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Types of electrical circuits
There are three main types of circuits encountered in a domestic situation. They are
ring circuits, radial circuits and lighting circuits.
Ring circuits
Most modern socket circuits are ring circuits or ring mains as they are sometimes referred
to. A cable leaves the consumer unit and travels to each socket on the main and when it reaches
the last socket it then returns to the consumer unit, thus creating a ring. The advantage of this system is that power can reach the sockets in
the circuit from both directions, which reduces the power load on the cables.
A ring circuit
can serve an area up to 110 square metres (120 square yards), 2.5mm2 cable is used
to wire the circuit and the circuit has a 30amp fuse or 32amp MCB on the consumer unit. It is
usual for a house to have one ring circuit upstairs and one ring circuit downstairs.
Ring circuits can have extra sockets added to them by adding a 'spur' onto a ring circuit. A
spur is a branch off the ring circuit, usually from an existing circuit, although a
junction box could also be used. Theoretically as many spurs as sockets could be added, but
the maximum load of the circuit (30/32amp) still exists).
With radial circuits the cable comes from the consumer unit and travels to each socket, simlar
to the ring circuit. However when the circuit reaches the last socket the cable ends, whereas a
ring main travels back to the consumer unit.
Radial circuits can therefore only serve
a smaller area. Using 2.5mm2 cable combined with a 20amp fuse/MCB an area of 20 square
metres (24 square yards) is permissible. For 4mm2 cable combined with a 32amp
MCB or a 30amp cartridge fuse (a re-wirable fuse is not allowed) an area of 50 square metres
(60 square yards) is permissible.
In a similar way to ring circuits spurs can be added at
points along the radial circuit if required. High powered appliances (cookers / showers) must
have their own radial circuit.
Lighting circuits are basically radial circuits. There are two distinct types of lighting,
circuit the loop-in circuit and the older junction box circuit. Most houses combine aspects
of both types of circuits. The loop-in circuit has a cable, running from light to
light terminating at the last light as in the conventional radial circuits and then single
cable run from the lights to the light switches.
The other type of lighting circuit has a
junction box for each light. The cable runs from the consumer unit to the first junction box
and then onto the next terminating at the last junction box. Then another cable is run from each
junction box to its light and another wire from the junction box to that light switch.
The cable used for a lighting circuit is 1mm2 or 1.5mm2 for long runs.
A 5amp fuse or 6amp MCB is used on the consumer unit for a lighting circuit. The maximum
load for a lighting circuit is 1200 watts, which amounts to 12 x 100 watt lights. If more
lights are needed then another lighting circuit should be used. It is usual to
have 2 lighting circuits in a house one upstairs and the other downstairs.
