Power circuits (page 1)

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The electricity for socket outlets is supplied by either of two types of circuit - ring main or radial circuit.

Ring main

This is the most usual type of circuit nowadays. A 2.5 mm sq. twin and earth cable runs from the consumer unit to the first outlet. From there it continues to the next and so on until the last outlet. From here, the cable continues back to the consumer unit once more where it connects to the same terminals as the cable at the beginning. This may seem odd, but in fact it provides for a reduced loading on the cables. The circuit is protected by a 30 amp fuse or a 32 amp MCB (miniature circuit breaker) In a house, there is usually one ring main for the downstairs outlets and one for the upstairs ones. As with all electrical installations, there are rules governing these circuits. Be sure you know the regulations before attempting any work. These include the following:

A ring may serve up to 100 m sq of floor area and, in theory, may have any number of sockets outlets or fused connection units directly connected to it. There is a limit to the length of cable permissible in a ring circuit. This is 60 metres for circuits protected by a cartridge fuse and 50 metres for circuits protected by an MCB.

Spurs

Spurs are extensions from a ring main using a single length of 2.5 mm sq. twin and earth cable. They may be connected via one of the socket outlets on the ring, or via a junction box at some point in the ring. The junction box method obviously has the advantage that it can be used to connect a spur at any point in the ring. Each spur may serve only one socket outlet or fused connection. The total number of spurs permissible on a ring is equal to the number of socket outlets on the original ring. Note that a double socket outlet counts as two for this purpose.

Continued...

• The mains supply
• Lighting circuits
• New sockets