In case of plain oil circuit breakers artificial control of arc is less therefore long arc length is necessary for arc quenching. Its necessary that final arc extinction should occur while the contact gap is still shorter. For this some arc control is incorporated and the breakers are then called arc control circuit breakers. There are two types of such breakers:

1. Self-blast oil circuit breakers – in which arc control is provided by internal means i.e. arc itself

facilitates its own extinction efficiently.

2. Forced blast oil circuit breakers – in which arc control is provided by mechanical means external to the circuit breaker.

Self-blast oil circuit breakers – In this type of breakers, the gases produced during arching are confined to a small volume by the use of an insulating rigid pressure chamber or explosion pot surrounding the contacts. The space available for the arc gases is restricted by the chamber so a very high pressure is developed to force the oil and gas through or around the arc to extinguish it. The magnitude of the pressure depends upon the value of fault current to be interrupted. The arc itself generates the pressure so such breakers are also called self-generated pressure oil circuit breakers.

The pressure chamber is relatively cheap and gives reduced final arc extinction gap length andarcing time as against the plain oil breaker. Different types of explosion pots are described below:

a. Plain explosion pot – it is a rigid cylinder of insulating material and encloses the fixed and moving contacts as shown in fig 8. The moving contact is a cylindrical rod passing through a restricted opening called throat at the bottom. When fault occurs the contacts get separated and an arc is struck between them. The heat of the arc decomposes oil into a gas at very high pressure in the pot. This high pressure forces the oil and gas through and around the arc to extinguish it.

If the arc extinction will not take place when the moving contact is still within the pot, it occurs immediately after the moving contact leaves the pot. Its because, emergence of moving contact will be followed by violent rush of gas and oil through the throat producing rapid extinction.

Limitation of this type of pot is that it cannot be used for very low or very high fault currents. With low fault currents, the pressure developed is small, thereby increasing the arcing time. And with high fault currents, the gas is produced so rapidly that the plot may burst due to high pressure. So this pot is used on moderate short circuit currents only where rate of gas evolution is moderate.

b. Cross jet explosion pot – Fig 9 shows the cross jet pot which is made of insulating material and has channels on one side that acts as arc splitters. The arc splitters help in increasing the arc length, thus facilitating arc extinction. When fault occurs, the moving contacts of the circuit breaker begins to separate and arc is 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 oil is forced across the arc path. The arc is therefore driven sideways into the arc splitters, which increase the arc length, causing arc extinction.

The cross jet explosion pot is used for interrupting heavy fault currents. For low fault currentst he gas pressure is small and consequently the pot does not give a satisfactory operation.

c. Self compensated explosion pot – This pot is a combination of plain explosion pot and cross jet explosion pot. So it can interrupt low as well as heavy short circuit currents. Fig 10 shows the self compensated explosion pot.

Forced Blast Oil Circuit Breaker

In this type of circuit breaker there is a piston attached to a moving contact.When fault occurs the moving contact moves and hence the piston associated with it also moves producing pressure inside the oil chamber.So the oil gets movement or turbulates and  quenches  the arc.

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