66. Metal sheathed electric supply lines-Precautions against excess leakage .-(1) The following provisions shall apply to electric supply lines (other than overhead lines) [* * *] for use at high or extra-high voltage:-
(a)The conductors 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 precautions taken where necessary to avoid corrosion of the sheathing:[Provided that in the case of thermo-plastic insulated and sheathed cables with metallic armour, the metallic wire or tape armour shall be considered as metal sheathing for the purpose of this rule:Provided further that this rule shall not apply to cables with thermo-plastic insulation without any metallic screen or armour.] [ Inserted by G.S.R. 1723, dated 21.11.1977 (w.e.f. 31.12.1977).](b)[ 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.] [ Substituted by G.S.R. 1074, dated 5.11.1985 (w.e.f. 16.11.1985).](c)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 rule provided that the foregoing provisions as to conductivity are complied with.(2)Nothing in the provisions of sub-rule (1) shall preclude the employment in generating stations, sub-stations and switch-stations (including outdoor sub-stations and outdoor switch-stations) of conductors for use at high or extra-high voltages which are not enclosed in metal sheathing or preclude the use of electric supply-lines laid before the prescribed date to which the provisions of these rules apply.
