Safety and stability of the electric power supply becomes more and more essential and crucial. Earthing method of the neutral point is one of the most important and effective factors for the above targets. With the increase of number and capacity for the power distribution cable lines , the single phase earthing fault current will increase greatly according to the line to earth capacitance. and the arc is hard to extinguish that will cause over voltage easily and lead to trip of power supply frequently. Sieyuan Electric is the only company to produce tap-change，capacitance-adjustable，thyristor phase-controlled arc suppression coil equipment，the market share is the top one in the world.