Motor Control Centers are the central nervous system of industrial facilities, and being comfortable around them separates journeyman-level industrial electricians from the guys who get the good work. An MCC is essentially a lineup of individual motor starters (buckets) in a common enclosure, with a common bus feeding all of them. Each bucket contains a disconnect, a motor starter (contactor plus overload relay), and control terminals.
Common failure modes: (1) Overload relay trip - check the motor nameplate FLA, verify the overload relay is set correctly, check for single-phasing or high ambient temperature. (2) Contactor failure - pitted contacts cause voltage drops, welded contacts cause motors to run when they shouldn’t. (3) Control circuit failure - a 120V control circuit powers the coil. If the coil doesn’t energize, check the control fuse, the stop/start circuit, and the interlocks. (4) Loose bus connections - thermal cycling loosens connections over time; MCC bus connections should be torqued on a maintenance schedule.
Safety on MCCs: treat all metal parts as energized until proven dead and locked out. The main bus in an MCC is not interrupted by individual bucket disconnects - if you need to work on the bus, the whole lineup has to come down.