Single core cables in conduction alternating current, can form annular magnetic field around the cable, steel belt or bare wire will form a closed loop path, forming eddy current, and eddy current will cause the cable heating, increase the loss of the cable.
Three - core cables pass through the three-phase alternating current, the sum of the three-phase current is zero, so no eddy is produced in the guide ring of the guide steel belt and naked steel wire, so the steel armor can be steel.
Need of special note is, single core cable is not can't armor, but is a magnetic strip or steel wire, often using the magnetic stainless steel belt and the permeability of stainless steel wire armoured can also be (single core cable used by stainless steel sheathed).
In addition, magnetic steel wire is not cannot be used for single core cable armoring, but will have to cover (extruded) insulation wire, then squeeze insulated wire for cable armoring, insulation will ring magnetic circuit for segmentation, can't form a circular vortex, is also no problem.