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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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    A half-wave rectified waveform will pass through a transformer and the only thing which does not is a waveform where dV/dt (the rate of change of voltage/current) is zero.

    It's not just about the transformer in the RCD though. As I understand it some AC designs held the contacts closed with a permanent magnet and released the contacts by using an electromagnet energised from the sense coil to create a cancelling magnetic field. Such designs are naturally more sensitive on one half wave than the other - possibly only ever tripping on one half of the waveform - and can give quite different operating times on 0 and 180 degree tests - but with a full a.c. residual waveform they'll still trip eventually (and within the required times). Give it a half-wave rectified residual waveform though and it might be 50:50 whether it'll trip at all.


      - Andy.
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  • 0
    A half-wave rectified waveform will pass through a transformer and the only thing which does not is a waveform where dV/dt (the rate of change of voltage/current) is zero.

    It's not just about the transformer in the RCD though. As I understand it some AC designs held the contacts closed with a permanent magnet and released the contacts by using an electromagnet energised from the sense coil to create a cancelling magnetic field. Such designs are naturally more sensitive on one half wave than the other - possibly only ever tripping on one half of the waveform - and can give quite different operating times on 0 and 180 degree tests - but with a full a.c. residual waveform they'll still trip eventually (and within the required times). Give it a half-wave rectified residual waveform though and it might be 50:50 whether it'll trip at all.


      - Andy.
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