The standard EN IEC 60216-1 Electrical insulating materials - Thermal endurance properties - Part 1: Ageing procedures and evaluation of test results, which is based on a European standard and subsequently published by the International Electrotechnical Commission (IEC), describes the general ageing conditions and procedures to be used to derive thermal endurance properties and provides guidance on using the detailed instructions and guidelines in other parts of the standard. Although originally developed for electrical insulating materials and simple combinations of such materials, the procedures are considered to have more general applicability and are widely used in the evaluation of materials not intended for use as electrical insulators.
In the application of this standard, a practically linear relationship is assumed between the logarithm of the time required to cause a specified property change and the inverse of the corresponding absolute temperature (Arrhenius relation). For the valid application of the standard, no transitions, especially no first-order transitions, should occur in the temperature range examined. In the remainder of this standard, the term insulating materials always means insulating materials and simple combinations of such materials.
Each test procedure shall specify the shape, dimensions and number of test specimens, the exposure temperatures and times, the property to which the temperature index relates, the determination methods, the end point and the derivation of the thermal endurance properties from experimental data. The property selected shall, if possible, reflect to a significant extent a function of the material in practical use. A selection of properties is also given in IEC 60216-2 Electrical insulating materials - Thermal endurance properties - Part 2: Determination of thermal endurance properties of electrical insulating materials - Selection of test criteria. To ensure uniform conditions, it may be necessary to specify the conditioning of the specimens after removal from the furnace and before measurement.
Selection of test properties for the temperature index If IEC material specifications are available, property requirements are usually given in terms of acceptable lower limits of temperature index values. If such material specifications are not available, a selection of properties and methods for assessing thermal resistance is included in IEC 60216-2. If no such method is available, an international, national or institutional standard or a specially developed method shall be used and shall be given in this order of preference.
In most cases, the required thermal endurance properties are for an estimated time of 20 thousand hours. However, such information is often required for other longer or shorter times. For example, in the case of longer times, the times given as requirements or recommendations in the text of this standard (e.g. 5 thousand hours for the minimum value of the longest time to the end point) should be increased by 20 thousand hours of the actual specification time. Similarly, the ageing cycle times should be varied by approximately the same amount. The temperature extrapolation should again not exceed 25 K. In the case of shorter specification times, the relevant times can be reduced by the same amount if necessary. Special care is required for very short specification times, since higher ageing temperatures can lead to non-linear temperature regions, which include transition points, such as the glass transition temperature or partial melting, for example. Very long specification times can also lead to non-linearity.
The thermal durability of materials may need to be characterized with different durability data (derived using different properties or endpoints) to facilitate adequate selection of the material with respect to its specific application in an insulation system.
There are two alternative ways in which the endpoint can be defined:
The end point should be selected to indicate the degree of deterioration of the insulation material which reduces the ability of the insulation system to withstand a stress encountered in actual service. The degree of deterioration specified as the end point of the test should be related to the safe value that is practically permissible for the desired material property.
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