Fast trip 10ma RCD

Discussion in 'Electricians' Talk' started by firstline37, Sep 13, 2008.

  1. firstline37

    firstline37 New Member

    When all the country is on a RCD , will someone invent a RCD that will discriminate between between a 30ma standard one.
    Ive been to a job with a dodgy Creda tumble dryer that has been repaired twice and even replaced once.Its still trips the RCD . I ran it through a plug in RCD and it tripped that and the main RCD straight away. 0.01 M to earth.
    For an intermittent fault an RCD plug adaptor that could discriminate would be great to test an appliance over time.
    Ive read a 10ma doesnt discriminate from a 30ma.
    Can anyone explain why not?
     
  2. Bandit12

    Bandit12 New Member

    its cos if it trips a 30 mA it will trip a 10 mA at the same time
    you would need a 30 mA with a time delay supplying the 10 mA like it used to be in the 16th with a TT system

    BETTER THAN 17 TH IN MY OPINION

    Anyone agree ??
     
  3. fast

    fast New Member

    if the device is working properly then it will discriminate. but you idea would only be useful if you had a faulty appliance that you cant trust so you would use a smaller rated rcd. but then again if the fault is a massive one then i don't matter what you do it will trip the lot.
    may be we will see you on 'dragons den' in the near future with your idea.

    top bird is that blond on there man. (debera)
     
  4. fast

    fast New Member

    bandit i agree. of course time delay. you will have to explain all this to the dragons
     
  5. 50Hz Shuffle

    50Hz Shuffle New Member

    If the device is working properly, it will not discriminate.

    An RCD is not a current limiting device.

    If the earth fault (leakage)current was 6mA only, a 10mA RCD would trip and a 30mA would not.

    If the earth fault (leakage) current was 16mA, there is no telling which one operate first - the same would apply if the earth fault current was 150A.
     
  6. wklivesvtime

    wklivesvtime New Member

    "An RCD is not a current limiting device."

    Whats that got to do with it?
     
  7. Bandit12

    Bandit12 New Member

    why would a 10mA RCD trip at 6mA ?
     
  8. wklivesvtime

    wklivesvtime New Member

    Cos they tend to let go a bit before the official rated trip current. However shuffle is getting a bit excited thinking a 30 will let go at 16ma - they dont usually ramp below 20.
     
  9. fast

    fast New Member

    how have you tested the dryer? was its heating elements warm or did you test it cold. in fact going right back to the start of this thread you haven't explained how you conducted the test,...??????????????????????????????????????????????????????????????????????????????????????
     
  10. Bandit12

    Bandit12 New Member

    Back 2 start

    0.01 M to earth.

    Think its goosed no need for 10 mA rcd

    I AM OUT

    as the dragons say
     
  11. CraigMcK

    CraigMcK Screwfix Select

    why would a 10mA RCD trip at 6mA ?

    Bandit, the standard says that they should trip somewhere between 50% & 100% of their rating, so a 30mA can trip at 15mA and be in tolerance.

    You can get 10mA RCD's, ABB make them. I think that they are specified in schools in some countries in Europe
     
  12. firstline37

    firstline37 New Member

    The fault was obvious , i shorted l and n and ir tested to earth, 0.01 M .( even though repair man said it was the installation)

    Maybe my post was misleading , i agree that a big short like this may take out both RCDs . However most RCD s i test seem to trip around 10ms on 5 x current.
    If the 10ma RCD could be made to Trip quicker (say 5ms) and on 10ma , would this give us discrimination to aid testing INTERMITTENT faults on appliances without effecting other circuits?
     
  13. fast

    fast New Member

    old on one second. what was the earthing system? TNCS?
     
  14. 50Hz Shuffle

    50Hz Shuffle New Member

    WK, I see you're refering to the theory and science written by your magic mice again.

    As for current limiting...it is precisely the fact that RCDs are not current limiting devices which is the reason that RCDs do not discriminate - and the reason why if you require RCDs to discriminate, you can have only two of them in series and one of them must be a Type S.

    And a 10mA RCD will 'let go' at 6mA because they 'tend' to go a bit before the official rated trip current. Of course it has nothing to do with the standards laid down in BS EN 61008/9 and BS4239 and is the very reason for the 0.5xIn test presribed in GN3 and the OSG.

    Eek eek...eh up, I think I hear your magic mice as well....:)
     
  15. Bandit12

    Bandit12 New Member

    Craig!!
    He said If the earth fault (leakage)current was 6mA only, a 10mA RCD would trip and a 30mA would not.

    10 mA RCD may trip and be ok but WOULD TRIP ???

    1. - General purpose non-delayed RCDs
    This is a general purpose type of RCD which is intended to operate very quickly AT ITS rated current. Three tests are required:

    a) - 50% of the rated tripping current applied for 2 s should not trip the device,

    b) - 100% of rated tripping current, which should not he applied for more than 2 s, must cause the device to trip within 200 ms (0.2 s), and

    c) - where the device is intended to provide supplementary protection against direct contact, a test current of 150 mA, applied for no more than 50 ms, should cause the device to operate within 40 ms.
     
  16. wally2

    wally2 New Member

    In practice I find the ramp test on a 30ma usually works out at 23ma to 25ma.
    Although the spec allows lower I tend to change it for one that trips around the norm (on new work)
     
  17. Clark Kent

    Clark Kent New Member

    You have have a van full, then wally2..?

     
  18. wally2

    wally2 New Member

    No I don't ext. Most ramp at what I want them to. Are you back on your own now or doing both from now on.
     
  19. fast

    fast New Member

    explain the test. where the reading was nort point somot.

    how did you do the test? was it tested from the plug top? was it a test from the fused spur? (where my suggestion about the earthling system would apply) was it tested from the dryer circuit board (still connected to the mains) what sort of tester did you use? was the dryer switched on? all these things do matter if we are going to solve the problem.
    electric is logical, you can ask a series of questions to get to the bottom of it.
     
  20. Clark Kent

    Clark Kent New Member

    No I don't ext. Most ramp at what I want them to.
    Are you back on your own now or doing both from
    from now on.


    1 As long as the RCD meets requirements then, I'm happy ;)


    2 I'm doing both, and it's killing me off (7 days a week), time for a rethink...


    ;)
     

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