Just before all the self isolation and not being closer than 6 foot to anyone else I had a chat with a couple of DNO engineers who were doing insulation tests on the HV cables going to/from a sustation at the end of the road the 11Kv cables are tested at 15 Kv to Earth not sure if they used AC or DC. Another thing was the that I never realised just how big the fuse holders for the outgoing LV cables are I asked what size the fuses were and they told me they were 400 amps. I was just being nosey but it's interesting to me to see just where my juice comes from which is a rather scruffy 500 KVA transformer
The cable testing would use DC. AC is not suitable because there is no simple way to tell the difference between most undesirable leakage, and entirely normal capacitive coupling.
Leakage of say 10ma at 11KV is serious as it may indicate imminent breakdown. A capacitive charging current of 10ma or more is normal according to the length of the cable.
Selecting the DC test voltage is important on XLPE Cables designed to operate on AC.
Typically, you probably don't want to be much above 5kV if you want to avoid irreparable damage within the XLPE - it causes cracks or so called "trees" within the insulation.
If it has to be tested above that 5kV limit level, then better to use very low frequency (VLF) AC testing
The cable testing would use DC. AC is not suitable because there is no simple way to tell the difference between most undesirable leakage, and entirely normal capacitive coupling.
Leakage of say 10ma at 11KV is serious as it may indicate imminent breakdown. A capacitive charging current of 10ma or more is normal according to the length of the cable.
Selecting the DC test voltage is important on XLPE Cables designed to operate on AC.
Typically, you probably don't want to be much above 5kV if you want to avoid irreparable damage within the XLPE - it causes cracks or so called "trees" within the insulation.
If it has to be tested above that 5kV limit level, then better to use very low frequency (VLF) AC testing
