Analysis and comparison of rod-plane and rod-rod gaps as direct voltage measurement devices

The direct voltage sparkover characteristics of the rod-rod and the rod-plane electrode gaps systems are presented and compared. It is shown that for both arrangements the linearity is limited by the effects of atmospheric humidity for small (<0.25 metre) and large (>1 metre) electrode separations. The characteristics are discussed in terms of leader growth in the prebreakdown discharge; this is humidity dependent and is shown to be more prominent in the rod-rod gap. The precision and useful range of measurements in the two gaps are compared, particularly in relation to the recent approval of the rod-rod gap as a device for direct voltage measurement by the IEC.