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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 1.5
2
.There is no
immediate damage or danger and the situation can continue to exist
for days.Tested in a complete luminaire at 110 % V
mains
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 (
l
Z
c
l
= 2.
l
Z
L
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.
5
147
3.18 Circuit breakers, fusing and earth leakage