Philips Electromagnetic Lamp manual 147, Non-standard conditions, Short-circuiting of the lamp

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3.18 Circuit breakers, fusing and earth leakage

Information on what lighting load a certain MCB can handle may be given by the MCB supplier, provided information about the cabling lay-out, lamp type and circuit is available.As a guide, a practical value for the figure (1) of the 10 A MCB type C represents 1500 W lighting load with the conventional gear.

2 Non-standard conditions

A fluorescent lamp circuit normally consists of four parts: lamp, ballast, starter and compensating capacitor.

The effects of short-circuiting one of these parts are:

1) Short-circuiting of the lamp

This has been described above: in the inductive circuit the mains current will be approx. 1.5 times the nominal value, which means an extra temperature rise of the ballast and cabling by a factor 1.52.There is no immediate damage or danger and the situation can continue to exist for days.Tested in a complete luminaire at 110 % Vmains the ballast temperature must be lower than 232 ºC, which guarantees a minimum lifetime of the ballast of 20 days in this situation. In most cases the mains fuse will not blow and the situation can only be solved by good maintenance.

In a capacitive circuit the current is even lower than the nominal value when the lamp is short-circuited. So then the described effects are not noticeable.

2) Short-circuiting of the ballast

As there is no current limit in this case, the lamp current will rise immediately to an undefined high value in the inductive circuit. If the current is not switched off by the mains fuse, the lamp will normally become an open circuit because (one of) the lamp electrodes will melt. In most cases this process is so quick that there will be no extra danger or damage. In practice, however, it happens that the ballast is partly short-circuited inside the copper coil, for example at the end of the ballast lifetime.This results in a higher ballast temperature and a higher lamp power.This process is cumulative and normally the mains fuse will not blow, while the ballast gets hotter and hotter until a fatal earth or winding breakdown occurs. For this reason, the ballast must be mounted in such a way that it can cause no danger during end-of-life failure.

Good maintenance can prevent blown-up lamps and burned-out ballasts. When in a capacitive circuit the ballast is short-circuited, the lamp

is only stabilised by the series capacitor. In most cases the lamp will extinguish, as the remaining impedance is too high ( lZc l = 2. lZL l).

In those cases where the lamp continues to work, the high capacitive peak currents through the lamp, will rapidly damage the lamp electrodes.The lamp will blacken at the lamp ends and sooner or later a lamp electrode will break, resulting in an open circuit.

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