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A simple thermo-mechanical model is applied to evaluate the influence of the nonuniformity of ZnO varistor disks used in surge arresters on their energy handling capability. Puncture is the dominating failure mode for slightly nonuniform disks, but cracking becomes more likely as the degree of nonuniformities increases. It is shown that minimization of the chance of a failure of varistor disks at high-current pulses can be achieved by adjusting their resistivity in the upturn region of the -characteristic. Simulation of the behavior of varistor disks under high-current 4/10 s pulses required by the ANSI standard tests shows that these tests provide very little information about the actual energy handling capability of the disks. This conclusion suggests that alternate test methods should be developed and included in the relevant standards.

Thermal and mechanical behavior of high power ZnO surge arresters under current pulses of various magnitude and duration is simulated. By solving heat transfer equations for a varistor disk with nonuniform electrical properties, we compute the time dependence of the temperature profile and the distribution of thermal stresses. The simulations identify failure processes and determine energy handling capability of varistor elements as a function of the applied current. The results are in good agreement with the experimental data and explain the energy handling dependence upon the surge intensity and duration.