In the case of a sudden drop in current, the transformer core may generate residual magnetism. For example, the Electrical Current Transformer suddenly cuts off the power supply and the secondary winding suddenly opens under the condition of high current. The core of the transformer has residual magnetism, which reduces the permeability of the core and affects the performance of the transformer. All transformers after long-term use should be demagnetized. Demagnetize the transformer before inspection. Demagnetization is the alternating excitation current through the primary or secondary winding to give the iron core an alternating magnetic field. Gradually increase the alternating magnetic field (excitation current) from 0 to make the iron core saturated, and then slowly reduce the excitation current to zero to eliminate remanence.