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Topic Title: RCD Fault Finding
Topic Summary: Low Neutral Earth Resistance
Created On: 03 July 2008 10:05 PM
Status: Read Only
Linear : Threading : Single : Branch
 RCD Fault Finding   - tkerby - 03 July 2008 10:05 PM  
 RCD Fault Finding   - ady1 - 03 July 2008 10:14 PM  
 RCD Fault Finding   - tattyinengland - 03 July 2008 10:16 PM  
 RCD Fault Finding   - truss - 04 July 2008 12:34 AM  
 RCD Fault Finding   - AJJewsbury - 04 July 2008 01:35 PM  
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 03 July 2008 10:05 PM
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Joined: 04 June 2002

I'm trying to fault find a residential installation where there seems to be a problem with the rcd tripping on an upstairs circuit.

If all the circuits are moved to the master circuit breaker and off the rcd, they operate fine

If the other two socket rings are moved on to rcd protection, everything is also ok

If the upstairs sockets are moved on to the rcd, energising any circuit, whether rcd protected or not (e.g. lighting or smoke alarm) will trip the rcd.

The installation is a tn-s and the rcd brand new as its been replaced in the fault finding although I dont think it was the problem to begin with. With only the upstairs sockets connected on the rcd and its neutral bus bar, I'm measuring about 2 ohms neutral to ground. I'm sure this leakage is causing the issue (although I dont understand why a non rcd protected circuit would cause it to trip)

I'm sure there is probably either a whisker from neutral touching the wall box in a socket but there are about 20 sockets on this main and some buried behind lots of building materials as the house is being refurbished. What is the best way of identifying the cause of the fault and tracing it to an individual piece of wiring or socket?

Tim Kerby CEng MIET

IET Council 2010-2013
Chairman, Scotland SE Young Professionals
 03 July 2008 10:14 PM
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Hi tkerby
I would do the usual end to end and R1+R2 tests. If ok, then do the usual IR tests. If ok i would IR between your disconnected RF tails and N at the board, Earth and Live (But turn all power off first and measure at 250V not 500) That will find your fault.
Either, as you suggest. A borrowed neutral or something faulty plugged in.

Happy hunting
Welcome to the forum


Resistance is futile.
 03 July 2008 10:16 PM
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You've identified the circuit at fault, carry out normal dead tests,part by part until you find the fault. If necessary drop out half the circuit and carry out dead tests, id which side of the ring the fault is on and the go one leg at a time till you chase down where the fault is. Voila. It's always worth going throught the sockets one by one and checking each ones tight connections and singns of dammage. Last week I had a neutral getting crushed against the metal transformer casing for a shaver socket, causing a neutral / earth fault - innevitably causing the RCD to trip.
 04 July 2008 12:34 AM
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It sounds to me like your upstairs ring is sharing a neutral with one of the non-RCD protected circuits.

I would disconnect all of the neutrals from the non-RCD Neutral bar & (carefully) energise the upstairs ring on the RCD side. You will probably find that by doing this, one of the disconnected neutrals will show a voltage to both earth and a connected neutral. Once you have found this you will know which circuit is sharing a neutral with the up ring. You could do the same test dead of course by measuring for continuity between the (disconnected) up ring circuit's neutral and the non RCD neutrals.

Once you have found the offending circuit i would look for recent additions such as a new light (outside?) or extractor fan somewhere which could have got it's line from a switch and it's neutral from the upstairs ring.

I could be wrong of course... (it has been known)

Good luck & Regards,

 04 July 2008 01:35 PM
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although I dont understand why a non rcd protected circuit would cause it to trip

You've got a fault between N and PE.

With no current being used, there's pretty much no voltage difference between N and earth, so little or no current flows across your fault.

But when the installation starts to draw current (especially in a non-TN-C-S setup) there'll be a small but significant voltage drop along the supply N (like every other conductor carrying current), which means that the N bars at the CU will now be at a little bit higher voltage than the earth bar. It doesn't matter which circuits are drawing current, or whether they're RCD protected or not.

The significant thing is that the fault is downstream of the RCD and isn't readily isolated by switching off single pole MCBs either! So now current can flow from the RCD's N output terminal to earth through your fault, the RCD seem an imbalance and quite rightly trips.

And we wonder why those parts of the world that have favoured RCDs for years (e.g. France) always use double pole MCBs...

- Andy.

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