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!

 

[David M]

You still need to clarify which conductors you IR tested between to get the 0.7Mohms (assuming this is what you meant). If you do thorough tests then you can start to eliminate the possibilities causing the fault (again assuming there is one). Its good practice to be methodical with your testing rather than just rushing and second guessing.

 

[shinypen]

Thanks everyone. Lesson learnt. My error on both.

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

again, my error with ir. Reading was as stated by others actually 0.7 m ohms. Looks like I've got an issue with the lighting insulation.

Methodical step by step testing the correct way I completely agree, but hadn't realised leaving cpc in met caused errors which lead me down the wrong path. Thanks.

 

[shinypen]

To clarify. Original ir test was cpc to line. Done all ir test combinations now.

 

[Anthony_wayne]

It's less the leaving the CPC in the main earth bar and more to do with the neutral conductor being left connected. The correct prescribed method is to leave the cpc connected, but remove both line conductors.

 

[David M]

Thanks everyone. Lesson learnt. My error on both.

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

again, my error with ir. Reading was as stated by others actually 0.7 m ohms. Looks like I've got an issue with the lighting insulation.

Methodical step by step testing the correct way I completely agree, but hadn't realised leaving cpc in met caused errors which lead me down the wrong path. Thanks.

I understand that you are relatively new to the industry (you mentioned this earlier in the thread) Inspection & testing isn't just about taking readings and checking them against what is accepted. Its also about understanding what the tests are for, why the readings are what they are and the implications of such, whether acceptable or not.
Fault finding requires understanding and knowledge of testing, also experience which has to be earned.
I have watched many a spark just go through the motions without any thought to why he/she is doing the tests in the first place.
That's why in my opinion methodical thorough testing and a clear understanding of what you are doing and looking for is vital.
Its great that you are questioning your readings, but its important that those readings are accurate, appropriate and correctly interpreted.

 

[Richard Burns]

It's less the leaving the CPC in the main earth bar and more to do with the neutral conductor being left connected. The correct prescribed method is to leave the cpc connected, but remove both live conductors.

It should be clear that this is the action to take for IR testing and that continuity testing should normally be undertaken with no possibility of parallel paths reducing the reading.

 

[Anthony_wayne]

It should be clear that this is the action to take for IR testing and that continuity testing should normally be undertaken with no possibility of parallel paths reducing the reading.

Agreed mate, though we were talking about his IR test so thought that was clear enough. I see a lot of experienced sparkies still disconnecting the CPC for the IR tests, they don't believe it changed until you show them GN3.

 

[shinypen]

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[shinypen]

All sorted now. Sorry for the confusion and thanks for all the input.

 

[westward10]

So this had nothing to do with R1+R2 then

 

[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.