The IEE tutorial workshop on

Earthing & Bonding techniques

for electrical installations.
Presentation by:
Geoff Cronshaw,
Chief Engineer,
Standards and Compliance,
IEE.
IEE tutorial workshop
1. Overview of earthed and unearthed
electrical systems.
2. Performance requirements of BS 7671.
3. Type of earthing systems, advantages,
disadvantages, and applications of
different systems, use of RCD’s.
1. Overview of earthed and
unearthed electrical systems
• It is necessary to determine the type of system, i.e.TN-S,
TN-C-S, TT, etc, of which the proposed installation will
form a part, before proceeding with the installation
design.
• A distributor is required on request to provide a
statement on the type of earthing (Reg 28 of the
Electricity Safety, Quality and Continuity Regulations)
and unless inappropriate for reasons of safety, the
distributor is required to make available his supply
neutral conductor or protective conductor for connection
to the consumer’s earth terminal (Reg 24).
• For a low voltage supply given in accordance with the
Electricity Safety, Quality and Continuity Regulations
2002 the supply system will be TN-S, TN-C-S(PME)
or TT, and most commonly for new supplies TN-C-S.
• TN-C and IT systems are both very uncommon in the
UK, the former because it requires an exemption from
the Electricity Safety, Quality and Continuity
Regulations which invoke special installation
arrangements and the latter because the source is not
directly earthed and this is not permitted for a low
voltage public supply in the UK (Reg 8).
• For TN-S, TN-C-S and TT systems the following
explanations should aid a full understanding of the
earthing arrangements and their scope of application.
The nomenclature of these system types is as follows:
T = Earth (from the French word Terre)
N = Neutral
S = Separate
C = Combined
 TN-S system earthing
system

(origin of
installation)
source of supply installation
L installation
equipment

N
PE
Exposed-
T 542-01-02 conductive-
source N-S part
earth
(separate neutral (N) and protective conductors
(PE) throughout the system)
 circuit protective
conductors

TN-S LABEL - Safety Electrical

Connection. Do not remove
Electricity
metal water
kWh company
pipe
isolator
S7 6E 165456
metal gas
pipe

L N NL
main switch

Main
earthing LABEL - Safety Electrical
100 A Connection. Do not remove
terminal

gas meter

Water Gas
service pipe service pipe
TN-S system

A TN-S system has the neutral of the source of

energy connected with earth at one point
only, at or as near as is reasonably
practicable to the source, and the consumer’s
earthing terminal is typically connected to the
metallic sheath or armour of the distributor’s
service cable into the premises or to a
separate protective conductor of, for instance,
an overhead supply
 TN-C-S system earthing
L

PEN N
link PE
542-01-03
source additional
earth source electrode
T
N-C S

(combined neutral and (separate neutral (N) and

protective conductor (PEN) protective conductors (PE)
in source of supply, with within installation)
PME applied)
 circuit protective

TN-C-S
conductors
LABEL - Safety Electrical
Connection. Do not remove

Electricity
company metal water
kWh
isolator pipe

S7 6E 165456
metal gas
pipe

L N NL
main switch

Main
100 A
earthing
LABEL - Safety Electrical
terminal
Connection. Do not remove

gas meter

Water Gas
service pipe service pipe
TN-C-S system

A TN-C-S system has the supply neutral

conductor of a distribution main connected
with earth at source and at intervals along its
run. This is usually referred to as protective
multiple earthing (PME). With this
arrangement the distributor’s neutral
conductor is also used to return earth fault
currents arising in the consumer’s installation
safely to the source. To achieve this, the
distributor will provide a consumer’s earthing
terminal which is linked to the incoming
neutral conductor.
TT system earthing

542-01-04
source
T T
earth consumer’s
earth
 circuit protective
TT conductors
LABEL - Safety Electrical
Connection. Do not remove

Electricity metal water

kWh
company pipe
Electricity Supply isolator
(usually overhead)
S7 6E 165456 metal gas
pipe

L N NL

main switch

100 A Main
earthing
terminal

gas meter

LABEL - Safety Electrical

Connection. Do not remove

Water Gas
Earth Rod service pipe service pipe
TT system

A TT system has the neutral of the source of

energy connected as for TN-S, but no facility
is provided by the distributor for the
consumer’s earthing. With TT, the consumer
must provide his or her own connection to
earth, i.e. by installing a suitable earth
electrode local to the installation.
2. Performance requirements
of BS 7671
• A wide variety of types of earth electrode are recognised by BS
7671. The most suitable type for a particular system will depend
upon a number of factors, the single most important of these being
the soil resistivity of the ground. If earth rods are to be driven it may
be necessary to go down several metres before a good conductive
layer is reached, especially where the water table is low. Rods can
only be as effective as the contact they make with the surrounding
material. Thus, they should be driven into virgin ground, not
disturbed (backfilled) ground. Where it is necessary to drive two or
more rods and connect them together to achieve a satisfactory result,
the separation between rods should be at least equal to their
combined driven depth to obtain maximum advantage from each rod.
• In some locations low soil resistivity is found to
be concentrated in the topsoil layer, beneath
which there may be rock or other impervious
strata which prevents the deep driving of rods, or
a deep layer of high resistivity. Only a test or
known information about the ground can reveal
this kind of information. In such circumstances,
the installation of copper earth tapes, or pipes or
plates, would be most likely to provide a
satisfactory earth electrode..
• Closely spaced buildings may sometimes make it
difficult to find ground suitable for driving an earth
electrode. Electrodes which employ suitable structural or
other underground metalwork, or the metal reinforcement
of concrete embedded in the ground may then be of
particular advantage. Whatever form an earth electrode
takes, the possibility of soil drying and freezing, and of
corrosion, must be taken into account. Preferably, testing
of an earth electrode should be carried out under the least
favourable conditions, i.e. after prolonged dry weather.
Further information on earthing principles and practice
can be found in BS 7430 : 1998 ‘Code of Practice for
Earthing’ (formerly CP 1013: 1965).
• Earthing conductors - The earthing conductor of
an installation is part of the earth current path. It
is therefore important that the conductor is
adequately sized and, particularly where buried
partly in the ground, of suitable material and
adequately protected against corrosion and
mechanical damage.
• The size of an earthing conductor is arrived at in
basically the same way as for a circuit protective
conductor, except that Table 54A of BS 7671
must be applied to any buried earthing
conductor. For a TN-C-S (PME) supply, the
designer will additionally require it to be no
smaller than the main bonding conductors.
• Sizing of circuit protective conductors.
There are several factors which may influence or
determine the size required for a circuit protective
conductor. A minimum cross-sectional area of 2.5 mm2
copper is required for any separate circuit protective
conductor, i.e. one which is not part of a cable or formed
by a wiring enclosure or contained in such an enclosure.

An example would be a bare or insulated copper

conductor clipped to a surface, run on a cable tray or
fixed to the outside of a wiring enclosure. Such a circuit
protective conductor must also be suitably protected if it
is liable to suffer mechanical damage or chemical
deterioration or be damaged by electro-dynamic effects
produced by passing earth fault current through it. If
mechanical protection is not provided the minimum size
is 4 mm2 copper or equivalent.
• BS 7671 provides two methods for sizing protective
conductors including earthing conductors (see also Table
54A). The easier method is to determine the protective
conductor size from Table 54G, but this may produce a
larger size than is strictly necessary, since it employs a
simple relationship to the cross-sectional area of the
phase conductor(s).
• The second method involves a formula calculation. The
formula is commonly referred to as the ‘adiabatic
equation’ and is the same as that used for short-circuit
current calculations (see Regulation 434-03-03). It
assumes that no heat is dissipated from the protective
conductor during an earth fault and therefore errs on the
safe side. Even so, application of the formula will in
many instances result in a protective conductor having a
smaller csa than that of the live conductors of the
associated circuit. This is quite acceptable.
• Regulation 543-01-03 states :
543-01-03 The cross-sectional area, where calculated, shall
be not less than the value determined by the following
formula or shall be obtained by reference to BS 7454:

I 2t
S=
where:
k
S is the nominal cross-sectional area of the conductor in mm2.
I is the value in amperes (rms. for a.c.) of fault current for a fault of negligible
impedance, which can flow through the associated protective device, due
account being taken of the current limiting effect of the circuit impedances
and the limiting capability (I2 t) of that protective device. Account shall be
taken of the effect, on the resistance of circuit conductors, of their
temperature rise as a result of overcurrent - see Regulation 413-02-05.
t is the operating time of the disconnecting device in seconds corresponding to
the fault current I amperes.
k is a factor taking account of the resistivity, temperature coefficient and heat
capacity of the conductor material, and the appropriate initial and final
temperatures.
3. Type of earthing systems,
advantages, disadvantages,
and applications of different
systems, use of RCD’s.
Protective multiple earthing (PME).
Such a supply system is described
in BS 7671 as TN-C-S.
• The Electricity Safety, Quality and Continuity
Regulations 2002 permit the distributor to combine
neutral and protective functions in a single conductor
provided that, in addition to the neutral to earth
connection at the supply transformer, there are one or
more other connections with earth.
• This protective multiple earthing (PME) has been
almost universally adopted by supply companies in the
UK as an effective and reliable method of providing
their customers with an earth connection. Such a supply
system is described in BS 7671 as TN-C-S.
 PME earth terminal
L1
L2
L3

PEN

PME earth
Source earth terminal Additional
L N PE earth
protective
conductor

Customer’s installation
• However, under certain supply system fault conditions
(external to the installation) a potential can develop
between the conductive parts connected to the PME
earth terminal and the general mass of earth.
• Supply system
There are multiple earthing points on the supply
network, and providing bonding within the building
complies with BS 7671 it is unlikely that such a
potential as described above would in itself constitute a
hazard.
• Additional earth electrode for PME supplies.
• In the unlikely event of the PEN conductor of the
supply becoming open circuit, touch voltages perhaps
causing some discomfort may arise on exposed metal in
customers installations downstream of the open circuit.
 • The effect can be mitigated by connection of a suitable
earth electrode to the main earth terminal of the
customers installation. The value of the resistance to-
earth necessary to limit the touch voltages to a given
value depends on the load and the network parameters:
Consumer’s
Line Re installation
L

Vs RL

PEN N
PEN Open-
PE
circuit
PEN

Vp
RB RA
Where:
Vs is the nominal supply (source) voltage
Vp is the touch voltage
Re is the external supply resistance
RL is the load resistance (Vs2 / wattage)
RA is the resistance of the additional earth
electrode including parallel earth (e.g.
water and gas pipes)
RB is the resistance to earth of the neutral
point of the power supply.
• Special locations
The Electricity Association provides guidance on PME
in its Engineering Recommendation G12/3,
‘Requirements for the application of protective multiple
earthing to low voltage networks’. The guidance given
in this Section is based on Part 6 of G12/3.
• Remote supplies.
A physically isolated building may be supplied via both
a long LV distribution main and a long individual
service. If these circumstances are coupled with an
unbalanced load a potential difference will be evident
between the PME terminal and true earth.
• This rise in neutral potential is transferred to metalwork
connected to the PME terminal, and may be noticeable
in such locations as shower areas, where simultaneous
contact with true earth (a wet concrete floor) and the
PME terminal (shower pipes) may be possible.
• G12/3 recommends that such floors should incorporate
a metal grid connected to the supplementary bonding
within the shower/bathing area. Certainly, if there is a
grid in the floor it must be connected to the local
supplementary bonding. An alternative approach would
be not to use the PME earthing terminal and to afford
protection by means of RCDs and an independent earth
electrode, that is, treat as part of a TT system.
• Sports pavilions
Where no shower area exists nor is likely to exist in a
sports pavilion, PME may be offered provided the
appropriate metalwork is bonded. Where a shower
exists PME should only be applied where there is an
earth grid in the floor of the shower area,
supplementary bonded to accessible metal pipework,
etc. An alternative would be to plumb the installation in
plastic pipes.
• Swimming pools
Swimming pools supplied with their own dedicated
service should be protected with an RCD. All
metalwork should be bonded and connected to an earth
electrode. Where a swimming pool forms part of a
residence, all metalwork and pipes supplying the pool
should be connected to an earth electrode and
segregated from the rest of the building. An RCD
should then be used to protect the supplies to the pool
area and the swimming pool installation treated as part
of a TT system. Where segregation of pipes and
metalwork around a pool is impracticable, e.g. in an
indoor pool, the installation of a metal grid around the
pool and the supplementary bonding of adjacent
metalwork is recommended together with an RCD in
addition to PME.Additional earth electrode for PME
supplies.
• Agricultural and horticultural premises
A PME terminal may be used in these premises
provided main equipotential bonding has been carried
out and the additional measures required by Section
605 of BS 7671 have been provided, e.g. reduced
disconnection times, supplementary bonding, RCD
protection. The general principles to be adopted for
separate buildings utilizing a PME terminal are as
follows:
(i) each separate building should have a designated
main earthing terminal within the building
(ii) any main equipotential bonding required in a
separate building to external services, structural
metalwork, etc, should be connected to the
designated earth terminal. The cross-sectional area
of the bonding conductor should be selected from
Table 54H of BS 7671 using the electricity
distributor’s supply neutral conductor as reference.
• Livestock/dairies
Particular care must be taken in areas where livestock
are housed as they are sensitive to small potential
differences.
• Building and construction sites, quarries, etc
It may be difficult to satisfy the electricity distributors
bonding requirements because of the large number of
parts of the building works that are extraneous-
conductive-parts. In these circumstances a PME
earthing terminal may not be provided. Regional
electricity companies may be prepared to offer a TN-S
supply to large sites requiring their own substation
where the earthing terminal can be connected directly
to the transformer neutral. For quarries, see also below.
• Caravan park and marinas
Regulation 9(4) of the Electricity Safety, Quality and
Continuity Regulations does not allow the combined
neutral and protective conductor to be connected
electrically to any metalwork in a caravan or boat. This
prevents PME terminals being used for caravans or
boat mooring supplies, although they may be used for
fixed premises on the sites, such as the site owner’s
living premises and any bars or shops, etc. Account is
taken of this in Section 608 of BS 7671 with respect to
caravans, and also will be for marinas when a section is
included in the standard. The extension of PME to
toilets and amenity blocks is not recommended as there
is a high probability of people going barefoot and being
in good contact with true earth through wet concrete
floors.
 Typical site distribution for a PME
supply, separation from PME earth at
main distribution board
TNC Supply (PME) Main Board Distributor pitch supply Pitch
outlets

100 mA or greater
general purpose
RCD in insulated
L L enclosure
30 mA RCD

PEN N
RCD RCD
PE

Installation earthing to
be separated from the
PME earthing

Resistance areas of installation

and supply earthing to be
separated
 Typical site distribution for a PME
supply, separation from PME earth at
pitch supply point

Isolator Distributor pitch supply point Pitch

Main Board
TNC Supply (PME) outlets
Overcurrent
device to
disconnect
O.4 s
30 mA RCD
L L
PEN N
RCD
PE

Cable protective conductor

(armour/sheath) separated from RCD
earth and caravan protective conductors
• Petrol filling stations
The reference publication is “Guidance for the design,
construction, modification and maintenance of petrol
filling stations”, published by the APEA and the
Institute of Petroleum, which recommends a TT supply
for hazardous areas.
A separate earth electrode and RCD or other alternative
arrangement is required to ensure the segregation of
petrol filling area earthing and that of the PME earth of
the distribution network. A PME earth may be used for
permanent buildings such as shops and
restaurants.Additional earth electrode for PME
supplies.
• Mines and quarries
A supply taken to an underground shaft, or for use in
the production side of a quarry, must have an earthing
system which is segregated from any system bonded to
the PME terminal.
• Supply terminating in a separate building
Occasionally a service will terminate in a position
remote from the building it supplies. The size of the
PME bonding in the building supplied must be related
to the size of the incoming supply cable, in accordance
with Regulation 547-02-01 and Table 54H of BS 7671.
If the size of the circuit protective conductor of the
cable between the supply intake position and the
building is less than that of the PME bonding
conductor, a suitable additional conductor will have to
be installed or the PME earth not used.
 Supply terminating in a separate
building

Main Equipotential
Bonding to be related
Remote supply Building to the neutral of the
termination Supplied main cable M

Supplier’s
cutout Size of protective
conductor/armouring to
Main earth be related to the neutral
Earth
terminal of the main cable M
terminal

Main supply
cable M
Sub main S
• PME and outside water taps
Under an open-circuit supply neutral condition the
potential of an outside water tap will rise above earth
potential. A person coming into contact with the tap
could receive an electric shock and the shock could be
severe, if that person were barefooted. The probability
of these two conditions occurring together is considered
to be so small that the use of PME where a metal
outside tap exists is not precluded. It is recommended,
however, that a plastic insert be provided in the pipe to
the outside water tap.
• High rise buildings
For guidance on the application of PME to high rise
buildings and building complexes, guidance should be
sought from the electricity supply company and
Engineering Recommendation G12/3.
• Highway power supplies
Street electrical fixtures were exempted from the
requirements of Regulation 7 of the former Electricity
Supply Regulations 1988 as amended. The exemption
allowed the minimum copper equivalent cross sectional
area of the main equipotential bonding conductor to be
reduced to 6 mm2 in specific circumstances. As the
Electricity Safety, Quality and Continuity Regulations
do not specify minimum sizes of main bonding
conductors, it is to be presumed that it would be
reasonable to allow the reduction in size for street
lighting equipment in the same circumstances as
before. Although BS 7671 at present has a minimum
size of 10 mm2 for supply neutral conductors of 35
mm2 csa or less, it would seem appropriate to reduce
the minimum size to 6 mm2 for supply neutral
conductors of 6 mm2 csa or less, see Regulation
547-02. However, the departure from Part 5 would
need to be recorded on the Electrical Installation
certificate.
A TT system
• A TT system has the neutral of the source of energy
connected as for TN-S, but no facility is provided by
the distributor for the consumer’s earthing.
• With TT, the consumer must provide his or her own
connection to earth, i.e. by installing a suitable earth
electrode local to the installation. The circumstances in
which a distributor will not provide a means of earthing
for the consumer are usually where the distributor
cannot guarantee the earth connection back to the
source, e.g. a low voltage overhead supply, where there
is the likelihood of the earth wire either becoming
somehow disconnected or even stolen.
• A distributor also might not provide means of earthing
for certain outdoor installations, e.g. a construction site
temporary installation, leaving it to the consumer to
make suitable and safe arrangements for which they are
fully responsible. The electricity distributor is required
to make available his supply neutral or protective
conductor for connection to the consumer’s earth
terminal, unless inappropriate for reasons of safety
(Reg 24). Construction site, farm or swimming pool
installations might be inappropriate unless additional
precautions are taken, such as an additional earth
electrode.
A TN-S system
• A TN-S system has the neutral of the source of energy
connected with earth at one point only, at or as near as
is reasonably practicable to the source, and the
consumer’s earthing terminal is typically connected to
the metallic sheath or armour of the distributor’s
service cable into the premises or to a separate
protective conductor of, for instance, an overhead
supply.
• Large consumers may have one or more HV/LV
transformers dedicated to their installation and installed
adjacent to or within their premises. In such situations
the usual form of system earthing is TN-S. It is not
necessary for the star (neutral) point of the transformer
LV secondary to be brought out at the transformer
cable box in order to earth this point. The usual and
most convenient means of earthing the source neutral is
for the connection to earth to be made at the first
accessible position in the LV system at which the
neutral is terminated, i.e. LV feeder pillar or main LV
switchboard.
Use of RCD’s
• Supplementary protection by residual current devices.
• With specific exceptions, supplementary protection by
a residual current device must be provided in every
circuit intended or reasonably expected to supply
portable equipment for use outdoors.
• Residual current devices do not prevent shock currents
flowing through the body, for a current must flow to
earth for the residual current device to detect. However,
provided the residual current device is sufficiently
sensitive and functionally tested periodically it will
operate sufficiently quickly to prevent injury.
• Automatic disconnection of supply
• BS 7671 prescribes automatic disconnection of the
supply to any circuit, i.e. distribution circuit or final
circuit, in which an earth fault may occur. This is
irrespective of how successful the earthing and bonding
provisions may be in keeping down the magnitude of
voltages that may appear between exposed-conductive-
parts and extraneous-conductive-parts conductive-parts
during an earth fault, since, even if there is no shock
risk, thermal damage may be caused to conductors or
their surroundings if earth fault current is allowed to
persist. Whatever the type of system, the operating
characteristic of each device for automatic
disconnection and the relevant impedance of the
associated circuit should be properly co-ordinated to
achieve the required disconnection time.
• For installations supplied at nominal voltages (Uo) in
the range 121 - 277 V to earth the prescribed maximum
disconnection times are generally 0.4 s or 5 s, but are
0.2 s for some special locations.
• An RCD may be used to provide indirect shock
protection in TN systems where the earth fault loop
impedance (Zs) is insufficiently low to operate a fuse or
circuit-breaker within the prescribed disconnection
time. An RCD may also be required for installations or
locations of increased shock risk, such as those in
Part 6 of BS 7671. Where more than one RCD is used
they and their circuits should be segregated, or if RCDs
are used in series the upstream device needs to be time
delayed or S type and the downstream a type for
general use, both to BS EN 61008 or BS EN 61009 to
achieve discrimination.
Mixed disconnection times
• The requirements for mixed disconnection times apply
for any final circuits supplied from the same
distribution board, distribution circuit or sub-main. This
includes adjacent boards supplied from a common
circuit or with a common bonding connection.
• Mixed disconnection times within an installation can
give rise to a particular problem. Consider two final
circuits of an installation as shown below:
 Final circuits having 'mixed' disconnection
times (Regulation 413-02-13)
distribution
board
exposed-conductive-part
circuit A
load A
L (indoors)
fixed
circuit B Load B

250 V
Uf
N

main earthing protective

terminal If conductor
in cord
E
Rc

further equipotential
main equipotential
bonding conductor
bonding
(if necessary)
conductor

extraneous-conductive-part

earth
• Other ways of avoiding or overcoming the problem
could include:
(i) design all circuits supplied from a common
distribution board or distribution circuit to achieve
disconnection within 0.4 s
(ii) use RCD (RCCB or RCBO) protection. This would
have to include, in the example illustrated, fixed
equipment circuit B. Fitting an RCD to socket-
outlet circuit A would not solve the possible
problem, since the RCD would be unaffected by the
appearance of the voltage Uf and any current this
might send through the body of a person who
makes simultaneous contact with the exposed-
conductive-part of load A and the nearby
extraneous-conductive-part.Additional earth
electrode for PME supplies.
• Automatic disconnection in a TT system Other than in
exceptional circumstances in which steel reinforcement
of underground concrete or similar structure is
available for use, in TT systems it is often difficult and
costly to achieve a sufficiently low earth electrode
resistance for overcurrent devices to be used for
indirect shock protection.
We have covered a brief
overview of earthing.

Thank you for listening, and

have you any questions?