Single Conductor Penetration Question

[synchro]

Wouldn't anything metallic, such as lock-nuts or flanges, bridge the gaps?

Non-ferrous locknuts as in Mike's graphic.

The slots are used to prevent a magnetic field induced by the current through an isolated phase conductor from circulating around it inside the ferrous metal sheet which has a high magnetic permeability (a measure of conductivity for magnetic fields). The slots prevent this circulating magnetic field by effectively creating a larger hole that surrounds all of the phase conductors, and so the conductor's magnetic fields within this larger hole will cancel out and therefore not have an effect.

Circulating magnetic fields induced in a ferrous metal will create hysteresis losses within that metal, as well as resistive losses from surrounding eddy currents, resulting in heating.

Metallic materials such as aluminum and zinc are good conductors of electrical currents, but not of magnetic fields like ferrous materials are. In fact, the magnetic permeability of aluminum and zinc is essentially the same as air, but that of steel is on the order of a thousand times higher. And so non-ferrous locknuts would not "short out" the magnetic fields between the sides of the slots, and so what infinity mentions about using them is correct.

 

[winnie]

I agree that non ferrous locknuts would not short magnetic flux.

They would provide a path for induced electrical currents.

The slots in the ferrous materials act like airgaps in an inductor core. They reduce but do not eliminate the magnetic circuit.

 

[Jpflex]

The code requires the phase conductors to be bundled together along with neutral and EGC where applicable for magnetic field cancelation (except very specific applications like underground in non-metallic conduit. If they are not bundled and there is ferrous materials adjacent, they will inductively heat the metal.

This if I remember correctly is called isolated phase runs, but where the conduits terminate with only 1 phase there must be a slit made between phase conduit knockouts or a non ferrous plate

 

[infinity]

Non-ferrous plates are used all of the time with MI cable and individual strand MC cables for things like fire pumps.