BREAKDOWN OF COMMERCIAL LIQUIDS

When a difference of potential is applied to a pair of electrodes immersed in an insulating liquid, a small conduction current is first observed. If the voltage is raised continuously, at a critical voltage a spark passes between the electrodes.

The passage of a spark through a liquid involves the following.

(a) Flow of a relatively large quantity of electricity, determined by the characteristics of the circuit,

(b) A bright luminous path from electrode to electrode,

(c) The evolution of bubbles of gas and the formation of solid products of decomposition (if the liquid is of requisite chemical nature)

(d) Formation of small pits on the electrodes,

(e) An impulsive pressure through the liquid with an accompanying explosive sound.

Tests on highly purified transformer oil show that

(a) Breakdown strength has a small but definite dependence on electrode material,

(b) Breakdown strength decreases with increase in electrode spacing,

(c) Breakdown strength is independent of hydrostatic pressure for degassed oil, but increases with pressure if oil contains gases like nitrogen or oxygen in solution.

In the case of commercial insulating liquid, which may not be subjected to very elaborate purifying treatment, the breakdown strength will depend more upon the nature of impurities it contains than upon the nature of the liquid itself.

These impurities which lead to the breakdown of commercial liquids below their intrinsic strength can be divided into the following 3 categories.

(a) Impurities which have breakdown strength lower than that of the liquid itself (ex: bubbles of gas).

Breakdown of the impurities may trigger off the total breakdown of the liquid.

(b) Impurities which are unstable in the electric field (ex: globules of water). Instability of the impurity can result in a low resistance bridge across the electrodes and in total breakdown.

(c) Impurities which result in local enhancement of electric field in a liquid (ex: conducting particles). The enhanced field may cause local breakdown and therefore initiate complete breakdown.