⑴ The coordination of circuit breaker and circuit breaker should consider the instantaneous release action value of the upper-level circuit breaker, which should be greater than the maximum expected short-circuit current at the outlet end of the lower-level circuit breaker. If the short-circuit current value is not much different, the superior circuit breaker can choose a trip unit with short time delay.
⑵ When the short-circuit current is greater than or equal to the setting value of the instantaneous release of the current-limiting circuit breaker, it will trip within a few milliseconds. Therefore, it is not suitable for lower-level protection appliances to use a circuit breaker to achieve selective protection requirements.
(3) For a circuit breaker with short delay, when its time limit is set at the maximum delay, its making and breaking capacity decreases. Therefore, in the selective protection circuit, consider choosing the short delay making and breaking capacity of the circuit breaker to meet the requirements.
⑷ It should also be considered that the short-circuit delay return characteristic of the upper-level circuit breaker and the action characteristic time curve of the lower-level circuit breaker should not intersect, and the short-delay characteristic curve and the instantaneous characteristic curve should not intersect.
⑸ When the circuit breaker and the fuse are used together, the coordination of the upper and lower levels should be considered. The ampere-second characteristic curve of the circuit breaker should be compared with the ampere-second characteristic curve of the fuse, so as to have protection selectivity in the case of short-circuit current.
⑹ When the circuit breaker is used for the protection of the distribution circuit, a circuit breaker with a long delay action overcurrent release should be selected. When a single-phase grounding short circuit occurs at the end of the line, the short circuit current is not less than the instantaneous or short delay overcurrent of the circuit breaker 1.5 times the set current of the trip unit.
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