Basic lighting circuits

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Of all the DIY jobs you can do in your home, tackling electrics seems to hold the most fears for the do-it-yourselfer. The truth is, carrying out electrical work is quite straightforward, so long as you are clear what you are doing and work carefully and methodically. Modern electrical wiring equipment has never been so easy to install or maintain. This leaflet explains how different power circuits work and how to recognise them.

Always consider 'safety' before you start any DIY project. Look for the 'Be Safe' sign and read the safety advice.

Power circuits

Your home probably contains several different power circuits. Some will supply wall-mounted socket outlets (also known as power points) into which you plug all sorts of small or portable electrical appliances. How these circuits are wired up will depend on the age of your house or on whether it has been rewired since it was first built. Other circuits will supply individual large appliances such as cookers, showers and immersion heaters.

Ring circuits

Houses built (or rewired) since the 1960s have their socket outlets supplied by one or more ring circuits. The circuit cable starts and finishes at the consumer unit, allowing current to flow round the circuit in either direction and so increasing its capacity. The circuit can supply an unlimited number of 13-amp socket outlets, for plugs with rectangular pins. Each circuit can supply rooms with a total floor area of up to 100sq m (1075sq ft), and is protected by a 30-amp fuse or circuit breaker in the main consumer unit..

Radial circuits

Before the ring circuit was introduced, each socket outlet was supplied by an individual cable running from the house fusebox. The outlets had round holes which accepted plugs with round pins. These are now obsolete and should be replaced. Modern homes may have radial power circuits supplying an unlimited number of 13-amp socket outlets. These are used if a ring circuit would be wasteful of cable. Each can supply rooms with a floor area of up to 20sq m (215sq ft) if protected by a 20-amp fuse or circuit breaker.

Combined system

Both ring circuits and modern radial circuits can supply additional socket outlets wired up as spurs or branch lines off the main circuit. This economises on the amount of cable needed to serve the main circuit, since only a single length of cable connects the spur socket to the circuit. Spur cables can be connected to the main circuit at an existing socket outlet, or by means of a junction box wired into the main circuit. Each spur can supply one single or one double socket outlet, and the circuit can have as many spurs as there are outlets on the main circuit.

Switched spurs

In fitted kitchens, slide-in appliances such as fridges, dishwashers and washing machines are often plugged into unswitched socket outlets located at the rear of the appliance recess. The socket is wired as a spur, with the cable running via a switch on the wall above the worktop which controls the appliance.

Appliance circuits

Power circuits supplying individual appliances are wired as radial circuits, with the cable running from an appropriately rated fuse or circuit breaker in the consumer unit to an isolating switch and then to the appliance. You are likely to have one circuit rated at 15 amps serving the immersion heater in the hot water cylinder, and circuits rated at 30 , 40 or even 45 amps supplying electric cookers and an instantaneous shower.

Identifying old wiring

If your house still has socket outlets that take round-pin plugs, or has any circuit wiring using rubber-sheathed and insulated cable, the electrical system should be professionally inspected without delay. You can check for the presence of old cables at the main fuse box, where they enter the unit.

If you suspect that your wiring may be unsafe or you notice anything unusual call in a professional electrician immediately.