Insulation resistance problem

[An Buachaillín Bán]

Hi all,

Stuck on another job here IR testing. It's a simple 3 bed semi and after short put L-N at board and testing to earth, I'm getting 0.16 Mohms.

I've disconnected main incoming neutral, no surge protection of any kind in the house, I've taken out all lamps and plugged out anything I could find plugged and the same reading is coming up. It's a small board thats also supplying a shower priority board next to it. I've the priority board turned off too just to see can I get the value up. Haven't been able to. Any ideas? Would a damp or wet socket cause a short and if so would it display a value like 0.16?

Thanks

 

[westward10]

0.16 Meg ohm is not a short. Test individual circuits to narrow it down.

 

[loz2754]

Are you doing a global IR test across all circuits?
You need to disconnect all the outgoing neutrals from the bus bar and test each circuit individually, so as to narrow down the low reading to a particular circuit(s).

 

[An Buachaillín Bán]

Are you doing a global IR test across all circuits?
You need to disconnect all the outgoing neutrals from the bus bar and test each circuit individually, so as to narrow down the low reading to a particular circuit(s).

Yes I'm disconnecting L and N from each circuit, joining them and testing to earth and so far so good. Are you saying then that it is probable that it is a neutral is at fault? Thanks

 

[loz2754]

It could be line to earth or neutral to earth.
You now need to test individually, not with line and neutral joined.
And be as sure as you can that there are no connected loads.

 

[An Buachaillín Bán]

It could be line to earth or neutral to earth.
You now need to test individually, not with line and neutral joined.
And be as sure as you can that there are no connected loads.

As you say that, as I was testing an individual circuit, had the 2x neutrals in a connector and joined to the top of mcb. Mcb was off and I then got a voltage reading reading of 230v! If I turned off the mcb next to it, it knocked off the voltage. I'm wondering is that an erroneous connection or was it an original ring circuit. Not used to ring circuits here in Ireland

 

[timhoward]

You want the main switch off for your own safety, especially if testing something that isn’t quite making sense.

Are you IR testing L+N to the CPC bar, or disconnecting the cpc from each circuit and testing to that?
(You should test to the CPC bar in case there’s a fault to something that’s bonded.)

 

[littlespark]

Test one circuit at a time… don’t just turn the breaker off, take the live out the breaker, and the neutral out the bar and test outwards from there.

Age of property? What’s the likelihood of rodent damage?

Connected loads? …. Indicator lights on dp switches, dimmer switches, extractor fans, smoke detectors, LED lamps and outside PIR sensors….. It’s difficult to identify every connected load nowadays.

 

[An Buachaillín Bán]

Test one circuit at a time… don’t just turn the breaker off, take the live out the breaker, and the neutral out the bar and test outwards from there.

Age of property? What’s the likelihood of rodent damage?

Connected loads? …. Indicator lights on dp switches, dimmer switches, extractor fans, smoke detectors, LED lamps and outside PIR sensors….. It’s difficult to identify every connected load nowadays.

Thanks for the reply. A little update, I disconnected all circuits, one at a time. Disconnected L from breakers, and corresponding N from neutral bar. I joined the two together and tested to MET and got good reading on all circuits. It is an old installation, 40 years plus. I didn't disconnect earths from MET as there are many single earth cables and I couldn't identify each one. Should I still test L to N, L-E, and N-E? I thought that because I got good readings with the way I tested then I was good to go.
Maybe I'm doing something wrong when performing the global/overall test.

 

[timhoward]

Disconnected L from breakers, and corresponding N from neutral bar. I joined the two together and tested to MET and got good reading on all circuits.

Good news

I didn't disconnect earths from MET as there are many single earth cables and I couldn't identify each one.

That's the correct way to do it. Always leave earths in.( I've had a screw connect live and bonded steel before now which wouldn't be picked up just testing the circuit on it's own.)

Should I still test L to N, L-E, and N-E?

No need if every circuit is testing fine independently L+N -> E, it won't tell you any more.

I thought that because I got good readings with the way I tested then I was good to go.

Agreed.
The only thing remaining is possibly repeating global test for your own sanity.
It will either reveal no problem (indicating that whatever caused it is not part of fixed wiring, or that it was a highly annoying intermittent fault) or if you get a low reading again, it would be good to understand why.
Are there any RCBO's with functional earth connections?
If not, I'd then be turning off breakers and removing all N's and repeating global test to prove it's not a fault inside the box.

 

[loz2754]

If the main switch is on when testing global IR then the neutral and earth will likely give a very low reading. Not that anyone would...