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The voltage generated across a zinc-oxide (ZnO) element, in which a lightning impulse current flows, has been computed with the finite-difference time-domain (FDTD) method. Also, the distribution of temperature in the ZnO element has been computed on the basis of FDTD-computed conduction current densities with a discretized heat equation. The ZnO element having a thickness of 37 mm and a diameter of 34 mm is represented with many 1 mm × 1 mm × 1 mm cubic cells, each of which has a nonlinear resistivity in each of the x , y, and z directions dependent on the electric field in each direction and the temperature in each cell. Waveforms of voltage appearing across the ZnO element and the temperature on the side surface of the ZnO element computed for a short lightning impulse current having a magnitude of 66 kA agree reasonably well with the corresponding measured ones.

Electromagnetic fields and the heat of a metal oxide varistor (MOV), in which a lightning impulse current flows, are calculated using the finite-difference time-domain (FDTD) method. The MOV is represented with small rectangular parallelepiped cells, each of which has a resistivity dependent on electric field and temperature. For this purpose, the expression of resistivity as a function of the electric field, proposed previously, is extended to include the dependence on temperature. The temperature dependence is based on voltages across an MOV for impulse currents of 0.5 to 10 kA at temperatures in a range from about 300 to 900 K, measured by Andoh et al. (2000). FDTD-calculated waveform of voltage across the MOV agrees well with the corresponding measured one for a short impulse current with a magnitude of about 4 kA and a duration of about 30 μs. In addition, the temperature on the surface of the MOV agrees well with the corresponding measured one. Further, calculations are carried out for the MOV with a nonuniform resistivity distribution, which roughly simulates deterioration or degradation of the MOV, for a long duration current having a magnitude of 5 kA. The proposed expression of resistivity, given as a function of electric field and temperature, is useful in studying electro-thermal calculations, which can provide insights into causes of MOV damages.