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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 mm
2
is laid down in
IEC 598, although this may be reduced to 0.4 mm
2
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
5
138
Fig. 132. Solid-core wire inside a
luminaire for fluorescent lamps. White
wires are used where the wiring is
visible from below.
Fig. 133. Flexible stranded wire inside
the pivoting base of a spotlight.
3.14 Electrical wiring