It is a very simple type of circuit breaker. In this type of circuit breaker, the contacts separate under the whole of the oil in the tank. That is why It is called Plain Break Oil Circuit Breaker.

Construction of Plain Break Oil Circuit Breaker

It consists of fixed and moving contacts. These contacts are enclosed in a weather tight tank. The tank contains oil up to a certain level. Air cushion is provided at the top of the oil. The air cushion provides sufficient room to allow for the reception of the arc gases without the generation of unsafe pressure in the dome of the circuit breaker. Another of the function air cushion, it absorbs the mechanical shock of the upward oil movement. A gas outlet from the tank is essential and some form of vent is fitted in the tank cover.

plain break oil circuit breaker
Plain Break Oil Circuit Breaker 

Working of Plain Break Oil Circuit Breaker

Under normal operating conditions, the fixed and moving contacts remain closed. At this instant, the breaker carries normal circuit current. When a fault occurs on any part of the system, moving contacts begin separate and an arc struck in the oil. An arc vaporizes the oil mainly into hydrogen gas. Hydrogen gas cannot remain in molecular form and is dissociated in its atomic form releasing lots of heat. The gas, thus liberated surrounds the arc and its explosive growth around it displaces the oil violently. As the moving contacts separate, the length of arc increases, the rate of gas formation decreases due to fall in temperature. In this circuit breaker, an arc extinguishes, when the distance between the fixed and moving contacts reaches a certain critical value.


  • This circuit breaker suffers from the defect of permitting rather long and inconsistent arcing times. As a result, such circuit breakers are suitable for low current and low voltage operations.
  • These circuit breakers do not permit high speed interruption.