Electricity generated in the power station comes into your home at 240 volts and is transmitted by a service cable which terminates in a sealed unit containing the Electricity Board’s service fuse. This is generally rated at 100 amps, but 60 or 80 amps is possible in older houses. This fuse provides a last defence in the unlikely event that your own fuses or M.C.B.s (miniature circuit breakers) fail to operate.
From the sealed fuse unit, the current passes through a meter, which records the amount used, and flows on through a consumer unit. This incorporates the fuses or M.C.B.s for all the house circuits. The cables of these circuits convey the current from the consumer unit (or, in older installations, the fuse board) to lights, switches and socket outlets. Each circuit has a specific current rating determined by the maximum load it is expected to carry. There are four standard current ratings—5 amps for lighting, and 15, 20 and 30 amps for power. A fifth rating—45 amps—has recently been introduced for “electricaire” central heating units and large electric cookers. Each of the two principal types of circuit— lighting and power—may be based on one or other of two wiring systems.
• RING FINAL CIRCUIT WIRING. In houses wired since 1947, most or all of the power is supplied by a ring final circuit— so called because the circuit is wired in the form of a continuous ring starting and ending at a single 30 amp fuse or M.C.B. in the consumer unit.
A ring final circuit can supply any number of 13 amp socket outlets and fixed appliances, provided only that the total floor area covered by the circuit does not exceed 100 square metres. In addition, branch cables called spurs may be run from the ring cable to supply socket outlets in remote positions—for example, a loft or basement. The number of spurs must not exceed the number of socket outlets and fixed appliances connected directly to the ring, and each spur may only supply one single or one double outlet. A spur is connected to the ring cable at the terminals of a ring socket outlet or at a 30 amp joint box inserted into the ring.
RADIAL WIRING. As with a ring final circuit, the function of a radial power circuit is to supply power for a number of 13 amp socket outlets and fixed appliances. However, whereas a ring final circuit terminates back at the consumer unit, a radial circuit terminates at the last outlet. Furthermore, the maximum floor area that may be covered by a radial circuit is much less than that for a ring final circuit. A radial circuit of 20 amp current rating may supply an unlimited number of outlets and appliances within an area of 20 square metres, and one of 30 amp rating may supply an unlimited number within an area of 50 square metres.
Generally, radial circuits are best used to supplement ring final circuits, particularly in a kitchen with a large range of electrical appliances. However, an appliance with a load in excess of 3,000 watts (3kW) should be wired on its own individual circuit protected at the consumer unit by a 30 or 45 amp fuse or M.C.B. In fact, all power circuits installed before 1947 were wired to just one socket outlet or fixed appliance apiece. The outlets were designed to take round-pin plugs of 2, 5 or 15 amps—the then standard current ratings— and the plugs did not contain their own fuses. If you have round-pin outlets and the circuits run from a number of separate fuse boxes, each with its own mains switch, then you can be fairly certain that the wiring is more than 55 years old. In that case, you should have the wiring checked as soon as possible, since it will certainly have deteriorated and may even be dangerous. Indeed, the electricity boards have declared that “any house over 25 years old may have dangerous wiring”.
• JOINT-BOX WIRING. In many houses built before 1966, the lighting circuit runs from a 5 amp fuse in the consumer unit to a series of joint boxes fixed between ceiling ioists. From each box run two more ;ables—one to the corresponding switch, the other to the ceiling rose.
• LOOP-IN WIRING. A modern lighting installation is most likely to use the loop-in system—so called because the supply cable runs direct from one ceiling rose to another Until all the roses are linked, or looped in, together. One cable is run from each ceiling rose to its corresponding switch.
A more Modern wiring. In a house with ring final circuit wiring, all the circuits run from a consumer unit with a single mains switch. Socket outlets are designed to take square-pin plugs of 13 amps and are connected by a continuous ring of cable. In most cases, one ring serves the upstairs socket outlets, and a second supplies those on the ground floor. In the same way, each floor is usually served by its own lighting circuit running direct from one ceiling rose to another. A cable goes from each ceiling rose to its corresponding light switch.
Very old wiring
In a house with pre-1947 radial wiring, the circuits run from a number of separate fuse boxes, each with its own mains switch. All the power circuits are connected to one socket outlet or one fixed appliance apiece, and the socket outlets are designed to take round-pin plugs of 2, 5 or 15 amps. The lighting circuits— usually one for each floor—run to joint boxes fixed between the ceiling joists. From each joint box one cable goes to the corresponding light switch and another goes to the ceiling rose.