It was once common to have separate main switches and fuse boxes for each circuit - lighting, the cooker, the immersion heater and, perhaps, a number for power outlets. When extra circuits were added, it was likely that an ill-assorted jumble of fittings and circuitry would result, making it unclear which circuit was which and what electrical load was being borne.

In modern practice, the compact consumer unit means that the fuse holders of correct amperage for the appliance or circuit are easily identifiable. The unit should be placed as close to the electricity meter as possible, and it is best mounted on a board to protect it from damp.

Cables are fed through the consumer unit usually through convenient 'knock-outs' - easily pushed out sections. The wires should be insulated by rubber grommets located in these holes to prevent the sheath covering the inner wire core from chaffing. One entry hole is used for the circuit wiring, and another for the meter 'tails', which are connected to your electricity board's meter.

Unless you are extremely confident about doing electrical work, this job should be left for a qualified electrician. In any case, you will have to have your unit inspected by the Electricity Board before it is connected to the mains.

Wiring a consumer unit is a professional job which should only be undertaken by experts and is not recommended for you to try for yourself. However, if you do decide that you do have the expertise to carry this work out yourself, after installing the new consumer unit, you MUST arrange for the Electricity Board to test the circuits and to connect the unit to the meter and the earth. Connection of the consumer unit to the meter and earth may only be done by your Electricity Board. Do not make these connections yourself under any circumstances.

New Consumer Units

• If you buy a new consumer unit, get one with MCBs (miniature circuit breakers) or cartridge fuses. Install a 20-amp fuse carrier or MC B for each heater circuit and a 15-amp fuse carrier or MCB for an immersion heater circuit.
• Do not mount the new consumer unit on the Electricity Boards' meter board even if there appears to be space for it.The new unit should be mounted on a separate board, close to the meter. Paint or varnish the mounting board on both sides to protect the unit from any dampness on the wall.

1 Mount the unit

The new consumer unit should be mounted on 18mm (Ain) plywood board. Cut the board to size, push through the 'knock-outs' in the back of the unit, lay the unit on the board and mark the position of the holes for the fuse ways, meter leads and earth wire. Drill 18mm (Ain) holes in the board for the cables. Screw the board to the wall using plastic insulators to space it and then fix the unit to the board.

2 Run the circuit cables to the unit

One at a time, run each circuit cable into the consumer unit and prepare the wires ready for connection. The circuits are wired in the same way to separate fuse ways with the red wire connected to the terminal on the fuse way. The black wire is connected to the neutral terminal block and the earth wire, which must be sheathed green/yellow, is connected to the earth block.

3 Cut back the sheath

To connect the cables to the consumer unit, cut back the sheath on the wires and connect the red cables to the terminal on the fuse carrier. Firmly twist the wires with a pair of pliers and tighten them securely in place so that no dangerous stray wires, which could cause short circuiting, are sticking out. Strip, twist and attach the black and yellow/green wires to their correct terminal points in the same way.

4 Meter leads

The meter leads use 16mm sq. single-core cable. The live conductor is insulated in red; the neutral is insulated in black. Both live and neutral single-core cables are also often sheathed in grey. Feed the meter leads into the consumer unit. Connect the red, live conductor to the live (L) terminal on the main isolating switch, and the black lead to the neutral (N) terminal on the main isolating switch. Connect 6mm sq. single-core cable to the earth block in the consumer unit. The other end must only be connected to the earth terminal by the Electricity Board. Do not under any circumstances attempt to make these connections yourself. Secure the cover onto the consumer unit and call your local Electricity Board, who will arrange to visit, test and connect the unit to your meter and to the Earth.