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.
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
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
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
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"Why not the best?"
Jimmy Carter
RE: Single phase cable terminations and gland plate material
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Single phase cable terminations and gland plate material
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
Bill
--------------------
"Why not the best?"
Jimmy Carter