1、BRITISH STANDARD BS 7831:1995 IEC 370:1971 Test procedure for thermal endurance of insulating varnishes Electric strength method (Implementation of CENELEC HD 570 S1 1990) UDC 621.315.617.1:621.317.333BS7831:1995 This British Standard, having been prepared under the directionof the Electrotechnical
2、Sector Board, was published underthe authority of the Standards Board and comes intoeffect on 15November1995 BSI 07-1999 The following BSI references relate to the work on this standard: Committee reference GEL/15/3/7 Draft announced in BSI News April1995 ISBN 0 580 24485 7 Committees responsible fo
3、r this British Standard The preparation of this British Standard was entrusted to Technical Committee GEL/15, Insulating material, to Subcommittee GEL/15/3/7, Varnishes and resins, upon which the following bodies were represented: British Ceramic Research Ltd. British Industrial Ceramic Manufacturer
4、s Association Electrical and Electronic Insulation Association (BEAMA Ltd.) Electricity Association Federation of the Electronics Industry Rotating Electrical Machines Association (BEAMA Ltd.) Transmission and Distribution Association (BEAMA Ltd.) Amendments issued since publication Amd. No. Date Co
5、mmentsBS7831:1995 BSI 07-1999 i Contents Page Committees responsible Inside front cover National foreword ii Foreword 2 Text of HD 570 S1 3 List of references Inside back coverBS7831:1995 ii BSI 07-1999 National foreword This British Standard has been prepared by Subcommittee GEL/15/3/7. It is ident
6、ical with IEC370:1971 Test procedure for thermal endurance of insulating varnishes Electric strength method published by the International Electrotechnical Commission (IEC) and has been endorsed by CENELEC as HD570 S1. A British Standard does not purport to include all the necessary provisions of a
7、contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Cross-references Publication referred to Corresponding British Standard HD 437 S1:1984 (IEC 212:1971) BS 2844:1995 Standar
8、d conditions for use prior to and during the testing of solid electrical insulating materials IEC 216 BS 5691 Guide for the determination of thermal endurance properties of electrical insulating materials Summary of pages This document comprises a front cover, an inside front cover, pages i and ii,
9、theHD title page, pages2 to8, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.HARMONIZATION DOCUMENT DOCUMENT DHARMONISATION HARMONISIERUNGSDOK
10、UMENT HD 570 S1 July 1990 UDC 621.315.617.1:621.317.333 Descriptors: Electrotechnical equipment, electrical insulation, electric strength, evaluation, thermal endurance English version Test procedure for thermal endurance of insulating varnishes Electric strength method Mthode dessai pour lvaluation
11、 de la stabilit thermique des vernis isolants par labaissement de la rigidit dilectrique Prfverfahren zur Beurteilung des thermischen Langzeitverhaltens von Isolierlacken Verfahren zur Prfung der elektrischen Durchschlagfestigkeit This Harmonization Document was approved by CENELEC on1990-06-11. CEN
12、ELEC members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for implementation of this Harmonization Document on a national level. Up-to-date lists and bibliographical references concerning national implementation may be obtained on application to the Ce
13、ntral Secretariat or to any CENELEC member. This Harmonization Document exists in three official versions (English, French, German). CENELEC members are the national electrotechnical committees of Austria, Belgium, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Nethe
14、rlands, Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom. CENELEC European Committee for Electrotechnical Standardization Comit Europen de Normalisation Electrotechnique Europisches Komitee fr Elektrotechnische Normung Central Secretariat: rue de Stassart 35, B-1050 Brussels 1990 Copy
15、right reserved to CENELEC members Ref. No. HD570 S1:1990 EBS7831:1995 BSI 07-1999 2 Foreword This Harmonization Document was approved by CENELEC on1990-6-11. The following dates were fixed: Contents Page Foreword 2 1 Introduction 3 2 Test specimen 3 3 Test apparatus 4 4 Ageing temperatures and times
16、 4 5 Testing procedure 5 6 Calculations 5 7 Report 6 Figure 1 Electrode material: brass 7 latest date of announcement of the HD at national level (doa) 1990-12-15 latest date of publication of aharmonized national standard (dop) 1991-06-15 latest date of withdrawal of conflicting national standards
17、(dow) 1991-06-15BS7831:1995 BSI 07-1999 3 1 Introduction 1.1 Scope This procedure covers a method for determining the relative thermal endurance of electrical insulating varnishes by means of coating on glass cloth and measuring electric strength before and after heat ageing. 1.2 Object To establish
18、 temperature indices to assist in determining the suitability of electrical insulating varnishes for use in electrical systems. 1.3 General This test method determines the retention of the electric strength of the varnish, coated on glass cloth, after ageing at elevated temperatures. In evaluating t
19、he suitability of application of insulating varnishes for electrical equipment, such physical and chemical properties as hardness, bonding strength, solvent resistance and thermoplastic flow are equally important. The evaluation of these properties, however, is not within the scope of this test meth
20、od. These properties must be evaluated separately by other test procedures. A major factor affecting the life of an electrical insulating varnish is thermal degradation. After a varnish has been weakened by thermal degradation, conditions such as moisture and vibration can cause failure of electrica
21、l equipment. An insulating varnish is effective in protecting electrical equipment only as long as it retains its physical and electrical integrity. The thermal degradation of the varnish results in changes to some of its properties. These changes may involve weight loss, porosity, crazing, embrittl
22、ement and loss of other mechanical characteristics. Thermal degradation of the varnish can be detected by a decrease in electric strength. It is, therefore, used as the failure criterion for this test method. Electrical insulating varnishes undergo flexing in service due to vibration and thermal exp
23、ansion. For this reason, a functional test may include flexing or elongation of the insulation. Two alternative methods are recommended in this procedure: Alternative I: A curved electrode system designed to elongate the outer surface of the varnish specimen approximately2%. This simulates flexing t
24、o which the varnish may be subjected in service. Alternative II: A flat electrode system. This method indicates the influence of thermal degradation only. The test specimen is not flexed (as in Alternative I) so that electrically weak points, which develop during thermal ageing, are determined witho
25、ut the effect of additional mechanical elongation. Results of tests using both methods give an indication of whether flexing after ageing has an essential effect on electric strength. In the test procedure, specimens are aged in ovens at elevated temperatures for specified periods. The specimens are
26、 then removed from the oven, cooled and tested for electric strength. At each temperature the thermal life is determined as the ageing time necessary for the electric strength to decrease to a pre-selected value. This value may be selected on the basis of some functional characteristic of the varnis
27、h for the intended application. The relative thermal endurance is then determined as a curve showing the relationship between ageing temperature and thermal life. 2 Test specimen Test specimens shall consist of panels of continuous filament, woven glass cloth coated with varnish by dipping. 2.1 Spec
28、imen preparation The glass cloth panels shall be cut from continuous filament, woven glass0.1mm to0.18mm thick with a weight per unit area of90g/m 2to140g/m 2and with20to26 ends and16 to24 picks per centimetre. (Where glass cloth having the specified picks and ends is not available in the country ma
29、king the test, the nearest standard cloth of that country shall be used.) (The dimensions of the curved electrode were designed to give an approximate2% elongation to the outer surface of a0.1mm thick glass cloth coated to0.175mm to0.185mm total thickness. It should be noted, therefore, that greater
30、 thicknesses will increase the elongation which, in turn, may significantly affect the ageing results.) The glass cloth shall be heat cleaned to remove binders. (A suggested heat cleaning procedure consists of heating the cloth24h at250C and24h at400C. Caution: heating above450C may damage the cloth
31、.) Each panel of glass cloth shall be15cm 30cm with the30cm dimension parallel to the warp threads of the cloth. Each panel shall be mounted and fastened in a suitable specimen-holding frame.BS7831:1995 4 BSI 07-1999 (Such a frame is made by using a1m length of corrosion-resistant wire approximately
32、1.7mm in diameter bent to form a rectangle having inside dimensions of15cm 30cm. The ends of the wire may overlap about5cm at one corner and be fastened together.) A set of twelve or more panels is required for each ageing temperature. A suitable fixture shall be used for holding the specimen frames
33、 in the oven in a vertical position with a minimum spacing of2.5cm. 2.2 Varnish dipping Test specimens shall be prepared by dipping the mounted glass cloth panels in the varnish. Specimens shall be prepared at room atmosphere or preferably at23 2 C and50 5% relative humidity (RH.) The consistency of
34、 the varnish shall be adjusted by trial so that two or more coats will give an overall increase in thickness of0.08 0.005mm over the cloth thickness. The panel shall be immersed in the varnish in the direction of the30cm length until bubbling stops. It shall be mechanically withdrawn at a uniform ra
35、te of10cm/min, and allowed to drain for h. Specimens shall be reversed endwise between subsequent dips to provide a more uniform coat. After each dip, the specimens shall be baked in the same vertical position as the last dip and at the temperature and time specified by the varnish manufacturer. 2.3
36、 Measuring apparatus The thickness shall be measured with a screw-type micrometer, having anvil and spindle surfaces of6mm to8mm in diameter and a mechanism for controlling the force between the anvil and spindle surfaces. In general, the force shall be10N. The average of the five measurements shall
37、 constitute the average thickness of the specimen. The micrometer shall be calibrated periodically; its accuracy shall be within34m. 3. Test apparatus 3.1 Alternative I Curved electrode test fixture The fixture shall be in accordance with the dimensions shown in Figure 1, page7. The electrodes shoul
38、d be of polished brass. The upper (movable) electrode shall have a total mass of1.8kg. Provisions shall be made to allow sufficient movement of the upper electrode or lower electrode so that intimate contact between the specimen and both electrodes is assured. This may be done by placing a soft rubb
39、er pad under the lower electrode. 3.2 Alternative II Flat electrode test fixture The electrodes shall consist of opposing cylindrical brass rods6mm in diameter with edges rounded to a radius of1.0mm(in). The electrode faces shall be smooth, flat and parallel and the electrodes shall be held exactly
40、opposite one another. The upper (movable) electrode shall have a total mass of50 2g. Any suitable holding and guiding arrangement meeting these requirements will be satisfactory. 3.3 Electric strength test set The electric strength test set shall be in accordance with IEC Publication243, Recommended
41、 Methods of Test for Electric Strength of Solid Insulating Materials at Power Frequencies. 3.4 Ageing ovens The air-circulating ovens shall be such that the temperatures may be controlled within the range of tolerances specified in IEC Publication212, Standard Conditions for Use Prior to and During
42、the Testing of Solid Electrical Insulating Materials. 4 Ageing temperatures and times The specimens shall be aged at not less than three, and preferably more, ageing temperatures covering a sufficient temperature range to establish the relative thermal endurance. The ageing temperatures shall differ
43、 by at least20degC. The lowest ageing temperature shall give a thermal life of at least5000h. An ageing temperature giving a thermal life of less than100h should not be used. To reduce the error in extrapolating thermal endurance data to obtain the temperature index, the lowest ageing temperature sh
44、ould be chosen so that an extrapolation exceeding20degC should not be necessary. See also IEC Publication216, Guide for the Preparation of Test Procedures for Evaluating the Thermal Endurance of Electrical Insulating Materials, Clause7, for a guide to selecting ageing temperatures. 1 32 -BS7831:1995
45、 BSI 07-1999 5 5 Testing procedure 5.1 Specimen thickness The average initial thickness of each specimen shall be measured at five points before heat ageing. The thickness shall be determined along the centre of the specimen parallel to its30cm length, using the apparatus described in Sub-clause2.3.
46、 5.2 Initial electric strength voltage One specimen from each set of specimens shall be conditioned for at least4h at23 2 C and50 5% RH and then tested for electric strength by the short-time test according to IEC Publication243, Sub-clause7.1 except that a rate of rise of500Vs 1shall be used. Six e
47、lectric strength measurements shall be made4.5cm apart starting4cm from one end of the specimen. In Alternative I, the specimen shall be inserted in the curved electrode fixture so that the warp threads are bent. In each alternative, the electrode is lowered slowly taking care to avoid injury to the
48、 specimen. 5.3 Ageing and specimen testing Five specimens shall be tagged with aluminium foil or otherwise permanently identified and placed in the specimen holding frames described in Sub-clause2.1. The fixture containing the specimen frames shall then be placed in the ageing oven and positioned so
49、 that it is at least10cm from the walls at any point and the specimens are held parallel to the direction of the air flow. One specimen shall be removed at the end of each of three ageing times equal to25%, 50% and100% of the estimated thermal life at the selected ageing temperature. After removal, the specimens shall be conditioned for at least4h at23 2 C and50 5% RH and then subjected to the electric strength test according to Alternative I or Alternative II. At the time of50% of estimated thermal life, five additio
