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Topic Title: Eddy Currents
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Created On: 23 January 2013 08:20 PM
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 23 January 2013 08:20 PM
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GB

Posts: 359
Joined: 04 September 2002

Gents
I need to take three single core cables (400mm) into a electrical cabinet (2mm thick steel) but the cores will need to enter the cabinet 300mm apart to hit the existing copper bus bars, I would generally look to cut a large section of the steel plate away and then add an aluminium gland plate c/w brass glands, unfortunately in this case two strengthening bars internal to the cabinet stop one large slot being cut (or three smaller slot with cut between)
My question is what distance is required arround the cables to prevent any possibility of eddy currents forming.
ie I can cut three holes say 150mm square then fit one ali plate over all three holes (plate say 800 x 300) with the brass gland for each phase going central to the 150mm hole, but is this acceptible? I cant find any literature with regards to this scenario.


Regards
 23 January 2013 09:06 PM
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Avatar for Martynduerden.
Martynduerden

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I would suggest you modify the top or the panel, three 400mm Singles tell me its not a cheap job to start with, is modifying the panel going to add much in the way of cost?

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Martyn.

Only a mediocre person is always at their best



www.electrical contractors uk.com
 23 January 2013 09:21 PM
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GB

Posts: 359
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There are space and as normal time considerations which make major modification impossible.
I was hoping that the steel being spaced from the cable by say 50mm all round and the cables going through a brass gland and the ali plate would negate the eddies but cant prove it!!!
 23 January 2013 09:28 PM
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perspicacious

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Try seeking confirmation from the manufacturer. If they email back with a "no" you can at least show the client and seek further instruction

The magnitude of any Eddy currents is likely to be significant (unless the cable is sized for VD rather than current)

Regards

BOD
 23 January 2013 09:32 PM
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perspicacious

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Anyone remember having to cut a short length of conduit, thread it to accept female bushes, de-burr and then see if the then prescribed capacity of seven 2.5 singles was possible by looping a single piece through and through, finally passing 15 A through it to demonstrate Eddy currents?

Regards

BOD
 24 January 2013 08:42 AM
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eclipse

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Is this an AWA cable? if so would an ally gland not be more suitable, also if not reliant on the armouring for earthing why not enter the cabinet through a stuffing gland and just single point bond the cable at the other end

-------------------------
Thanks

Alan.

Now what was that reg no?
 24 January 2013 01:05 PM
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GB

Posts: 359
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Alan
These will be 6381TQ (Rubber cables) so no armouring.

Regards
 24 January 2013 02:57 PM
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eclipse

Posts: 151
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Yes ok see where you're coming from, would it not be worthwhile speaking to a panel manufacturer regarding the strengthning bars and some reworking, maybe they could offer a solution.

-------------------------
Thanks

Alan.

Now what was that reg no?
 24 January 2013 03:13 PM
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BigRed

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Are the strengthening bars part of the cab, ie welded and supporting said bus bars or just to stop warp and twist? might be worth considering a thick piece of Tufnol, 5-6mm thick over a slot, and therefore re enforcing the case again? If it's rubber cable, big plastic stuffing glands will do, has been done on the bottom of a busbar chamber before for temp supplies.
 24 January 2013 06:31 PM
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GB

Posts: 359
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The cabinet supports are a folded section of the top plate spot welded.
I dont want to cut this section if at all possible and had hoped that there would be some information on distances between the cable and the ferrous metal ie if this distance is greater than say 50mm then all ok.
I do realise the effect is dependent on cable, load type, and material of the enclosure I guess I was hoping for a rule of thumb that said in all circumstances 50mm (or so) ok.
 25 January 2013 10:03 AM
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OMS

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If you want a very approximate RoT, then start with a spacing to the ferrous surround of 4 x the cable OD. This tends then to provide bigger non ferrous plates as a function of current flow (you have bigger cables for more current).

For 400mm2 single core rubber you probably want about 200mm radius from the cable centre point before you hit ferrous material.

If those struts impinge on this then can you slot them (ie cut right through them) and then braze them back to re provide the structural integrity (with a non ferrous filler). Failing that, can you cut through them and use the aluminium gland plate to connect each "half" so again you retain the strut integrity.

The above is a basic RoT - it's a bit overcautious, but will tend to align with what switchgear manufacturers do for initial setting out.

Obviously, the closer you keep the group of glands the better in terms of the overall gland plate size to still maintain the 4 x OD seperation from any ferrous surround.

Note you are also likely to have less problems with non armoured EPR as you have no sheath currents to contend with.

Obviously, the whole issue of protection to those unloading tails and means of earthing will need looking at as well.

Regards

OMS

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 25 January 2013 07:42 PM
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GB

Posts: 359
Joined: 04 September 2002

Many thanks.
Input always appreciated.
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