DC on AC supply

davezawadi (David Stone):


I have done a considerable amount of work on RCDs, and strangely enough, they show the properties I have outlined above. This is why I am very skeptical about this DC current alleged problem. There is an extremely low number of scenarios where an appliance can apply just the correct current to cause problems and have no other effect, either on itself or the CPD. Most of the cases shown in BS7671 table A53.1 are seriously flawed, to cause a problem they need a critical resistance in the alleged fault path, and the If shown is not anything like a short circuit. I have attempted to draw out this from several sources and they cannot define any realistic fault which is shown in A53.1 which will not trip a correctly rated CPD, either in the appliance or in the supply circuit.

David,


Some thoughts:

1. Regarding CPD operation. By "CPD" assume you are talking about OCPD (because, pedantically, an RCCB can be a CPD for protection against electric shock by ADS)?
   
   
   If that is the case, we need to consider that RCDs are also used in cases where an OCPD will not operate:
   
    - TT systems that might have very high earth fault loop impedances due to earth electrode resistances involved
   
    - Additional protection.
   
   
   Granted, for Additional protection, we would either be talking about accidental damage to a piece of equipment and contact with a certain point in it (but this is surely "carelessness by the user" in the same way as the broken lawnmower lead) - perhaps reducing the chance of this kind of thing happening.
   
   
   But that leaves an issue for ADS in TT systems.
   
    

• RCDs operate in N-E faults too. 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 !

davezawadi (David Stone):


I have done a considerable amount of work on RCDs, and strangely enough, they show the properties I have outlined above. This is why I am very skeptical about this DC current alleged problem. There is an extremely low number of scenarios where an appliance can apply just the correct current to cause problems and have no other effect, either on itself or the CPD. Most of the cases shown in BS7671 table A53.1 are seriously flawed, to cause a problem they need a critical resistance in the alleged fault path, and the If shown is not anything like a short circuit. I have attempted to draw out this from several sources and they cannot define any realistic fault which is shown in A53.1 which will not trip a correctly rated CPD, either in the appliance or in the supply circuit.

David,


Some thoughts:

1. Regarding CPD operation. By "CPD" assume you are talking about OCPD (because, pedantically, an RCCB can be a CPD for protection against electric shock by ADS)?
   
   
   If that is the case, we need to consider that RCDs are also used in cases where an OCPD will not operate:
   
    - TT systems that might have very high earth fault loop impedances due to earth electrode resistances involved
   
    - Additional protection.
   
   
   Granted, for Additional protection, we would either be talking about accidental damage to a piece of equipment and contact with a certain point in it (but this is surely "carelessness by the user" in the same way as the broken lawnmower lead) - perhaps reducing the chance of this kind of thing happening.
   
   
   But that leaves an issue for ADS in TT systems.
   
    

• RCDs operate in N-E faults too. 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 !