Protective Bonding (AM2 Exam)
Bonding is designed to limit the voltage level of any accessible electrically conductive parts in order to reduce the risk of electrical shock.
Main bonding is the electrical interconnection between the main earthing terminal and the incoming metallic services (water, gas or oil pipes) plus any other extraneous conductive parts of a building or an installation.
On the AM2 exam you will be presented with TN-C-S earthing arrangements.
According to regulation 544.1.1 and Table 54.8 of the IEE Wiring Regulations (BS7671 17th edition) the required size of the main bonding conductor is 10mm2 for supplies with a Neutral conductor of 35mm2 or less.
Because of this you will be required to use 10mm2 single core earth cable to connect both the incoming water pipe and the incoming gas pipe to the main earthing terminal inside the CU.