How to do MCCB maintenance

MCCB Maintenance:

Guidelines for Inspection and Preventive Maintenance of Molded Case Circuit Breakers (mccb) Used in Commercial and Industrial Applications. the following MCCB inspections and test procedures listed and explained in details.


  1. Inspection of MCCB enclosure
  2. Preventative maintenance //
    • Environmental evaluation
    • Interchangeable trip units
    • Wire connectors and connections
  3. Mechanical operation tests
  4. Insulation resistance tests
  5. Individual pole resistance test (Millivolt drop test)
  6. Inverse time overcurrent trip test (Overload current injection test)
  7. Instantaneous overcurrent trip test (High current injection test)
  8. Rated hold-in test (Full load injection test)
  9. Accessary device tests //
    • Shunt trip release
    • Undervoltage trip release
    • Electrical operator breaker assembly
    • Auxiliary switch
    • Alarm switch



But, generally, maintenance on molded case circuit breakers (MCCB) is limited to three basic actions:

  1. Proper mechanical mounting,
  2. Electrical connections, and
  3. Periodic manual operation

Most lighting, appliance, and power panel circuit breakers have riveted frames and are not designed to be opened for internal inspection or maintenance. All other molded case circuit breakers, that are UL approved, are factory-sealed to prevent access to the calibrated elements. An unbroken seal indicates that the mechanism has not been tampered with and that it should function as specified by UL.

A broken seal voids the UL listing and the manufacturers’ warranty of the device. In this case, the integrity of the device would be questionable. The only exception to this would be a seal being broken by a manufacturer’s authorized facility.

Molded case circuit breakers receive initial testing and calibration at the manufacturers’ plants. These tests are performed in accordance with UL 489, Standard for Safety, Molded Case Circuit Breakers, Molded Case Switches and Circuit Breaker Enclosures. MCCB, other than the riveted frame types, are permitted to be reconditioned and returned to the manufacturer’s original condition. In order to conform to the manufacturer’s original design, circuit breakers must be reconditioned according to recognized standards.

The circuit breaker can fail to open due to a burned out trip coil or because the mechanism is frozen due to dirt, dried lubricant, or corrosion. The overcurrent device can fail due to inactivity or a burned out electronic component.

Many problems can occur when proper maintenance is not performed and the breaker fails to open under fault conditions. This combination of events can result in fires, damage to equipment, or injuries to personnel.

Common sense, as well as manufacturers’ literature, must be used when maintaining circuit breakers. Most manufacturers, as well as NFPA 70B, recommend that if a molded case circuit breaker has not been operated, opened or closed, either manually or by automatic means, within as little as six months time, it should be removed from service and manually exercised several times. This manual exercise helps to keep the contacts clean, due to their wiping action, and ensures that the operating mechanism moves freely. This exercise however does not operate the mechanical linkages in the tripping mechanism.

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