Supplementary equipotential bonding is applied to reinforce an equipotential zone.

This equipotential zone may be:

1. A special location where there is an increased risk from electric shock.
2. Where the conditions can't be met for automatic disconnection, see here Supplementary bonding for automatic disconnection problems

Supplementary bonding in a bathroom

Under the 17th edition regulations (BS7671:2008) supplementary bonding in a bathroom may be omitted where certain criteria are met. These criteria are as follows:

• All circuits comply with the requirements for disconnection times
• All circuits are RCD protected by a 30mA device maximum
• All extraneous conductive parts in the location are effectively connected to the protective equipotential bonding.

The disconnection times for a TN or a TT system should not be difficult to meet, especially as all circuits need to be RCD protected. The effectiveness of the bonding can be determined by measuring the resistance between the extraneous conductive part and the MET. The product of the resistance and the current required to disconnect any circuit within the location should not exceed 50v. Where an RCD is employed, the current required to automatically disconnect the circuit can be taken as the operating current of the RCD, i.e. 30mA. This means if the resistance between the MET and the extraneous conductive part does not exceed 1666Ω then it can be considered to be effectively bonded.
If the resistance exceeds 22KΩ then the part can be taken as not an extraneous conductive part therefore will not need bonding.
If the resistance is between 1666Ω and 22KΩ then supplementary bonding will need to be employed as it is an extraneous conductive part not effectively connected to the protective bonding.
If there are circuits within the location which are not RCD protected i.e. installed under the 16th edition then supplementary bonding is still required.

Where supplementary bonding is required the following bonding conductor sizes should be used as a minimum:

Note, if there are any 'exposed to extraneous' bonds present the 'exposed to exposed' bond sizes should be used in place of the 'extraneous to extraneous' bond sizes or whichever is larger.