Lighting Circuits and Socket Outlets

Lighting Circuits and Socket Outlets Points : Lighting Circuits and Socket Outlets, Incandescent Lamp Circuits, Number of Points per Circuit, Practical Advice on Wiring, Common Mistakes in Installation, This section deals with a selection of final sub-circuits rated at not more than 15 amps. The planning and arrangement of these circuits, the number of points per circuit, the current rating and the cables required are described in detail. Incandescent Lamp Circuits Current Rating No cable or flexible cord must have a current rating less than that of the fuse which protects it. This means that if the protective fuse of a lighting circuit is 15 amps for example, then all the cables and cords in that circuit must be rated at 15 amps or more i.e. the cables would have to be at least 7/.029 and the cords at least 70/.0076.

If the Lighting circuit in a house was rated at 15 amps, then these heavy cables and cords would have to be fitted throughout in order to comply with the above Regulation. This would mean increased expenditure with no particular advantage.

Number of Points per Circuit Once the rating of a particular lighting circuit has been decided, it only remains fin1 the maximum number of lighting points that can be connected to it. Generally with incandescent lamps, the wattage of the lamps is not specified by the consumed if normal illumination is required, and so it can be assumed by the electrical engineer that 100 watts will be supplied by each lamp-holder.

Now the wattage of an incandescent lamp = current x supply voltage. Thus on a final sub-circuit, of 5 amp rating, with a 230 volt supply, the maximum number of light points that can be connected.

= Current rating of sub circuit X Supply voltage/Wattage of lamps to be connected

= 5 x 230/100 = 11.5

In actual practice, not more than 10 incandescent light points should be supplied by one final sub-circuit, for normal illumination purposes. If however, special high wattage lighting is required, the consumer should always specify the wattage of the bulbs to be fitted. For example, assuming the circuit is rated at 5 amps, the supply voltage is 230 volts, and there are 150 watt incandescent lamps to be connected, then the maximum number of lamps per circuit is

5 x 230/100 = 11.5

Alternatively, a consumer could specify tha the wanted only two lighting circuits for, a special installation, each with ten 150 watt incandescent lamps connected. In this case, the current rating of each circuit would be 10 x 150/230 = 6.5 amps i.e. each circuit would have a fuse rating of 10 amp and be wired with 31.036 cable. Practical Advice on Wiring However small the premises it is recommended that at least two lighting circuits be provided. This avoids the whole building being plunged into darkness when a fuse blows. A general guide is that there should be one lighting circuit for every l000sq.ft of floor area, Switches controlling lights in bathrooms should always be installed outside the bathroom, preferably just outside the bathroom door for convenience.

When flexible cord is used in a lamp pendant, the weight supported by the cord should not be greater than the values given in table at the back of this Manual. The weight should not be taken directly by the cable terminations, but some form of cord clamp should be provided in the lamp holder itself so that the weight is taken by the whole cord. Common Mistakes in Installation One quite common mistake occurs when a lighting extension is required in a factory or commercial building. Two cables are attached temporarily to the nearest switch fuse on the distribution board and run to their destination. More often than not, this switch fuse controls a power circuit and the fuses are 15 amp, whereas the cables used to the lighting extension are probably 3/0.29.

The mere fact that the fuse rating is larger than the rating of the cable it is supposed to be protecting is not even realized, or if it is, it is ignored. It can be seen how dangerous it is to ignore LE.E Regulation 108 from the following example: A factory lighting circuit, wired with 7/.029 cables, is protected by 15 amps fuses. If a pendant consisting of 231.0076 flexible cord is connected to this circuit and a Normal tungsten filament bulb fitted, there is no trouble. Unfortunately an electrical installation is often abused, especially by non-technical persons, and it is not uncommon for-.apparatus taking a large current to be connected to the lighting circuit.

Assume a 3 kW heater taking approximately 15 amps is plugged by means of an adaptor into the light socket. A current of 15 amps promptly flows through the flexible cord which is rated at 5 amps. The cord then gets dangerously hot and could easily cause a fire. No protection would be given by the circuit fuse since it would not blow until a current of about 30 amps flowed. Obviously precautions must be taken against this sort of thing happening, and it is recommended that at leas 701.0076, if not 110/.0076, flexible cords should be fitted at every point when the circuit is protected by 15 amp fuses. Another quite common mistake occurs when 3 plate ceiling roses arc used. Fig shows a typical wiring diagram using this type of ceiling rose and it can be seen that the live conductor is looped at the third terminal. When the cables are run in this way, it is recommended that the ceiling roses should be of the shrouded type and not of the type with all the terminals exposed.