The secondary side is absolutely not allowed to open the circuit, because once the open circuit, the primary side current I 1 all become magnetized current, cause M and e 2 sudden increase, cause core oversaturation magnetization, heat serious and even burn out the coil; at the same time, after magnetic circuit oversaturation magnetization, increase the error.

When the current transformer is working normally, the secondary side is connected in series with the current coil such as the measuring instrument and relay, the impedance of the current coil such as the measuring instrument and relay is very small, and the secondary side is approximately short circuit. The magnitude of CT secondary current is determined by the primary current. The magnetic potential produced by the secondary current is balanced by the magnetic potential of the primary current. If it is opened suddenly, the exciting electromotive force will change from a very small value to a very large value, and the magnetic flux in the iron core presents a heavily saturated flat-top wave. Therefore, the secondary side winding will induce a very high peak wave when the magnetic flux passes through zero, its value can reach thousands or even tens of thousands of volts, endangering the safety of staff and the insulation performance of the instrument. In addition, the secondary side open circuit makes the secondary side voltage up to several hundred volts, once touched will cause electric shock accident. Therefore, the secondary side of the current transformer is equipped with a short-circuit switch to prevent the secondary side open circuit. In the use of the process, the secondary side once the open circuit should be removed immediately circuit load, and then, power cut processing. Don’t use it again until everything is in order.