Gland plate for single core cables. 1

[preb]

It is generally specified to use non-magnetic cable glands for single core cables & mostly this leads to the use of Aluminium gland plates instead of the normal mild steel. As per my understanding, we are trying to reduce the eddy current loss. Aluminium also has some amount of eddy current loss (which is put for better use in our electro-mechnaical energy metres). So is it just that comparatively eddy current loss is less in aluminium than steel? Is there any other reason for using aluminium? Do hysteresis loss or any other factor(other than thickness of material) play any role in gland plate heating in the case of mild steel?

 

[waross]

A magnetic material means a much higher magnetic flux and as a result much higher eddy currents. Some hysteresis loss also with steel.

Bill
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"Why not the best?"
Jimmy Carter

 

[ppedUK]

The main requirement for a gland plate suitable for single core cables is that it must be Non-Magnetic, for the reasons waross gave, but the gland plate does not have to be Aluminium. Brass gland plates are sometimes used for the same reasons where brass stuffing glands have to be used.

The problem with aluminium cable glands (assuming you are using AWA cable) is that they tend to have very short threads (I don't know why!) and if installing very large cables, the threads are easily stripped during cable installation. Similarly, aluminium gland plates distort very easily with large cables, unless the plate is quite thick, but with a thick plate, you can't get the locknut on the gland because of the short thread.

I have seen HV switchgear that required a number of large single core cables to be terminated and this required steel glandplates (half inch thick) for mechanical strength, but these had Non-Magnetic (stainless steel) inserts welded in between the gland holes to break the magnetic circuits.

The key is to eliminate, or minimise the eddy current heating in the glandplate.

 

[ScottyUK]

Aluminium glands paired with aluminium armour avoid any problems of intermetallic corrosion. Brass gland plates are fairly common, and I've seen a few in glass reinforced plastic. Personally I prefer the brass type. It's typical to see a 6mm or 8mm brass plate where a 3mm steel one would be expected.

Anyone who can explain the short threads problem on aluminium gland I am interested in an answer too!


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

 

[MikeHalloran]

My theory is that electrical hardware with mechanical problems was almost always designed by an electrical engineer. They crack a book, run whatever equations they find there, and call it good. After all, anyone can see and touch mechanical stuff, so it must be simple, right?

Sorry.
No offense intended, present company excepted of course.



Mike Halloran
Pembroke Pines, FL, USA

 

[ScottyUK]

None taken. You guys would just make the glandplate from steel then watch the pretty colours as the metal gods made it glow red hot, wouldn't ya??

There is quite possibly an element of truth in what you say, especially some of the switchgear designs on the low voltage and medium voltage markets. At least, there has to be some explanation why they are so awful.


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

 

[MikeHalloran]

Hey, mechanical guys _like_ things that get real hot, or catch fire, or explode occasionally, or have immense mamm... never mind.

The glandplate argument also applies to getting the phase wires out of the (steel) generator head, which I have seen done by a trained electrical tech/assembler who mostly decides where to cut the holes by himself.
He says he _noticed_ that the metal between the holes got warm, but he didn't _say_ anything about it, much less _do_ anything.



Mike Halloran
Pembroke Pines, FL, USA

 

[RRaghunath]

well summarised, ppedUK.

 

[larsen7]

Can anyone clarify the benefits of the traffolite gland plate when glanding a PILCS/SWA/PVC multicore cable into an air box. The issue that is coming up is the degradation of the paper insulation within the cable make-up which could impact on the integrity of the glanding arrangement when using an air box. Normally this would not be a problem with a bitumen filled box, but the cable will now be glanded into an air box.

The cables shall be terminated using suitable metallic compression glands, to BS 6121, that have provision for clamping the cable armouring and seating on to the outer sheath of the cable.

Does anyone have any experience of using traffolite glandplates for PILCS cables and if so what are the benifits and potential problems? I have been told without reason that the traffolyte gland plate will be suitable for PILCS cables, but I need more justification that this.

An option is to cut the PILCS cable and joint it onto an XLPE type cable before glanding into the air box, but we want to limit implementation works and interruption of supplies as much as possible.

Graham

 

[ScottyUK]

Traffolite is a material for making engraved labels and nameplates in contrasting colours - do you mean Tufnol in either the fabric or the glass-filled variety?

Anywhere that you have single-core cables you will have a net external flux which will cause a circulating current in the gland plate. It is most pronounced in ferrous materials because of their magnetic properties. At low currents it's not much of a problem but with high currents it is because of the heating effect. The history of using non-ferrous gland plates for single-core cables goes back 'forever' and I've seen them in brass and tufnol and occasionally aluminium.

Your problem isn't so much the glandplate but the correct termination of the PILC cables in an air terminal box. If you have single core cables then use a non-ferrous glandplate. If the PILC is a multicore then use a standard steel glandplate.


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

 

[ppedUK]

larsen7

You make a slightly worrying statement when you mention "Degradation of the paper" in an air box. I assume that you will be sleeving the cores and cable inner with a proprietary heat shrink termination kit for paper insulation won't you? If not, you're onto a loser from the outset.

Gland plate material is not an issue with multicore, multiphase cables, providing that it has suitable mechanical strength.

Also you don't mention what voltage the cable is at. Worry less about the gland plate material (mechanical strength issues aside) and concern yourself more with the correct termination kit for a PILC/SWA cable in an air box, particularly at anything above LV level. You wouldn't normally use a compression gland on PILC cable anyway (although I have seen many instances of this) as the lead tends to 'cold flow' under compression. There are many other issues with terminations at HV level (such as the screening of the cores etc). Seek advice from an experienced cable jointer for your installation.

Scotty,

I think Tufnol was banned some years back, I haven't seen it used for at least 15-20 years. It has generally been replaced by GP03. But I agree, although traffolyte is available in quite thick sheets, you wouldn't normally use it for a gland plate.

 

[ScottyUK]

Now you mention it I have only seen the glass-filled epoxy type on recent jobs. Why was the phenolic/fabric version banned? I can imagine the dust from drilling it might cause some horrible lung diseases, and god knows what it would break down in to if it was on fire.


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

 

[larsen7]

Gents, many thanks for responding and providing some excellent pointers.

Your all spot on with regards to the gland plate. It appears that I was given poor information. The issue is the gland arrangement. We will be using a standard steel plate now, but I have to clear up the probs with the glanding of the PILCS multicore cables.

Remember this is a multicore cable 0.3" PILCS/SWA/PVC, there are 2 others of the same csa that need dealt with. The application is the reqplacement of a transformer fed lv switchboard. The other cable types are all XLPE/SWA/PVC multicores so they are not a prob.

The multicore PILCS cable makeup is:-
1)Stranded conductors 2)Impregnated paper insulation 3) Filler 4)Impregnated paper belt 5) Sheath lead 6) Bedding 7) Steel tape armour 8) Serving PVC

CMP glands do not do a gland arrangement for the PILCS multicore cable, and they recommend the cable be jointed to an XLPE tpye before being glanded using a standard compression gland. They do glands for 'cold flow applications, but this does not include for the paper insulation within the makeup and they have no confidence that this would work for the multicore paper insulated cable. I am awaiting to hear back from Hawke International. Did speak with ISP who supply jointing kits and other accessories, but they don't have any experience of the glanding part of the cable.

Has anyone heard of a stuffing gland or heat shrink gland?

Graham

 

[Marmite]

Larsen, It's not clear what voltage you are talking about. Traditionally PILC cables would be plumbed onto a brass wiping gland. These days you can use a top hat gland or a heatshrink gland. See attached link for Prysmian 11kV heatshrink termination.

http://www.cablejoints.co.uk/upload/Pirelli_11kv_Heat_Shrink_Cable_Terminations.pdf

For heatshrink glands have a look at

http://www.tycoelectronics.com/catalog/bin/TE.Connect?C=1&M=BYPN&TCPN=409534-001&RQPN=CES-1

Regards
Marmite

 

[larsen7]

For anyone interested the solution is to use a 'Bolt On Gland' and 'Heat Shrinkable Termination' for glanding the multicore PILCS cable into the air termination box on the new switchboard.The Steel Tape Armour on the cable will not pass through the gland with any sort of ease, therefore the armour will need to be terminated and an earth braid clamped to them. This is then clamped to the gland body (Worm Drive Clamp) providing an adequate earth connection. The outer heat shrinkable sleeve will ensure water/moisture tightness and will provide additional strain relief. To provide additional strain relief the cable will need to be supported below the glanding arrangement to prevent too much strain being applied to the cable lug connections. This can be a bespoke arrangement made up by the implementation contractor, effectively being a steel ring clamp connected to a metallic bar and anchored onto the switchboard frame. The ring clamp will be connected below the gland arrangement.

Many thanks for all your help with this problem.

Graham

 

[raithrovers1]

A stuffing gland is a gland usually used on unarmoured flexible cable. The gland has a rubber gromet that tightens against the cable when the gland is tightened thus giving a tight seal.

UPS engineer

http://www.powerups.co.uk