Working Principle of Vacuum Circuit Breaker

Circuit Breaker:

A device used to close/Open a electrical equipment to the power supply is called Circuit Breaker.

They are named by the cooling or arc quenching media used in the device. When an equipment is disconnected from a power supply, the potential strength will be more. This will produce heat in between the surfaces and cause the media to ionize. This is spark. If the spark continues and it will be called as Arc. The heat energy will damage the device.

To reduce the effect of heat generation, the area provided to the breaker will be increased. The time to separate the equipment from power supply will be minimized by using spring. However the arc can be controlled by cooling or using media which can withstand high potential strength.

Air, Oil ,Vacuum and SF6 gas are some of the quenching media used in a circuit breaker. The breaker is named based on them. They are Air Circuit Breaker (ACB), Oil Circuit Breaker (OCB), Vacuum Circuit breaker(VCB) and SF6 Circuit Breaker.

Vacuum Circuit breakers:

The vacuum circuit breaker interrupts the current in a vacuum. The contacts of a vacuum breaker are enclosed in a ceramic envelope or “bottle” that is evacuated to an extremely low atmospheric pressure, approximately .0023 in. Hg absolute. Although the physics of interruption are quite complex, the vacuum interrupter works because the arc requires a conducting path to sustain it, but within the vacuum bottle there are no gasses to ionize; therefore, for all practical purposes, there can be no conducting path and the arc cannot be sustained.

Vacuum Bottle of VCB

Vacuum Bottle of VCB

Figure shows a cutaway view of a single pole vacuum interrupter. The vacuum bottle has a stationary contact firmly mounted on one end of the enclosure, and a moving contact, which travels a very short distance from open to close (1 inch or less), is sealed to the other end of the envelope with a flexible bellow.

VCB Operation:

The operating mechanism is comprised of the mechanical and electrical components required to:

  1. Charge the closing spring with sufficient potential energy to close the circuit breaker and to store opening energy in the opening and contact pressure springs.
  2. Mechanisms to release closing and opening actions.
  3. Means of transmitting force and motion to each of three pole positions.

Vacuum Circuit Breaker-Parts

Vacuum Circuit Breaker Front View

  1. Operate all these functions automatically through electrical charging motor, cutout switches, antipump relay, close coil, open coil, and auxiliary switch.
  2. Provide indication of the circuit breaker status (open/closed), spring condition (charged/discharged), and number of operations.

The Back side of the VCB consists of Three phase Contacts and Vacuum bottle as shown in below figure.


Vacuum Circuit Breaker Back View

The Vacuum bottle is placed in between incoming and outgoing contacts of each phase. The incoming contacts (upper side in the figure) receive power from the Busbars. The outgoing contacts (lower side in the figure) deliver power to the loads when the breaker is closed.