Eddy current query

[jasonf987]

Morning Gents,

i have a 3 phase, 400V AC system. i want to use two 185mm single core cables per phase in the terminal box, i am however concerned about eddy current. should the gland plate be made from a non metallic substance e.g bakalite and what is the minimum distance between the cables to prevent eddy currents? I was thinking about 6x the diameter of the cable.

 

[Skogsgurra]

It all depends. But one thing is for sure, you do not decrease eddy currents by increasing distance between cables. Keep them tight instead, so that fluxes cancel.

I take it that the cables are 185 mm2. That usually doesn't mean a lot of eddy currents because of the relatively low current allowed in the cables.

There might be harmonics (if you are feeding a VFD, for instance) and that increases eddy currents. But for normal loads, there shouldn't be much to woory about. Bakelite (textile armoured) is a good choice if you want to avoid risks.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[RRaghunath]

Normally, I come across thick Aluminium Gland plates. Aluminium is surely better than Bakelite from the point of supporting cable load and also from the point of longevity.

 

[ScottyUK]

Brass or the glass-fibre laminated sheet are mechanically sound options. I think Skogs is thinking of Tufnol which is a phenolic resin reinforced by fabric rather than Bakelite which is a similar resin filled with wood flour.


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

 

[waross]

There are two causes of gland plate heating.
The first is magnetically caused and is eliminated by eliminating any magnetic encirclement of less than a balanced set of conductors. Thus aluminum or brass rather than steel plates. Another option is to cut slots in a steel gland pate between the cable entry holes. This may be done in groups of A-B-C-N. When cables are shielded or armored with a conducting material there is a voltage induced in the shield or armour. If the shields are shorted together at both ends a circulating current(not an eddy current)will flow in the shield or armour and the gland plates.
The solution is two fold.
1> Avoid magnetic encirclement by using a non magnetic gland plate, or slotting a steel plate.
2> Avoid connecting the cables shields or armours together at both ends by using an insulating gland plate at one end or insulating the shield or armour inside the connector or gland.
It is normal to use a conducting plate at the supply end to ground the shields or armours and use an insulating plate at the load end.

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

 

[dpc]

Glastic may be a good option for an insulating material:

http://www.glastic.com/fsl.htm

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[Skogsgurra]

Bill,

I learned someting new - again! Never thought about the naked armour being a source of possible extra heating.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

It's in our code here Gunnar.
A couple of excerpts to illustrate the magnitde.
The magnitude of the induced sheath voltage is relative to the current in the conductor.
The magnitude of the sheath current is a function of the induced voltage and the sheath impedance and increases in magnitude with increased conductor spacing.
If sheath currents are allowed to flow we must limit the cable ampacity to 70% of rated.
Increased cable spacing will increase the magnitude of the sheath currents.
Underground installations with each conductor in a separate duct have greater sheath currents than typical surface installations with closer cable spacing.
Below 200 amps per conductor, in air, at normal spacings, we are allowed to ignore the effect of sheath currents.
Thank you for the mention Gunnar.
Regards

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

 

[Skogsgurra]

I guess that it is in the code here, too. Physics is physics regardless of country.

So, I think that I should have known about it. Perhaps I am an outlaw?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

I wouldn't have thought so, ... until you mentioned it! You may know best.
LOL,


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

 

[CharlieGill]

We all live and learn...hopefully.
Charlie