Page 369 - Offshore Electrical Engineering Manual

356 CHAPTER 3 Switchgear

When any overpotential testing is undertaken, the test voltage should be increased
as rapidly as consistent, with its value being indicated by the test instrument. If the
voltage breaks down the insulation as it is being increased, the test set operator should
repeat the test, adjusting the voltage slowly enough to record the value at which the
breakdown occurs.
Table 7.3.2 gives the maximum DC site test voltage, and this value should be
maintained for 15 min. Test current values should be recorded at 30-s intervals for
the first 2 min and at 1-minute intervals thereafter. The test potential should then
be reduced to zero and earths applied for 10 min before continuing the next test.
Although it is usually obvious if the equipment fails this test, values of leakage cur-
rent equivalent to an insulation resistance greater than 100 MΩ are expected and
leakage current at the full test voltage should remain steady. If the leakage current
gradually increases during the test, this is likely to indicate an incipient failure or
the connection of a semiconductor or similar component which should have been
isolated before starting the test.
Commissioning checks for other forms of switchgear follow a similar pattern, but
the exceptions are discussed in the following sections.

OIL SWITCHGEAR OVER 1 KV
The mechanical check should include a visual check of the circuit breaker tank oil
levels and an examination of oil-filled bushings for any leakage. The electrical testing
should include an oil sample insulation test in accordance with BS EN 60422.

VACUUM CIRCUIT BREAKERS
The mechanical check should include a visual inspection of the interrupters for dam-
age or seal corrosion. If fitted, the wear gauge should be checked in each case.
The electrical testing should include vacuum integrity tests by applying a DC
overpotential voltage (usually around 20 kV). The manufacturer should be consulted
both as to the correct voltage to apply and to the safety precautions that should be
taken because of the possibility of generating X-rays during this test (refer to BS
6626).

SULPHUR HEXAFLUORIDE SWITCHGEAR
The mechanical inspection should include the following:

1. A check that the gas system is operating at the correct pressure.
2. If the switchboard is of the sealed type, topping up of sulphur hexafluoride
should be required infrequently by using a temporary connection to a gas