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Aluminium and eddie currents



3 phase +N single core, insulated, aluminium conductors (2 sets in parallel) pass through an aluminium plate (conductors are approx 3" apart, 2 rows of 4 conductors). Can/will eddie currents be produced in the plate? There's arguments it will, because that's how the old mechanical electricity meters work (current and voltage coils influencing an aluminium disc), and anecdotal tales of aluminium bars shooting out of energised coils, compared to others saying no, because aluminium is non-ferrous?


F
  • Single core cables are normally glanded into Aluminium gland plates so if they can go through one cable to a hole they certainly should be able to go through all through the same hole.
  • I too believe that you're OK putting single core cables through separate holes in aluminium. Yes you can induce some current in aluminium - as you can in other non-ferrous metals such as copper (in say the windings of a transformer) - but the effect is much smaller than with steel - so small that the effect can be neglected.


    Even with steel, you occasionally see meter tails on 100A supplies being (erroneously) taken through separate holes in steel DBs and the like without any obvious problem - so if the effect with a non-ferrous enclosure is even smaller, it's not really going to be a practical problem at all.

       - Andy.
  • When a cable carrying an un- cancelled current passes at right angles though a plate it does not induce currents in it - any current induced is parallel to the wires that create it - think of the primary and secondary windings in a transformer, very much parallel to each other.


    What you do induce at right angles to current is a magnetic field, that forms closed circular loops around the wire. 

    Only magnetic materials respond to that, and with AC, this magnetising and demagnetising at 50Hz can certainly give heating, (the groups of atoms that are like little elemental magnats get very hot and bothered swinging back and forth trying to keep up). The effect can be reduced by making the hole a loose fit on the wire, so the magnetic material is not in the bit where the field is strongest, but far better is to have flow and return currents in the same hole. For this purpose, "in the same hole" may be round holes joined by thin saw cuts to make a dog-bone shape, and if needed for IP rating or arc containment, the slot can be brazed or filled with epoxy. Or if not needed, just  left open. But this only matters for iron and steel, and even then, not all forms of stainless steel. So much for magnetism


    There is another  situation where you can come unstuck  with armoured cables, carrying unbalanced currents, such as single core AWA sharing a gland plate, and that is when currents are induced in the armour (because the armour is parallel to the central core of cores..... ) and then current arrives at the gland plate from the armour of one core, and leaves via the armour of another. For this the cure is insulated bushes at the gland plate, so the armour is interrupted on all but one phase, or swapping the armour connections around at intervals on a long length,  so that  current arriving on any one armour has spent a more or less equal fraction of the distance being on the armour of each phase in turn. Or just having very thick glanding plates so they do not resistively  heat, and de-rating the cables to allow for the current that will flow.


    So, if your plates are ally, no magnetics - but are you glanding off  multiple lots of armour to the same plate ?  - if so, think about the current that flows between the glands - that effect is resistive, NOT magnetic.