In order to study the operating characteristics of the current transformer, and confirm whether it will be full and affect the correctness of the maintenance operation when maintaining external faults from large currents, it can be tested by some testing methods. The direct test method is to bear a practical load on the secondary side, apply current from the primary side, and investigate the secondary current to find the full point of the current transformer. However, for maintenance-grade current transformers, the full point may exceed 15 to 20 times the rated current. When the current transformer becomes larger, it may be difficult to test on site. In addition, the full point of the current transformer can be measured through the volt-ampere characteristic test.

The fullness of the current transformer is the full current of the current transformer calculated because of the excessive magnetic flux density of the iron core. The test method of the volt-ampere characteristic is: the original side is disconnected, the current passes through the secondary side, and the voltage drop on the secondary side winding is measured. Because the primary side of the current transformer is open circuit, the primary side current has no demagnetization effect, and the iron core is easily full under small currents. Therefore, the volt-ampere characteristic test does not need to increase the large current, which is relatively easy to implement on site.