If the current transformer is open on the secondary side, the primary current will be used for excitation and the core will be severely saturated. The alternating magnetic flux will induce a very high voltage on the secondary coil, and its peak value can reach several thousand volts or even tens of thousands of volts. Such a high voltage acts on the secondary coil and the secondary circuit, which will seriously threaten personal safety and The safety of the equipment, even if the coil insulation is burned out due to overheating, the protection may not reflect the fault due to no current. For differential protection and zero-sequence current protection, the unbalanced current may cause malfunction due to open circuit. Therefore, the "Safety Regulations" stipulate that the current transformer is strictly prohibited to open during operation.
Why is the voltage transformer not allowed to open? Solution: The impedance inside the voltage transformer is very small. If the secondary circuit is short-circuited, a large current will appear, which will damage the secondary equipment and even endanger personal safety.