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DC on AC supply

nicemark
I have seen a number of videos which demonstrate the problems caused by DC currents on AC supplies with respect to RCD's. An issue which is apparently increasing with increased technology applications.  Please could someone explain how DC gets back into the neutral.


Almost all PSU's in all equipment begin with an isolating transformer. So getting a significant pulsed "dc" signal back that way is not impossible but tricky. (Pulses only occurring on one side of the cycle). 


My thoughts are that the DC is most often "created" by an unbalanced waveform, with a muted positive cycle, giving an overall DC flow.


Please could you direct me to, or give examples of how DC gets into the neutral, and/or just how much of an issue it can be in some homes.
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    perspicacious:
    What about equipment that dumps DC components down the protective conductor? And of course there are EV charging circuits where the pilot function which intentionally puts DC through the protective conductor !


    Isn't the equipment that dumps DC components down the protective conductor also an intentional act?


    Regards


    BOD (also pedantically!)


    Intentional or otherwise, it's permitted by product standards.


    Provided protective conductor current is below certain limits - fine.


    However, in the case of an N-E fault, particularly in TN systems, will transpose the current onto the Neutral. This kind of fault will not operate OCPDs which operate in line conductors only, and, the theory goes, will "blind" some RCDs, so they don't operate for this fault, and possibly not some L-PE or impedance to PE faults (perhaps via the human body) afterwards.


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  • 0
    perspicacious:
    What about equipment that dumps DC components down the protective conductor? And of course there are EV charging circuits where the pilot function which intentionally puts DC through the protective conductor !


    Isn't the equipment that dumps DC components down the protective conductor also an intentional act?


    Regards


    BOD (also pedantically!)


    Intentional or otherwise, it's permitted by product standards.


    Provided protective conductor current is below certain limits - fine.


    However, in the case of an N-E fault, particularly in TN systems, will transpose the current onto the Neutral. This kind of fault will not operate OCPDs which operate in line conductors only, and, the theory goes, will "blind" some RCDs, so they don't operate for this fault, and possibly not some L-PE or impedance to PE faults (perhaps via the human body) afterwards.


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