Power System / Maintenance / Maintenance of Minimum Oil Circuit Breakers

Maintenance of Minimum Oil Circuit Breakers

The oil is an insulating and arc quenching medium. In a conventional bulk oil circuits the oil servers as both insulating and arc quenching medium. But a minimum oil circuit breaker, only utilizes, the arc quenching properly of the oil. The solid core ceramic insulator provides necessary insulation to the circuit breaker whereas; the circuit breaker consists of an oil dashboard. This dashboard is an oil chamber, which contains required oil for quenching of arc between the contacts of the CB.

The minimum oil circuit breaker has two portions in its pole. The top portion is a porcelain chamber, contains the oil. The making and breaking of the contacts of the CB occur in this chamber. Whereas the bottom porcelain chamber is hollow, only supports the top chamber. Also, the moving contact lever runs through this bottom chamber.

The main advantage of a minimum oil circuit breaker is that, it uses less quantity of oil than a BOCB. Hence, the risk of fire hazard is low in a MOCB.

The following actions are necessary for proper maintenance of MOBC.

  1. We should check the breaker operation if any doubt arises regarding its operation. In that case we must check the operation breaker from both remote and weal. For that we must operate the circuit breaker electrically from remote (i.e. Control Room) and from local (i.e. switch yard). Also we check the proper operation of the breaker locally by pushing its mechanical on off push button or on off lever.If we found any coating developed on the sliding surface, we must remove it before we put the CB in the service.
  1. Also after a doubtful operation off the MOCB, we should contact off the CB, for any burning sign. Also we should not forget to looses and tighten the tip before we finally tight it.
  2. Also we should check the condition of arc quenching chamber. If we found any burning spot on the chamber body, we must remove it from the breaker. Then we rinse it with clean transformer oil. After that, only we refit the comer inside the breaking unit.

But if the extinguisher, chamber is in quite bad condition, we must replace it.

  1. We should clean the entire CB, including its mechanism box, gear assembly, insulators, breaker heads, power contacts, etc with non fluffy cloths. If there is any rust in any portion of the mechanism, we must remove it. After cleaning the gears and moving assemblies, we must properly lubricate them with proper grade grease.

Also, if there is any carbon disposition on the insulator body, we should clean it trichloro ethylene or acetone.

During lubricating, we should remember to avoid over breaking of gear wheels. Also we must keep the friction clutch, free from lubrication.

Also, we must lubricate the sliding parts, catches, rollers and operating magnet with lubricating oil not by grease.

Lastly, we must also follow the greasing and lubricating procedures provided in the user manual of the MOCB.

  1. We must check all the looking elements of tie rod assembly as well as of the other portions of the breaker mechanism. We need to do this at every six months. If we found any damaged or slightly damaged locking element, we must immediately replace it with new one.

Although after every over handling of MOCB we must replace all the looking elements, even these are damaged.

  1. During operations of circuit breaker, there many always are a mechanical shaking. These mechanical shaking may cause loosing of foundation bolt and after hardware bolts. So, we should check the tightness of all nuts and bolts at every 6 months. If these is may loose bolts, we need to immediately tight it.
  2. After that we must check the power connections of the CB. If there any oxidation, we should remove the oxide before re-tight it. We must always use spring washers where there is a chance of loose connection during heavy electrical over impulses.
  3. We must also check cleanliness of auxiliary switch at every half year. If any broken part is found we should replace it and also ensure the proper configuration of NOs and NCs after maintenance.
  4. The brushes of spring charging motor may get co-rotted due to excess friction. We must replace these brushes. We should do it at every six months.
  5. We need to oil level the MOCB poles, on the oil level glass fitted at the top head of the poles, on monthly basis. Also we need to search for any visible oil leakage on the CB pole.

If the oil level becomes low, we must top up with new oil. Again, if these is any visible oil leakage, we must plug it with proper action.

  1. Quarterly we inspect the MOCB for any defects in planting and door gasket of mechanism kiosk.

If we ignore it, there may be permanent holes in the kiosk and replace or repair the damaged gasket without neglecting it.

  1. In addition to all above mentioned steps, we should also check the duty cycle of the breaker operation at every year.

Leave a Comment