The following precautions shall be taken in case of electric supply lines other than overhead lines, for use at voltage exceeding 650 V, namely: –
(i) the conductors of the cable except the cable with thermoplastic or cross linked polyethylene, insulation without any metallic screen or armour shall be enclosed in metal sheathing which shall be electrically continuous and connected with earth, and the conductivity of the metal sheathing shall be maintained and reasonable 120 THE GAZETTE OF INDIA : EXTRAORDINARY [PART III—SEC.4] precautions shall be taken where necessary to avoid corrosion of the sheathing;
(ii) the resistance of the earth connection with metallic sheath shall be kept low enough to permit the controlling circuit breaker or cut-out to operate in the event of any failure of insulation between the metallic sheath and the conductor.
Explanation. – For the purposes of this regulation;
(a) in the case of thermoplastic insulated and sheathed cables with metallic armour, the metallic wire or tape armour shall be considered as metal sheathing; and
(b) where an electric supply line as aforesaid has concentric cables and the external conductor is insulated from an outer metal sheathing and connected with earth, the external conductor may be regarded as the metal sheathing for the purposes of this regulation provided that the foregoing provisions as to conductivity are complied with.