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).
The open core current transformer allows coupling to the power cable without disconnecting it from the circuit. They are the fastest to install and are used in intensities greater than 100A.