Abstract: No whole-stack flashover has been observed under the working voltage of about 2 MV in the PTS vacuum insulator stack, because of the uniform voltage distributions on different insulator rings, the effects of azimuthal transit-time and multi-stage structure. In this paper, two typical abnormal voltage waveforms are interpreted to be caused by the different flashover locations. The experimental results preliminarily validate the rules of transmission, attenuation and composition of transient electromagnetic waves in different mediums, caused by flashover. The stochastic occurrence of flashover presents a certain extent systemic discrepancy in two modules along with the increasing of experiment time, due to deposits accumulation on the surface of the insulator rings.