What is the principal working of current transformer supervision protection function in VAMP 255/245/230 relay's.

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The device supervise the external wiring between the device terminals and current transformers (CT) and the CT them selves. Furthermore, this is a safety function as well, since an open secondary of a CT, causes dangerous voltages.
The CT supervisor function measures phase currents. If one of the three phase currents drops below Imin< setting, while another phase current is exceeding the Imax> setting, the function will issue an alarm after the operation delay has elapsed.