Conduction and Breakdown in Gases(GASES AS INSULATING MEDIA)

The simplest and the most commonly found dielectrics are gases. Most of the
electrical apparatus use air as the insulating medium, and in a few cases other gases
such as nitrogen (N^, carbon dioxide (CO^, freon (CC12F2) and sulphur hexafluoride
(SF$) are also used.
Various phenomena occur in gaseous dielectrics when a voltage is applied.
When the applied voltage is low, small currents flow between the electrodes
and the insulation retains its electrical properties. On the other hand, if the
applied voltages are large, the current flowing through the insulation increases
very sharply, and an electrical breakdown occurs. A strongly conducting spark
formed during breakdown practically produces a short circuit between the
electrodes. The maximum voltage applied to the insulation at the moment of
breakdown is called the breakdown voltage. In order to understand the breakdown
phenomenon in gases, a study of the electrical properties of gases and the processes
by which high currents are produced in gases is essential.
The electrical discharges in gases are of two types, i.e. (O non-sustaining
discharges, and (U) self-sustaining types. The breakdown in a gas, called spark
breakdown is the transition of a non-sustaining discharge into a self-sustaining
discharge. The build-up of high currents in a breakdown is due to the process
known as ionization in which electrons and ions are created from neutral atoms
or molecules, and their migration to the anode and cathode respectively leads
to high currents. At present two types of theories, viz. (i) Townsend theory,
and (U) Streamer theory are known which explain the mechanism for breakdown
under different conditions. The various physical conditions of gases, namely,
pressure, temperature, electrode field configuration, nature of electrode surfaces,
and the availability of initial conducting particles are known to govern the ionization
processes.