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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 great many devices rectify the mains before use and do not use a transformer, though most are full wave rectifiers ~(bridges) but can create a pulsed DC fault if one diode fails or there is a short from on side of the output of the rectifier to earth or neutral.

    Some smoke alarms and emergency lights have transformerless power supplies

    Other devices have the potential to rectify because they use a triac, that may not be triggering equally on both half cycles.

    Several designs of PIR sensor, certain types of motor speed control and light dimmers come into this category.

    All of this is not new, and rectified mains faults are quite rare.

    Mike.
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    A great many devices rectify the mains before use and do not use a transformer, though most are full wave rectifiers ~(bridges) but can create a pulsed DC fault if one diode fails or there is a short from on side of the output of the rectifier to earth or neutral.

    Some smoke alarms and emergency lights have transformerless power supplies

    Other devices have the potential to rectify because they use a triac, that may not be triggering equally on both half cycles.

    Several designs of PIR sensor, certain types of motor speed control and light dimmers come into this category.

    All of this is not new, and rectified mains faults are quite rare.

    Mike.
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