How The Electricity Mains Supply Works

The mains electricity supply in a house or flat is in two parts. First the electricity board’s apparatus comprising the service fuse unit, the meter to register the electricity consumed, and, where night storage heating is installed, a time switch. This apparatus is sealed and must not be tampered with. You are responsible for any damage to the apparatus. Second, the consumer’s installation which includes everything on the house side of the meter.

Your equipment at the mains consists of a double-pole main switch and one or more fuses.

In the modern installation the main switch and the circuit fuses are contained in a single consumer unit. In some houses, especially in rural areas, there is also an earth leakage circuit breaker (ELCB) though in the latest models of consumer unit an ELCB is fitted in place of the conventional main switch even where the circuit is also protected by conventional earthing. In older installations, instead of a single consumer unit there are a number of main switch and fuse units — one for lighting, another for socket outlets, another for the cooker circuit and so forth.

Even in the modern installation a main switch and fuse unit is installed for a shower unit or a cooker circuit where these are added after the main installation has been completed and there are no spare fuses in the consumer unit.

The electricity supply in Britain is standardised at 240 volts a.c., 50Hz single phase two-wire for dwellings. All appliances and lamps bought should be suitable for this supply. For certain heavy-load appliances such as electric boilers, a three phase supply may have to be used.

Extending an installation

As a consumer you are permitted to make any additions or alterations to your installation. You do not have to obtain permission from the electricity board or any other authority before work is started.

When a major extension or a major alteration to an installation is completed, such as a new ring circuit or a rewire you should notify the electricity board so that the board can test the wiring before it is connected with the mains. The wiring should conform to the current edition of the Institution of Electrical Engineers (IEE) Wiring Regulations although these are not mandatory. If the wiring conforms with the regulations the board must connect the new wiring upon receipt of the necessary completed application form.

The basic installation

An installation comprises a number of separate circuits each having a specific current rating according to the circuit function: for example, lighting, ring circuit, cooker, and shower unit. All circuits originate at the consumer unit.

The consumer unit contains several ways, one for each circuit, fitted with either a fuse unit or a miniature circuit breaker (MCB). The average size is six-way, but the tendency is to fit eight-way consumer units to allow for extensions.

Fuses or MCBs of different current ratings are colour coded: 5 amps (A) (white), 15A (blue), 20A (yellow), 30A (red) and 45A (green), but not all consumer units have facilities for a 45A fuse.

Lighting circuits

Lighting circuits are wired using two methods: (I) the loop-in, and (ii) the joint box, methods.

Loop-in method

With the loop-in method 1.0 or 1.5mm2 twin and earth PVC sheathed feed cable is run from a 5A fuseway to each lighting point in turn and terminates at the last point. From each light a separate cable is run to the respective switch. The wires at the light are jointed at the ceiling rose, this serving as a joint box.

Joint box method

With the joint box method the feed cable from the 5A fuseway is run to a series of 15A four-terminal plastic joint boxes, one for each light and its switch. From each joint box two cables are run; one to the light, the other to the switch.

Ring circuits

A ring circuit is a multi-outlet 30A circuit supplying an unrestricted number of 13A socket outlets and fused connection units in a floor area not exceeding 100m2.

A 2.5mm2 twin and earth PVC sheathed cable starts at a 30A fuseway, is looped in and out of each 13A outlet and returns to the same fuseway, so forming a ring.

Spur cables of the same size may branch off the ring, each supplying one single or one double 13A socket outlet or one fused connection unit. A ring circuit can have as many spurs as there are original outlets (sockets and fused connection units) on the ring.

Radial circuits

A radial circuit is any circuit not wired in a ring such as circuits supplying a cooker, immersion heater or 13A outlets.

A multi-outlet radial circuit supplying 13A socket outlets and fused connection units can be of either 20A or 30A current rating; using 2.5mm2 and 4mm= cable respectively, each may supply an unrestricted number of 13A sockets within a limited area — 20sq m and 50sq m respectively.