Diesel-Generator Set Protection

They can be used for effective protection and control of diesel-generating sets (against overloads and short circuits).

Hoist I Elevator I Crane Control

They can withstand continuous vibrations of 2g and shocks up to 15g without affecting their performance. As such they offer the best protection for hoist / elevator / crane applications.

Furnace Control

These are suitable for high frequency systems up to 400 Hz. Hence they can be ideally used for control of high frequency furnaces. A de-rating factor of 0.8 is to be employed for arriving at the rated current of the MCCB. However it must be noted that fully magnetic MCCBs are not suitable for this application.

DC Power Supply Control

Rectifier panels which act as a source of d.c. supply can be conveniently protected by MCCBs. Moreover, MCCBs being suitable for both - a.c. as well as d.c. systems, control and protection of input as well as output sides of the rectifier is also possible.

Miscellaneous Marine Applications

They are ideally suitable for a variety of marine applications. These have cleared the moststringent performance, durability and environmental tests. While the durability tests include a, Vibration Test at 10g, Bump Test at 40g and Shock Test at 70g, the environmental test include the Damp Heat (steady state) Test, Damp Heat (cyclic) Test, Mould Growth Test and the Corrosive Atmosphere (salt spray) Test.

Miniature Circuit Breakers (MCBs)

MCBs are very small air circuit breakers also fitted in moulded plastic cases. They have current ratings of 5-100A and generally thermal overload and magnetic short-circuit protection. They have a very limited breaking capacity (300A) and are commonly used in final distribution boards (DBs) instead of fuses. The distribution board is supplied via a fuse or MCCB with the required breaking capacity. (Refer Figure 11.9).

Advantages

ü They are set to a pre-determined rating at the factory

ü It is easy to check if the breaker has tripped or not

ü The supply to the circuit is easily reinstated

ü Multi-pole units are available

ü They can discriminate between sustained and transient loads.

Disadvantages

ü They are costly

ü They have mechanical moving parts

ü Tripping heavy overloads causes distortion due to heat

ü Ambient temperature affects their characteristics

ü Regular tests are required to ensure their satisfactory operation

ü MCBs must be replaced if faults develop. No maintenance is generally possible

Figure 11.9 - A Miniature Circuit Breaker (MCB) operating during a Short-circuit

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