What are the characteristics of moulded case circuit breaker ？

Plastic-case Circuit Breakers are also known as device circuit breakers, all parts are sealed in a plastic shell, auxiliary contacts, undervoltage release and shunt release, etc., are modular. Because the structure is very compact, the plastic-case circuit breaker can not be repaired.

Most of them use manual operation, large capacity can choose electric closing. Due to the application of electronic overcurrent release, plastic-case circuit breakers can also be divided into class A and Class B. Class B has good three-stage protection characteristics, but due to price factors, the market share of class A products using thermal magnetic release is higher. Plastic-case circuit breaker is the contact, interrupter, release device and operating mechanism are installed in a plastic shell, generally do not consider maintenance, suitable for the branch protection switch, over current release device has two kinds of thermal magnetic and electronic, general thermomagnetic plastic-case circuit breaker is a non-selective circuit breaker, only overload long delay and short circuit instantaneous two protection methods. Electronic plastic-case circuit breaker has four protection functions: overload long delay, short delay, short instantaneous and ground fault. Some of the new products of electronic mould-case circuit breakers also have regional selective interlocking function. Most of the moulded case circuit breakers are manually operated, and some have motor operating mechanisms.

Working principle of plastic-case circuit breaker

There are two types of interlocking devices: mechanical interlocking and electrical interlocking. How it works:

(1) The mechanical interlock device generally uses a wire rope or lever mechanism to change the mechanical position (can also use a multi-function program lock) to ensure that the operating handle of the isolation switch cannot be moved before the circuit breaker cuts off the power supply.

(2) Electrical interlocking device Electrical interlocking generally has two types of interlocking, one is to control the handle of the isolation switch through the linkage auxiliary contact on the operating mechanism (normally open or normally closed). When the circuit breaker is not disconnected, the isolation switch handle cannot be operated.

Another type of electrical interlock is to use the linkage auxiliary contact (normally open or normally closed) on the distance switch operating mechanism to control the circuit breaker. When the handle of the isolation switch is pulled, the linkage auxiliary contact (normally open or normally closed) causes the circuit breaker to operate to cut off the circuit, thereby preventing accidents with the load pulling distance switch.

Plastic-case circuit breaker function

1. Open and disconnect no-load and load current in the high-voltage circuit under normal conditions;

2. When the system fails, it can cooperate with the protection device and automatic device to quickly cut off the fault current to prevent the expansion of the accident to ensure the safe operation of the system.

Under normal circumstances, the plastic-case circuit breaker itself does not have lubrication problems, and it is often its operating mechanism that needs lubrication. The function of the thermal relay is to automatically cut off the power supply when the motor is overloaded. The structure of the thermal relay is composed of two metal sheets with different expansion coefficients. When the current is too large, the expansion coefficient will expand first and play the role of cutting off the power supply. The thermal relay has manual reset and automatic reset after operation. Fuse function: When the circuit fails or is abnormal, the current continues to rise, and the increased current may damage some important devices or valuable devices in the circuit, and may burn the circuit or even cause a fire.

Characteristics of moulded case circuit breaker

1. Rated limit short-circuit breaking capacity Icu: There are two types of breaker breaking capacity indicators: rated limit short-circuit breaking capacity Icu and rated operation short-circuit breaking capacity Ics. Ics, as a characteristic parameter, does not simply consider the breaking capacity of the circuit breaker, but as a breaking indicator, that is, after breaking several short circuit faults, it can ensure its normal operation.

2. Current limiting breaking capacity: When the circuit breaker is short-circuited, the contact quickly opens to generate an arc, which is equivalent to a rapidly increasing arc resistance in the line, thereby limiting the increase in fault current. The less time the circuit breaker is broken, the closer the Ics is to the Icu, the better the current limiting effect, and the adverse effects of the electromagnetic effect, electric effect and thermal effect caused by the short circuit current on the circuit breaker and electrical equipment can be greatly reduced, and the service life of the circuit breaker can be extended.

3. Short circuit protection: Short circuit protection is short circuit instantaneous trip. It is necessary to pay attention to adjusting the setting value of the protection in time after the load change to prevent the setting value from being too small and frequent tripping to affect the power supply quality, or the setting value is too small to prevent the line and equipment from being effectively protected.

4. Overload delay protection: overload delay protection means that the load current exceeds the limited range of the equipment and the risk of burning the equipment, and the protection device can cut off the power supply within a certain period of time. There is a process of heat accumulation, and the protection action does not need to be too rapid. For short periods of overcurrent, the protection should not operate.

5. Isolation function: The isolation function requires that the leakage current after the circuit breaker is disconnected will not cause harm to the person and equipment. After multiple short-circuit trips, the performance of the switch decreases and the leakage current increases. For the human body, the leakage current below 30mA is safe, and in a harsh environment, the leakage current of more than 300mA lasts for more than 2 hours, which may cause the insulation damage to occur phase short circuit and then cause a fire.