What is meant by B, C, D, K and Z curves in MCBs?

MCB (Miniature circuit breaker) is a device designed to protect a circuit from short circuits and overcurrents. 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 trip curve type

MCB trip 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 In.   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 In 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 Curve

MCB Trip Curves click to Enlarge

Related Searches:
1. What is Class B curve in MCBs?
2. What is Class C curve in MCBs?
3. What is Class D curve in MCBs?
4. What is Class K curve in MCBs?
5. What is Class Z curve in MCBs?
6. What is meant by MCB trip class?

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This Post Has 3 Comments

  1. Very good explanation. I understood the concept.
    Thank you.

    1. Thank you, Mr. Sanket. Kindly browse through our articles. Please subscribe or follow us on twitter/facebook for instant updates.

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