I think it depends whether the cable is buried or not.

Regulation 543.6.1 says:

543.6.1  Where overcurrent protective devices are used for fault protection, the protective conductor shall be incorporated in the same wiring system as the live conductors or in their immediate proximity.

So provided the external cpc is in its immediate vicinity, it could be OK to provide a reinforcing cpc.

BUT Regulation 522.8.10 could be said to imply that if the cable is buried directly in the ground, the armour needs to be able to act as a cpc ... and that would also potentially imply an internal "additional cpc" is also out. Depending on how you view Regulation. 522.8.10 may also be interpreted that a suitable duct (for example, high impact resistance"twinwall" or similar) is able to provide mechanical protection for buried cables, so if using that you can ignore the fact the armour has to be a cpc.

BUT THEN AGAIN, nothing to say any cpc has to be able to take all the fault current, and can't be re-inforced by additional parallel conductors.

If you are using the reinforcing cpc method, the earth fault loop impedance of the combined line conductor and cpc are calculated in accordance with PD IEC/TR 50480. This is referenced from Appendix 4 to BS 7671, and further guidance can be found in the IET Electrical Installation Design Guide.

I am not aware of any prohibition on use of a separate CPC in conjunction with SWA Cable in the UK. Overseas regulations may differ.

5 core SWA is hard to find in the larger sizes, so for a 3 phase 4 wire circuit, a 4 core SWA and external CPC is the norm.

This might come from 521.5.1  - the armour of a steel wire armoured cable being considered a ferrous enclosure - and that reg's requirement for all conductors, including the protective conductor, to be within the ferrous enclosure.  However some years ago a specific exemption was added to allow an additional protective conductor in parallel with SWA (but not steel conduit etc). So the comment might have been a correct interpretation a few years ago, but is clearly allowed now.

There's also been some debate on how to size the additional conductor - whether it needs only to 'make up' what's missing from the SWA or whether it needs to be the full required c.s.a. by itself (on the basis that currents don't necessarily divide nicely between ferrous and non-ferrous conductors). Bit that's a different debate (I think the conclusion was that it's somewhere between the two).

   - Andy.

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

gkenyon said:

BUT Regulation 522.8.10 could be said to imply that if the cable is buried directly in the ground, the armour needs to be able to act as a cpc

The interesting phrase is, "suitable for use as a protective conductor". That might be read as excluding any cable whose armour is insufficient as a CPC, in which case a conduit or duct would be required and of course the extra CPC would be within the same duct.

I don't think that 543.6.1 is particularly relevant because "wiring system" as defined includes, "parts which secure and, if necessary, enclose the cable" so fastening the extra CPC under the same cleats or cable ties, etc. would include it in the same wiring system.

AJJewsbury said:

There's also been some debate on how to size the additional conductor - whether it needs only to 'make up' what's missing from the SWA or whether it needs to be the full required c.s.a. by itself (on the basis that currents don't necessarily divide nicely between ferrous and non-ferrous conductors). Bit that's a different debate (I think the conclusion was that it's somewhere between the two).

Everything you need to know to determine the proportion of the fault current in the armour and separate cpc is included in Annex NA.4.5 of PD CLC/TR 50480 (see Appendix D, section D5-5, of the 2022 IET Electrical Installation Design Guide which contains the formula you need to use.

gkenyon said:

BUT Regulation 522.8.10 could be said to imply that if the cable is buried directly in the ground, the armour needs to be able to act as a cpc ... and that would also potentially imply an internal "additional cpc" is also out. Depending on how you view Regulation. 522.8.10 may also be interpreted that a suitable duct (for example, high impact resistance"twinwall" or similar) is able to provide mechanical protection for buried cables, so if using that you can ignore the fact the armour has to be a cpc.

mmmm..... i hesitate to put my interpretation of this reg to the guy who I had assumed wrote it? 

I've always interpreted this reg to say something along the lines of,

If the cable is buried directly in the ground without the use of proper duct (my interpretation here - Twin walled smooth bore duct)  then the cable shall be armoured and that armour shall be earthed. I've always assumed that the use of the words, "earthed armour or metal sheath or both" allowed for the use of cabling other than our standard SWA or AWA, Lead sheath maybe? Copper sheath of a Pyro (m.i.m.s) cable I suppose. Has to be........

I tend to use SWA as standard in buried (ducted) supplies, but I've always assumed that this regulation did not preclude the use of other cables types as long as it was in proper duct (or conduit - but who buries conduit? and does it need to be earthed too?) 

And as for 543.6.1, I've always understood the use of those words in the last part of the reg, " .......or in their immediate proximity" to allow for that parallel earth specifically used commonly with SWA supplies. Its why I thought that was there for?

Its My take on it anyway........

jbrameld said:

'simply' a matter of using the resistances of each to figure out the current split

As mapj1 says, it's not a simple resistive split, but rather an 'impedance split'.

Just a reminder that, whilst we generally ignore impedances for conductors with csa up to 16 mm², with the exception of 2c 1.5 mm² SWA, the csa of the armour is >16 mm² and reactance is important in the calculations involving the armour (used as cpc or in parallel with a copper conductor as cpc) for 50 Hz mains circuits.

Of course if we think that 16mm2 limit is related to current handling capacity,  or more accurately heating while handing fault currents, then we should also allow for the fact that the resistance, and so the heating, of the steels in SWA can be that much higher. 

The resistivity of  steel typically varies with carbon content from 100 to 1000 x 10-9. while drawn copper has a much lower resistance than even the best steel -  17.2 x10-9 ohm.metre at 20C.

The resistivity of something is just the resistance between opposite faces of a 1m cube of it - but a 1m length of 1m2 cross section is a pretty inflexible sample converting m2 to mm2 and turning the nano-ohns to milli-ohms is more practical !

According to JPs authoritative text on the topic, IEE GN8 suggests using a factor of 8 for the ratio of the resistivity of copper to steel - though  this may assume more conductive steel is  used in SWA than for average construction steel.

In an case to get the equivalent earthing impedance to 16mm2 of copper needs  well over 200mm2 of steel. More dragons...

Mike