Comparative tests on thermal endurance of enamelled copper and aluminium wires, using the conventional ageing test procedure and the equalized ageing process method

1) In spite of the complicated ageing behaviour of wire coatings the EAP method produces results of satisfactory reliability. 2) Effect of the base metal on the thermal endurance of the coating varies very much, depending on the coating type. Whereas the effect on poly(vinyl acetal) seems to be negligible, it is very significant on AI overcoated polyester, giving to Aluminium wires 15 ? 20 K higher rating than to Copper wires. Effect on poly(ester imide) was found to be temperature-dependent, the difference diminishing towards lower temperature and probably being insignificant at the service temperature range. 3) Atmospheric humidity in the air surrounding the ageing ovens may have a very significant influence on ageing test results, especially on condensation polymers. Undoubtedly this may be one reason for the large scatter of twist test results sometimes obtained; in addition it may distort even the shape of the Arrhenius plot, e.g. due to seasonal variations in the absolute humidity. As a consequence, some kind of humidity control should always be used when ageing tests on materials susceptible to hydrolytic degradation are performed.