R1+R2 continuity both ways

[shinypen]

Hi. Testing an old lighting circuit today after a c/u change. First few readings were all fine. Then on one rose I got a reading of 0.5 ohms switch closed (fine) and 186 ohms switch open.

Is this indicative of an insulation problem or something else? I did an IR test and got 0.75 ohms but it was a bit rushed and couldn't be sure it wasn't just an error. That is very low surely? I'll do that again tomorrow.

I'd be grateful for suggestions!

 

[weevilward]

If you're not taking the neutral of the circuit you are testing out of the neutral bar at the CU, then you are effectively IR testing every circuit connected to that neutral bar. If any one of those circuits has a low IR then this will bring down your reading. Surge protectors plugged into a ring main will also show a low IR to earth. I think Powerbreakers show this too??? Also a combination of circuits with acceptable but low IR can combine to give an overall low IR that fails.

Always best to take both live conductors out at CU before IR testing. And I tend to test at 250v first to make sure there's nothing still plugged in or wired in before giving it full 500v. If L-N still shows continuity and I cannot find the thing wired into the circuit then I will just IR test st 250v with L and N tied, to avoid frying the thing. Chances are it's a TV amp or smoke detector or PIR or something equally sensitive to meggering.