Conduction and breakdown in Gases
1 – 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 nitrogen (N2), carbon dioxide (CO2), freon (CC12F2) and sulphur hexafluoride (SF6) 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 they insulation retains, it's electrical properties. On the other hand, if the applied voltages are large, the current flowing through the insulation increases very sharply 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 current are produced in gases is essential.
The electrical discharges in gases are of two types:-
(i) Non-sustaining discharge.
(ii) self- sustaining.
The breakdown in a gas, called spark is the transition of 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 current. At present two types of theories, viz.
(i) Townsend theory
(ii) Streamer theory are known
'Which explain the mechanism of breakdown under different condition?
The various physical condition 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.