When there is a large difference in the expected short-circuit current at the outgoing terminals of the upper and lower circuit breakers, and the instantaneous release is set, the setting current of the instantaneous release of the upper circuit breaker shall be greater than the expected short-circuit current of the lower circuit breaker to ensure selective protection.

当上下级断路器距离较近，出线端预期短路电流差别很小时，则上级断路器宜选用带有短延时脱扣器延时动作，以保证有选择配合。

When the distance between the upper and lower circuit breakers is close and the expected short-circuit current difference at the outlet end is very small, the upper circuit breaker should select the delayed action of the release with short delay to ensure selective cooperation.

当上下级保护电器都采用选择型断路器时，为保证上下级之间的动作选择性，上级断路器的过载长延时和短路短延时的整定电流，宜不小于下级相应保护整定值的1.3倍。

When the upper and lower protection appliances adopt selective circuit breakers, in order to ensure the action selectivity between the upper and lower levels, the setting current of overload long delay and short-circuit short delay of the upper circuit breaker should not be less than 1.3 times of the corresponding protection setting value of the lower level.

上下级保护断路器都选择非选择型断路器时，应加大上下级之间的断路器的脱扣器整定电流的级差值，一般按下述原则确定。

When non selective circuit breakers are selected for upper and lower protection circuit breakers, the level difference of release setting current of circuit breakers between upper and lower levels shall be increased, which is generally determined according to the following principles.

1）上级保护电器的长延时脱扣器整定电流，宜不小于下级长延时脱扣器整定电流的2倍。

1) The setting current of the long time delay release of the upper protection appliance should not be less than 2 times of the setting current of the lower long time delay release.

2）上级保护断路器的瞬时脱扣器整定电流，宜不小于下级瞬时脱扣器整定电流的1.4倍。

2) The setting current of the instantaneous release of the upper protection circuit breaker should not be less than 1.4 times of the setting current of the lower instantaneous release.

3）末级非选择型断路器，其短路瞬时脱扣器整定电流应尽量小，但应躲过短时出现的过负荷尖峰电流。

3) For the final non selective circuit breaker, the setting current of the short-circuit instantaneous release shall be as small as possible, but it shall avoid the overload peak current in a short time.

当上级保护是选择型断路器，而下级保护是非选择型断路器时，应符合如下条件：

When the upper protection is a selective circuit breaker and the lower protection is a non selective circuit breaker, the following conditions shall be met:

1）上级保护断路器的短路短延时脱扣器的整定电流，应不小于下级保护断路器的短路瞬时脱扣器整定电流的1.3倍。

1) The setting current of the short-circuit short-time delay release of the upper protection circuit breaker shall not be less than 1.3 times of the setting current of the short-circuit instantaneous release of the lower protection circuit breaker.

2）上级保护断路器瞬时脱扣器整定电流应大于下级保护断路器出线端单相短路电流的1.2倍。

2) The setting current of the instantaneous release of the upper protection circuit breaker shall be greater than 1.2 times of the single-phase short-circuit current at the outgoing end of the lower protection circuit breaker.

上级保护断路器是选择非选择型断路器，下级保护断路器采用选择型断路器时，不能保证下级保护先动作。

The upper protection circuit breaker is a non selective circuit breaker. When the lower protection circuit breaker adopts a selective circuit breaker, the lower protection cannot be guaranteed to act first.

当下级保护断路器出口端短路电流大于上级的瞬时脱扣器整定电流时，为保证选择性，下级保护断路器宜选用限流型断路器。

When the short-circuit current at the outlet of the lower protection circuit breaker is greater than the setting current of the upper instantaneous release, in order to ensure selectivity, the lower protection circuit breaker should select the current limiting circuit breaker.