VCB(Vacuum Circuit Breaker)

the use of vacuum circuit breaker is mainly restricted to  system voltage below 38 kV. The characteristics of vacuum as medium and cost of the vacuum CB does not makes it suitable for voltage exceeding 38 kV. In the past for higher transmission voltage Oil Circuit Breaker (OCB) and Air Blast Circuit Breaker (ABCB) were used. These days for higher transmission voltage levels  SF6  Circuit Breakers are largely used. OCB and ABCB have almost become obsolete.  In fact in many installations SF6  CB is used for lower voltages  like 11 kV, 6 kV etc.. Sulphur Hexafluoride symbolically written as SF6  is a gas which satisfy the requirements of an ideal arc interrupting medium. So SF6  is extensively used these days as an arc interrupting medium in circuit breakers ranging from 3 kv  upto 765 kv class. In addition to this SF6 is used in many electrical equipments for insulation. Here first we discuss in brief, some of the essential properties of  SF6 which is the reason of it's extensive use in circuit breakers SF6 gas has high dielectric strength which is the most important quality of a material for use in electrical equipments and in particular for breaker it is one of the most desired properties. Moreover it has high Rate of Rise of dielectric strength after arc extinction. This characteristics is very much sought for a circuit breaker to avoid restriking. SF6 is colour less, odour less and non toxic gas. SF6  is an inert gas. So in normal operating condition the metallic parts in contact with the gas are not corroded. This ensures the life of the breaker and reduces the need for maintenance. SF6 has high thermal conductivity which means the heat dissipation capacity is more. This implies greater current carrying capacity when surrounded by SF6 . The gas is quite stable. However it disintegrates to other fluorides of Sulphur in the presence of arc. but after the extinction of the arc the SF6  gas is reformed from the decomposition. SF6 being non-flammable so there is no risk of fire hazard and explosion.  MCCB,VCB,ACB all are of different type with same  functioning. all of them are used to isolate the power or to trip the power at the time of fault in line and thus save the circuit from further damage and isolate unheathy part from healthy one. if the circuit if of low rating we can go ahead with MCCB but for high rating circuit we go for costlier CB that is ACB/VCB because they are more reliable and have long life. Circuit2017 circuit2017.