The molded case circuit breaker can automatically cut off the current when the current exceeds the tripping setting. Plastic shell refers to the use of plastic insulator as the shell of the device, which is used to isolate between conductors and grounded metal parts.

塑壳断路器通常含有热磁跳脱单元，而大型号的塑壳断路器会配备固态跳脱传感器。其脱扣单元分为：热磁脱扣与电子脱扣器。常用的，额定电流共有以下几种

Molded case circuit breakers usually contain thermal magnetic trip units, while large models of molded case circuit breakers will be equipped with Solid-state trip sensors. The tripping unit is divided into thermal magnetic tripping and electronic tripping. The common rated currents are as follows:

工作原理：低压断路器的主触点是靠手动操作或电动合闸的。

Working principle: the main contact of low-voltage circuit breaker is closed manually or electrically.

主触点闭合后，自由脱扣机构将主触点锁在合闸位置上。过电流脱扣器的线圈和热脱扣器的热元件与主电路串联，欠电压脱扣器的线圈和电源并联。当电路发生短路或严重过载时，过电流脱扣器的衔铁吸合，使自由脱扣机构动作，主触点断开主电路。当电路过载时，热脱扣器的热元件发热使双金属片上弯曲，推动自由脱扣机构动作，主触点断开主电路。当电路欠电压时，欠电压脱扣器的衔铁释放，也使自由脱扣机构动作，主触点断开主电路。当按下分励脱扣按钮时，分励脱扣器衔铁吸合，使自由脱扣机构动作，主触点断开主电路。

After the main contact is closed, the free tripping mechanism locks the main contact in the closing position. The coil of the overcurrent release and the thermal element of the thermal release are connected in series with the main circuit, and the coil of the undervoltage release is connected in parallel with the power supply. In case of short circuit or serious overload of the circuit, the armature of the overcurrent release is pulled in to make the free tripping mechanism act, and the main contact disconnects the main circuit. When the circuit is overloaded, the thermal element of the thermal release generates heat, bends the bimetallic sheet, pushes the free release mechanism to act, and the main contact disconnects the main circuit. When the circuit is under voltage, the armature of the under voltage release releases, which also makes the free release mechanism act, and the main contact disconnects the main circuit. When the shunt release button is pressed, the armature of the shunt release is pulled in to make the free release mechanism act, and the main contact disconnects the main circuit.

微型断路器由操作机构、触点、保护装置(各种脱扣器)、灭弧系统等组成。其主触点是靠手动操作或电动合闸的。

The miniature circuit breaker is composed of operating mechanism, contact, protection device (various releases), arc extinguishing system, etc. The main contact is closed manually or electrically.

主触点闭合后，自由脱扣机构将主触点锁在合闸位置上。过电流脱扣器的线圈和热脱扣器的热元件与主电路串联，欠电压脱扣器的线圈和电源并联。当电路发生短路或严重过载时，过电流脱扣器的衔铁吸合，使自由脱扣机构动作，主触点断开主电路。当电路过载时，热脱扣器的热元件发热使双金属片上弯曲，推动自由脱扣机构动作。当电路欠电压时，欠电压脱扣器的衔铁释放。也使自由脱扣机构动作。

After the main contact is closed, the free tripping mechanism locks the main contact in the closing position. The coil of the overcurrent release and the thermal element of the thermal release are connected in series with the main circuit, and the coil of the undervoltage release is connected in parallel with the power supply. In case of short circuit or serious overload of the circuit, the armature of the overcurrent release is pulled in to make the free tripping mechanism act, and the main contact disconnects the main circuit. When the circuit is overloaded, the thermal element of the thermal release generates heat, bends the bimetallic sheet and pushes the free release mechanism to act. When the circuit is under voltage, the armature of the under voltage release is released. It also makes the free tripping mechanism act.