Why do regulation require a 10mm² main bond to the incoming water and gas supply pipes. ?
The reason may be to protect the CPC of a cable inside the house.
If the incoming water supply is metallic and the incoming Neutral rises above True Ground potential then current will flow through the circuit in the diagram. There is no fuse or MCB in the circuit. The current is determined by the potential difference between the incoming Neutral and the water main ( at True Ground ) and the impedance of the metallic water main. The smallest conductor in the circuit is the 1.5 mm² CPC in the 2.5 mm² ( or 4 mm² ) feed to the immersion heater.
With a water main impedance of 0.1 ohms and a potential difference of 10 volts the current will be in the order of 100 amps through the CPC.
One assumes the regulations have to cover every conceivable set of circumstance which could include a metallic supply pipe at 0.1 ohm and more than 10 volts difference.
Putting a lpw impedance cable that can safely (?) carry 200 amps from MET to Water Main would seem to be the solution that covers all eventualities in that the majority of the current flows through the 10 mm² bond cable and CPC does not have to carry the entire current.
A similar circuit exists from MET via a gas boiler and pipe work to both gas and water supply pipes
The reason may be to protect the CPC of a cable inside the house.
If the incoming water supply is metallic and the incoming Neutral rises above True Ground potential then current will flow through the circuit in the diagram. There is no fuse or MCB in the circuit. The current is determined by the potential difference between the incoming Neutral and the water main ( at True Ground ) and the impedance of the metallic water main. The smallest conductor in the circuit is the 1.5 mm² CPC in the 2.5 mm² ( or 4 mm² ) feed to the immersion heater.
With a water main impedance of 0.1 ohms and a potential difference of 10 volts the current will be in the order of 100 amps through the CPC.
One assumes the regulations have to cover every conceivable set of circumstance which could include a metallic supply pipe at 0.1 ohm and more than 10 volts difference.
Putting a lpw impedance cable that can safely (?) carry 200 amps from MET to Water Main would seem to be the solution that covers all eventualities in that the majority of the current flows through the 10 mm² bond cable and CPC does not have to carry the entire current.
A similar circuit exists from MET via a gas boiler and pipe work to both gas and water supply pipes