The maximum primary current of the protection transformer that can meet the requirements of the accuracy level under the rated load is called the rated accuracy limit primary current. The accurate limit coefficient is the ratio of the rated accurate limit primary current to the rated primary current. When the primary current is large enough, the iron core will saturate and fail to reflect the primary current, and the accurate limit coefficient represents this characteristic. The accuracy level of the protection transformer is 5P and 10P, which means that the allowable current error is 1% and 3% when the primary current is rated accurately and the composite error is 5% and 10% respectively. When the line fails, the inrush current generates heat and electromagnetic force, and the current transformer for protection must withstand it. When the secondary winding is short-circuited, the effective value of the primary current that the current transformer can withstand within one second without damage is called the rated short-term thermal current. When the secondary winding is short-circuited, the peak value of the primary current that the current transformer can withstand without damage is called the rated dynamic stable current.