Oil circuit breakers use oil as an insulating medium to extinguish arcs. They consist of current carrying contacts enclosed in a strong metal tank filled with transformer oil. When faults occur, the contacts separate and an arc is struck, vaporizing the oil into gas which helps extinguish the arc. There are two main types: bulk oil circuit breakers which use a large quantity of oil for arc extinction in a grounded tank, and low oil circuit breakers.

1. OIL CIRCUIT BREAKER
• It is a type of circuit breaker which uses oil as a
dielectric or insulating medium for arc extinction.
• The contacts of the breaker are made to separate
within an insulating coil.

2. CONSTRUCTION
• It consists of current carrying contacts enclosed in a
strong, weather-tight earth metal tank.
• The tank is filled with transformer oil, that act as an
arc extinguishing medium and 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.

3. CONT..
• The breaker tank is securely bolted for carrying out
the vibration caused on interrupting very high current.

4. WORKING
• During normal working conditions, the contact
of the oil circuit breaker is closed and carry
currents.
• Under fault conditions, the contacts of the
breakers are moving apart ,and an arc is struck
between the contacts.
• Due to this large amount of heat is liberated ,
and very high temperature is reached which
vaporises the oil into gas.

5. TYPES OF OIL CIRCUIT BREAKER
There are two types of oil circuit breaker
1)Bulk oil circuit breaker
2)Low oil circuit breaker
BULK OIL CB:
A breaker which uses large quantity of oil for arc extinction is
called bulk oil circuit breaker.
Also known as dead tank-type CB because their tank is held at
ground potential.
The quantity of oil depends on the system voltage.