An RCD tripping puzzler that I can't understand

Discussion in 'Electricians' Talk' started by ProDave, Nov 14, 2009.

  1. ProDave

    ProDave New Member

    Newish installation, wired about 4 years ago to 16th edition. Split load CU with most circuits on RCD side, lighting, smoke alarms etc on non RCD side.

    The problem is in the bathroom. There's a 9KW electric shower. There's also an extractor fan above the shower, an SELV fan with transformer in the loft space above the bathroom.

    The fan is fed from the output of the shower isolator via a switched FCU. The owner arrantly wanted it that way so the fan only comes on when the shower is in use, not every time the lights were turned on.

    Everything runs fine, lights, shower, fan all operate.

    But, when you have finished your shower and come out of the bathroom EVERY time you turn off the shower isolator switch, the RCD trips. RCD can be re set immediately and does not trip again until next time the shower isolator is turned off.

    If you turn the fan off buy turning off it's FCU, and then turn off the shower isolator, the RCD does not trip.

    I can't find any shorts or IR problems, and it has been working fine for a couple of years. It's only started tripping recently.
  2. unphased

    unphased Screwfix Select

    Try changing the transformer for the fan. I think that will solve the problem. :)
  3. ProDave

    ProDave New Member

    Yes that may well do.

    But apart from anything, I want to understand the mechanism that's causing the trip.

    The transformer only has a L and N connection, there's no CPC connection on it. So exactly HOW can it be faulty to cause this trip, and why ONLY at switch off, and only when turned off with the double pole shower isolator and not when turned off with the single pole switch on the FCU.
  4. JP.

    JP. Screwfix Select

    Interesting. Maybe the shower switch contacts are out of sync when switching off thus creating some of imbalance within the SELV bit.
  5. JP.

    JP. Screwfix Select

    One other thing..have you IR'd stuff and accidentally IR'd the SELV ****** on the wrong tester voltage?..not sure if this could do anything but you never might have flashed the windings or summit and somehow the N is going down to E (although no earth connected) via the steam. Probably not possible but you never know..especially if there is a parked CPC.
  6. nottsspark

    nottsspark New Member

    sounds like possible sensitive rcd, have had the same thing happen to me b4 with a shower and changed it and it was fine, also id disconnect the cable from the shower and then try switchin on off, eliminating the shower as the problem
  7. mot4321

    mot4321 New Member

    I'm not a domestic electrician and this may be a no brainer, but could the freewheeling fan generate a voltage causing an over sensitive rcd to trip, on the other hand if the shower isolator is twin pole that shouldn't feedback to cu, is it twin pole or maybe faulty? just a though good luck
  8. Removed 4

    Removed 4 New Member

    So exactly HOW can it be faulty to cause this trip, and why ONLY at switch off....

    Dave, brush-up on Lenz's Law......

    The transformer might not be 'faulty' as such, when used on a non-RCD circuit. It might be a cheap and nasty device with no damping of the inevitable collapsing magnetic flux when switched off. The same problem occurs with certain fluorescent fittings having ancient ballasts supplied by an RCD.

    The transformer having been saturated with its initial 'charge' will produce an opposing transient spike at the moment of disconnection - quicker than you can say Jack Robinson - and certainly quicker than the action of the switch.

    I'm not surprised that it doesn't trip when the local FCU is operated but it does when the shower switch is used: The additional circuit length all the way back to the RCD only serves to amplify that inductive spike.

    As I said, it's all in Lenz's law......

  9. cliffy brown

    cliffy brown New Member

    beat me to it loooch
  10. ProDave

    ProDave New Member

    Okay, Lenz's law.

    But firstly the "extra circuit length" is all of 30cm.

    And secondly this effect would just cause the switch to arc and the extended current flow would still be from L to N, i.e NO imbalance so RCD should still not trip.

    So my only thought now is could the back emf be causing the shower switch to arc to EARTH, that would cause an imballance and trip?
  11. Removed 4

    Removed 4 New Member

    Dave, don't confuse back-EMF with Inductive (transient)Spikes, those terms are not synonymous.

    A transient spike has no need of a reference to earth in order to upset an RCD.

  12. cliffy brown

    cliffy brown New Member

    and again
  13. firstline37

    firstline37 New Member

    I would try a new double pole switch on the shower.
    Seem to remember this on an outside light with double pole switch tripping an RCD.
  14. Greg McCormack

    Greg McCormack New Member

    The problem is either a loose wire somewhere there is power passing through - make sure all the consumer board terminations are tight OR the RCD is too sensitive and needs changing.

    What has prompted me to contribute to this thread five years later - is the concern that nobody has seemed to point out that you are in breach of three regulations. 1) Part L of the Building REgulations - Heating and Ventilation - but I suppose if you have a dispensation from the owner then that it is admissible. But you are in breach of the wiring regulations in two ways 2) The wiring regulations state - a heating load ( such as a shower ) needs a dedicated cabling and it is not permissible to connect another load to this cable. ( again this isn't a big issue - but is a DEVIATION that should be mentioned on the certificate that is issued AND would warrant mentioning on an EICR ) but 3) Is a breach of the wiring regulations with regard to the amperage allowed to go through the cabling to the fan which is probably only 1.5sqmm at most - which can only cope with around 16 Amps - and not the amperage of the shower ( 40 Amp ). THERE HAS BEEN NO MENTION OF A FUSED SPUR - so I can only assume that there isn't one - and thus you are in breach of the regulations.
  15. Coloumb

    Coloumb Well-Known Member

    It might be ok if an adiabatic equation was done. Even on a 40a breaker I reckon a fault would be cleared before the cable got fried. **** design but perhaps not "unsafe".
  16. dobbie

    dobbie Well-Known Member

    The fan is fed from the output of the shower isolator via a switched FCU
    Stated in third paragraph
  17. unclemonkey

    unclemonkey New Member

    rcd's detect imbalances not just including earth faults. Sound like a malfunctioning RCD to me. fit new rcd

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