Q & A of the Day - Is there a need for equipotential bonding in this wooden clubhouse?

Question: My local bowling club has recently had some work done on the electrical and plumbing equipment fitted in the wooden clubhouse. The hut is fed from another building by a 2c SWA cable (about 50m). This is terminated into a consumer unit having two sections - one for lights and one for sockets/heaters. Each section of the board is protected with a 30mA RCD.

The water pipe entering the hut is plastic, but the kitchen plumbing is in copper - this is linked to the toilet block via plastic pipes, which then change back to copper in the two toilets. An electric heater is fitted in the toilets.

Does this installation need equipotential bonding?

Testing from the sink to the kettle chassis in the kitchen, we find that we have 900kohm. Testing from the pipes in the kitchen to earth, we have 500kohm. Should both copper piped areas have equipotential or supplementary bonding?

Answer: Chapter 6 of IEE Guidance Note 8 (Earthing & Bonding) details that where the designer elects to accept the let-go threshold of 10mA as a safe level and where the resistance between the conductive part concerned and the Main Earthing Terminal (MET) is above 22kohms, the conductive part need not be considered to be an extraneous-conductive-part which introduces an earth potential as defined in BS7671 and would therefore not require main protective bonding.

The results of your on-site testing between the metallic pipework in the kitchen and earth (500kohms) indicate that the water service pipework concerned need not be considered to be an extraneous-conductive-part, it would therefore not need to be bonded.

Supplementary bonding to a metallic kitchen sink is generally not required as a kitchen is not defined in BS 7671 as a location requiring additional fault protection. Furthermore, a metallic sink will almost certainly not, in its own right, be an extraneous-conductive-part.

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