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).
In the busbar current transformers, the current cable to be measured must pass through the central hole, without the need to cut it. They are the most used because of their good Precision/Price ratio for intensities greater than 50A.