Supplementary protective 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.

Information on supplentary bonding in a location containing a bath or shower can be found here:Bathroom Supplementary Bonding

The other type of supplementary bonding where conditions can't be met for automatic disconnection supplementary bonding is applied to limit the touch voltage between the point of utilisation and the Main Earth Terminal (MET) of the building to 50v. As the exposed and extraneous conductive parts should be bonded back to the MET, the maximum voltage apparent between surfaces during a fault should be 50v. Nowadays where the condition for a fuse/MCB to disconnect cannot be met the circuit may be backed up by an RCD (but not an RCBO) therefore negating the need for this type of supplementary bonding.