How does a Molded Case Circuit Breaker Work?
The power system requires a high degree of reliability. During operation, the system may have some abnormal conditions or may cause troublesome problems. Some of these situations are beyond control and cannot be avoided. Therefore, there is a need for an efficient device to detect such failure conditions and react immediately to minimize damage to the device. Molded case circuit breaker (MCCB) can help protect equipment. A molded case circuit breaker (MCCB) is a device that protects low-voltage power distribution systems from overloads and short circuits. Molded case circuit breaker allows quick reactivation of a circuit after clearing a short circuit or overload. MCCBs (molded case circuit breakers) can easily identify the difference between an overcurrent and a short circuit. It allows a slight overcurrent for a while, but turns on faster as the current level increases. MCCB can be used to complete protection and disconnection functions.
The structure of the MCCB
As shown in the picture below, the hook engages the latch of the common trip shaft via a roller trigger. The common tripping shaft is supported by a support arm fixed on the base of the overcurrent tripping device in a freely rotating state. Each pole is provided with a time-delayed trip bimetal element for overcurrent detection and tripping. and an electromagnet for instantaneous tripping. The bimetal is heated and bent in the direction of the arrow and rotates the common trip shaft clockwise. When the latch is disengaged, the hook also rotates clockwise to release the bracket. The electromagnet consists of a fixed iron core surrounding the conductor, a movable iron core, and a retracting spring that continuously applies force to the movable iron core in the separation direction. When the overcurrent exceeds a certain limit, the moving iron core will be sucked by the return spring, and the tripping lever will turn the common tripping shaft clockwise to release the cradle. Since each pole is fitted with a bimetal and an electromagnet, overcurrent on any pole will affect the common trip axis, so all poles can be tripped at the same time without phase failure.
Working principle of MCCB
If an overcurrent continues to flow, the bimetal will heat up and bend. When the bimetal strip reaches a certain working temperature, the tripping operation will be carried out according to the displacement of the bimetal strip.
The graph below shows the relationship between bimetal temperature, current and time. As the current value increases, the time to reach the operating temperature becomes shorter.
Bimetal temperature current and time
When this relationship is plotted on the current operating time scale, an inverse time trip characteristic as shown in the figure below can be obtained.
Inverse time tripping characteristics
In the event of a short circuit, the circuit must be disconnected immediately. In this case, the electromagnetic trip device will trip the circuit instantaneously before the bimetallic curve. The instantaneous tripping current value is generally set to more than 10 times the rated current to avoid unnecessary actions caused by instantaneous overcurrent, such as the excitation inrush current of the transformer or the starting current of the induction motor.
The entire unit of the MCCB is enclosed with plastic case insulators, so when the load current is switched, no arcing is released. Also, this type is safe because the live parts are not exposed. The contact switching speed is constant regardless of the handle switching speed. The load current can be switched safely. Even if the overcurrent flows only to one pole, all poles are disconnected at the same time, so there is no possibility of phase failure.
