Thermoplastics offer several favorable properties that make these materials an interesting alternative for use in metal enclosed medium voltage switchgear. In general they have a high dielectric strength, and by filling the polymer matrix with glass fibers a good mechanical strength can also be obtained. Injection molded dumb-bell shaped samples from four different commercially available thermoplastics were subjected to 10 MPa tensile stress at 105 and 135 degrees centigrade for up to six months, and the creep of 75 mm long sections was recorded during this period with an accuracy in the micrometer range. The creep showed a logarithmic relationship with time, and the elongation after a continuous mechanical stress throughout the 30 years life time of the switchgear is estimated to be well below 0.5% for the materials tested. Hence, the creep properties for these thermoplastics are found to be suitable for use as structural components in medium voltage switchgear, including in the parts that maintain the high contact force on a closed vacuum interrupter over the entire life time.

During lightning impulse type testing of medium voltage switchgears up to two spark-overs are accepted. Consequently, when selecting conducting composites for use in screens and shields of such equipment it is important that their surfaces not degrade after a spark-over, as this normally will reduce the overall dielectric withstand level. Lightning impulse tests where one electrode is coated with polycarbonate based conducting composites have been carried out. With conductive filler of stainless steel fibers heavy surface damage occurs during spark-overs, causing fibers to stick out and thereby lowering the dielectric strength. Carbon fibers filler shows a similar behavior, but to a much lesser extent. For polycarbonate filled with carbon powder some kind of conditioning effect takes place, causing the dielectric strength of the system to increase, and no surface damage is observable even after dozens of spark-overs.