Oil circuit breakers are used for ratings up to 72.5KV. A tank contains the huge amount of oil. Such breakers are used for indoor metal-clad draw-out types of switchgear. Oil is used as arc extinction medium as well as insulation medium. It is also known as transformer oil.The electrodes are separated in the oil-filled tank.
An arc-controlling device is fitted to assist the arc extinction process.
The construction of a breaker is such that arc-control devices makes the flow of gases produced during arc quenching axial or radial with respect to arc. Disadvantages of bulk oil circuit breakers: a) Large quantities of oil is required for arc quenching and insulation purposes. b) After a specified number of operations, the oil should be replaced. c) The size of tanks for ratings above 36KV is too large.
These disadvantages led to the invention of minimum oil circuit breakers. Minimum oil circuit breakers do not require steel tanks and require only 1/10 of the oil used by bulk oil circuit breakers.