WHAT SHOULD BE THE VALUE OF EARTH RESISTANCE? – Part 3 – NATIONAL ELECTRICAL CODE, 2011

In continuation to the Part 1 & Part 2 of this article, published in my LinkedIn pages on 07-06-2021 & on 14-06-2021 respectively, in this part of the article, we will explore the provisions of the National Electrical Code, 2011 (Reaffirmed in 2016), published by the Bureau of Indian Standards.

3.1. Cl. 3.0.9 of Part 1/Section 14 of the National Electrical Code, 2011, mentions that “It is recommended that the value of any earth system resistance shall not be more than 5.0, unless otherwise specified”.

(Author’s Note: At last, a particular maximum value for earth system resistance has been specified in a Government Document. Yes! It is only in this National Electrical Code, 2011, that a maximum earth resistance value of 5 Ohms is specified. Please note that it is, again, the value of earth system resistance, which means that it is the combined resistance of all the earth electrodes in the installation, linked by an earth grid conductor. In this connection, please refer to Cl. 4.3.7 of Part 1/Section 14 of the National Electrical Code, 2011, which suggests all earth electrodes to be connected to the earth grid, during the measurement of the resistance of the earthing system).

3.2. Cl. 3.1.1.2 of Part 1/Section 14 of the National Electrical Code, 2011, mentions that “The earth system resistance should be such that when any fault occurs against which earthing is designed to give protection, the protective gear will operate to make the faulty portions of plant harmless”.

(Author’s Note: Even though in Cl. 3.0.9, the NEC prescribes a particular value for the earthing system resistance, here, in this Clause, it is vague again, simply mentioning that the earth resistance should be sufficiently low enough to cause the protection to operate, during earth fault conditions).

3.3. Cl. A-1 of ANNEX A of Part 1/Section 14 of the National Electrical Code, 2011, mentions that “Where a driven or buried electrode is used, the earth resistance shall be as low as possible”.

(Author’s Note: Here again, the Code is vague in simply mentioning that the earth resistance shall be as low as possible, without assigning a particular value to the earth resistance).

3.4 Cl. 9.8 of Part 1/Section 15 of the National Electrical Code, 2011, mentions that “The whole of the lightning protective system should have a combined resistance to earth not exceeding 10 ohms before any bonding has been effected to metal in or on the structure or to surface below ground”.

(Author’s Note: Again, for the Lightning Protection System Earthing of non-industrial buildings, the Code specifies a maximum earth resistance of 10 Ohms, before bonding the same to any other earth grid).

3.4. Cl. C-3.3 of ANNEX C of Part 7 of the National Electrical Code, 2011, mentions that “The resistance value of the earth termination network should be maintained permanently at 10 Ω or less”.

(Author’s Note: Again, for the Lightning Protection System Earthing of structures with explosive or highly inflammable materials, the Code specifies a maximum earth resistance of 10 Ohms).

Conclusion: Now, at least, in this National Electrical Code, 2011, some definitive values are specified for the value of earth resistance for electrical earthing system and for lightning protection earthing system.

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(Post-script : Please re-read the post-scripts 1 & 2 of Part 1 of this article).

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K. SIVAKUMAR

21-06-2021