Positive voltage spikes caused by a fast decrease of the plasma current are observed in runaway dominated discharges in the small Brazilian Tokamak TBR-1[Kuznetsov et al., Phys. Plasmas 6, 4002 (1999)]. Comparison of the measured voltage spike with the value given by the solution of the one-dimensional diffusion equation for the toroidal electric field permits one to infer the plasma conductivity and initial electric field distribution. An additional runaway generation due to voltage spikes can be important. In this case, both the decay time of the voltage spike and the total toroidal current perturbation can decrease substantially.