The three main tests that are carried out as part of MCCB maintenance are described below.
Insulation Resistance
This test involves disconnecting the MCCB and testing the insulation between phases and across the supply and load terminals. If insulation resistance has dropped below the values recommended by the circuit breaker manufacturer, the MCCB will not be able to provide reliable protection.
Contact Resistance
This test consists of testing the resistance of the electrical contact. As in insulation testing, the measured values must be compared with manufacturer provided values. Under normal conditions, the contact resistance is a very low value since the MCCB must allow operating current through with a minimal voltage drop.
Tripping Test
This test consists on simulating overcurrent and fault conditions, and observing the response of the MCCB. Since this test involves high current and the MCCB will heat, it must be carried out last – otherwise the insulation and contact resistance measurements will be altered by temperatures. Normally, this test consists of two parts:
Molded Case Circuit Breaker Maintenance
Since molded case circuit breakers protect important loads that typically draw high currents, periodic maintenance and testing are fundamental in order to guarantee reliable operation. Normally, maintenance procedures for MCCBs include:
Lubrication
Since this procedure requires opening the MCCB, it is only possible for models that allow it. Adequate lubrication ensures that the manual disconnection switch and the internal moving parts will operate smoothly. This is very important during a fault, where the internal mechanism must operate within a short timeframe.
Cleaning
Dirt can cause a deterioration of MCCB components over time. If dirt includes a conducting material, there is also the risk of creating a path for current and causing an internal fault. Normally, a vacuum cleaner is used to remove dirt from an MCCB.
Adequate cleaning helps with visual inspection, since damage signs may not be so obvious if the molded case circuit breaker is dirty.
