Influence of the wavefront of impulse voltages on the sparkover of rod gaps and insulators

The influence of the duration of the wavefront of an impulse voltage on the sparkover of rod gaps, up to 80 cm, and insulators has been determined for voltages of both polarities up to 500 kV and for times to crest from 2 to 140 µs. The pattern of sparkover as the wavefront (T1) increases is a complicated one; for small gaps, the positiveimpulse sparkover first rises and then falls as the wavefront increases, the reverse behaviour to that of large gaps.Withstand voltages follow curves similar in shape to those of the critical-sparkover-voltage curves as T1 varies, and the value of rod gaps for co-ordination of insulation tests is questioned. The effectiveness of a radiation source in sphere-gap voltage measurement is demonstrated and its use again recommended.