Why metal pipework is bonded, what main and supplementary bonding do, and why a plumber must not remove it.
Why pipework matters electrically
Metal pipe can form part of a building’s protective bonding system or provide another route for electrical current. Cutting or removing it may interrupt that route, even though the plumbing work itself is sound.
Main protective bonding connects extraneous-conductive-parts to the main earthing terminal. These are parts outside the electrical installation that can introduce a potential, usually earth potential. An incoming metal service may qualify; a metal fitting does not qualify merely because it is metal. A plastic incoming service changes the assessment, but other earth connections must still be considered.
Bonding reduces dangerous voltage differences between connected parts a person could touch together. It does not promise that every part remains at zero volts or that a fault can never produce a dangerous touch voltage. Supplementary bonding is an additional local measure with its own requirements.
Before cutting or replacing pipe
Plan how the required electrical continuity will be maintained. A plastic insert can interrupt a previously continuous metal path. Existing bonding must not simply be removed or left disconnected; have the arrangement assessed and any required repair made and verified by a person competent in that electrical work.
Where the work requires a temporary continuity link, fit it across the intended break before opening the pipe, with effective contacts. It is not the finished permanent bond and does not replace the gas-work precautions or assessment of electrical hazards. Protective and bonding conductors can carry current, including diverted neutral current even when a property’s main switch is off. Treat evidence of dangerous voltage or current as a reason to stop and obtain the appropriate assistance.
What the CPC does
A Class I appliance’s circuit protective conductor provides the intended path for fault current, allowing its protective device to disconnect within the required time. It is not the circuit’s normal load return, but equipment leakage or functional currents can flow through it in normal operation.
A loose or broken CPC can leave a metal case dangerously energised under a fault. The appliance may continue working, so its operation gives no assurance that the protective path is sound. An RCD or an accidental parallel path does not excuse a missing required CPC.
Green-and-yellow identification shows the intended protective function. It does not prove the conductor has been correctly connected: continuity and the connections need verification. Electrical competence is required for the assessment and work; the plumbing repair alone does not certify electrical safety.