a. The magnetic fluxes produced by the three phases in a multi core power cable almost cancel put each other, since the vector sum of these currents at any instant is zero and practically there is no residual magnetic flux around the cable. In case of single core cable, the magnetic flux induces the voltage in the metallic sheath.
b. When the cable conductor is carrying alternating current, for safe and reliable operation, the metallic sheath must be grounded. If the metallic sheath is at one end the potential of the unearthed end could be much above the earth potential. If both ends are grounded, a circulating current is induced in the metallic sheath.
c. The maximum acceptable induced voltage under normal load current operation is limited by corrosion and safety considerations.
d. Code of practice of earthing (IS 3043) as well as electricity council London recommended permissible induced voltage level of 65 Volts.
Hence keeping above all points in mind metallic sheath and armour of all multi core power cables shall be earthed at both end equipment and switchgear end. Sheath and armour of single core power cable shall be earthed ate switchgear end only. The sheaths of shielded control cables should be grounded at both ends to eliminate induced potentials.