Time For an Electrical Refit
Wiring up plugs and changing fuses are among the simplest tasks in electrical work. However, the rules of safety are all-important. Electricity, the great unseen, should be the servant and not the master. The basics of fuses, plugs and sockets and the electrical ratings-the safe working limits-are described.
In the interests of safety it is essential that certain rules are followed whenup electrical equipment. Properly wired equipment also gives more reliable service. Electricity should be your ‘slave’. Just at the flick of a switch you can command instant light, heat, sound and vision. Improperly used it can also be a deadly enemy.
Plugs and sockets
Equipment to be used is connected into the house supply using plugs and sockets. Sockets must always be on the supply side. This system may be a ring main which uses square-pin plugs rated at 13A, or the older, round-pin plugs rated at 2A, 5A and 15A. These use unfused plugs, unlike the ring-main system, which makes the former more complex and less safe. For these reasons it is no longer used; all new houses employ the ring-main system.
Fuse boxes and fuses It was once common practice to have separate main switches and fuse boxes for each circuit-lighting, the cooker, the water heater, and, probably, several for power outlets. When extra circuits were added, a jumble of ill-assorted fittings and circuitry would result, producing the likelihood of hazard, since it was no longer clear which circuit was which and what electrical load was, consequently, being borne.
In modern practice, a consumer’s power-supply control unit, or consumer’s unit, is generally used. This is a compact fuse-box unit with fuse holders of the correct amperage for the appliance or circuit.
Permanent connection between switches and fuses is made by a solid busbar, made of copper, which is a broad metal strip able to handle a high current. A short bus-bar connects to a heavy neutral block; this is provided with a number ofholes and climbing for the neutral wires of each circuit. For the earth connections, a similar brass block is used.
It is most important to use a fuse of the correct rating, since the fuse is the deliberate ‘weak link’ in a circuit. Keep a supply of fuse wire near the consumer unit, or, in the case of cartridge fuses, an accessible box of these.
Another type of fuse box is called the miniature circuit breaker, or MCB. This has an automatic switch to control and to protect the installation. If too many appliances are switched on and overload the circuit, it automatically breaks contact and switches off. It is not then possible to re-engage the circuit until the fault is corrected or the circuit load is reduced to safe level.
The MCB can also be used to switch circuits in an installation, which enables these to be isolated at will.
The consumer’s unit should be located close to theauthority’s own meter. It is best mounted on a board to protect it from damp.
Cables are fed through into the consumer’s unit usually through ‘knockouts’, which, as the name suggests, are easily pushed-out sections. The wires should be insulated by rubber grommets located in these holes to prevent chafing. One entry hole is used for the circuit wiring and the other for the meter ‘tails’ which are connected to the company meter.
To connect up a consumer’s unit, cut back the sheath on the wires and connect the red cables to one side of the fuse. Firmly twist the wires with a pair of pliers, and then tighten these securely in place so that no dangerous stray wires, which could cause short circuiting, stick out.
There are several categories of fuse rating and these are colour coded on the front of the fuse holder. The colours are white, lighting; blue, electric fire; yellow, water heater; red, ring main; and green, cooker. Make sure also that the correct grade of cable is used and that this cor-responds with these fuse values.
When connecting the ring main, note that you have two sets of wires-the beginning and the end of the ring circuit.
It is a good idea to fix a label on to the consumer’s unit-these are often provided -to identify the individual circuits.
Finally, connect the main earth wire and meter tails. You need a length of about lm of green-covered 25mm – single-strand cable for the earth lead, and also lm each of red and black insulated 16mm – seven-strand cable for the tails.
Connect the green wire to the earthing block on the consumer’s unit and the red and the black wires into the bottom of the main switch. Take care to see that the polarity is correct; red = live and black = neutral. The tails and earth are then connected by the electricity authority into the company’s fuse box.
All wiring circuits in the house are fused. This includes a ring-main system which has a main circuit fuse for each ring as well as fused plugs. Fuse ratings are always given in amps. This indicates the maximum current it can sustain without failure.
New fuse systems use unpluggable, rewirable fuse carriers, or cartridge fuse carriers. Some employ earth-leakage detector, main-circuit breakers.
Older systems have wall-mounted re-wireable fuses with unscrewable safety covers. With the old system it is possible to get at the fuse without switching off the supply, so great care must be exercised when replacing fuse wire, to make sure that the mains supply is switched off at the main fuse box.
With the new system the mains has to be switched off before the fuse cover can be removed.
There are three ratings of fuse wire used for rewiring fuses, and these can often be bought with the three types on one card. The thicker the wire, the higher its rating; these are always clearly marked.
The three types are:
• 5 amp, for lighting circuits;
• 15 amp, for heating circuits and
• 30 amp, for power circuits. The last is the one used for ring-main systems.
Always use the correct rating when rewiring a fuse. If a fuse persistently fails, then-do not try rewiring with a higher rating. This condition indicates a fault in the circuit which must be corrected.
In a house which has a ring-main system, this only applies to the circuits carrying the three-pin wall-mounted sockets.
The lighting circuits are not part of the ring main, and are rewired using the 5A rating fuse wire.
10. November 2011 by admin
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