The iron core made of nanocrystalline ribbon has high magnetic permeability and low coercivity. It is especially sensitive to leakage current in the initial stage of magnetization. The leakage switch made with this iron core has a small operating current. At the same time, it has good resistance to high current impact. Because the nanocrystal um is an order of magnitude higher than that of permalloy and has high sensitivity, the number of primary winding turns can be reduced for leakage switches that require the same sensitivity, thereby reducing the volume of the leakage switch accordingly. It not only saves costs but also meets technical requirements.