The first curve in the graph shows the response of a thermal trip unit. The curve shows that the circuit breaker’s tripping time reduces with an increase in the current. The thermal region shows the response of the bimetallic contact trip unit during overcurrent. While the Y-axis represents the tripping time of the circuit breaker on a logarithmic scale.
The horizontal X-axis represents the multiples of the current flowing through the circuit breaker. The following figure shows a chart of a trip curve.
Therefore, the circuit breaker selected should not trip so fast that it creates a nuisance & it should not trip so late that it causes any damage. The circuit breaker should be able to tolerate the inrush current & it should provide some delay before tripping. Inrush current is the huge current draw during the starting of a motor that causes voltage dips in the main line. The overcurrent can happen under normal conditions such as the inrush current of a motor.
But it should not trip so fast & unnecessary that it becomes a problem. The current flowing through the bimetallic strip heats up contact & trip the circuit breaker.Ĭircuit breakers are used for tripping the power supply as quickly as possible in case of overcurrent. It uses a bi-metallic contact that bends with a change in temperature. Thermal Tripping Unit: the thermal tripping unit is used for protection against overloading. A circuit breaker has two types of tripping unit i.e. The circuit breaker instantly cut off the power supply to reduce further damage. The fault currents generated due to these fault conditions can damage the electrical devices as well as cause fire in a building that can also pose danger to human life. It breaks the circuit in case of overloading & short circuit.
Types of Circuit Breaker Based on Tripping CurvesĪ circuit breaker is an electrical device that provides protection against fault current.In this way, classification selection is made according to different rated currents. For places with poor heat dissipation conditions, a 120A circuit breaker may be selected. P = UICOSφ (power = voltage × current × power factor)įor example: a three-phase motor with a rated power of 45KW, the calculated current I = 45000/( X380X0.8) = 85.5AĪ circuit breaker with rated current In=100A can be selected. Calculation of rated current of three-phase load In this way, classification selection is made according to different rated currents.Ģ. In case of poor heat dissipation conditions in the distribution cabinet or distribution box, the capacity should be reduced to 0.7-0.8 for use, and 160A circuit breakers should be selected. P = UI COSφ (power = voltage × current × power factor)įor example: the total capacity of single-phase load electrical equipment is 20KW, the line rated current is calculated according to the formula: I =20000/(220x0.8) = 113.6AĪccording to the rated current of 113.6A, a circuit breaker with a rated current of 140A should be selected. Calculation of rated current of single-phase load