If any plastic pipework or joint fittings are used, then the earth
bonding should by-pass it and be connected to the metalwork where any
valves or tap connections have been made to all appliances, this
includes all points on the water and gas distribution systems.

If plastic pipework (of 1m length or more) is used, then the taps/metal
bath/radiators etc. should NOT be supplementary bonded. Doing so reduces
electrical safety. If the internal pipework of the house is substantially
metal, but with a plastic supply (i.e. MDPE), then the internal pipework
should be main bonded.

Christian.