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intermittent RCD tripping advice please

Détective de fuite

Hello there everyone,

I wonder if anyone would be kind enough to advise on the best course of action with a fault that my father has.

He’s had an intermittent RCD tripping fault since late last summer.

The RCD regularly trips anything from 2 hours to 4-5 days. There doesn’t appear to be any obvious reason for the trips - appliances, weather, time of day etc.

Installation is twelve years old and was all newly installed when their house was renovated. The CU is a Europa and has two rows of 10 slots, each with RCCB protection. The bottom row is the one that keeps tripping - top is fine.

They have had a few things added fairly recently like AC and a new microwave. The microwave seems the closest to the timeline of problems starting.

They do have a Whirlpool bath that needed a new control lead (low voltage logic control stuff), due to water rusting the male/female connector, but that has now been replaced - no water ingress to the main PCB control box. However, Dad found that when turning the bath supply back on that it would sometimes trip straight away, but normally it stays on when idle. I became suspicious of the bath when I learned of the rusted control cable and tripping and advised him to fully isolate the bath to see if the tripping stopped. Isolating the bath does seemed to have reduced the frequency of the tripping, but not stopped it (maybe it’s just adding a tad more leakage, which just pushes the RCD over the limit?). There are a few boxes under the bath (PCB control box, LED control box, air blower/heater). Note: the bath wasn’t connected when the latest IR test was performed. However, I’ve seen the power trip just by turning on their outside lights, for example.

First electrician came a few weeks ago and did some very basic testing. He was on his way home from another job and didn’t have much time. He tested the RCD, said it was fine, but replaced it anyway (Clipsal) to rule it out. He told Dad at the time that it was a N-E leak and that it could be anything causing it. He tried for a while to get him back, but he was too busy with an industrial install.

A different electrician visited this week and spent two hours there looking at the install. I had the chance to speak with him this morning. He said that the IR test didn’t look good on three of the circuits, which are all the ring mains. He didn’t test leakage. He tested the outdoor sockets which were all fine. Dad thinks the meter said 360 for one of the ring mains, but no idea if this is correct - take that with a pinch of salt! He also discovered that top RCD was stuck, but freed it and said it was fine (it hasn't tripped since, but the bottom has twice).

He recommended that Dad worked one room at a time, unplugging everything, leaving it for a week and using this approach to try and locate the problem. Dad isn’t keen on this as he’s a bit older and not the most technical person! However, I’m happy to spend some time there to do some testing, if it will help.

Given the randomness of the tripping, I asked him if he thought it might be caused by cumulative leakage across all the ring mains or a single faulty appliance. He felt it was a possibility.

When Dad wasn’t keen on the testing option, he suggested upgrading the existing CU with RCBOs. Cost is ~£700. As I believed would be the case, he confirmed that doing this won’t necessarily fix the problem, but it would potentially pinpoint the circuit - although, at this point, surely we are suspecting the ring mains, or in particular the two ring mains on the bottom CU row that trips. He told me that it is not cost effective for him to diagnose the fault, but he felt there was value in somebody like me doing the testing and was supportive of that idea.

Would it be worthwhile getting a clamp meter and testing all of the appliances on the ring mains - starting with everything connected to the sockets on the two ring mains of the bottom CU row that is tripping? I understand that I’ll need to adapt an extension lead so I can clamp just the L-N.

What about clamping the L-N at the CU, taking the background readings of the leakage? Could you leave the clamp on and then go around and unplug things one at a time, looking for a large drop and potential source of the problem(s)?

I can understand that you might have a faulty appliance on one ring main, but all three? Likewise, how likely would it be that all three ring main/circuit cables are damaged in some way?

Appreciate any insights, thoughts or advice on this problem.

Many thanks in advance,
Richard

  • 0 in reply to Détective de fuite

    There may not be a fault at all, juts a piff poor design of too many final circuits running through one sensitive R.C.D. Possibly a shared neutral then. Or a spike on the supply when the inductive load is turned off which the bottom R.C.D. does not like. I'd go for a new all R.C.B.O. consumer unit myself. Go on, bite the bullet. You know it makes sense.

    Z.

  • 0

    Switching off inductive loads like fluorescent lamps often does cause a short surge in voltage - while that won't directly cause an imbalance the RCD would see, the increase in voltage on the L&N lines can increase the leakage to earth via either via things like filtering capacitors or any existing N-PE faults. Such spikes are very short lived - a very small fraction of a second - so probably wouldn't register on a clamp meter.

    If that N-PE fault is still there I'd try to track that down first. If you still have a problem (e.g. due to all the filtering in the AC equipment etc) - then as Zoomup says it'd be a matter of divide and conquer with the RCD protection - and split the loads between a larger number of 30mA RCDs.

       - Andy.

  • 0

    Thanks guys.

    One question: Can I use the ability that I can trigger the fault with the florescent lights to determine if I've found a faulty appliance? Say I isolated all the appliances on the ring mains and then I flick the light switch to get it to trip. If it doesn't trip the RCD could I then turn things back on, one at a time to 'find' the faulty appliance. Does this logic work at all?

  • 0 in reply to Détective de fuite

    So, what happens, if say, there is a faulty appliance and the fault only develops half way through a  cycle for example such as in a washing machine? Also we refer to "ring mains" as  ring final circuits these days. I think that you are wasting your time here.

    Z.

  • 0 in reply to Zoomup

    To answer the O/P.

    Personally, I'd completely disconnect the old florescent lights and see if that improves things. They could be temporarily replaced with a conventional cheap batten lampholder and ordinary light bulb for the purposes of elimination. Don't forget, just switching off the lights at the light switch isn't actually isolating the whole light fitting, but merely disconnecting only the live conductor. See further below -

    Zoom,

    Agree up to a point - the only way in that instance is to let the appliance go through it's full working cycle. I.E. Wash, spin, dry, empty water, end.

    However, nothing is wasting time if the choice is to have power or no power when it comes to tracing a fault.

    Just a quick question - to the O/P. When you say you are 'isolating' the appliances, are you actually physically unplugging them from their socket outlets? This needs to be done because 'isolating' by just switching off the socket switch isn't actually 'isolating' at all. The neutral and earth are still connected to the consumer unit via the neutral and earth plug pins unless the socket outlet is equipped with a double pole switch. To save tail chasing, make sure that appliances are physically removed from the sockets themselves.

    Overall, it sounds as if there are 2 consumer units? Or at least a double tier consumer unit, with each row of circuits being protected by a single RCD main switch? Not a good idea for fault discrimination as you are finding out. Am suspecting the cumulative leakage currents are approaching or have reached the RCD trip threshold, and no amount of just replacing the RCD will stop the tripping even if no real fault is actually present.

  • 0

    Interesting results

    add up all those 3mA and 4ma & see how many are one any one of your RCD.
    Note that an RCD with a 30mA threshold is guaranteed to trip before 30mA and it may operate at an imbalance as low as 15mA, but actually usually when we test them they do their thing somewhere in the low 20s, varying a bit with age and from test to test.

    I think it may be worth considering that this might be a case where there is no one 'killer fault' but a situation where there are a number of items that in some unlucky  combinations tip it over the edge.


    How practical is it to re-organise the circuits onto more RCDs - is there room for another one in the box ? Or are RCBOs made for that box ? that would in effect mean one RCD per circuit. If not is there room for another box along-side ?

    The lights if they are quite new, may be a red herring, except in saying that your RCDs are near the edge and the transient of starting takes you over.

    Mike

  • 0

    One question: Can I use the ability that I can trigger the fault with the florescent lights to determine if I've found a faulty appliance?

    Quite possibily ... but with a few caveats. As has already been mentioned, be careful of the situation changing around you - all sorts of things can switch on or off or vary their loads automatically, which can potentially be misleading. Also the N-PE fault may not be in a pluggable appliance - high resistance faults can easily occur in fixed wiring or fixed appliances especially where damp or corrosion is about (outside lights?) or there can be multiple faults all making variable contributions that can seem to give illogical results (somehow a lot of faults seem to come in threes). So it may be possible, but it probably would be a lot easier with an insulation tester - so you can have a measurable quantity to judge changes against.

       - Andy.

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