Strange test results - advice requested

[SolarCity]

Okay, I've got a pretty unusual problem that I've had over the last couple of days and I'd welcome some advice on this.

Currently we are carrying out electrical works on barn conversions into single and two bed homes. Fairly small, single storey.

Carried out second fix - sockets, lights (with lamps removed) etc. No current drawing equipment at all.

I started testing on Friday afternoon and did a global insulation resistance test which gave a figure of 5M ohm. Obviously I investigated further and found odd figures on different circuits:

280M ohm on one of the ring mains approximately. The figure fluctuates - not just rise, I mean rise and fall.
5M ohm on one of the lighting circuits.
300M ohm on the other lighting circuit.

The 5M ohm lighting circuit was isolated to the two bathrooms - one giving a reading of 5M ohm and the other giving a reading of 250.

Investigated the 5M ohm circuit which consisted of four unlamped spot lights and initially got odd readings on different sections. We found we could actually affect the readings by moving the cables around. Now we seem to be getting a reading of 500 rising to 1000+. Cables feel free, so not trapped.

This would be unusual enough but we are getting similar issues in four of the other units. We have a few circuits with very low readings - 5M ohms and a few with everything between 100 and 1000.

Checked readings with another testing and they corroborate the readings so it's not a tester issue.

Any thoughts?

I know that the figures are "acceptable" but surely this points to a problem that needs putting right.

 

[HandySparks]

Do you have any unused cable left which you can IR test? It would be unusual, but a batch of faulty cable would explain why the odd readings are present in multiple buildings.

 

[SolarCity]

We don't have any cable left from the original delivery, but we do have cable drums from further deliveries. They all test totally clear.

I have thought about faulty cable, and it still seems the most likely explanation to me, but we are getting faults on the 1.0mm and the 2.5mm. Not impossible, but surely this makes it far less likely to be the cause?

 

[Pete999]

Have you checked the connections on your MFT? could be iffy connections

 

[SolarCity]

That was my first thought. But I checked with another tester and the results were identical.

 

[Pete999]

That was my first thought. But I checked with another tester and the results were identical.

Were you using the same leads?

 

[SolarCity]

Were you using the same leads?

No, different leads.

 

[Pete999]

No, different leads.

Interesting Watson, well that knocks that theory on the head doesn't it? checked any connections on the actual installation, basic questions I know but when you can't find the answer it's usually staring you right in the face.

 

[David M]

When you say insulation test are theses readings only between L & E ?

Is there any damp, i.e wet plaster, condensation...?

If moving the cable affects the readings then this would initially lead me to think there is a trapped cable / loose cable.

 

[Marvo]

I'd also suspect high relative humidity/damp but it's summer in the UK so maybe unlikely. Are the cables in free air or in conduit, if in conduit maybe something soapy and moist was used as lubricant to assist with pulling.


**edit** I just read this post back to myself and I can only apologise for the last sentence, it didn't translate very well

 

[Pete999]

Okay, I've got a pretty unusual problem that I've had over the last couple of days and I'd welcome some advice on this.

Currently we are carrying out electrical works on barn conversions into single and two bed homes. Fairly small, single storey.

Carried out second fix - sockets, lights (with lamps removed) etc. No current drawing equipment at all.

I started testing on Friday afternoon and did a global insulation resistance test which gave a figure of 5M ohm. Obviously I investigated further and found odd figures on different circuits:

280M ohm on one of the ring mains approximately. The figure fluctuates - not just rise, I mean rise and fall.
5M ohm on one of the lighting circuits.
300M ohm on the other lighting circuit.

The 5M ohm lighting circuit was isolated to the two bathrooms - one giving a reading of 5M ohm and the other giving a reading of 250.

Investigated the 5M ohm circuit which consisted of four unlamped spot lights and initially got odd readings on different sections. We found we could actually affect the readings by moving the cables around. Now we seem to be getting a reading of 500 rising to 1000+. Cables feel free, so not trapped.

This would be unusual enough but we are getting similar issues in four of the other units. We have a few circuits with very low readings - 5M ohms and a few with everything between 100 and 1000.

Checked readings with another testing and they corroborate the readings so it's not a tester issue.

Any thoughts?

I know that the figures are "acceptable" but surely this points to a problem that needs putting right.

Any hidden Junction Boxes? you said the readings fluctuated when you moved the cables, could be loose connections, just a thought.

 

[telectrix]

barn conversions. could it be mice/rat/squirrel attacks on the cables?

 

[SolarCity]

Hi fellas.

No junction boxes nor mice attacks.

We actually isolated one of the faults to a bathroom lighting circuit. Just four spots from a switch, no leds. We seemed to be able to affect the reading by moving the cable.

The cable was free enough to pull a replacement cable through - which we did. Reconnected the new cable and the fault cleared. No sign of damage or moisture on the replaced cable - and it also now tests clear.

I've not come across anything like this before. The plaster is totally dry and has been for a couple of weeks now.

 

[telectrix]

must be dodgy cable the. last year i had a 40m length of 3 core 6mm SWA where the brown (L) conductor went completely O/C within 2 weeks of installation. had to be dug up and replaced. as it was a gypsy site, no problem in getting grunts to dig up. lol.

 

[LeeH]

No, different leads.

New batteries? I've had that problem before.