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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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  • Three days ago I said:

    ”The question that Graham asked was quite specific:

    ”However, if the lamp is damaged, and the user is being protected against accidental contact with live parts, say after the rectifier, would the type AC RCD operate is perhaps another?”

    in that he said “after the rectifier”, so assume there are some diodes charging a capacitor, which discharges to supply the DC current, and the input current is 25 mA at 230 volts, what is the DC output current and voltage?”.

    So presumably after the rectifier the current and voltage is peak, not RMS? So if the current and voltage is 25 mA at 230 volts AC measured RMS, then after the rectifier it should be 35,35 mA at 325 volts pulsed DC unless there are a few more electronic components to drop the voltage?

  • in reply to Sparkingchip

    I think not. The "average" rectified current must be lower than the upstream a.c. because there is none for 10 ms in every 20 ms cycle. The RMS current of the pulsed DC is only 17.7 mA.

    Some maths to follow. :-)

  • in reply to Chris Pearson

    Some misunderstanding there. If it's a single loop its the same current all the way around the loop. And superposition.. 

    Lots of careful misuse of apparently similar terms! Worse than trying to find a true earth..

  • in reply to Philip Oakley
    Philip Oakley said:
    If it's a single loop its the same current all the way around the loop.

    Quite so. If the (earth) fault is after the rectifier, the "pulsed d.c." flows through the RCD, which is where the problem lies. If the type AC RCD does not trip, lives could be at stake.

    Philip Oakley said:
    And superposition

    OK, so you have a 30 mA RMS half-wave rectified (pulsed d.c.) current and superpose 6 mA of smooth d.c. What is the combined RMS current?

  • in reply to Chris Pearson
    Chris Pearson said:
    If the type AC RCD does not trip, lives could be at stake

    Which is the point.. (why we don't like RCDs that become 'blinded' by average DC flows)

    The simplistic assumptions (e.g. no inductive/capacitive energy storage/filtering) produce worst case figures. 

    The very sharp corners between half cycles aren't that realistic and, if Fourier analysed, imply a high frequency harmonics content, which tends to be filtered by the general wiring etc (without additional components).

    BigClive did some video on the workings of diode bulbs - worth looking at. I can't remember if the ones he looked at had a half or full wave rectification.

    There are problems of nomenclature in the attempted mixing of the two signals, in that often the first includes the second (commonly because of an arbitrary zero marking).

    A pulsed DC is a smooth DC and a pure AC (i.e. 'centred' about that smooth DC level)

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  • in reply to Chris Pearson
    Chris Pearson said:
    If the type AC RCD does not trip, lives could be at stake

    Which is the point.. (why we don't like RCDs that become 'blinded' by average DC flows)

    The simplistic assumptions (e.g. no inductive/capacitive energy storage/filtering) produce worst case figures. 

    The very sharp corners between half cycles aren't that realistic and, if Fourier analysed, imply a high frequency harmonics content, which tends to be filtered by the general wiring etc (without additional components).

    BigClive did some video on the workings of diode bulbs - worth looking at. I can't remember if the ones he looked at had a half or full wave rectification.

    There are problems of nomenclature in the attempted mixing of the two signals, in that often the first includes the second (commonly because of an arbitrary zero marking).

    A pulsed DC is a smooth DC and a pure AC (i.e. 'centred' about that smooth DC level)

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