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.

當上下級斷路器距離較近，出線端預期短路電流差別很小時，則上級斷路器宜選用帶有短延時脫扣器延時動作，以保證有選擇配合。

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.

當上下級保護電器都采用選擇型斷路器時，為保證上下級之間的動作選擇性，上級斷路器的過載長延時和短路短延時的整定電流，宜不小于下級相應保護整定值的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.

上下級保護斷路器都選擇非選擇型斷路器時，應加大上下級之間的斷路器的脫扣器整定電流的級差值，一般按下述原則確定。

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）上級保護電器的長延時脫扣器整定電流，宜不小于下級長延時脫扣器整定電流的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）上級保護斷路器的瞬時脫扣器整定電流，宜不小于下級瞬時脫扣器整定電流的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）末級非選擇型斷路器，其短路瞬時脫扣器整定電流應盡量小，但應躲過短時出現的過負荷尖峰電流。

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.

當上級保護是選擇型斷路器，而下級保護是非選擇型斷路器時，應符合如下條件：

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）上級保護斷路器的短路短延時脫扣器的整定電流，應不小于下級保護斷路器的短路瞬時脫扣器整定電流的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）上級保護斷路器瞬時脫扣器整定電流應大于下級保護斷路器出線端單相短路電流的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.

上級保護斷路器是選擇非選擇型斷路器，下級保護斷路器采用選擇型斷路器時，不能保證下級保護先動作。

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.

當下級保護斷路器出口端短路電流大于上級的瞬時脫扣器整定電流時，為保證選擇性，下級保護斷路器宜選用限流型斷路器。

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.