Cable routing through slotted hole

There is no magnetic reason not to do this - the problem with cable bunches is purely thermal, so long as the net current in the hole sums to zero there will be no overall magnetic field.


As an aside about the mechanism ..


Note that the metal box hole problem is almost never an eddy current problem, despite misleading pictures in a few text books from organisations that should know better.

There will be no electric curent in any steel (or other metal) that is at right angles to the direction of current flow - - the slotted hole thing is required to prevent magnetic losses. (eddy currents are parallel to the currents that cause them, not at right angles - eddy current can be an issue in long runs of AWA however - but in the case the current in the armour really is parallel to the driving current in the core.)

If you want a picture, the effect is that all the little dipole magnets in the steel swing round like so many atom scale compasses, trying to magnetise the steel to form a complete nose to tail loop of N-S-N-S... surrounding the current, and then having to undo it and reverse direction  50 times a second and getting all hot and bothered doing so. When there is no net current, they do not feel the urge to surround it.


Once you have this off pat, you realise you can hacksaw cut between the holes in a cast iron or steel plate to break the magnetic path, and fill the cuts

in with solder or braze metal to restore the IP rating,  which if it was a current thing , would not work.


regards Mike.

Thanks Mike


Just to clarify, when I said a slotted hole I didn’t mean two holes with a hacksaw cut between them often referred to as a dog bone shape. I was meaning a proper slot in trunking the same width the whole way along like 25mm or 32mm to allow for Multiple circuits into a distribution board

from the point of view of magnetism, steel is sooo much more magnetic than air or copper, that a dog bone, a rectangle  or a round hole  of the same area, or anything in between are very much alike.

When the data for the material says   'mu is about a thousand ' , 

That means one mm of air gap is as hard to jump as a full metre flux path in the steel, so the magnetic loop tends to prefer to go the long way round in the magnetic material, and not jump the gap even if you cut really thin slots. The other effect of this is that a really diddly gap on re-building a transformer completely trashes its performance.

I think that I can see the reason for concern.


Think of it another way. Suppose that you make up a brass or non-metallic gland plate. You can put the cables through where you like because the magnetic fields will not be circulating in the brass - they will be in the steel casing of the DB or other equipment. Provided that the cables go through the same hole (of whatever shape) there will be no problem.


However, I did wonder what would happen in an extreme case - let's have a 100 m x 25 mm long slot with all the lines at one end and all the neutrals at the other. Mike (or anybody else) would that make a difference?

You are right,  your  thought experiment 100 metre slot of 4000:1 aspect ratio, would find some of the flux jumping the gap, and the inductance it would create around a pair of wires would depend on the position of the wires - However, in terms of heating the plate,  it would still be fine -  just that the magnetic field would look much the same in the very long slot, or an infinitely long one, you'd need an enormous current density in the wire to get enough field to warm up the steel. and by then the copper heating would probably be the main concern.

for a Steel of mu factor 1000, slots of a few thousand: length to width ratio are where the 'might as well be an infinite slot ' effect starts to kick in.

The other way of looking at this is how thin can the slit be in the dogbone, and I'd expect the cancellation to be starting to look less than perfect at a few hundred to one - so the un accompanied  hack saw cut of perhaps a touch under a mm wide probably ought to made a bit bigger, when it gets near a foot long.

Yes, there is a limit, but it is a long way from the normal case.

(and not all steel has a mu of 1000 either, but the low mu ones are not lossy, so there is less heating anyway...)