Minimum Oil Circuit Breaker Construction Working Advantages & Disadvantages (MOCB)

Low oil Circuit Breakers (MOCB)
Q. Draw neat diagram MOCB. Explain its working.
Q. Draw a neat labeled diagram of MOCB.

In case of bulk oil circuit breaker oil is used for two functions.

- Firstly it acts as an arc-quenching medium and
- Secondly it insulates the live parts from earth.

But for arc extinction purpose a very small amount of oil is sufficient while the major part is utilized for insulation purposes.
Thus the oil required in case of bulk oil circuit breaker is large and need frequent replacement.
This increases the overall cost; tank size and weight of or the breaker as well as it also increases the fire risk and maintenance problems.

Figure (a)

Above drawbacks of bulk oil circuit breakers are removed in minimum oil circuit (also known as MOCB) breaker in which a small quantity of oil is used for arc extinction and for insulation purpose a solid material is used.

Construction of Minimum Oil Circuit Breaker :

Figure (a) shows the cross section of a single-phase minimum oil circuit breaker. The construction is divided into two compartments. Both the compartments are separated from each other but both are filled with oil.
The upper compartment is mainly a circuit-breaking compartment while the lower one is the supporting compartment.
The arrangement is made in such that two compartments are separated by a partition and oil from one chamber is prevented from mixing with the other chamber.
This arrangement permits two advantages.

- Firstly, the circuit breaking chambers requires a small volume of oil which is just enough for arc extinction.

- Secondly the amount of oil to be replaced is reduced, as the oil in the supporting chamber does not get contaminated by the arc.
(i) Supporting chamber :

Supporting chamber is made up of insulating material such as porcelain and is mounted on a metal chamber.
It is filled with oil and care is taken such that oil in the circuit-breaking compartment does not get mixed with it.
The oil inside the supporting chamber and the annular space formed between the porcelain the insulation and Bakelite paper is employed for insulation purpose only.

(ii) Circuit-breaking chambers :

It is also made up of porcelain enclosure and is mounted on the top of the supporting compartment.
Circuit breaking chamber is filled with oil and has the following parts :

(a) Upper and lower fixed contacts.

(b) Moving contacts

The moving contact is hollow and includes a cvlinder, which moves down over a fixed piston.
The function of turbulator is to control the arc device. It has both axial and radial vents. The axial venting ensures the interruption of low currents whereas radial venting helps in the interruption of heavy currents.

(iii) Top chamber :

Top chamber is made up of metal and is mounted on the circuit-breaking chamber.
The function of top chamber is that it provides expansion space for the oil in the circuit-breaking compartment.
The top chamber is also provided with a separator, which prevents any loss of oil, by centrifugal action caused by circuit breaker operation during fault conditions.

Working of Minimum Oil Circuit Breaker :

Under normal operating conditions, the moving contact remains engaged with the upper fixed contact.
As soon as fault occurs on the system, the moving contact is pulled down by the tripping springs and arc is struck between the two contacts.
The heat of arc causes vaporisation of the oil and produces gases under high pressure.
Due to this action oil is pass through a central hole in the moving contact and forcing a series of oil through the respective passage of the turbulators.
This causes successively quenching of arc by the effect of separate stream of oil moving across each section in turn and bearing away its gases.

Advantages of Minimum Oil Circuit Breaker :

Q. State any four advantages of MOCB.

A Minimum oil circuit breaker has the following advantage over a bulk oil circuit breaker :

It requires small quantity of oil (only for are quenching purpose since for the insulation purpose solid materials are used).
It requires small space, since the quantity of oil required is less and a small separation of contact is sufficient to quench the arc.
There is less risk of fire.
Maintenance problem are reduced.

Disadvantages of Minimum Oil Circuit Breaker :

A low oil circuit breaker has the following disadvantage as compared to a bulk oil circuit breaker.

There is a difficulty of removing the gases from the contact space in time.
Due to smaller quantity of oil, the degree of carbonization is increased.
The dielectric strength of the oil deteriorates rapidly due to high degree of carbonization.

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