I n t e r n a t i o n a l T e l e c o m m u n i c a t i o n U n i o n

ITU-T K.85
TELECOMMUNICATION (11/2011)
STANDARDIZATION SECTOR
OF ITU

SERIES K: PROTECTION AGAINST INTERFERENCE

Requirements for the mitigation of lightning

effects on home networks installed in customer
premises

Recommendation ITU-T K.85

Recommendation ITU-T K.85

Requirements for the mitigation of lightning effects on home networks

installed in customer premises

Summary
Recommendation ITU-T K.85 gives the requirements for home network equipment and installations
in customer premises. It covers the classification of interfaces, resistibility of equipment, impact of
installation practices, installation of surge protective devices (SPDs) and a risk assessment of
equipment damage according to IEC 62305-2. The risk assessment of user injury will be covered by
a separate Recommendation.

History
Edition Recommendation Approval Study Group
1.0 ITU-T K.85 2011-11-13 5

Rec. ITU-T K.85 (11/2011) i

 FOREWORD
The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of
telecommunications, information and communication technologies (ICTs). The ITU Telecommunication
Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical,
operating and tariff questions and issuing Recommendations on them with a view to standardizing
telecommunications on a worldwide basis.
The World Telecommunication Standardization Assembly (WTSA), which meets every four years,
establishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on
these topics.
The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1.
In some areas of information technology which fall within ITU-T's purview, the necessary standards are
prepared on a collaborative basis with ISO and IEC.

NOTE
In this Recommendation, the expression "Administration" is used for conciseness to indicate both a
telecommunication administration and a recognized operating agency.
Compliance with this Recommendation is voluntary. However, the Recommendation may contain certain
mandatory provisions (to ensure, e.g., interoperability or applicability) and compliance with the
Recommendation is achieved when all of these mandatory provisions are met. The words "shall" or some
other obligatory language such as "must" and the negative equivalents are used to express requirements. The
use of such words does not suggest that compliance with the Recommendation is required of any party.

INTELLECTUAL PROPERTY RIGHTS

ITU draws attention to the possibility that the practice or implementation of this Recommendation may
involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence,
validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others
outside of the Recommendation development process.
As of the date of approval of this Recommendation, ITU had not received notice of intellectual property,
protected by patents, which may be required to implement this Recommendation. However, implementers
are cautioned that this may not represent the latest information and are therefore strongly urged to consult the
TSB patent database at http://www.itu.int/ITU-T/ipr/.

 ITU 2012
All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the
prior written permission of ITU.

ii Rec. ITU-T K.85 (11/2011)

 Table of Contents
Page
1 Scope ............................................................................................................................ 1
2 References..................................................................................................................... 1
3 Definitions .................................................................................................................... 2
3.1 Terms defined elsewhere ................................................................................ 2
3.2 Terms defined in this Recommendation ......................................................... 3
4 Abbreviations and acronyms ........................................................................................ 3
5 Lightning damage ......................................................................................................... 3
6 Requirements ................................................................................................................ 4
6.1 Classification of ports ..................................................................................... 4
6.2 Resistibility ..................................................................................................... 5
6.3 Safety .............................................................................................................. 6
6.4 Earthing .......................................................................................................... 6
6.5 Risk assessment .............................................................................................. 6
6.6 Installation of overvoltage protection ............................................................. 7
6.7 Wiring installation .......................................................................................... 8
6.8 Special requirements ...................................................................................... 8
6.9 Use of wireless and optical fibre .................................................................... 8
Annex A – Principles of surge protection ................................................................................ 9
A.1 Surge coupling into external telecommunication lines .................................. 9
A.2 Surge coupling into internal cabling............................................................... 9
A.3 Impact of actual installations .......................................................................... 9
A.4 Separate earth electrodes ................................................................................ 10
A.5 Philosophy of protection................................................................................. 10
Annex B – Installation practices which may result in damage ................................................ 11
Annex C – Coordination .......................................................................................................... 12
C.1 Telecommunication SPD coordination........................................................... 12
C.2 Mains SPD coordination................................................................................. 13
C.3 Internal port coordination ............................................................................... 14
Annex D – Risk assessment ..................................................................................................... 15
D.1 Explanation of terms....................................................................................... 15
D.2 Risk management ........................................................................................... 18
Appendix I – Risk assessment example ................................................................................... 21
I.1 General ........................................................................................................... 21
I.2 Building characteristics .................................................................................. 22
I.3 Characteristics of the services ........................................................................ 22
I.4 Characteristics of the internal systems ........................................................... 23
I.5 Zones definition in the structure ..................................................................... 25

Rec. ITU-T K.85 (11/2011) iii

 Page
I.6 Expected dangerous events to the structure.................................................... 25
I.7 Risk assessment for the unprotected structure................................................ 25
I.8 Selected protection measures ......................................................................... 27
I.9 Risk assessment related to the protected structure ......................................... 27
I.10 SPDs ............................................................................................................... 28
Appendix II – Assessment of protection needs ........................................................................ 29
II.1 Single port equipment..................................................................................... 29
II.2 Earthed equipment .......................................................................................... 29
II.3 Non-earthed equipment .................................................................................. 29
II.4 Level of protection provided by an MSPD..................................................... 29
II.5 Consideration of loop areas ............................................................................ 30
Bibliography............................................................................................................................. 32

iv Rec. ITU-T K.85 (11/2011)

Introduction
This Recommendation provides all requirements to minimize disruption and damage to home
networks. To ensure a reliable home network service it is essential that a number of interrelated
requirements are met. These include the following.
All equipment complies with the appropriate resistibility level of [ITU-T K.21].
• The equipment ports have been correctly classified in accordance with [ITU-T K.75].
• Overvoltage protection, when required, has been installed according to [ITU-T K.66]. This
includes the use of both multiservice surge protective devices (MSPDs) and primary (point
of entry) protection. It also includes ensuring correct earthing and bonding.
• A risk assessment of equipment damage is performed in accordance with [IEC 62305-2].

Rec. ITU-T K.85 (11/2011) v

Recommendation ITU-T K.85

Requirements for the mitigation of lightning effects on home networks

installed in customer premises

1 Scope
This Recommendation applies to home networks installed in customer premises. Service to the
home network can be provided by optical fibre, wireless, hybrid fibre/coaxial (HFC) cable or x
digital subscriber line (xDSL). This Recommendation predicts the frequency of damage to the home
network (telecommunications) equipment within the structure.
NOTE – Consideration of loss of the telecommunication service is contained in [ITU-T K.72]. Consideration
of injury of the service user, within the structure, will be considered in a future ITU-T Recommendation.

2 References
The following ITU-T Recommendations and other references contain provisions which, through
reference in this text, constitute provisions of this Recommendation. At the time of publication, the
editions indicated were valid. All Recommendations and other references are subject to revision;
users of this Recommendation are therefore encouraged to investigate the possibility of applying the
most recent edition of the Recommendations and other references listed below. A list of the
currently valid ITU-T Recommendations is regularly published. The reference to a document within
this Recommendation does not give it, as a stand-alone document, the status of a Recommendation.
[ITU-T K.12] Recommendation ITU-T K.12 (2010), Characteristics of gas discharge tubes
for the protection of telecommunications installations.
[ITU-T K.21] Recommendation ITU-T K.21 (2008), Resistibility of telecommunication
equipment installed in customer premises to overvoltages and overcurrents.
[ITU-T K.44] Recommendation ITU-T K.44 (2008), Resistibility tests for telecommunication
equipment exposed to overvoltages and overcurrents – Basic Recommendation.
[ITU-T K.65] Recommendation ITU-T K.65 (2004), Overvoltage and overcurrent
requirements for termination modules with contacts for test ports or SPDs.
[ITU-T K.66] Recommendation ITU-T K.66 (2004), Protection of customer premises from
overvoltages.
[ITU-T K.67] Recommendation ITU-T K.67 (2006), Expected surges on telecommunications
and signalling networks due to lightning.
[ITU-T K.71] Recommendation ITU-T K.71 (2011), Protection of customer antenna
installations.
[ITU-T K.72] Recommendation ITU-T K.72 (2011), Protection of telecommunication lines
using metallic conductors against lightning – Risk management.
[ITU-T K.75] Recommendation ITU-T K.75 (2008), Classification of interface for
application of standards on resistibility and safety of telecommunication
equipment.
[IEC 60364-4-44] IEC 60364-4-44 ed. 2 (2007), Low-voltage electrical installations – Part 4-44:
Protection for safety – Protection against voltage disturbances and
electromagnetic disturbances.
<http://webstore.iec.ch/webstore/webstore.nsf/artnum/038219>

Rec. ITU-T K.85 (11/2011) 1

[IEC 60950-1] IEC 60950-1 ed. 2 (2005), Information technology equipment – Safety –
Part 1: General requirements.
<http://webstore.iec.ch/webstore/webstore.nsf/artnum/035320>

[IEC 61643-21] IEC 61643-21 ed. 1.1 Consol. with am1 (2009), Low-voltage surge protective
devices – Part 21: Surge protective devices connected to telecommunications
and signalling networks – Performance requirements and testing methods.
<http://webstore.iec.ch/webstore/webstore.nsf/ArtNum_Popup/42865!opendocument>

[IEC 61643-22] IEC 61643-22 ed. 1 (2004), Low-voltage surge protective devices – Part 22:
Surge protective devices connected to telecommunications and signalling
networks – Selection and application principles.
<http://webstore.iec.ch/webstore/webstore.nsf/artnum/033373>

[IEC 62305-2] IEC 62305-2 ed. 2 (2010), Protection against lightning – Part 2: Risk
management.
<http://webstore.iec.ch>

[IEC 62368-1] IEC 62368-1 ed. 1 (2010), Audio/video, information and communication
technology equipment – Part 1: Safety requirements.
<http://webstore.iec.ch/webstore/webstore.nsf/ArtNum_Popup/45465!opendocument>

3 Definitions

3.1 Terms defined elsewhere

This Recommendation uses the following terms defined elsewhere:
3.1.1 home network [b-ISO/IEC 15044]: Internal network for digital and analogue information
transport in home or business premises of similar complexity, providing defined access points and
using any medium in any topology.
3.1.2 level [b-ETSI ES 201 468]: Measures on a relative scale how important it is that the
equipment operates as specified.
NOTE – Two levels are defined for the purpose of the present document and are designated as level 1 and
level 2.
Level 1 should be selected if the equipment has moderate failure consequences. Equipment has
moderate failure consequences when:
– a failure causes limited inconvenience;
– repairs may be made without compromising the responsibilities of the network operator.
Level 2 should be selected if the equipment has severe failure consequences. Equipment has severe
failure consequences when:
– failure compromises the function of vital, centralized systems, or services of a
commercially sensitive or security related nature;
– repair or restoration costs are high, or the time the equipment is out of service is
unacceptably long;
– corruption of charging or billing information occurs.

2 Rec. ITU-T K.85 (11/2011)

3.2 Terms defined in this Recommendation
This Recommendation defines the following terms:
3.2.1 multiservice surge protective device (MSPD): A surge protective device (SPD)
containing both telecommunications and mains protection. It may also include port protection for
video or Ethernet.
3.2.2 tracer/locate wire (optical fibre cable): Conductive wire in an optical fibre cable that
allows a buried cable to be located with a metal detector.

4 Abbreviations and acronyms

This Recommendation uses the following abbreviations and acronyms:
CLP Cell Loss Priority
CPE Customer Premises Equipment
GDT Gas Discharge Tube
HFC Hybrid Fibre/Coaxial (cable)
LEMP Lightning ElectroMagnetic Pulse
LPS Lightning Protection System
LT/NT Line Termination/Network Termination
MET Main Earth Terminal
MOV Metal-Oxide Varistor
MSB Main electrical Switchboard
MSPD Multiservice Surge Protective Device
NTD Network Termination Device
PE Power Earth
POTS Plain Old Telephone Service
SELV Safety Extra-Low Voltage
SPC Surge Protective Components
SPD Surge Protective Device
USB Universal Serial Bus
xDSL Any of the various types of Digital Subscriber Lines

5 Lightning damage
Damage to equipment can occur as a result of:
• surges entering on metallic services, with respect to the local earth;
• lightning strikes to antennas;
• lightning current entering the ground at or near the building with respect to remote earth;
• induction into indoor cabling due to nearby lightning.

Rec. ITU-T K.85 (11/2011) 3

6 Requirements
To limit the risk of misunderstanding by the supplier/manufacturer, it is recommended that the
equipment specification for overvoltage protection and safety includes a schematic diagram
showing the likely or possible applications of the equipment. This diagram should specifically
address connections of the equipment within the building and especially its interconnection to any
lines, or other equipment type connected to lines, that are external to the building. This should
include any antenna connections.
Some conditions might come with a small increase in item cost but could significantly increase
application and reduce life cycle cost.

6.1 Classification of ports

[ITU-T K.75] provides information on classifying plain old telephone service (POTS), Ethernet and
video ports. A mains port is always an external port.
Correct classification of ports is important. If an internal port is subject to conducted lightning
surges or a.c. surges, there will be equipment damage and human safety issues.
Please note that when specifying the above overvoltage and safety requirements it is also very
important to clearly advise the vendors where the equipment is to be used and what is expected to
be connected to it (for example, whether it will or will not involve any connections to cabling that
exits the building). Manufacturers of customer equipment in particular assume that the equipment
ports will only be connected to intra-building cabling. A problem then occurs if the network
operator, or the customer, decides to extend the service to an outbuilding. Equipment designed for
connection to intra-building cabling may have insufficient isolation from the point of view of safety
and insufficient resistibility to overvoltages, when connected to inter-building cables. To ensure the
equipment use is not limited, the best way to proceed may be to assume that the port will connect to
an inter-building cable, unless it is clear that it will not. The types of ports that could be connected
to inter-building cabling include:
• POTS/xDSL;
• Ethernet;
• coaxial cable ports.
For those ports that may connect to inter-building cables, classify the ports as follows:
• For [IEC 60950-1] (safety) testing, use TNV-3 for ports with a working voltage higher than
the safety extra-low voltage (SELV); or TNV-1 for ports with a working voltage lower than
or equal to SELV (see [IEC 60950-1] for definitions).
• For [IEC 62368-1] (safety) testing the applicable value of the transient voltage on an
external circuit shall be determined using Table 15 of [IEC 62368-1]. Where more than one
location or condition is applicable, the highest transient voltage applies.
• For resistibility testing, classify it as external.
NOTE – Where approved engineering practices have been implemented, a lower port classification may be
used. An example involves the use of a non-removable galvanic isolator with the equivalent of double
insulation between the coaxial cable and the coaxial cable port.

4 Rec. ITU-T K.85 (11/2011)

6.2 Resistibility
[ITU-T K.21] has two levels of resistibility: "basic" and "enhanced". [ITU-T K.44] gives advice on
the selection of "basic" and "enhanced".
The required level of resistibility can be determined in Figure 1. Information on "Special
requirements" is contained in [ITU-T K.44]. Non-compliance with [ITU-T K.66] could include:
• non-bonded earths;
• difficulty in installing SPDs at the building entry;
• bonding wires in excess of 10 m.

Identify the equipment and the

number and type of ports

Determine the power

induction limits

Earthing and bonding No

to [ITU-T K.66]?

Yes Primary protection No

Yes
installed to [ITU-T K.66]
when required?

Does the equipment Yes

have more than one external
port type?

No

Are the power

Yes
induction limits >
600 V?

No
''Basic'' ''Enhanced'' ''Special''
K.85(11)_F01

Figure 1 – Flowchart for determining required level of resistibility

The resistibility requirements of [ITU-T K.21] in conjunction with [ITU-T K.44] ensure that the
equipment can be protected by the installation of an MSPD or primary protector.
It is expected that protection for ports connected to external cables will only be installed if required
and therefore the "inherent" tests in Table 2 of [ITU-T K.21] are required.
It is necessary to determine the special test protector to be used when checking compliance with
[ITU-T K.21]. Information on selecting the agreed primary protector, and hence the special test
protector, is given in Annex C of Recommendation [ITU-T K.12]. If the product can be used within
the customer premises there is a likelihood that the customer may connect it via an MSPD or may
request primary protection to be installed. This sets the worst-case scenario for determining the
"agreed primary protector" as per [ITU-T K.44].

Rec. ITU-T K.85 (11/2011) 5

6.3 Safety
All equipment shall comply with the latest version of [IEC 60950-1] or [IEC 62368-1]. It should be
noted that [IEC 62368-1] is the latest safety standard and that it will eventually replace
[IEC 60950-1]. National deviations of these standards exist; country specific requirements need to
be complied with in these countries.
Safety can be achieved by a combination of insulation and earthing, see [ITU-T K.75].
It is preferable to use reinforced or double insulation, or suitable isolation, between people and an
external telecommunications interface instead of basic insulation plus a hard-wired earth. This
eliminates the need for a hard-wired earth (see clause 6.4).

6.4 Earthing
Where port insulation/isolation is inadequate, the equipment will need to be earthed. Earthing may
also be required for functional (e.g., signalling or testing) purposes.
There are four methods of earthing and these are:
1) no earthing;
2) earthing via the mains plug (i.e., connection to protective earth);
3) hard-wire earthing (i.e., direct bonding to protective earth);
4) connection to a stand-alone earthing system/electrode.
Methods 1 and 2 have both advantages and disadvantages and the choice may depend on national
requirements of a.c. wiring rules. Method 2 must only be used where equipment earthing is not
required if the equipment is unplugged, i.e., if unplugging the equipment removes the safety hazard.
When purchasing or specifying customer premises equipment, preference should be given to
method 1 and, where viable, method 2, due to the high cost of the implementation of method 3.
Method 4 is most likely not suitable for customer premises equipment due to the consequences of a
lack of bonding to protective earth. The cost of installing a hard-wired earth should be part of any
business case and intention to buy equipment.
The method of earthing will impact the overvoltage resistibility requirements and safety
requirements of equipment. It is usually cheaper to provide the safety isolation within the
equipment than to rely on earthing, unless the earthing/bonding is required for other purposes.
It is worthwhile making it clear to the manufacturer/vendor whether you will accept equipment
safety requiring a hard-wired connection to protective earth (requiring installation labour and/or
perhaps limiting locations where it can be installed in the building) or whether your preference is
for safety provided by isolation/insulation only.

6.5 Risk assessment

A risk assessment of equipment damage should be performed according to [IEC 62305-2]. Input
data to perform a risk assessment include:
• strike density
• building dimensions
• external cable types and lengths
• building type
• internal wiring attributes
• equipment resistibility level
• power system type
• existence of SPDs.

6 Rec. ITU-T K.85 (11/2011)

Regardless of the risk assessment result, if the building has a lightning protection system (LPS)
installed, primary protection, bonded to the protective earth (PE), should be installed at the point of
entry for all services; and all metallic pipes should be bonded to the PE at the point of entry.
The information required to perform a risk assessment according to [IEC 62305-2] is given in
Annex D.
A risk assessment example is given in Appendix I.
6.5.1 Internal cable interfaces
Perform a risk assessment according to Annex D. There are three scenarios.
1) Ethernet interfaces complying with [ITU-T K.21]: Use the relevant requirement,
e.g., "basic" requirement from Table 2 of [ITU-T K.21], as the withstand voltage.
2) Ethernet interfaces not complying with [ITU-T K.21]: The withstand voltage is not known.
Check with the manufacturer. In situations where the value cannot be determined,
[IEC 62305-2] suggests that PMS be set to 1.
3) The equipment at one end on the cable has a level of insulation, i.e., no earth and no power
connection. In this case the risk can be considered quite low.
If the risk of damage to the internal cable interface, for unshielded cable situations, is above
acceptable levels install an MSPD as described in clause 6.6.3. If the risk of loss for a shielded
cable or coaxial cable situation is considered too high, install an MSPD as described in clause 6.6.3
or use a cable with the necessary shield resistance.
6.5.2 External cable interfaces
Perform a risk assessment according to Annex D. If the risk of damage to the external cable
interface, due to flashes near the service line, is above acceptable levels, install an MSPD. If the risk
of loss due to flashes to the service line is above acceptable levels, also install SPDs at the point of
entry.

6.6 Installation of overvoltage protection

When required by the risk assessment, MSPDs and primary protection should be installed according
to [ITU-T K.66]. The MSPD protects the equipment and the primary protection protects the MSPD.
The worst-case overvoltages and overcurrents are provided in [ITU-T K.67]. These voltages and
currents can be used to dimension SPDs and conductors, etc.
6.6.1 Primary protection
The SPD requirements and installation requirements may be controlled by the network operator or
regulator. Country requirements must be observed. To prevent high current surges entering the
structure due to lightning strikes to services, it is important to install the primary protection at the
point where the services enter the building.
6.6.2 MSPDs
MSPDs with a mains connection may need to comply, depending on national interpretation, with
either [IEC 60950-1] or [IEC 62368-1]. Installation should be in accordance with [ITU-T K.66].
Incorrect use of MSPDs may actually increase the probability of damage.
NOTE – The type of MSPD referred to in this Recommendation relies on a protective earth connection for
safety. If used without a protective earth users may be exposed to surges exceeding a few hundred volts.

Rec. ITU-T K.85 (11/2011) 7

6.6.3 Internal port protection
When protection of internal ports is required there are two possibilities.
1) Ports with a coordination requirement may be protected by the use of an appropriate SPD or
MSPD.
2) Ports without a coordination requirement must be protected in accordance with
[IEC 61643-21] and [IEC 61643-22].

6.7 Wiring installation

Every attempt should be made to install wiring, earthing and bonding according to [ITU-T K.66]
and [IEC 60364-4-44]. Where this cannot be achieved and the risk assessment indicates a need for
protection, see clause 6.8.
Equipotentialization for a timber floored building, when required, can be achieved by adding a ring
earth around the building (see [ITU-T K.66]).

6.8 Special requirements

Where the earthing and bonding requirements of [ITU-T K.66] and [IEC 60364-4-44] cannot be
achieved, an engineering solution may be needed. Possible engineering solutions are:
• special resistibility requirements
• isolation devices, e.g., high voltage a.c. isolation mains transformers.

6.9 Use of wireless and optical fibre

Operators can provide a wireless service to provide communication within the home network. This
is a method of protection. An optical fibre solution could also be used.
Operators can also provide a wireless backup solution to maintain availability in case of loss of the
optical fibre network.

8 Rec. ITU-T K.85 (11/2011)

 Annex A

Principles of surge protection

(This annex forms an integral part of this Recommendation.)

To be able to provide effective protection, it is necessary to understand how surges are coupled onto
a circuit and their possible impact on the equipment.

A.1 Surge coupling into external telecommunication lines

Consider a lightning strike coupling a surge into an external telecommunication line (see
Figure A.1, surges S3 and S4). This coupling can be inductive or conductive. Generally speaking, it
is expected that inductive coupling will not result in currents greater than 10 to 35 A or voltages
greater than a few kV (see Table 5 of [ITU-T K.67]). For conductive coupling, the total cable
current/voltage is unlikely to exceed 1000 A or 100 kV, unless the strike point to the cable is within
approximately 100 m of the building. If there is no primary protection installed, the equipment will
be subjected to the lightning surge entering the building. If primary protection is installed at the line
termination/network termination (LT/NT), the equipment may be subjected to one or more of the
following surges:
1) the let-through voltage of the primary protection (surge does not operate the primary
protector);
2) a chopped waveform caused by the primary protector operating;
3) the voltage caused by the primary protector current conducted in the primary protector bond
wire.

A.2 Surge coupling into internal cabling

The magnitude of the voltage induced into internal cabling depends on many factors such as
lightning strike current, closeness of the strike, size of the loop, type of cable and building shielding
(surges S1 and S2 in Figure A.1). [ITU-T K.67] provides information on likely maximum voltages.
Due to the complexity of the required calculation, a risk assessment should be performed in
accordance with [IEC 62305-2] . The current rating of SPDs and bonding conductors should be
determined using [ITU-T K.67]. Table 2 of [ITU-T K.67] shows that induced short circuit currents
of up to 6 kA 10/350 μs are possible within internal cabling due to lightning flashes to the building.
This is likely to be more than the capability of MSPDs. If a very high reliability is required a
specialized protection design may be necessary.

A.3 Impact of actual installations

It is likely that the primary protector may be some distance from the equipment and that the primary
protector installation will not be perfect. The issues are:
• Inductive drop across the SPD connecting leads: It is not expected that this will be a
problem with network termination devices (NTDs) complying with [ITU-T K.65].
[ITU-T K.21] makes allowance for a total of 1 metre of connecting lead for mains SPDs
during coordination testing.
• The current conducted in the bonding conductor between the SPD and the main earth
terminal (MET): The maximum recommended length of bonding conductor is specified in
[ITU-T K.66]. The length of this bonding conductor affects the current sharing between the
primary protector and the equipment SPD, or an SPD or MSPD installed at the equipment.
If the equipment has high input impedance, the peak voltage at the equipment is
proportional to the length of the bonding conductor.

Rec. ITU-T K.85 (11/2011) 9

• The voltage induced in the cabling between the protection frame and the equipment: The
magnitude of this voltage will depend on the magnitude of the current, the closeness of the
lightning strike and whether the cable is shielded or unshielded.
• Surge reflection at equipment port: Some IEC standards consider voltage doubling at the
equipment due to reflections a major consideration. Due to the relatively short distance
between the primary protection and the equipment, any voltage doubling for high
impedance equipment will only occur for a short time (< 1 µs) and is unlikely to cause
insulation breakdown. Any current doubling into low impedance equipment will also only
occur for a short time and will be easily handled by the equipment SPD.

A.4 Separate earth electrodes

Figure A.1 shows two separated earth electrodes. Unless these are bonded together, a potential
difference may occur between the electrodes and damage the ports of equipment A and
equipment B. A ring earth can be a solution for equipotentialization (see [ITU-T K.66]).

A.5 Philosophy of protection

The philosophy for protection of customer premises equipment (CPE) is as follows:
• Install an MSPD to protect the equipment when necessary. An MSPD can protect the
equipment against surges magnetically coupled into the building wiring and service cables
and lines. It may also provide some level of protection of the equipment against direct
strikes to the service plant, provided the strike point is more than a few hundred metres
from the building.
• Install primary protection at the building entry point on both the telecommunication and
power services, to protect the MSPD when the risk assessment requires it.

S1
S3
Power line S2

MSB
S4
Loop a Loop b

Primary
MET protector
bond wire
S3 Auxiliary
LT/NT earth
Telecommunication line Equip.A Equip.B electrode

PE electrode MSB – Main electrical switchboard

MET – Main earth terminal
LT/NT – Network termination device
PE – Power earth K.85(11)_FA.1

Figure A.1 – Surge coupling

10 Rec. ITU-T K.85 (11/2011)

 Annex B

Installation practices which may result in damage

(This annex forms an integral part of this Recommendation.)

The following installation practices should not be used as they can damage equipment:
a) Internal port connected to an external cable. The solutions are:
i. connect the external cable to an external interface;
ii. install an SPD according to [IEC 61643-21] and [IEC 61643-22];
iii. use an isolation box to provide the necessary isolation.
b) Internal port connected to an external cable no longer in use. The solutions are:
• disconnect and earth the external cable. Rather than earthing the copper conductors,
they may be insulated to a level higher than the cable insulation.
c) Internal port connects to cable connected to associated equipment within the building but
the associated equipment is bonded to a separate earth system. To prevent damage, all earth
references must be bonded to the same point, usually the PE. Possible solutions are:
i. bond all earths to the PE;
ii. where it is necessary to have two or more separated earth points, the interconnection
interfaces must be classified as "external" and primary protection installed, when
necessary (or install a primary protector SPD according to [IEC 61643-21] and
[IEC 61643-22]).
d) The interconnection of an external interface of equipment A to an internal interface of
associated equipment (equipment B), where the external interface of equipment A has
inadequate isolation to a second external interface which is exposed to conducted
(10/700 µs) surges. In this case the internal interface of the associated equipment
(equipment B) is also exposed to 10/700 µs surges. The solutions are:
i. only connect internal interfaces to internal interfaces and external interfaces to external
interfaces;
ii. use an isolation box to provide the necessary isolation.
e) Internal port is connected to internal cable which has a gas discharge tube (GDT), bonded
to a separate earth connected to it. The solution is:
• remove the GDT.

Rec. ITU-T K.85 (11/2011) 11

 Annex C

Coordination
(This annex forms an integral part of this Recommendation.)

Coordination may be required between the following SPDs:

• a telecommunications primary protector, normally a GDT, and an MSPD;
• an SPD installed in the MSB and an MSPD;
• an MSPD and an internal port.

C.1 Telecommunication SPD coordination

Figure C.1 shows a primary protector GDT with a 10 m bond wire and an MSPD connected to earth
via 10 m of power earth wire. The simulation shows that the operation of the MSPD GDT does not
affect the voltage at the customer lightning protection (CLP) GDT. The CLP GDT will always
operate. In this simulation the current in the MSPD GDT is less than 1/3 of the total current.

12 Rec. ITU-T K.85 (11/2011)

 R10
10 × 0.043 Vgdt2
+ L1 –

Vgdt1 R1 10 × 1u Part3 –
+
R7 10 × 0.00287 L2
2 + – 10 × 1u
I1 L3
Part2 R2 10 × 1u
+ –
10 × 0.00688

K.85(11)_FC.1a

Mains conductor 2.5 mm (6.88 mohm/m)

CLP bond conductor 6 mm² (2.87 mohm/m)
Two conductors of 0.5 mm telecom conductor (43 mohm/m)

COORD.CIR Temperature = 27 Case = 1

4.00K
Total current
3.00K

CLP GDT current

2.00K
CLP GDT voltage
1.00K
MSPD GDT current

0.00K
MSPD GDT voltage

–1.00K
0u 10u 20u 30u 40u 50u
Left Right Delta slope
I(I1) 0.000K 0.574K 0.574K 1.149e+007
V(VGDT1) 0.000K –0.212K –0.212K –4.244e+006
I(R1) 0.000K 0.719K 0.719K 1.439e+007
V(VGDT2) 0.000K –0.099K –0.099K –1.982e+006
I(R2) 0.000K –0.039K –0.039K –7.775e+005
T 0.000u 50.000u 50.000u 1.000e+000
K.85(11)_FC.1b

Figure C.1 – Simulation of a 10 m bond wire primary protector GDT with an MSPD
connected to earth via 10 m of power earth wire

C.2 Mains SPD coordination

In Figure C.2 an EPCOS B80K275 metal-oxide varistor (MOV) has been used as the primary
protector and S20K275 MOVs in the MSPD. The N-E MOV in the MSPD is not shown as it does
not conduct current in this type of installation. The current ratio between primary to secondary
MOV is 6:1. This current ratio is determined by the lower bulk resistance of the larger B80K MOV
and the impedance of the telecoms wiring and earth conductor and the 1 mA voltage of the MOVs.
NOTE – This simulation has been performed with identical MOVs. In practice the 1 mA voltage can vary
by +/– 10%. This can affect the current sharing.

Rec. ITU-T K.85 (11/2011) 13

 R13 R18
L12 10u
0 – 10 × 0.0069 V3

+
L13 10u R2
– X7 X4

+
S20K275 S20K275
10 × 0.0069
K3 V4
K2
COUPLING = 0.8
R1 parameters (L12, L13, L10)
0.0069
L14 + L10 10u
– R310 × 0.0069

+
1u –

R3 is set to 1E9 ohms to simulate hight impedance ports and to 0.1 ohm to simulate low
impedance ports.
X10 Mains conductor 2.5 mm (6.88 mohm/m).
R7 Generator parameters are 3 kA short circuit and 6 kV open circuit with a time to half
I1 2 B80K275
value of 350 μs.
K.85(11)_FC.2a

COORSPOW.CIR Temperature = 27 Case = 1

80.00K

63.00K

46.00K

29.00K

12.00K

–5.00K
0u 10.00u 20.00u 30.00u 40.00u 50.00u
Left Right Delta slope
I(R1) 67.310K 15.396K –51.914K –1.254e+009
I(R2) 10.060K 0.001K –10.058K –2.429e+008
I(R3) 10.060K 0.001K –10.059K –2.429e+008
T 8.586u 50.000u 41.414u 1.000e+000
K.85(11)_FC.2b

Figure C.2 – Simulation of an EPCOS B80K275 MOV primary

protector with S20K275 MOVs in the MSPD

C.3 Internal port coordination

This will need to be determined for each type of port. The IEC 61643 series provides information
on designing such an SPD.

14 Rec. ITU-T K.85 (11/2011)

 Annex D

Risk assessment
(This annex forms an integral part of this Recommendation.)

[IEC 62305-2] provides the tools to determine the frequency of damage of equipment. In
this Recommendation the risk of damage to telecommunication equipment within the structure is
considered. The loss of the telecommunications network is considered in [ITU-T K.72]. Injury of
the service user is currently under study in ITU-T.

D.1 Explanation of terms

D.1.1 Damage
D.1.1.1 Source of damage
The lightning current is the primary source of damage.
In general, the following sources are distinguished by the strike attachment point (Figure D.1):
S1: flashes to a structure;
S2: flashes near a structure;
S3: flashes to a service (which includes telecommunication lines);
S4: flashes near a service (which includes telecommunication lines).

Source of damage Striking point

S1

S3

S2

S4

K.85(11)_FD-1

Figure D.1 – Lightning as source of damage

D.1.1.2 Types of damage
A lightning flash may cause damage depending on the characteristics of the structure to be
protected. Some of the most important characteristics are: type of construction, contents and
application, type of service and protection measures provided.
In general, for practical applications of the risk assessment, it is useful to distinguish between three
basic types of damage which can appear as the consequence of lightning flashes. They are as
follows:

Rec. ITU-T K.85 (11/2011) 15

D1: injury to living beings by electric shock;
D2: physical damage;
D3: failure of electrical and electronic systems.
This Recommendation is only concerned with damage D3, and in particular the failure of
telecommunication equipment.
A lightning flash may cause equipment damage depending upon the characteristics of the
telecommunication network, power network and the structure. Some of the most important
characteristics are:
• type of telecommunication network, e.g., buried or aerial cable, screened or unscreened
cable, cable shield installed or not installed;
• type of power network, e.g., buried or aerial cable, screened or unscreened cable, cable
shield installed or not installed, neutral bonded or not bonded to power earth (PE);
• type of structure construction e.g., timber, brick or reinforced concrete;
• whether or not protection measures have been used at the structure or on the services.
D.1.1.3 Types of loss
Each type of damage, alone or in combination with others, may produce a different consequential
loss in the structure to be protected. The type of loss that may appear depends on the characteristics
of the structure itself and its contents. The following types of loss shall be considered:
L1: loss of human life (including permanent injury);
L2: loss of service to the public;
L3: loss of cultural heritage;
L4: loss of economic value (structure, content, and loss of activity).
This Recommendation is only concerned with loss L2, loss of service to the public, specifically the
risk of damage to telecommunication equipment installed within the structure.
D.1.2 Risk and risk components
D.1.2.1 Risk
The risk R is the value of a probable average annual loss.
To evaluate risk, R, the relevant risk components (partial risks depending on the source and type of
damage) shall be defined and calculated.
The risk, R, is the sum of the risk components.
The risk components, as defined in [IEC 62305-2], which can cause all types of loss, are listed in
clauses D.1.2.2. to D.1.2.5.
D.1.2.2 Risk components for a structure due to flashes to the structure
Direct lightning flashes to the structure to which the telecommunication network is connected can
cause the following risk component:
RA: Component related to injury to living beings caused by electric shock due to touch and step
voltages inside the structure and in the zones up to 3 m outside the structure. Loss of type L1 and, in
the case of structures holding livestock, loss of type L4 with possible loss of animals may also arise.
NOTE – In special structures, people may be endangered by direct strikes (e.g., top level of garage parking
or stadiums). These cases may also be considered using the principles of this Recommendation.
This Recommendation does not consider RA.

16 Rec. ITU-T K.85 (11/2011)

RB: Component related to physical damage caused by dangerous sparking inside the structure
triggering fire or explosion, which may also endanger the environment. All types of loss (L1, L2, L3
and L4) may arise.
RC: Component related to failure of internal systems caused by lightning electromagnetic pulse
(LEMP). Loss of type L2 and L4 could occur in all cases along with type L1 in the case of structures
with risk of explosion and hospitals or other structures where failure of internal systems
immediately endangers human life.
D.1.2.3 Risk component for a structure due to flashes near the structure
RM: Component related to failure of internal systems caused by LEMP. Loss of type L2 and L4
could occur in all cases, along with type L1 in the case of structures with risk of explosion and
hospitals or other structures where failure of internal systems immediately endangers human life.
D.1.2.4 Risk components for a structure due to flashes to a line connected to the structure
RU: Component related to injury to living beings caused by electric shock due touch voltage inside
the structure. Loss of type L1 and, in the case of agricultural properties, losses of type L4 with
possible loss of animals could also occur.
This Recommendation does not consider RU.
RV: Component related to physical damage due to lightning current transmitted through or along
incoming lines (e.g., fire or explosion triggered by dangerous sparking between external installation
and metallic parts generally at the entrance point of the line into the structure). All types of loss (L1,
L2, L3, L4) may occur.
RW: Component related to failure of internal systems caused by overvoltages induced on incoming
lines and transmitted to the structure. Loss of type L2 and L4 could occur in all cases, along with
type L1 in the case of structures with risk of explosion and hospitals or other structures where
failure of internal systems immediately endangers human life.
NOTE 1 – The lines considered in this assessment are only the lines entering the structure.
NOTE 2 – Lightning flashes to or near pipes are not considered as a source of damage based on the bonding
of pipes to an equipotential bonding bar. If an equipotential bonding bar is not provided, such a threat must
also be considered.
D.1.2.5 Risk component for a structure due to flashes near a line connected to the structure
RZ: Component related to failure of internal systems caused by overvoltages induced on incoming
lines and transmitted to the structure. Loss of type L2 and L4 could occur in all cases; along with
type L1 in the case of structures with risk of explosion and hospitals or other structures where
failure of internal systems immediately endanger human life.
NOTE 1 – The lines considered in this assessment are only the lines entering the structure.
NOTE 2 – Lightning flashes to or near pipes are not considered as a source of damage based on the bonding
of pipes to an equipotential bonding bar. If an equipotential bonding bar is not provided, such a threat must
also be considered.
D.1.3 Composition of risk components related to a structure
Risk components to be considered for each type of loss in a structure are listed below.
R1: Risk of loss of human life:
R1 = RA1 + RB1 + RC11) + RM11) + RU1 + RV1 + RW11) + RZ11) (1)
1)
Only for structures with risk of explosion and for hospitals with life-saving electrical equipment or
other structures when failure of internal systems immediately endangers human life.

Rec. ITU-T K.85 (11/2011) 17

R2: Risk of loss of service to the public:
R2 = RB2 + RC2 + RM2 + RV2 + RW2 + RZ2 (2)
R3: Risk of loss of cultural heritage:
R3 = RB3 + RV3 (3)
R4: Risk of loss of economic value:
R4 = RA42) + RB4 + RC4 + RM4 + RU42) + RV4 + RW4 + RZ4 (4)
2)
Only for properties where animals may be lost.
This Recommendation will only consider risk R2 and only that due to damage D3 (failure of
electrical and electronic systems). This Recommendation also focuses more on the "frequency of
damage" rather than on "loss".

D.2 Risk management

D.2.1 Basic procedure
The following procedure shall be applied to risk management:
a) identification of the structure and the relevant services and their characteristics;
b) identification of the risk components to be used (e.g., loss of service);
c) identification of sources of damage:
i. flashes to structure (S1)
ii. flashes near the structure (S2)
iii. flashes to a telecommunication service (S3)
iv. flashes to non-telecommunication services (S3)
v. flashes near a telecommunication service (S4)
vi. flashes near a non-telecommunication service (S4);
d) evaluation of risk R2;
e) evaluation of need of protection, by the comparison of the risk R2 with the tolerable
risk, RT. This Recommendation will compare the frequency of damage F2 with the tolerable
frequency of damage FT.
D.2.2 Tolerable risk RT
It is the responsibility of the authority having jurisdiction to identify the value of tolerable risk.
A representative value of tolerable risk, RT, against the loss of telecommunications service within
the structure due to lightning is given in Table D.1.

Table D.1 – Typical values of tolerable risk RT

Types of loss RT(y–1)
Loss of service to the public 10–3

In this Recommendation the frequency of damage will be calculated. A value of FT = 0.1 is

suggested.

18 Rec. ITU-T K.85 (11/2011)

D.2.3 Specific procedure to evaluate the need for protection
For risk R2 the following steps shall be taken:
– identification of the structure and the relevant services and their characteristics;
– identification of the tolerable risk RT;
– identification of the risk components to be used (e.g., loss of service);
– identification of sources of damage:
i. flashes to the structure (S1)
ii. flashes near the structure (S2)
iii. flashes to a telecommunication service (S3)
iv. flashes to non-telecommunication services (S3)
v. flashes near a telecommunication service (S4)
vi. flashes near a non-telecommunication service (S4);
– calculation of risk R2 using the relevant risk components;
– calculation of need of protection, by the comparison of the risk F2 with the tolerable
risk, FT.
Figure D.2 shows the flow chart to evaluate the protection needs and for selecting the protection
measures of telecommunication lines.

Rec. ITU-T K.85 (11/2011) 19

 Identify:
- the structure to be protected;
- the tolerable risk FT;
- the service delivery medium
e.g., copper, coaxial cable, optical
fibre or wireless;
- all external cables;
- all internal cables.

Yes Is it wireless?

No

Calculate the risk components

Fd = FC and Fw

Refer to [ITU-T K.71]

Yes Install protective measures

Fd ≥ FT?
against direct flashes to
external cables and/or to
structure a)
No

Calculate the risk component

Fi = FM and FZ

No Service within the

Fi > FT – Fd?
structure is protected

Yes

Can SPDs be installed No

to reduce FM and FZ
Calculate the new value of the
risk components FM + FZ
Yes
Install SPDs
according to clause 6.6

FC = ND × PC FM = NM × PM
RW = NL × PW RZ = NI × PZ
a)
SPDs must also be installed at the equipment to protect against FC K.85(11)_FD.2

Figure D.2 – Procedure for selecting protection measures in a structure

20 Rec. ITU-T K.85 (11/2011)

 Appendix I

Risk assessment example

(This appendix does not form an integral part of this Recommendation.)

I.1 General
The structure to be considered is the customer's building with a power line and an optical fibre
service. This is shown pictorially in Figure I.1.

S1
S3
Power line
S2

L – N wiring loop, 1a L/N – E wiring loop, 1b

Hard
MSB drive
S4
c/d – earth
wiring loop, 2b Printer
MET

c – d wiring loop, 2a
ONT HGW USB
Optic fibre Ethernet

PE electrode MSB – Main electrical switchboard The hard drive, HGW and printer are
MET – Main earth terminal powered from a common power-board
HGW – Home gateway K.85(11)_FI.1
ONT – Optical network terminal

Figure I.1 – Schematic of building wiring showing inductance loops

Damage to telecommunication equipment can occur and F2 should be evaluated.

Tables I.1 to I.9 follow the risk assessment example in [IEC 62305-2] and may not include all
mechanisms of damage. For example, depending on the method of terminating the optical fibre
tracer/locate wire, a strike to the wire may cause damage to equipment. Additional items such as
this may need to be considered.
The following clauses report the results of the risk assessment to the structure in accordance with
[[IEC 62305-2] and the possible protection measures in order to reduce the risks below the tolerable
values. An understanding of [IEC 62305-2] is required to understand this appendix.
The frequency of damage will be calculated considering the following port types:
• mains port (external);
• telecommunication ports (external);
• Ethernet ports (unshielded internal);
• USB ports (shielded internal).

Rec. ITU-T K.85 (11/2011) 21

I.2 Building characteristics
The main building characteristics are reported in Table I.1.

Table I.1 – Building characteristics

Reference
Parameter Comment Symbol Value
(Note)
Dimensions (m) (Lb · Wb · Hb) 20.0×20.0×20.0
Location factor Isolated Cdb 1.0 Table A.1
Probability PA PA 1.0 Equation B.1
Additional protection measures None PTA 1.0 Table B.1
Characteristics of structure None PB 1.0 Equation B.2
Shield at the structure boundary None KS1 1.0 Equation B.5
Shield internal to the structure None KS2 1.0 Equation B.6
2
Ground flash density 1/km /year Ng 3
NOTE – Table and equation references are to [IEC 62305-2].

I.3 Characteristics of the services

There is one metallic service entering the building: an aerial unshielded low voltage power line
(1000 m).
The characteristics of the power line entering the structure are reported in Table I.2 together with
the calculated values of the collection areas (AL and AI) and the expected dangerous events
(NL and NI).

22 Rec. ITU-T K.85 (11/2011)

 Table I.2 – Characteristics of the power line
Reference
Parameter Comment Symbol Value
(Note)
Soil resistivity (Wm) ρ 1000
Length (m) Lc 1000
Height (m) Hc 6
Line installation factor Isolated CI 1.0 Table A.2
Line type factor No CT 1.0 Table A.3
Line environmental factor Aerial CE 1.0 Table A.4
Shield resistance per unit length (Ω/km) Unshielded
Probability PC None PC 1.0 Equation B.3
Probability PSPD None PSPD 1.0 Table B.3
Number of conductors entering the structure m 2
2
Collection area for lightning to the line (m ) AL 40 000 Equation A.9
2
Collection area for lightning near the line (m ) AI 4 000 000 Equation A.11
Number of direct lightning to the line NL 0.12 Equation A.8
Number of dangerous lightning near the line NI 12.0 Equation A.10
Dimensions of the adjacent structure (m) None (La·Wa·Ha)
Number of direct lightning to the adjacent NDa 0.0
structure
NOTE – Table references are to [IEC 62305-2].

I.4 Characteristics of the internal systems

The main characteristics of the internal systems connected to the power line are reported in
Table I.3.

Table I.3 – Main characteristics of the internal installations

Reference
Parameter Comment Symbol Value
(Note 1)
Power service port
Shield resistance per unit length (Ω/km) Unshielded
Withstand voltage of the equipment 2.5 kV KS4 0.4 Equation B.7
Installed coordinated SPD protection Not installed PSPD 1 Table B.3
Probability factor due to direct lightning to CLD 1.0 Table B.4
the structure
Probability factor due to direct lightning CLI 1.0 Table B.4
near structure
Probability factor due to direct lightning to PLD 1.0 Table B.8
the line
Probability factor due to lightning near the PLI 0.3 Table B.9
line
Probability factor due to strike to structure PC 1.0 Equation B.2
Probability factor due to a strike to the line PW 1.0 Equation B.10

Rec. ITU-T K.85 (11/2011) 23

 Table I.3 – Main characteristics of the internal installations
Reference
Parameter Comment Symbol Value
(Note 1)
Probability factor due to a strike near to PZ 0.3 Equation B.11
the line
L–N
Characteristics of internal wiring 0.5 m² loop KS3 0.01 Table B.5
–5
Shielding factors PMS 1.6×10 Equation B.4
–5
Probability of failure PM 1.6×10 Equation B.3
L/N – E
Characteristics of internal wiring 0.5 m² loop KS3 0.01 Table B.5
–5
Shielding factors PMS 1.6×10 Equation B.4
Probability of failure PM 1.6×10–5 Equation B.3
Telecommunications service port n.a.
USB port
Shield resistance per unit length (Ω/km) Unshielded (Note 2)
Withstand voltage of the equipment 100 V KS4 10.0 Equation B.7
Installed coordinated SPD protection Not installed PSPD 1 Table B.3
Factor CLD CLD 1.0 Table B.4
c–d
Characteristics of internal wiring 0.5 m² KS3 0.01 Table B.5
Shielding factors PMS 0.01 Equation B.4
Probability of failure PM 0.01 Equation B.3
c/d – e
Characteristics of internal wiring 0.5 m² KS3 0.01 Table B.5
Shielding factors PMS 0.01 Equation B.4
Probability of failure PM 0.01 Equation B.3
Ethernet data port
Shield resistance per unit length (Ω/km) Unshielded
Withstand voltage of the equipment 1 kV KS4 1.0 Equation B.7
Installed coordinated SPD protection Not installed PSPD 1 Table B.3
Factor CLD CLD 1.0 Table B.4
e–f
Characteristics of internal wiring 0.5 m² KS3 0.01 Table B.5
Shielding factors PMS 0.0001 Equation B.4
Probability of failure PM 0.0001 Equation B.3

24 Rec. ITU-T K.85 (11/2011)

 Table I.3 – Main characteristics of the internal installations
Reference
Parameter Comment Symbol Value
(Note 1)
e/f – e
Characteristics of internal wiring 20 m² KS3 0.2 Table B.5
Shielding factors PMS 0.04 Equation B.4
Probability of failure PM 0.04 Equation B.3
NOTE 1 – Table and equation references are to [IEC 62305-2].
NOTE 2 – The rationale for the classification of the USB cable as unshielded is explained in Appendix II.

I.5 Zones definition in the structure

In the building, there is only one zone (Zone n.1) whose characteristics are reported in Table I.4.

Table I.4 – Characteristics of zone n.1

Reference
Parameter Comment Symbol Value
(Note)
Loss due to physical damage n.a. Lf 0.01 Table C.8
Loss due to failure of internal system n.a. Lo 0.001 Table C.8
Risk of fire Ordinary rf 0.01 Table C.5
Protection against fire No rp 1.0 Table C.4
NOTE – Table and equation references are to [IEC 62305-2].

I.6 Expected dangerous events to the structure

The number of expected dangerous events for the building is reported in Table I.5.

Table I.5 – Expected number of dangerous events

Reference
Parameter Comment Symbol Value
(Note)
Collection area for structure (m2) AD 16,510 Equation A.2
2
Collection area near structure (m ) AM 825 000 Equation A.11
Number of direct lightning strikes to the ND 0.05 Equation A.8
structure
Number of dangerous lightning strikes NM 2.5 Equation A.10
near the structure
NOTE – Clause references are to [IEC 62305-2].

I.7 Risk assessment for the unprotected structure

I.7.1 Risk assessment of equipment damage (related to R2)
The frequency of damage can be calculated as follows:
Frequency of damage = FC + FM + FW + FZ (I.3)

Rec. ITU-T K.85 (11/2011) 25

The values of the probability factors P and of the losses L are reported in Table I.6.

Table I.6 – Risk R2: values of the probability factors

Probability Value
PB n.a.
PC –
External power port 1.0
Internal USB port 1.0
Internal Ethernet port 1.0
PM –
External power port 1.6×10–5
Internal USB port 0.01
Internal Ethernet port 0.04
PV (power) n.a.
PW (power) 1.0
PZ (power) 0.3

The values of the risk components related to the building are reported in Table I.7.

Table I.7 – Risk F2: values of the frequency of damage

Risk components Frequency of damage
FC (Note) 0.05
FM (Note)
Power port 4 × 10–5
USB port 0.025
Ethernet port 0.1
FW (power) (Note) 0.12
FZ (power) (Note) 3.60
Total
Power port 3.8
USB port 0.075
Ethernet port 0.15
NOTE – FC = ND × PC
FM = NM × PM
FW = NL × PW
FZ = NI × PZ

The risk F2 is greater than the tolerable value assumed to be equal to 0.1 (1 damage per 10 years).
Therefore, protection measures are necessary. Importantly it is estimated that 3.8 damages to the
power port will occur per annum mainly due to surges induced into the power line.

26 Rec. ITU-T K.85 (11/2011)

The estimated risk of damage to the Ethernet port is 0.15m greater than the suggested tolerable
value of 0.1. However, the calculation FM is based on a vertical loop and a withstand voltage of
1 kV. This is most likely a worst-case situation and it is expected that damage to the Ethernet ports
will be lower than indicated in the table. Therefore, it is assumed the Ethernet port will not need
protection.

I.8 Selected protection measures

A protected power board or mains SPD can be installed at the equipment in order to reduce the risk
components FM and FZ of the power port.

I.9 Risk assessment related to the protected structure

I.9.1 Assessment of the risk R2 of loss of service to the public
The values of the relevant probability factors P are reported in Table I.8.

Table I.8 – Risk R2: values of the probability factors

(protected structure)
Probability Value
PC 1.0
PM power port 1.6×10–6
PW (power) 1.0
PZ (power) 0.03

The values of the risk components related to a protected building are reported in Table I.9.

Table I.9 – Risk R2: Values of the risk components

related to a protected building
Risk components Frequency of damage
FC 0.05
FM
Power port 4×10–6
USB port 0.025
Ethernet port 0.1
FW (power) 0.12
FZ (power) 0.360
Total
Power port 0.53
USB port 0.075
Ethernet port 0.15

The selected protection measures reduce the frequency of damage to the mains power port but not
below the tolerable value of 0.1. In practice however, it is expected that the additional protection
will provide a higher level of protection than the assigned 0.1 reduction factor.

Rec. ITU-T K.85 (11/2011) 27

I.10 SPDs
I.10.1 Selection of SPDs
The expected overcurrent values in the installation point must be considered when selecting SPDs,
as indicated in [ITU-T K.67] and Annex E of [IEC 62305-2]. According to Table 2 of
[ITU-T K.67], the worst-case induced current due to a flash near the structure is 100 A 10/350.
According to Table E.2 of [IEC 62305-2], 5 kA 8/20 is the worst-case mains surge on the mains
conductors caused by a lightning strike near the power line. This should be handled by a 5 kA 8/20
MOV type protector.

28 Rec. ITU-T K.85 (11/2011)

 Appendix II

Assessment of protection needs

(This appendix does not form an integral part of this Recommendation.)

Appendix II considers the damage issues with single port equipment and mains powered earthed
and non-earthed equipment. Equipment powered from an external power adaptor may be earthed or
non-earthed, depending on the equipment and power supply design.
Three types of equipment need to be considered:
• Single port equipment, e.g., POTS;
• mains powered earthed equipment;
• mains powered non-earthed equipment.
Equipment powered by a power adaptor will be considered according to whether the power source
supplies an earth to the equipment.
It also looks at the issue of non-earthed power points.

II.1 Single port equipment

Single port equipment has a high level of isolation to earth and only needs an SPD connected a – b.
It will normally be protectable by a GDT connected across the symmetric pair.

II.2 Earthed equipment

Earthed equipment is expected to contain inherent earthed surge protective components
(SPCs)/SPDs on the mains and any other external ports and will generally have a good level of
protection as equipotential bonding is provided in the equipment (see Figure II.1). There is a
possibility of damage to the internal port due to current returning to the MET via the earth wire
causing the equipment to rise in potential.
When used with an earth, the equipment should be reliable in most installations and there is
minimal safety risk. When used without an earth, damage levels are likely to be significant and
safety risks exist for both an a.c. earth fault situation and due to lightning.

II.3 Non-earthed equipment

Non-earthed equipment generally relies on insulation barriers between different types of ports for
reliability and safety. If this insulation barrier is broken down damage is likely to occur. There is no
equipotential bonding between the ports (see Figure II.2).
Adding an MSPD with protection for all ports will provide equipotential bonding. If the MSPD is
plugged into an earthed power point, protection of the equipment and its users should be effective.
If the power point is not earthed, equipment damage levels may not be reduced and a possible safety
risk for users exists for lightning as the mains double insulation transformer will be breached by the
main SPDs.

II.4 Level of protection provided by an MSPD

Available MSPDs may have protection only for external ports or protection for both external ports
and internal ports. The risk assessment will indicate which ports needs protection. Some MSPDs
may also have coordination elements.

Rec. ITU-T K.85 (11/2011) 29

Equipment with earthed SPDs includes:
• MSPD with coordination elements. In this case the surge current entering the equipment
will be significantly reduced. The reduction factor PMSPD could be assumed to be 0.1.
• MSPD without coordination elements. For external symmetric pair ports the coordination
element in the equipment should ensure operation of the MSPD GDT and the reduction
factor PMSPD could be assumed to be 0.1. For mains power ports, the current will be shared
by the MSPD and the equipment MOVs. In this case the reduction factor PMSPD could be
assumed to be 0.5. While it may seem that the mains port of the equipment is at risk of
damage, the inherent protection level can be quite high depending on the size of the
protection element used.
Equipment without earthed SPDs. In this case current should not enter the equipment and the
reduction factor PMSPD could be assumed to be 0.1.

II.5 Consideration of loop areas

It can be seen in the bottom diagram of Figure II.3 that the induced voltage on transformer isolated
ports connected by a short cable may be higher than expected. If the risk assessment indicates that
protection is needed for ports connected to long internal cables, protection may also be required for
ports connected to short cables. It is suggested to always use a loop area of 10 m² for Ethernet ports.
[IEC 62305-2] determines the reduction factor KS3 for shielded cables assuming that the withstand
voltage conductor to screen is 1000 V. The internal shielded cable test of [ITU-T K.21] applies the
test voltage to the screen resulting in a conductor to screen voltage considerably less than the test
voltage. In this case the unshielded loop area is used to determine the reduction factor KS3.
For shielded cable USB ports, a loop area of 0.5 m² is recommended. In this case KS3 = 0.01,
according to [IEC 62305-2]. Now generic USB ports have a resistibility (withstand) level of 100 V.
In this case KS4 =10, according to [IEC 62305-2].

1.5 kV isolation
L
SPD SELV

SPD SPD
N
1.5 kV isolation

1.6*230 V TNV-3
E
K.85(11)_FII.1

Figure II.1 – Earthed equipment

• Equipment is earthed and safe according to [IEC 60950-1].
• It is understood that surge currents in the earth wire due to surges on the line can cause the
equipment to rise in potential with respect to the MET.
• Risk of damage on internal ports due to mains current requires either mains primary
protector or all ports (external and internal) to connect via an MSPD.
• If the earth connection is missing from the power point, safety may be compromised. The
SELV circuit may be exposed to lightning surges.
• [ITU-T K.21] requirements ensure protection up to the inherent test levels.

30 Rec. ITU-T K.85 (11/2011)

 3 kV isolation
L
SPD SELV

N
1.5 kV isolation

TNV-3

K.85(11)_FII.2

Figure II.2 – Non-earthed equipment

• Equipment is safe according to [IEC 60950-1].
• It is understood that surge currents in the mains conductors can cause the primary winding
of the mains port transformer to rise in potential with respect to the MET.
• If the insulation is breached damage and safety issues may occur.
• [ITU-T K.21] requirements ensure protection up to the inherent test levels.
• When protection is required, install an MSPD or primary protection on all external ports.
• If the earth connection is missing from the power point, the MSPD may create a safety
issue. The SELV circuit may be exposed to lightning surges.
• An alternative protection method is to use high insulation values on all external ports. This
is only effective against induced surges.
Earthed

Large loop, induced voltage ≅ V Small loop, induced voltage << V

Stress ≅ V/2 for transformers Stress << V/2 for transformers
Unearthed

VUSB

Large loop, induced voltage = V

Stress = V/4 for all transformers including ports for
equipment connected with a short cable
With shielded cable, V USB << V K.85(11)_FII.3

Figure II.3 – Induced voltage depends on the equipment type

Rec. ITU-T K.85 (11/2011) 31

 Bibliography

[b-ISO/IEC 15044] ISO/IEC 15044 (2000), Information technology – Terminology for the
Home Electronic System (HES).
<http://webstore.iec.ch/webstore/webstore.nsf/ArtNum_Popup/26335!opendocument>

[b-ETSI ES 201 468] ETSI ES 201 468 V1.3.1 (2005), Electromagnetic compatibility and Radio
spectrum Matters (ERM); Additional ElectroMagnetic Compatibility (EMC)
requirements and resistibility requirements for telecommunications
equipment for enhanced availability of service in specific applications.
< http://pda.etsi.org/pda/queryform.asp>

32 Rec. ITU-T K.85 (11/2011)

 SERIES OF ITU-T RECOMMENDATIONS

Series A Organization of the work of ITU-T

Series D General tariff principles

Series E Overall network operation, telephone service, service operation and human factors

Series F Non-telephone telecommunication services

Series G Transmission systems and media, digital systems and networks

Series H Audiovisual and multimedia systems

Series I Integrated services digital network

Series J Cable networks and transmission of television, sound programme and other multimedia signals

Series K Protection against interference

Series L Construction, installation and protection of cables and other elements of outside plant

Series M Telecommunication management, including TMN and network maintenance

Series N Maintenance: international sound programme and television transmission circuits

Series O Specifications of measuring equipment

Series P Terminals and subjective and objective assessment methods

Series Q Switching and signalling

Series R Telegraph transmission

Series S Telegraph services terminal equipment

Series T Terminals for telematic services

Series U Telegraph switching

Series V Data communication over the telephone network

Series X Data networks, open system communications and security

Series Y Global information infrastructure, Internet protocol aspects and next-generation networks

Series Z Languages and general software aspects for telecommunication systems

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