Construction and Working :
The oil acts both as an arc extinguishing medium and as an insulator between the live parts and earth.
At the top of the oil air is filled in the seal vessel to serve as cushion to accommodate the displaced oil on formation of gas around the arc and also to absorb the mechanical shock of the upward movement of oil.
The breaker tank is securely bolted to an adequate foundation to bear out the vibrations caused on interrupting very heavy currents.
An ample head of oil above the arcing contacts is necessary to provide substantial oil pressure at the arc; and to prevent occurrence of the chimney effect.
A chimney of gas from the arc to the oil surface is produced which comes in contact with the earthed tank. If this gas is partially ionized and is of low dielectric strength, an arc will strike between the contact and the earthed tank with serious consequences.
Hence an appreciable amount of oil depending upon the operating voltage should always exist between the contact and the tank.
A gas outlet from the tank is essential and some form of vent is fitted in the tank cover. The position of the vent is carefully chosen so that the partially ionized gases which come out of the vent do not harm the personnel and also do not cause flash-over to the neighbouring equipments.
Important feature of such breakers is that no special device is used for controlling the arc caused by the moving contact.
Fig. shows a double break plain oil circuit breaker which is perhaps the most familiar of all oil circuit breakers.
The vertical break principle also permits the use of a cylindrical oil tank requiring relatively small floor area.
The plain break principle involves the simple process of separating the current carrying contacts under oil with no special control over the resulting arc other than the increase in length caused by the moving contact.
For successful interruption, a comparatively long arc length is essential so that the turbulence in the oil caused by the pressures generated by the arc, may assist in quenching it.
Under normal operating conditions, the fixed and moving contacts remain closed and the breaker carries the normal circuit current.
On occurrence of a fault the moving contacts are pulled down by the protective system and an arc is struck between the contacts and a large amount of heat is liberated and a temperature of about 5,000° absolute is reached which vaporizes the surrounding oil into gas.
The gas thus liberated surrounds the arc and its explosive growth around it displaces the oil violently.
As the moving contacts separate, the length of the arc increases, the rate of gas formation decreases due to fall in temperature.
The arc is extinguished when the distance between the fixed and moving contacts reaches a certain critical value, depending upon the arc current and recovery voltage.
Based on energy balance principle, final extinction of arc takes place at a current zero when the power input to the arc is less than that dissipated between the contacts. Such breakers thus suffers from the defect of permitting rather long and inconsistent arcing time (up to 100-150 ms).
From practical point of view the speed of the break should be as high as possible because a certain break distance has to be reached before interruption is likely to occur and the sooner it is achieved the smaller the energy released in the breaker and the less mechanically strong a breaker will have to be designed.
Disadvantages:
The main drawback of a double break plain oil circuit breaker is that a problem of unequal voltage distribution across the breaker is introduced and thus uneven sharing of the total interrupting duty is caused.
One break may share 70 to 80 percent of the interrupting duty.
If high value resistors are employed instead of capacitors, damping will be achieved.
The resistance values are generally of the order of 10-100 k ohm.
Advantages:
The main features that have an important bearing upon the performance of a plain break oil circuit breaker are:
1. Critical length of break.
2. Speed of contact movement. In order to quench the arc earlier the speed should be high and this is the reason that two break circuit breakers are preferred.
3. Head of oil above contacts, which is governed by the operating voltage of the breaker.
4. The clearance between the live contacts and the earthed pressurized tank.
Among the factors mentioned above that contribute to increased rupturing capacity are a considerable head of oil and large clearances; for this reason large rupturing capacity tends to require large circuit breakers.
Applications:
The plain break oil circuit breakers are widely employed on low voltage dc and ac circuits.
Such circuit breakers are not considered suitable for short-circuit rating exceeding 250 MVA at 11 kV.
Also such breakers are not considered suitable for high speed interruption, therefore, these cannot be employed for autoreclosing