Thanks all.

Will check the referenced regs when I get back to my Big Brown Book.

AJJewsbury Andy - surely if trying to figure out how much fault current would travel down the SWA vs a parallel/additional CPC then it's 'simply' a matter of using the resistances of each to figure out the current split - although the resistance of the SWA would be affected by how good the gland terminations are, and would possibly be more susceptible to corrosion - especially outdoors.

Jason.

surely if trying to figure out how much fault current would travel down the SWA vs a parallel/additional CPC then it's 'simply' a matter of using the resistances of each to figure out the current split.

Quite, except that the word 'magnetism' seems to strike fear into the hearts of those classically trained in electricity only, who presumably slept through the EM part of the lecture and woke up near the end, when all the things that go wrong were being discussed leading to irrational worries about eddy currents, additional inductance and confusion about if we are cutting grooves into gland plates to interrupt lines of magnetic field or eddy currents.
Now wires inside tubular magnetic cores do indeed experience substantially higher inductance, as the magnetic field around the wire is augmented by the elemental magnets in the tube wall lining up in rings nose to tail spinning around the wire and reversing 50 times a second. So steel conduits with outbound current up one tube and return down another are a very bad idea, while if the tubes were plastic or indeed any non magnetic metal the inductane would be largely unaffected.

Worse, the sort of steel used in conduits does not have a particular magnetic or electrical spec, so  the heating associated with that regular magnetising and demagnetising, as well as any eddy currents flowing parallel to the currents, are not easily quantified, except as 'here be dragons, avoid them' Single core wire armour is always aluminium or copper, never iron or steel,  just in case this is a problem.

But the steel wires of an SWA do not really form a continuous cylinder enclosing the wires  - really there are air gaps, slots  and bunching and the magnetic effect is more like split tube with a distributed 10% gap, rather than welded conduit.  As such the extra impedance in the fault loop for an external CPC versus one up the middle is only a small fraction of the case if closed rings or short tubes of steel had been threaded on the wire, and not that significant, at least at 50Hz,  and for a CPC we are not really pushing the current rating to its limit..

Oh and at really high currents once all the magnetic domains (elemental magnets in the iron or steel) have aligned, and there are no more to move, the inductor saturates anyway, and the in effect the inductance is falls to its free air value.

So 'here be dragons, but really not very big ones' The  final Zs should ideally be verified at AC not as a an R1/R2 at DC, or at least expect the AC case to have a lower PSSC, and allow slack in the ADS design for that.

Mike

surely if trying to figure out how much fault current would travel down the SWA vs a parallel/additional CPC then it's 'simply' a matter of using the resistances of each to figure out the current split.

Quite, except that the word 'magnetism' seems to strike fear into the hearts of those classically trained in electricity only, who presumably slept through the EM part of the lecture and woke up near the end, when all the things that go wrong were being discussed leading to irrational worries about eddy currents, additional inductance and confusion about if we are cutting grooves into gland plates to interrupt lines of magnetic field or eddy currents.
Now wires inside tubular magnetic cores do indeed experience substantially higher inductance, as the magnetic field around the wire is augmented by the elemental magnets in the tube wall lining up in rings nose to tail spinning around the wire and reversing 50 times a second. So steel conduits with outbound current up one tube and return down another are a very bad idea, while if the tubes were plastic or indeed any non magnetic metal the inductane would be largely unaffected.

Worse, the sort of steel used in conduits does not have a particular magnetic or electrical spec, so  the heating associated with that regular magnetising and demagnetising, as well as any eddy currents flowing parallel to the currents, are not easily quantified, except as 'here be dragons, avoid them' Single core wire armour is always aluminium or copper, never iron or steel,  just in case this is a problem.

But the steel wires of an SWA do not really form a continuous cylinder enclosing the wires  - really there are air gaps, slots  and bunching and the magnetic effect is more like split tube with a distributed 10% gap, rather than welded conduit.  As such the extra impedance in the fault loop for an external CPC versus one up the middle is only a small fraction of the case if closed rings or short tubes of steel had been threaded on the wire, and not that significant, at least at 50Hz,  and for a CPC we are not really pushing the current rating to its limit..

Oh and at really high currents once all the magnetic domains (elemental magnets in the iron or steel) have aligned, and there are no more to move, the inductor saturates anyway, and the in effect the inductance is falls to its free air value.

So 'here be dragons, but really not very big ones' The  final Zs should ideally be verified at AC not as a an R1/R2 at DC, or at least expect the AC case to have a lower PSSC, and allow slack in the ADS design for that.

Mike