MCB is a device designed to protect a circuit from short circuits and over currents. Trip curves of MCB's (B, C, D, K and Z curves) tell us about the trip current rating of Miniature Circuit breakers. Trip current rating is the minimum at which the MCB will trip instantaneously. It is required that the trip current must persist for 0.1s.
MCB TYPE  TRIPPING CURRENT 
TRIP CURVE CLASS B

Above 3 to 5 times rated current. Suitable for cable protection

TRIP CURVE CLASS C

Above 5 to 10 times the rated current. Suitable Domestic and residential applications and electromagnetic starting loads with medium starting currents

TRIP CURVE CLASS D

Above 10(excluding 10) to 20 times the rated current. Suitable for inductive and motor loads with high starting currents.

TRIP CURVE CLASS K

Above 8 to 12 times the rated current. Suitable for inductive and motor loads with high inrush currents.

TRIP CURVE CLASS Z

Above 2 to 3 times the rated current. These type of MCBs are highly sensitive to short circuit and are used for protection of highly sensitive devices such as semiconductor devices.

An MCB with trip curve class B means that the MCB trips at as soon as the current rises above 3 to 5 times its rated current I_{n}. Similarly, MCB with trip curve class C means that the MCB trips at as soon as the current rises above 5 to10 times its rated current I_{n} and so on..
In some applications, frequent current peaks occur for a very short period (100ms to 2s). For such applications class K type fuses shall be used. Class K type fuses are used in circuits with semiconductor devices.
MCB Trip Curves

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