The thermal-mechanical test is one of the most significant tests for composite insulators. Here we have a partial section of International Standard IEC 61109 2008: Insulators for overhead lines – Composite suspension and tension insulators for a.c. systems with a nominal voltage greater than 1 000 V – Definitions, test methods, and acceptance criteria.
IEC standard has been accepted and used by a great number of countries to test composite insulators and its fittings. We will quote part of its test, including test specimens, test methods, and acceptance criteria so that we can more about the thermal-mechanical pre-stress test of composite insulator:
In section 10.3.2 Thermal-mechanical pre-stress, the standard writes:
“Before commencing the test, the specimens shall be loaded at the ambient temperature by at least 5 % of the SML for 1 min, during which the length of the specimens shall be measured to an accuracy of 0,5 mm. This length shall be considered to be the reference length. The specimens are then submitted to temperature cycles under a continuous mechanical load as described in Figure 1, the 24 h temperature cycle being performed four times. Each 24 h cycle has two temperature levels with a duration of at least 8 h, one at (+50 ± 5) °C, the other at (–35 ± 5) °C. The cold period shall be at a temperature of at least 85 K below the value actually applied in the hot period. The pre-stressing can be conducted in air or any other suitable medium. The applied mechanical load shall be equal to the RTL (at least 50 % of the SML) of the specimen. The specimen shall be loaded at ambient temperature before beginning the first thermal cycle. NOTE The temperatures and loads in this pre-stressing are not intended to represent service conditions, they are designed to produce specific reproducible stresses in the interfaces on the insulator. The cycles may be interrupted for maintenance of the test equipment for a total duration of 2 h. The starting point after any interruption shall be the beginning of the interrupted cycle. After the test, the length shall again be measured in a similar manner at the same load and at the original specimen temperature (this is done in order to provide some additional information about the relative movement of the metal fittings). ”
To do such a test, the relevant testing machine is essential. The thermal-mechanical testing machine is needed for heating up the composite insulator as required above and turn cycle of temperature change for periods. The “thermal-mechanical tester” produced by Contune International, the leading testing machine supplier, is able to test composite insulators, ceramic disc insulator strings as per IEC or ANSI requirements on this test, the process of which is controlled by computer for ambient temperature, specific mechanical load, time and cycles. To meet different demands of users, this machine can even be customized to test one to four strings of insulators at one time so that the test efficiency will be high and users can save much time on the test of multi-specimens situations.
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