Vacuum Circuit Breaker (VCB)

Introduction of VCB

        A vacuum circuit breaker VCB  is such kind of circuit breaker where the arc quenching takes place in vacuum.The operation of opening and closing of current carrying contacts and associated arc interruption take place in a vacuum chamber in the breaker which is called vacuum interrupter.The vacuum interrupter consists of a steel arc chamber in the center symmetrically arranged ceramic insulators.The vacuum pressure inside a vacuum interrupter is normally maintained at (10)-6 bar.
        Vacuum circuit breaker are employed for outdoor applications ranging from 22kv to 66kv,
Even with limited rating say 60 to 100MVA ,they are suitable for majority of application in rural areas.

Construction of VCB
  •   It consist of fixed contact, moving contact and arc shield mounted inside a vacuum chamber.
  • The movable member is connected to the control mechanism by stainless steel bellows.This enables the permanent sealing of the vacuum chamber so as to eliminate the possibility of leak.
  • A glass vessel or ceramic vessel is used as the outer insulating body.
  • The arc shield prevents the deteriorating of the interval dielectric strength by preventing metallic vapours falling on the cover.
Working of VCB
 
  • When the breaker operate, the moving contact separates from the fixed contact and arc is struck between the contacts.The production of arc is due to the ionization of metal ions and depends very much upon the material of contacts.
  • The arc is quickly extinguished because the metallic vapours, electrons and ions produced during arc are diffused in a short time and seized by the surfaces of moving and fixed members of shields.
  • Since the vacuum has very fast rate of recovery of dielectric strength, the arc extinction in a vacuum breaker occurs with a short contact separation.
Advantages of VCB
 
  • The vacuum circuit breaker has a long life.
  • The vacuum circuit breaker is fast in operation.
  • No exhaust of gas to the atmosphere.
  • No fire hazard .
  • Unlike Oil circuit breaker (OCB) the explosion of vcb is avoided.This enhances the safety of the operation personnel.
  • VCB is almost maintenance free.
Disadvantages of VCB
 
  • It is uneconomical at voltage exceeding 38K volts.
  • The cost of the breaker becomes excessive at higher voltage above 38KV more than two numbers of the circuit breaker are required to be connected in series.
  • VCB production is uneconomical if produced in small quantities.
Maintenance of VCB

           A vacuum circuit breaker needs less maintenance and no inflammable substance that support fire compared with that of oil in the Oil circuit breaker.
General

 Clean the circuit breaker to remove dust and dirt.The envelop of the vacuum interrupter and insulating materials should be wiped with a clean cloth.
Vacuum
 
  • After every 5000 switching of load current check the contact for wear.Replace if the contact piece has worn out.
  • With stand test at 22KV for one minute.
Operating Mechanism
  •  Apply a small amount of oil to rotary sections ,sliding surfaces,pins,coupling section etc.Use grease for the operating shaft and bearings and machine oil for other parts.
  • Check for rust and wear for coupling section.
  • Visual check of spring for rust and deformation.
  • Check clampimg section for loosened bolts and nuts ,broken spring washer and snap pins etc.
  • Check control circuit for loosened connections.
  • Check control switches for contacts are damaged by arcing.

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