8.1 Do away with old rumours

Let us first tidy up an old rumour which has it that fluorescent lamps consume a vast lot of electricity during start-up or warm-up – nobody specifies this precisely – and should therefore rather be kept in operation instead of turning them off when not needed for a shorter period. This rumour refers to magnetic ballast operation, since it is older than the invention of electronic ballasts, and is, of course, a balderdash, while the conclusive advice is largely correct: How could you ever draw such a high current out of a properly designed and fused system that within a few seconds a substantial amount of energy comes to be consumed? But still, even senior experienced electricians propagate this misconception, even though already their apprentices should be able to calculate that this is impossible. In fact, during pre-heating, when nearly all of the lamp apart from the filaments is shorted, the current is about 35% higher than the rating. Yet this is almost entirely reactive current. The reactive power during pre-heat actually rises about 90% and during cold operation about 30% above that of normal operation. The active – and thereby costly – share of the power approaches its rated value only slowly from below (Fig. 8.1). The truth about the story is, however, that it is not economical to switch fluorescent lamps very often because this contributes much to their ageing. This ageing effect, though, depends very much on the preheating conditions (being optimal with electronic starters) and is with today’s high quality lamps often found to be of minor impact in practice than the theory of fluorescent lighting wants it.

Fig. 8.1: Power intake of a 58 W fluorescent lamp with class B1 magnetic ballast during inrush, cathode pre-heat and warm-up phases: Moderate reactive overcurrent, no excess active power above rating at all!