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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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    There's also a particular issue with electric vehicle charge points as they use d.c. signalling between a pilot wire and PE to communicate between the charger and the vehicle (for things like agreeing the charge rate and detecting whether the c.p.c is intact) - a simple pilot wire to N fault (plausible as they run in the same flex and terminate into the same wiring enclosures) risks running d.c. around the N-PE loop via the supply's N-PE link (and hence through the N coil of any RCDs in the circuit).

       - Andy.
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    There's also a particular issue with electric vehicle charge points as they use d.c. signalling between a pilot wire and PE to communicate between the charger and the vehicle (for things like agreeing the charge rate and detecting whether the c.p.c is intact) - a simple pilot wire to N fault (plausible as they run in the same flex and terminate into the same wiring enclosures) risks running d.c. around the N-PE loop via the supply's N-PE link (and hence through the N coil of any RCDs in the circuit).

       - Andy.
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