Moulded Case Circuit Breakers (MCCBs)

These are small, compact air circuit breakers fitted in a moulded plastic case. They have a lower current rating than ACBs (30-1500A) and generally a lower breaking capacity. Figure 11.8 (a) is a pictorial representation of various MCCBs and Figure 11.8(b) depicts a sectional view of the same.

Figure 11.8(b) - Sectional View of a Moulded Case Circuit Breaker

MCCBs usually have an adjustable thermal overload setting and an adjustable or fixed magnetic over-current trip for short-circuit protection built into the case. An under voltage; trip coil may also be included within the case.

A hand-operated lever usually closes MCCBs but a motor closing arrangement can also the incorporated. MCCBs are claimed to be reliable, trouble free and require negligible maintenance. If the breaker remains 'On' for long periods, it should be tripped and closed a few times in order to free the mechanism and clean the contacts. Terminals should be checked for tightness otherwise damage due to overheating would occur. The front cover of larger MCCBs (around 400A rating) can usually be removed, interior dust blown out and the contracts dressed with a file if required.

Following tripping after a short-circuit fault, the breaker should be inspected for damage, checked for correct operation, and its insulation resistance measured. A reading of at least 4 to IO is usually required. Any other faulty operation usually requires replacement or overhaul by the manufacturer. MCCBs can be used for every application on board a ship – from generator breakers to small distribution breakers. The limited breaking capacity may demand that a 'back-up' fuse be fitted for very high prospective short-circuit fault levels. Some applications are mentioned in the paragraphs that follow:

Feeder Protection

They are ideally suited for outgoing feeder circuits on distribution boards. The selected MCCB should have adequate interrupting capacity and the MCCB current rating should be equalto that of the load.

Capacitor Control

They are suitable for controlling LT capacitors for power factor correction. To avoid clipping while switching on the capacitors, the MCCB rating should be around 1.5 - 2 times the rated current of the capacitors so as to cater to initial inrush currents.

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