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To do things well, some aspects have to be considered:
- First of all, capacitors for discharge lamp circuits have to fulfil the
requirements as specified in IEC publications 1048 and 1049.The use
of PCB (chlorinated biphenyl) is forbidden.
- It is recommended that capacitors which have some approval marks,
such as VDE, KEMA, DEMKO or ENEC be used.
- Normally every lamp circuit is compensated by its own capacitance.
Only in some special cases group or central compensation for more
lamp circuits can be a better solution.
- In case of failure of the parallel capacitor (open or short-circuited)
the lamp behaviour is not affected. Regular control of the mains
currents and/or power factor (λ or cos ϕ) is advisable.
- In case of failure of the series capacitor the lamp behaviour is
immediately affected.This type of capacitor must create an open circuit
in case of failure, so that the lamp will be extinguished.
- The lifetime of capacitors depends on the capacitor voltage and
capacitor case temperature.Therefore capacitors with the correct
voltage marking (parallel 250 V with a maximum capacitance
tolerance of +/- 10% or series 450 V with a maximum capacitance
tolerance of +/- 4 %) and within the specified temperature range
(normally - 25 ºC to + 85 or 100 ºC) should be used.
Used within the specifications, capacitors with the VDE marking will
have a lifetime equal to that of ballasts: 30 000 hours or 10 years.
- If a specified parallel capacitance value occasionally is not available,
the next higher value can be used, provided that the value is not more
than 20 per cent above specification.
Two general types of capacitors are currently in use: the wet and the
dry type.
Wet capacitors available today contain a non-PCB oil and are equipped
with internal interrupters to prevent can rupture and resultant oil
leakage in the event of failure. So a clearance of at least 15 mm above
the terminals has to be provided to allow for expansion of the capacitor.
In case of failure, these capacitors will result in an apparent open circuit,
which means the mains current drawn by the circuit approximately
doubles in case of a parallel capacitor.This can cause a fuse to blow, a
circuit breaker to open, but will have no further detrimental effect.
Used as a series capacitor, the open circuit of the failing capacitor will
extinguish the lamp.
Dry,metallised-film capacitors are relatively new to the lighting industry
and are not yet available in all ratings for all applications. However,
they are rapidly gaining popularity because of their compact size and
extreme ease of installation and are, therefore, widely used nowadays.
During its lifetime this type of capacitor gradually loses its capacitance,
resulting in a gradually increasing mains current when used as a
parallel capacitor. In the end the capacitor acts like an open circuit.
For the series capacitor a capacitance loss of only 5% during its
lifetime can be accepted, so the dry capacitor is not recommended
for series applications.
Dry capacitors are more sensitive to voltage peaks than wet capacitors.
In critical applications (mains supply containing peaks, frequent
switching, high level of humidity or condensation) the wet capacitor is
advisable.
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3.2 Capacitors