IEC , Electrical insulating materials – Thermal endurance properties – Part 2: Determination of thermal endurance properties of electrical. The text of the International Standard IEC was approved by IEC I.S. EN EN: COMBINED PDF. The text of the International Standard IEC was approved by IEC .. I.S. EN EN: COMBINED PDF.
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This part of IEC specifies the experimental and calculation IEC , Electrical insulating materials – Thermal endurance properties – Part 2. As from 1 January all IEC publications are issued with a designation .. IEC , Electrical insulating materials – Properties of. Home; BS EN Add to Cart. Printed Edition + PDF; Immediate download; $; Add to Cart IEC
It gives acceptance tests and in-service monitoring tests for both unloaded and loaded multi-chamber ovens and conditions of use. Definitions Rate of ventilation N: Number of air changes per hour in the exposure chamber at room temperature that indicates how many times per hour the oven replaces air; regulated with ventilation opening apertures.
Temperature fluctuation: The maximum change in temperature at one point in the exposure volume over a perion of time.
Temperature difference: Maximum difference of temperature between any two points in the exposure volume at any one time. Depends on the temperature uniformity of heating elements, their placement in the oven and how the air circulates.
Temperature variation: Difference between the highest temperature and the lowest temperature measured in the exposure volume over a period of time. Temperature deviation: Calculated difference in the exposure temperature from the intended value due to the combination of the temperature difference, temperature fluctuation and the error in the measurement of temperature.
Time constant: Measure of time taken for the temperature of a standard specimen to approach the exposure volume temperature, the rate of speed at which the standard specimen is heated from room temperature to any oven temperature.
It is the main parameter of oven, which affects the rate of specimen temperature increase is the air circulation inside oven. The test methods and service requirements The oven chamber has to be made of suitable materials and all electrical and other auxiliary elements have to be easily accessible for maintenance.
Inner oven part should be constructed of suitable anti-corrosion material, which has no absorption properties. All joints have to be leak-proof and not to corrode. Interior surfaces must be easy to clean.
The oven door and front of oven chamber must be capable of being closed with enough downforce in order to closure be tight. If it is necessary, oven closure should have sealing, the inside of the oven be reliably separated from the atmosphere, when the doors are closed. Ovens should have safety device that turns it off, when the temperature inside the oven significantly rises above the set temperature, preventing accidental loss of experimental date and specimens emergency thermostat.
Rate of ventilation: Rate of ventilation is determined by measuring the additional power required to oven chamber with opened air vents to keep the set temperature, compared to the case, when the oven is maintained at the same temperature with all vents closed. There is need to seal all the air vents, doors, openings for thermometers and specifically where the ventilator shaft enters in the oven.
Electricity consumption meter watt-hour meters with a resolution of 1. The oven chamber is heated to the test temperature. Measure the ambient temperature at distance of 2 m from the oven, approximately of the oven air inlet at least 1 meter from any solid object. When the oven temperature stabilizes, measure the power consumption for a period of time, such as half an hour. It was subsequently found that the statistical confidence index included in the TEP was not widely understood or used.
However, the statistical tests were considered essential, particularly after minor modifications to make them relate better to practical circumstances: the concept of the halving interval HIC was introduced to indicate the rate of change of ageing time with temperature. TEP was then abandoned, with the TI and HIC being reported in a way which indicated whether or not the statistical tests had been fully satisfied. At the same time, the calculation procedures were made more comprehensive, enabling full statistical testing of data obtained using a diagnostic property of any type, including the particular case of partially incomplete data.
Simultaneously with the development of the IEC series, other standards were being developed in ISO, intended to satisfy a similar requirement for plastics and rubber materials.
These are ISO and ISO respectively, which use less rigorous statistical procedures and more restricted experimental techniques. A simplified calculation procedure is described in IEC Although originally developed for use with electrical insulating materials and simple combinations of such materials, the procedures are considered to be of more general applicability and are widely used in the assessment of materials not intended for use as electrical insulation.
In the application of this standard, it is assumed that a practically linear relationship exists between the logarithm of the time required to cause the predetermined property change and the reciprocal of the corresponding absolute temperature Arrhenius relationship. For the valid application of the standard, no transition, in particular no first-order transition should occur in the temperature range under study.
Throughout the rest of this standard the term "insulating materials" is always taken to mean "insulating materials and simple combinations of such materials". IEC , Standard conditions for use prior to and during the testing of solid electrical insulating materials IEC , Electrical insulating materials — Thermal endurance properties — Part 2: Determination of thermal endurance properties of electrical insulating materials — Choice of test criteria IEC , Electrical insulating materials — Thermal endurance properties — Part 3: Instructions for calculating thermal endurance characteristics IEC all Parts 4 , Electrical insulating materials — Thermal endurance properties — Part 4: Ageing ovens IEC , Electrical insulating materials — Thermal endurance properties — Part Ageing ovens — Single-chamber ovens IEC , Electrical insulating materials — Thermal endurance properties — Part 8: Instructions for calculating thermal endurance characteristics using simplified procedures 1 1 ———————— To be published.
IEC IEC , Guide for the statistical analysis of ageing test data — Part 1: Methods based on mean values of normally distributed test results 3 3.
The abscissa is usually graduated in a non-linear Celsius temperature scale oriented with temperature increasing from left to right.
IEC —9— 3. This standard is concerned only with type 2. IEC 3. IEC — 11 — 4 Synopsis of procedures — Full procedures The standardized procedure for the evaluation of thermal properties of a material consists of a sequence of steps, as follows. It is strongly recommended that the full evaluation procedure, as described below and in 5.
Diagnostic procedures may be non-destructive or destructive determinations of a property or potentially destructive proof tests see 5.
The full experimental and evaluation procedures are given in Clause 5 and as far as 6.
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A simplified procedure is given in IEC The chosen property should reflect, in a significant fashion if possible, a function of the material in practical use. A choice of properties is given in IEC To provide uniform conditions, the conditioning of specimens after removal from the oven and before measurement may need to be specified.
If such material specifications are not available, a selection of properties and methods for the evaluation of thermal endurance is given in IEC If such a method cannot be found, an international, national, or institution standard, or a specially devised method should be used, and in that order of preference. IEC In the majority of cases, the required thermal endurance characteristics are for a projected duration of 20 h.
However, there is often a need for such information related to other longer or shorter times. In cases of longer times, for example, the times given as requirements or recommendations in the text of this standard for example, 5 h for the minimum value of the longest time to end-point shall be increased in the ratio of the actual specification time to 20 h.
In the same way, the ageing cycle durations should be changed in approximately the same ratio. The temperature extrapolation again shall not exceed 25 K. 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, for example, glass transition temperature or partial melting, with consequent non-linearity. Very long specification times may also lead to non-linearity see also Annex A. See IEC 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. 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 in an insulation system.
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. The material specifications or the test standards 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. 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.
IEC — 13 — 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. 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, molding, curing, pre-conditioning, are performed in the same manner for all specimens.
Instructions for an adequate number of specimens are given in IEC Generally, the following instructions 5. 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.
Where the test criterion for non-destructive or proof tests is based upon the initial value of the property, this should be determined from a group of specimens of at least twice the number of specimens in each temperature group. For destructive tests, see 5. However, further guidance will be found in IEC For graphical derivation and in some other cases the treatment of data may be simpler if the number of specimens in each group is odd.
Further guidance will be found in IEC When the criterion is an absolute property level, n d is usually given the value of zero, unless reporting of the initial value is required.
IEC 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. In some cases for example, 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 , the initial value is the arithmetic mean of the test results.
To reduce the uncertainties in calculating the appropriate thermal endurance characteristic, the overall temperature range of thermal exposure needs to be carefully selected, observing the following requirements if the required thermal endurance characteristics are for a projected duration of 20 see also 5.
For some materials, it is not possible to achieve a time to end-point of less than h while retaining satisfactory linearity. However, it is important that a smaller range of mean times to end-point will lead to a larger confidence interval of the result for the same data dispersion. Relevant and detailed instructions on how to proceed using non-destructive, proof or destructive test criteria are provided in 5.
Table 1 gives guidance in making initial selections. A number of recommendations and suggestions, useful in establishing times and temperatures, will be found in Annex B. Unless otherwise specified, IEC shall apply.
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.
IEC 5. However, environmental conditioning, the influence of atmospheres other than air and immersion in liquids such as oil may be important, but these are not the concern of this standard. 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 controlled and equal to the absolute humidity corresponding to standard atmosphere B according to IEC This, or other specified conditions, shall then be reported.
There are two alternative ways to define the end-point value: As a percentage increase or decrease in the measured value of the property from the original initial state. This approach will provide comparisons of materials, but does not include relationship of the property value required in normal operation, as is the case b.
Therefore, the actual numerical value from test in the original material state should be stated. As a fixed value of the chosen property.
This value might be selected with respect to usual service requirements. End-points of proof tests in cyclical ageing are therefore stated in most cases as a fixed values of the property e. The chosen property should reflect, in a significant fashion if possible, a function of the material in practical use.
A choice of properties is given in . By standard  to provide uniform conditions, the conditioning of specimens after removal from the oven and before measurement may need to be specified.
If the test specimens size or the specimens forms are dependent on the temperature exposure, then test methods that are not depend on these effects shall be used. If 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 . For detructive and non-destructive tests, for each exposure temperature and for each heat ageing period, the value of the chosen property is plotted as a function of the logarithm of the time Figure 3.
Cigré brochure 323 ageing of cellulose in mineral
The point at which this graph intersects the horizontal line representing the end-point criterion is taken as the time to failure. The end-point, limit value should be chosen so that describes the insulating material deterioration degree, which reduces the insulation system ability to withstand stresses occurring in the normal use.
The degree of deterioration, which is herein referred as threshold tested value, should be close to allowable material properties safe value, which is required in practice. The accuracy of endurance test results depends largely on the number of specimens aged at each temperature.
Instructions for an adequate number of specimens are given in . Figure 3: Determination of the time to reach the end-point at each temperature — property variation according to IEC For selection of temperatures and exposure times apply different rules.
As mentioned above, to determine TI should be specimens exposed to at least three, however preferably four temperatures, which cover wide enough temperature range to establish Arrhenius relationship between the times to reach threshold value for a given material and reciprocal thermodynamic absolute temperature. Determination of the relative temperature endurance, i.
If, after ageing, the results for the reference EIM are found to be significantly different from earlier experience, this may indicate changes in material or equipment.Relatively stringent requirements are set for these products in line with the expected harsh usage conditions.
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The end-point, limit value should be chosen so that describes the insulating material deterioration degree, which reduces the insulation system ability to withstand stresses occurring in the normal use. This approach will provide comparisons of materials, but does not include relationship of the property value required in normal operation, as is the case b.
The large statistical scatter of test data which was found, together with the frequent occurrence of substantial deviations from the ideal behavior, demonstrated the need for tests to assess the validity of the basic physico-chemical model. However, the statistical tests were considered essential, particularly after minor modifications to make them relate better to practical circumstances: the concept of the halving interval HIC was introduced to indicate the rate of change of ageing time with temperature.
In the case of proof tests with incomplete data usually censored at the median , it may be possible to obtain a sufficient increase in data group size by continuing the exposure until further test specimens have failed the proof test.
From thermal endurance data obtained for reference control material is determined the time, at which is TI equal to the known value — a correlation time. These tests have been designed to test all important aspects of the data which might invalidate derivation of thermal endurance characteristics, as well as to decide whether a failure to satisfy the statistical requirements is of practical significance.
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