Earthing and Bonding in a Timber House with a Plastic Incoming Main
A detached timber-framed house in St. Thomas has a plastic incoming water main and a plastic incoming gas service pipe. The consumer unit is at the front of the house; the metallic water installation inside the property includes about 12 m of copper pipe feeding bathrooms and a kitchen. A metallic gas pipe runs 6 m inside the property to the boiler. The electrician proposes to bond both internal metallic pipes to the main earthing terminal with 6 mm² cable, and states that because the incoming services are plastic, no main protective bonding is required at all.
1. Evaluate the electrician's statement that no main protective bonding is required.
Read the worked answer
The statement is only partly right and is dangerously over-simplified.
Main protective bonding is required for extraneous-conductive-parts - metalwork that is not part of the electrical installation but is liable to introduce a potential, generally earth potential, into the installation. An incoming metallic service pipe is the classic example, but it is not the only one.
Where the incoming water and gas mains are plastic, the metallic pipework inside the property may still be an extraneous-conductive-part if it is in contact with earth - for example, where copper pipes are buried in or in contact with the ground, or where they are bonded to other earthed metalwork. The correct approach is to test, not assume: measure the resistance from the pipe to the main earthing terminal or to a known earth. If the measured resistance is below the threshold used to identify an extraneous-conductive-part (commonly 23 kilohms when tested at a suitably low voltage), the pipe must be main-bonded.
If the internal pipework is genuinely isolated from earth by the plastic main and by insulating sections, bonding is not required - but that conclusion must be recorded with test evidence, not asserted.
2. Assuming bonding is required, specify the conductor size and connection points, and explain the risk of using 6 mm² for both pipes.
Read the worked answer
Main protective bonding conductor size depends on the cross-sectional area of the supply neutral conductor or the earthing conductor, whichever is relevant. For a typical single-phase domestic supply with a neutral of 25 mm² or less, the minimum main protective bonding conductor is 10 mm²; where the neutral exceeds 35 mm², 16 mm² is required. A 6 mm² bonding conductor is therefore likely to be undersized for main protective bonding, though 6 mm² is the minimum for supplementary bonding.
Connection points must be at the point of entry to the premises, or as close as practicable to it, on the consumer's side of any insulating section, and before any branch. Each connection must be made with a permanent, reliable clamp of a type suitable for the pipe material - a brass clamp on copper is fine, but a clamp designed for copper must not be used on plastic or on aluminium without checking compatibility. The bonding conductor must be continuous, run to the main earthing terminal, and be protected against mechanical damage and corrosion.
Using 6 mm² risks the bonding conductor overheating and failing under fault current, leaving pipework at a dangerous potential. It also fails to provide the low-impedance path the bonding is intended to create, undermining the disconnection times relied on elsewhere in the installation.