Sizing main protective bonding conductors,  guidance.

Contractors may, on occasion, be confused regarding the minimum sizing of main protective bonding conductors required for the differing earthing systems that exist.

The sizing of the main protective bonding conductor depends on a number of factors, including the type of earthing system (TT, TN-S, TN-C-S etc.) and the conductor material used.

Main protective bonding conductors should not be confused with earthing conductors or circuit protective conductors. Their functions are quite distinct, and although they may appear to be the same due to size and colour, the sizing requirements are quite different. Indeed, Regulation 543.1.1 specifically excludes the sizing of protective bonding conductors from the requirements for sizing all other protective conductors.

In this article we will consider the specific requirements for the sizing of main protective bonding conductors.

Non-PME supplies 

For installations fed by a non-PME supply, such as a TN-S or TT earthing system, Regulation 544.1.1 requires main protective bonding conductors to have a cross-sectional area (csa) of not less than half that required for the earthing conductor and not less than 6 mm2, but need not be more than 25 mm2if the conductor is of copper or, if of another metal, a csa affording equivalent conductance.

This means, for example, that line conductors of 4 mm2, 6 mm2, and 10 mm2 would require the main protective bonding conductor to have a minimum csa of 6 mm2. No other calculation is required where the conductor is copper. The table above shows the correct sizing of main protective bonding conductors for TN-S and TT earthing systems.

For example, where the meter tail (the line conductor) has a csa of 25 mm2, the minimum size for the main protective bonding conductor (copper) must be 10 mm2.

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PME supplies 

In many ways, determining the size of the main protective bonding conductors in a TN-C-S system is much simpler.

For those installations having a PME supply, Regulation 544.1.1 requires main protective bonding conductors to be sized according to the copper equivalent csa of the supply neutral conductor and not the line conductor. These minimum values are to be found in Table 54.8 of BS 7671. The contractor, therefore, merely has to read from the table.

For example, where the meter tail (this time the neutral conductor) has a csa of 25 mm2, the minimum size for the main protective bonding conductor (copper) must be 10 mm2.

Equivalent conductance 

One phrase used in Regulation 544.1.1 is ‘equivalent conductance’.

It is permitted to use conductors other than copper as a bonding conductor. However, all other usable materials will have poorer conductivity. The question therefore arises, how is equivalent conductance to be determined?

Fortunately there is guidance that exists. From IET Guidance Note 8, ‘equivalent conductance’ requirements will be met if the csa (Sm) is not less than that given by the equation:

Where:

  • is the minimum csa required for the protective bonding conductor (in a metal other than copper)
  • Sc is the minimum csa required for a copper protective bonding conductor
  • Pm is the resistivity of the metal from which the main bonding conductor is made
  • Pc is the resistivity of copper

For example, this would typically mean the minimum csa for a steel main protective bonding conductor where the ratio Pm/Pc is 8.5, would be required to be 8.5 times that of copper, and for an aluminium conductor where the ratio is 1.68, the minimum csa would be required to be 1.68 times that of copper.

As an example, if the incoming meter tail for an electrical installation having a TN-S earthing system (line conductor) was 25 mm2 and the main protective bonding conductor was steel, the minimum csa of steel would have to be 8.5 x10, which would be 85 mm2. This is because the minimum csa of main protective bonding copper conductor is 10 mm2.

The installer should always be wary about choosing to use the steel wire armouring of a cable as a main protective bonding conductor. The armouring is commonly large enough to use as a circuit protective conductor but is rarely large enough to act as a protective bonding conductor. 

See the difference between earthing and bonding