Why Flashover for + polarity is less than - polarity?

[cuky2000]

We know that if a positive polarity surge is applied to rod in air with the plane grounded, the flashover voltage will be substantially lower than if a negative polarity surge is applied to the rod with the plane grounded.

However, it is not obvious why.

Your comments or provide a reference will be appreciated

Thanks

 

[rtechn]

Imagine the rod is positive: due to their higher mobility, electrons will move towards the rod, leaving behind them a positive space charge. This will cause an increase of the electric field at the region near the plane. It is as though the rod extended and the gap diminished.

If the rod is negative, when electrons flow away, positive charge will be left near the rod, increasing the field (easier corona) there, but reducing the ionization region into the gap, which demands a higher voltage to cause the flashover.

A good reference is the book High Voltage Engineering - Fundamentals (E. Kuffel, W.S. Zaengl, J. Kuffel), 2nd edition, pages 357 to 359.