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The current form of partial discharge (PD) in a void was divided into fast and slow components, with short and long fall times, respectively. Comparing these with the current shape of PD in a void between two metal surfaces, in which the slow component was not observed, it was suggested that the formation of fast component in a usual void was due to the instantaneous discharge between the discharge areas of the PD, i.e. the region to which the carriers in a PD process can instantaneously propagate, and the slow one might be due to the low conductivity in the regions outside the discharge areas. An equivalent circuit was proposed to describe the effect and numerical simulations were carried out. It was suggested that the fall part of the first PD after polarity reversal might reflect the surface conductivity change in aging process.