DC on AC supply

You are making another error Andy. The trip signal is not DC referred, it is an alternating current, although the mean depends on the waveform, which is the magnetic field reversal point. A true RMS measurement of this signal will give the trip current, and the waveform is definitely AC, although not a sine wave. It also appears to be thought that a small DC current prevents transformer action completely. This is incorrect, although it will change the exact trip current. A current difference that is larger than the DC present will still give a difference signal, just not as big and possibly not as well shaped as may be expected. I refer you to any basic textbook on magnetism that deals with flux mechanisms. A strong magnet may be made to stop any attraction completely by winding a coil around it and applying sufficient current to cancel the magnetic field. That is the mechanism we are discussing, NOT some magic saturation property, which does not exist.


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. It is true that a fault in the correct value range can increase the trip time or protection current, but how it can happen is never defined. The danger area, if any, is very low power devices, which are inevitably class 2, so they cannot cause this variety of single fault. Something is seriously wrong in the State of Denmark! (Shakespeare 1564-1616). He obviously anticipated BS7671.

You are making another error Andy. The trip signal is not DC referred, it is an alternating current, although the mean depends on the waveform, which is the magnetic field reversal point. A true RMS measurement of this signal will give the trip current, and the waveform is definitely AC, although not a sine wave. It also appears to be thought that a small DC current prevents transformer action completely. This is incorrect, although it will change the exact trip current. A current difference that is larger than the DC present will still give a difference signal, just not as big and possibly not as well shaped as may be expected. I refer you to any basic textbook on magnetism that deals with flux mechanisms. A strong magnet may be made to stop any attraction completely by winding a coil around it and applying sufficient current to cancel the magnetic field. That is the mechanism we are discussing, NOT some magic saturation property, which does not exist.


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. It is true that a fault in the correct value range can increase the trip time or protection current, but how it can happen is never defined. The danger area, if any, is very low power devices, which are inevitably class 2, so they cannot cause this variety of single fault. Something is seriously wrong in the State of Denmark! (Shakespeare 1564-1616). He obviously anticipated BS7671.