Lamp caps

a lampholder. The screw thread is the neutral which should never be at line voltage, and it
is possible to hold the bulb without touching the thread, so that there are two reasons why
there should not be any danger. Nevertheless, if the circuit wiring is of the wrong
polarity, which is not an entirely unknown fault to occur, and at the same time an
inexperienced person is clumsy with the bulb, there is a risk of shock. However, lamps
inserted into E14 and E25 lampholders to BS EN 60238 do not make contact with the

outer terminal until the last twist if the lamp. Therefore in this case, BS 7671 IEE Wiring
Regulations states that it is not necessary to connect the phase conductor to the centre
contact, although for conformity it is advisable that the centre contact is always
connected to the phase conductor. The bayonet cap is used as the standard in the UK up
to 150W, which is the largest bulb likely to be used in domestic premises, although 200W
lamps are available. Some continental countries use the Edison screw for all sizes.
The characteristics of incandescent lamps are shown in Figure 7.2. The graph shows
how very sensitive the life is to change of voltage. For example an increase of 2½ per
cent above normal voltage increases the efficiency by 2½ per cent and the light output by
7 per cent but reduces the life by 20 per cent. The graph also shows that for voltages
below normal the light output falls more rapidly than the voltage, which is something to
be borne in mind when one considers the voltage drop that can be accepted in the circuit
wiring to the lamps.