117. Earthing metal, etc .-(1) All metallic sheaths, coverings, handles, joint boxes, switchgear frames, instrument covers, switch and fuse covers or boxes, all lampholders (unless efficiently protected by an insulated covering made of fire resisting material), and the frames and bedplates of generators, transformers and motors (including portable motors), shall be earthed by connection to an earthing system in the manner prescribed in rule 115.
(2)Where cables are provided with a metallic covering constructed and installed in accordance with clause (d) of rule 122, such metallic covering may be used as a means of connection to the earthing system.(3)All conductors of an earthing system shall have conductivity, at all parts and all joints, at least equal to 50 per cent of that of the largest conductor used solely to supply the apparatus, a part of which it is desired to earth:Provided that no conductor of an earthing system shall have a cross-sectional area less than 0.15 sq. cm. except in the case of the earth conductor of a flexible cable used with portable apparatus where the voltage does not exceed 125 volts, and the cross-sectional area and conductance of the earthcore is not less than that of the largest of the live conductors in the cable.(4)All joints in earth conductors and all joints in the metallic covering of cables shall be properly soldered or otherwise efficiently made.(5)No switch, fuse, or circuit-breaker shall be inserted in any earth conductor.(6)This rule shall not apply (except in the case of portable apparatus) to any system in a mine in which the voltage does not exceed 30 volts.
