Automatic Disconnection of Supply (A.D.S.) and Equipotential Bonding

Before any additions or modifications can be carried out on an electrical installation, the Regulations require that certain safety features are present in the existing installation. The most fundamental of these are Automatic Disconnection of Supply (A.D.S.) and Equipotential Bonding.

Automatic Disconnection of Supply (A.D.S.)

In order for electrical current to flow, there has to be a voltage difference between two points, and a conductive path between them. In a cooker, for example, the voltage between the live and neutral wires is approximately 230 volts. The cooker element has a path through it for the electricity, and all the current flows through it to produce heat.

Consider what would happen if a live wire within the cooker was to come in contact with the outer case. The case of the cooker would then become live. A person standing on the ground (which is at 0 volts) touching the cooker would become part of the path from the 230V cooker case to the ground, and would get an electric shock.

In order to avoid this, the case of the cooker is deliberately connected to earth. If the case were to become live, the current flows along the earth wire. This fault would then be detected by the fuse or circuit breaker, which would interrupt the supply to the cooker and remove the hazard. This is known as Automatic Disconnection of Supply, or A.D.S.

Equipotential bonding

Most buildings have other services besides electricity, usually water and gas. These services are usually delivered in metal pipework. As metal is a conductive material, any fault where the pipework becomes live would be carried throughout the building. Likewise, a fault even in another building that is served by the same pipework could cause dangerous voltages to be transmitted to neighbouring properties.

Even if the pipework stays at 0 volts, a faulty appliance with a live case would be at a different voltage so a person touching the two would provide a path for the electricity, and would get an electric shock.

In order to protect against this, any pipework is electrically connected (bonded) together and also bonded to the earthing system of the electrical installation. This ensures that all connected metalwork (including metal cased appliances) is at the same voltage at all times, so no current can flow.

You can check to see if there is equipotential bonding within your property. The connection is made using a purpose-made clamp (as shown to the left), and should be within 600mm (2 ft) of the entry of the pipework to the property. It should be on the consumers side of the pipework (i.e. after the water stop tap, or gas meter). There should be a green and yellow cable connecting the clamp to the main earthing terminal (near to the electricity meter or fuseboard).

If it is missing, in poor condition or you cannot find it, then it is vital that you get it installed by a competent electrician. Bonding clamps are often removed during refurbishments etc because their importance is misunderstood.

More information can be found in the leaflet 'Why do earthing and bonding need to be checked?' downloadable from the Electrical Safety Council website.