Supplementary protective bonding is applied to reinforce an equipotential zone.

This equipotential zone may be:
  - A special location where there is an increased risk from electric shock.
  - 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 electronic RCD such as an RCBO - see note) therefore negating the need for this type of supplementary bonding.

Note:
In TN systems RCBOs should operate in overcurrent mode for fault protection. In overcurrent mode they are voltage independent, whereas in RCD mode they are not, and they obtain the voltage they need to supply the electronics inside from the supply circuit. The IEC stipulate that voltage dependent RCDs should operate at voltages as low as 50V, so that if the voltage collapses in a fault condition they will still work. But if the neutral is lost it is possible that an RCBO will not operate in RCD mode with an earth fault. So the design for a TN system is supposed to ensure that RCBOs operate in overcurrent mode for fault protection, and for this reason the earth fault loop impedence restrictions are the same as for an MCB, and a Type C or D RCBO may not provide a 0.4s disconnection time.