Earthing and bonding requirements when relocating the gas meter

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.

Firstly, 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.

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).

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).

Table 1. Minimum size of earthing conductor:

CSA of line conductor of main tails ‘S’

Minimum CSA of earthing conductor

16 mm2

same as S

16 mm2 , 35 mm2

16 mm2

S . 35 mm2

S 3 0.5

 

Accordingly, it is usual to find a 16mm2 earthing conductor connecting the main earthing terminal (usually within the consumer unit) to the system 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.

Table 2. Minimum size of main bonding conductors:

CSA of supply neutral conductor

CSA of main protective bonding conductors

35 mm2

10 mm2

35 mm2 , 50 mm2

16 mm2

50 mm2 , 95 mm2

25 mm2

95 mm2, 150 mm2

35 mm2

S . 150 mm2

50 mm2

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 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.

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 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.

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 0.05Ω.

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, 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.

To Sum Up:

  • IET Wiring Regulations, BS 7671 relating to additions and alterations requires the earthing and bonding arrangements to be adequate.
  • 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.
  • 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.
  • It is usual to find a 16.00 mm2 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 80/100 A installation, the main protective bonding conductor should be 10 mm2.
  • 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.
  • 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).