The ring main

In the exam you may have to interpret a diagram of the ring main, similar to this one.

The mains supply

The wiring in a house connects all appliances together in parallel. This is so that each appliance has the mains supply of 230 volts across it, and also so that they can all be switched on and off independently.

The consumer unit contains the main switch and the fuses for all of the fixed circuits, such as the power ring circuit and the lighting circuit.

The power sockets in a house are connected by means of a ring circuit. In a ring circuit the live, neutral and earth wires form a loop of cable going from the consumer unit to all of the sockets in turn and then back to the consumer unit.

• The live wire carries current to the house/appliance at a high voltage.
• The neutral wire completes the circuit and carries current away at low/zero voltage.
• The earth can carry current safely into the ground if a fault develops in a metal framed appliance.
• Many electrical appliances, including cookers and washing machines, have metal cases.
• The earth wire creates a safe route for the current to flow through if the live wire touches the casing.

If a live wire inside an appliance, such as a cooker, comes loose and touches the metal casing it could cause an electric shock. The earth terminal is connected to the metal casing, so in that instance the current would go through the earth wire instead of causing an electric shock. A strong current surges through the earth wire because it has a very low resistance. This breaks the fuse or trips the MCB and disconnects the appliance. Switches and fuses are placed into the live wire.

What are the advantages of using a looped parallel circuit?

• There are two paths for the current which means the cables can be thinner.
• Less current is carried in each part of the cable.
• Convenience – sockets can be placed anywhere on the ring, and therefore can be operated separately at a voltage of 230 V.

A looped parallel circuit