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Union of India - Section

Section 76 in The Indian Electricity Rules, 1956

76. [ Maximum stresses, factors of safety

.-(1) The load and permissible stresses on the structural members, conductors and ground wire of self-supporting steel lattice towers for overhead transmission lines shall be in accordance with the specifications laid down, from time to time, by the Bureau of Indian Standards.
(2)Overhead lines not covered in sub-rule (1) shall have the following minimum factors of safety:-
(i) for metal supports 1.5;
(ii) for mechanically processed concrete supports 2.0;
(iii) for hand-moulded concrete supports 2.5;
(iv) For wood supports 3.0.
Explanation 1.-The minimum factors of safety shall be based on such load as may cause failure of the supports to perform its function (assuming that the foundation and other components of the structure are intact).Explanation 2.-The load shall be equivalent to the yield point stress or the modulus or rupture, as the case may be, for supports subject to bending and vertical loads and the crippling load for support used as struts.Explanation 3.-The strength of the supports of the overhead lines in the direction of the line shall not be less than one-fourth of the strength required in the direction transverse to the line.Explanation 4.-The minimum factor of safety for the stay wires, guard wires or bearer wires shall be 2.5 based on the ultimate tensile strength of the wire.Explanation 5.-The minimum factor of safety for conductors shall be 2 based on their ultimate tensile strength. In addition, the conductor tension at 320C, without external load shall not exceed the following percentages of the ultimate tensile strength of the conductor:
Initial unloaded tension 35 per cent
Final unloaded tension 25 per cent
Provided that the conductors having a cross-section of a generally triangular shape, such as conductors composed of 3-wires the final unloaded tension at 320C, shall not exceed 30 per cent of the ultimate tensile strength of such conductor.
(3)For the purpose of calculating the factors of safety specified under sub-rule (2)-
(a)The maximum wind pressure shall be as specified in the relevant Indian Standards;
(b)For cylindrical bodies, the effective area shall be taken as full projected area exposed to wind pressure; and
(c)The maximum and minimum temperature shall be as specified in the relevant Indian Standard.
(4)Notwithstanding anything contained in sub-rules (2) and (3), in localities where overhead lines are liable to accumulations of ice or snow the appropriate Government may, by order in writing specify the loading conditions for the purpose of calculating the factor of safety.]