30mA RCD fun and games

[Ben]

Any help on the following would be much appreciated. We moved into a 10 year old house earlier this year, the RCD has been fine up until last Friday, having said that it is a 30mA rated RCD for the whole house (groan). Since last Friday it has gone of 4-5 times a day.

Over the weekend I tried to identify which particular circuit was causing the problem by turning off circuits (I realise this does not stop N-E leaks on those circuits from tripping the RCD...). The RCD tripped with both lighting circuits off and with the central heating circuit off. Oh and it also tripped with the washing machine completely disconnected from the supply.

There seems to be no pattern as to which device was being turned on/off when the trip occurs. My suspicion is that something late last week started leaking enough current to earth to sensitise the RCD and now the system trips randomly when 'something' turns on or off.

OK, so the possible culprits:

1. Light in bedroom: We have a 2 month old baby who last week sounded like he was not happy with the dry atmosphere in our bedroom so (don't laugh!) I boiled a kettle in our room for a couple of minutes. (No, it did not end up like a sauna). The RCD didn't trip at that point but I'm wondering whether moisture got into the light fitting causing an increase in the overall leakage... note that based on the fact that the RCD has tripped when the lighting circuits were off, this leakage would have to be N-E not L-E. The fitting itself is 3 x 40W halogen (I think) bulbs with an earthed metal body...

1. Lights or sockets in the garage: The garage is attached to the house and has a couple of sets of sockets which are run off the kitchen socket circuit and 4 strip lights which I assume are run off the downstairs lighting circuit. It rained like mad last Friday and over the weekend... so I was wondering whether dampness had got in 'somewhere'... there are no obvious leaks in the garage. Note we have had no problem with this before.

2. Emmersion heater on the hot water tank. I noticed quite a bit of corrosion around the electric 'backup' heating element on the hot water tank. This has it's own circuit.

So, last night I:

a. Removed the light fitting from our bedroom and took a hairdryer to the circuitry beneath... hopefully got rid of any moisture.

b. Connected the fridge/freezer to the ring main rather than the kitchen sockets circuit and turned off the kitchen socket circuit.. (thus 'disconnecting' the sockets in the garage and all kitchen appliances... I wonder whether the electric igniter on the gas hob has got 'leaky'?)

c. Completely removed the wiring from the emersion heating element

Since then... no trips.. hoorah!

So I'm wondering what I do next... I'm hoping that it was the light in the bedroom as this would be by far the most easy to fix.. however if this is the problem then I am suprised that it didn't immediately trip? I'm also worried that I may not have found the cuplrit but simply reduced the amount of extra 'valid leakage current'?

Any ideas about how to diagnose further where the problem maybe? I'm not sure I'm not particularly happy about opening up the consumer unit.... (otherwise I assume one approach would be to attempt to measure the N-E/L-E impedances on the circuits).

The follow up to this is that once I've got to the bottom of this I'm intending to improve our consumer unit. Which of the following is best and roughly how much should I expect to pay to:

1. Change CU to split-load system and use a 100mA RCD. At least place lights on non-RCD side!

1. Leave current CU in place but replace RCD with 100mA unit and replace 'required' circuits with their own 30mA RCD/MSB 'thingy'

Should I also think about getting all the garage circuitry onto a separate circuit... now that could be painful!

Finally (is anyone still reading?) I'm not to keen on replacing the RCD etc myself... how the hell do I assess an electrician as 'OK' apart from rejecting anyone with a Stetson hat on! :-)

Sorry for the long post but I wanted to get as much info down as possible!

Ben

[Dave Plowman (News)]

That's the most likely culprit IMHO.

[usenet]

That seems incredibly unlikely, otherwise we'd all have RCDs popping off whenever we boil kettles.

Somewhat more likely.

Elements of all sorts can go leaky.

Not allowed, sockets which might be used for outside equipment *must* have 30mA RCD protection.

This is better and within the regulations. Do you actually need a 'whole system' RCD though? It's only required if you have a TT supply, i.e. one with a local earth. This is only likely if you have an overhead supply.

Probably a good idea, but could be done in the longer term.

[meow2222]

You cant usually diagnose a fault by guesswork.

I'd sort out the CU arrangement first, not last. Theres a good chance that will solve your nuisance trips. If not, it will narrow it down to the one problem circuit. Alternatively you can continue with your divide and conquer to narrow it down before any work.

A whole house 30mA RD with immersion and garage lectrics on it is truly poor design, this was just waiting to happen.

The simple way to find the problem is to open the CU (with power off and all breakers off) and test each circuit L to E resistance with a multimeter. Before the purists complain, a simple multimeter will pick up the problem in almost every case.

However if youre not happy about opening the CU, let a sparky do it. Obviously doing so you'd be working on partly live electrics, so its not for someone that doesnt fully understand what theyre doing.

NT

[Chris Bacon]

Oh dear, then mine will give problems... I've only been waiting for about 12 years, though. Not one "trip" so far.

[Ben]

I thought this was a prime candidate, however... having taken off the wiring I then measured L-E and N-E resistance at the connections on the hot water tank and found neither of these had a resistance that my multimeter could measure (I think it's max resistance is 20 or

50Mohms). Maybe my multimeter's voltage is too low to 'stimulate' the low resistance... he said clutching at straws!

[Andy Wade]

IT would help greatly to know what type of earthing you have - is it TN-C-S (PME), or TN-S (earthed to sheath of supply cable) or TT (no mains earth supplied & you rely on your own earth electrode)?

Remainder of reply assumes TN.

[snip details]

It's better to change only one thing at a time. Try restoring these items one by one.

If you can venture inside the CU and disconnect the relevant neutral(s) for each circuit you isolate, that would help a lot. In any case I suspect there's a definite fault that can be pinned down. My money would be on it being either either a neutral-earth short in the wiring somewhere, or else the immersion heater (or similar) is on its way out.

You need to do some insulation resistance tests with a proper 'Megger' (500 V insulation tester). You won't get very far without getting inside the CU, so maybe it's time to call a sparks.

Not permitted (unless you have separate RCD protection for sockets, and then unorthodox).

Unless TT-earthed, in which case a minimum of two RCDs is needed.

Possible, but not "best" - and the 100 mA RCD would need to be a time-delayed (Type S) item, to "afford discriminative operation" as the jargon has it.

Better (still assuming TN) would be a new split load unit with a 30 mA RCD and everything except socket circuits on the non-RCD side. (Individual RCBOs might be "best" be the cost might put you off.)

And bear in mind that it might only be a new immersion heater that you need, or that the problem might only be a neutral wire trapped under a screw in a wiring box.

Difficult to comment without knowing (a) how it's wired now and (b) what goes on in your garage...

Strikes me you know enough of the principles to detect blatant bullshit. Get hold of copies of the IEE On-Site Guide and/or the newer Electrician's Guide to the Building Regulations and study...

[Mathew Newton]

Faulty RCD? ;-)

[Chris Bacon]

Bah. It works if I press "test", also it has tripped when I had my fingers inside a socket to screw some wires in (the downstairs ring). Bang! it threw "off", just over my head.

[ARWadsworth]

And not a lot of effort is needed to remove it from the circuit. As there will be a double pole switch operating it there is no need to remove any wires as the N-E would be seperated when the switch is turned off.

Adam

[Ben]

Life is never easy :-) Got home and turned on the kitchen socket circuit. Couple of hours later, RCD trips... heyho.

At least I now know which circuit is at fault.... all I have to do now is determine what part of it! I assume I'm looking for a L-E (it didn't trip when the circuit was switched off) and that the most likely culprits are the garage circuitry or maybe the gas ignition. I think the only appliance actually plugged in when it tripped tonight was the hob extractor fan. Will do a quick check of that plug with the multimeter, assuming that turns up a blank it'll have to be a call out to a sparky.

[John Rumm]

Alas not certain - a N-E fault could show up because the N may (quite normally) rise a few volts when the circuit is loaded (depending on how far you are along the circuit from the CU, and also on how "stiff" your supply impedance is).

Things with heating elements and things involving water in close proximity to power are always likely contenders. Needless to say the average kitchen is often full of things that match those descriptions.

You could always carry on playing the stepwise refinement game a little more if required. You may find for example breaking the ring at a socket half way round the circuit, disconnecting one end of the circuit at the CU and running a temporary lash up cable from the CU to the break in the circuit to make a new ring using the new cable plus half of the old circuit.

Also if you can't find anything with a DMM, is there any chance you could borrow a megger from someone?

[Ben]

Point taken....

Well I've now unplugged everything from the sockets, the only thing left 'powered' on is the gas ignition (as it looked quite a challenge to get 'into' the gas hob last night).

Makes sense.

I don't know whether this is good or bad 'wiring practice' but looking in the garage I found that it was a spur from the kitchen socket circuit. The spur had a two socket block from which another spur leads round the garage to a set of two 'two socket blocks' (i.e. the garage has 3 sets of 2 socket blocks).

I thought that I may as well check that the the latter wiring was not at fault so simply disconnected the wires connecting the first '2 socket block' to the two 'two socket blocks'. Thus the only 'live' wiring in the garage is appox 1.5m length to the first 2 way block. Does this 'spur' arrangement sound OK?

I'm sure that the fault will end up being behind the most difficult to get at tiled part of the kitchen :-)

Not wishing to start a 'ruck' but can I perform anything useful testing wise using a DMM? Presumably I'm reliant on the voltage being generated by the DMM being large enough to 'stimulate' the fault? I assume the approach would be to disconnect each part of the ring circuit in turn measuring L-E and N-E on each isolated section?

Hmm... good question... unless the isolated garage wiring is at fault I fear getting hold of a megger or, more likely, getting hold of a man with a megger will be the next step! If I do manage to get hold of a megger is it relatively straight forward using them?

Thanks for all the advice thus far.... I'm very impressed with uk.d.i.y! Ben

[Tony Williams]

This is very similar to a Saga (with a capital-S) that I had, and posted here for help a year or so ago.

Random RCD trips started just after the incoming supply cable was replaced and I spent days, tracing red herring after red herring. I realised early that it was an N-E short, but took days to consider the possibilty that it could be in the cabling, rather than an appliance.

So I powered-down the consumer unit, opened all MCB's, and lifted all Neutrals off the commoning-block in the CU. Then used a DMM to ohm each separate Neutral-Earth.

Sure enough one N-E showed continuity, the T&E going to the kitchen ring. Unplugging everything off that ring made no difference, so it *was* in the cabling.

Fortunately I had a low-resistance ohmeter as well, and was able to target the lowest N-E resistance to one particular socket in the kitchen. Took the socket off, no problems in there.

Then I looked up at a row of new nails in the wall and idly tapped one probe of the ohmeter across the heads. Found a continuity to one of them, 4ft above the socket and about 6 inches to the left. Pulled the nail back and the N-E continuity disappeared.

[Ben]

So the latest in this particular Saga is that the RCD has tripped and hence it's not in the bit of garage wiring I managed to isolate last night.

I assume to do this I:

1. Power down the CU, open all MCBs, lift the kitchen socket neutral off the common block. Test for low resistance on N-E or L-E... then to isolate:
2. Determine the order of the sockets in the ring and open each in turn, 'break the ring', test N-E and L-E and do this round the loop until I get a low resistance ...

Then I know it's in the last section. Is this correct?

[tony sayer]

And that was what killed that woman a few years ago which was what led to part P....

[Phil]

They could have just legislated that everyone entering a kitchen has to wear rubber wellies and have one hand tied behind their back....would have made more sense.

[tony sayer]

In article , Phil writes

Thought that was for bedrooms......

[John Rumm]

The first spur should be taken as a fused connection in this case. Assuming there is a fuse for the spur (since you can take a spur from a spur without protection from overload for the cablw) and we are talking about an attached or integral garrage then we are on the borderlines of good practice - depends on what the garrage power is used for.

If it is a detached garrage then it is a pretty naff way of doing the wiring to it.

What about lights in the garrage? Are there any and how are they powered?

Yup good move. Garrages and outbuildings in general not being heated etc are more likely to suffer problems with dampness getting into fittings and are hence are a typical place for the origin of these types of faults...

Safe bet ;-)

You can perform plenty of useful testing with a DMM. It is only the cases where that fails to show up anything conclusive that the megger comes in handy in that it will allow you to eliminate some catagories of fault that you can't eliminate with the DMM.

Yup that would do it for starters...

Much the same as you would use a DMM on resitance range with the obvious provisio that you are now playing with 500V and not

[meow2222]

be aware its all too easy to unintentioanlly export dangerous voltages to metalwork around the house with one. Meggers are something best not used by the untrained imho. A multimeter will find the problem in almost every case.

NT