I hope you do not mean Steel wire armour, and single core.

Not only does the armour form shorted turn of a 1 turn transformer - and this is why we gap and rotate the armours part way along on long runs of AWA by the way (or use insulating gland plates at one end on short runs)  but then there is the additional problem of magnetic losses in the steel armour. (not to be confused with eddy current - they flow along the wire parallel to the main current, while the magnetic losses are the same if you were to slice the armour into isolated washers, and relate to the magnetic flux spiralling one way and then the other about the current, and reversing with the mains cycle.)


Ideally get flow and return currents under the same armour or as soft skinned  singles in a common enclosure, even if that means parallel cables,  this is essential with SWA, and very desirable with other armours.

If you must use armoured singles, then the armour must  be non-magnetic, and the shorted turn formed by the armour gapped in some way on all but the shortest run.


Note that both the armour voltage and the magnetic losses rise with frequency, so can give really unpleasant effects on VFDs, thought for your heater I assume it is simple resistive load and 50Hz.

The only place for steel armoured singles is in DC circuits.

Stephen,

Normal practice to minimise eddy currents is to install the three single core phase cables in a trefoil arrangement and then every so often switch the phase rotation.

Thus start   L1           then switch to  L1

                L2  L3                             L3  L2

and keep switching at regular intervals.

As an aside, have you not thought of using several three core cables in parallel - this should prevent the problem.

Thanks for the replies.  This was done by our customer, who installed a heater we supplied.  It's a very secure site so I wanted to do my research before I get back on site.  The job has been tampered with by a number of people with half backed ideas already, so I want to get the theory well understood.


Stephen