Electromagnetic Effect on Armoured Cable

The same as if you had a single core 900mm2 (steel wire?) armoured cable (all three conductors are at the same potential and in close proximity to each other within the screen so act as one from an electromagnetic point of view).

Can you provide more details on your application and elaborate on your question (unless I have already answered your question above).

Dear Mr Jack of all trades, Thank you for the answer, this is existing setup, 3*300 mm squire Cables are connected to the 16.5 MVA furnace transformer primary side for each line. and operational current is around the 850Amps, the cable temperature rising up 75 to 80 centigrade[Ambient temp is 45 centigrade] is it normal. My question is temp rising due to load or electromagnetic effect.

A temperature rise of 75-80 deg C is much too high for three core 300mm2 conductors running in parallel, which should handle 850A fine (so its likely not load current). The high temperature is likely due to bad (high resistance) joints or connections, possibly also due to nearby or conducted heat from the furnace. Check all your cable connections and joints.

Also check your cable screen for circulating currents (just in case).

Dear Jack of all trades, Cable are having Only end terminations, in between there is no joints, cable length around 150 meters, and cables are away from the furnace area, all three cables are getting equal temperature, Pl explain me how I can check the circulating current. Thank you.

The easiest way is to use a current clamp meter to measure the current running thru the cable screen.

http://en.wikipedia.org/wiki/Current_clamp

Attach the current clamp meter over the screen only (not the screen and the cable) to measure the current. If you are earthing both ends of the cable screen this is probably what is causing your cable to overheat.

http://en.wikipedia.org/wiki/Ground_loop_(electricity)

http://www.eng-tips.com/viewthread.cfm?qid=186838&page=10

etc.

If this is the case you should only earth the cable at the transformer side and insulate the cable screen at the furnace side as induced voltage (which could reach a lethal potential) on the unearthed furnace side of the cable screen could be present. It is swapping one problem for another, but it is necessary if you want to use single core armoured cable in this way.

For more information also consult your local electrical wiring standards and regulations as the requirements are also listed in there.

Dear jack of all trades, thank you very much for your guide lines.

Are all connections (joins) tight and what are the currents in each of the three leads ?

Yes! The armour is bound to heat up due to what is called "eddy-current heating". When the three cores of the cable are connected to three different phases and assuming balanced loading of the phases, the fluxes would cancel out and there would not be any residual flux. But, when you connect all the three cores to the same phase, only one phase flux would be there and there would not be any neutralising flux from the other two phases. This entire phase flux would magnetically link with the armour and induce a voltage in the armour. And, when this armour is having a closed path the induced voltage would circulate currents in the armour which causes the heat and the temperature rise. Solution would be to break the closed path of the armour (i.e.) do not earth the armour at both ends. Earth at only one end. Let the armour not even touch any metallic object at the other (non-earthed) end.

But, in this arrangement ensure that the induced voltage is less than the tolerable touch voltage else it can cause shock hazard.

Dear electrical expert65, Thank you very much.