Small Oil Volume Breaker

 As the system voltages and fault levels increased the Bulk Oil Breakers required huge quantities of insulating oil and
 became unwieldy in size and weight.

 This added enormously to the cost of a power system.  Simultaneously improvements were made in the technique
 of ceramics.

 The function of oil as insulating medium in the Bulk Oil Breakers was transferred to the porcelain containers.

 Only a small quantity of oil was used to perform its functions as arc quenching medium. This led to the development
 of small oil volume or low oil content breakers in the continent of Europe.

 Like the Bulk Oil Breakers these have also since then passed through many stages of development with varying designs
 of the arcing chambers. Today the small oil volume breakers are available for voltages up to 36 kV and the fault levels
 associated therewith.
 Contrary to the operation of the impulse type Circuit Breaker, such as air blast Circuit Breaker, in which arc extinction
  and dielectric  recovery
 are affected by means of an external quenching medium, the process of arc extinction in  the small
oil volume
 Circuit Breaker is of internal
 thermo- dynamic origin.

 During the tripping operation an arc strikes in oil between the moving contact and the fixed contact's
 This arc is elongated vertically in the explosion pot until the distance traveled is sufficient to withstand the
 voltage between contacts.

 The increase in internal pressure due to the Splitting up
 and vaporization of oil by the arc creates a rapid movement of the extinguishing

 medium round the arc This self-quenching effect causes a rapid cooling of the ionized column along its whole

 Length due to partition of the explosion pot and the dielectric recovery is sufficiently rapid.

 To prevent the arc restricting after a natural Passage Through zero. The electric arc itself has, therefore,

 Supplied the necessary energy for its own extinction. There are now numerous

 manufacturers of small oil volume breakers                                                                    

 However, to illustrate the principles of working, the sectional view of working portion of 170 kV 3500 MVA.
 Breakers of

Fig (4)


 M/s Delle France have been shown in Fig. (4) the most important part of the breaker is its extinguishing chamber.
 This takes the form of an insulating cylinder containing oil, in the axis of which moves the contact rod and within
 which breaking occurs.

 The arcing chamber is supported at its base by a casing enclosing a mechanism whose function is to

 move the contact rod  According to the impulses given by the control mechanism. In the on position, the current flows from
 the Upper current terminal (1) to the contact fingers, (2) Follows the movable contact rod (7) and reaches the current terminal (10)
 across the lower contact fingers (8). At the beginning of the stroke and before breaking, the contact rod strongly pulled down.

 Wards by the tripping springs, starts a high speed opening motion.
 Then, an arc strikes between the contact rod tips (6) and the stationary

 Arcing ring (3) protecting the upper contact fingers.

 At this moment gases escape without hindrance towards top of the apparatus.

 The contact rod rapidly reaches a very high linear speed; it moves
 the arc downwards and forces it to enter the explosion pot (5) where it is maintained rectilinear and is elongated in a direction opposite
 to the   release of gases towards fresh oil. Since the arc is as short as possible the arc voltage is minimized and the energy dissipated is
   reduced.  
  Still, since the gases can no longer develop freely, they generate a considerable pressure in the explosion pot (5), thus producing a violent
 upward  axial blast of oil vapor, exhausting the highly ionized gaseous mass.

 The optimum distance is thus obtained, the jet of oil causes the dielectric strength to be rapidly increased, and at the
 following current zero, the arc is impeded from restricting and the breaking is thus achieved.

 The explosion pot (5) is intended to withstand high pressures.
 It is partitioned into several components by means of discs whose function is to retain a certain quantity of fresh oil while the first break
 is proceeding; this allows a second break to occur with complete
 safety at the full short circuit current. The low oil content Circuit Breakers require separate current Transformers of wound type. Still at
 all  voltages from 33 kV and above the costs of these breakers
 inclusive of current Transformers compete favorably with that of the Bulk Oil Breakers.