Strange Wiring Fault Causing RCD To Trip

securespark · 5 Jan 2007

Came across a strange fault today.

Trouble is, I have not had time to investigate thoroughly, ie do IR etc...

There is a 16A radial with an immersion heater & a bedroom socket on it, protected by an MEM RCD (AD80HE).

Whatever you plug in to the socket (even Class II equipment) causes the RCD to trip. The circuit is on a split-board, and is wired correctly (ie no crossed neutrals).

The RCD is testing OK, save for the 5x test, which it fails. I have replaced it temporarily with a stock item off the van but still (as I expected) the fault exists.

Has anyone got any ideas what the fault may be?

ricicle · 5 Jan 2007

Has it all worked correctly before.Just thinking some polarity problem somewhere.Or fault on immersion perhaps.Does the RCD only trip when an appliance is plugged in that socket only?

securespark · 5 Jan 2007

Of course, I only have the customer's memory to rely on, but he said that the socket has been working in the past, but, because it is at the back of a built-in 'robe, it has not been used recently until now.

RCD only trips when that socket is used, yes. Even switching on the IH is no problem, so I am ruling out the IH ATM.

The socket is fed with a single 2,5 and I even suspected N/E reversal at the other end of that cable, but that is not the case.

Adam_151 · 5 Jan 2007

1) Not something as simple as a swapped neutral and cpc in the socket, or a junction box, etc, is it?

2) Does the immersion heater trip the RCD?

3) Whats the supply earthing type?

4) how is it cabled? from DB to immer isolator and then onto socket? vice versa? branched radial at JB or DB? What cable lengths are involved?

5) Haev you called a priest to try and excorise the circuit?

ricicle · 5 Jan 2007

I think its going to be bloody obvious as soon as you get your IR tester out

plugwash · 5 Jan 2007

i take it the immersion heater does not trip the RCD.

i'd guess that there is an earth-neutral reversal somewhere (should be easy enough to test for this) possiblly in a hidden junction box.

a N-E fault is also possible but unless its a very high resistance one i would not expect the socket to be the only thing that trips the RCD.

ricicle · 5 Jan 2007

I'm thinking perhaps the N-E is reversed in the socket and during the time that the socket was not used a new RCD board has been fitted and is now making the problem apparent?

securespark · 5 Jan 2007

Adam_151 said:
1) Not something as simple as a swapped neutral and cpc in the socket, or a junction box, etc, is it?

Well, I swapped N & E at the socket and it still tripped when an appliance plug was inserted.

2) Does the immersion heater trip the RCD?

No.

3) Whats the supply earthing type?

TN-S.

4) how is it cabled? from DB to immer isolator and then onto socket? vice versa? branched radial at JB or DB? What cable lengths are involved?

Well, not ripped up boards yet, but the socket has only one cable supplying it, so it either EOL or from a JB. Cable lengths (logic applied here!) are no more than 30m. The immersion switch is downstairs, there is another upstairs (both at rear of house) and the socket is at the front of the house. The CU is also at the front, so I suppose the socket feed could be tapped off the 2,5 leaving the CU? Will know more on Monday...

5) Have you called a priest to try and excorise the circuit?

Not yet! Although this possible solution seems increasingly likely!

plugwash said:
a N-E fault is also possible but unless its a very high resistance one i would not expect the socket to be the only thing that trips the RCD.

The RCD does not trip when inserting an appliance plug with the CB off, so that suggests it is not an N/E fault, doesn't it?

securespark · 5 Jan 2007

ricicle said:
I think its going to be b****y obvious as soon as you get your IR tester out

But why only that socket? And why even with Class II does it still trip?

ricicle · 5 Jan 2007

securespark said:
ricicle said:

I think its going to be b****y obvious as soon as you get your IR tester out

Click to expand...

But why only that socket? And why even with Class II does it still trip?

From what I've read that socket is on a radial with the the immersion, so all the other sockets are on other ccts right?

securespark · 5 Jan 2007

Yes, but they are all protected by the MEM RCD.

RMS · 5 Jan 2007

securespark said:
ricicle said:

I think its going to be b****y obvious as soon as you get your IR tester out

Click to expand...

But why only that socket? And why even with Class II does it still trip?

It does sound like a N-E fault. As soon as you use an appliance and draw current from the live and return via the neutral then there must be leakage onto the earth causing the RCD to operate.

I'm sure an insulation resistance tester will save the day.

securespark · 5 Jan 2007

So, with a Class II appliance attached to the socket, where is it leaking to?

ricicle · 5 Jan 2007

Secure,does the RCD trip as soon as you plug the appliance in or is it when you actually turn the appliance on (ie draw current)

RMS · 5 Jan 2007

With no load connected to the circuit then no current will flow. RCD is healthy.

When you plug the Class II appliance in then you draw current through the circuit from the live through the appliance and back via neutral. If there is N-E fault in the circuit(not the appliance) then current will leak to earth in the circuit wiring and cause an imbalance within the RCD coils causing it to open.

But why does the immersion element not cause the RCD to trip. Maybe the the fault is beyond this point in the circuit?

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Strange Wiring Fault Causing RCD To Trip

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