shows the partial discharge occurred by loosened metal bolts in the cable head of power equipment inside, and the part connected to the bus bar. When the signal pattern shown in Fig. 5 occurs [13 , 14] , the metal and its connecting part should have been intensively checked. The measurement signals were successively measured and stored by PRPD method during the 1,000 cycles. The unit of the vertical axis is 100 mV/div, the unit of the horizontal axis is 2 ms/div, and the synchronous signal of 60 Hz and the measured signals are shown by nesting.