Strange fault we had yesterday

[oscar21]

I sorted the fault fairly quickly as it happens but only because I'd come across it once before and at the time a few people said it couldn't possibly happen they way I described it, but its happened twice now.

Both jobs done by us, full re-wires.

The first time was during the 16th edition, RCD on socket side of the board, no RCD on the lights etc. Yet when the lights were switched on the RCD tripped.

Second time yesterday was similar, dual RCD board this time, Right hand RCD would randomly trip and also this time the test button wouldn't work on this RCD, other RCD behaved as normal. I concluded that when the socket circuit on the left RCD was used the right hand RCD one would trip.

Can anyone guess the fault, it is quite simple if you think about it. Its not what everyone is going to say and it has a lot to do with it being an empty house and not a lot of load on the various circuits.

 

[timhoward]

I’m assuming it’s not neutrals swapped as it allows loads to work some of the time.

So my guesses are
An interconnection between circuits from both sides away from the CU.
Or one leg of ring mixed up with another radial at board.

 

[timhoward]

But now thinking about test button not working on an RCD that does work….that has to be a clue!

 

[oscar21]

But now thinking about test button not working on an RCD that does work….that has to be a clue!

Yes it does, I swapped the two RCD's round and the fault was still the same, so RCD's not faulty. There were no interconnections or mixed up wiring either, every termination was in the correct breaker and this applies to all the points as well, I thought at first one leg of the upstairs sockets got mixed up with the downstairs or something like that but it was all wired perfectly.

 

[timhoward]

Did the test button on other RCD work?
Edit - you said it works
What is earthing system please?

 

[oscar21]

Yep, when the RCD's were swapped round the test button on the "faulty" RCD worked again and the one that was functioning fine became the "faulty" one.

TN-S.

Just to add, the test button did work if you pressed it 3 or 4 times so it was hit and miss, my mate was convinced the RCD was faulty until we swapped them round.

 

[plugsandsparks]

Closest i have had to that is when Roland had N in one paw and E in the other. Had similar bizarre behaviour of test button and random tripping.

 

[timhoward]

RH RCD top connections backwards?
So the return N current from LHS will imbalance the coil on right?

 

[oscar21]

RH RCD top connections backwards?
So the return N current from LHS will imbalance the coil on right?

Kind of on the right track, with the first fault back in about 2006 on a split load board it was a simple N-E fault on the socket circuit caused by the socket screw nicking the neutral wire. Because it was a brand new empty flat there wasn't anything plugged into the sockets to cause the RCD to trip even though there was a fault on it. Yes testing would have shown this up first but back in them days it was a lot more laxed and it was get the power on to spot anything obvious and then do all the tests. Nowadays you would be thrown off site for even daring to mention doing it this way.

When the lights were switched on most of the current went down the unprotected neutral as it should but because current takes every path and not just the easiest, enough current must have back fed through the RCD to give enough of an imbalance to cause it to trip, so zero current on the live through the RCD but 30mA or more on the neutral would make it trip. This was in the days of fillament lamps as well so an appreciable load on the circuit.

 

[westward10]

You really put all lights on one RCD and power on the other?

 

[littlespark]

You really put all lights on one RCD and power on the other?

i think he said it was a split board…. Sockets on rcd, lights on non-rcd side.

 

[oscar21]

You really put all lights on one RCD and power on the other?

Just one RCD for the sockets, back in the dark days lighting and cable didn't need RCD protection, I would imagine today's H&S rule makers would need a long sit down at even the suggestion of doing that today.

 

[oscar21]

Anyway the fault was a N-E short on the kitchen sockets which sat on the "faulty" RCD, it meggered clear at the time but the kitchen was being tiled so when whoever it was that tightened the sockets onto the new tiles it nicked the neutral wire. Because there wasn't anything plugged into the kitchen sockets it didn't trip and there wasn't enough current draw on the lights to make it trip either.

The only sockets that were being used were the living room ones on the left RCD, when anything was plugged into them there must have been enough current being diverted through the right hand RCD to trip it, obviously the living room RCD stayed on as there wasn't any fault on that side of the board so everyone was oblivious to it tripping anyway.

As for the test button not working I can only guess that because the neutral side of the RCD was effectively linked out, when the test fault current was introduced to the RCD some of it must have back fed back through the RCD on the neutral side to confuse it, probably right on the limit so it was hit and miss whether the button worked or not.

We repaired the kitchen fault and everything was fine and dandy again. It does make a good case for RCBO boards though.

 

[westward10]

Just one RCD for the sockets, back in the dark days lighting and cable didn't need RCD protection, I would imagine today's H&S rule makers would need a long sit down at even the suggestion of doing that today.

I misread it.