Maintaining Molded Case Circuit Breakers: Key Steps to Ensure Proper Operation

Molded case circuit breakers (MCCBs) play an essential role in electrical distribution systems. They help prevent hazards and minimize damage by interrupting the circuit upon detecting excess current. As with any electromechanical device, MCCBs require periodic maintenance to function properly over their service life. This article outlines three key areas of focus for MCCB maintenance.

Visual Inspections

Visual inspections of MCCBs should be performed regularly, at least once a month or according to the manufacturer’s recommendations. Check that the MCCB shows no signs of physical damage like dents, cracks, or corrosion. Examine the housing for any discoloration that could indicate overheating. Check all electrical connections for tightness, as loose or oxidized connections can pose a fire risk and impair proper tripping of the MCCB.

Ensure the MCCB handle and button spring back to the “ON” position after they have been turned to "OFF" or "TRIPPED”. This confirms that the internal trip mechanism is moving freely and releasing properly. Note any unusual wear or noises, as this may indicate components like the trip unit or contacts need replacement. Log and report any issues found so corrective actions can be taken promptly.  Visual inspections take little time but are crucial for identifying conditions that could compromise safety if left unaddressed.

Mechanical Operational Tests

In addition to visual checks, mechanical operational tests should be performed on each MCCB at least once a year or in accordance with the manufacturer’s specifications. The tests are intended to ensure all parts of the MCCB are functioning properly through the complete range of manual and automatic tripping operations.

Open and close the MCCB contacts manually using the handle to ensure smooth operation. Activate all of the release functions - undervoltage, overload, and short circuit trips - to verify the trip unit and other internal mechanisms operate correctly and promptly trip the MCCB to the “OFF” position. Check for proper indications on the MCCB display before and after each type of trip.

Correct any issues before returning the MCCB to service. Recurrent problems could signify that replacement of internal components may soon be required.

Additional Maintenance for Electronic MCCBs

Where electronic components have been incorporated for functions such as trip unit control or communications, additional maintenance procedures are needed. In many electronic MCCBs, the trip unit settings can be viewed and adjusted through a display and control module. The settings should be checked against the system specifications to make sure they remain within the proper range for optimal protection and selectivity.

Batteries used for displays, trip unit memory, or other purposes will require periodic replacement to avoid operational issues. Surge protection components may similarly need replacement at certain intervals recommended by the manufacturer. Fuses used to protect printed circuit boards within the electronic units should also be checked.

Performing thorough and regular maintenance as directed by the MCCB manufacturer is vital to providing maximal protection and a long total service life. Most maintenance procedures are relatively straightforward but should only be completed by fully trained personnel using proper safety precautions at all times. With the high power levels controlled and critical protection functions performed by MCCBs, maintenance is not an area where chances should be taken.