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IS 9677 (1980): Guide for limits of temperature-rise of thewindings of electrical equipment when tested by differentmethods [ETD 1: Basic Electrotechnical Standards]

IS t 9677 - 1980

Indian Standard GUIDE FOR

LIMITS OF TEMPERATURE-RISE OF THE WINDINGS OF ELECTRICAL EQUIPMENT

WHEN TESTED BY DIFFERENT METHODS

Electrotechnical Standards Sectional Committee, ETDC 1

Chairman Representing

SHRI J. S. ZAVERI Bharat Bijlee Ltd, Bombay

Members

SHRI V. K. BATRA National Physical Laboratory ( CSIR ), New Delhi SHRI T.L. BHATIA Indian Posts and Telegraphs Department ( Ministry

SHRI J. M. NEOOY ( Altcrnnfc ) of Communications ), New Delhi

SHRI V. S. BHATIA Low Voltage Switchgear and Controlgear Sectional Committee, ETDC 57, IS1

Biro N. DAYAL COL K. V. KUDVA ( Alternate )

Ministry of Defence ( DGI )

DIREOTOR STANDARDS ( ELEOTRI- Research Designs & Standards Organization, CAL) ( Ministry of Railways ), Lucknew

DI~ECTOB ( TED ) Central Electricity Authority ( Department of Power ) ( Ministry of Energy ), New Delhi

DEPUTY DIBEOTOR ( TED ) ( Alfcrnatc ) SHRI M. L. DONQRE Power Cables Sectional Committee, ETDC 59, ISI SHRI R. D. J&IN Conductors and Accessories for Overhead Lines

Sectional Committee, ETDC 60, IS1 SHRI S. K. KA~LIWAL Bharat Heavy Electricals Ltd, Bhopal

SHRI PRAKASH SINGH ( Alternate ) SHRI S. K. MUKHERJEE National Test House, Calcutta

SHRI B. MUKHOPADHYAY ( Alternatr ) SHBI D. V. NARKE Transformers Sectional Committee, ETDC 16, ISI SHBI N. NATE Relays Sectional Committee, ETDC 35, ISI SHRI H. M. PAI High Voltage Switchgear and Controlgear Sectional

Committee, ETDC 58, IS1 SHRI D. N. PURANDARE Electrical Wiring Accessories Sectional Committee,

ETDC 44, IS1 SHRI S. G. RAMAOHANDRA Rotating Machinery Sectional Committee, ETDC

15, ISI

( Confinucd on page 2 )

@ CopVrighl 1981 INDIAN STANDARDS INSTITUTION

This publication is protected under the Indian Copyright Act ( XIV of 1957) and reproduction in whole or in part by any means except with written permission of the publisher shall be deemed to be an infringement of copyright under the said Act.

IS : 9677 - 1980

( Cenfinued from page 1 )

Members Rejwcsanting

SHRI V. L. SASTRY Electronics and Radar Development Establishment ( Ministry of Defencc ), Bangalore

SHRI K.V. BEAT ( Ahrnotr ) DR S. M. SEN Institution of Engineers ( India ), Calcutta DB H. V. K. ~JDUPA Secondary Cells and Batteries Sectional Committee,

ETDC 11, IS1 SHBI H. N. VENKOBHA RAO Central Electrochemical Research Institute ( CSIR ),

Karaikudi SERI Y. MAHADEVA IYER ( Alternate )

SHBI M. P. WAQH Indian Electrical Manufacturers’ Association, Bombay

SHRI S. D. M~HEKA~ ( Aftsrnafe ) SHRI S. P. SACHDRV, Director General, IS1 ( Ex-o&o Mcmbcr )

Director ( Elec tech ) ( Secretary )

IS : 9677 - 1980

Indian Standard GUIDE FOR

\ LIMITS OF TEMPERATURE-RISE OF THE WINDINGS OF ELECTRICAL EQUIPMENT

WHEN TESTED BY DIFFERENT METHODS

0. FOREWORD

0.1 This Indian Standard was adopted by the Indian Standards Institution on 18 December 1980, after the draft finalized by the Electrotechnical Standards Sectional Committee had been approved by the Electro- technical Division Council.

0.2 The safe and reliable operation of an electrical equipment is achiev- ed by ensuring amongst other things that under normal conditions of operation, the steady state temperature of the equipment is maintained within the limits for which the electrical insulation is designed. This factor ultimately decides the safe current ( rated current ) which can pass through the equipment without the temperature of the insulation exceeding the safe limits.

0.3 The temperature-rise of the windings in an electrical equipment depends on several factors such as the categories of duty of the equip- ment; intermittent or continuous, the degree of exposure of the winding to the atmosphere, the enclosures and their proximity, conditions of ventilation, cooling, etc. These factors make it necessary to specify the permissible limits of temperature-rise of the windings and a precise method to ensure that temperature-rise of the winding remains within the safe limits.

0.4 This standard has been brought out with a view to provide guidance on the limits of temperature-rise of the windings when tested by different methods such as thermometer, resistance and embedded temperature detectors.

0.5 The measurement of temperature of the winding by using the mercury-in-glass thermometer is not always reliable owing to features such as contact surface available and the accessibility. The choice of the method also depends on the accuracy desired. Also, while specifying these values by thermometer method, note is to be taken of the fact that

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IS : 9677 - 1980

the method would not be able to take into account the ’ hot-spot ’ tem- perature of winding. In this standard, guidance has been provided on the limits of temperature-rise measured by the following three methods:

a) Thermometer method,

b) Resistance method, and

c) Embedded temperature detector method.

NOTE - Embedded temperature detectors (E. T. D. ) arc resistance thermometers or thermocouples built into the equipment appropriately during construction at points which are inaccessible after the equipment is completed.

0.6 Even though it is observed that in most of the cases, while specifying the requirements relating to temperature-rise of the electrical equipment, the factors mentioned in this guide are taken care of, there are instances where adequate differentiation of the limits by the three methods have not been made. Moreover, the presumptions that are to be valid regarding hot spot temperature, etc, are not clearly brought out in certain cases. It is felt that a common standard would provide adequate guidance while specifving the limits in the individual specifications and, would serve as a useful guide in case of any divergent views. It isr however, not intended to enforce literally the provisions of this guide fos the individual specifications and the relevant Sectional Committee would be free to use the guide to the extent it is relevant.

0.7 While formulating this standard, information contained in IS : 1271- 1958* has been used.

0.8 While it is generally desirable not to exceed the limits specified in IS : 1271-1958*, due to special consideration, not provided in this guide, and dictated by the needs of specific equipment or specific design/ application of same equipment, more stringent or liberal limits may be imposed in the individual specifications along with a clarification for the same.

0.9 It is also recognized that this phenomenon of temperature-rise is an important criteria for parts other than windings of electrical equipment, such as contacts, terminals metallic parts acting as springs, etc. How- ever, the recognized method for temperature rise in all such cases is the contact thermometers.

0.10 This standard shall be read in conjunction with IS : 9678-1980t.

*Classification of insulating materials for ekctrical machinery and apparatus in relation to their thermal stability in service.

tMcthods of measuring temperature-rise of electrical equipment.

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IS : 9677 - 1980

0.11 For the purpose of deciding whether a particular requirement of this standard is complied with, the final value, observed or calculated, expressing the result of a test, shall be rounded off in accordance with IS : 2-1960*. The number of significant places retained in the rounded off value should be the same as that of the specified vaiue in this standard.

1. SCOPE

1.1 This standard specifies guidelines for the limits of temperature-rise for windings in electrical equipment measured by the following methods:

a) Thermometer method,

b) Resistance method, and

c) Embedded temperature detector method.

2. GENERAL CONSIDERATIONS

2.0 As indicated in 0.3, the temperature-rise in a winding depends on several factors. While all these factors are kept in view while laying down the limits ‘for individual equipment, guidance on general consi- derations applicable for all cases is given below.

2.1 Reference Ambient Temperature

2.1.1 The limits of temperature-rise are generally applicable, over a reference ambient temperature and for the limits of altitude specified in the individual standards.

2.1.2 Unless otherwise specified in the individual standards, the standard reference ambient temperature for electrical equipment is 4O”C, at an altitude not exceeding 1 000 metres.

NOTE -The basis for arriving at the standard reference ambient temperature of 40% and the assumptions made are covered in IS : 9676-1980t.

2.2 Methods of Measurement of Temperature-Rise

2.2.1 The methods of measurement of temperature-rise, and the special precautions to be observed are covered in IS : 9678-1980$

*Rules for rounding off numerical values ( revised 1. $Rcference ambient temperature for electrical equipment. fMethods of measuring temperature-rise of electrical equipment.

5

IS : 9677 - 1980

2.2.2 The limits of temperature-rise are specified in the individual specifications in relation to the method most appropriate and suitable for the winding under test. While the rise in temperature is the difference between the steady state temperature of the winding and the cooling medium temperature, the limits shall have to be based on the particular method which has been indicated.

2.2.3 In the thermometer method of measurement, the temperature is determined by thermometers applied to the accessible surfaces of the winding. The term thermometer also includes non-embedded thermo- couples and resistance-thermometers, provided they are applied to the points accessible to the usual bulb thermometers. As, by this method, only the temperature at the accessible surface is measured, the limits of temperature-rise specified for the hot-spot shall be lowered unless other- wise specified, by 15”C, to account for the gradient allowance.

2.2.4 In the resistance method of measurement, the temperature-rise of the windings is determined by the increase in the resistance of the windings. The change in the resistance of the winding corresponds to average temperature-rise of the winding and therefore limits of temperature-rise specified are 5°C lower than the class of insulation to compensate for hot-spots.

2.2.5 Embedded temperature detector employs requisite number of detectors built into the windings, suitably distributed around the winding. The location of detectors are so chosen that the temperatures someasured are indicative of the hot-spots in the winding. Embedded temperature detectors shall therefore be placed as far as possible at the hot spot. Due to constructional constraints, if it is not possible to place them at the hot spot, suitable allowance shall have to be made on the limits of temperature measured.

NOTE - Unless otherwise specified, embedded temperature detectors, shall be of the resistance temperature detector type.

3. SPECIAL CONSIDERATIONS

3.0 It is generally desired that the limits of temperature-rise specified be adhered to, to the extent possible. Actual application of the electrical equipment involves many varieties of cases, and more stringent or liberal limits may be applicable departing from the values specified in the individual specifications. Suitable guidelines are hence provided in the individual specifications of electrical equipment in order to cater to such specific cases.

3.1 Operating Altitude - No adjustment in the limits of temperature- rise are required if the altitude does not exceed 1 000 metres. However, for altitudes different from normal service altitude, allowances on the limits shall be, as specified in the relevant specifications.

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