4.2 Susceptibility to disturbances

This is a short chapter. Fluorescent lamps operated with magnetic ballasts are almost entirely unsusceptible to commonplace network disturbances. The high inductance connected in series with the lamp suppresses surges, peaks and harmonics, i. e. if the likes of those are present in the line voltage they will be able to drive only a fraction of the proportional current through the lamp.

What may cause trouble – not really damages but flickering of the light – are voltage dips. Especially if these cover more or less one semi wave, during this semi wave the current will drop over-proportionally with the voltage dip: Since current starts to drop during the voltage dip, voltage drop across the lamp rises (Fig. 2.1) and leads to an acceleration and amplification of the current decrease. Subsequently, the full flux density of a normal current peak is by far not reached, which turns the next (opposite) current semi-wave into a milder form of starting process with excess current peak (Fig. 4.5) if the voltage is normal during that next semi wave. This way a positive flicker may occur, i. e. excess brightness above normal peak value, even though the voltage is normal during this particular semi wave and has even been sagging the semi-wave before. If the inrush current or other current peak caused by some nearby device (Fig. 4.4) happens to hit more or less equal parts of two subsequent semi-waves, only a normal current sag with consequential brightness sag occurs, but also slightly amplified beyond the proportional magnitude because of the non-linear behaviour (Fig. 4.6).

Fig. 4.4: Test configuration to provoke a voltage sag

Fig. 4.5: Current peak caused by an asymmetric voltage dip

Fig. 4.6: Current dip caused by an approximately symmetric voltage dip

Basically the same occurs when DC impact causes a slight voltage asymmetry in the network. Old hair dryers, when operated at half power, normally use only one semi wave, and when the network resistance is high, a fluorescent lamp with magnetic ballast operated on the same circuit may flicker visibly. After all a measurement showed that a direct voltage content of 6 V, representing 2.7% of the line voltage rating, caused a direct current of 92 mA to flow through a ballast and lamp circuit, representing 18.1% of the rated lamp current.

The »negative resistance« of the lamp also leads to an overproportional variance of brightness with deviation from rated voltage, while electronic ballasts including compact fluorescent lamps (CFL) claim to compensate this by means of their electronic control. What remains left of this promise will be discussed in section 7.2. Still, the loss of brightness at undervoltage is a lot less than with incandescent lamps, the efficiencies of which, performing poorly anyway, drop dramatically when operated below the rated power input.

But this is virtually all that may happen with magnetic ballasts. Adequate means to reduce the voltage flicker – and over-dimensioning of cables and especially transformers is in many cases enough to achieve this – just need to be provided, which will be necessary for other sensitive devices anyway and as a second effect reduce energy losses. Damages or failures of lamps or ballasts on account of poor power quality do not occur.