The 3 phases and relevant Neutral seem to go through the same hole hole as the lives, so no - no eddies here. 

The 3 phases and relevant Neutral seem to go through the same hole hole as the lives, so no - no eddies here. 

Even if they all went through individual holes, there is no scope for eddy currents (*) - you need a long conductor parallel to the primary current for that (conduit, SWA armour that sort of thing). There would however, be scope for heating of the steel via magnetic hysteresis loss,  (the magnetic field forms rings around the conductor, currents are parallel to it) but in this case the net current through any one hole averages out to zero, assuming the right neutral is bundled with the right phases, so the net magnetic field is  only a modest imbalance, and can safely be neglected.

Magnetic loss is actually a pretty weak effect in any case, and to heat a thin steel plate like that sort of trunking, even with small hole and closely fitting wire, needs many tens even low hundreds of amps of unbalanced current to get a discernible temperature rise, and the effect falls quite rapidly with increasing gap between wire and the hole, and with thinner metal sheet.

regards Mike

* apologies  if pointing this out seems petty, but the magnetic side of things is sometimes not well taught and often misunderstood, and  I think it is important to be clear what sort of effects can and cannot occur in that geometry, as it avoids the wrong sort of solution.  Its not helped that some official-ish documents also get magnetic and resistive heating effects confused.

Magnetic loss is actually a pretty weak effect in any case,

To the point that the DPC for AMD4 proposed dropping the all through the same hole requirement entirely for steel DBs and the like as long as the rated current is below 200A.  Oddly the exception doesn't seem that it would apply to steel trunking, just the DBs themselves.

   - Andy.

I missed that, but it seems very sensible.

Of course you could make a brass/aluminium/Paxolin/etc. gland plate.

In the few cases where steel plate has to be dog boned to link holes carrying opposing un-cancelled currents, a single hacksaw cut will normally do it to topologically turn two holes into one dumb-bell shaped one - air being thousands of times less magnetic than steel you don't need much of a gap to break the field lines from closing around any one conductor. This can then be left open or filled with a suitable  non magnetic material such as epoxy resin, or a line of hard solder or braze metal and ground flat, depending on the environmental / blast proofing requirements of the enclosure.

Mike

mapj1 said:

This can then be left open or filled with a suitable  non magnetic material such as epoxy resin, or a line of hard solder or braze metal and ground flat, depending on the environmental / blast proofing requirements of the enclosure.

Or depending upon one's attention to detail. For some reason, there is a distribution circuit in my home which uses insulated and sheathed singles. So I have dutifully dog-boned the holes and because the circuit leaves through the top of the DB, I felt duty-bound to fill the cut with Araldite.

In these circumstances, cocktail sticks come into their own.