Neutral to CPC low reading on lighting circuit EICR

Typically the EICR I fitted in this afternoon because I'd hoped it would be fairly simple (new build home, 2007), turned out to be the only recent one that's thrown up a major FI.

Downstairs lighting circuit, 1mm T&E, currently on a 6A MCB

R1+R2 fine, insulation test L&N -> E an immediate 0.0MOhm

Removed every lamp and still getting 0.26 Ohm resistance Neutral to Earth.

L-N and L-E both test >999 MOhm

With some further isolation I managed to narrow it down to what appears to be a single run between the ceiling fitting in the back room (3 plate wiring) and the adjacent kitchen switch (2 plate wiring) and fairly sure there's nothing else in between in terms of fittings.

Am I missing anything obvious, or does this sound like nail/screw through the cable bridging N & E?

The house has been ill treated by tenants, but no obvious signs that the wiring has been tampered with - though odd how they used loop in wiring in one room and neutral at switch in the other rooms....

Does go to show the value of doing a proper EICR though - Live testing only would have missed this - though it would have bitten anyone in the arse who had just put RCD board in without properly testing first....

Fun follow up for 10 points - What would you code this incoming cable carefully installed to be a trip hazard?

Test at every accessible point N to E, you might be able to narrow the location down. Possibly with careful measurements and luck to an approx. distance between the two lowest results.

To add, if one of the rooms is a kitchen, any possibility of unused cabling e.g. for cabinet lights not fitted, cables hidden behind wall units. Perhaps even a wall unit screw through a cable.

SJD said:

Test at every accessible point N to E, you might be able to narrow the location down. Possibly with careful measurements and luck to an approx. distance between the two lowest results.

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Not long been home but was planning to dig out the resistance tables and try to work out how far 0.12 Ohm is on 1mm - the last reading I got from one end of what I think is the dodgy leg- it's a new build so board not floorboards and of course downstairs lighting, not nicely in the loft....

I was doubting myself at one point, but definitely getting the readings with the circuit totally disconnected from the CU end and failing IR with or without earth connected, so only thing I can see it being is a screw or nail through the cable - might even have been there since it was built...

Fortunately property is currently empty so could leave it off until I can work things out...

Dartlec said:

Fun follow up for 10 points - What would you code this incoming cable carefully installed to be a trip hazard?

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How did the incomer end up like that?

DPG said:

How did the incomer end up like that?

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AFAIK that's how Crest Homes installed it 13 years ago!

Dartlec said:

AFAIK that's how Crest Homes installed it 13 years ago!

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Nice!

Dartlec said:

Fun follow up for 10 points - What would you code this incoming cable carefully installed to be a trip hazard?

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DPG said:

Nice!

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There are words for that incoming supply but I would not like to use them in case ladies are present

Dartlec said:

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James said:

There are words for that incoming supply but I would not like to use them in case ladies are present

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I've seen similar in a new build completed within the last year too - except it was carpet that time.

Wonder what the common denominator could be cough new build cough.

I'm assuming the floor layers followed the DNO install and cut the floor round it, but even on the base floor the DNO shouldn't have connected it - I'll be using SWA clamps on it I think, but that can only mitigate it so far.

Anyone do an earthed steel version of this? ?

Multi-tools are so handy.

SJD said:

To add, if one of the rooms is a kitchen, any possibility of unused cabling e.g. for cabinet lights not fitted, cables hidden behind wall units.

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That's exactly what it was last time I had this. I've also known front door / back door outside lights removed and cable literally sawn off at the wall, shoved back 5mm and sealed over.
I guess you've had every light switch off by now but an earthed back box and a facia screw through the N is another contender.

BTW I make it about 2.7m if that measurement was 2 x 1mm cores (as opposed to a nulled wander lead)