Handbook of Electrical Installation Practice - BeKnowledge

658 Handbook of Electrical Installation Practice
Before any examination always ensure that the capacitor has been disconnected
from the supply, and then wait for at least one minute (capacitors up to 660V) or
five minutes (capacitors operating at voltages higher than 660V) and then ensure
discharge is complete by measuring the voltage between terminals. Finally, short all
terminals together before testing. Shorting the terminals to earth is not effective for
adequate discharge.
The following points should be observed when carrying out any capacitor
maintanance.
Physical examination
(1) Examine externally that there is no damage or leakage of impregnant.
(2) Check that all cables are securely fixed and that all earth bonds are tight.
(3) Measure, if possible, the running temperature of the capacitor.
Testing
(1) Examine all insulators for signs of tracking, clean terminals and check for
tightness.
(2) Ensure that discharge resistors, where fitted, are in order.
(3) Check all connections for tightness.
(4) Measure insulation resistance of the terminals to case.
(5) If a capacitance bridge is available measure the capacitance prior to energising.
If such a device is not available the capacitor should be energised and
the line current measured by means of a ‘clip-on’ ammeter. The current
measured should be compared with the current obtained from the following
equation:
I = kvar
3V
where V is line-to-line voltage.
In considering the question of maintenance it is important to know that most
capacitors lose output during the course of their life. Some capacitors fail completely
after a number of years of operation. The failure mode of most capacitors is
a gradual loss of output without any noticeable signs of defect. Some capacitors,
however, especially of the bulk oil-filled variety, can fail with disastrous
consequences.
Many capacitors manufactured between the early 1950s and the late 1970s are
filled with a liquid whose chemical name is polychlorinated biphenyl. It has a variety
of trade names, such as, Aroclor, Biclor, Pyraclor, Askarel etc. The use and disposal
of this fluid is covered by stringent regulations, including the ‘Disposal of Poisonous
Wastes Act’. Legislation was introduced under section 100 of the Control of
Pollution Act 1974, to implement the sixth amendment (85/467 EEC) to directive
76/769 EEC, such that from 30 June 1986 it was illegal to manufacture, buy, sell or
install pcb capacitors. Existing pcb filled capacitors could continue to remain in use
only while they remained in a serviceable condition without leakage.