How To Find A Neutral Earth Fault

Ane of those Ignorance is Bliss / Wish I'd Never Started This stories.
The dominate wanted an Induction Hob and no spare slots on eight-way Consumer Unit. Besides EXTREMELY old (although probably as safe as information technology always was - I thought), then change it for a CU with more slots. And, given that MCBs are now the virtually bones option and not wanting the risk of a single RCD with no inkling where the trip(s) might be coming from, I decided to use RCBOs. Ok - expensive. Simply you can always immediately tie issues to just one circuit.

So yesterday I stuck the affair in. And everything is OK including the new hob (with its dedicated radial using 6mm cablevision to run across its max load of 32A!!) just EXCLUDING a lighting circuit that immediately tripped the RCBO. Investigating revealed the cause directly away: a Neutral to Earth mistake that had probably existed for a VERY long time but without any RCDs in the arrangement, remained undetected.

Worse, several of the connections that might be incorrect are lost in ceiling voids: the roses on several of the pendants have only i TC&E cable in them, indicating that some cowboy in the afar by moved pendants merely (presumably) substituted junction boxes in the void instead of extending all the cables to the new positions. Understandable but irritating, since I've no thought of the original locations and the JBs are almost certainly under fitted carpets, bathroom flooring, etc.

In the calculator networking world, there'south a gadget called a TDR for locating (Ethernet) cable impairment only equally far as I know, the just tool bachelor for mains wiring is an ohm meter. At two locations where I've constitute a full complement of cores in the ceiling rose, the resistance Neutral to Earth is near two ohms. This is most the same value as on the end of the cable at the consumer unit.

Needless to say, beingness lighting and quite old, it'southward a radial excursion, not a ring. But I can't at present work out what the road is.

I guess the only adept affair to come out of this is that 8 out of 9 circuits are working OK with RCBOs! If I'd chosen a split CU with ii RCDs, I'd have spent a lot longer fiddling effectually to isolate the faulty circuit.

Anyone recommend an constructive process for localising the mistake to a smaller area than the whole circuit?
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You can get and employ them on mains wiring, nosotros employ them all the time on the DNO networks up to 33kV for fault finding.
There is ane sat in my car outside equally I type this reply
It would exist helpful if you could tell united states what your test results for this excursion were before yous changed the consumer unit and the test results after you lot changed the consumer unit.
Simply disconnecting all the wires at what yous retrieve may be half way along the radial (record which wire is which for refitting) and so cheque each half to find which has the fault.
Then divide the faulty half - and so on,

Have you separate circuits for upstairs and downstairs?
Often the landing low-cal will take alive from downstairs and neutral from upstairs circuits. This volition cause tripping.
The whole of the affected excursion is on the First floor. The stairs/landing lighting is on the downstairs circuit and is OK on a unlike RCBO.
Other test results to this betoken are irrelevant: at that place's definitely (very) low resistance Neutral to Globe on the (single) cable leading to the start of the circuit. And the measured resistance N to E further downwards the excursion is similar.

Slightly comforted by the idea that a TDR might help. However, since I don't know the exact route, being told that in that location'southward a glitch 25 metres from instrument would not be much assist!
Also finding the speed of propagation might be an result, though a exam on a known length of like cable volition get yous into the ballpark
- Thanks x 1
The first thing I would check, if you lot oasis't already, is EVERY ceiling rose/fitting.
I did a CU change non so long back where testing revealed a neutral world fault on lighting - fortunately for me it was in the very first rose that I checked - rose had been screwed back on pinching the neutral and CPC together.
The problem with finding the 'route' of a radial circuit is that, a lot of the time they tin be wired in a 'spider' - in other words, the main feed into a JB and and then the feeds to individual lights fed from hither.

If it'south first floor lighting, can you non admission well-nigh of the cable in the loft?
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Yup - that's the sort of possibility that worries me! Actually the house is iii floors total but half the first and all the second is on a secondary CU - I think there was a separate flat on the upper floors at i time. And so, when it was rewired later, they followed the same organization.
And so the faulty circuit is between the fitted carpets, elevation bathroom floor, etc. on the 2nd floor and the finished ceilings on the First, with the added complication of 4 out of 5 ceiling fittings that were moved, leaving a hidden JB, earlier I got the firm.

This could get messy!
You said this was an old install, how old? What kA where the previous breakers, 3 or half dozen?
Are yous happy with the ELI & IRT readings taken for the install when y'all did the board change?

Have yous done an IRT test on the faulty cct ?
If bad IRT then it could be trapped,damaged or deteriation of cable not a bad connection. Carve up downwardly what you lot can get to end to end and eliminate the bad section and supersede, forget looking for bad connections in a sealed ceiling or floor.

Can you lot renew the section or circuit? I am sure your boss will understand that its got to be sorted and if y'all cant get to subconscious wiring it needs renewing. Not your fault you are doing it right to BS7671.

I have seen with lighting circuits spare neutrals used for earths and vice versa so any contradistinct or added surface area may be the culprit of a crossed connection. Break it down, evidence each conductor & IRT examination it for harm etc.
It could be as simple as a neutral at a switch (T&E method) non in a connector touching back on the earthed backbox

And if it was a borrowed neutral from another cct it would trip both circuit RCBO's not 1.

Proficient luck, no pressure mate its only for the boss!
I guess that it is 'nuff', really. Even if the OP had tested the excursion, and found the N-Due east error, prior to irresolute the CU (which I presume is what the 'digs' are about), he would nevertheless presumably accept been looking for advice in relation to the same question he has asked - how to locate the source of the known Northward-E fault. I can't come across that whatsoever other standard test results would aid you, or anyone else, to answer the question, would they?
Kind Regards, John.
Although pre-testing may have given prior warning it would withal need correcting so really where one is non giving quotes and you lot know the task needs doing come what may then to pre-test only tells you if fault is yours or existed earlier and so little point.
As to finding there are two methods.
One is remove neutral rose at a time or supplant rose at a fourth dimension.

At such a low ohm then easy to adhere a battery bell or low-cal and only remove one at a time. One time fault cleared replace in aforementioned order as removed in the hope of reducing the area nether investigation.
Thing is, John, if he was doing this for anyone else, he'd be in a fleck of bother now - because, regardless of the price to himself, he'd have to find the fault.
Had he washed the absolute minimum testing before a cu bandy (i.e. Whole Board IR Examination), he'd accept known there was a fault 2 minutes after getting his screwdriver and tester out.

So he could have arranged to find the fault Before fitting the new board.

The other thing, of course, is that he manifestly hasn't washed the tests required for 'Initial Verification' of the CU - i.eastward. IR testing..........otherwise he wouldn't have got to the point where the lighting excursion was tripping the RCBO - it shouldn't have been energised in the outset place.Hence, the 'Digs'........shoddy piece of work to say the least.
Maybe, simply as I've just posted - there's NO excuse for not doing the relevant dead tests earlier energising!!
The skilful old 'bang' exam........
No argument near any of that. Withal, my point there is that horse has already bolted, so nothing much is to be gained, at least in relation to this OP, by trying to close the gate - except in a headmasterly sense!
Kind Regards, John.