Bulk Oil Circuit Breaker

Definition: A circuit breaker that uses a large amount of oil for arc extinction and insulation purposes is known as a Bulk Oil Circuit Breaker. This type of circuit breaker is also called a Dead Tank Circuit Breaker since its tanks are grounded at the same potential as the earth. It is also known as a Plain Break Circuit Breaker.

The amount of oil used in a bulk oil circuit breaker (BOCB) varies depending on the design and specifications of the breaker. Generally, the amount of oil is determined based on factors such as the voltage rating, interrupting capacity, and cooling requirements of the circuit breaker. If the output rating of a circuit breaker is 110kV, then the quantity of oil used is between 8000 to 10000 Kg. On the other hand, if the output rating of the circuit breaker is 220kV, then the quantity of oil used is 50000 kg.

The bulk oil circuit breaker(BOCB) utilizes oil for two distinct purposes.

1. For arc extinction
2. For insulating the current-carrying parts

In a BOCB, one-tenth of the oil is used for arc extinction, and the remaining is used for insulation.

Construction of bulk oil circuit breaker 

The circuit breaker’s fixed and moving contacts are immersed in insulating oil in a metal tank at ground potential. The diagram of the bulk oil circuit breaker is shown below.


Under normal operating conditions, the fixed and moving contacts of a circuit breaker are in a closed position. However, in case of a fault in the system, a protective mechanism is triggered, causing the moving contacts to separate from the fixed contacts. This separation results in an arc production between the two contacts. A significant amount of heat energy is dissipated from the arc into the oil, causing it to vaporize and decompose.

As the gaseous pressure inside the oil increases, the insulating oil gets displaced from around the fixed and moving contacts. This, in turn, causes the inner wall of the bulk oil circuit breaker to withstand higher pressure. To accommodate this, the metal tank of the BOCB is made strong enough to withstand the increased pressure of the oil.

The top covers of the BOCB are tightly bolted to the tank. The entire structure must be properly fitted with a concrete foundation, or else the breaker will dislocate during the interruption of higher fault currents.

The gas vent on the top cover removes oil gases safely. The breaker’s oil tank is not completely filled, leaving space for oil expansion.

Working of bulk oil circuit breaker 

When a fault occurs, the protective mechanism of the breaker separates the fixed and moving contacts, creating an arc between them. The arc length increases when the fixed and moving contact separates, and it causes an increase in the arc resistance. This results in a lower temperature and reduced formation of gases around the arc between contacts due to increased arc resistance.

During the operation of a BOCB, the arc quenching process occurs when the arc current passes through the zero-crossing point. This leads to the extinguishing of the arc and the interruption of the fault current. The circuit breaker employs hydrogen gas to cool down the arc, which helps in the arc quenching process in the oil circuit breaker.

Advantages of BOCB

  1. The oil used in these breakers provides excellent thermal conductivity.
  2. The oil extinguishes the arc quickly and efficiently, leading to a self-restoring arc interruption process.
  3. BOCBs can handle high currents and interrupt large fault currents effectively. This makes them suitable for high-voltage applications.

Disadvantages of BOCB 

  1. The oil used in BOCBs poses a fire hazard in the event of a fault.
  2. Transporting a BOCB is difficult due to its size.
  3. It requires a larger space area when compared to a minimum oil circuit breaker. BOCBs typically have larger physical dimensions than modern circuit breakers using alternative technologies such as vacuum or SF6.
  4. The BOCB has a long and inconsistent arcing time.
  5. The bulk oil circuit breaker can not interrupt current at high speed. BOCBs generally have slower interruption speeds compared to vacuum or SF6 circuit breakers.
  6. Compared to SF6 gas, oil’s dielectric strength is relatively lower, which may limit the voltage rating and interrupting capacity of BOCBs, especially for higher voltage applications.
  7. BOCBs are more susceptible to adverse weather conditions such as extreme temperatures, humidity, and contamination from dust or moisture.

Applications of bulk Oil circuit breaker 

  1. The bulk oil circuit breaker is used for voltages up to 33kV.
  2. These circuit breakers are only suitable for low-voltage applications where high breaking capacity is not required.
  1. Air Break Circuit Breaker
  2. SF6 Circuit Breaker

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