Page 339 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Application and uses of air circuit breaker

• It is used for protection of plants
• It is used for common protection of electrical machines
• Air circuit breaker is also used in electricity sharing system upto 15KV
• Also used in low as well as high voltage and current applications.

• It is used for protection of transformers, capacitors and generators.
Types of air circuit breaker
• Plain air circuit breaker
• Air blast circuit breaker
Plain air circuit breaker: In this circuit breaker a chamber is fitted surrounding the contact. The chamber is
known as “arc chute”. The arc chute will help in achieving cooling. Arc chute is made from some refractory
material.

The arc chute is divided into a number of small compartments by using metallic separation plates called arc
splitters and behave as a mini arc chute as in Fig 3. Initial arc will split into a series of arcs and make the arc
voltages higher than system voltage. They are preferable choice in low voltage application.

Fig 2 Fig 3

Oil circuit breakers (OCB)
Circuit breakers which uses the insulating oil (e.g transformer oil ) as an arc quenching medium is called as oil
circuit breaker. The main contacts of the OCB are opened under the oil and an arc is struck between them. The
heat of the arc evaporates the surrounding oil and dissociates it into gaseous of hydrogen at high pressure.
The hydrogen gas occupies a volume about one thousand times that of the oil decomposed. The oil is, therefore,
pushed away from the arc and an expanding hydrogen gas bubble surrounds the arc region of the contacts. The
arc extinction is completed by two processes. Firstly, the hydrogen gas has high heat conductivity and cools the
arc, thus aiding the de-ionization of the medium between the contacts.
Secondly, the gas sets up turbulence in the oil and forces it into the space between contacts, thus eliminating the
arc as in Fig 4. The result is that arc is extinguished and circuit current is interrupted.
The advantages of oil as an arc quenching medium
i It absorbs the arc energy to decompose the oil into gases which have excellent cooling properties.
ii It acts as an insulator and permits smaller clearance between main contacts.

iii The surrounding oil presents the cooling surface in close proximity to the arc.

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