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We racked our brains for ages over this and in the end have left it but it's still niggling me and I wanted to see if anyone could shed some light on it (I'm probably being thick)

We just had a small job to extend a radial to add in a couple more sockets, it's in a school so the board is the usual mismatch of circuits from the last 40 years. All went fine until we came to test with the fluke, when we got a very low reading testing insulation between phase and neutral.

Phase - Earth was 19MOhm
Neutral - Earth was 19MOhm
Phase - Neutral read as 0MOhm

Running a resistance test we found that there was a resistance of around 40Ohm between the phase and neutral.

After trying everything we could think of I decided to test some of the other circuits and half of them had the same dodgy reading. I disconnected the neutral feed to this board in case the problem was being picked up from elsewhere in the school but it made no difference.

In the end all we could do was test our side of the new installation (which was spot on) and let the school know that it could do with being looked at.

Could this be as simple as we'd missed unplugging something on the circuit somewhere or has anyone else got any more ideas? We're back tomorrow to do a different job so would like to have another look at it if possible.
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The fact that the Line-CPC, and Neutral-CPC are fine with the Line-Neutral at 0Mohms then the chances are that the is still some load on the circuit, especailly the the 40Ohm Resistance reading aswell. Would double check everythin is unplugged and FCU's switched off.

If it really bugging you, you could split the ciruit to find with half its on, then spilt that half till eventually you will locate where the problem is, or witch two points its between


Sounds like there is something still connected in the circuit.

What you could do is try is a continuity test first hand to see if there is definately anything connected between conductors i.e. between phase & neutral. If there is (most likely) it is just a case of tracking it down.

I would check the circuit pre the extention. You never know someone may have spured off the circuit and hidden a FCU with neon?

If it had of been a lighting circuit, I would have said there is a lamp still in the circuit @ 40ohms.

Like Paul said its a case of further investigation.
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Thanks guys, will defo have another look tomorrow as it's been bugging me. We traced as much as we can and unplugged everything but the cables are all in the wall and could easily have been dropped through the floor or something daft as you say. The whole place could do with a rewire really but as always money is the issue.

I'm glad you both think it comes down to something plugged in as I couldn't think of anything else that could cause it. They've probably run the downstairs lights and a few cookers off it :)


I = V/R
I = 230/40
I = 5.75 Amps

Would i be right n saying that the load isn't likely to be a neon light on a fcu or a small light with the 5.75A reading,

or is this just totally irrelevant in what im saying 5.75A..


sounds like a small cooker IS being run off it, not just the neon!!



Have a gander outside and see if there are any PIR lights that may be connected to your circuit?


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