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This Regulation is aimed at mitigating the effects that are caused by unwanted magnetic fields (eddy currents).
They did not include multicore SWA cables in with the single core SWA cables, simply because multicore SWA cables are acceptable for a.c. circuits.
It is quite obvious that SWA is considered to cause unwanted magnetic fields, otherwise there would be no constraints against using the single core SWA cables.
They haven't specifically mentioned multicore SWA cables, just as they haven't specifically mentioned galvonised steel conduit, galvonised steel trunking or even maleable iron conduit as all of these are covered by the term 'ferromagnetic enclosure'.
Now you have stated that the practice of running a separate earth has been in use for generations.
Are you sure that it has been for that long, and do you know whether the separate earth is a CPC, or a bonding conductor?
The armour while acceptable for a CPC, is not acceptable for bonding, as such a separate conductor is required.
I think you have basically said the same as me other than your last line.
Where does it say that you can't use the armour as a bonding conductor.
I've always thought that (assuming all the calcs allow) you can use the armour- as a cpc, bonding conductor, earth conductor or indeed all these at once.
In other words, if you were running a sub main you could use the armour as a combined cpc and bonding conductor.