(3) Select the rated primary current of the current transformer according to the current of the tested line; select the rated secondary current of the transformer according to the rated current of the measuring instrument.

Like voltage transformers, current transformers and current measuring instruments have minimal error around their rated values. Therefore, the rated primary current of the current transformer should be equal to or slightly larger than the measured current, and the rated secondary current should be equal to the rated current of the measuring instrument. If the magnitude of the measured current varies, the variation range should be 10%. 120% of the rated current range, otherwise a multi-variable ratio current transformer should be used, and the current ratio should be changed accordingly. When the current is less than 10% of the rated current, the error between the current transformer and the measuring instrument is very large; when the current is greater than 120% of the rated current, the current transformer and the measuring instrument may be burned out.