In this circuit breaker the contacts are separated under the whole of the oil in the tank. There is no special arc control system other than increasing length caused by separation of contacts. Arc extinction occurs when a critical gap is reached between the contacts. Double break plain oil circuit breaker is shown in fig 7 below

It consists of fixed and moving contacts enclosed in a strong weather-tight earthed tank containing oil up to a certain level and an air cushion above the oil level. The air cushion collects arc gases without generation of unsafe pressure in the dome of circuit breaker. It also absorbs mechanical shock of the upward oil movement. Under normal operating conditions the fixed and moving contacts remain closed and the circuit breaker carries the normal circuit current. When fault occurs, the moving contacts are pulled down by protective system and an arc is struck which vapourises the oil mainly into hydrogen gas.

The following processes facilitate the arc extinction:

1. The hydrogen bubble generated around the arc cools the arc and aids deionistion of the

medium between the contacts.

2. The gas sets up turbulence in the oil and helps in eliminating the arcing products from the arc

path.

3. As the arc lengthens due to the separating contacts, the dielectric strength of the medium is

increased.

Disadvantages: There is no special control over the arc other than increase in length by, separating the moving contacts. And for successful interruption long arc length is required. They have long and inconsistent arcing times and they do not permit high speed interruption. Due to these disadvantages its only used for low capacity installations for low voltages not exceeding 11 kV

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