67. Connection with earth .- [(1) All non-current carrying metal parts associated with HV/EHV installation shall be effectively earthed to a grounding system or mat which will:-
(a)limit the touch and step potential to tolerable values;(b)limit the ground potential rise to tolerable values so as to prevent danger due to transfer of potential through ground, earth wires, cable sheath, fences, pipe lines, etc.;(c)maintain the resistance of the earth connection to such a value as to make operation of the protective device effective.(1-A) In the case of star-connected system with earthed neutrals or delta-connected system with earthed artificial neutral point:-(a)the neutral point of every generator and transformer shall be earthed by connecting it to the earthing system as defined in rule 61(4) and hereinabove by not less than two separate and distinct connections:Provided that the neutral point of a generator may be connected to the earthing system through an impedance to limit the fault current to the earth:Provided further that in the case of multi-machine system neutral switching may be resorted to, for limiting the injurious effect of harmonic current circulation in the system;(b)in the event of an appreciable harmonic current flowing in the neutral connection so as to cause interference, with communication circuits, the generator or transformer neutral, shall be earthed through a suitable impedance;(c)in case of the delta-connected system the neutral point shall be obtained by the insertion of a grounding transformer and current limiting resistance or impedance; wherever considered necessary at the commencement of such a system.][(1-B) In case of generating stations, extra high voltage sub-stations and extra high voltage industrial installations, the system neutral earthing and protective frame earthing may be, if system design so warrants, integrated into common earthing grid provided the resistance to earth of combined mat does not cause to exceed the step and touch potential beyond its permissible values.] [ Inserted by G.S.R. 793(E), dated 3.12.2002 (w.e.f. 4.12.2002).](2)Single-phase high or extra-high voltage systems shall be earthed in a manner approved by the Inspector.(3)In the case of a system comprising electric supply lines having concentric cables, the external conductor shall be the one to be connected with earth.(4)Where a supplier proposes to connect with earth an existing system for use at high or extra-high voltage which has not hitherto been so connected with earth, he shall give not less than fourteen days' notice in writing together with particulars to the telegraph authority of the proposed connection with earth.(5)[(a)] [ Sub-R. (5) renumbered as Clause (a) thereof and Clause (b) inserted by G.S.R. 1723, dated 21.11.1977 (w.e.f. 31.12.1977).] Where the earthing lead and earth connection are used only in connection with earthing guards erected under high or extra-high voltage overhead lines where they cross a telecommunication line or a railway line, and where such lines are equipped with earth leakage relays of a type and setting approved by the Inspector, the resistance shall not exceed 25 Ohms.(b)[ Every earthing system belonging to either the supplier or the consumer shall be tested for its resistance to earth on a dry day during dry season not less than once a year. Records of such tests shall be maintained and shall be produced, if required, before the Inspector or any officer appointed to assist him and authorised under sub-rule (2) of rule 4-A.] [Sub-R. (5) renumbered as Clause (a) thereof and Clause (b) inserted by G.S.R. 1723, dated 21.11.1977 (w.e.f. 31.12.1977). ](6)In so far as the provisions of rule 61 are consistent with the provisions of this rule, all connections with earth shall also comply with the provisions of that rule.
