partial discharge inception voltage Ui
applied voltage at which repetitive partial discharges are first observed in the test object,
when the voltage applied to the object is gradually increased from a lower value at which no partial discharges are observed In practice,
the inception voltage Ui is the lowest applied voltage at which the magnitude of a PD pulse quantity becomes equal to or exceeds a specified low value.
NOTE For tests with direct voltage, the determination of Ui needs special considerations. See clause 11.