They are normally conducted at twice normal voltage to earth, if 230v to earth then tested at 500vDC.
The tests do not deteriorate the insulation, as 230v cables are normally rated 600v anyway.
3 phase 400v motors can be tested @ 500v to earth, but 1000v between phases.
The tests do not stress the good insulation, but pick up deterioration already present.
It also depends on the standards of the country the cables were manufactured.
I've sailed on 10 year old Japanese ships, were I was afraid to touch the insulation on cables, because it just crumbled when touched, leaving bare wires.
Edit.
600v rated cable is RMS value, peak is RMS x 1.414 = 848v. Which is 1696v between them. (1200v RMS).
As ships electrician for nearly 40 years, I tested the insulation of equipment every month, without any problems due to testing.
Yes you may cause a little more stress than the normal circuit voltage, however, that is why you test the cable at approximately 2 X rated voltage of the insulation. Motor insulation is slightly different in that it is extremely thin and is generally not tested above its peak rated voltage.
For cables carrying power, typical switching surges, near-by or direct lightning strikes, fault clearing anywhere on the circuit causes stress on the insulation that may be enough to fail the cable. You test the cable to find the beginning of a possible fault or to fail the cable under a controlled condition. Failing the cable with a Megger test is much less damaging to the rest of the circuit than a full fault that must be cleared by a fuse or circuit breaker.
Hope this helps,
Newton1Law
YES , high voltage eventually pinches through the weakest point along the cable. Do this test not more than once per year.
no ..for high quality cables ...
Just wondering if frequent electrical insulation tests can eventually damage or reduce the usefull lifetime of the wiring in electrical installations (especially if the applied test voltages are over 1000v).
