ELECTRICAL
TECHNICAL
Relocation, relocation
There are many occasions where the gas
engineer is required to relocate the gas
meter, usually from the existing position
inside the property to a new outside
cabinet. Graham Wretham explains what
should happen regarding the existing
earthing and bonding requirements.
F
irstly, it is important to
note the fundamental
requirements of the
IET Wiring Regulations,
BS 7671 relating to additions
and alterations to an installation.
The particular Regulation
states: ‘No addition or alteration,
temporary or permanent,
shall be made to an existing
installation unless it has been
ascertained that the rating and
the condition of any existing
equipment, including that of
the distributor, will be adequate
for the altered circumstances.
Furthermore, the earthing
and bonding arrangements, if
necessary for the protective
measure applied for the safety
of the addition or alteration,
shall be adequate.’
Accordingly, if the
bonding conductors are
repositioned or changed, they
must be adequate.
Protection against
electric shock
Chapter 41 of the Regulations
deals with the requirements
for protection against electric
shock. A protective measure
should generally consist of an
appropriate combination of a
provision for basic protection
and an independent provision
for fault protection.
The most common measure
for fault protection is ‘Automatic
Disconnection of Supply’ where
fault protection is provided by
protective earthing, protective
equipotential bonding and
automatic disconnection in
case of a fault.
Let’s consider protective
earthing requirements first.
58 › INSTALLER › October 2013
Protective earthing
Exposed conductive
parts should be connected
to a protective conductor.
A circuit protective conductor
(cpc) should be run to and
terminated at each point in
wiring and at each accessory
or (Class 1) item of electrical
equipment (see Figure 1).
Earthed
metal case
Earth
Figure 1. Protective earthing.
While the gas engineer will
probably have no influence on
the presence and adequacy
of these circuit protective
conductors, he/she should at
least ascertain the presence
and adequacy of the main
earthing conductor that
connects the installation
earthing to the Distribution
Network Operator’s earthing
system, which will usually be a
TN-C-S system (see Figure 2).
The cross sectional area
of this earthing conductor is
generally selected from the Table
54.7 provided (see Table 1).
Accordingly, it is usual to find
a 16 mm2 earthing conductor
connecting the main earthing
terminal (usually within the
consumer unit) to the system
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earthing. If the conductor is
present but undersized, British
Gas advice is that the consumer
should contact a competent
electrician.
Protective
equipotential bonding
In each installation, main
protective bonding conductors
should connect extraneous
conductive parts to the
main earthing terminal.
Such extraneous conductive parts
include:
• Water service pipes
• Gas installation pipes
• Other service pipes
and ducting
• Central heating and air
conditioning systems
• Exposed metallic structural
parts of the building
• The lightning protective system
Where TN-C-S (PME) conditions
apply, the main equipotential
bonding conductor should be
selected in accordance with the
neutral conductor of the supply
and Table 54.8 of the Wiring
Regulations.
Typically, for an 80/100A
installation, the main protective
bonding conductor is likely to
be 10mm2.
The minimum main bonding
conductor sizes specified in
Table 54.8 (see Table 2) were first
introduced into the Electricity
Supply Regulations on October
1988. Smaller minimum sizes
were accepted by some
electricity distributors prior to
that date, but sizes given in Table
54.8 were introduced to reduce
the likelihood of main bonding
conductors overheating due
to the PME network circulating
currents, which the conductors
may have to carry continuously
or for long periods.
››If the conductor
is present but
undersized, British
Gas advice is that
the consumer should
contact a competent
electrician.‹‹
If an installer/designer finds an
existing installation where PME
conditions apply – for example
the main bonding conductors
have a CSA of 6mm2 rather
than 10mm2 – they may, having
carefully considered all the
circumstances, conclude that
the deficiency does not pose
a significant risk to the users.
For example, if there is no
evidence of overheating of
the existing main bonding
CSA of line conductor of
main tails ‘S’
Minimum CSA
of earthing conductor
S < 16 mm2
same as S
16 mm2 , S < 35 mm2
16 mm2
S.
35 mm2
S 3 0.5
Table 1. Minimum size of earthing conductor.
CSA of supply neutral conductor
CSA of main protective
bonding conductors
S < 35 mm2
10 mm2
35 mm2 , S < 50 mm2
16 mm2
50 mm2
95 mm2
25 mm2
95 mm2 , S < 150 mm2
35 mm2
,S<
S.
150 mm2
50 mm2
conductors and terminations, it
may be reasonable to assume
that the existing arrangements
are adequate for any network
circulating current – unless the
main bonding conductors were
only recently installed. However,
the deficiency must be recorded,
together with the reasons why
the installer has concluded that
the deficiency does not pose a
significant risk to the users.
Termination of
bonding conductors
The main equipotential
bonding conductor connection
to any gas or other service
should be made as near as
practicable (preferably within
600mm) to the point of entry
to the premises. Connections
must be made on the consumer’s
side of any insulating insert –
before any branch pipework
and on hard metal.
Figure 3 shows the correct
position of the bonding
conductor connection, which
should be at the point of entry
into the building or at the first
available position where the
gas installation pipe becomes
accessible within the building
– although this may be some
distance away from the gas
meter. Accordingly, most
engineers accept the bonding
conductor termination to be
within the meter cupboard.
The author
Graham Wretham is
Managing Director of Redford
Charles Ltd, an electrical
training and consultancy
company in Kent.
Previously the NICEIC
Inspecting Engineer for Kent,
Graham was later promoted to
Senior Engineer and was also
the NICEIC’s representative
on various external and
international technical
panels. Graham gained
further promotion to NICEIC
Operations Manager and then,
following a restructure, to
Operations Director.
Graham left the NICEIC in
2005 to start his own electrical
and consultancy business,
Redford Charles Ltd, which
has become one of the leading
electrical training companies
in the south east and works for
a number of blue chip clients.
max
600m
Equipotential
bonding
Internal meter installation
(low pressure fed)
Figure 2a.
Internal gas meter.
Equipotential
bonding connection
(non-preferred position)
Meter installation in
an external meter box
(low pressure fed)
Equipotential
bonding connection
(preferred position)
Figure 2b.
External gas meter.
Table 2. Minimum size of main bonding conductors.
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Every core of a cable shall be
identifiable at its terminations
and preferably throughout its
length. The bi-colour green and
yellow shall be used exclusively
for identification of a protective
conductor. Moreover, a label
referring to BS 951 with the words
‘Safety electrical connection – do
not remove’ must be permanently
fixed in a visible position at the
point where every bonding
conductor is connected to an
extraneous conductive part (see
Figure 4).
Safe working procedures
In order to minimise the risk of
electric shock to occupier and
operative, it is stressed that the
electrical installation for the
premises must be isolated by
way of the main double-pole
switch disconnector while the
existing main protective bonding
conductor is disconnected from
the gas pipe.
Accordingly, the occupier
should be advised, before any
work commences, that the
electrical installation will need to
be isolated during these works.
Inspection and testing
The inspection, which is
supplemented by testing, should
include at least the checking
of the following items (where
relevant to the installation and
where necessary during erection):
• Connection of conductors
• Identification of conductors
• Selection of conductors
• Correct connection of
accessories and equipment
• Methods of protection
against electric shock – basic
protection:
– Insulation of live parts
– Barriers or enclosures
• Methods of protection against
electric shock – fault protection:
– Automatic disconnection
of supply
– Presence of earthing
conductors
– Presence of circuit protective
conductors
– Presence of protective
bonding conductors
• Prevention of mutual
detrimental influence
• Presence of appropriate devices
for isolation and switching
correctly located
• Labelling of protective devices,
switches and terminals
• Selection of equipment and
protective measures appropriate
to external influences
• Adequacy of access to
switchgear and equipment
• Presence of danger notices and
other warning signs
• Presence of diagrams,
instructions or similar
information
• Erection methods
Testing follows, and is
supplementary to, the inspection
process and shall be undertaken
Figure 3. Typical earthing
and bonding arrangements.
only when the inspection has
confirmed that it is safe to do so.
The relevant test is:
• Continuity of protective
conductors including main
and supplementary
equipotential bonding.
Testing procedures
Before carrying out a test
to confirm continuity of the
protective conductor (including
main and supplementary
equipotential bonding), it
is necessary to avoid the
measurement of parallel paths.
Accordingly, it may be preferable
to disconnect the conductor
before measurement is taken.
Metal
water pipe
Circuit protective
conductors
The test instrument to be used is
an Ohmmeter with a low ohms
range (see Figure 5).
There is generally only one
option for undertaking this
continuity test for protective
bonding conductors – the
‘wander lead’.
The wander lead method is
normally implemented when
testing the continuity of Protective
Bonding Conductors and is
undertaken by connecting a long
wander lead to the disconnected
protective bonding conductor
from the main earthing terminal
within the distribution board (see
Figure 6).
With this long lead and the
other lead of the instrument, make
a connection between the remote
connection and the extraneous
conductive part under test.
WARNING:
It is important to
remember that while this test is
being conducted, the installation
should be turned off as there is
no measure of fault protection in
place at the time of testing.
It is also important to remember
that when a long wander lead
is used, its resistance value
should be subtracted from the
instrument reading or the test
instrument should be nulled.
The final outcome of this test
is that the resistance value of the
protective bonding conductor
must not exceed 0.05 Ω.
Metal
gas pipe
04771 3
Label:
Electrical connection
Do not remove
kWh
16mm2
Figure 4. Modern BS 951 Bonding Clamp
(photo courtesy of Legrande.)
10mm2
Main earthing
terminal
Figure 5. Test instruments.
Gas
meter
Water service
pipe
Gas service
pipe
› IET Wiring Regulations, BS 7671 relating to additions
and alterations requires the earthing and bonding
arrangements to be adequate.
› The main equipotential bonding conductor connection
to any gas or other service should be made as near as
practicable to the point of entry to the premises.
Main earthing
terminal
BS 951 clamp
with label
(see Regulation
514-13-01)
› It is usual to find a 16.00mm2 earthing conductor
connecting the main earthing terminal (usually within the
consumer unit) to the system earthing.
› A main protective bonding conductor shall connect the gas
installation pipe (extraneous conductive part) to the main
earthing terminal. Typically, for an 80/100A installation,
the main protective bonding conductor should be 10mm2.
Figure 6. Continuity testing of bonding conductors.
Label:
Electrical connection
Do not remove
TO SUM UP
› The gas engineer should ascertain the presence and
adequacy of the main earthing conductor that connects the
installation earthing to the Distribution Network Operator’s
earthing system, which will usually be a TN-C-S system.
S7 6E 258965
100A
a Minor Electrical Installation
Works Certificate should be
issued. Similar documentation
incorporating the same degree of
information could be issued – for
example, an invoicing document
that included the type of work
undertaken and the premises
in which the work was done.
Additional information such as
the type of earthing system, fault
protection and essential tests may
need to be provided. n
› The most common measure for fault protection is
‘Automatic Disconnection of Supply’ where fault protection
is provided by protective earthing, protective equipotential
bonding and automatic disconnection in case of a fault.
Electricity
supplier isolator
Main
isolating
switch
Certification and reporting
The requirements state that
any alterations or extensions
of an existing main protective
bonding conductor should be
documented on a Minor Electrical
Installation Works Certificate or
equivalent in-house document.
Defects or omissions – with
regard to the existing earthing
and bonding arrangements –
should be referred back to a
supervisor/manager before
work proceeds.
Accordingly, for the addition of
protective bonding conductors,
max
600m
BS 951 clamp
with label
(see Regulation
514-13-01)
max
600m
› The bonding conductor termination will use a BS 951
clamp together with a label with the words ‘Safety
electrical connection – do not remove’ permanently fixed
in a visible position at the point of each connection.
› While the existing main protective bonding conductor is
disconnected from the gas pipe, the electrical installation
for the premises must be isolated by way of the main
double-pole switch disconnector.
› A continuity test, using a low resistance ohmmeter, should
be undertaken on the main protective bonding conductor.
› A Minor Electrical Installation Works Certificate should
be issued for the alteration/additional work undertaken
(or a similar documentation incorporating the same
degree of information).
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