Voltage Level

Classification of Voltage Levels - Indian & International Practices

by: K.Sivakumar, Manager - Training, Switchgear Training Centre, Larsen & Toubro Limited, Coonoor, T.N.

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Synopsis: As electrical professionals, many of us have been coming across the terms LV or LT, MV, HV or HT & EHV or EHT in our day-to-day professional lives. But, are we aware of the limits of these voltages as per Indian Standards and/or International Standards? This article aims to anlayse the limits set for these voltages in Indian as well as International Standards.

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Foreword: Voltage levels have been defined and classified by various National and International Standards as also by certain electric power utility companies. The following are the definitions given in various Indian & International Standards for AC Voltages.

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India: As per the Indian Electricity Rules, 1956, vide Rule 2(av), the following are the limits:

Low Voltage: Not exceeding 250V

Medium Voltage: Not exceeding 650V

High Voltage: Not exceeding 33000V

Extra High Voltage: Exceeding 33000V

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But, in IS 13234 - 1992, which is the "Guide for short-circuit current calculation in Three-phase A.C. Systems" - it is mentioned vide Table I that:

Low Voltage: 100V to 1000V

Medium Voltage: > 1kV to 35kV

High Voltage: > 35kV to 230kV

As can be seen from the above, there is no harmony between the IS and the IE Rules in this issue.

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France: Vide a publication dated 14^th November 1998, the French Regulations define three voltage levels as below:

Low Voltage: < 1kV

High Voltage A (HTA): > 1kV but </= 50kV

High Voltage B (HTB): > 50kV

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CENELEC (The European Electrotechnical Standardisation Committee): Vide a publication dated 27^th July 1992, the CENELEC recognises the following voltage levels:

Low Voltage: < 1kV

Medium Voltage: > 1kV but </= 35kV

High Voltage: > 35kV

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IEC (International Electrotechnical Commission): In IEC 60071, the following ranges are defined:

Low Voltage: < 1kV

High Voltage Range I: > 1kV but </= 245kV

High Voltage Range II: > 245kV

The acronyms EHV (Extra High Voltage) and UHV (Ultra High Voltage), though are being used elaborately in India, have never been officially defined in any of the above three standards.

USA:

i) Vide IEEE Std. 241-1990 (IEEE Recommended Practice for Electric Power Systems in Commercial Buildings), Clause 3.1.1.2,

Low Voltage: A class of nominal system voltages 1000V or less

Medium Voltage: A class of nominal system voltages greater than 1000V and less than 100000V

High Voltage: A class of nominal system voltages equal to or greater than 100000V or less than 230000V

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ii) Vide ANSI C84.1-1989 (Voltage ratings for electric power systems and equipment),

Low Voltage: 120V to 600V

Medium Voltage: 2400V to 69000V

High Voltage: 115000V to 230000V

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iii) Vide ANSI C92.2-1987 (Power Systems - Alternating Current Electrical Systems and Equipment Operating at Voltages above 230kV Nominal - Preferred Voltage Ratings)

Extra High Voltage: 345000V to 785000V

Ultra High Voltage: 1100000V

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Note: IEEE - Institution of Electrical and Electronic Engineers

ANSI - American National Standards Institute

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Conclusion: As can be seen from the above, there is no clear cut demarcation for classification of various voltage levels in AC Electric Power Transmission and Distribution. Also, there is no single standard across the globe, with various countries defining various limits for these voltages. And, as in the case of India and USA, even within one country, two standards do not match, when it comes to these limitations. It is wished that the electrical community across the globe meets and takes a decision soon to bring harmony across the countries of the world in standardizing the limits for various voltage classifications.