7.12.1 - Low voltage discharge lighting

The very high luminous efficiency of discharge lamps has led to their almost universal application for industrial and commercial premises; the introduction of low rated types as direct replacements for filament lamps is beginning to see their wider use in domestic situations.

Discharge lamps are those which produce light as a result of a discharge in a gas. Included are:

Fluorescent
Really low pressure mercury vapour lamps, very widely used for general lighting in homes, shops, offices, etc.

High pressure mercury
Provide a very intense lighting level for outside use in situations where the (sometimes) poor colour rendering is not important.

Low pressure sodium
The most efficient lamp of all, but its poor colour (orange) light output limits its use to street and road lighting

High pressure sodium
The acceptable golden light colour enables the lamp to he used for road and outside lighting in areas where better colour rendering is needed, as well as for large indoor industrial applications.

Discharge lamps, unlike their incandescent counterparts, require control gear in the form of chokes, ballasts, autotransformers and transformers. These devices result usually in a lagging power factor, which is corrected, at least partially, by connecting capacitance across the supply. This control gear should be positioned as close as possible to the lamps. Because of low power factor and the inductive/capacitive nature of the load, switches should be capable of breaking twice the rated current of a discharge lamp system, and maximum demand is calculated by using a multiplying factor of 1.8 {6.2.1}.

Electronic devices are becoming increasingly common to provide high voltage pulses to assist discharge lamps to strike (start). These pulses can cause problems with insulation breakdown in some types of cable, particularly low voltage mineral insulated types.