The FS in the current transformer represents the instrument security factor, which is only applicable to measurement-level current transformers. The specific regulations are as follows:
　　FS = rated instrument limit primary current / rated primary current;
　　The value is determined only when the user requests it, and the recommended value is 5, or 10;
　　 is mainly to protect the secondary instrument when the system fault current passes through the current transformer. The smaller the FS, the safer the secondary instrument.
　　The primary current of the rated instrument limit is the minimum primary current with a composite error greater than or equal to 10% under the rated load.
　　 The current transformer for measurement is a current transformer specially used to measure the current and electrical energy of the power system. Under normal working conditions, it should meet the specified accuracy level requirements to ensure accurate measurement. However, the current transformer used in the power system often has a large overcurrent flowing through the primary winding due to system failure or operation. In this case, it is hoped that the secondary current will no longer increase strictly in proportion to prevent the instruments and meters connected to the secondary circuit from being impacted by large currents. Therefore, the instrument security factor requirements are put forward for the current transformer for measurement.
　　 The so-called instrument safety factor (FS) refers to the ratio of the instrument safety current to the rated primary current. The safety current of the instrument refers to the minimum primary current value of the current transformer for measurement under the rated secondary load with a composite error of not less than 10%. This means that the larger the composite error under the instrument's safety current, the better. Generally, the required size of the instrument security factor depends on the overload capacity of the secondary instrument. As the measurement accuracy of the instrument increases, the overload capacity of the instrument decreases, so the instrument security factor of the instrument transformer is required to be small.