Page 340 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

The disadvantages of oil as an arc quenching medium.

i It is inflammable and there is a risk of a fire.
ii It may form an explosive mixture with air.
iii The arcing products (e.g. carbon) remain in the oil and it deteriorates the quality of insulating oil.
iv Periodic checking and replacement of the insulating oil is required.

Types of oil circuit breakers
i Plain break oil circuit breakers
i Arc control oil circuit breakers.
iii Low oil circuit breakers
Plain break oil circuit breakers: In plain- break oil circuit breaker the main contacts are placed under the whole
oil in the tank. There is no special system for arc control other than the increase in length of separation of the
contacts. The arc extinction occurs when a critical gas is reached between the contacts.

The plain - break oil circuit breaker is the oldest type and has a very simple construction. It consists of fixed and
moving contacts enclosed in a strong weather- tight earthed tank containing transformer oil upto a certain level
and an air cushion above the oil level.
The air cushion provides sufficient room to arc gases without the generation of unsafe pressure in the circuit
breaker. It also absorbs the upward oil movement. Fig 5 shows a double break plain oil circuit breaker. It is called
a double break because it provides two breaks in series.

Fig 4 Fig 5

Principle of working
Under normal operating conditions, the fixed and moving contacts remain closed and carries the normal circuit
current. When a fault occurs, the moving contacts are pulled down by the tripping mechanism and an arc is
produced which vaporizes the oil into hydrogen gas. The arc extinction is completed by the following processes.
i The hydrogen gas bubble generated around the arc, cools the arc.
ii The gas sets up turbulence in the oil and helps in eliminating the arc.

iii As the arc lengthens due to the separation of contacts, the Arc voltage is increased.
The result is at some critical gap, the arc is extinguished and the circuit current is interrupted.
Disadvantages
i There is no special control over the arc other than the increase in gap length.
ii These breakers have long and inconsistent arcing times.

iii The speed of interruption is less.

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CITS : Power - Electrician & Wireman - Lesson 106-116 CITS : Power - Electrician & Wireman - Lesson 106-116