PURPOSE:To increase the efficiency in welding by operating majority time and minority time mean values on each arc short circuit period, then by performing the previewing of a burn through based on the ratio of the majority time mean value to minority time mean value. CONSTITUTION:A counter 2 and latch part 3 are provided for the short circuit detecting part 1 which detects the short circuit of an arc. The short circuit detecting part 1 generates the short circuit detecting signal by receiving the arc voltage, sends out a gate signal to the counter 2 while the arc generating term and feeds a latch signal to the latch part 3 and an interrupt signal to CPU 4 as well. The counter 2 counts the clock that an oscillating part 5 outputs and the short circuit period is stored in the latch part 3. The CPU 4 operates the mean value and ratio of the short circuit period of minority time and majority time and previews the burn through by the comparison of the ratio thereof with the prescribed threshold value. With this method the prevention of the burn through is made possible and the welding efficiency is increased.
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