They are used for the measurement of alternating current, when the current to be measured is too large to apply directly to the measuring instrument.
The standard output currents are /1A and /5A. The precision is indicated by the class, the most used being class 0.5 (0.5% error) and class 1 (1% error).
With the class of wound primary transformers, it is necessary to interrupt the current cable to be measured in order to insert the primary of the transformer in serial connection. It is used to measure intensities of up to 60A.