The origin of the frequency-dependent Mott–Schottky behavior observed in a wide range of ZnO-Bi2O3 varistor systems has been investigated. Lumped parameter/complex plane analysis of two-probe ac electrical data indicates that several trapping relaxations contribute to the measured MOV grain-boundary admittance in the frequency range, 10−2 Hz≤f≤107 Hz. Furthermore, this approach allows the development of an equivalent circuit representation which incorporates these trapping phenomena in a systematic manner.

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The ac electrical response of the grain boundary in the ZnO–Bi2O3‐based varistor system (in the frequency range 10−2≤f≤107 Hz) has been analyzed using complex plane analysis techniques as a function of temperature (90°≤T≤150°C) and electric field (E60%lmA/cm2≤E≤ElmA/cm2). The high‐temperature/field data were best represented in the impedance plane. The lumped parameter equivalent circuit representation, containing a voltage variable resistive element associated with the grain boundary, was in agreement with a model previously reported in the literature. The resulting equivalent circuit representation of the grain‐boundary response consisting of a resistance and a capacitance are interpreted in terms of various phenomena occurring at the electrical thickness of the grain boundary.

The conditions leading to thermal runaway in metal oxide varistors are discussed in terms of its power dissipation. An equivalent circuit is used to rationalize the similarities and differences between the DC and 60 Hz ac behavior.