The method of using applied force of magnetic field to current is called magnetic quenching. When a larger electric power is required to blow the arc into the arc extinguish chamber, a special magnetic blow coil is adopted, which usually has a turn to several turns. In order to make the magnetic field more concentrated in the arc region to increase the blowing force, the center of the coil has a core, in the two ends the arc-extinguishing chamber are arranged the guide plate in parallel. Magnetic blow-out coil connects with contact circuit in series, the current through the coil is the arc current, and coil current generates the magnetic flux, through the core, guide magnetic clamp, forming circuit between the two contacts. A strong magnetic flux is generated between the two contacts, and the arc is driven by force in the magnetic field, leaving the contact quickly results in stretching and cooling on the arc angle, to force the arc extinguishing.