More RCD tripping woes

[Pander]

Hi to all, new to the forum, this is my first post so please be gentle with me.
OK here is my query, I am trying to help a friend with a small but irksome problem of nuisance tripping of the RCD, at first it appeared to be random with the toaster being heavily involved, first thoughts were toaster gone bung, so a new one was purchased but the problem remained, I suggested testing the sockets, they were tested with a Rapitest socket tester ( I know you guys are not keen but that was all that was available at the time ) all sockets tested OK except one double that tested OK with the tester plugged in and socket turned on, but when the socket was turned off with the tester still plugged in it was showing a neutral fault, could this socket be the cause of the random tripping or are those testers not that reliable because I thought the testers only worked when it had power to it. Any ideas on what might be going on.
Thanks in advance.

The C/U is a Siemens Stratum 200
RCD 80A 30ma

 

[securespark]

Neutral fault could be a dicky switch on the socket, Swap it out and retest.

As for the fault, you really need an MFT (multi-function tester) to get to grips with it. Failing that, you need a competent spark!

The MFT will do several things:

Check the Insulation Resistance of the fixed wiring on the RCD side to make sure there are no short circuits present.

Check the RCD for correct operation and Ramp Test, which ensures it is not tripping when it should not.

Also you should use it to ensure you have an acceptable EFLI at each outlet.

When IR testing you need to disassociate all loads from the circuit.

 

[Pander]

Hi securespark,
thanks for the quick response, I think the most obvious route to take would be your first suggestion to swap out the socket and retest, I will try to get over to him at the weekend and do the swap as I don't think he would be confident enough to do it himself.
Out of interest could a neutral fault cause the RCD to trip at random Also as I do not have the correct test equipment for a ramp test or an earth fault loop impedance test on the RCD if the problem is not solved with the swapping of the socket I think it maybe time for him to get a sparky.
Thanks again.

 

[EFLImpudence]

Out of interest could a neutral fault cause the RCD to trip at random

I am not quite sure what your socket tester classes as a 'Neutral fault' other than it is not connected or has voltage on it.

N.B. These socket testers CANNOT detect Neutral to Earth faults.

To be pedantic a neutral fault can't cause a 'random trip'. A random Neutral fault when it happens may cause the RCD to trip (depending on the type of fault).

if the problem is not solved with the swapping of the socket I think it maybe time for him to get a sparky.

Very wise.

Returning to your original question. The RCD could trip because of accumulated earth fault currents which could have been sent 'over the limit' by the toaster but the main fault may be with another appliance.

The socket testers do not work when the socket is turned off (i.e. no power) so it does seem likely that this socket could be faulty.

 

[JohnW2]

The socket testers do not work when the socket is turned off (i.e. no power) so it does seem likely that this socket could be faulty.

... or wired with L/N reversed (and a single pole switch). With the socket switched off, the tester would then still have an L supply (and earth, as a return path) but would see an open-circuit N (because of the switch), which could be the 'neutral fault' it was reporting. However, if that were the case, then one would expect it to report the L/N reverse when the socket was switched on - which is contrary to what the OP described.

Kind Regards, John.

 

[EFLImpudence]

... or wired with L/N reversed (and a single pole switch). With the socket switched off, the tester would then still have an L supply (and earth, as a return path) but would see an open-circuit N (because of the switch), which could be the 'neutral fault' it was reporting. However, if that were the case, then one would expect it to report the L/N reverse when the socket was switched on - which is contrary to what the OP described.

Yes, I was thinking more along the lines of a double pole switch, because faulty, only switching off the Neutral.

However, for Pander, both of our examples are coincidental and not the cause of the tripping fault.

 

[Pander]

Hi guys,
don't you just love forums ? I do! cracking advice, great banter, an all together great experience, anyhow back to the original post, the testing is not being carried out by myself at the moment info is being passed to me via phone, text ETC, I am just trying to help from afar. It would seem to be that the switch on the socket is indeed faulty and when the tester is inserted into the socket and the socket turned on the tester indicates socket OK three lights lit ( the tester works on a lights lighting up system you know the ones ) when the socket is turned off two lights lit, which indicates a neutral fault on the tester, that was the basis of my question as to whether a neutral fault could cause random tripping, I was a little puzzled though that the tester in question could show a fault when not powered but that appears to have now been answered by the fact that the socket is faulty and is giving false readings, I have now obtained an MFT and will try to do some further testing over the weekend after replacing the socket.

 

[PrenticeBoyofDerry]

As a short term remedy, you could consider linking out the the socket outlet, ie disconnecting the current face plate and connecting up the cables in a connection block. (that is providing it needs continuity to another outlet, ie part of a ring final circuit or part of but not the end of a radial circuit, if not just terminate the ends)
Then replace face plate just as a cover for safety from live conductors and see if the RCD still trips out. If it does not trip, we can assume it is the socket outlet plate, if it does the problem lies elsewhere.

 

[bernardgreen]

that was the basis of my question as to whether a neutral fault could cause random tripping,

On its own a neutral to earth fault is un-likely to trip an RCD but when there is current flowing in another circuit then the neutral earth fault wil divert some of that current in the neutral away from the RCD and the resulting in-balance will trip the RCD.

Neutral_Earth_trip

• bernardgreen
• 16 Jan 2011
• 48

If for example the neutral earth fault is in a lighting circuit the RCD could trip when an appliance is turned on in the kitchen. The immediate thought is the fault has to be on the kitchen circuit.

Random tripping of an RCD requires a check on all circuits. It is complicated even further when long runs of cables are involved and inter-conductor capacity adds to leakage currents.

 

[JohnW2]

On its own a neutral to earth fault is un-likely to trip an RCD but when there is current flowing in another circuit then the neutral earth fault wil divert some of that current in the neutral away from the RCD and the resulting in-balance will trip the RCD.

Whilst that's totally true, in practice there is nearly always some current flowing in some other circuit all the time (frdges, freezers, time switches, bell transformers etc. etc. - even if there are not lots of electronic appliances on 'standby') - so, in practice, a N-E fault will usually result in immediate operation of an RCD.

Furthermore, socket testers of the type we're talking about usually cannot detect a N-E fault (it would not be easy to implement) - so a 'neutral fault' indicated by such a tester usually means an absent/disconnected neutral.

Kind Regards, John.