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
This value is searched RTE . IEC standard  covers the minimum requirements for ventilated and electrically heated single-chamber ovens, with or without forced gas circulation, for thermal endurance evaluation of electrical insulation. It covers ovens designed to operate over all of part of the temperature range from 20 K above ambient to K. Next appropriate standard  covers the minimum requirements for precision ovens for thermal endurance evaluation of electrical insulating materials and other appropriate applications.
It covers ovens designed to operate over all or part of the temperature range from 20 K above room temperature up to K. Last standard for ageing ovens  covers the minimum requirements for ventilated and electrically heated multi-chamber ovens used for thermal endurance evaluation of electrical insulation and of any other appropriate thermal conditioning application where the use of single-chamber ovens is inappropriate.
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. All sealer elements are then removed, the input and output vents are estimated partially open in order to achieve the desired rate of air exchange.
Measure the power consumption for about half an hour, the same way as before. If necessary, change the ventilation opening apertures position and measurement is repeated until the required air exchange rate is reached.
Rates in the range 5 to 20 changes per hour shall be made available through the exposure chamber. Temperature change: Nine thermocouples is placed into the empty oven, composition of iron-constantan or chromel-alumel, made from wires with a diameter of 0,5 mm, while wires connection should not be longer than 2,5 mm; the ventilation openings and apertures are set to achieve the desired air exchange from 5 to 20 times per hour. 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.
Prepare a number of specimens following the instructions in 5. If necessary, determine the initial value of the property as specified in 5. Divide the specimens by random selection into as many groups as there are exposure temperatures. Establish the exposure temperatures and times in accordance with the instructions of 5.
Place one group for exposure in each of the ovens complying with 5. NOTE 1 It is suggested that individual specimens be identified to simplify their return to the correct oven after each test. NOTE 2 Attention should be given to the recommendation in 5.
Some test properties may require measurement at the oven temperature, in which case the ageing is continuous.
Apply the appropriate test to each specimen and then return the group to the oven from which they came, at the same temperature as before, and expose for a further cycle. Continue the cycles of temperature exposure, cooling and application of the test until the average measured value for the specimens in the group has reached the end-point specified and provided at least one point beyond the end-point.
IEC Evaluate the results as listed in 6. At the end of each cycle, remove all specimens from the oven. After each removal, allow the specimens to cool to room temperature and then subject each one to the specified proof test.
Return specimens which have withstood the proof test to the oven from which they came, at the same temperature as before, and expose for a further cycle.
If the results show that this time to end-point is likely to be reached in about 10 periods of exposure, there is no need to alter the period of exposure originally selected. If the results do not show this, the period may be changed so that the median result may be expected in at least seven cycles preferably about 10 provided this change in cycle time is made before the fourth cycle.
The cycles of temperature exposure may be continued until all specimens have failed, so that a more complete statistical analysis may be made see IEC Evaluate the results, as listed in 6. See 5. After each removal, allow the group of specimens to cool to room temperature unless otherwise specified. For materials in which a significant variation of properties with temperature or humidity is expected, unless otherwise specified, condition the specimens overnight in standard atmosphere B of IEC Test the specimens and plot the results and the arithmetic mean of the results or a suitable transform thereof against the logarithm of exposure time as given in IEC The analysis of TI data is based on the assumption that there is a linear relation between the logarithm of the time to end-point and the reciprocal of the thermodynamic ageing temperature.
The method of evaluation of TI results is by the numerical procedure detailed in IEC together with a graphical presentation as shown in Figure 1. A simplified procedure is available in IEC The thermal endurance of an electrical insulating material is always given for a specific property and end-point. If this is disregarded, any reference to thermal endurance properties ceases to be meaningful since the properties of a material subjected to thermal ageing may not all deteriorate at the same rate.
Consequently, a material may be assigned more than one temperature index or halving interval derived, for example, from the measurement of different properties.
If a time different from 20 h has been used for deriving the TI, the relevant time expressed in kh shall be stated, followed by kh. Times to end-point, x - and y -values General 6.
From these values, if necessary by interpolation see Figure 2 , obtain the time to end-point and calculate its logarithm as the y -value to be used in 6. A time to end-point within the first ageing period shall be treated as invalid. In this case, either a start again with a new group of specimens, or b ignore the specimen and reduce the value ascribed to the number m i of specimens in group i by one. If the end-point is reached for more than one specimen during the first period, discard the group and test a further group, paying particular attention to any critical points of experimental procedure.
IEC to end-point are calculated using a mathematical procedure described in detail in 6. This procedure is based on the assumption that the ageing rate of all specimens aged at one temperature is the same and can therefore be determined from the ageing rate of the property means of the successive groups tested. An approximately linear region of the ageing graph is selected Figure 3 and a line parallel to the mean ageing graph drawn through each time, property point.
The intercept of this line with the end-point line gives the logarithm of the required time to end-point see Figure 4. NOTE The ageing graph is formed by plotting the value of property or a suitable transform of its value against the logarithm of the exposure time.
It is necessary to ensure that the intercept of the regression line with the time axis gives the same value as the mean of the intercepts of the individual lines. The procedure is carried out numerically, and appropriate statistical tests are introduced.
The y -values derived are used in the calculations of 6. Figure 4 — Destructive tests — Estimation of time to end-point 6. BS EN ? IEC For destructive tests, the same procedure shall be used, applied to the hypothetical values of y obtained as in 6. The regression analysis for slope and intercept of the thermal endurance graph and the tests for deviations from linearity shall be as in 6. 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.
A simplified procedure, requiring a limited subset of the following tests is reported in IEC If either of these conditions is not met, the value of TI cannot be reported.
In order to carry out valid calculations, one or more further group s of specimens shall be aged at such a lower temperature as will enable the conditions to be met. This difference is dependent on the scatter of the data points, the deviations from linearity in the regression analysis, the number of data points and the extent of extrapolation.
The general calculation procedures outlined here and detailed in IEC are based on the principles set out in IEC These may be briefly expressed as follows see 3.
IEC — 23 — 2 The values of the deviations of the logarithms of the times to end-point from the linear relation are normally distributed, with a variance which is independent of the ageing temperature. The first assumption is tested by the so-called Fisher test F -test. In this test, a test parameter F is calculated from the experimental data and compared with a tabulated value F 0. If not, the assumption is a priori rejected, but, since in special cases it is possible to detect a statistically significant non-linearity which is of little practical importance, the calculations may, under specified conditions, be continued in a modified way for details, see IEC In the case of destructive tests 6.
Calculation procedures and suitable restrictions have been developed to meet these circumstances and are given in detail in IEC A flow chart and decision table setting out the procedures and conditions are given in Annexes A and B of IEC Failures at the end of the first ageing cycle cannot be accepted.
Either a new group, possibly with a shorter cycle time, should be started, or the first cycle failure ignored and the nominal size of the group reduced by one for example, a temperature group of 21 would be treated in the mathematical process as 20; see 6. In either case, the specimen preparation technique should be carefully examined.
In all cases, ageing shall be continued until more than one-half of the test specimens in each group have failed to pass the proof test. It is not necessary for all groups to be equal in size or for equal numbers to have failed. It is permitted for these conditions not to be satisfied in specified circumstances either a small extrapolation or linearity test at significance level 0, may be permitted; see 6.
IEC Thermal endurance graph and thermal endurance characteristics Calculate the temperature? In the same way, calculate the temperature? The difference? Calculate the temperature? Using the points? Using Equations 46 to 50 of 6. Plot these time-temperature pairs on the thermal endurance graph and draw a smooth curve passing through these points. On the same graph, plot the ageing temperatures, the times to end-point measured or hypothetical , and the mean times.
The thermal endurance characteristics are as derived in the calculations of 6. NOTE 1 This table is intended primarily for cyclic proof testing and non-destructive tests, but may also be used as a guide for selection of suitable time intervals for destructive tests.
In this case, cycle times of 56 days, or even more, may be required. NOTE 2 When extending the test program by submitting additional specimens to ageing at temperatures below the lower of the originally planned ageing temperatures, a temperature interval of 10 K and a cycle duration of 42 days for TI determination should be considered. If the data dispersion is not high through inadequate experimental technique, the effect of the high dispersion can be overcome by the use of a larger number of data values, i.
This does not necessarily imply a complete repeat of the experimental work, since it is possible if material is available to test further specimens and add the results to the original data. These further tests may be at lower or intermediate temperatures but should not generally be at higher temperatures than originally selected.
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. The size of the confidence interval is roughly proportional to the square root of the reciprocal of the total number of data values.
This model is valid when the selected end-point of the diagnostic property is correlated with a particular degree of molecular change in the material which is subject to ageing. The validity of the model is, therefore, not dependent on the more stringent condition of a linear relationship between the level of the diagnostic property and the degree of molecular change.
In addition to the above-mentioned basic assumption, some general assumptions regarding the chemical mechanisms of the thermal ageing need to be satisfied: a the material or combination of materials should be uniform in the macro-physical sense; b the thermal degradation should proceed in a homogeneous phase; c the ageing reaction should be essentially irreversible.
It may arise from inadequate experimental technique for example, oven temperature errors ; such non-linearity may be corrected by further testing.
However, in many cases, the deviations arise from the ageing behavior of the material; this happens with many thermoplastic materials or other materials where the ageing temperature range includes, or is close to, a transition temperature of some kind, or where there is more than one ageing mechanism at work.
In such cases, it may be possible to obtain an acceptable result by further testing at a lower temperature. This will have the effect of decreasing the extrapolation, which is one of the influences in determining the size of the confidence interval, and also make the errors associated with the non-linearity less serious. IEC — 27 — It is also possible that acceptable results will be obtained where further testing at a lower temperature has been carried out by removing the results at the highest temperature s , since the deviations may only become significant at the higher temperatures.
If these expedients are not successful, it will be necessary to test at a temperature low enough for extrapolation not to be required. The row in table 1 corresponding to the estimated TI shows suggested ageing times in days at oven temperatures which appear at the head of the respective columns. Early results of the ageing test may motivate an adjustment of ageing cycles or additional ageing temperatures.
It is advisable to distinguish between: — — cyclic and continuous ageing; destructive, non-destructive and proof deterioration. If this rule results in changes of mechanism for example, when a transformation point like melting or softening is exceeded , then the maximum exposure temperature will need to be limited. In such cases, or if the value of HIC is known or expected to be less than 10 K, the difference between the levels of ageing temperature may need to be reduced, but to not less than 10 K so that oven temperature tolerance effects will be acceptable.