It is strictly forbidden to use AC air circuit breakers in the DC circuit; because AC arcs and DC arcs have different arc extinguishing mechanisms, the arc extinguishing chambers of AC and DC vacuum circuit breakers are fundamentally different. AC air circuit breakers do not have the ability to extinguish DC short-circuit arcs. Ability. Therefore, it is strictly prohibited to use AC air circuit breakers in the DC circuit. Tests and operating practices have proved that the breaking capacity of AC circuit breakers is only 1/5-1/8 of the breaking capacity of DC circuit breakers. When the rated current is breaking, the burning of the contacts caused by the direct arc occurs from time to time, and the full breaking time The uncertainty of is also a difficult problem in level difference coordination.
When using AC and DC dual-purpose air circuit breakers, its performance must meet the requirements of breaking DC circuit short-circuit current and guide selectivity.
In the DC power supply system, it is necessary to prevent the fuse and the air circuit breaker from being mixed in the same branch. Because the level difference between the circuit breaker and the fuse is difficult to match, the tripping speed of the air circuit without time limit basically does not change with the current. , And the action of the fuse has inverse time characteristics. Regardless of whether the circuit breaker is installed before or after the fuse, it will always lose its action selectivity within a certain range of short-circuit current values. Therefore, this combined protection method should be avoided as much as possible. In particular, the fuse can not be used after the automatic air circuit breaker. In this way, the level difference is more difficult to prevent the loss of action selectivity when the circuit fails.
Due to the characteristics of high breaking capacity, fast breaking speed, short arcing, small size and complete specifications, DC circuit breakers are currently widely used in complete sets of DC power supply devices in substations. Products from different manufacturers should not be mixed as much as possible. Due to the dispersion of action characteristics, action selectivity may be lost. Therefore, in principle, the DC circuit breakers of a plant or station should be selected from the same manufacturer’s series.
