A single core armoured cable will always have a layer of aluminium wire armour (AWA) instead of steel wire armour (SWA). This is because the steel in Armoured cable wire has a much lower conductivity and therefore higher resistance than aluminium. If it were used in a single core cable the magnetic field generated would induce an electric current in the armour (eddy current) and combined with the increased resistance.
AWA is non-magnetic and has a much better conductivity and lower resistance, so can conduct these induced currents to earth more efficiently than steel. SWA armoured cable wire is used in multicore armoured cables because the electromagnetic fields from the neighbouring cores effectively cancel each other out, meaning less current is induced into the armour.
TAG: Armoured cable wire