External influence tripping RCD on TN-S system?

[pcboffinuk]

Does anyone have experience of intermittent faults that trip an RCD, that are caused by influences external to the house?

I have an elderly couple who had a new consumer unit fitted about 2 years ago (not by me - needless to say no cert although it does appear to have been installed more or less correctly).

The cutout is a little unusual compared to the ones I normally see as it had two separate incoming cables, Neutral and Phase, which are both clamped to provide a TN-S.







The clamps may look like a DIY install, but I suspect not as I see a lot of TNS done this way around here.

The problem is that the RCD is regularly tripping for no obvious reason. They noticed recently that when the neighbour used her hedge trimmer it tripped 3 times in 20 minutes.

I only had chance to have a quick look on Friday and I checked all the circuits IR (fine), ring continuity (Fine except for one high P-P which I will resolve but I don't think is connected to the fault) and the RCD (ramp test tripped at 27mA and I/5I tests all fine).

The installer came back a year or so ago and moved the cooker circuit to the non RCD side to try to eliminate this fault, which it failed to do. All that is left on it is a new shower install and two RFCs.

I am going back to check out a couple of appliances (Fridge, Freezer) which could possibly be causing the fault, but they are not particularly old.

I have run into enough coincidences to know you can not rely on things like noticing a neighbour using a hedge trimmer, but has anyone run into ways that the external supply can cause random RCD tripping? I am sure I have heard something about it but couldn't find an exact match with a search. The earth does not seem to be shared via a split cut out, though I haven't had access to next door to check yet. However, the neighbours supply is apparently on the opposite wall to this one so the earths could well be as good as linked underground.

I realise that RCDs in theory should only monitor the downstream supply, but is there a way that something 'upstream' could cause frequent tripping, or should I forget that and concentrate inside?


On a possibly related note, the resistance of Neutral to Earth external to the supply was about 0.6. Obviously TN-C-S wuold have a low resistance, but TN-S? I am guessing that they tie the outer sheath to earth at the same point as they tie the Neutral to supply for PME?

Any suggestions welcome

Gavin

 

[Steve]

The two cables are not neutral and phase. One feeds the cutout with L and N, and the other feeds next door.

Does next door have an RCD? Try a nice friendly chat with them.

 

[plugwash]

Does anyone have experience of intermittent faults that trip an RCD, that are caused by influences external to the house?

Maybe a high resistance neutral-earth fault somewhere (possiblly inside equipment). They can cause strange behaviour of RCDs in a TN-S or TT system as the neutral-earth voltage varies.

Surges can also drive more current down a fault.

You need to establish how much leakage there is in the installation. If it is more than about a quarter of the RCDs rating it only takes a moderate surge to push it up into the region where the RCD might trip.

Also if you have not already done so you need to test the RCD.

On a possibly related note, the resistance of Neutral to Earth external to the supply was about 0.6.

On a pure TN-S I would expect the N-E resistance to be similar to Ze in a TN-S fed from a system that also feeds TN-C-S install I would expect it to bea little lower than Ze so this figure sounds reasonable.

 

[pcboffinuk]

You need to establish how much leakage there is in the installation. If it is more than about a quarter of the RCDs rating it only takes a moderate surge to push it up into the region where the RCD might trip.

Also if you have not already done so you need to test the RCD.

The RCD tests fine , both in isolation and with the sockets connected.

Ramp test was 27mA or so, I & 5I times were well within limits - 27 and 9ish from memory.

I plan to use my PAT tester to test the fridge and freezer, but apart from those this is an elderly couple with not a lot of electrical stuff plugged in. Anything that could have been dubious like garden pond has been isolated for months and the problem still occurs.

Is there any way of confirming if there might be an external problem that could be tripping the RCD. I hope to get access to the neighbour to check that nothing silly has been done there.

If as Steve says the second cable is supplying next door then the two houses are effectively sharing the same earth via the sheath of the cable between then - is that a 'good thing'?

Supposedly the neighbour has had a new CU fitted, presumably with RCD, so could a leakage on her side be adding to the problem on my clients side?

 

[johnney]

Ive had similar problems in the past and rcd hasn't tripped on half test change rcd could be mechanical fault with it.

 

[JohnD]

how about a pic of the CU?

 

[pcboffinuk]

I didn't take one of the CU as its a bog standard Wylex split load board, but returning next week so will take some more piccies then.

I may try another rcd, but intermittent problems mean that its always difficult to know if you've fixed the problem or not