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

Superceded by the 17th edition regulations, see here Supplementary equipotential bonding (17th edn regulations)

Supplementary bonding in a bathroom
The most common type of supplementary bonding joins together the exposed and extraneous conductive parts within a special location such as a bathroom.
The objective of this is to re-inforce the local equipotential zone, keeping those metalic items within the location which have the ability to inject a potential, at or very close to the same potential. The bonding can be carried out in close proximity to the bathroom i.e. in an adjacent airing cupboard if required. This type of supplementary bonding is done localy to the location and does not connect to the Main Earth Terminal (MET).

The theory behind this type of bonding is if a person who has lower body resistance due to wet skin and lack of clothing who simultaneously comes into contact with two extraneous/exposed parts can't form a circuit between the two with enough current flowing to cause harm.
Only metalic parts which can introduce a potential into the equipotential zone require supplementary bonding. These include metalic pipework such as hot/cold water, radiator, metalic building structure and the CPCs of any circuits supplying class 1 (earthed metalic) or class 2 (double insulated) equipment within the zone. SELV lights do not require supplementary bonded. The reason behind bonding the CPC of a circuit supplying class 2 equipment is in case the piece of equipment should ever be changed for a piece of class 1 equipment in the future. Items supplied only by plastic pipework are not extraneous and do not require bonding unless they are in contact with the building structure.

Another example of an equipotential zone could be looked upon as a Faraday cage. If a Faraday cage is suspended above the ground on insulators so it is not in contact with the general mass of earth, a person put inside the cage and the cage then livened up to 230v above the potential of true earth, the person in the cage will be able to walk around the cage without receiving a shock. This is because all of the metalwork is at the same potential hence no current can flow through the person. If a metallic object is introduced into the cage it needs to be in contact with the cage in order for it to be at the same potential and for the person to remain safe (equipotential bonding). If a metallic item is introduced into the cage which is not in contact with the metal cage but is in contact with true earth the result is a person coming into contact it will receive a 230v shock.

Further information can be found here: Link to IEE document and Niceic link for bonding -nice diagrams

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.