Why metal pipework is bonded, what main and supplementary bonding do, and why a plumber must not remove it.
Plumbers work on metal pipework that can be tied into a building electrical safety system, so a basic grasp of earthing and bonding is essential, even though the electrical installation work itself is for a competent electrician.
Bonding means connecting metal parts together with a protective conductor so they all sit at the same electrical potential. If everything touchable is at the same potential, there is no dangerous voltage between two things a person might touch at once.
Main (protective) bonding connects the incoming metal services, such as metal water and gas pipes where required, to the building main earthing terminal. Supplementary bonding additionally links exposed and extraneous metal parts together in higher-risk locations.
The plumbing trap here is breaking a bond without realising. If you cut into bonded metal pipework, remove a section, or insert a non-conducting component, you can interrupt the bonding or an earth path that was relying on that metal.
A particular case is replacing metal pipe with plastic. Plastic does not conduct, so a length of plastic in what used to be a continuous metal run can break the electrical continuity that bonding depended on.
The rule for a plumber is never to leave bonding worse than you found it: maintain continuity, refit bonding clamps correctly, and where electrical work or assessment is needed, bring in a competent electrician. Electrical installation work in dwellings is controlled under Approved Document P; confirm bonding requirements against the current standards.
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