Insulators under a high voltage

Introduction

Air is one of insulators that prevent charge exchange between two charged conductors. Parallel plates is an example where the air is insulating the negatively charged plate from he positively charged one. Nevertheless, air does keep this properties when the voltage difference between the plates causes air ionization. the process will produce free electrons accelerates toward the positive plate.Enough number of these electrons will make the air a conductor and sparking is observed on the plates.

Stages of Sparking

Sparking does not happen suddenly, but it microscopically has stages defined as the following:

Avalanche formation

Physically is formulated using Townsend's primary ionization coefficient ([math] \alpha [/math]), a relationship is built combining the number of moving electrons (n) over a distance (dx) to produce a new free electrons (dn):

[math] dn = n .\alpha. dx [/math]

Solving the equation will give the number of the free electrons produced is calculated as : [math] n = n_0 e^{-\alpha n} [/math]

References

Dr JP Holtzhausen, Dr WL VoslooHigh "Voltage Engineering Practice and Theory" (online drift copy)