Minimum Oil Circuit Breaker (MOCB)

Minimum Oil Circuit Breaker (MOCB)



  • The supporting, current interrupting and top chamber are made up of porcelain. Hence, clearance between live parts and enclosure is small and requires less quality of oil, hence the breaker is called as Minimum oil circuit breaker (MOCB). 
  • The chambers are completely filled with oil. The upper chamber of oil is does not come to lower.
MOCB

  • The fixed contact is enclosed in the quenching chamber. Moving contact makes sliding contact with lower fixed contact. The operating rod is operated by operating mechanism.the three poles are operating simultaneously. The voltage ratings are from 3. 6 kv to 420 kV. 
  • Wnen the contacts carrying current are separated in the oil the arc is developed. The heat of arc causes decomposition of oil and gases are formed. These gases expand due to heating of arc. The gas flowing near the contact zone causes cooling and splitting of the arc and extinguishes the arc.
  • After the arc is quenched at last current zero instant, fresh dielectric medium is inserted between the contacts.

Advantages:-

  1. It requires lesser quantity of oil.
  2. It requires smaller space.
  3. Maintenance is less.
  4. Fire risk is reduced.
  5. Cost per breaking capacity in MVA is less. 
  6. Suitable for both manual and automatic operation. 

Disadvantages:-

  1. Smaller quantity of oil is used so the degree of Carbonisation is increased. 
  2. It is difficult to remove gases from the space between contacts. 
  3. Carbonization is more so oil deteriorates rapidly due to frequent operations.
  4. Difficulty in locating CT.
  5. possibility of fire and explosion.

Maintenance of oil circuit Breakers (OCBs):-

The main inspection is:
1. Checking of contacts.
2. Checking of dielectric strength of oil.
  • Due to the switching ON-OFF on load and in case of tripping in faults, the contacts get burnt by arcing. The insulating oil also lost by some amount and loses it dielectric strength due to carbonization. This results in reduction rupturing capacity of the breaker.
Following is the maintenance schedule (every after 3 months/6 months).
  1. Inspect the condition of contacts, if burnt replace.
  2. check dielectric strength of oil seeing the colour of oil. If badly discolored then take it out for filtration or replace it by new one. Good oil withstands 30 KV for 1 minute in a testing set of 4 mm gap.
  3. Check the insulation for possible damage and clean the surface to remove carbon deposits with fabric cloth.
  4. 0il level to be checked, if gone down make up the level by fresh oil.
  5. Mechanisms shall be checked frequently.














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