What is First pole-to-clear factor for medium voltage breaker

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For three-phase circuits, the TRV (Rated Transient Recovery Voltage) refers to the pole that breaks the circuit initially, in other words the voltage across the terminals of the first open pole. The ratio of this voltage to a single phase circuit voltage is called the first pole-to-clear factor, it is equal to 1.5 for voltages up to 72.5 kV
(isolated neutral of the supply circuit).