Oil circuit breaker is such type of circuit breaker which used oil as a dielectric
or insulating medium for arc extinction. In oil circuit breaker the contacts of
the breaker are made to separate within an insulating oil. When the fault occurs
in the system the contacts of the circuit breaker are open under the insulating oil,
and an arc is developed between them and the heat of the arc is evaporated in
the surrounding oil. The oil circuit breaker is divided into two categories
- Bulk Oil Circuit Breaker
- Low Oil Circuit Breaker
Construction of Oil Circuit Breaker
Oil circuit breaker is very easy in construction. It consists of current carrying
contacts enclosed in a strong, weather-tight earth metal tank and the tank is
filled with transformer oil. The oil is both acts as an arc extinguishing medium
and as an insulator between the live part and earth.
At the top of the oil, air is filled in the tank which acts as a cushion to control
the displaced oil on the formation of gas around the arc and also to absorb
the mechanical shock of the upward movement of oil. The breaker tank is
securely bolted for carrying out the vibration caused on interrupting
very high current. Oil circuit breaker consists gas outlet which is fitted in the
tank cover for the removal of the gases.
Working Principle of Oil Circuit Breaker
During the normal operating conditions, the contact of the oil circuit breaker
is closed and carry the current. When the fault occurs in the system, the contacts
of the breaker are moving apart, and an arc is struck between the contacts.
Due to this arc, a large amount of heat is liberated, and a very high temperature
is reached which vaporises the surrounding oil into gas. The gas, thus liberated
surrounds the arc and its explosive growth around it displace the oil violently.
The arc is extinguished when the distance between the fixed and moving contact
reaches a certain critical value, depends on the arc current and recovery voltage.
The oil circuit breaker is very reliable in operation, and it is very cheap. The most
important feature of oil circuit breaker is that no special devices are used for controlling
the arc caused by moving contact. The oil as an arc quenching medium has certain
advantages and disadvantages
Advantages of Oil as an Arc Quenching
- The oil has a high dielectric strength and provides insulation between the contact after the arc has been extinguished.
- The oil used in circuit breaker provides a small clearance between the conductors and the earth components.
- The hydrogen gas is formed in the tank which has a high diffusion rate and good cooling properties.
Disadvantages of Oil as an Arc Quenching
- The oil used in oil circuit breaker is inflammable and hence, cause a fire hazard.
- There is a risk of formation of explosive mixture with air.
- Due to decomposition of oil in the arc, the carbon particles is generated which polluted the oil and hence the dielectric strength of the oil decreases.
Maintenance of oil circuit breaker
After a circuit breaker has interrupted by short-circuit current, sometimes their
contacts may get burnt due to arcing. Also, the dielectric oil gets carbonized in
the area of the contacts, thereby losing its dielectric strength. This results in
the reduced breaking capacity of the breaker. Therefore, the maintenance of
oil circuit breaker is essential for checking and replacement of oil and contacts.