EN 60216-6:2004

STANDARDElektrični izolacijski materiali - Lastnosti toplotne vzdržljivosti - 6. del: Ugotavljanje indikatorjev toplotne vzdržljivosti (TI in RTE) izolacijskega materiala po metodi s fiksnim časovnim okvirom (IEC
60216-6:2003)Electrical insulating materials - Thermal endurance properties - Part 6: Determination of thermal endurance indices (TI and RTE) of an insulating material using the fixed time frame method (IEC 60216-6:2003)©
Standard je založil in izdal Slovenski inštitut za standardizacijo. Razmnoževanje ali kopiranje celote ali delov tega dokumenta ni dovoljenoReferenčna številkaSIST EN 60216-6:2004(en)ICS29.035.01

EUROPEAN STANDARD
EN 60216-6 NORME EUROPÉENNE EUROPÄISCHE NORM
March 2004 CENELEC European Committee for Electrotechnical Standardization Comité Européen de Normalisation Electrotechnique Europäisches Komitee für Elektrotechnische Normung
Central Secretariat: rue de Stassart 35, B - 1050 Brussels
© 2004 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 60216-6:2004 E
ICS 17.220.99; 29.035.01
English version
Electrical insulating materials -
Thermal endurance properties Part 6: Determination of thermal endurance indices (TI and RTE)
of an insulating material using the fixed time frame method (IEC 60216-6:2003)
Matériaux isolants électriques -
Propriétés d'endurance thermique Partie 6: Détermination des indices d'endurance thermique (TI et RTE)
d'un matériau isolant en utilisant
la méthode de trame de durées fixes (CEI 60216-6:2003)
Elektroisolierstoffe -
Eigenschaften hinsichtlich
des thermischen Langzeitverhaltens Teil 6: Bestimmung der thermischen Langzeitkennwerte (TI und RTE) eines Isolierstoffes unter Anwendung
des Festzeitrahmenverfahrens (IEC 60216-6:2003)
This European Standard was approved by CENELEC on 2004-03-01. CENELEC members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the Central Secretariat or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CENELEC member into its own language and notified to the Central Secretariat has the same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.

- 3 - EN 60216-6:2004 Foreword
The text of document 15E/221/FDIS, future edition 1 of IEC 60216-6, prepared by SC 15E, Methods of test, of IEC TC 15, Insulating materials, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC as EN 60216-6 on 2004-03-01.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement
(dop) 2004-12-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn
(dow) 2007-03-01
Annex ZA has been added by CENELEC. __________
Endorsement notice
The text of the International Standard IEC 60216-6:2003 was approved by CENELEC as a European Standard without any modification. __________

(normative)
Normative references to international publications
with their corresponding European publications The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. NOTE When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD applies. Publication Year Title EN/HD Year IEC 60212 1971 Standard conditions for use prior to and during the testing of solid electrical insulating materials
HD 437 S1 1984 IEC 60216-1 2001 Electrical insulating materials - Properties of thermal endurance Part 1: Ageing procedures and evaluation of test results
EN 60216-1 2001 IEC 60216-2 1990 Part 2: Choice of test criteria
HD 611.2 S1 1992 IEC 60216-3 2002 Part 3: Instructions for calculating thermal endurance characteristics
EN 60216-3 2002 IEC 60216-4-1 1990 Part 4: Ageing ovens - Section 1: Single-chamber ovens
HD 611.4.1 S1 1992 IEC 60216-4-2 2000 Part 4-2: Ageing ovens - Precision ovens for use up to 300 °C
EN 60216-4-2 2000 IEC 60216-4-3 2000 Part 4-3: Ageing ovens - Multi-chamber ovens
EN 60216-4-3 2000 IEC 60216-5 2003 Part 5: Determination of relative thermal endurance index (RTE) of an insulating material
EN 60216-5 2003 IEC 60493-1 1974 Guide for the statistical analysis of ageing test data Part 1: Methods based on mean values of normally distributed test results - -

NORME INTERNATIONALECEIIECINTERNATIONALSTANDARD 60216-6Première éditionFirst edition2003-12Matériaux isolants électriques – Propriétés d'endurance thermique – Partie 6: Détermination des indices d'endurance thermique (TI et RTE) d'un matériau isolant en utilisant la méthode de trame de durées fixes Electrical insulating materials – Thermal endurance properties – Part 6: Determination of thermal endurance indices (TI and RTE) of an insulating material using the fixed time frame method Pour prix, voir catalogue en vigueur For price, see current catalogue© IEC 2003
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Copyright - all rights reservedAucune partie de cette publication ne peut être reproduite ni utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie et les microfilms, sans l'accord écrit de l'éditeur. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the publisher. International Electrotechnical Commission,
3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, SwitzerlandTelephone: +41 22 919 02 11 Telefax: +41 22 919 03 00 E-mail: inmail@iec.ch
Web: www.iec.ch CODE PRIX PRICE CODE XCommission Electrotechnique InternationaleInternational Electrotechnical Commission

60216-6 © IEC:2003 – 3 – CONTENTS FOREWORD.5 1 Scope.11 2 Normative references.11 3 Terms, definitions, symbols and abbreviated terms.13 3.1 Terms, abbreviations and definitions.13 3.2 Symbols and abbreviated terms.19 4 FTFM protocol – Principles and objectives.21 4.1 Principles of FTFM protocol.21 4.2 Objectives of FTFM protocol.21 5 TI determination.235.1 Ageing procedures.23 5.2 Ageing times and temperatures.23 5.3 Test specimens.25 5.4 Diagnostic tests.27 5.5 Selection of end-points.27 5.6 Establishment of initial property value.27 5.7 Ageing conditions.29 5.8 Procedure for ageing.29 6 Calculation procedures.31 6.1 General principles.31 6.2 Precision of calculations.33 6.3 Derivation of temperatures equivalent to property values.33 6.4 Regression analysis (temperature on time).39 6.5 Statistical tests.43 6.6 Thermal endurance graph.47 7 Calculation and requirements for results.49 7.1 Calculation of thermal endurance characteristics.49 7.2 Reporting of results.49 8 Report.51 9 RTE Determination – Objectives.51 10 Additional symbols.5311 Experimental procedures.53 12 Calculation procedures.55 12.1 General principles.55 12.2 Input data.5512.3 RTE.5712.4 Confidence limits.57 12.5 Extrapolation.61 13 Results and report.6113.1 Results of statistical and numerical tests.61 13.2 Result.6113.3 Report.61

60216-6 © IEC:2003 – 5 – Annex A (normative)
Decision flow chart.63 Annex B (normative)
Decision table .65 Annex C (informative)
Statistical tables.67 Annex D (informative)
Suggested ageing times and temperatures.73 Annex E (informative)
Figures.77 Annex F (normative)
Statistical significance of the difference
between two regression estimates.83 Figure E.1 – Property–temperature graph with regression line.77 Figure E.2 – Thermal endurance graph.77 Figure E.3 – Ageing times and temperatures in relation to thermal endurance graph.79 Figure E.4 – Ageing times and temperatures in relation to thermal endurance graph.79 Figure E.5 – Ageing times and temperatures in relation to thermal endurance graph.81 Table B.1 – Decision table.65Table C.1 – χ2-function.67 Table C.2 –t–function.67 Table C.3 – F-function, P = 0,05.69 Table C.4 – F-function, P = 0,005.71 Table D.1 – Ageing temperatures and ageing periods .73

60216-6 © IEC:2003 – 7 – INTERNATIONAL ELECTROTECHNICAL COMMISSION ____________ ELECTRICAL INSULATING MATERIALS – THERMAL ENDURANCE PROPERTIES –Part 6: Determination of thermal endurance indices (TI and RTE)
of an insulating material using the fixed time frame method FOREWORD 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with may participate in this preparatory work. International, governmental and non-governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for Standardization (ISO) in accordance with conditions determined by agreement between the two organizations. 2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international consensus of opinion on the relevant subjects since each technical committee has representation from all interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any misinterpretation by any end user. 4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications transparently to the maximum extent possible in their national and regional publications. Any divergence between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter. 5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any equipment declared to be in conformity with an IEC Publication. 6) All users should ensure that they have the latest edition of this publication. 7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and members of its technical committees and IEC National Committees for any personal injury, property damage or other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) andexpenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is indispensable for the correct application of this publication. 9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent rights. IEC shall not be held responsible for identifying any or all such patent rights. International Standard IEC 60216-6 has been prepared by subcommittee 15E, Methods test, of IEC technical committee 15: Insulating materials. The text of this standard is based on the following documents: FDIS Report on voting 15E/221/FDIS 15E/225/RVD Full information on the voting for the approval of this standard can be found in the report on voting indicated in the above table. This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

60216-6 © IEC:2003 – 9 – IEC 60216, under the general title Electrical insulating materials – Thermal endurance properties, consists of the following parts: Part 1:
Ageing procedures and evaluation of test results Part 2:
Choice of test criteria Part 3:
Instructions for calculating thermal endurance characteristics Part 4:
Ageing ovens Part 5:
Determination of relative thermal endurance index (RTE) of an insulating material Part 6:
Determination of thermal endurance indices (TI and RTE) of an insulating material using the fixed time frame protocol The committee has decided that this publication remains valid until 2008. At this date, the publication will be
• reconfirmed; • withdrawn; • replaced by a revised edition, or • amended.

60216-6 © IEC:2003 – 11 – ELECTRICAL INSULATING MATERIALS – THERMAL ENDURANCE PROPERTIES –Part 6: Determination of thermal endurance indices (TI and RTE)
of an insulating material using the fixed time frame method 1 Scope This part of IEC 60216 specifies the experimental and calculation procedures for deriving the thermal endurance characteristics, temperature index (TI) and relative thermal endurance index (RTE) of a material using the “fixed time frame method (FTFM)”.
In this protocol, the ageing takes place for a small number of fixed times, using the appropriate number of ageing temperatures throughout each time, the properties of the specimens being measured at the end of the relevant time interval. This differs from the procedure of IEC 60216-1, where ageing is conducted at a small number of fixed temperatures, property measurement taking place after ageing times dependent on the progress of ageing. Both the TI and the RTE determined according to the FTFM protocol are derived from experimental data obtained in accordance with the instructions of IEC 60216-1 and IEC 60216-2, as modified in this standard. The calculation procedures and statistical tests are modified in relation to IEC 60216-3 and IEC 60216-5.
2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. IEC 60212:1971, Standard conditions for use prior to and during the testing of solid electrical insulating materials IEC 60216-1:2001, Electrical insulating materials – Properties of thermal endurance – Part 1: Ageing procedures and evaluation of test results IEC 60216-2:1990, Guide for the determination of thermal endurance properties of electrical insulating materials – Part 2: Choice of test criteria IEC 60216-3:2002, Electrical insulating materials – Thermal endurance properties – Part 3: Instructions for calculating thermal endurance characteristics IEC 60216-4-1:1990, Guide for the determination of thermal endurance properties of electrical insulating materials – Part 4: Ageing ovens – Section 1: Single-chamber ovens IEC 60216-4-2:2000, Electrical insulating materials – Thermal endurance properties – Part 4-2: Ageing ovens – Precision ovens for use up to 300 °C

60216-6 © IEC:2003 – 13 – IEC 60216-4-3:2000, Electrical insulating materials – Thermal endurance properties – Part 4-3: Ageing ovens – Multi-chamber ovens
IEC 60216-5:2003, Electrical insulating materials − Thermal endurance properties – Part 5: Determination of relative thermal endurance index (RTE) of an insulating material IEC 60493-1:1974, Guide for the statistical analysis of ageing test data – Part 1: Methods based on mean values of normally distributed test results 3 Terms, definitions, symbols and abbreviated terms For the purposes of this document, the following terms, definitions, symbols and abbreviations apply. 3.1 Terms, abbreviations and definitions 3.1.1 assessed thermal endurance index
ATE
numerical value of the temperature in degrees Celsius, up to which the control material possesses known, satisfactory service performance in the specified application
NOTE 1 The ATE of a specific material may vary between different applications of the material. NOTE 2 ATE is sometimes referred to as “absolute” thermal endurance index. 3.1.2 ageing temperature temperature in degrees Celsius at which a group of specimens is thermally aged 3.1.3 end-point temperature temperature in degrees Celsius at which a specimen is considered to have reached its end-point after ageing for the specified time 3.1.4 candidate material material for which an estimate of the thermal endurance is required to be determined NOTE The determination is made by simultaneous thermal ageing of the material and a control material. 3.1.5 central second moment of a data group sum of the squares of the differences between the data values and the value of the group mean divided by the number of data in the group 3.1.6 95 % confidence limit statistical parameter, calculated from test data, which with 95 % confidence constitutes an upper or lower limit for the true value of a quantity estimated by statistical analysis NOTE 1 This implies that there is only 5 % probability that the true value of the quantity estimated is actually larger (or smaller) than the upper (or lower) confidence limit. NOTE 2 In other connections, confidence values other than 95 % may sometimes be used, e.g. in the linearity test for destructive test data.

60216-6 © IEC:2003 – 15 – 3.1.7 control material material with known assessed thermal endurance index (ATE), preferably derived from service experience, used as a reference for comparative tests with the candidate material 3.1.8 correlation coefficient number expressing the completeness of the relation between members of two data sets, equal to the covariance divided by the square root of the product of the variances of the sets NOTE 1 The value of its square is between 0 (no correlation) and 1 (complete correlation). NOTE 2 In this standard, the two data sets are the values of the independent variable and the means of the dependent variable groups. 3.1.9 correlation time of RTE estimated time to end-point of the control material at a temperature equal to its ATE in degrees Celsius 3.1.10 correlation time of TI hypothetical time to end-point used to calculate TI NOTE Its usual value is 20 000 h. 3.1.11 covariance of data sets for two sets of data with equal numbers of elements where each element in one set corres-ponds to one in the other, the sum of the products of the deviations of the corresponding members from their set means, divided by the number of degrees of freedom 3.1.12 degrees of freedom number of data values minus the number of parameter values 3.1.13 destructive test diagnostic property test, where the test specimen is irreversibly changed by the property measurement, in a way which precludes a repeated measurement on the same specimen 3.1.14 end-point line line parallel to the temperature axis intercepting the property axis at the end-point value 3.1.15 halving interval
HIC numerical value of the temperature interval in kelvin which expresses the halving of the time to end-point taken at a time equal to TI 3.1.16 regression analysis process of deducing the best fit line expressing the relation of corresponding members of two data groups by minimizing the sum of squares of deviations of members of one of the groups from the line

60216-6 © IEC:2003 – 17 – 3.1.17 regression coefficients coefficients of the equation derived by regression analysis 3.1.18 relative thermal endurance index RTE estimate of the thermal endurance of a candidate material, made by thermal ageing simultaneously with the control material, as described in this standard NOTE The value of RTE is the value of the temperature in degrees Celsius at which the estimated time to end-point of the candidate material is the same as the estimated time to end-point of the control material at a temperature equal to its ATE. 3.1.19 significance probability of a value of a statistical function greater than a specified value NOTE The value is equal to (1 – p) where p is the cumulative distribution function value. Significance is conventionally printed in upper case (P). 3.1.20 standard deviation square root of the variance of a data group or sub-group 3.1.21 standard error of an estimate of the true value of a data group property value of the standard deviation of the hypothetical sampling population of which the group property may be considered to be a member NOTE For an estimate of the group mean, the standard error is equal to the group standard deviation divided by the square root of the number of data in the group, and indicates the uncertainty in the estimate of thetrue value of the mean. This standard is concerned only with means and the difference between two means.
3.1.22 temperature index
TI
numerical value of the temperature in degrees Celsius derived from the thermal endurance relationship at a time of 20 000 h (or other specified time) 3.1.23 temperature group (of specimens) number of specimens being exposed together to thermal ageing at the same temperature in the same oven NOTE Where there is no risk of ambiguity, either temperature groups or test groups may be referred to simply as “groups”. 3.1.24 test group (of specimens)
number of specimens removed together from a temperature group for destructive testing NOTE Where there is no risk of ambiguity, either temperature groups or test groups may be referred to simply as “groups”. 3.1.25 thermal endurance graph graph in which the logarithm of the time to reach a specified end-point in a thermal endurance test is plotted against the reciprocal thermodynamic (absolute) test temperature

60216-6 © IEC:2003– 19 –3.1.26thermal endurance graph papergraph paper having a logarithmic time scale as the ordinate and values proportional to thereciprocal of the thermodynamic (absolute) temperature as the abscissaNOTE
The ordinate is in powers of ten (from 10 h to 100 000 h is often a convenient range). The abscissa isusually graduated in a non-linear (Celsius) temperature scale oriented with temperature increasing from left toright.3.1.27time group (of specimens)all test groups removed for testing at the same time3.1.28variance of a data groupsum of the squares of the deviations of the data from a reference level defined by one or moreparameters divided by the number of degrees of freedomNOTE
The reference level may, for example be a mean value (1 parameter) or a line (2 parameters, in this caseintercept on the axis of the independent variable and slope).3.2 Symbols and abbreviated termsThe following symbols are used in the calculations of Clauses 6, 7 and 12.SymbolDescriptionClauseaRegression coefficient: intercept of regression line with x-axis6.4.3bRegression coefficient: slope of regression line relative to y-axis6.4.3rbParameter derived from b for calculation of cYˆ6.5.3cParameter in calculation of χ26.5.1FF-distributed variance ratio for linearity test6.3.3, 6.5.2g, h, i, jIndexing parameters for regression calculations6.3, 6.4HICHalving interval7.1kNumber of ageing times6.1.1NTotal number of xij
values6.4.2niNumber of xij values in time groupi6.1.1PSignificance of the value of a statistical test functionAnnexes A,B, and CpeEnd-point property value6.3pghProperty value h in temperature group g (time group iimplied)6.3gpMean property value in temperature groupg (time group iimplied)6.3qBase of logarithms in calculation of χ26.5.1rNumber of temperature groups selected in time group i6.3.2r2Square of correlation coefficient6.4.3s2Total (non-regression) variance of x-values6.5.2

60216-6 © IEC:2003– 21 –SymbolDescriptionClause21gsVariance of property values in temperature group g (time groupiimplied)6.3.22asValue ofs2 adjusted to allow for acceptable non-linearity.6.5.22rsParameter derived from s2 for calculation of cYˆ6.5.3tStudent's t-distributed stochastic variable6.5.3TC, TCaLower confidence limit of TI or TIa (see 2as above)6.5.3tp, NValue of twith probability p andN degrees of freedom6.5.3xijValue ofx, index numberj , in time group i6.3.4xGeneral mean of x-values6.4.2cXXˆ,ˆEstimate ofx, and its confidence limit6.5.3yiValue of y for time groupi6.1.1yGeneral mean of y-values6.4.2cYYˆ,ˆEstimate ofy, and its confidence limit6.5.3zijReciprocal kelvin temperature for ijϑ6.1.1()y2µCentral 2nd moment of yvalues6.4.2νTotal number of property values in time group (i implied)6.3.2χ2χ2 distributed variable for variance equality (Bartlett's) test6.5.1ijϑAgeing temperature for specimen group jin time group i6.1.10Θ273,15 = kelvin value of 0 °C6.1.1iτAgeing time for time group i6.1.14 FTFM protocol – Principles and objectives4.1 Principles of FTFM protocolThe FTFM (fixed time frame method) protocol is based upon the principle that thermal ageingfor determination of thermal endurance characteristics is carried out over a small number offixed times, with a sufficient range of ageing temperatures at each time to ensure that theend-point is reached in a satisfactory manner.In this it differs from the conventional procedure (fixed temperature frame), where a smallnumber of ageing temperatures is employed, with ageing being carried out with testing atintervals, until the end-point has been reached.4.2 Objectives of FTFM protocolThe objective of the protocol is to achieve the following advantages:

60216-6 © IEC:2003 – 23 – The determination of thermal endurance characteristics is completed in a fixed, pre-determined time. This enables much more efficient planning of the determination, and will often have substantial commercial advantage. A simple TIdetermination will be completed in 5 kh, whereas by the conventional protocol, it may be necessary for ageing to be considerably prolonged past this time to achieve the end-point at the lowest chosen ageing temperature. Each temperature to end-point (i.e. time-group mean) in the thermal endurance regression is based on a number of temperatures which may be any number between three (3) and the number of temperature groups in a time group.
Since the largest source of systematic error in the conventional (fixed temperature frame) protocol is temperature error (actual indication error or temperature distribution error), systematic errors can be considerably reduced. Errors from this source can lead to results which are either inaccurate or invalid through incorrect assessment of linearity. 5 TI determination
5.1 Ageing procedures Each test procedure should specify the shape, dimensions and number of the test specimens, the times of exposure, the property to which TIis related, the methods of its determination, the end-point, and the derivation of the thermal endurance characteristics from the experimental data. The chosen property should, if possible, reflect in a significant fashion a function of the material in practical use. A choice of properties is given in IEC 60216-2. To provide uniform conditions, the conditioning of specimens after removal from the oven and before measurement may need to be specified. 5.2 Ageing times and temperatures In the majority of cases, the required thermal endurance characteristics are for a projected duration of 20 000 h. However, there is often a need for such information related to other, longer or shorter times. In cases of longer times, the times given as requirements or recommendations in this standard (e.g. 5 000 h for the minimum value of the longest ageing time) shall be increased in the ratio of the actual specification time to 20 000 h.
In cases of shorter specification times, the related times may be decreased in the same ratio, if necessary. Particular care will be needed for very short specification times, since the higher ageing temperatures may lead into temperature regions which include transition points, e.g. glass transition temperature or partial melting, with consequent non-linearity. Very long specification times may also lead to non-linearity. Recommendations for ageing times and temperatures are given in Annex D.

60216-6 © IEC:2003 – 25 – 5.3 Test specimens 5.3.1 Preparation
The specimens used for the ageing test should constitute a random sample from the population investigated and shall be treated uniformly.
Since processing conditions may significantly affect the ageing characteristics of some materials, it shall be ensured that, for example, sampling, cutting sheet from the supply roll, cutting of anisotropic material in a given direction, moulding, curing, preconditioning, are performed in the same manner for all specimens. The material specifications or the standards for the diagnostic test methods will contain all necessary instructions for the preparation of specimens.
The thickness of specimens is, in some cases, specified in the list of property measurements for the determination of thermal endurance. See IEC 60216-2. If not, the thickness shall be reported. Some physical properties are sensitive even to minor variations of specimen thickness. In such cases, the thickness after each ageing period may need to be determined and reported if required in the relevant specification.
The thickness is also important because the rate of ageing may vary with thickness. Ageing data of materials with different thicknesses are not always comparable. Consequently, a material may be assigned more than one thermal endurance characteristic derived from the measurement of properties at different thicknesses.
The tolerances of specimen dimensions should be the same as those normally used for general testing. Where specimen dimensions need smaller tolerances than
those
normally used, these special tolerances should be given.
Screening measurements ensure that specimens are of uniform quality and typical of the material to be tested. 5.3.2 Number of specimens The accuracy of endurance test results depends largely on the number of specimens aged at each temperature. The total number of specimens (N) is derived as follows:
dcbaN+××=where
a is the number of specimens in a test group undergoing identical treatment at one temperature and discarded after determination of the property (usually five);
b is the number of treatments, i.e. total number of exposure temperatures, at one time; c is the number of ageing time levels;
d is the number of specimens in the group used to establish the initial value of the property. Normal practice is to select d = 2awhen the diagnostic criterion is a percentage change of the property from its initial level. When the criterion is an absolute property level, d is usually given the value of zero, unless reporting of the initial value is required.

60216-6 © IEC:2003 – 27 – It is good practice to prepare additional specimens, or at least to provide a reserve of the original material batch from which such specimens may subsequently be prepared. In this way, any required ageing of additional specimens in case of unforeseen complications will introduce a minimum risk of producing systematic differences between groups of specimens. Such complications may arise, for example, if the thermal endurance relationship turns out to be non-linear, or if specimens are lost due to thermal runaway of an oven.
5.4 Diagnostic tests If IEC material specifications are available, property requirements in terms of acceptable lower limits of TI values are usually given. If such material specifications are not available, a selection of properties and methods for the evaluation of thermal endurance is given in IEC 60216-2. If such a method cannot be found, an international, national or institution standard or a specially devised method should be used in that order of preference. In this case, the diagnostic test shall be stated in the report, including the property, measurement procedure and end-point. 5.5 Selection of end-points The thermal endurance of materials may need to be characterized by different endurance data (derived using different properties and/or end-points), in order to facilitate the adequate selection of the material in respect of its particular application. See IEC 60216-2.
There are two alternative ways in which the end-point may be defined:
a) As a percentage increase or decrease in the measured value of the property from the original level. This approach will provide comparisons among materials but bears a poorer relationship than item b) to the property values required in normal service. For the determination of the initial value, see 5.6. b) As a fixed value of the property. This value might be selected with respect to usual service requirements. End-points of proof tests are predominantly given in the form of fixed values of the property.
The end-point should be selected to indicate a degree of deterioration of the insulating material which has reduced its ability to withstand a stress encountered in actual service. The degree of degradation indicated as the end-point of the test should be related to the allowable safe value for the material property which is desired in practice. 5.6 Establishment of initial property value
Select the specimens for the determination of the initial value of the property to constitute a random subset of those prepared for ageing. Before determining the property value, these specimens shall be conditioned by exposure to the lowest level of ageing temperature of the test (see 5.2), for two days (48 ± 6) h. NOTE In some cases (e.g. very thick specimens) times greater than two days may be necessary to establish a stable value. Unless otherwise stated in the method for determining the diagnostic property (for example, parts of material specifications dealing with methods of test, or a method listed in IEC 60216-2), the initial value is the arithmetic mean of the test results.

60216-6 © IEC:2003 – 29 – 5.7 Ageing conditions 5.7.1 Ageing ovens Throughout the heat ageing period, ageing ovens shall maintain, in that part of the working space where specimens are placed, a temperature with tolerances as given in IEC 60216-4-1. Unless otherwise specified, IEC 60216-4-1 shall apply. IEC 60216-4-2 and IEC 60216-4-3 may be specified in special cases. The circulation of the air within the oven, and the exchange of the air content should be adequate to ensure that the rate of thermal degradation is not influenced by accumulation of decomposition products or oxygen depletion (see 5.7.2).
5.7.2 Environmental conditions Unless otherwise specified, ageing shall be carried out in ovens operating under a normal laboratory atmosphere. However, for some materials very sensitive to the humidity in the ovens, more reliable results are obtained when the absolute humidity in the ageing oven room is maintained at the value
equal to the absolute humidity of standard atmosphere B according to IEC 60212. This, or other specified conditions, shall then be reported.
NOTE The effects of special environmental conditions such as extreme humidity, chemical contamination or vibration in many cases may be more appropriately evaluated by insulation systems tests. Although environmental conditioning, the influence of atmospheres other than air and immersion in liquids, such as oil, may be important, these are not the concern of this standard.
5.7.3 Conditions for property measurement Unless otherwise specified, the specimens shall be conditioned before measurement, and measured under conditions as specified in the material standard specification. 5.8 Procedure for ageing Establish a testing scheme, as for example outlined in Annex D. Prepare a number of specimens following the instructions of 5.3.2. If necessary, determine the initial value of the property as specified in 5.6. Divide the specimens by random selection into test groups appropriate for the testing scheme. Place the appropriate numbers of groups in each of the ovens at the required temperature. NOTE Attention should be given to the recommendation in Annex D (Note 2) to prepare extra groups of specimens should the thermal endurance characteristics of the material be unsuited to the basic recommendation of Annex D. After each ageing time, select at random one group from the appropriate ageing ovens and remove it from the ovens. Allow to cool to room temperature, unless otherwise specified. If specified, condition for the specified time in the specified atmosphere, and test the specimens by the specified test procedure. It is recommended to carry out calculations as data become available, particularly for the shortest exposure time. Evaluate the results as specified in Clause 6.

60216-6 © IEC:2003 – 31 – 6 Calculation procedures 6.1 General principles 6.1.1 Thermal endurance calculation The general calculation procedures and instructions given in 6.4 are based on the principles set out in IEC 60493-1, modified as follows (see 3.7.1 of IEC 60493-1:1974):
a) The relation between the mean of the reciprocals x of the thermodynamic (absolute) temperatures at which the specified end-point is reached and the logarithm y of the ageing time is linear. b) The values of the deviations of the values of x obtained from the linear relation are normally distributed with a variance which is independent of the ageing time. The data used in the general calculation procedures are obtained from the experimental data by a preliminary calculation. Calculation data comprise values of z, y, nandk, where ()0/1,zijij+=ϑ= reciprocal of thermodynamic value in K of ageing temperature ijϑin °C;ii2ylog= = logarithm of value of ageing time in h (iτ);ni = number of z values in group number i aged for time iτ;k = number of ageing times or groups of x values. NOTE Any number may be used as the base for logarithms, provided consistency is observed throughout calculations. The use of natural logarithms (base e) is recommended, since most computer programming languages and scientific calculators have this facility. 6.1.2 Property value – Equivalent temperature transform (Calculation of hypothetical ageing temperature derived from the value of a property)When destructive test criteria are employed, each test specimen is destroyed in obtaining a property value; for this reason, time and/or temperature values necessary to reach end-point cannot be directly measured. To enable estimates of the times to end-point to be obtained, the following assumptions are made that in the vicinity of the end-point (for one ageing time): a)
the relation between the mean property values and the reciprocals of the thermodynamic temperatures is approximately linear; b)
the values of the deviations of the individual property values from this linear relation are normally distributed with a variance which is independent of the temperature; c)
the curves of property versus reciprocal of the ageing (thermodynamic) temperature for the individual test specimens are straight lines parallel to the line representing the relation of item a) above. For application of these assumptions, an ageing curve is drawn of the data obtained at each of the ageing times. The curve for each ageing time is obtained by plotting the mean value of property for each specimen group against the reciprocal of its ageing temperature (thermodynamic). If possible, ageing is conducted at sufficiently high and low ageing temperatures that at least one group mean is above and at least one below the end-point level.
NOTE A non-linear temperature scale graduated in °C is usually employed as
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