Table of contents

1) Definition

2) Class B Trip curve 

3) Class C Trip curve

4) Class D Trip curve

5) Class K Trip curve

6) Class Z Trip curve

7) Class A Trip curve

8) Importance of MCB Trip Curve Types 

9) Trip Curves for Other circuit breakers 


 

Definition

The MCB trip curves, also known as the I-t tripping characteristic consist of two sections viz, overload section and short circuit section. The 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.

 

Class B trip curve

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

Class C trip curve

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.

Class D trip curve

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.

 



Class K trip curve

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.

Class Z trip curve

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

Class A trip curve

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

MCBs with trip curve class B and trip curve class C is the most commonly used ones. MCBs with Class C trip curves 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 electronic 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.

 

Importance of MCB trip curve types

It is important to choose an appropriate MCB current rating and trip curve in order to safeguard the circuit from damage during faults. Hence it is necessary to calculate the short circuit current and inrush current before choosing an appropriate MCB rating. If the chosen MCB rating is much higher than required, then it may not trip in the event of a fault. Similarly, if the MCB is underrated, then it may cause nuisance trips, for example even the starting currents or inrush currents may trip the MCB.