Handbook of Electrical Installation Practice - BeKnowledge

528 Handbook of Electrical Installation Practice
conductors and between each conductor and sheath. No breakdown should
occur.
The test voltages given are intended for cables immediately after installation and
not for cables that have been in service. When testing is required for cables that
have been in service, the cable manufacturers should be consulted for appropriated
test conditions which will depend on the individual circumstances. A lower voltage
should be applied taking in account the age, environment, history of breakdowns
and purpose of the test.
If switchgear or other apparatus cannot be disconnected from the ends of the
cable during the test, the value of test voltage must be the subject of agreement with
all parties concerned.
During the testing period special precautions must be taken to keep all persons
away from any exposed parts which may be charged at the high voltage.
If cables with special anti-corrosion sheathing have been used and it is thought
that the sheathing could have become damaged during cable laying operations it is
sometimes beneficial to apply a voltage test between the cable sheath and the
ground before the cable is terminated. The value of test voltage should be no more
than 4kV per mm of oversheath thickness, with a maximum value of 10kV applied
for 1 minute.
If a cable should break down as a result of h.v. testing, the position of the breakdown
must obviously be found so that a repair or replacement can be carried out.
If the test set can provide sufficient power the fault can often develop into a low
resistance path which may be located after the removal of the h.v. by conventional
methods using resistance bridge techniques. More often than not, the fault of high
resistance remains, which can involve problems of location. Sometimes the position
of the fault can easily be detected by ear if the test voltage is left connected, preferably
at a lower value than the full test value.
Various makes of sophisticated fault location gear are available for finding high
resistance faults and most cable manufacturers offer a fault location service.
One quite effective method is to charge a large capacitance by the h.v. test set
through a resistance and then discharge it into the faulty cable core through a spark
gap. The voltage and size of spark gap are arranged so that the capacitor discharges
every few seconds. By walking along the cable the position of the fault can usually
be heard as the insulation breads down each time the capacitor discharges.A stethoscope,
consisting of microphone, amplifier and earphone, is useful for detecting the
fault position in buried cables.