Sorry, I am not quite picking up on what Mike is saying, can you just clarify if this is touch current as measured through a body model?

Sorry, I am not quite picking up on what Mike is saying, can you just clarify if this is touch current as measured through a body model?

I think, which is why I queried it, they seem to allow any no of amps to flow in the CPC, so long as it is not flowing L-E, In other words L and N currents balance - i.e. in a standard worded like that,  you are not regulating  the shock a victim would receive if the CPC broke and their body replaced it, but the rather the result of an RCD-like measurement of residual supply current, phase currents minus neutral currents  sort of thing. 

On a stand alone item like a kettle on an insulated worktop, the two are the same, but not for anything that has any degree of installation that allows a second earth path.

As a silly extreme imagine a double wound transformer with its primary wired L-N and its secondary wired between the supply CPC and some plumbing or building steel that is casually earthed , The CPC current is not derived from a pole of the supply, so L-N currents balance but a CPC current  is very much flowing , and if the voltages and series resistances were chosen right then breaking the cpc and holding both sides could be painful ;-) 

As far as I know most PA testers actually do measure CPC current, with varying degrees of error on non-sinusoidal waveforms, and for the purposes of the CE mark, the test gear that gets used in the labs does too - well at least it does on the stuff we CE mark,which is not electric  car chargers to be fair.

It just seems a strange way to set a limit that is really about shock protection, hence the query.

Mike.

mapj1 said:

I think, which is why I queried it, they seem to allow any no of amps to flow in the CPC, so long as it is not flowing L-E, In other words L and N currents balance - i.e. in a standard worded like that,  you are not regulating  the shock a victim would receive if the CPC broke and their body replaced it, but the rather the result of an RCD-like measurement of residual supply current, phase currents minus neutral currents  sort of thing. 

Agreed ... that is effectively the approach taken in some standards.

mapj1 said:

As a silly extreme imagine a double wound transformer with its primary wired L-N and its secondary wired between the supply CPC and some plumbing or building steel that is casually earthed , The CPC current is not derived from a pole of the supply, so L-N currents balance but a CPC current  is very much flowing , and if the voltages and series resistances were chosen right then breaking the cpc and holding both sides could be painful ;-) 

Yes, I do see where you're going. But that's a number of things going wrong in sequence rather than instantaneously, so therefore not "single fault conditions" or "additional protection".

mapj1 said:

It just seems a strange way to set a limit that is really about shock protection, hence the query.

BS EN IEC 62368-1 is very interesting. Doesn't align with BS EN 61140 at all really - especially where "SELV" and "PELV" are concerned, hence limiting the use of electronic supplies for certain special locations in BS 7671:2018+A2:2022.