RCD randomly tripping

Discussion in 'Electricians' Talk' started by jgw1977, Nov 4, 2021.

  1. jgw1977

    jgw1977 Member

    Hager CD285U RCD keeps randomly tripping. I have a consumer unit and one side keeps randomly tripping even at night when most things are completely off.


    The CU is split - one side has the lighting and water heater, and the side which trips has the extension, hob/oven, ground and first floor sockets.


    Could the RCD be failing? I’ve tried switching random things off, but it’s still randomly tripping.
     

    Attached Files:

  2. Bogle Crag

    Bogle Crag Screwfix Select

    First thing I look for in this situation is water ingress, any outside lights,sockets,pond pumps
     
  3. quasar9

    quasar9 Screwfix Select

    You say “when most things are completely off” . What items are still running ? Also, it’s a strange in which the load has been distributed across the two RCDs. Each should support some sockets and lights so when tripped, at least the whole house is not plunged into darkness.

    isolate all items. Unplug them where possible or switch them off at FCU or isolator and then see if it trips. Classic Culprits are, heating elements in appliances, water using appliances like washing machines or dishwasher which may trip at some point in their wash cycle via water ingress

    faulty RCD’s are not unknown. You could get an electrician to swap it with a new one. It may or may not solve the problem.

    keep a note and times of trips, a pattern may emerge that narrows your search.
     
  4. Sparkielev

    Sparkielev Screwfix Select

    As above un plug any appliance that has a element, motor, check outside socket if you have one, the consumer unit has been badly designed the sockets should be split either side, RCD faults can be a pain in the ares to find if it keeps happening then get a reputable electrician
     
  5. MGW

    MGW Screwfix Select

    I had an RCD tripping, I also had insulation tester and RCD tester and it showed as being A1, but when changed the problem stopped, but not all RCD's are equal, the X-Pole was claimed to trip between 90 - 100% where the requirement is 50 - 100% and it also had a built in warning to show when getting near to trip point. But unless an EATON consumer unit can't fit one. We also have type AC, A, F and B and the type F is claimed not to be affected with high frequency that could result when using inverter drives, and I know my house fridge/freezer and fridge both inverter drive, as is the washing machine.

    We are told we should split the install into circuits, as necessary, to take account of danger that may arise from the failure of a single circuit such as a lighting circuit and reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation.

    This means we arrange it so if a RCD fails due to a fault with some thing plugged in, it will not also plunge one into darkness, and it is not safe to have wandering leads up/down stairs, so home is normally split for sockets side to side, but logistics mean this is not really an option with lights, so many homes are wired up/down for sockets, or they use a high integrity consumer unit so at least 3 RCD's/RCBO's.

    A RCBO is a RCD and MCB combined.

    Well that is the theory, but in the days of 100 mA RCD's they rarely tripped, so when we went to 30 mA many electricians tried to get away with just fitting 2, and to be fair often it works. I fitted RCD's to my last house around 1992 and only 2 one for each Wylex fuse board, I would get a bout of tripping, maybe 3 times a week, then nothing for next 2 years, and no fault found. As a result after loosing freezers full of food, this house is all RCBO 14 of them, and yes they have tripped, but was good reason roof leaked. But it did not trip the one powering the freezers.

    Most of our homes are a compromise, and we have to decide what we are willing to pay for. A all RCBO consumer unit can cost three times more than a unit with just 2 RCD's. You have to decide how far to go.

    So trip can be caused spikes on the supply, damp, and many more, but we are told normal use there should be no more than 9 mA drain to earth, but closer it is to limit the more likely a spike can trip it, some RCD's are more resistance to spikes, and a surge protection device will likely stop some spikes, which can also increase the life of LED bulbs. But in real terms we guess, since I have not had a LED fail and have SPD fitted, I assume it has helped, but don't know, it is a lottery.

    It may be a clamp on meter with a scale which can show 0.001 amp can locate a problem, or an insulation tester can find a problem, or even seeing water lodged in an outside light. We can have some luck and realise the kettle is faulty, although with a neutral - earth fault it is easy to be mislead and blame some high power item, and in real terms it is just some toast stuck in the toaster.

    In the main switches only switch the line, and we want both live feeds both line and neutral disconnected, so where you can unplug rather than just switch off. I worked with two RCD's for over 25 years, I kept saying I was going to fit a modern consumer unit but never did, the old consumer units and fuse boxes did not have the option of fitting RCBO's but today we do have the option, so I took the option and am glad I did. Wish I had fitted type A rather than type AC, but unless forced they will remain as type AC now.
     
  6. quasar9

    quasar9 Screwfix Select

    Well explained MGW, and you are right in most sockets only single pole , which switch the live side leaving neutral connected. However you can get double pole versions too for about 50pence extra. I recently specified 40 odd Crabtree DP switched sockets so appliances can be properly isolated.
     
  7. MGW

    MGW Screwfix Select

    With class II it really does not matter, but to explain class I and class II I thought was going to confuse.

    I think the whole idea of two RCD's is flawed, but many homes wired that way, line - earth fault reasonably easy, neutral - earth much harder, as it appears that some high powered item is at fault, when it is in fact some low powered item like the toaster.

    The use of mineral insulated heaters is often the root of the problem, be it in a washing machine, frost free freezer, kettle or oven, the seal on end is easy damaged and then it absorbs moisture. Single pole switching helps find the fault, as even if a relay activated the element if the neutral is still connected you can do an insulation test earth to neutral and still find it.

    My clamp-on does measure meg ohm, but at 9 volt, so often to find fault you need 250 or 500 volt, this upload_2021-11-4_11-36-53.png meter will find the fault, but they cost £30 upwards and 500 volt does make one say ouch. So not sure if a good idea for non electricians.
     
  8. jgw1977

    jgw1977 Member

    Thanks for your help - this might be a dumb question, but if I turn off all the outside lights at the switch and the power still trips - does this rule out the issue being the outside lights? Just trying to narrow it down a little.
     
  9. sparky steve

    sparky steve Screwfix Select

    No. Your switch will more than likely be single pole, and therefore will only disconnect your switched live. Water ingress in your outside lights may be creating a short between neutral & earth.
     

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