The heat distortion test evaluates the ability of an insulator or jacket to maintain its physical structure without deformation under elevated temperature conditions. Distortion of the insulation or outer jacket can create an uneven insulative surface on a conductor; areas with reduced insulation coverage are more susceptible to damage. Distortion also increases the possibility for air bubbles to form within the insulation material, leaving the wire or cable more vulnerable to partial discharge.
The test set-up varies slightly by sample size but the general procedure remains largely the same. Each sample is conditioned in an oven, 1 hour on its own, 1 hour in the thickness gauge jig. Thickness measurements are made both prior to and after the oven conditioning and the percent distortion is calculated.