Mutual inductor, also known as instrument transformer, is the general name of current transformer and voltage transformer. It can turn high voltage into low voltage and large current into small current, so it can be used for measurement or protection system. Its main function is to convert high voltage or high current into standard low voltage (100V) or standard low current (5A or 1A, referring to rated value), so as to realize standardization and miniaturization of measuring instruments, protective equipment and automatic control equipment. At the same time, mutual inductor can also be used to separate high voltage system to ensure the safety of the body and equipment.