Effects of Voltage on the Performance of Lamps

Naturally, all lamps are designed to produce their rated luminous output at their rated voltage. An over-voltage on an incandescent lamp produces a brighter and whiter light because the filament temperature is increased. Its operating life is, however, drastically reduced. A mere 5% increase over its rated voltage will reduce the lamp life by 50%. Conversely, reduction in the voltage will increase the operating life of a GLS lamp but it produces a duller, reddish light. Lamps rated at 240V are often used in a ship's lighting system operating at 220V. This 'under-running' should more than double the lamp life.

Similar effects on light output and operating life apply to discharge lamps but if the supply voltage is drastically reduced (below 50%), the arc discharge ceases and they will not re-strike until the voltage is raised to nearly its normal value. Fluorescent tubes will begin to flicker noticeably as the voltage is reduced below their rated value.

The normal sinusoidal a.c. voltage waveform causes discharge lamps to extinguish at the end of every half cycle, i.e. every 10ms at 50Hz or every 8.33ms at 60Hz. Although this rapid light fluctuation is not detectable by the human eye, it can cause a stroboscopic effect whereby rotating machinery in the vicinity of discharge lamps may appear to be stationary or rotating slowly, which could be dangerous to operators.

There are a few methods to alleviate a stroboscopic problem. A few of them are:

(i) Use a mixture of incandescent and discharge lighting in the same area.

(ii) Use twin discharge lamp fittings with each lamp wired as a 'lead-lag' circuit, e.g. the lamp currents are phase-displaced so that they go through zero at different times; hence the net light output is never fully extinguished.

(iii) Where a 3-phase supply is available, connect adjacent discharge lamps to different phases (e.g. red, yellow and blue) so that the light in a given area is never extinguished.