Could people check my thinking on this please? I've been ponding how the L-N voltage monitoring approach might be able to distinguish broken PENs from other kinds of faults (e.g. L-PEN shorts) - as broken PENs should usually taken more seriously (either disconnect quicker or be more reluctant to re-connect).

My train of thought goes like this (with plenty of assumptions):

A L-PEN fault (or L-N or L-PE within an installation) - presuming fault of negligible impedance etc., and on a TN system of course will, tend to turn the supply conductors into a potential divider - so (presuming equal sized conductors) we get about half of Uo at the point of the fault. On that phasor diagram, a fault on L1 would be at the red dot:

we'd see the same at points downstream, but view from things tapped off upstream, the L & N conductors would still have some voltage difference between them:

so I notice that N could be closer to the centre, but never further away than Uo/2.

Similarly considering faults on L2 or L3:

so we get a triangular area within which N must remain for L-PEN faults.

So viewed as a voltage difference from our chosen line (L1 in this case), is it fair to say that the L-N voltage can't exceed the distance to the furthest point of that triangle from the end of the L1 vector?

Or in other words, if L-N exceeds (say) 318V, then we must have an open-PEN event? (OK, plus or minus a chunk to deal with Uo not being precisely 240V, and a few wobbles for inductive things happening), but as a general principle?

Of course there will be other characteristics too (typically normal shorts should be cleared pretty promptly, whereas open-PENs can go on for hours), but if we were looking for something that if present could show immediately that we were definitely seeing an open-PEN event, would this do?

    cheers

           - Andy.

(tweaked for less transparent diagrams)

For pure resistive faults I agree with this logic and inductance is not really a issue until we get to the really fat cables where R is so low the inductance starts to take effect.
Of course we could be on the faulty branch that gets the reduced line voltage ;-)
And, how much do we care if our fault detector disconnects for the wrong kind of fault - my feeling would be that a false positive trip may be prefferable to a false negative non trip of an open PEN were for some reasons of lucky load balance the voltage pull is not so great.

And of course there may be an over-voltage due to something else happening at the supply side. but that is likely to be very short lived - the transformer cores wil saturate if there is much of an HV over-voltage.

Mike.

For pure resistive faults I agree with this logic and inductance is not really a issue until we get to the really fat cables where R is so low the inductance starts to take effect.
Of course we could be on the faulty branch that gets the reduced line voltage ;-)
And, how much do we care if our fault detector disconnects for the wrong kind of fault - my feeling would be that a false positive trip may be prefferable to a false negative non trip of an open PEN were for some reasons of lucky load balance the voltage pull is not so great.

And of course there may be an over-voltage due to something else happening at the supply side. but that is likely to be very short lived - the transformer cores wil saturate if there is much of an HV over-voltage.

Mike.

And, how much do we care if our fault detector disconnects for the wrong kind of fault

That's something else I've been pondering - I'm probably biased in that at home we suffered from a couple grumbling DNO cable faults over the last few years - each one took literally months to finally locate and fix properly. So we went through many events where the lights would flicker a few times, or the power go off for a few minutes. If I'd had an EV and each time it had happened overnight I'd woken up to an unusable car I wouldn't have been happy.

   - Andy.

For pure resistive faults I agree with this logic

thanks taking the time Mike - much appreciated.

   - Andy.