Fig. 132. Solid-core wire inside a luminaire for fluorescent lamps. White wires are used where the wiring is visible from below.
The diameter (or rather the cross-sectional area) of the wire must be matched to the strength of the current flowing through it.A wire whose area is too small has a resistance that is too high and it will become warm, the resulting heat loss reducing the efficiency of the luminaire. A minimum nominal cross-sectional area of 0.5 mm2 is laid down in IEC 598, although this may be reduced to 0.4 mm2 in certain cases where space for internal wiring is severely restricted
(see IEC 598, section 5.3.1).
Of particular importance with regard to insulation material and thickness is, of course, its temperature resistance. Here it must be borne in mind that it is not only the temperature of the air in the luminaire that matters, but also that of components with which the insulation may come in contact, such as ballast and lampholders.The insulation of the wire used must be resistant to all such temperatures, not only under normal conditions of operation, but also in the presence of a fault condition.
Not all sorts of insulation are suitable for use in luminaires. For example, simple PVC (polyvinyl chloride) insulation is only heat-resistant up to 90 ºC. It contains a softener, which can vaporise, making the insulation brittle and therefore prone to damage. Moreover, the evaporated softening agent attacks a number of plastics used in the manufacture of luminaire housings.There is, however, an inexpensive PVC insulation that is heat-resistant up to a temperature of 105 ºC, and which is safe in this respect.Where temperatures in excess of 105 ºC can arise, yet another kind of PVC insulation is usually employed, one that is resistant up to 115 ºC.
Where still higher temperatures may be encountered, as in floodlights for example, siliconerubber (170 ºC to 200 ºC) and PTFE (polytetrafluorothene) (250 ºC) insulating materials are available. Extra
Fig. 133. Flexible stranded wire inside the pivoting base of a spotlight.
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