Eddy Currents in Gland Plates 7

[pilts]

I have an application where I will be taking a 100A 400V 3phase supply to a new distribution board. The cabling will be single core cables in non-trefoil formation. The gland plate is ferrous (steel). I do not believe that eddy current heating will be a prblem in this application, but how do I prove this, or is there a rule of thumb to apply before I need to change the gland plate to a non-ferrous material?
Any help or ideas would be appreciated.

 

[don01]

Pilts,
This is an issue which came up on our plant installing some dbs at around the 400 amps each. The contractor we were using, who has really been around the ridges, pointed this same issue out. His view was simple. All the petro chem plants insist on non ferrous, our main panel gland plates are non ferrous so why try to save 10 bucks of aluminum. (Also its easier to drill out than steel)

I don't know of any rules but I will ask and post them if I get a sensible response from my contacts.

Regards
Don

 

[wowski]

This is a problem which we have found in the past, especially with large services. There have been a couple of ways around it. (i) as stated before use non-ferrous gland plates , i.e. aluminium or brass, but you will also have to be careful with your choice of gland materials. Aluminium is known to react corrosively with other dis-similar metals.
(ii)the other solution which which was suggested to us was to cut a slot in the gland plate to prevent circulating eddy currents.

 

[sparks22222]

Pilts,

Have the same problem at the moment with eddy currents in a heavy installation. Contractors used light steel plate for glands instead of brass or aluminium. Always make sure your plates are of this material.
Gland for incoming cable heated to a non- touchable temperature. You have been warned!!!

Sparks

 

[resqcapt19]

The use of a ferrous gland plate appears to be a violation of section 300.20(B) of the National Electrcical Code.
"(B) Individual Conductors. Where a single conductor carrying alternating current passes through metal with magnetic properties, the inductive effect shall be minimized by (1) cutting slots in the metal between the individual holes through which the individual conductors pass or (2) passing all the conductors in the circuit through an insulating wall sufficiently large for all of the conductors of the circuit. "
Don

 

[RajT]

As a rule of thumb 200amps is the upper limit so 100amps as in your case should be fine.
You may also wish to adopt the idea of cutting slots between each of the three single core cables.

 

[MDom]

see also thread 238-12547
B.r.

 

[sparks22222]

WE HAVE ELIMININATED THE HIGH EDDY CURRENTS CAUSING THE MAIN INCOMING GLAND TO HEAT UP BY CHECKING ALL EARTH CABLES. WE FOUND SOME EARTHS WERE NOT CONNECTED PROPERLY. THIS MAY CAUSE HIGH CURRENTS IN EARTH

 

[1x1x1x]

Your eddy current losses are depending on the used material and the harmonic content of the current. If the current has harmonic components, the losses will be much higher. The best solution could be the use of fiberglass material, which will eliminate the eddy current. However if this is not an option make sure, that no "closed loop" metal is around the conductors and use a stainless steel material which will minimize the eddy current losses.

 

[purplepete]

Gents,
this is offered for interest only.
Some years ago we had a 400 amp 3 phase motor fed by three number 240 sq mm single core cables. They were run in magnificent trefoil with the earth conductor nested in the valley of two of the conductors...the SAME conductors over a route of approximately 100 metres. The motor DS bearing failed through electrolysis within twelve months. The earth conductor was found to be carrying 65 amps (induced). Only when we separated the earth by about 18 inches from the trefoil array did we reduce it to about 4 or 5 amps.
Cheers

 

[jbartos]

The current NEC also includes the following:
"FPN: Because aluminum is not a magnetic metal, there
will be no heating due to hysteresis; however, induced currents
will be present. They will not be of sufficient magnitude
to require grouping of conductors or special treatment
in passing conductors through aluminum wall sections."
Although, a higher harmonic content may increase the induced currents.