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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?

  • 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.

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  • 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 Ross C

    Just catching up on earlier points.

    One thing we forget is that current induced/produced magnetisation, the H field, is directly proportional to the instantaneous ("DC") current via the permeability constant. This then produces the total magnetisation M by the ferromagnetic effect in the materials. That current (I) relationship is not via the rms (equal average energy I^2.R), rather it's via the average magnetisation (I.avg).

    Thus the discussions about equivalent rms current in a half cycle (energy to the load) don't carry through to RCD blinding effect of continuous DC test currents. If I remember correctly there is pi/4 factor in there, which could fit the rationale for the 6mA DC test current  (continuous). There may also be secondary effects to add for the types of ferromagnetic saturation that (unreported) have been found in blinded RCDs.

    I've not had time to try out any simulations of the ferromagnetic saturation effects. https://physics.stackexchange.com/questions/60194/equation-describing-magnetic-hysteresis  With luck it may give some further insights.

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