HD 570 S1:1990

SLOVENSKI STANDARD
01-oktober-1998
Test procedure for thermal endurance of insulating varnishes - Electric strength
method (IEC 60370:1971)
Test procedure for thermal endurance of insulating varnishes - Electric strength method
Prüfverfahren zur Beurteilung des thermischen Langzeitverhaltens von Isolierlacken -
Verfahren zur Prüfung der elektrischen Durchschlagfestigkeit
Méthode d'essai pour l'évaluation de la stabilité thermique des vernis isolants par
l'abaissement de la rigidité diélectrique
Ta slovenski standard je istoveten z: HD 570 S1:1990
ICS:
29.035.01 Izolacijski materiali na Insulating materials in
splošno general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

NORME
CEI
INTERNATIONALE
IEC
INTERNATIONAL
Première édition
STAN DARD
First edition
1971-01
Méthode d'essai pour l'évaluation de la
stabilité thermique des vernis isolants
par l'abaissement de la rigidité diélectrique
Test procedure for thermal endurance
of insulating varnishes —
Electric strength method
© IEC 1971 Droits de reproduction réservés —
Copyright - all rights reserved
Aucune partie de cette publication ne peut être reproduite ni
No part of this publication may be reproduced or utilized in
utilisée sous quelque forme que ce soit et par aucun
any form or by any means, electronic or mechanical,
procédé, électronique ou mécanique, y compris la photo-
including photocopying and microfilm, without permission in
copie et les microfilms, sans l'accord écrit de l'éditeur.
writing from the publisher.
International Electrotechnical Commission 3, rue de Varembé Geneva, Switzerland
Telefax: +41 22 919 0300 e-mail: inmail@iec.ch IEC web site http: //www.iec.ch
CODE PRIX
Commission Electrotechnique Internationale
J
PRICE CODE
International Electrotechnical Commission
IEC
Men Pour prix, voir catalogue en vigueur
• •
For price, see current catalogue

—3—
CONTENTS
Page
FOREWORD
PREFACE
Clause
1. Introduction 7
2. 9
Test specimen
Test apparatus
3.
4. Ageing temperatures and times
5. Testing procedure
6. Calculations
7. Report
— 5 —
INTERNATIONAL ELECTROTECHNICAL COMMISSION
TEST PROCEDURE FOR THERMAL ENDURANCE
OF INSULATING VARNISHES - ELECTRIC STRENGTH METHOD
FOREWORD
The formal decisions or agreements of the I E C on technical matters, prepared by Technical Committees on which all the
1)
National Committees having a special interest therein are represented, express, as nearly as possible, an inte rnational
consensus of opinion on the subjects dealt with.
They have the form of recommendations for international use and they are accepted by the National Committees in that
2)
sense.
3) In order to promote this inte rnational unification, the I E C expresses the wish that all National Committees having as
yet no national rules, when preparing such rules, should use the I E C recommendations as the fundamental basis for these
as national conditions will permit.
rules in so far
4) The desirability is recognized of extending international agreement on these matters through an endeavour to harmonize
as national conditions will permit. The National
national standardization rules with these recommendations in so far
Committees pledge their influence towards that end.
PREFACE
This Recommendation has been prepared by Sub-Committee 15B, Endurance Tests, of I EC Technical
Committee No. 15, Insulating Materials.
A first draft was discussed at the meetings held in Tel Aviv in 1966 and in Warsaw in 1967. As a result
of this latter meeting, a final draft was submitted to the National Committees for approval under the
Six Months' Rule in June 1968. Amendments were submitted to the National Committees for approval
under the Two Months' Procedure in August 1970.
The following countries voted explicitly in favour of publication:
Korea (Republic of)
Australia
Belgium Netherlands
Canada South Africa
Czechoslovakia Sweden
Denmark Switzerland
France Turkey
Israel United States
Italy of America
Japan Yugoslavia
—7—
TEST PROCEDURE FOR THERMAL ENDURANCE
OF INSULATING VARNISHES - ELECTRIC STRENGTH METHOD
1. Introduction
1.1 Scope
This procedure covers a method for determining the relative thermal endurance of electrical insu-
lating varnishes by means of coating on glass cloth and measuring electric strength before and after
heat ageing.
1.2 Object
To establish 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 the suitability of application of insulating
varnishes for electrical equipment, such physical and chemical properties as hardness, bonding
ow are equally important. The evaluation of these
strength, solvent resistance and thermoplastic fl
properties, however, is not within the scope of this test method. 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 electrical equipment. An insulating varnish is effective in protecting electrical equip-
ment 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, embrittlement 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 expansion.
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 approxi-
mately 2 %. This simulates flexing to which the varnish may be subjected in service.
Alternative II:
A flat electrode system. This method indicates the in fluence 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 without the effect of additional mechanical elongation.

—9—
Results of tests using both methods give an indication of whether flexing after ageing has an essen-
tial effect on electric strength.
In the test procedure, specimens are aged in ovens at elevated temperatures for specified periods.
The specimens are then removed from the oven, cooled and tested for electric strength. At each tem-
perature 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 varnish 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 Specimen preparation
The glass cloth panels shall be cut from continuous filament, woven glass 0.1 mm to 0.18 mm thick
with a weight per unit area of 90 g/m2 to 140 g/m2 and with 20 to 26 ends and 16 to 24 picks per
centimetre. (Where glass cloth having the specified picks and ends is not available in the country
making the test, the nearest standard cloth of that country shall be used.)
(The dimensions of the curved electrode were designed to give an approximate 2 % elongation to the
outer surface of a 0.1 mm thick glass cloth coated to 0.175 mm to 0.185 mm total thickness. It should
be noted, therefore, that greater thicknesses will increase the elongation which, in turn, may signifi-
cantly affect the ageing results.)
The glass cloth shall be heat cleaned to remove binders. (A suggested heat cleaning procedure
consists of heating the cloth 24 h at 250 °C and 24 h at 400 °C. Caution: heating above 450 °C may
damage the cloth.)
Each panel of glass cloth shall be 15 cm x 30 cm with the 30 cm dimension parallel to the warp
threads of the cloth.
Each panel shall be mounted and fastened in a suitable specimen-holding frame.
(Such a frame is made by using a 1 m length of corrosion-resistant wire approximately 1.7 mm in
diameter bent to form a rectangle having inside dimensions of 15 cm x 30 cm. The ends of the wire
may overlap about 5 cm 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 in the oven in a vertical position
with a minimum spacing of 2.5 cm.
2.2 Varnish dipping
Test specimens shall be prepared by dipping the mounted glass cloth panels in the varnish. Speci-
mens shall be prepared at room atmosphere or preferably at 23 ± 2 °C and 50 ± 5 % relative humidity
(RH.)
...