Impulse waves are specified by defining their rise or front time, fall or tail time to 50% peak value, and the value of the peak voltage. Thus 1.2/50 µs,(micro sec) 1000kV wave represents an impulse voltage wave with a font time of 1.2 µs,(micro sec) fall time to 50% peak value of 50 µs,(micro sec) and a peak value of 1000kV.