core cables for single-point bonding (earthing of metal
sheath and/or earthing of SVL).
Single-core cables for direct earthing at terminations, earthing
joints and between SVL and earth.

Maximum length criteria Bonding leads shall be as short as possible. In general, ≤ 3m.
In exceptions where bonding leads are ≥ 3m is not possible,
special considerations of situation and calculations are made.

Type of insulation (XLPE, PVC, XLPE (high flexibility of bonding lead required) GKN - Type for
PE) Coaxial and GN-Type for Single-core)
Outer-sheath with semi-conductive In general: NO semi-conductive layer
layer or graphite layer If specified by operator (e.g. in Tunnel applications)
manufacturers are ready to extrude a SC- or FR- and/or
Halogen free layer on top of outer- sheath.
Watertight Bonding cables: NO
Water tightness achieved by design of contact at the joint and
link box
LINK BOXES
Location where link boxes are Link boxes are installed inside the joint-bay, or in a manhole
installed adjacent to the joint-bay or in a cabinet above joint bay and
above ground.
Are the link boxes accessible? YES

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Waterproof test The link boxes are installed above ground or above floor level
to assure that box will not be submersed by water ---> IP 54.
For Export only: When link boxes installed below ground level:
--> Waterproof test is required an ingress protection rating
IP68 according to IEC60529.
In addition, a transparent two component Compound is used
for filling of the compartment of the box where SVL s are
accommodated and also area of Cable-Glands.
Internal arc test Internal arc test is not required. Link boxes in our country are
designed not to leak flames/fire.
CALCULATION CRITERIA
Method used for calculating The following methods and rules are used:
induced voltage sheath (i.e. (a) Electromagnetic Transient Calculations by special
CIGRE Documents, EMTP/ATP, Software EMTP / EMTPRV / ATP
etc.) (b) Guidelines, standards and rules issued by our country
associations and committees of: Electrotechnical Engineers,
Power producers, and Network operators
(c) Technical Brochures and Guidance’s of Cigré and IEEE,
Electra 128 (Cigre WG 21.07, 1990) and ANSI/IEEE Std. 575-
1988
Sheath voltage limits during In normal operation the induced standing voltage on metal
normal operation sheath and support structures shall not exceed 400V. Person
Contact protection is required.
(Without person contact protection the max. Voltage is 50
Volts)
SVL selection criteria during fault Rated Voltage Ur (kV): 3.7, 5.0, 6.2, 7.5, 8.7
conditions Continuous Voltage Uc (kV): 3, 4, 5, 6, 7
The TSO specifies the maximum sheath fault currents in
kA/time in seconds. Selection of the SVL and their supply /
installation is duty and guaranteed by the cable manufacturer,
who calculates the specific situation.
Residual Voltage (Ures) kV 3.1, 2.5, 3.7, 10, etc. HV - Cable
Manufacturer specifies the max. Permissible Voltage to
assure Protection of the cable outer sheath.

SVL selection criteria during Rated Voltage Ur (kV): 3.7, 5.0, 6.2, 7.5, 8.7
transient overvoltage conditions Residual Voltage Ures (kV): 10.0, 13.3, 16.7, 20.0, 23.3
(lightning and switching) (At Impulse 10kA Waveform 8/20μs)
The size of the SVL shall limit the residual voltage [Ures]
which shall not damage the protection sheath of the HV-cable.
The most harmful transient must be considered for size
design.
(Remark: Decision regarding installation of fault counter is
with TSO)
Are you considering internal cable Internal cable faults are in terms of overvoltages considered
fault conditions into selection as not to be the most critical.
criteria of SVLs?
AFTER INSTALLATION TEST (BEFORE COMMISSIONING)
Outersheath voltage tests 10kV / 1 min as specified in IEC 60229
Bonding system connections Bonding system connection tests before commissioning are
not common, usually visual inspection
MAINTENANCE TEST
Outersheath voltage tests (off line) IEC 60229 or reduced voltage e.g. 10kV / 1 min or 5kV / 1min
decision by TSO
Check leakage current.
Bonding system connection (off Visual Inspection annually.
line) Sheath Voltage test including test of bonding leads.
Visual Check of fault counter, if installed

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Current by metal screens (on line) Occasionally / During DC voltage test

SVL tests (off line)

C.12. Response 12
ITEMS
BONDING SCHEMATICS
Solid bonding It's typically used on lower voltage systems.
Single-point bonding It's only used in cases where the installation length is less
than or equal to 3 drum lengths and it can be demonstrated
that cross bonding would be uneconomic and the losses from
solid bonding would be too high.
Two distinct types of single point bonding are used – namely
that the single point bond may be either at the circuit, or in the
midpoint.
Sectionalized cross-bonding It's used.
Continuous cross-bonding It's not used.
Hybrid bonding Cross-bonding systems may be installed with single point
bonded sections at one or both ends of
the circuit. This is typically in cases where the drum lengths
required to install the circuit do not align well with
a fully cross bonded system.
Direct cross-bonding without link It's not used.
box
WITHSTAND VOLTAGE LEVEL OF BONDING COMPONENTS
Screen to ground impulse Different values than IEC 62067 annex G and IEC 60840
withstand level for joints (kVp) annex G standard:
• 17,5 kVp for 33 kV and 66 kV rated system voltage
• 22,5 kVp for 132 kV rated system voltage
• 37,5 kVp for 275 kV rated system voltage
• 55 kVp for 400 kV rated system voltage
Screen interruption impulse Different values than IEC 62067 annex G and IEC 60840
withstand level for joints (kVp) annex G standard.:
• 35 kVp for 33 kV and 66 kV rated system voltage
• 45 kVp for 132 kV rated system voltage
• 75 kVp for 275 kV rated system voltage
• 110 kVp for 400 kV rated system voltage
Screen to ground DC withstand 25 kV according to IEC 62067 annex G and IEC 60840 annex
level for joints (kV) G standard
Screen interruption DC withstand 25 kV according to IEC 62067 annex G and IEC 60840 annex
level for joints (kV) G standard
Screen to ground AC withstand
level for joints (kV rms)
Screen interruption AC withstand
level for joints (kV)
Impulse withstand level for
outersheath (kVp)
DC withstand level for outersheath
(kV)
AC withstand level for outersheath
(kV)
Impulse withstand level for • 75 kVp between phases and 37.5 kVp phase to earth for
bonding cables (kVp) 132 kV and bellow rated system voltage.
• 95 kVp between phases and 47.5 kVp phase to earth for
275 kV rated system voltage.
• 125 kVp between phases and 62.5 kVp phase to earth
for 400 kV rated system voltage

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

DC withstand level for bonding • Insulation of single-core bonding cables: 25 kV during 1

cables (kV) min
• Outer insulation of concentric bonding cables: 25 kV
during 1 min

AC withstand level for bonding • Inner insulation of concentric bonding cables for 275 kV
cables (kV) and bellow rated system voltage: 15 kV during 5 min
• Inner insulation of concentric bonding cables for above
275 kV rated system voltage: 20 kV during 5 min
DC withstand level between metal
screen cable and metal enclosed
GIS (kVp)
AC withstand level between metal
screen cable and metal enclosed
GIS (kVp)
Impulse withstand level for post
insulator (kVp)
DC withstand level for post
insulator (kV)
AC withstand level for post
insulator (kV)
Sheath Voltage Limiters (SVLs)
Type of resistor (Silicon Carbide,
Zinc Oxide, others)
Typical rated voltage (Ur) installed
Type of connection in sectionalized
joints (delta connection, star
connection with neutral grounded,
star connection without neutral
grounded)
Nominal discharge current (kA)
(wave 8/20µs)
Line discharge according to
60099-4
Are SVLs installed inside link
boxes?
BONDING LEAD CABLES
Type of bonding cable: where Bonding cables are typically concentric.
single-core or concentric bonding
leads are used?
Maximum length criteria Bonding leads are kept as short as possible to minimize
impedance.
Type of insulation (XLPE, PVC,
PE)
Outersheath with semi-conductive
layer or graphite layer
Watertight
LINK BOXES
Location where link boxes are
installed
Are the link boxes accessible?
Waterproof test
Internal arc test
CALCULATION CRITERIA
Method used for calculating induce
voltage sheath (i.e. CIGRE
Documents, EMTP/ATP, etc.)

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Sheath voltage limits during A range of different standing voltage limits are applied.
normal operation For example, C55/4 specifies, for maximum load current:
• 65V for systems with rated voltage at 132kV or below
• 150V for systems with rated voltage of 275kV and 400kV
(although this does not align with what is done at these
voltage).
The utility does not impose a formal limit on standing voltage,
but designs must have suitable engineering justification.
SVL selection criteria during fault
conditions
SVL selection criteria during
transient overvoltage conditions
(lightning and switching)
Are you considering internal cable
fault conditions into selection
criteria of SVLs?
AFTER INSTALLATION TEST (BEFORE COMMISSIONING)
Outersheath voltage tests
Bonding system connections
MAINTENANCE TEST
Outersheath voltage tests (off line)
Bonding system connection (off
line)
Current by metal screens (on line)
SVL tests (off line)

C.13. Response 13
ITEMS
BONDING SCHEMATICS
Solid bonding This type of bonding is used on distribution cables but not at
transmission voltage levels except on submarine cables
where no other alternatives exist.
Single-point bonding This configuration for shield/sheath bonding is used for short
circuits (substation get-away) and by some utilities as
"sectionalized single point bonding". Single-point bonding
may also be used when otherwise cross-bonded systems do
not have the correct number of sections (divisible by 3).
Sectionalized cross-bonding This is very common, although some utilities have been
forced to use sectionalized single point bonding when they
have not considered minor section lengths.
Continuous cross-bonding This configuration is rarely or never used.
Hybrid bonding For "hybrid", we assume this to mean a mix of 2 or more types
of bonding. This is common for longer circuits where there
may be a number of sections that cannot evenly be divided
into major cross-bonding sections, or where there is an
unusual length of cable, say from a long HDD section.
Direct cross-bonding without link This is uncommon. I am not aware of any applications.
box
WITHSTAND VOLTAGE LEVEL OF BONDING COMPONENTS
Screen to ground impulse IEC 62067 and IEC 60840 as well as Association of Edison
withstand level for joints (kVp) Illuminating Companies (AEIC) requirements in CS9.
Screen interruption impulse IEC 62067 and IEC 60840 as well as Association of Edison
withstand level for joints (kVp) Illuminating Companies (AEIC) requirements in CS9.
Screen to ground DC withstand Maximum of 25kVDC, although 10-15kVDC may be used for
level for joints (kV) field tests on reels before cable installation or after laying
tests.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Screen interruption DC withstand Maximum of 25kVDC, although 10-15kVDC may be used for
level for joints (kV) field tests on reels before cable installation or after laying
tests.
Screen to ground AC withstand IEC 62067 and IEC 60840 as well as Association of Edison
level for joints (kV rms) Illuminating Companies (AEIC) requirements in CS9.
Screen interruption AC withstand IEC 62067 and IEC 60840 as well as Association of Edison
level for joints (kV) Illuminating Companies (AEIC) requirements in CS9.
Impulse withstand level for In accordance with AEIC CS9
outersheath (kVp)
DC withstand level for outersheath In accordance with AEIC CS9
(kV)
AC withstand level for outersheath In accordance with AEIC CS9
(kV)
Impulse withstand level for In accordance with AEIC CS9
bonding cables (kVp)
DC withstand level for bonding In accordance with AEIC CS9
cables (kV)
AC withstand level for bonding In accordance with AEIC CS9
cables (kV)
DC withstand level between metal In accordance with AEIC CS9 and IEEE 48
screen cable and metal enclosed
GIS (kVp)
AC withstand level between metal In accordance with AEIC CS9 and IEEE 48
screen cable and metal enclosed
GIS (kVp)
Impulse withstand level for post
insulator (kVp)
DC withstand level for post
insulator (kV)
AC withstand level for post
insulator (kV)
Sheath Voltage Limiters (SVLs)
Type of resistor (Silicon Carbide, Not specified but generally zinc oxide
Zinc Oxide, others)
Typical rated voltage (Ur) installed ~3.5kV to 12.5kV
Type of connection in Star (each phase individually grounded in grounding link
sectionalized joints (delta boxes)
connection, star connection with
neutral grounded, star connection
without neutral grounded)
Nominal discharge current (kA) 10kA
(wave 8/20µs)
Line discharge according to Usually based on 60099 since many link boxes with SVLs are
60099-4 supplied by overseas manufacturers.
Are SVLs installed inside link Yes
boxes?
BONDING LEAD CABLES
Type of bonding cable: where Both single core and concentric bonding cables are used.
single-core or concentric bonding
leads are used?
Maximum length criteria Generally, less than 10m
Type of insulation (XLPE, PVC, PE, PVC
PE)
Outersheath with semi-conductive Bonding cables generally do not have an outer semi-
layer or graphite layer conductive layer or graphite; transmission cables will have
these features.
Watertight There is no metal radial moisture barrier on most bonding
cables. Longitudinal water blocking is sometimes included if
specified.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

LINK BOXES
Location where link boxes are Generally, link boxes for splices are located within the
installed manholes where the power cable joints are located. A few
utilities bring the link boxes outside of manholes. Link boxes
for conventional termination structures are located on the
structures. Link boxes for riser poles will be located on the
man davit arm or pole.
Are the link boxes accessible? The link boxes are usually located in manholes (for splices).
Waterproof test NEMA or other enclosure standard may be used.
Internal arc test Not generally required.
CALCULATION CRITERIA
Method used for calculating induce
voltage sheath (i.e. CIGRE
Documents, EMTP/ATP, etc.)
Sheath voltage limits during Utilities tend to select values from 50V to 240V, with 120V
normal operation being common since this is AC common "exposed" voltages
with normal wall outlets. Higher voltages may be allowed
since bonding cables are rated for 600V.
SVL selection criteria during fault Specified based on duration of fault and fault current limit.
conditions
SVL selection criteria during The maximum expected voltage during normal operation is
transient overvoltage conditions used. Manufacturers are frequency used to evaluate options.
(lightning and switching)
Are you considering internal cable Generally, not by most utilities.
fault conditions into selection
criteria of SVLs?
AFTER INSTALLATION TEST (BEFORE COMMISSIONING)
Outersheath voltage tests 10-25kVDC.
Bonding system connections Sometimes, more often on systems of 230kV or greater.
MAINTENANCE TEST
Outersheath voltage tests (off line) Some utilities may perform once every 3-5 years, but not that
common.
Bonding system connection (off Mechanical connections are checked, but bonding cables are
line) generally not checked or tested unless a known problem is
being investigated.
Current by metal screens (on line) Generally, the access points to check current in bonding leads
are not accessible as they are within manholes. Safety
practices for most transmission cable using utilities is not to
enter energized splice vaults / manholes where bonding
cables would be accessible to measure currents. Concentric
bonding cables cannot be tested with clamp-on ammeters.
SVL tests (off line) Cost to troubleshoot or evaluate SVLs is not justified. If a
visual inspection indicates a possible problem, the SVL(s) will
just be replaced.

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

APPENDIX D. Review of service experience – Survey

details (2)
Eight (8) responses to the questionnaire shown in Table 4.1 were received. Detailed responses are
included below with country names of the responses removed.

D.1. Response 1
Maintenance of cable bonding systems Your company:
This spreadsheet is intended for collecting data from different cable supplier and operators all over
the world. The questionnaire is related only to maintenance of cable bonding systems. If some of
the fields in the spreadsheet seems superfluous please leave it blank or copy the answer from one
cell into another.
The answers will be part of a Cigré Technical Brochure delivered at a later stage by Working Group
B1.50. Answers will be anonymized if requested.
Solid bonding Single point Cross bonding Other bonding
bonding schemes?
Bonding methods
Please give a brief
description of how
the different
bonding methods
are made
Scheduled maintenance on bonding systems
What kind of Once per year
scheduled the following is
maintenance is performed:
performed? Online Visual
- What tasks are inspection:
performed and 1.Link boxes and
with what interval? their covers
- What equipment 2.SVL's
is being 3.Link boxes:
maintained? bars and
(e.g. link boxes, connections
earthing boxes, 4.Grounding
bonding leads, cables
SVLs, supporting 5. Terminal Base
insulators, connector
grounding points, Thermo-vision
earth continuity inspection
conductors, etc.) Measurement:
- What tests are Link boxes bars
performed and currents
with what interval? Off-Line
- Are there Verification,
different cleaning and
maintenance retighten:
requirements seen 1. Link boxes and
from different their covers
manufacturers? 2.SVL's
3.Link boxes:
bars and
connections
4. Grounding
cables
5. Terminal Base
connector

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

SVL's Electrical
Test and leak
testing
Measurement:
1. Link boxes
bars currents
2. SVL's Nitrogen
pressure
Unscheduled maintenance (after cable fault)
What kind of
maintenance is
performed after a
cable fault?
- What tasks are
performed?
- What equipment
is being
maintained?
(e.g. link boxes,
earthing boxes,
bonding leads,
SVLs, supporting
insulators,
grounding points,
earth continuity
conductors, etc.)
- What tests are
performed?
- Are there
different
maintenance
requirements seen
from different
manufacturers?
Other comments:
If you have any other comments related to maintenance of cable bonding systems, please write it
here.

D.2. Response 2
Maintenance of cable bonding systems Your company:
This spreadsheet is intended for collecting data from different cable supplier and operators all over
the world. The questionnaire is related only to maintenance of cable bonding systems. If some of
the fields in the spreadsheet seems superfluous please leave it blank or copy the answer from one
cell into another.
The answers will be part of a Cigré Technical Brochure delivered at a later stage by Working Group
B1.50. Answers will be anonymized if requested.
Solid bonding Single point Cross bonding Other bonding
bonding schemes?
Bonding methods
Please give a brief We use solid We use single Done using We use sheath
description of how bounding by point bonding by single conductor arrestors and
the different jumpering the 350kcmil copper or coaxial cable also one circuit
bonding methods joint as well as or running a with link box with sheath
are made typing this to parallel wire current
ground monitoring
depending on system
route
Scheduled maintenance on bonding systems

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

What kind of Physical exam Physical exam Physical exam

scheduled
maintenance is
performed?
Here under:
- What tasks are
performed and with
what interval?
- What equipment ALL ALL ALL ALL
is being
maintained?
(e.g. link boxes,
earthing boxes,
bonding leads,
SVLs, supporting
insulators,
grounding points,
earth continuity
conductors)
- What tests are Jacket tests are Jacket tests are Jacket tests are Jacket tests are
performed and with done at 3kV DC done at 3kV DC done at 3kV DC done at 3kV DC
what interval? once every 3 once every 3 once every 3 once every 3
years years years years
- Are there No No No No
different
maintenance
requirements seen
from different
manufacturers?
Unscheduled maintenance (after cable fault)
What kind of We undertake
maintenance is sheath arrester
performed after a testing as well as
cable fault? jacket tests
Here under:
- What tasks are
performed?
- What equipment We check cable We check cable We check cable We check cable
is being condition by condition by condition by condition by
maintained? physical exam, physical exam, physical exam, physical exam,
(e.g. link boxes, exam link boxes, exam link boxes, exam link boxes, exam link boxes,
earthing boxes, clean and clean and clean and clean and
bonding leads, restore, jacket restore, jacket restore, jacket restore, jacket
SVLs, supporting tests on the tests on the tests on the tests on the
insulators, section, SVL section, SVL section, SVL section, SVL
grounding points, tests tests tests tests
earth continuity
conductors)
- What tests are DC tests on DC tests on DC tests on DC tests on
performed? jacket system jacket system jacket system jacket system
- Are there These done case These done These done case These done case
different by case case by case by case by case
maintenance
requirements seen
from different
manufacturers?
Other comments:
If you have any other comments related to maintenance of cable bonding systems, please write it
here.
We have experienced challenges at time where we have used coaxial cable originally to make all

103
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

the bonding connection that restoration time can increase if you do not stock the appropriate
quantity of such cable. The link boxes will not allow entry of more than 4 cables!

D.3. Response 3
Maintenance of cable bonding systems Your company:
This spreadsheet is intended for collecting data from different cable supplier and operators all over
the world. The questionnaire is related only to maintenance of cable bonding systems. If some of
the fields in the spreadsheet seems superfluous please leave it blank or copy the answer from one
cell into another.
The answers will be part of a Cigré Technical Brochure delivered at a later stage by Working Group
B1.50. Answers will be anonymized if requested.
Solid bonding Single point Cross bonding Other bonding
bonding schemes?
Bonding methods
Please give a brief Bonded through Mainly used Traditional cross N/A
description of how earthing box. Only where cable bonding has
the different grounded at system cannot mainly been
bonding methods substations be divided into used. Now
are made an equal direct cross
number of three bonding is used
minor sections. (first major
Grounding is section in each
done in the field end is traditional
to an ECC, cross bonding).
SVLs located at
substations.
Scheduled maintenance on bonding systems
What kind of Visual inspection of Visual Visual
scheduled grounding box and inspection of inspection of link
maintenance is connections. DC grounding box, boxes and
performed? voltage test of jacket. box with SVLs connections. DC
No standardized and voltage test of
interval connections. DC jacket at 5 kV.
voltage test of
jacket.
- What tasks are
performed and
with what interval?
- What equipment
is being
maintained?
(e.g. link boxes,
earthing boxes,
bonding leads,
SVLs, supporting
insulators,
grounding points,
earth continuity
conductors)

104
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

- What tests are

performed and
with what interval?
- Are there
different
maintenance
requirements seen
from different
manufacturers?
Unscheduled maintenance (after cable fault)
What kind of DC voltage test of DC voltage test DC voltage test
maintenance is jacket of jacket of jacket
performed after a Link boxes are
cable fault? opened,
including visual
inspection.
- What tasks are
performed?
- What equipment
is being
maintained?
(e.g. link boxes,
earthing boxes,
bonding leads,
SVLs, supporting
insulators,
grounding points,
earth continuity
conductors)
- What tests are
performed?
- Are there
different
maintenance
requirements seen
from different
manufacturers?
Other comments:
If you have any other comments related to maintenance of cable bonding systems, please write it
here.

A maintenance schedule is under preparation and will hopefully be issued in 2017-2018.

D.4. Response 4
Maintenance of cable bonding Your company:
systems
This spreadsheet is intended for collecting data from different cable supplier and operators all over
the world. The questionnaire is related only to maintenance of cable bonding systems. If some of
the fields in the spreadsheet seems superfluous please leave it blank or copy the answer from one
cell into another.
The answers will be part of a Cigré Technical Brochure delivered at a later stage by Working Group
B1.50. Answers will be anonymized if requested.
Solid bonding Single point Cross bonding Other bonding
bonding schemes?
Bonding methods
Please give a Only for links No ampacity limit Screen Sometimes, single
brief with ampacity interruptions are point bonding (at
description of below 600 A (to If length is too long, always one or two ends) is
how the limit induced an intermediate performed at mixed with cross
different current) grounding is joints bonding. It

105
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

bonding necessary sometimes

methods are For safety For new cables, decreases the
made reasons, if one For new cables, when needed, number of major
of the earthing SVL used at one or SVLs are ZnO sections, or may
cable at an both ends are ZnO 12 kV, always help moving a joint
ending point is 15 kV, always bay to a preferred
accidentally For every rated location.
removed, the ECC is always voltage, cross-
induced voltage made of copper, 1 bonding is now
must be kept kV-insulated performed
below maximal (U1000R02V) directly inside
values. the joint bay,
Therefore, one Cross-sections are except for the
or two normalized: first and the last
(maximum) - 63 kV / 90 kV: 120 major section
intermediate mm² (closest to cable
grounding points - 225 kV: 150 mm² ends), which
may be added - 400 kV: 185 or must still be
300 mm² protected with
SVLs

If voltage is <=
90 kV and if the
link connects two
substations (no
tower on the
line), every
cross-bonding is
performed
directly in the
bay, and SVLs
are no longer
needed

Bonding leads
are made of
copper, insulated
20 kV, and the
section must
withstand the
short circuit
current.

Inside joint bay:

unipolar (8 m
maximum)
When using a
dedicated pit and
SVLs: coaxial
(10 m maximum)
Scheduled maintenance on bonding systems
What kind of Every 5 years Every 5 years
scheduled (approx.), more or (approx.), more or
maintenance is less depending on less depending on
performed? the link importance. the link importance.
Link must be de- Link must be de-
energized prior to energized prior to
visiting, in order to visiting, in order to
access the pits access the pits
- What tasks Pits are opened Pits are opened
are performed Water is evacuated Water is evacuated

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

and with what (if needed) (if needed)

interval? Visual inspection is Visual inspection is
performed performed
SVLs are SVLs are
systematically systematically
removed (to be removed (to be
tested later) and tested later) and
replaced by tested replaced by tested
ones ones
- What Only SVLs are Only SVLs are
equipment is (automatically) (automatically)
being removed and sent removed and sent
maintained? to a dedicated lab to a dedicated lab
(e.g. link to be tested. to be tested.
boxes, earthing If they are still If they are still
boxes, bonding working, they will be working, they will be
leads, SVLs, re-installed on re-installed on
supporting another link. another link.
insulators,
grounding Otherwise, every Otherwise, every
points, earth defective defective
continuity equipment noticed equipment noticed
conductors, during visual during visual
etc.) inspection shall be inspection shall be
replaced. replaced.
- What tests The company The company
are performed doesn't perform any doesn't perform any
and with what test on bonding test on bonding
interval? systems (only systems (only
commissioning commissioning
tests and after tests and after
reparations) reparations)
A dedicated A dedicated
laboratory does all laboratory does all
the testing the testing
regarding SVLs. regarding SVLs.
- Are there No No
different
maintenance
requirements
seen from
different
manufacturers?
Unscheduled maintenance (after cable fault)
What kind of After every fault, the bonding system is always checked
maintenance is
performed after
a cable fault?
- What tasks Same as for schedule maintenance
are performed?
- What Same as for schedule maintenance
equipment is
being
maintained?
(e.g. link
boxes, earthing
boxes, bonding
leads, SVLs,
supporting
insulators,
grounding

107
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

points, earth
continuity
conductors,
etc.)
- What tests Same as for schedule maintenance
are performed?
- Are there No
different
maintenance
requirements
seen from
different
manufacturers?
Other comments:
If you have any other comments related to maintenance of cable bonding systems, please write it
here.

The company has decided not to perform any jacket test once the link is commissioned.
This test can nevertheless be done on demand, if necessary

D.5. Response 5
Maintenance of cable bonding systems Your
company:
This spreadsheet is intended for collecting data from different cable supplier and operators all over
the world. The questionnaire is related only to maintenance of cable bonding systems. If some of
the fields in the spreadsheet seems superfluous please leave it blank or copy the answer from one
cell into another.
The answers will be part of a Cigré Technical Brochure delivered at a later stage by Working Group
B1.50. Answers will be anonymized if requested.
Solid bonding Single point Cross bonding Other bonding
bonding schemes?
Bonding methods
Please give a brief Short cable length used as a Long length of Combination of XB
description of how specially for the closing length cables and single point
the different transformer cable. when the minor bonding.
bonding methods section of the
are made XB is not equal;
for transformer
connections
with high load
requirements
Scheduled maintenance on bonding systems
What kind of
scheduled
maintenance is
performed?
- What tasks are Check on XB, outer sheet test. Every 3 years
performed and
with what interval?
- What equipment link box, earth box, bonding lead, SVL, outdoor end terminations
is being
maintained?
(e.g. link boxes,
earthing boxes,
bonding leads,
SVLs, supporting
insulators,
grounding points,

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

earth continuity
conductors, etc.)

- What tests are metal sheath test: 5 kV/10 min; interval 3 years
performed and all others: more or less visual inspections; interval varies between 1-3 years
with what interval?
- Are there no, all manufactures offer the same scheme
different
maintenance
requirements seen
from different
manufacturers?
Unscheduled maintenance (after cable fault)
What kind of
maintenance is
performed after a
cable fault?
- What tasks are testing and visual inspections
performed?
- What equipment visual check on components
is being
maintained?
(e.g. link boxes,
earthing boxes,
bonding leads,
SVLs, supporting
insulators,
grounding points,
earth continuity
conductors, etc.)
- What tests are oversheath testing 5 kV/10 min/ voltage withstand test at lower test voltage
performed? than SAT
- Are there no not really
different
maintenance
requirements seen
from different
manufacturers?
Other comments:
If you have any other comments related to maintenance of cable bonding systems, please write it
here.

Online PD measurement for 380 kV cable systems.

D.6. Response 6
Maintenance of cable bonding systems Your company:
This spreadsheet is intended for collecting data from different cable supplier and operators all over
the world. The questionnaire is related only to maintenance of cable bonding systems. If some of
the fields in the spreadsheet seems superfluous please leave it blank or copy the answer from one
cell into another.
The answers will be part of a Cigré Technical Brochure delivered at a later stage by Working Group
B1.50. Answers will be anonymized if requested.
Solid bonding Single point Cross bonding Other bonding
bonding schemes?
Bonding methods

109
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

Please give a brief It's frequently It's used in Sectionalized A mix between
description of how used medium short links. cross-bonding is single-point and
the different voltage links (up the most common sectionalized
bonding methods to 45 kV). bonding schematic cross bonding is
are made In 66 kV links and used in long links. carried out in
above is used in The screens are some projects.
short links (i.e. usually only Single-point
transformer links crossing and not bonding can be
inside power transposing the performed on
station...) cables. either one or both
Continuous cross- sides of the link.
bonding is very
rarely used.
Scheduled maintenance on bonding systems
What kind of Maintenance of cable systems is performed as in the following:
scheduled 1. Cable path survey (link on):
maintenance is Visual inspection of cable route to detect dangerous activities to cable link
performed? (civil work close to cable path, attempted theft, etc.). Frequency of cable path
- What tasks are survey depends on type of cable route.
performed and with • Zone 1 (urban areas with several utility company infrastructure):
what interval? Every 2 days.
- What equipment • Zone 2 (urban areas with few utility company infrastructure): Every
is being week.
maintained? • Zone 3 (rural areas): Every month.
(e.g. link boxes, • Zone 4 (poorly accessible areas): Every 6 months.
earthing boxes, 2. Visual inspection (link on):
bonding leads, Visual check of all accessible components (terminations, joints, link boxes,
SVLs, supporting SVLs, metal support and cleats, earthing connection, bonding lead cables,
insulators, etc.) is done every year.
grounding points, 3. Thermography inspection (link on):
earth continuity Thermography inspection is done in outdoor terminations, sealing end
conductors, etc.) terminations, link boxes and joints (if they are accessible) every year.
- What tests are 4. Bonding system connection inspection (link off except metal sheath
performed and with current measurement):
what interval? The following tasks are done in all links every five years:
- Are there • Visual check of all connections (connections inside link boxes, earth
different connections,
maintenance • Outersheath DC test (5 kV DC during 1 minute)
requirements seen • Check SVL’s integrity (apply reference voltage and measure current).
from different • Check continuity of ECC cable.
manufacturers? • Earth resistance measurement in joint-bay and transition tower.
• Metal sheath current measurement in terminations with metal screen
connected to earth (line on).
5. Tan δ measurement (line off)
Links are older than 10 years Tan δ measurement will be done. Frequency
test depends on results.
6. Links with oil cables (OF cable or MI cables)
• Oil analysis:
Oil will be analyzed in link with oil cables (OF or MI cables) every five years.

• Check pressure oil and alarm

Pressure oil and alarms will be checked in link with oil-filled cables every
three years.
Unscheduled maintenance (after cable fault)
What kind of
maintenance is
performed after a
cable fault?
- What tasks are - Visual check of all accessible components (terminations, joints, link boxes,
performed? SVLs, metal support and cleats, earthing connection, bonding lead cables,
etc.…)

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 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

- What equipment - Whatever is accessible

is being
maintained?
(e.g. link boxes,
earthing boxes,
bonding leads,
SVLs, supporting
insulators,
grounding points,
earth continuity
conductors, etc.)
- What tests are - Outersheath DC test (5 kV DC during 1 minute).
performed?
- Are there - The company establishes the same maintenance requirements regardless
different of cable manufacturer.
maintenance
requirements seen
from different
manufacturers?
Other comments:
If you have any other comments related to maintenance of cable bonding systems, please write it
here.

D.7. Response 7
Maintenance of cable Your company:
bonding systems
This spreadsheet is intended for collecting data from different cable supplier and operators all over
the world. The questionnaire is related only to maintenance of cable bonding systems. If some of
the fields in the spreadsheet seems superfluous please leave it blank or copy the answer from one
cell into another.
The answers will be part of a Cigré Technical Brochure delivered at a later stage by Working Group
B1.50. Answers will be anonymized if requested.
Solid Single point Cross bonding Other bonding
bonding bonding schemes?
Bonding methods
Please give a Not used Earthing of metal Earthing of metal cable In case of cable
brief description for HV cable sheath by sheath by bonding route cannot be
of how the Cable bonding leads and leads and configured as one
different Systems disconnectable disconnectable Links or multiple CB-
bonding ≥60kV Links at one side of at both sides of HV- major sections, a
methods are HV-Cable-Line. Cable-Line. By SBP configuration is
made Sheaths are Insulating joints, the used as first and/or
protected by SVLs screen continuity is last section of line.
at the non-earthed interrupted. Coaxial Consequently (SB)
side. bonding leads and and (SPB) are
In SPB cross link boxes are combined.
arrangements an used to cyclic change
ECC must be used. screen connection of
Frequently used is each phase. Phases
Midpoint bonding as are mainly in Trefoil
a special formation. When Flat
arrangement of formation no
SPB. Transposition of
cables. Protection of
Sheaths of HV- screen of each phase
Cable links from against overvoltages
OHL to GIS: SVLs -->3 SVL's
are on OHL-side. accommodated in
Except one TSO Cross link box. Box
installed always above

111
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

ground or sidewall of
manhole/joint bay

Scheduled maintenance on bonding systems

What kind of
scheduled
maintenance is
performed?
- What tasks NA Annual visual Annual visual Annual visual
are performed inspection inspection inspection
and with what
interval?
- What Not used -Disconnectable -Disconnectable links -Disconnectable
equipment is for HV links -Earthing boxes links
being Cable -bonding leads -bonding leads -Earthing boxes
maintained? Systems -SVLs -SVLs -bonding leads
(e.g. link boxes, -Supporting -Supporting insulators -SVLs
earthing boxes, insulators -Grounding points -Supporting
bonding leads, -Grounding points insulators
SVLs, -Plexiglas Cover at -Grounding points
supporting GIS (Touch -Earth continuity
insulators, protection) conductors
grounding -Earth continuity
points, earth conductors
continuity
conductors,
etc.)
- What tests NA (a) Visual inspection (a) Visual inspection (a) Visual inspection
are performed (annually) (annually) (annually)
and with what (b) DC voltage test (b) DC voltage test of (b) DC voltage test
interval? of screen and SVL screen and SVL of screen and SVL
1 min at continuous 1 min at continuous 1 min at continuous
voltage (typical 4,8 voltage (typical 4,8 kV) voltage (typical 4,8
kV) (every 5 to 10 (every 5 ... 10 Years) kV) (every 5 to 10
Years) Years)
- Are there NA Based on Based on Based on
different recommendations recommendations of recommendations
maintenance of cable cable Manufacturer the of cable
requirements Manufacturer the Engineers of TSO are Manufacturer the
seen from Engineers of TSO free to decide on Engineers of TSO
different are free to decide maintenance are free to decide
manufacturers? on maintenance requirements. on maintenance
requirements. requirements.
Recently established
Recently company is working on Recently
established setting up rules and to established
company is working unify maintenance company is working
on setting up rules procedures for all on setting up rules
and to unify TSO's Networks and to unify
maintenance ≥60kV maintenance
procedures for all procedures for all
TSO's Networks TSO's Networks
≥60kV ≥60kV
Unscheduled maintenance (after cable fault)
What kind of
maintenance is
performed after
a cable fault?
- What tasks Visual Check / Visual Check / Visual Check /
NA
are performed? cleaning / cleaning / cleaning /

112
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

check of functioning check of functioning of check of functioning

of SVL SVL of SVL
- What
equipment is
being
maintained?
(e.g. link boxes,
earthing boxes,
bonding leads, Check of the Check of the
Check of the complete
SVLs, NA complete bonding complete bonding
bonding System e.g.
supporting System e.g. all System e.g. all
all Components.
insulators, Components. Components.
grounding
points, earth
continuity
conductors,
etc.)
- What tests After Repair of After Repair of
After Repair of cable:
are performed? cable: sheath test cable: sheath test
sheath test acc. IEC
acc. IEC 60229 acc. IEC 60229
60229 including
including bonding including bonding
bonding leads.
leads. leads.
NA Link boxes and
Link boxes and Link boxes and
disconnections "open"
disconnections disconnections
"open" 10 kV / 1 "open" 10 kV / 1
10 kV / 1 min
min (previously: (previously: 25kV/ 1 min)
min (previously:
25kV/ 1 min) 25kV/ 1 min)
- Are there
different
maintenance
requirements No No No No
seen from
different
manufacturers?
Other comments:
If you have any other comments related to maintenance of cable bonding systems, please write it
here.
Note:
CB- boxes/ SVLs are mounted at wall of joint pits or manholes or at structure of terminations in
order to be easy accessible

D.8. Response 8
Maintenance of cable bonding systems Your company:
This spreadsheet is intended for collecting data from different cable supplier and operators all over
the world. The questionnaire is related only to maintenance of cable bonding systems. If some of
the fields in the spreadsheet seems superfluous please leave it blank or copy the answer from one
cell into another.
The answers will be part of a Cigré Technical Brochure delivered at a later stage by Working Group
B1.50. Answers will be anonymized if requested.
Solid bonding Single point Cross bonding Other bonding
bonding schemes?
Bonding methods
Please give a brief All three phases N/A All phases are N/A
description of how are solid bonded crossed and
the different to earth. SVLs are fitted to
bonding methods limit metal sheath
are made voltages during a
fault.
Scheduled maintenance on bonding systems

113
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

What kind of Yearly, 3 years. N/A Yearly, 3 years. N/A

scheduled Depending on Depending on
maintenance is local local
performed? circumstances. circumstances
and length of
major sections.
- What tasks are SVL measurement N/A SVL N/A
performed and with and all joint measurement and
what interval? resistance all joint resistance
readings, bonding readings, bonding
lead and lead and
supporting supporting
insulator insulator
inspections all at inspections all at
three yearly three yearly
intervals. intervals.
- What equipment Link boxes, SVLs, N/A Link boxes, SVLs, N/A
is being Link Pillars, Link Pillars,
maintained? Bonding Leads, Bonding Leads,
(e.g. link boxes, Supporting Supporting
earthing boxes, Insulators and all Insulators and all
bonding leads, Connections. Connections.
SVLs, supporting
insulators,
grounding points,
earth continuity
conductors, etc.)
- What tests are SVL measurement N/A SVL N/A
performed and with and joint measurement and
what interval? resistance joint resistance
readings at three readings at three
yearly intervals. yearly intervals.
- Are there No N/A No N/A
different
maintenance
requirements seen
from different
manufacturers?
Unscheduled maintenance (after cable fault)
What kind of Serving test and N/A Serving test and N/A
maintenance is oil pressure oil pressure
performed after a system. Nothing system. Nothing
cable fault? on the earthing on the earthing
system system
- What tasks are As above N/A As above N/A
performed?
- What equipment N/A N/A N/A N/A
is being
maintained?
(e.g. link boxes,
earthing boxes,
bonding leads,
SVLs, supporting
insulators,
grounding points,
earth continuity
conductors, etc.)
- What tests are N/A N/A N/A N/A
performed?
- Are there N/A N/A N/A N/A
different

114
 TB 797 - Sheath bonding systems of AC transmission cables - Design, testing, and maintenance

maintenance
requirements seen
from different
manufacturers?
Other comments:
If you have any other comments related to maintenance of cable bonding systems, please write it
here.

Extended major sections will involve yearly testing this can be caused by link box reduction
schemes.

115
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