Finite element modelling of the electrical impulse induced fracture of a high voltage varistor

In testing and in service, varistors are subjected to very short (μs range) high current pulses. Due to the inertia effects that appear on rapid Joule heating dynamic stress waves are generated, which can cause brittle failure. An analytical solution for the one-dimensional case was presented recently by Vojta and Clarke. In this work a full three-dimensional analysis of an axisymmetrical varistor has been performed using Finite Element Simulation. The reflections of the stress waves from the bases and the shell of a varistor and their interference are analyzed. The resulting stress field and its development with time is much more complex than in the 1D case. The aspect ratio of the varistor has been shown to have a strong influence on the amplitude of the mechanical stresses and can be varied to minimize the maximum stress level reached. Damping has been considered but found to be negligible in realistic cases.