The following points highlight the three main types of oil circuit breakers. The types are: 1. Plain Break Oil Circuit Breakers 2. Arc Control Oil Circuit Breakers 3. Low Oil or Minimum Oil Circuit Breakers.

Type # 1. Plain Break Oil Circuit Breakers:

The plain oil circuit breaker is very simple in construction. It consists of current carrying contacts enclosed in a strong (in order to withstand the large gaseous pressure developed owing to dissipation of large energies), weather tight (to keep moisture out), earthed metal tank and immersed in oil, called the transformer oil. 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.

Double break plain oil circuit breaker is called a double break breaker because it provides two breaks in series. The two breaks in series provide rapid arc lengthening without the need for a specially fast moving contact speed, and the total gap distance at the end of stroke can conveniently be made ample. 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 suffer 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.

The main drawback of a double break plain oil circuit breaker is that a problem of unequal voltage distribution across the breaks is introduced and thus uneven sharing of the total interrupting duty is caused. One break may share 70 to 80 per cent of the interrupting duty.

In order to explain the above statement consider Fig. 10.3 (a) which represents the capacitance C1 between the fixed and moving contacts and capacitance C2 between the moving contact and earth; while Fig. 10.3 (b) represents its equivalent circuit.


If I is the fault current, then in Fig. 10.3 (b),

If we take C2 four times C1 we get V1 = 5V2. Thus, the voltage across one gap is about 83.4% of the system voltage. In order to equalise this voltage across the gaps, high resistances or capacitors are connected across them, as shown in Fig. 10.4. If C1 = 10 µµF, C2 = 40 µµF and C3 = 50 µµF are connected as shown in Fig. 10.4. Now –

Thus V1 becomes only 1.66 times that of V2. However, if C3 be taken of higher value the difference between V1 and V2 can be reduced further.

If high value resistors are employed instead of capacitors, damping will be achieved. The resistance values are generally of the order of 10-100 kΩ.

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.

All the factors 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. 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 auto-reclosing.

Type # 2. Arc Control Oil Circuit Breakers:

In case of a plain break oil circuit breaker, there is no control over the arc other than increase in length caused by the moving contact. However, it is necessary and desirable that final arc extinction should occur while the contact gap is still short. For this purpose, some arc control is to be provided and the breakers provided with arc control are called the arc control circuit breakers.

Arc control breakers are of two types, namely:

1. Self-blast or self-generated pressure oil circuit breakers in which arc control is provided by internal means i.e., the arc itself is used for its own extinction efficiently.

2. Externally generated pressure or forced blast or impulse oil circuit breakers in which arc control is provided by mechanical means external to the circuit breaker.

1. Self-Blast or Self-Generated Pressure Oil Circuit Breakers:

In such breakers use of pressure generated by the arc itself is made in speeding up the movement of the oil into contact space immediately after the current zero. The high pressure generated by the arc causes an immediate displacement of oil into the space between the contacts after the arc current goes to zero. This is achieved by surrounding the contacts by a pressure chamber or pot. The pressure developed by the oil depends upon the value of current interrupted.

Such circuit breakers have the following advantages:

i. The pressure chamber is relatively cheap to make.

ii. Length for the critical gap is reduced.

iii. Arcing time is reduced.

iv. The breaking capacity of the circuit breaker is increased.

The design of the chamber or pot should be such that the pressure developed is sufficient to quench the arc even at low values of current but not so much as to break the pot on heavy current.

This has led to the manufacture of a variety of pots, some of which are discussed below:

(a) Plain Explosion Pot:

It is a rigid cylinder of insulating material enclosing the fixed and moving contacts, closed at the top but with a restricted opening, called throat, at the bottom. The moving contact is a cylindrical rod passing through the throat. On occurrence of fault, the contacts separate, an arc is struck between them and oil is decomposed, by the heat of the arc, into a gas
at very high pressure in the pot.

This high pressure forces the oil and gas through and round the arc to extinguish it. If the arc is not extinguished while the moving contact is still within the pot, it happens just after the moving contact leaves the pot due to axial high velocity blast, released through the slot. Since the plain explosion pot performs axial extinction of the arc, it is sometimes called the axial extinction pot.

This device has the effect of controlling the nature of the turbulence to a certain extent, thereby giving a certain degree of consistency.

This type of pot cannot be used either for very low or for very large currents. With low fault currents, the pressure developed is small, thereby increasing the arcing time. On the other hand, with large fault currents, the gas is produced so rapidly that explosion pot is liable to burst due to high pressure. Thus the breaking capacity of such a circuit breaker is limited.

(b) Cross-Jet Explosion Pot:

It is made of insulating material and has channels on one side which act as arc splitters. The function of the arc splitters is to provide an increased arc length corresponding to a given amount of contact travel, and also to provide cutting edges across which the arc is attenuated, weakened and finally interrupted.

Oil movement is checked by the pressure of the arc itself until a current zero (the pressure then is very low) when the contacts of the pot are ejected across the arc path. The pot is called the cross-jet explosion pot because it performs cross or transverse extinction of the arc.

On occurrence of a fault, the breaker moving contact begins to separate and with the separation of contacts, the arc is initially struck in the top of the pot. The gas generated by the arc exerts pressure on the oil in the back-passage. When the moving contact uncovers the arc splitter ducts, fresh dielectric oil is forced across the arc path. The arc is, thus, driven sideways into the arc splitters which increase the arc length, causing arc extinction.

The performance of cross-jet explosion pot is very good when heavy fault currents are to be interrupted. However, for low fault currents, the gas pressure is small and consequently the pot does not give a satisfactory operation. This type of oil circuit breaker is suitable for interrupting heavy currents at high voltage (66,000V).

(c) Self Compensated Explosion Pot:

It is combination of the cross-jet explosion pot and the plain explosion pot and thus operates quite satisfactorily both at heavy currents as well as low currents. It consists of two chambers, the upper being a cross-jet explosion pot with two lateral orifices while the lower is a plain explosion pot.

When the short circuit current is low, the rate of generation of the gas is also quite low and as the pressure builds up the tip of the moving contact has time to reach the bottom chamber and there is only a small pressure leakage through the lateral jets because the movement of the oil to them is obstructed by the right angel bends and also by the presence of the arc.

The arc is quenched by the plain explosion pot action. When the short-circuit current is heavy the rate of generation of gas is very high and the device operates as a cross-jet explosion pot, arc extinguishes when the first or second lateral orifice is uncovered by the moving contact. Hence the arc is being self-compensated for low as well as high values of short-circuit currents.

(d) Oil Blast Explosion Pot:

This is an example of axial extinction and double break. Such a pot consists of mainly three compartments the upper fixed contact, intermediate contact and low hollow moving contact, which are in contact under pressure. It has two chambers—the upper and the lower and they are connected through holes, when the circuit breaker is closed. On occurrence of fault instead of one, the two lower contacts move downwards together and an arc is struck between the top fixed and intermediate moving contact.

Because of this arc a high pressure is developed in the upper tank and there is no relief from this pressure until the intermediate contact reaches its top after its maximum travel, now, the lower moving contact detaches from the intermediate contact and another arc is struck between the intermediate and lower moving contacts. The pressure developed by the arc is subsided by the movement of the oil through the lower moving contact which is hollow. When the arc current goes to zero, the oil is forced through the arc and it is extinguished. Long arcing time is the only drawback of this type of oil circuit breaker.

2. Externally Generated Pressure or Forced Blast or Impulse Type Oil Circuit Breakers:

In the self-blast oil circuit breakers, the arc itself generates the pressure required to force the oil across the arc path. The major drawback of such breakers is that arcing times tend to be long and inconsistent with fault currents considerably less than rated currents. It is due to generation of reduced gas at low values of fault currents. This problem is overcome in forced-blast oil circuit breakers in which the necessary pressure is developed by external mechanical means independent of the magnitude of the fault currents to be interrupted.

In this type of circuit breaker, the oil pressure instead of being created by the arc is created mechanically by the piston-cylinder arrangement. The movement of the piston is mechanically coupled to the moving contact, thus automatically oil pressure is generated and thus high speed interruption is attained.

The performance of such a breaker at low currents is more consistent than with self-blast type since the oil pressure developed is independent of the fault current to be broken. An important advantage of this design over the other conventional design is that the quantity of oil required is reduced to one-quarter. The possibility of current chopping can be avoided by using resistance switching.

Circuit breakers have also been built up which combine the self-blast and forced blast.

Type # 3. Low Oil or Minimum Oil Circuit Breakers:

The design and special features of what may be described as the bulk oil type of circuit breaker for voltage from the lowest up to 380 kV. But in case of bulk oil circuit breakers the quantity of oil required reaches very high figures with the increase in system voltage. For example, a 110 kV, 3,500 MVA breakers may need 8 to 12 thousand kg of oil, while a breaker of the same rating output for 220 kV may need 50 thousand kg of oil.

These large quantities of oil are subject to the carbonisation, sludging, etc., which occurs due to arc interruption and other causes, reducing (in time) the insulating properties and requiring regular maintenance. Not only this but also the expenses, tank size and weight of the breaker are increased. In case of a floor mounted large bulk oil circuit breaker, access to the contact system is not easy and is obtained only through access parts in the tank side after the oil has been pumped away to storage.

In bulk oil circuit breakers the oil performs two functions. Firstly, it acts as an arc extinguishing medium and secondly it insulates the live parts from earth. Fortunately the quantity of oil actually required for arc extinction is only about one-tenth of the total, the rest being used for insulation. This fact led to the development of low oil content or live tank circuit breaker in which the arc chamber contains a minimum quantity of oil and is mounted on a porcelain insulator to insulate it from the earth, the arc chamber itself is, in fact of bakelite paper enclosed in porcelain, so that such a circuit breaker appears as in Fig. 10.10, the lower porcelain being the support and the upper porcelain enclosing the contacts.

Such type of a circuit breaker has the added advantage that it requires less space than the bulk oil type, an important feature in large installations. Such circuit breakers are less suitable where very frequent operation occurs because degree of carbonisation produced in the small volume of oil is far more severe than in the conventional bulk oil circuit breakers and this leads to deterioration in the dielectric strength across the contact system in the open position. As regards quenching of the arc the oil behaves identically in bulk oil as well as in low-oil content circuit breakers. The methods of arc extinction, therefore, are basically same in both types, the difference lies in the application of these methods to achieve the desired arc extinction by various constructions of arc control device or interrupters.

Reverting to Fig. 10.10 it will be seen that the upper arcing chamber comprises a synthetic resin bonded paper cylindrical enclosure within a porcelain insulator. An annular space between these is filled with oil as an insulating medium but, again, this is physically separated from that in the arcing chamber and thus cannot be contaminated.

The circuit breaker is of the single-break type in which a moving contact tube moves in a vertical line to make or break contact at the upper fixed contacts mounted within the arc control device. A lower ring of fixed contacts is in permanent contact with the moving arm to provide the other terminal of the phase unit.

Within the moving contact tube is a fixed piston, which, as the tube moves downwards on opening, forces the column of oil inside the tube into the arc control device. This has two effects, firstly, a partial pressure balance is ensured, so that the pressure generated inside the arc—control device has little effect on the acceleration of the moving contact and, secondly, the amount of cavitation caused by the removal of the moving contact is controlled and the efficiency of arc-extinction is increased.

The “turbulator” arc-control device is built up of oil-impregnated vulcanized fibre plates held under compression by tension members, the plates being arranged to form a series of vents on one side of the arc, and a series of oil pockets on the other. The fixed arcing tip is so arranged that when the circuit breaker opens, the arc is drawn in front of the vents where it can be most easily extinguished.

To ensure that oil vaporised by the arc is replaced quickly when the arc is extinguished a filling valve is fitted in the top casting of the arc-control device. This valve, lightly spring-loaded, closes when pressure is set up within the device, but as this pressure dies away when the arc is extinguished, the valve opens to allow an inrush of oil.

When a circuit breaker closes on to a fault there is always a danger that pre-arcing will occur as the moving contact approaches the fixed contacts. In this design this possibility is minimised by fitting inserts of anti-tracking material on the inside edges of the “turbulator” plates.

The upper arcing chamber contains a separator which eliminates, by centrifugal action, loss of oil when the breaker operates under fault conditions. It has a spring-loaded vent valve, a breather to prevent moisture from entering the circuit breaker, and a safety diaphragm, under the domed cover, designed to lift and protect the circuit breaker from damage if excessive pressure should arise in the circuit breaking compartment.

Circuit breakers of this type can be used either out-of-doors or indoors, and be mounted either on pedestals or at ground level. At certain voltages and interrupting capacities, two breaks per phase are used.

Current transformers, which are incorporated in the bushings of the dead tank type, usually, have to be mounted separately in the live-tank type. These are usually of post type with low-oil content, the transformer being housed within the porcelain housing and filled with oil under vacuum. Low oil-circuit breakers are now available for all voltages and for the highest breaking capacities.

The low oil circuit breakers have the advantages of requirement of lesser quantity of oil, smaller space requirement, smaller tank size, smaller weight, low cost, reduced risk of fire and reduced maintenance problems.

However, it suffers from the following drawbacks when compared with a bulk oil circuit breaker:

(i) Increased degree of carbonisation due to smaller quantity of oil.

(ii) Rapid deterioration of dielectric strength of oil due to high degree of carbonisation.

(iii) Difficulty in removal of gases from the contact space in time.