Faurn 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. Unless otherwise specified, IEC shall apply. In this case, cycle times of 56 days, or even more, may be required. A simplified procedure is available in IEC 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. This publication does not purport to include all the necessary provisions of a contract.
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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.
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. This may be investigated and possibly corrected. In any case, the simultaneous ageing of reference and candidate will at least partially compensate for systematic changes.
RTE has been introduced in the updated standard  to reduce systematic errors in the TI determination. 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. From thermal endurance data obtained for test unknown material is TI in correlation time caluclated.
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.
Eight thermocouples are placed into the eight oven corners at a distance of 50 mm from the wall. The ninth thermocouple is placed at the geometric center of oven.
At least mm thermocouple wire must be inside the oven to reduce the conduction heat dissipation from the thermocouple. The oven chamber is then heated to the maximum operating temperature, fixed for a minimum of 16 hours.
During the one complete thermal cycle the temperature of all nine thermocouples is measured with an accuracy of 0. Sufficient number of measurement is made, so during one cycle is possible to determine the maximum, the minimum and the average temperature of each thermocouple. This value is recorded as set-up oven temperature.