1. The problem of material purity of electric conductor or or contact itself leads to excessive working resistance and contact resistance, which leads to excessive temperature rise.

2. The problem of C distribution can be checked by referring to the relevant national standards of electrical alloys, whether the metallographic distribution of C in contacts is uniformly distributed in dendritic form or not. Otherwise, excessive oxidation will occur when local contacts work, forming oxide layer, increasing resistance and local burning loss, which will lead to the change of cross-section area, thus leading to the change of temperature rise.

3. Welding problems, the use of welding material resistance is too high, or the contact itself welding silver layer is too thin, and the welding is not reliable (welding deviation, virtual welding), indirectly causing contact problems, so that the temperature rise is too high.

Molded case circuit breaker can automatically cut off the current when the current exceeds the jump setting. Plastic housing refers to the use of plastic insulators as the shell of the device, used to isolate between conductors and grounding metal parts. Plastic case circuit breakers usually contain thermomagnetic trip units, while large plastic case circuit breakers are equipped with solid state trip sensors. The release unit is divided into thermomagnetic release unit and electronic release unit.

Working principle

The main contacts of low-voltage circuit breakers are operated manually or electrically. After the main contact is closed, the free release mechanism locks the main contact at the closing position. The coil of the overcurrent release and the thermal element of the thermal release are in series with the main circuit, and the coil and the power supply of the undervoltage release are in parallel.

When the circuit is short-circuited or seriously overloaded, the armature of the over-current tripper is sucked, which makes 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 bends the bimetal sheet, pushes the free release mechanism to move, and the main contact disconnects the main circuit.

When the circuit is under-voltage, the armature of the under-voltage tripper is released, which also makes the free tripping mechanism act and the main contact disconnects the main circuit.

When the release button is pressed, the armature of the release device is sucked, so that the free release mechanism can move and the main contact can disconnect the main circuit.