Phase and neutral cables entering a metal enclosure through separate holes

Question

I have come across an Installation with the supply single AWA cables glanded off into a aluminium plate. Its a 800A installation with three phase and neutral cables all passing through separate holes so Eddy currents are a concern. 
 Anybody got any views ?

mapj1 · Accepted Answer

It used to be common on work that had to preserve a level of non flammability and IP rating, to have slot cut to dog-bone the gland holes, then re-filled with braze metal (non magnetic) then ground flush so the glands sat down nicely. Epoxy resin in the slots, tape over them or leaving a gap for things to get in are the lower cost options. 
 Or of course if there was no fire problem, magnesium aluminium alloy, or even tufnol. M.

AJJewsbury · Answer

gaps or gas ? 
 
 Opps, well spotted. Corrected now. 
 - Andy.

Chris Pearson · Answer

"Eddy" currents occur in steel plates, so aluminium should not be a problem.

mapj1 · Answer

If they all come in through the same non-magnetic plate, then there is no risk at all of magnetic heating as there may be with a steel plate close to the current cores, But if the armours are in effect all linked metallically at both ends then there will be current in the armour loop - like one turn transformer with a very large air core. If there is no evidence of heating of the plates or glands, then there is nothing to worry about. In that sense providing the thickness of the plate and the tightness of the glands is good enough for the current loop there is no issue.

JE 1872 · Answer

We had someone highlight this on one of our sites, when we looked into the design specification/as fitted drawings etc, it was established that the cables were installed in a trefoil configuration (equidistant in the shape of an equilateral triangle) and was suitable for this type of supply cable arrangement. Is this possibly the same here ?

broadgage · Answer

Disagree, any conductive material including aluminium is a problem.

mapj1 · Answer

The currents are due to the voltage induced along the length of the armour, and how much current flows rather depends on what is done at the other end. Nice drawings but no formulae here . 
 Lots of words and formulae but less pictures here The bit on induced armour voltage is near the end of the rather long page. Just above the glossary of symbols. But in practice if the gland plate is aluminium too, and thicker than twice the AWA strand thickness and all done up tight, it will run cool - and the table of voltage drop in bs7671 assume this grounded at both ends configuration and makes the allowance in the current rating and voltage drop for the armour losses it creates. (you can up the current rating a tiny bit with insulating gland plates or core rotation.) Mike.

AJJewsbury · Answer

JE 1872 said: Is this possibly the same here ? 
 Similar but different. Yes they're both the result of magnetic fields created by the individual cores and arranging things so they cancel out as far as reasonably possible before causing too much trouble - but the detail differs. Trefoil is preferred as a cable fixing layout mainly because it cancels out the magnetic fields along the length of the cable run (between the 3 lines at least)- which reduces the "interference" (not the proper word) between one core and the others - that's a good thing especially where large currents are involved as the interference can hamper the current flow (i.e. adds to the cable's impedance) - which is just not what you want when ADS depends on overcurrent protective devices. Where smaller currents are involved (and where cable resistance dominates over reactance) it's less of an issue - as a result it's common to see singles thrown into conduit or trunking with no particular control of their relative positioning. The OP's question related to the regulations around ferrous enclosures - again down to magnetic fields - but the effect is that of heating the metal around the hole (like a crude induction heater) - as long as all the conductors pass through the same overall hole the magnetic fields cancel out well enough (I suspect the steel channels the magnetic fields together to some extent)- the exact layout of the cores as they pass through the hole is of lesser importance. The effect was much more noticeable with the old thick-walled cast-iron switchfuses from early part of the last century - modern thin steel sheet enclosures are much less vulnerable. Where you do have a ferrous (i.e. iron or steel) enclosure and need to gland single-core cables into it, the trick is to make one big hole rather than three (or four) little ones and fill the gaps with something non-ferrous - e.g. three small holes in an aluminium plate, which in turn covers one big hole in the steel enclosure. 
 - Andy.

Phase and neutral cables entering a metal enclosure through separate holes

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