why should all non-test items be short-circuited and grounded?

Why do all non-test items need to be short-circuited and grounded when testing the on-load tap-changing switch?
Because the equivalent model of the transformer is equivalent to an R, L series network, DC charging is used during the on-load test. Since the current at both ends of the inductor cannot change suddenly, a long charging time is required to saturate the inductor and stabilize the current. For example, the main transformer may take more than half an hour. In order to quickly saturate the inductor coil and speed up the test, the medium-voltage side and low-voltage side of the transformer can be short-circuited. This will accelerate the saturation of the transformer and achieve the purpose of rapid testing. During the injection of current, induced voltage will be generated on the medium and low-voltage sides. To ensure personal safety, short-circuiting and grounding are required.