Sparkover voltage of sphere gaps under standard lightning and switching impulse voltages

The influence of impulse shape, gap spacing and of humidity on the sparkover voltage of sphere gaps consisting of 25cm diameter spheres has been studied for both polarities under standard waveshapes of 1.2/50 mu s and 250/2500 mu s. Results are compared with the values proposed by the IEC standard and an attempt to explain the breakdown mechanism and especially how it is influenced both by humidity and the waveshape has been made. Further suggestions are given concerning their use as calibrating gaps, especially under switching impulses, Also, under certain conditions, the inclusion in the calibrating gaps of 15cm gaps is proposed.