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Regulation stating a type AC RCD can not be upstream from a type A RCD

Hi

I found an EV charger today with built in type A RCD + RDC-DD connected to a type AC RCD in the consumer unit, the AC RCD is also protecting 3 other circuits including sockets. I know this is incorrect because the type AC RCD could be blinded by DC currents, but I am struggling to find a regulation to reference when providing information to the customer?

Thanks

Alan

Parents
  • I quite see all your comments Andy, but you have missed the subtly covered up error with this idea. I will explain:

    First are we providing additional protection or Earth fault protection? In my view it is additional protection because we have the normal Earthing system to cope with Earth faults (I am excluding TT installations for the moment).

    The question is then "where is the DC"? The only equipment which could have an actual DC current imposed on the mains waveform is possibly an EV that has this imposed deliberately, but this current is isolated from the mains supply, and for this to matter is probably false due to Kirchhoff's law.

    The only way that an RCD can detect anything using the standard differential transformer is that there is a direct imbalance in the phase and neutral current, and the only way to stop this working is to have sufficient alternative magnetic flux that essentially saturates the core, or at least significantly reduces the available permeability so reducing the sensitivity. The list of faults in Fig A53.1 (note I referenced the wrong one above now edited) may introduce an asymmetry in the mains current drawn by each half cycle, but is this DC? The point here is that the net field in the transformer during either half cycle is zero unless the currents in L and N do not balance, and as you know RCDs trip with a single cycle with imbalance although this may need to be repeated more than once (Remember the 0/180 selection on RCD testers?).

    The question is now what does Fig A53.1 actually mean? Is the indicated fault path actually an accidental contact with a live conductor by a person, which I doubt, or some kind of fault in the electronics? I doubt it is a person because these terminals are enclosed in all appliances, so it must be a double fault although not shown as such, so that a person becomes the only Earth connection and a leakage fault is also present. In principle I don't care if an appliance leaks to Earth a little, it is not fundamentally dangerous, and if it leaks a lot will pop the CPD.

    You may disagree with my discussion above, which is fine, but you need to look at the actual operation in an analytical way and this is difficult, but I would like to hear the argument fully explained as probably would others.

    It is alleged in this table and associated regulations that electronics means DC, and yet the only diagram where the Earth current could actually be DC is number 7 because of the smoothing capacitor, probably being several microfarads so that there is not a significant AC component to the leakage. Please consider the live and neutral currents carefully to see if there is a difference.

Reply
  • I quite see all your comments Andy, but you have missed the subtly covered up error with this idea. I will explain:

    First are we providing additional protection or Earth fault protection? In my view it is additional protection because we have the normal Earthing system to cope with Earth faults (I am excluding TT installations for the moment).

    The question is then "where is the DC"? The only equipment which could have an actual DC current imposed on the mains waveform is possibly an EV that has this imposed deliberately, but this current is isolated from the mains supply, and for this to matter is probably false due to Kirchhoff's law.

    The only way that an RCD can detect anything using the standard differential transformer is that there is a direct imbalance in the phase and neutral current, and the only way to stop this working is to have sufficient alternative magnetic flux that essentially saturates the core, or at least significantly reduces the available permeability so reducing the sensitivity. The list of faults in Fig A53.1 (note I referenced the wrong one above now edited) may introduce an asymmetry in the mains current drawn by each half cycle, but is this DC? The point here is that the net field in the transformer during either half cycle is zero unless the currents in L and N do not balance, and as you know RCDs trip with a single cycle with imbalance although this may need to be repeated more than once (Remember the 0/180 selection on RCD testers?).

    The question is now what does Fig A53.1 actually mean? Is the indicated fault path actually an accidental contact with a live conductor by a person, which I doubt, or some kind of fault in the electronics? I doubt it is a person because these terminals are enclosed in all appliances, so it must be a double fault although not shown as such, so that a person becomes the only Earth connection and a leakage fault is also present. In principle I don't care if an appliance leaks to Earth a little, it is not fundamentally dangerous, and if it leaks a lot will pop the CPD.

    You may disagree with my discussion above, which is fine, but you need to look at the actual operation in an analytical way and this is difficult, but I would like to hear the argument fully explained as probably would others.

    It is alleged in this table and associated regulations that electronics means DC, and yet the only diagram where the Earth current could actually be DC is number 7 because of the smoothing capacitor, probably being several microfarads so that there is not a significant AC component to the leakage. Please consider the live and neutral currents carefully to see if there is a difference.

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