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

MCB (Miniature circuit breaker) is a re-settable 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 current at which the MCB will trip instantaneously. It is required that the trip current must persist for 0.1s.

The trip curve, also known as I-t tripping characteristic consists of two sections viz, overload section and short circuit section. Overload section describes the trip time required for various levels of overload currents and the short circuit section describes the instantaneous trip current level of MCB.

Read More: Miniature Circuit Breaker (MCB) – Principle of operation

TRIP CURVE CLASS B

The MCB with class B trip characteristics trips instantaneously when the current flowing through it reaches between 3 to 5 times rated current. These MCBs are suitable for cable protection.

TRIP CURVE CLASS C

MCB with class C trip characteristics trips instantaneously when the current flowing through it reaches between 5 to 10 times the rated current. Suitable Domestic and residential applications and electromagnetic starting loads with medium starting currents.

TRIP CURVE CLASS D

MCB with class D trip characteristics trips instantaneously when the current flowing through it reaches between Above 10(excluding 10) to 20 times the rated current. Suitable for inductive and motor loads with high starting currents.

TRIP CURVE CLASS K

MCB with class K trip characteristics trips instantaneously when the current flowing through it reaches between 8 to 12 times the rated current. Suitable for inductive and motor loads with high inrush currents.

TRIP CURVE CLASS Z

MCB with class Z trip characteristics trips instantaneously when the current flowing through it reaches between 2 to 3 times the rated current. These type of MCBs are highly sensitive to short circuit and are used for the protection of highly sensitive devices such as semiconductor devices.

TRIP CURVE CLASS A

MCB with class A trip characteristics trips instantaneously when the current flowing through it reaches between 2 to 3 times the rated current. As like Class Z MCBs, these are also highly sensitive to short circuit and are used for the protection of semiconductor devices.

MCB Trip Curves

MCBs with trip curve class B and trip curve class C are the most commonly used ones. Class C MCBs can be found in the lighting power distribution boards in residential and commercial buildings. It trips as soon as the current rises between 5 to 10 times its rated current. Class B MCBs are used in the protection of electronics devices such as PLC, DC power supplies etc. in control panels. It trips as soon as the current rises between 3 to 5 times its rated current.

In some applications, frequent current peaks occur for a very short period (100ms to 2s). For such applications, class Z type MCBs shall be used. Class Z type MCBs are used in circuits with semiconductor devices.

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 13 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.

  2. Very good mcb make , what Amps load trip make

    Thanks

  3. Very good. Nice explain.. Good job

  4. Explanation is good but your second paragraph doesn’t match the charts. It looks like it is the B-curve that trips between 3-5 times its rated current, and C-curve that trips between 5-10 times its rated current.

  5. Very good, thanks

  6. very good …..thanks

  7. Thanks very much

  8. Very good explanation

  9. Is this curves is applicable to Rccb ?

    1. No. These curves are applicable for mcbs only.

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