Impulse flashover voltages are given for air-gaps, string insulators and post insulators which might be used in transmission systems at voltages from 132 to 400 kV. For a given length of air-gap, the flashover voltage with 1/50 μs impulses is least between a positive point and an earthed plane, and for this arrangement the 50% flashover voltage for gaps above lm is about 525 kV/m. For a negative point the voltage is considerably higher, the proportionate difference decreasing as the gap length increases. For point-point gaps and for suspension-insulator strings, on which flashover takes place clear of the intermediate metal fittings between insulator units, there is less polarity effect, and the voltage/length characteristics are intermediate between the positive and negative characteristics for point-plane gaps. For air-gaps and such insulator strings the voltage range over which the flashover probability is fractional is small. For long post insulators, over which flashover takes place via the intermediate metal fittings, the fractional-flashover-probability voltage range may be larger and the increase of flashover voltage with length may be smaller than for suspension strings. The polarity effect decreases with length of gap or insulator.