Help! IR test results driving me mad

[Bongo3000]

Hi all

I went round to a house with a view to move a switchline in lounge from 1 side of a door to the other. I tested insulation resistance and got 0.38Mohms L-E. Ok, just a case of identifying which of the cables is causing the problem, sort it out and move the switchline. The circuit is a combination of 3 plate and junction box for the light to the dining room. After checking and testing all the exposed accessories - switches and roses without any joy I lifted some boards upstairs and moved onto the junction boxes (there are 2). I though I'd found it when I found a nail in the wall just above the switch in the dining room. Unfortunately it was a red herring, the nail didn't go in deep enough to touch the cable so back to the drawing board.

Here's the bit that's got me - tested at the junction box and got low IR. removed all the conductors and tested all individually (L-E, N-E, L-N) all came out at >99Mohms except one cable for the loop which was giving 10Mohms. Tested again at the CU to make sure all the other conductors were tested, again >99Mohms. I put all the conductors back into NEW junction box and again got 0.38Mohms.

I'm now turning myself inside out and would welcome any suggestions. Apologies for the rambling.
As an aside 1 of the cables in the loop has green goo which I know is going need me to let the customer know that parts of the circuit will need replacing anyway.

Thanks,
Jason.

 

[telectrix]

might be that green goo that's causing the low reading. it may have a degree of conductivity.

 

[SimpleSimon]

The cable giving 10 ohms, wasn't the cable feeding the rest of the house with all lamps, dimmer switches still attached?

Id be more inclined to go with Tel and the green goo.

 

[Bongo3000]

the cable giving 10ohms was the loop between the JB and the lounge at the front of the house. I'd ruled it out as being the problem.

 

[telectrix]

wasn't it 10 Megohms?

 

[Bongo3000]

yes, sorry slip of the keyboard 10Mohms, hence i'd ruled it out.

 

[telectrix]

not a FCU with neon on the circuit?

 

[Bongo3000]

might be that green goo that's causing the low reading. it may have a degree of conductivity.

So if I clean the goo up from both ends and reconnect, am I right in thinking that the goo does not breakdown the insulation of the individual conductors or does it affect the IR within the cable as well?

Thanks, JAson.

 

[Bongo3000]

not a FCU with neon on the circuit?

No FCU with a neon. I shorted L/N and tested to E as well just to be sure.

 

[telectrix]

clean goo and try it. that's all you can do short of replacing affected cable/s.

 

[Bongo3000]

Thanks all. I'll clean it up and give it a go. Cheers, Jason.

 

[Paul.M]

So if I clean the goo up from both ends and reconnect, am I right in thinking that the goo does not breakdown the insulation of the individual conductors or does it affect the IR within the cable as well?

Thanks, JAson.

You may have a very small crack in the insulation probably on a bend under the floor and like Tel says giving a bit of conductivity.

Test it at 250V and see if the reading is different?

 

[telectrix]

bend me shape me (amen corner)

 

[Amp David]

Just had the same today regard this green goo stuff. What is it anyway. Only seems to be on the lighting which is wired with the old stranded T&E.

But then also to make my day, both ring circuits and central heating after the fused spur are all giving o M Ohms when test L/N to E, so looks like i'm having to go back and try and figure out what the cause of this is.

Thought the boiler could of had a fault seeing as the heating wiring was showing low M Ohms, but the when both rings and lighting showed the same it really cheered me up.

Also measuring continuity between earth bar and each MCB for dodgy circuits is giving 1.31 to 1.48 ohms???

 

[Paul.M]

http://www.electriciansforums.net/e...s-electrical-regulations/32250-green-goo.html

Have a look at this thread, Sintra's post (5) gives a lot of info.