Single Core cables and Steel Gland Plates

[SparkyLarks]

I have an applications where I'm installing 3 single core XLPE/AWA Cables.

The cables will be installed into an existing Steel Gland Plate and I need to calculate the possible circulating currents in the plate. (I know it should be a non-ferrous but it;s not really an option.

Does anyone know how to do this?

I know cutting slots in the gland plate will reduce the circulating current, but where do I cut the slots?

Is it two slots, across the width of the gland plate between the Cables.
Or

One slot across the length of the gland plate through the middle of the holes in the gland plates.

 

[eti]

Sparky

The eddy currents can be calculated using the CSA of the conductor, distance to the plate, materials, air gap in gland, primary (circuit) current etc. The calculation isn't very complex, but I'm not sure the answer will tell you very much.

The traditional solution as you rightly point out(other than another gland plate) is to cut slots between the glands. i.e. if you have three glands the first cut goes between the gland 1 and gland 2, the second cut goes between gland 2 and gland 3.

It tends to weaken the plate, so you can make the cut but leave a small piece of steep uncut in the centre of the line. The other thing to consider is the ingress of moisture and duct. some sort of insulating material along the inside of the plate usually works.

Hope this helps.

 

[waross]

It is recommended that the connecting hardware or glands including the lock-nuts be non-magnetic.

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

 

[SparkyLarks]

Waross, I'll be using brass glands.

eti,I was hoping to be able to figure out what the heating effect due to eddy currents would be. Before I make cuts in the gland plate I would like to be sure that it is necessary.

Should the Cables enter the Gland in Trefoil? Would that cause much of a reduction in eddy currents?

 

[ScottyUK]

I'd have expected you to use one gland per cable, not try to put them through one gland in trefoil. Anyway, why can't you do this properly using a non-ferrous glandplate?


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

 

[desertfox]

Hi SparkyLarks

Before you put cuts in the gland plate make sure there is no internal arc requirement, putting slots in the gland plate might invalidate internal arc protection.

desertfox

 

[Marmite]

Why not take the singles through the gland plate in a trefoil group and use a three finger heatshrink breakout? Desertfox makes a very valid point. Often cable boxes are designed to deflect the blast away from the operator in the event of failure. Some use brass bolts at the rear and stainless at the front, for example. You could mess up this important safety feature of the design completely by weakening the gland plate. Personally I would look at other options.
Regards
Marmite

 

[SparkyLarks]

The cables enter the switchgear through a cable box at the back of the switchgear. The cable box is about 400mm x 300mm x 300mm There is an existing 3-core 240sqmm SWA cable entering the cable box.

An aluminium plate could put onto the switchgear but it would be difficult.

I was hoping to find a way to calculate the heating due to eddy currents and see if for a steel plate the heating would be excessive.

My question about bringing the cables in in trefoil wither in a single gland, or with three bras glands arranged in a trefoil arrangement as close together as possible, is would that significantly reduce the heating effect?

 

[Marmite]

If you take the cables through the gland plate in a single gland in close ie touching trefoil then there shouldn't be any heating of the gland plate.
What voltage is this installation? Your cable box is quite small. If it is compound filled now, then you might not have enough clearance to do a dry termination without using a cold pouring compound such as guruflex. That would rule out cutting the gland plate.
Regards
Marmite

 

[SparkyLarks]

The voltage is 3.3kV,

The cable box isn't filled, just air.

 

[ScottyUK]

I'm now even less certain what the difficulty is to get a fab shop to make up a new glandplate in brass or GRP? I could almost find an excuse with a compound box but for an air box the only one I can think of is someone cocked up the order and didn't spec a non-ferrous glandplate and doesn't want to pay to put it right. Not a very good reason for bodging the job.


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

 

[SparkyLarks]

Scotty

I agree, and I'll be on site tomorrow to have a good look at it. If a non ferrous gland plate needs to and can go in then it will.

But it brings me back to my key question, how do I calculate the heating that will be seen for a given gland plate.

While I don;t doubt for a second that a steel gland plate will see greater circulating currents and heating than an aluminium/brass one, I like to be able to quantify things out.

How can I prove that the steel will heat up, and by how much?

 

[ScottyUK]

You basically have an air-gapped transformer driven by a current source. You need to estimate the flux linkage which crosses the air gap (ok, it's actually cable insulation but to a transformer it's an air gap) and work from there. It's almost certainly easier to consider the glandplate which forms the secondary winding as a steel ring of finite dimensions and calculate the losses in it rather than try to work with the glandplate itself. If you had 100% flux coupling then the secondary current would equal the primary current, but the reality is that air-gap transformers have lousy coupling and you're probably down at about 30% coupling or less. In my opinion it is a model best refined by controlled lab tests - I don't think you'll get an accurate equation as there are too many unknowns although I'm certainly interested if anyone has one to offer.



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

 

[prc]

I can check and give some indication if you can give current through the single core cable, pitch of the cable and the hole you are making for the brass cable gland.

The best way is to replace the gland plate by an aluminum or non magnetic stain less steel plate( 306 or 316).In case you make slots,the correct way is as explained by eti.But how can you allow an open slot that may affect the integrity of the cable box. One way is make a slot of atleast 50 mm wide and then fill it by welding a stain less steel strip of same width.

 

[waross]

The Canadian Electrical Code used to require that some method be used to reduce eddy currents and heating when the current was above 200 Amps per cable. Below 200 Amps, measures to reduce heating where recommended but not required.
I'm on vacation and can't check the current code wording.

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