Transient voltages are detected as impulsive or low frequency oscillatory transients with a rise time of less than 0.25 ms and a duration from 130 µs to half-cycle. 

They are extracted from the input signal as overshoot voltages after removing the regular signal components. 

      The impulsive transient begins when the instantaneous impulsive voltage rises above the threshold defined in percent of the nominal voltage amplitude, and ends when it is equal to or below the threshold minus the hysteresis voltage defined in percent of the impulse threshold. 

     The impulse magnitude is the maximum instantaneous impulsive voltage measured during the transient. 

The duration of an impulsive transient is the time difference between the beginning and the end of the impulse.

For example, in the case of 

4-wire L-N wiring 

with 120V RMS nominal L-L secondary voltage,

the default 20% phase-to-neutral impulse 

threshold is 69.3V * 1.414 * 0.2 = 19.6 V secondary, 

and the hysteresis is 19.6V * 0.05 = 1V.