RCD query

Discussion in 'Electricians' Talk' started by Johnupatree, Jul 5, 2021.

  1. Johnupatree

    Johnupatree New Member

    Daughter has moved house and I’m just looking it over for the list of dad jobs I’ll be doing!

    One thing she has found odd is the electrics tripping, while she waits for an electrician to respond I thought I’d ask for opinions from you chaps.

    MCB on left is her cooker. Any combination of two is fine, but all three trip the RCD. I’ve no idea on RCDs and just curious, is it not the right rating/type etc


    Attached Files:

  2. seneca

    seneca Screwfix Select

    It's an 80 amp rcd so I doubt it's a rating problem, more likely an accumulation of earth leakage, what else is connected apart from the cooker?
  3. MGW

    MGW Screwfix Select

    All appliances should have less than 2.5 mA to earth unless special arrangements are made, and a 30 mA RCD should hold at 15 mA and trip at 30 mA so three appliances should not trip the RCD. However only using a clamp-on ammeter that will measure 0.001 amp can one know for sure what the leakage is.

    If you measure the resistance to earth you can get an approximate value, but the meters are DC so only approximate as supply is AC and in the main they use 500 volt, we expect any appliance to measure over one meg ohm.

    So if all three allow 5 mA to earth then they could trip the RCD, which is why I use RCBO's but one way or another you need some thing to measure the leakage to work out which items are faulty.
    seneca likes this.
  4. Johnupatree

    Johnupatree New Member

    Connections as far as I can make out are upstairs/downstairs rings and cooker. No immersion. Nothing other than that. It’s a small house, no garage.
  5. Johnupatree

    Johnupatree New Member

  6. The Happy Builder

    The Happy Builder Screwfix Select

    If the RCD trips whilst the cooker is turned on or another high current appliance there could well be a neutral fault on one of the circuits.
  7. Johnupatree

    Johnupatree New Member

    Thank you, that makes sense. Sadly I’m not well up on electrical work to even know what to do, so I’ll wait for the electrician to sort it! Had hoped it was a simple DIY sort of problem, obviously not. Thanks again mate.
  8. Tony Goddard

    Tony Goddard Screwfix Select

    Cooker elements, which are packed with a dry mineral powder, which can wick moisture, are notorious for causing insulation faults and random tripping, but then again it could be one of a 100 things.

    Hopefully a spark armed with an insulation tester will be able to find it.

    If it's cumulative leakage, but no circuit is so bad as to be a fault, then replacing the RCD with a main switch and replacing the MCBs with RCBos (a combined min RCD/MCB per circuit) may be an answer.
    Bazza-spark likes this.
  9. Lectrician

    Lectrician Screwfix Select

    Sounds like a neutral to earth fault. The more load applied, the more current flows through the N to E fault until it trips,
  10. MGW

    MGW Screwfix Select

    As said likely a neutral earth fault, however that means that you need some one with an insulations tester to find the fault. The problem is homes are wired on the cheap, if we used all RCBO (that's a combined RCD and MCB) then there would be a good chance we could work out what is tripping a RCD and since the total leakage is higher less likely they will trip, my house has 14 RCBO's and when the roof leaked I loss 4 lights and 6 sockets which I could manage without until roof fixed.

    Design can help, again my house the sockets in top two floors split side to side, so if I have a side fail, I can run essential equipment with extension leads without them going up or down stairs.

    But house electrics is a compromise, I have lived with just two RCD's in the house for some 25 years or more, and during that time I have lost around 4 freezers full of food, so when it came to fit RCD's to this house, I decided an extra £300 for all RCBO will pay for its self if it stops one freezer full of food from being lost, but 1992 when fitted in last house RCBO's were just not around, neither were SPD (Surge protection device) so I had a 14 year old son who had decided he wanted to be a radio ham, and I wanted to protect him as much as I could, so fitted two RCDs.

    As to mineral insulated heating elements they are known for a fault when the seals fail at the ends and they absorb moisture, some times one can drive out the moisture they are hydrophilic that is they have an affinity to water, but heat can drive it out again, however it will return again and again, so only real cure is replace the element.
  11. Johnupatree

    Johnupatree New Member

    Thanks for the replies, very much appreciated and informative.
    Electrician due today/tomorrow so we’ll soon find out, but I’m sure it will be something like the cooker ring being due for replacement.
    Many thanks again for the time and patience in replying.
  12. Jim Kirk

    Jim Kirk Member

    General RCDs have two settings Load current and Tripping current.

    RCDs are used for two purposes - protection against electric shock - sockets bathrooms etc, the RCD rating must be 30mA or less

    and Protection against fault current - TT system mainswitch they can be of any load rating or any trip rating. i.e 100mA,300mA,500mA.

    RCDs do not operate under short circuit conditions like a fuse or MCB as the L and N current are the same value

    RCBOs are a two component device L-N short circuit/fault and earth leakage trip

    100A/30ma RCD(G), 100A/100mA RCD(S) , 100A/300mA RCD(S) - these can all be used as the mainswitch in a TT consumer unit, the trip value dependent upon the electrode resistance value. Only the first on can be used as a sole RCD.

    A 30mA RCD is never recommended as consumer unit main switch due to the low value causing summated and nuisance tripping.

    Sockets must be 30mA protected so all necessary accessories should have independent 30mA RCDs

    In a TT sysytem that would involve two RCDs in tandem, the 100mA or above mainswitch and a 30mA RCBO for sockets etc.

    However it cannot be guaranteed that the 30mA RCD trip time will preceed the 100mA trip time as required. RCBO and socket trip times should trip before the larger device but are not ajustable and area of the general(G) type.Where devices are in tandem such as a TT cons unit, higher rated devices should be (S) type seleable where the trip time can be adjusted to lag the 30mA device. This needs an RCD tester to test the 30mA device, before selecting the (S) device trip time.

    The whole system of multple RCDs in a consumer unit is absolute nonsense and CU mainswitches should not have any RCD component


    The preferred, safest, most trouble free design sysytem is 100A mainswitch/s if split units are required, with individual circuit potection devices the consumer unit ways

    MCBs where no RCD potection is required/desired

    Water heaters
    Cooking apliances(DP cooker control switches cannot/should not incorporate a socket outlet)
    Smoke alarms, CO and CM Alarms, Fire alarms/Intrude alarms
    Radial circuits for garage and outside socket supplies where the RCDs should be at the point of use

    RCBOs where RCD protection is required

    Lighting circuits
    Bathroom circuits
    Socket circuits
    Circuits with cables buried in walls.
  13. Johnupatree

    Johnupatree New Member

    Hi Jim,
    Am I reading this correctly, that the cooker MCB should not be protected by the RCD as shown in the picture? If so how should the setup be? Puzzled and thankful her electrician is due to visit tomorrow!
  14. Jim Kirk

    Jim Kirk Member

    Generally cookers have mineral powder insulated elements which are notorius for tripping 30mA RCDs if they have not been used for a while or if they draw damp. All electric cookers and ovens should be on an MCB not RCD circuit.

    But this is complicated by the fact that some older cooker control switches had a 13A included in the faceplate - this then requires to be connected on an RCD protected circuit due to the presence of the socket. If this is the case and it is cooker that is causing the trip then this switch would have to be replaced with a DP swith only.

    Although you have provided a limited photo I suspect that the main switch on the consumer unit is a DP switch only and the rest of the circuits are connected to the mainswith and the two ring circuts and the cooker are connected to the RCD.

    If this is the case then I'm afraid you consumer unit does not meet the Regs requirements post 2011. as the lights and some other circuits now require RCDs. 2018 Regs are way over the top and now even require surge protection so a replacement CU would be expensive as it will invariably require full or partial rewiring. Your electrican will advise on the best design for a repacement as we have no Part P here in Scotland

    Do not worry too much about trying to be compliant - safe trouble free operation is your aim at present.

    You have 3 RCD circuits - if the all work withot tripping individually or any 2 from 3 work OK then the problem is summated leakage tripping.

    So is the cooker cicuit has 15mA leaking it should not trip on its own as they are calibrated to trip beytween 15-30mA only.

    But you say any 2 from 3 work OK so a combination of 10Ma + 7Ma on the two sockets circuits will not trip either but if all three are switched on the total leakage will be 32mA which should trip the device - this is called summated tripping because it is the sum if the three circuits which will trip the device and this should only happen when the third circuit is energised.

    This does not happen if indivudal RCBOs and MCBs are used as in my last post as the faulty circuit will trip one device only.

    All 3 circuit require to be tested and and rectified if found to be the cooker and the switch has a socket incorporated then this must be replace with a switch only and the circuit moved to the mainswitch protection only, if the cooker tests ok.

    If the cooker fails the insulation test then energising it on a non RCD cicuit and operting all the elements in turn to dry out normally cures tripping problems unless an element has shorted out.
  15. The Happy Builder

    The Happy Builder Screwfix Select

    Generally it would be an old cooker if it has solid hobs, most people have had halogen or induction hobs now, so the “cooker issue” of damp hobs generally isn’t issue anymore.

    But over enthusiastic cleaning of a cooker due to a house/flat sale or change of tenancy can be an issue due to damp and/or damage, as can letting the plums boil over or spilling Prosecco.
  16. Johnupatree

    Johnupatree New Member

    Thank you for the reply Jim, nicely explained and makes sense, I’m learning a bit from this thread!
    Daughter is looking at going for an induction hob in the near future so I’m guessing the element issue will be eliminated, if indeed that’s what it is. The knowledge I’ve gained is a help in not having the wool pulled over my eyes when we speak to her new sparkle lol
  17. Johnupatree

    Johnupatree New Member

    Ah the age old plums boiling over problem, something that is happening less and less as I get older.
    Mrboomal likes this.
  18. Tony Goddard

    Tony Goddard Screwfix Select

    Jims explanation is spot on - having a 30mA RCD as a main switch is always asking for trouble, The best approach by far is to use RCBo's for those circuits requiring and RCD, and for those that don't (as jim lists) an MCB.
    This has the huge advantage of preventig the whole system going down from a fault or cumulative leakage.
    If your electrician advises replacing the CU, go for the RCBo/individual circuit option over the "split load" design, it costs marginally more (that margin is decreasing) but is a better job, and go for a bigger board than you actually need so as to give capacity for future modifications.
  19. MGW

    MGW Screwfix Select

    That was the case, however today we have to use RCD's on nearly every circuit, as wires not visible or protected enough to allow under current regulations to have no RCD protection.

    The regulations says "Every installation shall be divided into circuits, as necessary, to reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation." it also says a lot more, but that is the bit which seems to be over looked.

    The basic idea is no circuit should have more than 1/3 of the leakage that the RCD will allow as a base figure, 3.5 mA is the limit (543.7.1.1) for equipment using a standard socket, so as long as no more than 3 sub circuits are supplied from a single 30 mA RCD then we should be OK, but RCBO's are expensive and standard consumer units will only allow for two RCD's plus where things like solar panels and electric vehicle charging is concerned the RCD needs to be of a type which is not made as a single width RCBO unit, or special disconnecting equipment must be used, to auto disconnect if over 6 mA of DC is detected, so as it stands, as an industry we have a problem.

    It is pointless trying to say what a designer should do, when it is near impossible to fully comply, so we have a compromise, and the compromise is in the main between likelihood of tripping compared to cost to stop unwanted tripping taking into account the dangers involved. The regulations love that phrase "take into account" does not say you must do something, just take it into account.

    So you decide how much to pay and you have to accept if you have done it on the cheap you may in the next 25 years loose a freezer full of food due to a RCD tripping. But even with all RCBO's as I have, there are still problems as I only have type AC, could have paid more and had type A, but simply can't buy type F or type B single width RCBO's so I am taking a chance that my type A will not be affected by DC when required to trip.

    We are finding appliances which say used type A, Bosch central heating boilers for example, but as yet not seen any plug in appliance ask for type A, although we are warned inverter drive washing machines, and freezers should use type A not type AC.

    So I have taken the attitude which maybe wrong, that the chance of DC and a earth leaking fault at the same time is rather low when using 14 RCBO's to protect the house, we are given the phrase "potentially dangerous" and all 230 volt AC is potentially dangerous, but that is the phase which is used, and we have to ensure the electrics in the home are not potentially dangerous, or of course dangerous, so in a new installation we have no real option but use RCD's either stand alone or combined with the MCB as a RCBO.

    And in real terms any item with over 3.5 mA leakage we see as being faulty when used in the home. In theory when hard wired we can allow 10 mA, but this is over the 9 mA permitted for a 30 mA trip, so we consider one meg ohm or 0.00023 mA is the normal standard. The problem is AC not same as DC and the insulation resistance is measured with DC.

    Confused, no wonder, even in the trade we are confused, watch these pages for debates on Landlord EICR.
  20. Tony Goddard

    Tony Goddard Screwfix Select

    I would generally RCD protect every circuit unless to do so was detrimental or potentially dangerous (such circumstance as loss of supply to say a fire panel could cause harm)
    I have used the RCBo / main switch set up since around 2008, which was the last time I fitted a split load board.
    Landlord EICRs seem to be easy, most of those doing them are just looking to do CU changes, sharp intake of breath followed by "sorry guv, need a new consumer unit" is the approved methodology.

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice