Introduction of circuit breaker
A circuit breaker is a switching device capable of closing, carrying, and disconnecting the current under normal circuit conditions and closing, carrying, and disconnecting the current under abnormal circuit conditions (including short-circuit conditions) within a specified period of time. A circuit breaker can be used to distribute power, infrequently start an asynchronous motor, protect the power line and motor, and automatically cut off the circuit in case of serious overload, short-circuit, or under-voltage faults, and its function is equivalent to the combination of a fuse switch and an under-thermal relay. The function is equivalent to the combination of a fuse switch and an under-thermal relay, etc. And there is generally no need to change parts after breaking the fault current. At present, they have been widely used.
Circuit breakers are generally composed of contact systems, arc extinguishing systems, operating mechanisms, release devices, shells, etc. According to their structure, circuit breakers are divided into miniature circuit breakers, molded case circuit breakers, and frame circuit breakers. By function, it is divided into AC circuit breakers and DC circuit breakers.
The differences between AC and DC circuit breakers.
The main difference between DC circuit breakers and AC circuit breakers is the ability to de-energize. As the difference of the arcing and arc extinguishing process between Ac and DC circuit breaker, the same rating of the AC and DC circuit breaker is not exactly the same ability to turn on and off the DC power supply, the use of AC circuit breakers instead of DC circuit breakers or AC and DC circuit breakers mixed is one of the main reasons for the protection of cross-level misoperation.
Because AC has a zero-crossing point at every cycle, it is easy to extinguish the arc at the crossing point, while the DC switch does not have a zero-crossing point, the ability to extinguish the arc is very poor, so it is necessary to add an additional arc extinguishing device. In short, DC is difficult to extinguish the arc, while AC is easy to extinguish the arc with an overshoot of zero.
DC and AC circuit breakers are not interchangeable. The differences as below:
1.- The different number of main contacts, two for DC and three for AC.
2.-The structure of the arc extinguishing device is different between the main contact of DC and AC electrical appliances, the DC contact arc extinguishing structure is mainly magnetic blowing type, the AC contact arc extinguishing structure is mainly grid type.
3.-Generally, the same grade DC open can be used temporarily for AC occasions, but the AC open should not be used casually for DC occasions, a short circuit is likely to produce larger sparks. So when we don’t have an AC circuit breaker on hand, we can use a DC circuit breaker to temporarily fill the gap, but we can’t always use it, because the DC kairos ability requires high.
AC circuit breakers can be derived to protect DC circuits, three changes must be noted.
1.- Overload and short circuit protection.
(1).Overload long-delay protection.
Using thermal-activated (bimetallic elements) for overload long delay protection, the action source is I2R, the RMS value of AC current is equal to the average value of DC, so it does not require any modification to use. However, for high-current specifications, the secondary side of the current transformer can not be used because the transformer cannot be used in the DC circuit for current heating.
If the overload long-delay release is fully electromagnetic (hydraulic, i.e., oil-cup type), the delayed release characteristics should be changed, and the minimum action current should be larger to 110%-140%, therefore, the AC full electromagnetic release cannot be used in DC circuit (if it is to be used, it will have to be redesigned).
(2).-Short circuit protection.
Hot – electromagnetic AC circuit breaker short circuit protection is the use of magnet system, which is used after the filtered rectifier circuit (DC), the need to multiply the original AC set current value by a coefficient of 1.3. Full electromagnetic short-circuit protection is the same as the thermal-active electromagnetic type.
2.-circuit breaker accessories, such as shunt release, Undervoltage release, electric operating mechanism, etc.; shunt and Undervoltage are voltage coils, as long as the voltage value is the same, they can be used for AC system without any change, they can be used for DC system. Auxiliary and alarm contacts are common for AC and DC systems. It should be redesigned when the electric operating mechanism used for DC.
3.-As the shortage of characteristics of over zero, DC short-circuit current (or even a small multiple of the fault current) on and off; arc extinguishing is difficult, so the wiring should be used two or three poles in series, increasing the fracture, so that the fracture bear part of the arc energy.
Theoretically, due to the different characteristics of AC and DC, AC and DC circuit breakers cannot be crossed using, but in some special environments, they can be used temporarily instead of permanently. When using circuit breakers, we need to choose the right circuit breaker for the circuit, rather than just using it.