Creep behaviour of commercial glass fiber filled thermoplastics for use in medium voltage metal enclosed switchgear

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.