Circuit breaker is a kind of protective device for the line, which mainly protects the line from damage due to overload, short circuit and undervoltage. Its function is to automatically trip and cut off the load power supply in case of short circuit or overload of the line; It can also open and close when the fault occurs. In the distribution system, when overload occurs on the line, the load current value (called overcurrent limit) will be limited within a certain period of time, so that the load current does not exceed the specified limit value.
Structure and characteristics
Installation and use method
There are many types of circuit breakers, which are generally divided into the following according to the size of short-circuit current they can withstand:
"Circuit breaker" is used for rated voltage below 500V; "Fuse" shall be used for rated voltage of 1000V and above; "Thermal relay" is used for rated voltage of 1100V and above.
The main structure of the circuit breaker is to protect the protected equipment by making and breaking operations.
1. Basic structure
2. Working principle
3. Selection and use of circuit breaker
2、 Working principle
Under the rated voltage, the circuit breaker can realize the short-circuit fault in the motor, transformer, bus and other systems, as well as the overload protection and short-circuit protection under the overload and underload conditions through the on-off characteristics when the current is 2-3 times of the rated current and 2-4 times of the rated power.
When the circuit breaker is closed, because the motor and the circuit are in no-load state, the circuit breaker will generate a large current at the moment of closing, making the arc extinguishing chamber hot and the arc generation temperature can reach about 1000 ℃. Because the arc duration is short, the arc extinguishing system will not have fusion welding.
3、 Structure and characteristics
1. The ability of circuit breaker to break short circuit current and overload current: the circuit breaker can pass 100% of the rated current of the rated voltage, and can pass the rated total power or the maximum output short circuit current.
2. Overload capacity of circuit breaker: when the circuit breaker is in normal use, its overload capacity is 3 times the rated power current; When the circuit breaker is overloaded, its overload capacity is 5 times the rated power current.
3. Breaking capacity of circuit breaker: under different load conditions, for the same motor, when the load is light, its breaking capacity is greater than when the load is heavy, but for the same motor.
4. Short circuit withstand current of circuit breaker: under normal conditions, the short circuit withstand current is proportional to the length of the protected line; In case of accident, the short-circuit withstand current is inversely proportional to the length of the protected line.
4、 Installation and use method
1. First, the type of circuit breaker should be selected according to the distribution line and transformer used.
2. Install the switch on the fixed frame with screws and fix it.
3. Install the breaking power switch of the circuit breaker inside.
4. Power-on test: check whether there is current on the breaking power switch after it is connected, if not, press the closing button to connect the power; If there is current after connection, press the opening button to connect the power supply.
5. Hang the manual operating handle at the middle position, then hang the operating handle of the circuit breaker at the lower position of the manual operating handle, and then select the circuit breaker with appropriate size and performance according to the required breaking current.
5、 Fault analysis
1. Judgment of short circuit fault: when the short circuit fault of the circuit breaker occurs, it should be judged first according to the size of the short circuit current, the opening time of the circuit breaker and the action time of the main protection of the circuit breaker. If there is no arc, flame and overvoltage, it can be judged that the circuit breaker is normal;
2. Protection setting: select the setting parameters according to the response characteristics of the protection mechanism action in case of line fault. If the opening time of the circuit breaker is too short (such as within 20 seconds), the setting value should generally be 100~150 ms, otherwise it is easy to cause false tripping;
3. Overload fault of circuit breaker: generally caused by short circuit on the power supply side or load torque on the load side. The following methods can be adopted for such fault:
(1) Through calculation, we can determine whether there is a certain relationship between the load current and the opening time of the circuit breaker. Through comparison, we can determine the load current;