Miniature/Low Oil Circuit Breaker

Miniature Oil Circuit Breaker
Photo Credit - ABB Group

In the bulk oil circuit breaker, the oil has to perform two functions, first of all it act as an arc quenching medium and secondly it insulate the live part from earth. It has been found that only small percentage of oil is actually used for arc extinction while the major part is utilized for insulation purposes. For this reason the quantity of oil in bulk oil circuit breaker reaches a very high as the system voltage increases. This not only increases. This not only increases the expenses, tank size and weight of the breaker but it also increases the fire risk and maintenance problems.
A low oil circuit breaker employs solid materials for insulation purposes and uses a small quantity of oil which just sufficient for arc extinction. Regarding quenching the arc, the oil behaves identically in bulk as well as low oil circuit breaker. By using suitable arc control devices the arc extinction can be further facilitated in an low oil circuit breaker.

Construction

Fig. Miniature/Low oil circuit breaker
Photo Credit - Circuit Globe

Above figure shows the cross section of a single phase low oil circuit breaker. There are two compartment separated from each other but both filled with oil. The upper chamber is the circuit breaking chamber while the lower one is to prevent from mixing with the other chamber. This arrangement permit two advantages. First of all, the circuit breaking chamber 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.

Supporting chamber
It is a porcelain chamber mounted on a metal chamber. It is filled with oil which is physically separated from the oil in the circuit breaking chamber. The oil inside the supporting chamber and annular space formed both the porcelain insulation and bakelised paper is employed for insulation purposes only.
Circuit breaking chamber
It is a porcelain enclosure mounted on the top of the supporting compartment. It is filled with oil and has the following parts:-

Upper and lower fixed contact.
Moving contact
Turbulator
The moving contact is hollow and includes a cylinder which moves down over a fixed piston. The turbulator is an arc control device and has both axial and radial vents. The axial venting ensures the interruption of low currents whereas radial venting vents help in the interruption of heavy current.


Top Chamber
It is metal chamber and is mounted on the circuit breaking chamber. It provides expansion space for the oil in the circuit breaking compartment. The top chamber is also provided with a separator which prevent any loss of oil by centrifugal action caused by circuit breaker operation during fault condition.

Operation

Under normal operating conditions the moving contact remains engaged with the upper fixed contact. When fault occurs the moving contact is pulled down by tripping springs and an arc is struck. The arc energy vaporizes the oil and produces gases under high pressure. This action contains the oil to pass through a central hole in the moving contact and results in forcing series oil oil through the respective passage of turbulation. The process of turbulation is ordinary one in which the section of arc are successively quenched by the effect of separate streams of oil moving across each section in turns and bearing away in gases.

Advantages

1. It requires lesser quantity of oil.
2. The space required is small.
3. There is reduced risk of fire.

Disadvantages

1. Due to smaller quantity of oil the degree of carbonisation is increased.
2. There is difficulty of removing the gases from contact space in time.