Insulation resistance

[Marvo]

A surge arrestor would see 500volts as a surge which it would allow at least a portion of the waveform to pass safely to earth. It's possibly a good job there were surge arrestors left plugged in because you're likely to damage electronics if you run around IR testing at 500 volts on circuits with appliances still connected. I'd count myself lucky and do some more reading on correct testing procedures if I were you.

 

[Regs]

A surge arrestor would see 500volts as a surge which it would allow at least a portion of the waveform to pass safely to earth. It's possibly a good job there were surge arrestors left plugged in because you're likely to damage electronics if you run around IR testing at 500 volts on circuits with appliances still connected. I'd count myself lucky and do some more reading on correct testing procedures if I were you.

Ive been taught and told as long as you don't fire down line to neutral at 500v it would be fine.

 

[Octopus]

This is basic stuff really.

Test IR at 250v first, 90%+ of low readings are due to appliances plugged in and always go round and unplug EVERYTHING, some sockets are SP isolators, as are some FCU's.

Never fire 500V at a circuit with really low IR unless you are 100% certain everything is unplugged.

And yes I mean test the circuit in question only, all 3 to each other.

 

[Regs]

This is basic stuff really.

Test IR at 250v first, 90%+ of low readings are due to appliances plugged in and always go round and unplug EVERYTHING, some sockets are SP isolators, as are some FCU's.

Never fire 500V at a circuit with really low IR unless you are 100% certain everything is unplugged.

And yes I mean test the circuit in question only, all 3 to each other.

If you still had a low reading after unplugging everything then last resort you would disconnect FCU's and once all avenues had been exhausted then you would have to put it down as cable insulation then?

 

[Marvo]

You assume that once you've unplugged everything and switched off any isolators you're seeing the IR reading for the circuit cabling and the actual sockets only but the problem you have is the hidden appliances you don't find such as TV antenna amplifiers in lofts etc will give false readings.

 

[Regs]

Going back to what i asked originally, i thought the insulation resistance test before fitting an rcd was for nuisance tripping not to see if you had over 1 M.O.