Abstract
A quantitative theory of breakdown between concentric spheres in atmospheric air is developed, in which it is assumed that a conducting corona cloud, produced at the inner sphere, expands until electron avalanche growth is only just possible at its surface. Breakdown of the gap occurs if the corona sphere radius exceeds a fixed fraction of that of the outer sphere. This fraction differs for positive and negative impulses. Secondary emission processes are included where necessary.
The theory applies to rise times such that equilibrium is established between the applied impulse voltage and avalanche growth at the corona cloud surface (1 μs for negative impulses and 100 μs for positive).
It compares well with experiment for small concentric (hemi) spheres where data are available. It is extended to rod-plane and sphere-plane gaps, where good agreement with experiment is also found. The enhanced strength of sphere gaps is successfully explained and the theory extends naturally to the case of uniform field breakdown.
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