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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

Reply
  • 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

Children
  • 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?

  • 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

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