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Answer:
Regulation 601-04-01 requires local supplementary bonding to be provided between the circuit protective conductor of each circuit supplying Class I and Class II equipment in zones 1,2 and 3 and extraneous conductive parts in the zones. These parts would normally include metal pipes supplying sinks and radiators, metallic baths and shower basins etc. Isolated items of metal work such as hand rails, metal window frames and items supplied with plastic pipes are not considered to be extraneous conductive parts and do not require bonding.
Supplementary bonding conductors must be sized in accordance with Regulations 547-03 and in most instances particularly where the conductors are protected within the fabric of the building, they can be less than 4mm2 in size.