Ask the expert: To bond or not to bond?

How can you establish whether a conductive part is an extraneous-conductive-part?

The 15th Edition of the Wiring Regulations had us bonding most metal items that stood still; now one of the most misunderstood areas of protective bonding is deciding whether a conductive part is actually an extraneous-conductive-part. The presence of metal alone does not automatically mean that bonding is required.

BS 7671 defines an extraneous-conductive-part as a conductive part that is liable to introduce a potential, generally Earth potential, and which does not form part of the electrical installation.

That definition is important. A metallic water pipe, structural steelwork or heating system may be conductive, but the real question is whether it can introduce Earth potential into the installation.

Consider a water service supplied by a metallic service pipe buried in the ground. That pipework is likely to have a connection with the general mass of Earth and would therefore normally be considered extraneous. However, if the incoming water service is plastic and the internal pipework changes to copper after entering the building, the copper pipework should not automatically be treated as extraneous. Unless another connection with Earth exists, it may simply be isolated metalwork.

Start with inspection

Visual inspection should therefore be the starting point. Establish how the service enters the building and identify any insulating sections, plastic fittings or other possible connections with Earth.

Where the situation remains uncertain, resistance measurement can help. Industry guidance commonly uses a measurement between the suspected conductive part and the MET, with 22 kΩ frequently quoted as an indicative value based on a 10 mA body-current criterion.

However, electricians should avoid treating 22 kΩ as a simple pass or fail figure. It is also important at this point to confirm the correct test equipment is being used. Most multifunction testers will not have a suitable scale to reflect this result; therefore, additional test equipment may be needed.

A low resistance reading does not necessarily prove that the metalwork is extraneous. For example, a copper heating pipe connected to a boiler may show very low resistance to the MET through the boiler CPC. The test has then identified a connection through the electrical installation rather than a genuine connection with Earth.

Parallel paths through CPCs, boilers, pumps, existing bonding and connected equipment must therefore be considered when interpreting results.

Conclusion

The fundamental question remains: Can this conductive part independently introduce Earth potential into the installation?

Protective bonding is intended to limit dangerous potential differences. It is not about bonding every piece of metal that happens to be present. Good inspection, appropriate testing and engineering judgement are therefore essential before reaching for the bonding conductor.

cef.co.uk

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This article appeared in Electrotechnical News September/October 2026 edition – read it here