The IET is carrying out some important updates between 17-30 April and all of our websites will be view only. For more information, read this Announcement

Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Suggested Answer
  • Locked Locked
  • Replies 17 replies
  • Helpful Answers 1 Helpful answer
  • Subscribers 37 subscribers
  • Views 6587 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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

Parents
  • 0

    You seem to have a pretty good handle on the situation.

    A clamp meter should work - but you will need one that reads down to a few mA fairly accurately - many ordinary (i.e. cheap) ones don't. The only  drawback of that approach is that a decent amount of leakage has to be present while you're using the meter - which isn't always the case with intermittent problems.

    A N-PE fault sounds quite likely - it can be anywhere (even inside an appliance) and can sometimes be present even when the appliance is switched off (if the switching is the usual single pole)  - but only makes the RCD trip when the voltage on N rises - usually due to voltage drop - i.e. when other appliances are drawing current. The difficulty can be compounded by appliances that have some sort of automatic control - i.e. anything from washing machines which switch different bits on an off and the program progresses or heating systems under thermostatic controls.

    If it we me I'd go down the IR route and track down the N-PE issue for starters  - a single fault affecting several circuits isn't unknown - often cables run together from the consumer unit and through a common hole/notch in joists so a single mis-placed nail can easily pierce several cables.

    If it is just cumulative leakage currents (Air Con might be a good suspect for that too) - there might be some possibility of re-arranging circuits between the two RCCBs to balance things a bit better (but keep an eye on overall loads of each, especially if the RCDs are rated <100A).

       - Andy.

Reply
  • 0

    You seem to have a pretty good handle on the situation.

    A clamp meter should work - but you will need one that reads down to a few mA fairly accurately - many ordinary (i.e. cheap) ones don't. The only  drawback of that approach is that a decent amount of leakage has to be present while you're using the meter - which isn't always the case with intermittent problems.

    A N-PE fault sounds quite likely - it can be anywhere (even inside an appliance) and can sometimes be present even when the appliance is switched off (if the switching is the usual single pole)  - but only makes the RCD trip when the voltage on N rises - usually due to voltage drop - i.e. when other appliances are drawing current. The difficulty can be compounded by appliances that have some sort of automatic control - i.e. anything from washing machines which switch different bits on an off and the program progresses or heating systems under thermostatic controls.

    If it we me I'd go down the IR route and track down the N-PE issue for starters  - a single fault affecting several circuits isn't unknown - often cables run together from the consumer unit and through a common hole/notch in joists so a single mis-placed nail can easily pierce several cables.

    If it is just cumulative leakage currents (Air Con might be a good suspect for that too) - there might be some possibility of re-arranging circuits between the two RCCBs to balance things a bit better (but keep an eye on overall loads of each, especially if the RCDs are rated <100A).

       - Andy.

Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines