Oil Circuit Breakers :
Q. State the required properties of oil in CB.
Q. What is Oil Circuit Breaker.

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Web resuQ. What is Oil Circuit Breaker

Properties of oil in C.B. :

Working principle of Oil Circuit Breakers :

In case of oil circuit breakers, medium used for are quenching is some insulating oil.
In this case the contacts of circuit breakers are opened under oil and an arc is struck between them.
The heat developed due the arc evaporates the surrounding oil and dissociates it into a substantial volume of gaseous hydrogen at high pressure.
Due to this the oil is pushed away from the are and an expanding hydrogen gas bubble surrounds the are region and adjacent portion of the contact.
The are extinction is facilitated mainly by two processes

Firstly the hydrogen gas has high heat conductivity and cools the are thus aiding the de-ionisation of the medium between the contacts.
Secondly the gas set up turbulence in the oil and forces it into the space between contacts, thus it eliminates the arcing products from the arc path. The result is that arc is extinguished and circuit current interrupted.

There are two types of oil circuit breakers such as :

Bulk Oil Circuit Breakers :
Construction of Oil Circuit Breakers :

Figure shows the construction of plain break oil circuit breaker.
It consists of a strong weather tight-earthed tank containing oil up to certain level in which fixed contact and moving contacts are separated. Air cushion is provided above the oil level.
The air cushion provides sufficient room to allow for the reception of the are gases without the generation of unsafe pressure in the dome of the circuit breaker.
It also absorbs the mechanical shock of the upward oil movement. Double break plain oil circuit breaker is called a double break because it provides two breaks in series.

Working of bulk Oil Circuit Breakers:

Under normal operating conditions the fixed and moving contacts remains closed and breaker carries the normal circuit current.

When a fault occurs, the moving contacts are pulled down by the protective system and an arc is struck which vaporises the oil mainly into hydrogen gas.
In plain break oil circuit breaker contacts are separated under the whole of the oil in tank.
The are extinction process occurs when certain critical gap between the contacts is reached.
There is no special arrangement for arc control other than the increase in length caused by the separation of contacts.

The are quenching process occurs by the following ways in Bulk oil circuit breaker :

The hydrogen gas bubble generated around the are cools the arc and aids the deionization of the medium between the contacts.
The gas sets up turbulence in the oil and help in eliminating the arcing products from the are path.
As the are lengthens due to the separating contacts the dielectric strength of the medium is increased.

Advantages of Oil Circuit Breakers :
Q . State advantages of Bulk oil circuit breaker (BOCB).

Air cushion is provided above the oil level.
The air cushion provides sufficient room to allow for the reception of the arc gases without the generation of unsafe pressure in the dome of the circuit breaker.
It also absorbs the mechanical shock of the upward oil movement.

Disadvantages of Oil Circuit Breakers :

These breakers do not permit high-speed interruption.
There is no special arrangement for arc control other than the increase in length by separating the moving contacts. Therefore, for successful interruption, long arc length is necessary.
Quantity of oil required is large and needs frequent replacement since for every operation of circuit breaker the quality of oil deteriorates.
Arcing time is large.

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