Low-voltage circuit breakers are also called automatic air circuit breakers, or automatic switches for short. Low-voltage circuit breakers are functionally equivalent to a combination of knife switches, fuses, thermal relays, and undervoltage relays. Their function is not only to frequently turn on or off the circuit during normal operation, but also overload, short circuit, or loss of voltage in the circuit When a fault occurs, it can automatically trip to cut off the faulty circuit. It is a protective appliance that can automatically cut off the fault of the circuit. Low-voltage circuit breakers are different from contactors in that they allow short-circuit current to be cut off, but allow a lower number of operations.
1. The structure and basic working principle of the low-voltage circuit breaker The low-voltage circuit breaker is mainly composed of three parts: the main contact and the arc extinguishing system, various trip units, the operating mechanism and the free trip mechanism.
a. Main contact and arc extinguishing system The main contact and arc extinguishing system are the actuators of the circuit breaker, which are used to make and break the main circuit. The main contact is made of arc-resistant alloy (such as silver-tungsten alloy), and the arc extinguishing grid is used to extinguish the arc.
b. Trip unit The trip unit is the sensing element of the circuit breaker. When the circuit fails, the trip unit passes the free trip mechanism to break the contact.
Depending on the type of signal sensed, the trip unit has the following types:
1) Shunt release. For the tripping device used for remote opening, the shunt release is short-term working, and its coil is not allowed to be energized for a long time. Therefore, the coil is energized during the opening, and the coil should be powered off after the opening.
2) Loss of pressure release. The coil of the voltage-loss release is connected in parallel to the circuit. During normal operation, the armature draws in. When the circuit voltage is too low or disappears, the armature opens, driving the free trip mechanism to trip the circuit breaker, thereby achieving the purpose of voltage loss protection.
3) Overcurrent release. The coil of the overcurrent release is connected in series in the circuit. When instantaneous over-current or short-circuit current occurs in the circuit, the armature acts and drives the free trip mechanism to trip the circuit breaker, thereby achieving the purpose of over-current or short-circuit protection.
4) Overload release. When an overload current occurs in the circuit, the bimetallic strip of the overload release bends, driving the free tripping mechanism to trip the circuit breaker to achieve the purpose of overload protection. In actual use, not every type of circuit breaker has the above four types of trip units, which depends on the occasion of use of the circuit breaker.
c. Free trip mechanism and operating mechanism The free trip mechanism and operating mechanism are the mechanical transmission parts of the circuit breaker. Their function is to realize the tasks of automatic tripping and manual closing of the circuit breaker after the signal is received by the trip unit. But the instantaneous action when the contact is on or off has nothing to do with the operating speed of the handle.
2. The main technical parameters and typical products of low voltage circuit breakers
a. Main technical parameters
1) Rated voltage refers to the allowable voltage of the circuit breaker during long-term operation in the circuit. Generally, it is equal to or greater than the rated voltage of the circuit.
2) Rated current refers to the allowable continuous current of the circuit breaker during long-term operation in the circuit.
3) On-off capability refers to the short-circuit current value that the circuit breaker can make and break under the specified voltage, frequency and specified line parameters (AC circuit is power factor and DC circuit is time constant).
4) The breaking time refers to the time required to cut off the fault current, which includes the inherent breaking time and the arcing time.
b. Typical products of low-voltage circuit breakers Here we introduce typical products of low-voltage circuit breakers-frame circuit breakers and device-type circuit breakers. Frame-type circuit breaker is installed in a frame structure base after all components are insulated. Frame-type circuit breakers generally have higher short-circuit breaking capacity and higher dynamic stability, so they are mostly used as the main protective switch of the circuit. At present, there are DW10 series circuit breakers and DW15 series circuit breakers designed and produced by our country.
The device-type circuit breaker has an insulated plastic casing, the contact system, the arc extinguishing chamber and the trip unit are all installed in the casing, and the manual handle is exposed, which can be opened or closed manually or electrically. The device-type circuit breaker also has high breaking capacity and dynamic stability and relatively complete selective protection functions, which are widely used in distribution lines, and can also be used to control infrequent start motors and lighting circuits.
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