R1+R2 continuity both ways

[shinypen]

Well yes but I didn't want to confuse things further, though I'd about used up all my help for one week.

I was taking a R1+R2 reading with the cpc still on the met and all mcbs open . I know this is wrong and can give multiple paths. Just a mistake. This was giving me a continuity reading (albeit a high one) when the light switch was open. I can't quite get my head around this. Was the high R1+R2 measured actually the path back through the sub-station??? Is that what you were saying Richard?

If Neutral and earth are connected in the CU bars and the main switch is closed then when the light switch is open the meter could measure a resistance loop, via the supply neutral earth connection, through the load and this could give a high reading of the sort of level you have measured.

I misread you Anthony and thought you were talking about continuity. Sorry.

No damp areas? Almost like there is something else in the circuit? Or when you IR'd did you have cpc on the bar still with neutral connected?

 

[Octopus]

The cpc continuity is now fine. I'd left cpc in cu met bar. When I removed it the readings were fine.

Hum .... I always test for IR with all the CPC's connected...

 

[Richard Burns]

I was taking a R1+R2 reading with the cpc still on the met and all mcbs open . I know this is wrong and can give multiple paths. Just a mistake. This was giving me a continuity reading (albeit a high one) when the light switch was open. I can't quite get my head around this. Was the high R1+R2 measured actually the path back through the sub-station??? Is that what you were saying Richard?

You can see from this diagram that if you were measuring continuity with the circuit fully connected but only the MCB open then the reading with the light switch closed would give a resistance close to the R1+R2 you are trying to measure, if you open the light switch then the only path left is through the load back to neutral and if the main switch is closed then the path can go via the neutral earth link back to the tester.
The high resistance of this path will only lower the R1+R2 resistance reading slightly when measured with the light switch closed (resistors in parallel).

 

[shinypen]

Thanks so much for posting this. Very helpful. The bit I couldn't get my head around is the main switch was open in my case. This would negate this path back?

I think I've tracked it down. It seemed to be a suspect pendant (it did have a small crack in it). In any case when I bypassed the pendant the rogue reading went away.

 

[Richard Burns]

Thanks so much for posting this. Very helpful. The bit I couldn't get my head around is the main switch was open in my case. This would negate this path back?

I think I've tracked it down. It seemed to be a suspect pendant (it did have a small crack in it). In any case when I bypassed the pendant the rogue reading went away.

In the case of the main switch being open then this would probably indicate a neutral earth fault on the circuit (or a switch which was not turning off effectively). The appearance of a resistance reading in the say 20 to 900 ohm range tends to indicate measurement through a resistive load, by whatever means. Though this is a broad generalisation.

 

[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.