Impulse voltages

        Impulse voltages are characterized by polarity, peak value, time to front(tf), and time to half the peak value after the peak(tf). The time to front is defined as 1.67 times to time between 30% and 90% of the peak value in the rising  portion of the wave. According to IS: 2071 (1973), standard impulse is defined as one with tf=1.2us, tt=50us(called 1.2/50us wave). The tolerances allowed are +or-3% on the peak value,+or-30% in the front time(tf), and +or-20% in the tail time(tt).