A Miniature Circuit Breaker (MCB) is a resettable protective device that prevents electrical circuits from catching fire and causing damage to personnel and property. It is a device designed to isolate a circuit during an overcurrent event without using a fusible element.
There are two types of overcurrent events; a thermal overload and a short circuit.
MCB’s tripping characteristics are represented graphically in a trip curve chart. The chart shows the response of the thermal and magnetic trip element to various overload and short circuit situations.

Example 1: Thermal Tripping Characteristic
To determine the time it takes for the breaker to trip with a 20A load
The breaker will trip under a thermal overload between 10 and 100 seconds. The breaker is guaranteed to not trip before 10 seconds and will not take longer than 100 seconds to trip. The breaker may trip at any time between 10 and 100 seconds. 

Example 2: Magnetic Tripping Characteristic
To determine the time it takes for the breaker to trip with a 70 short circuit.
The breaker will trip under a short circuit between .001 and.01 seconds. The breaker is guaranteed to trip no later than .01 seconds for any short circuit equal to 70A. 
There are several types of MCB curves that manufacturers provide for applying circuit protection in different applications. The most common curves are B, C, and D. One MCB manufacturer also produces a K and Z curve.
The K and D curve breakers are both designed for motor applications where ampacity rises quickly and momentarily during “startup.” Both curves can “ride through” the momentary inrush of current and prevent nuisance tripping while providing protection to the circuit.
The K and D curve MCB have almost identical tripping characteristics. The magnetic element tripping characteristics are identical between the two curves and the thermal element tripping characteristics have a slight variation.
Example:
To determine the time it takes for the breaker to trip with a 20A load.

Let's now compare this to a 10A K Curve Breaker with a Thermal Overload at 20A
The K Curve Breaker will trip under a thermal overload between 6 and 350 seconds. The breaker is guaranteed to not trip before 6 seconds and will not take longer than 350 seconds to trip. The breaker may trip at any time between 6 and 350 seconds.
Example:
Both breakers have an element that will trip between 10 and 15 times rated current. Both breakers will trip under a short circuit between .001 and .01 seconds. And both breakers are guaranteed to trip no later than .01 seconds for any short circuit equal to 100A or greater. 
A Miniature Circuit Breaker (MCB) is a resettable protective device that prevents electrical circuits from catching fire and causing damage to personnel and property. It is a device designed to isolate a circuit during an overcurrent event without using a fusible element.
There are two types of overcurrent events; a thermal overload and a short circuit.
MCB’s tripping characteristics are represented graphically in a trip curve chart. The chart shows the response of the thermal and magnetic trip element to various overload and short circuit situations.
Example 1: Thermal Tripping Characteristic
To determine the time it takes for the breaker to trip with a 20A load
The breaker will trip under a thermal overload between 10 and 100 seconds. The breaker is guaranteed to not trip before 10 seconds and will not take longer than 100 seconds to trip. The breaker may trip at any time between 10 and 100 seconds.
Example 2: Magnetic Tripping Characteristic
To determine the time it takes for the breaker to trip with a 70 short circuit.
The breaker will trip under a short circuit between .001 and.01 seconds. The breaker is guaranteed to trip no later than .01 seconds for any short circuit equal to 70A.
There are several types of MCB curves that manufacturers provide for applying circuit protection in different applications. The most common curves are B, C, and D. One MCB manufacturer also produces a K and Z curve.
The K and D curve breakers are both designed for motor applications where ampacity rises quickly and momentarily during “startup.” Both curves can “ride through” the momentary inrush of current and prevent nuisance tripping while providing protection to the circuit.
The K and D curve MCB have almost identical tripping characteristics. The magnetic element tripping characteristics are identical between the two curves and the thermal element tripping characteristics have a slight variation.
Example:
To determine the time it takes for the breaker to trip with a 20A load.
The breaker will trip under a thermal overload between 10 and 100 seconds. The breaker is guaranteed to not trip before 10 seconds and will not take longer than 100 seconds to trip. The breaker may trip at any time between 10 and 100 seconds.
Let's now compare this to a 10A K Curve Breaker with a Thermal Overload at 20A.
The K Curve Breaker will trip under a thermal overload between 6 and 350 seconds. The breaker is guaranteed to not trip before 6 seconds and will not take longer than 350 seconds to trip. The breaker may trip at any time between 6 and 350 seconds.
Example:
Both breakers have an element that will trip between 10 and 15 times rated current. Both breakers will trip under a short circuit between .001 and .01 seconds. And both breakers are guaranteed to trip no later than .01 seconds for any short circuit equal to 100A or greater.
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