The reason for using aluminum armor in single corearmoured cable is to avoid the formation of ferromagnetic closed circuits in the cable sheath, thereby reducing induced currents and related heating issues.
Why is steel armor not used for single core armoured cable?
When anarmoured cable conductor passes current, magnetic field lines are generated around it, and the number of magnetic field lines is proportional to the magnitude of the current passing through the conductor. In a three core bundled cable, when the three-phase currents are balanced, there are no magnetic lines passing through the cable sheath armor. In a single core armoured cable, even if the three-phase currents are balanced in the system, due to the protective layers of the three cores, magnetic field lines pass through each protective layer. The number of magnetic field lines passing through the protective layer is proportional to its magnetic permeability. Steel strip is a ferromagnetic material with high magnetic permeability, so more magnetic field lines pass through it..
for A single core cable with steel tape armor , When a current passes through his conductor, the high permeability steel tape passes through more magnetic field lines. As the current is an alternating current of 50Hz, a voltage is induced in it, which is proportional to the speed of the magnetic field line change and proportional to the magnetic conductivity. When the steel tape forms a closed loop at both ends of the cable line, an induced current is generated in it, which is actually proportional to the load current and has a considerable value. Due to the induced current generating heat, it is necessary to reduce the current carrying capacity of the cable in order to ensure its safe operation.
To avoid this situation, the protective layer of a single core armoured cable does not use steel tape armor, but uses non-magnetic material aluminum armor, and does not form a ferromagnetic closed circuit in the protective layer
