Bulk Oil Type Breaker

In Bulk Oil Circuit Breaker oil serves a two-fold purpose, i.e., as means of extinguishing
the arc and also for providing insulation between the live parts and the metallic tank.

This is the oldest amongst the three types having been developed towards close
of the nineteenth century.
In its simplest form the process of separating the current carrying contacts
was carried out under oil with no special control over the resulting arc
other than the increase in length caused by the moving contact's

As the power systems began to develop resulting in higher voltages and higher
fault levels, plain break type breaker could no longer keep pace with the requirements.

Various methods of controlling the breaking process were investigated and developed.

This led to the development of controlled break oil Circuit Breaker.
This employed pressure chamber and is still widely used because it is relatively
cheap to make and gives greatly improved performance in terms
of final extinction, gap length and arcing time, as against the plain break
oil Circuit Breaker. Various designs exist according to the preferences
and requirements of individual manufacturers and designations such
as ‘Cross Jet Type’, ‘Explosion Pot’ and ‘Baffle pot’, etc.


Many oil Circuit Breakers feature special arc control devices most of which are based
on the simple pressure chamber principle but incorporate certain modifications
 aimed at improving the breaking capacity.

Depending on the working principle of these special pressure chambers
the breakers are designated as: impulse oil Circuit Breakers deign grid breakers,
 breakers with double arc pressure chambers and axial jet pressure
chamber oil Circuit Breakers.


For general illustration, a view of the contact actuating mechanism of 33 kV,
type OKM, bulk oil breaker manufactured by M/s English Electric Co. is shown in Fig (1)

The contacts are actuated by a lever assembly L housed within the top-plate
and connected to the lifting bridge N by links M.


The beam lever assembly is pivoted on a shaft H fixed in bearings in the top-plate and is operated by a tie rod G connected by an adjustable coupling J to the vertical pull-rod K from the Circuit Breaker operating

mechanism. An oil seal F is fitted to prevent leakage from top-plate and an indicator arm is operated by a pin E on the driven end of the beam lever. The lifting bridge N which carries the lift rods Q and moving contacts R moves vertically on guide I, rods D fixed in the top-plate, At the top end of each guide rod and fastened to the top plate by clips A is an accelerating spring C.
These springs are compressed by the lifting bridge during the closing stroke and provide a throw off force when the breaker is tripped open. The mechanism is prevented from over traveling the closed position by adjustable stops
B in the top-plate. At the lower end of each guide rod is an oil dashpot assembly P.

These oil buffers arrest the downward or contact opening movement. The working part of the breaker is cylindrical chamber known as an interrupter pot. The view of the interrupter is shown in Fig (2) the interrupter pot is screwed and locked on to an interrupter top block.  The interior of the chamber is fitted with insulating dividing plates which form labyrinths and oil flow passages. Assembled in the top of the chamber is the fixed spring loaded cluster type contact, the fingers of which are arranged in a circular formation to engage with the moving contact which is of the solid rod of candle type.
Alternate cluster fingers are extended to form arcing contacts.

These parts carry the arc current and protect the normal current carrying parts from burning. The moving contacts are clamped by pinch bolts at each end of a cross bar which is bolted to the lift rod.

The separation of the contacts and drawing out of the arc take place in the interrupter pot which almost completely restricts the movement of the oil within it.


The internal space available for gas is thus little more than that swept out by the moving contact, and a pressure is set up which depends upon the rate of gas production and its rate of flow through the vents.


The pressure rise and the condition resulting there from are believed to play a large part in giving this type of oil Circuit Breaker a very much higher breaking capacity than the plain break type.


             Fig (1)www.sayedsaad.com

                            Fig (2)www.sayedsaad.com