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An investigation of the temperature and time dependencies of polarization currents in zinc oxide varistors reveals two thermal activation energies that are present at 8 and 140 million electron volts and an additional thermally activated level at 0.66 electron volts. The investigations are performed up to high temperatures, with greater precision of measurement. The measured values are plotted on an Arrhenius graph that yields thermal activation energies. The results are consistent with theoretical predictions.