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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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    davezawadi (David Stone):

    Graham re your 1:

    Yes, there is a potential problem with contacting internal parts of equipment, but this is not really similar to running over the lawnmower cable is it? It is a deliberate act to either fiddle with something exposed because the casing has been removed or broken, and then to produce a suitable DC current which means touching something at a suitable DC potential and Earth at the same time. Whilst this might be possible, it is outlandishly unlikely, and clearly stupid. You might call it additional protection, but there have to be limits somewhere.

     

    But that is the function of additional protection, as stated in BS 7671 (Regulation 415.1.1, my underline) "The use of RCDs with a rated residual operating current not exceeding 30 mA is recognized in AC systems as additional protection in the event of failure of the provision for basic protection and/or the provision for fault protection or carelessness by users."


    I do agree that people should have more sense, but that's where we are now I guess.
    TT is a rather different case, the RCD is primary Earth fault protection, but so far has been pretty trouble-free. Whether fitting much more expensive RCDs against possible DC currents needs evidence, which we don't have. As I said above, getting just the right current to prevent the operation of a type A or AC is not easy, and is therefore fairly unlikely. Failure of internal components depends on the design, but most SMPS are isolated and therefore need a very particular kind of fault to leak DC from mains components isolated from Earth by 3mm at least. Failure of rectifiers etc. will cause excessive current draw and not Earth leakage.

     

    But there's still the N-E fault to a protective conductor with DC components to consider. And in this case the RCD is our only fault protection. It might be a different case, but it's a case that relies more heavily on RCD.

    Re 2:

    Electric cars are becoming more and more serious design problems. Why someone (who is presumably lost in the mists of time and standardisation minutes) decided it was a good idea to use the CPC as a signaling conductor is quite beyond reason. It saves one pin on an already special multiway connector. The result is that every installation costs at least £100 more than it need of done to save £1 elsewhere. C'est la vie.


    Well, the protective conductor communication is actually used as a safety feature to ensure the protective conductor is continuous - or so the latest version of BS EN 61851-1 says in Annex ZZ (if memory serves).


    Whether this is necessary because of plug-in petrol hybrids, well I don't think BS EN 61851-1 goes into that.


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  • 0
    davezawadi (David Stone):

    Graham re your 1:

    Yes, there is a potential problem with contacting internal parts of equipment, but this is not really similar to running over the lawnmower cable is it? It is a deliberate act to either fiddle with something exposed because the casing has been removed or broken, and then to produce a suitable DC current which means touching something at a suitable DC potential and Earth at the same time. Whilst this might be possible, it is outlandishly unlikely, and clearly stupid. You might call it additional protection, but there have to be limits somewhere.

     

    But that is the function of additional protection, as stated in BS 7671 (Regulation 415.1.1, my underline) "The use of RCDs with a rated residual operating current not exceeding 30 mA is recognized in AC systems as additional protection in the event of failure of the provision for basic protection and/or the provision for fault protection or carelessness by users."


    I do agree that people should have more sense, but that's where we are now I guess.
    TT is a rather different case, the RCD is primary Earth fault protection, but so far has been pretty trouble-free. Whether fitting much more expensive RCDs against possible DC currents needs evidence, which we don't have. As I said above, getting just the right current to prevent the operation of a type A or AC is not easy, and is therefore fairly unlikely. Failure of internal components depends on the design, but most SMPS are isolated and therefore need a very particular kind of fault to leak DC from mains components isolated from Earth by 3mm at least. Failure of rectifiers etc. will cause excessive current draw and not Earth leakage.

     

    But there's still the N-E fault to a protective conductor with DC components to consider. And in this case the RCD is our only fault protection. It might be a different case, but it's a case that relies more heavily on RCD.

    Re 2:

    Electric cars are becoming more and more serious design problems. Why someone (who is presumably lost in the mists of time and standardisation minutes) decided it was a good idea to use the CPC as a signaling conductor is quite beyond reason. It saves one pin on an already special multiway connector. The result is that every installation costs at least £100 more than it need of done to save £1 elsewhere. C'est la vie.


    Well, the protective conductor communication is actually used as a safety feature to ensure the protective conductor is continuous - or so the latest version of BS EN 61851-1 says in Annex ZZ (if memory serves).


    Whether this is necessary because of plug-in petrol hybrids, well I don't think BS EN 61851-1 goes into that.


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