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Up front S type RCD tripping

Grobbyman
Rural setting. Two adjacent houses from the same pole.

The one I've been called to has had intermittent tripping problems on the 100mA S type RCD, it's TT, since just before christmas.

Seems to be no rhyme or reason and consumer unit 30 mA RCD's (two) not affected.

Called in on Friday and tested the upfront and tripping times good, ramp tested at 60mA.

Disconnected the only circuit not covered by the consumer unit RCD'd (garage) and asked them to keep an eye on it.


Called Friday night and told me it's tripping again.


Asked next door, who are in the process of rebuilding, whether anything new about christmas time.


Seems they got a new induction hob with boost facility then.


Turned it on, pressed the boost and low and behold next doors S type tripped.


Seems to do it everytime.


Doesnt affect the premises RCD's (30mA's).


Any explanations?


Had a look at the pole can't see anything amiss.


Regards


George
  • 0
    It would take more than just a change on the N to cause an RCD to trip - it'd need some means of current escaping from your installation - perhaps poor insulation to PE (perhaps within an appliance) or just capacitive coupling if the changes in the supply frequency have sharp enough edges to behave similar to high frequency a.c.


    High frequency pulses and capacitors L or N to PE might be easier to explain the lack of selectivity with the 30mA RCDs - different types (AC, A, F, B etc) respond quite differently to short pulses and more modern units seem better at ignoring short glitches - and the behaviour isn't always entirely encapsulated by the nominal type (e.g. some marked AC actually behave more like A types in some circumstances).


    Or it might be some effect quite independent of the wiring - there have been reports of 2-way radios tripping RCBOs for instance.


       - Andy.
  • 0
    Sorry, didn't manage to get back there today.

    Ran up against the most horrible emergency light install I've ever seen.

    Hope to call in again tomorrow and get some tests done then.

    Will let you know how it goes.

    George
  • 0
    My money is on a neutral fault on the DNO pole as the primary fault.


    Worst emergency lights, those in a pub without a permanent live, only a switched live.
  • 0
    I hope so. Might be able to see something tomorrow. The owners already been in touch with UK power. They gave the usual "well nobody else has complained".


    This is a pack house. 250 double 6ft florescents over quite an area.

    And as you said every random em light fed from the switched supply.

    Also supply's taken to contactor boxes and as the contactors have failed, and more lights added, it looks like a rat's nest.

    Some of the supply cables seems to be running so hot the insulation has burnt off.

    Still nobody can see it so it must be ok.


    George
  • 0
    Managed to get back to the premises today.

    Ra 8.8Ohms.

    Clamp meter on the tails between 8 and 10mA.

    Asked the next door neighbour to turn on his induction hob and oven.

    Clamp meter didn't even flicker and no trip on the RCD.

    I mentioned this to the tennant who assured me her husband was certain it was the neighbours hob, what more can you say.


    George


  • Former Community Member
    0 Former Community Member
    Loose connection on the suppliers neutral would be my guess


    Don't suppose you checked the insulation resistance on the house with the new Hob ?


    Regards


    OMS
  • 0
    No sorry OMS.

    They didn't want my covidy body on the premises.

    I did watch through the window as he put the hob and oven on.

    Wish me luck with UK power!


    George

  • Former Community Member
    0 Former Community Member
    Grobbyman:

    Thanks for that.

    may take another s type just to give it a try.

    Hi, do you know what type of S type RCD is currently installed and what type are the 30 mA RCDs?

  • 0
    It's an MCG s type.

    THink the 30mAs are Proteus.


    George
  • 0
    I think Weirdbeard was thinking classification type of the RCDs (i.e. what kind of residual waveforms they should and shouldn't respond to) rather than the manufacturers - i.e. are they AC, A, F, B etc? - if it's not in letters it should be in symbols (e.g. sine wave, chopped sine wave and/or horizontal lines)

      - Andy.

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