Are you referring to the practice of installing an aluminum plate in the side of an enclosure to enter single conductor cables? If so, entering the cables through a ferrous plate has a couple of adverse effects. One is the magnetic hysteresis of typical enclosure steel. The steel typically has much greater hysteresis and heating due to hysteresis becomes troublesome. Add to that, enclosure steel is often much thicker than transformer laminations and eddy currents are much greater and add to the heating. I have seen paint burned off enclosures when single conductor cables were improperly entered.
Aluminum is non-magnetic so hysteresis is not an issue.
The magnetic permeability of steel is thousands of times higher than aluminum. There is so little magnetic fux influencing the aluminum that eddy currents are almost non-existent when compared to the eddy currents in a steel plate.
An exception may be Iso-Phase bus where the currents are so high that eddy current heating even in aluminum may become noticeable.
Bill
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"Why not the best?"
Jimmy Carter
