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My war against dual rcd boards

Newfutile
As each (RCD) Residual Current Device must not have more than 30% leakage current on it.
It's hard to see how dual (RCD) Residual Current Device boards can be fitted at all these days.
i come across so many dual rcd boards with solar, evse and heat pumps on them, these have all been recently installed.
i wonder if BS7671 should state: Dual (RCD) Residual Current Device boards shall not be fitted, unless it can be shown (and documented) that they are suitable for the combined leakage currents expected .
otherwise developers will keep specifying them and i will have keep educating them on the many reasons a type A rcd should not be shared with other equipment!.
maybe manuafactuers instructions should also state : not suitable for a shared rcd, for certain equipment.
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  • If you were to do a Type A RCD test on the RCDs installed in the consumer unit, what test current does the installation tester deliver?

  • in reply to Sparkingchip
    Sparkingchip said:
    If you were to do a Type A RCD test on the RCDs installed in the consumer unit, what test current does the installation tester deliver?

    Would that not depend on whether you were '. . . doing a Type A RCD test' or '. . . testing a Type A RCD in accordance with BS 7671 (643.7.1) and/or (643.8)' ?

    In the former case it would be an AC waveform with superimposed pulsed DC whilst in the latter case it would be an AC waveform.

     - Ross

  • in reply to Ross C

    As in the photo, what amperage is the pulsed DC current test current that the tester will deliver for a X1 30 mA Type A RCD test?

Reply
  • in reply to Ross C

    As in the photo, what amperage is the pulsed DC current test current that the tester will deliver for a X1 30 mA Type A RCD test?

Children
  • in reply to Sparkingchip

    A multiplier of 1.4 comes to mind, so with the instrument set as in the photo (30 mA × 1 × 1.4) = 42 mA ?

    - Ross

  • in reply to Ross C

    1.414 the square root of 2, so 42.42 mA.

  • in reply to Sparkingchip

    I thought that A type RCDs only needed to cope with 6mA of d.c.?

       - Andy.

  • in reply to Ross C

    Hang on. BS 7671 643.8 specifies, "alternating current test at rated residual operating current (IΔn)". No more, no less.

    ETA: your instrument's calibration certificate will show the actual current which it delivers.

  • in reply to AJJewsbury
    AJJewsbury said:
    I thought that A type RCDs only needed to cope with 6mA of d.c.?

    The DC component of the Type A test current is 6 mA DC.

    - Ross

  • in reply to Chris Pearson
    Chris Pearson said:
    Hang on. BS 7671 643.8 specifies, "alternating current test at rated residual operating current (IΔn)". No more, no less.

    Recall however that:

    • BS 7671 (643.7.1) and (643.8) require, as stated above, the application of an alternating test current, which is achieved with the instrument set to Type AC.

    • BS EN IEC 61557-6, the standard for RCD test instruments, allows the alternating test current to be -0 % / +10 % relative to the RCD rated residual operating current.

    - Ross

  • in reply to Ross C

    There is a good summary of the requirements for RCD testing in accordance with BS 7671 in Issue 91 (July 2022) of the IET Wiring Matters magazine:

    https://electrical.theiet.org/wiring-matters/years/2022/91-july-2022/changes-to-rcd-testing-in-bs-76712018plusa22022/

    - Ross

  • in reply to Ross C
    Ross C said:
    A multiplier of 1.4 comes to mind, so with the instrument set as in the photo (30 mA × 1 × 1.4) = 42 mA ?

    Why?

  • in reply to Sparkingchip

    I'm not sure of the exact derivation, but the IET Wiring Matters article concurs:

    'When the Type A setting is selected on the instrument, a half wave pulsating residual test current superimposed on a smooth direct  current of 6 mA is produced, which effectively applies a 1.4 multiplier to the rated residual current (IΔn). For example, if the 30 mA setting is selected, the RCD will be subjected to a test current of 42 mA (30 x 1.4 = 42 mA)'

    - Ross

  • in reply to Ross C

    This is messy. A half wave rectified AC is not there at all for half the time, and the relationship between its RMS value, peak value and average (ho yes, unlike an AC the average is not zero..) is not a simple thing.
    1.4 sounds like the ratio of the peak to the RMS of the AC before it was rectified, but without the qualifier, it is far from clear. 

    (

    A Sine wave of 30mA RMS has 
    42mA pk, 84mA p-p and zero average voltage

    Remove say the -ve half cycles and this becomes

    15mA RMS, (half the heating power, as off exactly half the time)
    42mA pk and p-p are the same (as everything below the line removed)
    and average of about 14mA (~33% of peak, derivation by integration is left to the student *) 

    )

    More importantly, I have no idea what they actually intended people to use in the spec.

    42mA pk,

    42mA RMS

    or 42mA average ?


    Mike.

    * I'm not that mean, look here where it is demonstrated for you in some lecture notes from a college in Babylon, I think .... 

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