John Peckham wrote an article on using the armour of SWA cable it is on IET web site and he talks about the use of armour as an earth. He states that 2.5mm² 4 core has a CSA of 20mm sq but I know in practice this changes between manufactures.
Batt cables give diameter of armoured wires as 0.9mm and overall diameter over armour as 11.9mm so I would guess on twelve strands? So 12 x 0.636 = 7.6mm² which seems somewhat short of size given by John Peckham.
Expressing the Maximum armour conductivity expressed as a percentage of one phase conductor for 2.5mm² 4 core PVC is 92% XLPE 96% or 7.7 and 7.9 ohms per Km compared with 7.41 ohms for copper conductor.
This could of course change with different makes of cable but from that information it would seem enough not to really worry about with 2.5mm² 4 core.
However as the cable sizes increase the difference between the copper and SWA resistance increases and at around 95mm² 4 core according to John Peckham it is no longer enough.
Looking at chart at 15mm² 4 core the copper is half the resistance of the SWA and I would consider the use of 5 core cable.
Long runs always worry me as one can so easy reach a point when the magnet part of the MCB is no longer of any use and one expects problems. However in practice where overloading has occurred it is normally in the enclosure where the cores are in free air where they melt rather than in body of the cable.
Maybe Sparx can add to this as to be honest I did not really take enough notice of things like ELI when I was in heavy industry it was normally an emergency and we replaced cable then thought about what we should have done. We hoped the design engineer had got it right in first place. With 16th Edition we started to measure ELI but only when we had a meter and often we failed to make out the installation certificate. Around 1995 people started to do things closer to the book and with the BS7671:2001 the following of rules really started to become the norm. By that time I was not working with SWA as much and retired in 2004 so although I have taken C&G2382 not really in game any more.
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