An oil circuit breaker (OCB) is a type of circuit breaker that uses oil as an arc quenching medium to interrupt the flow of electrical current in a circuit. It is mainly used for high voltage power systems, typically above 33 kV (kilovolts).

The basic construction of an oil circuit breaker consists of a chamber filled with insulating oil, where the contacts and arc control devices are immersed. When the circuit breaker is closed, the contacts allow the flow of electrical current through the circuit. However, when a fault occurs, such as a short circuit or overload, the contacts are quickly opened, creating an arc.

The oil within the circuit breaker chamber plays a vital role in extinguishing the arc. It provides insulation and acts as a cooling medium. As the arc is formed, the oil surrounding it vaporizes, generating a high-pressure gas that rapidly cools and deionizes the arc. This process helps to quickly extinguish the arc and restore the circuit to its normal state.

There are primarily two types of oil circuit breakers:

  1. Bulk Oil Circuit Breaker (BOCB): In this type, the insulating oil is stored in a separate container, and it flows into the circuit breaker chamber when the contacts open to create an arc. The oil is then recirculated through a filtering system to remove any impurities or gas byproducts before being reused.
  2. Minimum Oil Circuit Breaker (MOCB): This type uses a smaller quantity of oil compared to BOCBs. The oil is contained within the circuit breaker chamber and is only used for arc extinction. The MOCB design reduces the amount of oil required and simplifies the maintenance process.

It’s important to note that due to environmental concerns and advancements in technology, oil circuit breakers have been largely phased out in favor of more modern alternatives such as vacuum circuit breakers and SF6 (sulfur hexafluoride) circuit breakers. These alternatives offer better performance, safety, and environmental friendliness.