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Single phase cable terminations and gland plate material

Single phase cable terminations and gland plate material

Single phase cable terminations and gland plate material

(OP)
Hi

Consider three singlephase cables that need to be terminated at the gland plate of an indoor switchgear cubicle. Each cable has a metallic sheath. At the gland plate the cable is stripped and the sheath tied to the earthbar.

Question I was asked by an electrician (and which really got me thinking) is should the cables be terminated in linear or trefoil format and what is the difference? The gland plate in question was made of mild steel.

I reasoned that every current carrying conductor produces a magnetic field. This magnetic field is not contained by the sheath. Since the gland plate is non-ferrous it has a magnetic permeability equivalent to that of air. Thus magnetic field in plate same as that in surrounding air. The magnetic field gives rise to an electric field the latter being perpendicular to the gland plate and thus no real eddy currents are expected in the gland plate.

But to be safe the gland plate is cut unto two equal halves to reduce eddy currents? Am I correct?

Also, trefoil formation would be better as the resulting external magnetic field would be much reduced. But linear formation is more practical?

Appreciate your comments.

Thanks.

 

RE: Single phase cable terminations and gland plate material

You pretty much have it pinned down: linear configuration would suit the switchgear terminals, but trefoil would have better magnetic cancellation. I've never seen anyone use trefoil configuration at switchgear because conductors big enough to warrant single cores are usually too rigid to bend within a modern cable box.

Single cores will induce circulating currents in any closed ferrous loop which encloses the conductor. The closed loop will then get hot, possibly very hot. The ferrous glandplate is bad news and should have been specified as brass or aluminium, or a glass/resin composite. Cutting it in half is a poor solution - any chance of getting a new glandplate made in a non-ferrous material?
  

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If we learn from our mistakes I'm getting a great education!
 

RE: Single phase cable terminations and gland plate material

If the cables are run in a flat configuration, the terminations may as well be flat. Even if the cables are run trefoil, a flat configuration at the gland plate will not make much difference. A hacksaw cut between the three cable holes in a steel plate will make one hole as far as induction heating is concerned. This does not reduce the strength of the plate very much. Cutting the plate in half is a bad idea.
Above a few thousand amps, you may want to consider an aluminum plate.
Hi Scotty. I did an installation with about 12 cables plus neutrals. I arranged the holes in two parallel rows about two inches apart. The holes were on about 2 inch centers but one row was offset from the other row. Then I was able to use a series of trefoil patterns. I agree with you, trefoil is not common for only three conductors.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Single phase cable terminations and gland plate material

That's neat Bill. What size cables? We normally only see singles once it gets above 300mm2 and with solid-stranded cable that's not exactly easy to manipulate from the entry point onto the palms in the barely-big-enough cable boxes we get today. I guess with 12 plus neutrals it was a big cable box!
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Single phase cable terminations and gland plate material

(OP)
Hi Scotty, waross

Correction, it was an Al gland plate. Sorry. Question I still have though from a purely physics perspective is this:

If the cable is perpendicular to the gland plate then the  magnetic field will be parallel to the gland plate. But this implies that the electric field is perpendicular to the gland plate and so how can eddy currents be induced in the gland plate?

Maybe I need to study Maxwell's Laws again but that's a whole house of pain exercise!

Thanks.  

RE: Single phase cable terminations and gland plate material

They were locomotive cables. Very bulky. I think about 353 MCM. Almost 1 inch in diameter. These are designed for power connections between multi unit North American diesel locomotives. Flexible and tough. They are also used in some offshore generator power panels. Our little diesel plant used a lot of it.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

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