How Does a Molded Case Circuit Breaker Work?

The recent power systems require a high degree of reliability. While operating, a system may develop some abnormal condition or may create troublesome issues. Some of these situations are beyond human control and cannot be avoided. Hence, an effective device is needed to detect such fault conditions and react instantly to minimize the damage to the equipment. A molded case circuit breaker (MCCB) can help you protect your equipment.

The molded case circuit breaker (MCCB) is a device that protects low voltage power distribution systems against overloads and short circuits. A molded case circuit breaker allows a circuit to be reactivated quickly after a short circuit or overload is cleared.

How Does an MCCB Work?

An MCCB (molded case circuit breaker) can easily recognize the difference between an overcurrent and a short circuit. It allows a slight overcurrent for a while, but as the current level increases, it opens faster. MCCB is available to accomplish protection and disconnection functions.

First, let’s have a look at the structure of an MCCB.

As the example shown in the below figure, the hook is engaged with the latch of the common trip shaft through the roller trigger. The common trip shaft is supported in a freely rotating state by the support arm fixed on the base of the overcurrent trip device.

Each pole is provided with a bimetal element for time delay tripping for detecting overcurrent and tripping. And an electromagnet for instantaneous tripping.

The bimetal is curved in the arrow direction by heat and rotates the common trip shaft in the clockwise direction. When the latch is disengaged, also the hook rotates in the clockwise direction to release the cradle. The electromagnet consists of a fixed core enclosing a conductor, a movable core, and a retracting spring which constantly applies force to the movable core in the separating direction. When overcurrent exceeds a limit, the movable core will be attracted against the retracting spring, and the common trip shaft will be rotated in the clockwise direction by the tripping rod to release the cradle. Since the bimetal and electromagnet are provided for each pole and overcurrent on any pole affects the common trip shaft, all poles can be simultaneously tripped without open phases.

Now let’s continue with its working principle

If overcurrent flows continuously, the bimetal will receive heat and curve. When the bimetal reaches a certain operating temperature, the tripping operation will be performed according to the displacement of the bimetal.

The below figure shows the relationship between bimetal temperature, current, and time. As the current value increases, the time to reach the operating temperature becomes shorter.

When this relationship is plotted on the current-operating time scale, inverse time-tripping characteristics can be obtained as shown in the below figure.

Upon the occurrence of a short circuit, it is necessary to break the circuit immediately. In this case, the electromagnetic trip device will instantaneously trip the circuit before the bimetal curves. The instantaneous tripping current value is generally set to 10 times or more than the rated current to avoid unnecessary operation due to transient overcurrent, such as magnetizing inrush current of the transformer or starting current of the induction motor.

The entire unit of an MCCB is enclosed with a molded case insulator, so when switching a load current, the arc is not discharged. In addition, this type is safe as the live section is 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 simultaneously disconnected, so there is no possibility of phase failure.