RCD tripping and no idea why

Discussion in 'Electricians' Talk' started by HewsonSwansea, Oct 10, 2021.

  1. HewsonSwansea

    HewsonSwansea New Member

    Problem:
    RCD seems to trip randomly and I'm not sure why. It seems to be related to the kitchen sockets and the fridge in some way.

    Background:
    A few weeks ago, my electricity kept tripping. When I looked in the consumer unit, the RCD was down, but it wouldn't go back up unless the kitchen switch was down. As it seemed to be tripping at random times of day, I hedged my bets on it being the ancient fridge causing the problem. I got a brand new fridge and the problem was solved straight away.

    This week, some builders moved an electrical socket in an upstairs bedroom. This seemed to trigger the old problem. We have triple checked the socket and had the floorboards up to check that they haven't put a nail through a wire or pulled a wire out of the consumer unit, but it all seems fine. The electricity now trips randomly, but seemingly, only at night. As before, the RCD is down and won't go back up until the kitchen switch is down as well. There is no appliance, heater or boiler on a timer. So, I thought it must be the new fridge. The new fridge is now on an extension lead to another room and this seems to have temporarily resolved the problem.

    The Consumer Unit:
    The consumer unit has a 32 amp MCB (green arrow in attached photo) that supplies all sockets in the house across 3 floors, except the kitchen. The kitchen is supplied by a 20 amp MCB (pink arrow) - I suspect this is too low and contributing to the problem. The 6amp (red arrow) supplies all the lights in the house across 3 floors. There is also a mysterious 32amp (blue arrow) that seemingly does nothing.

    Other notes:
    • The boiler, fridge and oven can be switched on to max with the kettle boiling, but nothing trips.
    • The tripping does not seem to be related to use of any appliance - it tends to happen at night, when nothing is being used. I do not currently heat the house and this is not on a timer. The only things running through the night are the fridge/freezer, wifi, and a filter and heater for a fish tank. I have quadruple checked that there is no water dripping down the wires and into the fish tank etc. The fish tank is not on the kitchen sockets.
    • It doesn't look like the oven was put in correctly - there is no isolation switch. To access the socket, you have to pull out the oven. When I moved into the house, they hadn't even bothered to plug the oven in.
    • There are no signs of damp/water ingress in the kitchen or around the consumer unit, or anywhere else that I have noticed.
    • The builders had a couple of different plug-in testing devices. They suggested that all the kitchen sockets and the socket they tried were all working correctly. (Although perhaps this isn't the most accurate method).
    • Whilst the builders were here, the RCD did trip once in the day. The fridge was already on the extension lead, but they had power tools plugged in and I assumed it was an overload or something. The RCD reset without any difficulties, whereas usually, it takes several attempts to go back up.
    • The whole house was rewired in 2018, but I don't think those electricians were responsible for the cooker (it is the only electrical socket that doesn't match the others and it wasn't put in correctly).
    • When I moved in, I discovered that the internet cable and phone line into the property had been cut. There are also two sets of outside lights that have never worked. They were in a state of disrepair when I moved in, and looked like they would not have worked for several years. I got an electrician to have a look and he thought that the wires had probably been cut and rendered over, so they couldn't be repaired, instead we would need new lights and wiring.
    • There is a mysterious light switch in the hallway, which doesn't seem to do anything. It doesn't switch on the hallway light. I don't know if it is from a previous porch area, or if it is for the outside lights? There is an outside light switch near the front door, which doesn't work, but I assume it was for the outside lights.
    • The building work involved a wall coming down and a new ceiling. A light switch was moved and a socket was moved. All the problems started after the electricity was switched off to allow them to move the socket (the new ceiling and light switch happened two days after the onset of problems)
    • I believe that the consumer unit was installed in 2018 when the house was rewired. (I moved in in 2020)
    Any ideas?
    So, I have informally approached a few electricians (friends-of-friends) and they can't quite work out what is going on. At the moment, there seems to be a waiting list of approx. 3 weeks to get an electrician where I live and I'm trying to avoid paying the fees for an emergency electrician if possible. It would help to know if any of you have any ideas about what could be the cause of the problem, so I can share these with the electrician when they come in a few weeks. I'd welcome any suggestions that you may have.
    Thanks
     

    Attached Files:

  2. Tony Goddard

    Tony Goddard Screwfix Select

    This is what we call nuisance tripping - it can have many root causes, excess leakage and intermittent neutral earth faults are common causes. Even to a seasoned pro like me these can be awkward to find.

    Test gar can be helpful, but often the fault isn't present for very long, so you would have to be lucky to find it. I would run insulation tests on the house wiring, but clearly you cant do that.

    You can however use some deductive reasoning to work out what may be going on. here are some tips:-

    Does it always happen at a similar time? if so what item would be switching on around that time, an outdoor light, an immersion heater etc..

    Has anything happened that could have caused it to recur, here in the South West it rained like crazy last weekend, that has seen my phone ringing with similar problems..

    Check outdoor light fittings and external junction boxes, is there condensation inside, likewise the shed or garage, any rodent nibbling on cables..

    In the house, have you put up any pictures lately, drilled any holes??

    Try unplugging appliances that are not used, or overnight, does that abate the tripping?

    You could carefully, and with the power off remove all the switch and socket plates, are all the wires tightly home, is there any cut or damaged wire insulation, have any wires been trapped (particularly by faceplate screws), again any sign of dampness.

    If none of the above bear fruit, then you will need an electrician - they will test the insulation of the wiring to see if there is a leakage of current from live & neutral across to earth.

    Often it can be cumulative leakage from appliances, those with heating elements in, and some with smoothing circuits, surge protectors etc. can naturally leak current, this is OK until the cumulative load exceeds the 30mA to trip the RCD. Cookers, Washing Machines, Diswashers, Surge Protected extensions, PCs etc all can be expected to have a little leakage, if one is getting out of hand it can pull the whole lot over the limit.

    If you can borrow a modern PAT tester you can check the appliances yourself.

    Best of luck finding it - its quite satisfying to track a fault down!!
     
    HewsonSwansea likes this.
  3. Tony Goddard

    Tony Goddard Screwfix Select

    Might be a red herring but you state.

    "A light switch was moved and a socket was moved. All the problems started after the electricity was switched off to allow them to move the socket (the new ceiling and light switch happened two days after the onset of problems)"

    I'd be looking behind those, sometimes the screws used to hold the plate on chaff a wire inside - remember to switch off the main switch before taking them off, could be a screw through a cable also - so many possibilities!!

    You should have had a minor works certificate for each the socket and light switch, on that the insulation resistance for the circuit will be noted in Megohms.
     
    HewsonSwansea likes this.
  4. MGW

    MGW Screwfix Select

    Not got 18th edition but 17th says
    To comply with that the electrician needs to measure the imbalance and decide how many RCD's are required to ensure under normal operation the leakage is no more than 9 mA, I will hold up my hands and admit I have never measured it, mainly as I don't have a suitable meter. However the RCD tester does test with 1/2 rating and if tested with all circuits connected then there should be no problem, but although often done, wiring a home with just two RCD's is really taking a bit of a chance, odd with the old 100 mA type S (means delayed) trips were rare, but the 30 mA used today do seem rather sensitive.

    So personally I have 14 x RBCO's which is a MCB and RCD combined, this reduces the chance of tripping and also means if it does trip most things carry on working.

    The RCD does not measure current to earth, it compares the current in the line and neutral wires and if both lives are nearly the same it does not trip. So if either current leaks from line or neutral it trips, line is not too much of a problem as turning off MCB's or unplugging you can normally work out a line to earth fault. The problem is the neutral to earth fault.

    Neutral and earth are nearly the same voltage as each other, the voltage difference increases the more power that is used, so a neutral earth fault on toaster due to bit of damp toast can cause the RCD to trip when kettle is used, as the kettle uses more power than the toaster.

    Switching off with a neutral - earth fault often does not help, as often neutral is not switched, so step one unplug anything not in use.
     
    HewsonSwansea likes this.
  5. robertpstubbs

    robertpstubbs Screwfix Select

    Don’t rule out the possibility of the RCD being faulty.

    I recently had a faulty RCBO, although that was constantly tripping even with no load attached, rather than tripping intermittently.

    I don’t know how accurately they are manufactured (ie how close to the 30mA trip level they really are) or whether they deteriorate over time?
     
    HewsonSwansea likes this.
  6. Bob Rathbone

    Bob Rathbone Screwfix Select

    I agree with Tony, go to the socket that was moved and check to see if the cables are pushing against the back box fixing screws. Before you put the socket back on cover the head of the box fixing screws with a couple of layers of tape. Also look for damage to the cable insulation.
     
    HewsonSwansea likes this.
  7. HewsonSwansea

    HewsonSwansea New Member

    Thanks for your suggestions. Thought it might help clarify things to answer some of the questions you mentioned here:

    It tends to happen at night when I'm asleep, but there is nothing that switches on at that time. The heating is off completely. I don't have any outside lights, there's nothing on a timer. No shed or garage or anything outside with any electricity running to it. No evidence of rodents or damp that I've seen. Haven't put any screws or nails into the wall recently.

    The only thing I've noticed is that the fridge being on an extension lead connecting to a different circuit seems to abate the tripping. I can try unplugging all the other appliances and plugging the fridge back in to the kitchen socket to see if that makes a difference.

    I guess it's time to start unscrewing sockets and having a look inside. Have previously had a look inside the socket the builders have moved and it all seems fine. It's almost as thought turning off the electricity so they could do that has restarted the old problem with the old fridge. Doesn't feel any different from that at all, but now I have a brand new fridge, so I'd be pretty unlucky if that was faulty as well.
     
  8. HewsonSwansea

    HewsonSwansea New Member

    Thank you - this does sound possible. It doesn't seem to be related to a specific socket or appliance that I can find, which makes me wonder if it's a fault in the consumer unit, but I'll have a go at taking the sockets apart and see if I can find any problems in those.
     
  9. HewsonSwansea

    HewsonSwansea New Member

    Thank you, I will plug the fridge back into the kitchen socket tonight and unplug everything else to see if it trips again.
     
  10. HewsonSwansea

    HewsonSwansea New Member

    Thank you for all your help. Just as an update incase you're interested:
    It looked like things were getting better. I switched off all the sockets before going to bed and it seemed to stop tripping. Then we had some stormy weather and it was tripping 5 times a day. I finally got round to getting the sockets off the wall. Two were fine, but inside the socket for the boiler, I found a lot of rust and a revolting pile of dead larvae of some sort. I've cleaned out the socket, but will get the rusty knockout box replaced and the damp investigated.
    Thanks for all your help.
     

    Attached Files:

  11. Bazza-spark

    Bazza-spark Screwfix Select

    Thank you for the update, it is always useful for us to find out what the cause was.
     
    HewsonSwansea likes this.
  12. The Happy Builder

    The Happy Builder Screwfix Select

    @HewsonSwansea

    It looks like a blue neutral wire behind that socket is not terminated correctly in the yellow cable connector and the end of it is resting on the metal back box.

    That would cause an intermittent RCD trip.
     
  13. HewsonSwansea

    HewsonSwansea New Member

    Thank you, I'll have to have another look.
     
  14. sparko69

    sparko69 Screwfix Select

    The neutral (blue) in the yellow in line connector might also not be pushed firmly into the connector and that would also cause tripping so that needs checking by someone who is competent
     
  15. The Happy Builder

    The Happy Builder Screwfix Select

    Do it with the electric supply off.
     
  16. Bazza-spark

    Bazza-spark Screwfix Select

    Sorry, I missed something here. Thought it was sorted.
     
  17. The Happy Builder

    The Happy Builder Screwfix Select

    You missed that he opened up the socket and probably moved the wires inside it which may have stopped the tripping because the neutral conductor is no longer touching the metal back box, but hasn’t actually solved the problem.
     
  18. sparko69

    sparko69 Screwfix Select

    Hence why I suggested checking the termination of the in line connector.
     
  19. HewsonSwansea

    HewsonSwansea New Member

    Thanks everyone. The problem does seem to be resolved for now. It could be the debris that I removed or inadvertently moving some wires, I can't say for sure, but I do have an electrician coming next week anyway. I'm going to get him to look at the socket, the 20 amp kitchen MCB and fitting a cooker isolation switch too. Just happy to have some improvement and a vague idea of what might have been the cause. Thanks for your help.
     

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