PURPOSE: To enable the withstanding of a high voltage after the conduction of a half wave, by blowing an SF6 gas against a gap while a thin carbon rod short-circuited between electrodes with the conduction of the half wave is sublimated.

CONSTITUTION: Prior to the testing of a breaker, when a command is sent to an photoelectric converter O/E through an optical fiber 41 to attract a mobile iron piece 38 with an electromagnet 40, a disc 35 starts rotation with a motor M, a gear box G and a wheel 36. A thin carbon rod 36 housed in a groove on the circumference of a cylinder 30 dips into a dent 44 by its own gravity when matching the position of a tiny hole 43 provided at an electrode 20 to cause a short-circuiting between electrodes. An air flow is introduced between the electrodes from a pressure generating section through an insulation nozzle 46. Thereafter, as a voltage source current is fed to a sample breaker 1, the carbon rod sublimates instantaneously, allowing current to conduct between the electrodes 20 and 21 in arc near the subsequent zero point but the current is shut down at the next zero point thereof. Thus, a high voltage developing between the electrodes afterward can be withstood.