The symbol of miniature circuit breaker is MCB. The two ends of its main contact are isolated by air, and the air plays the role of isolation and insulation between the two electrical heads or between the moving and static contacts in MCB. Using air to isolate and insulate can save materials, simplify design, and facilitate installation and use, killing three birds with one stone. When MCB is open, the short distance between its moving and stationary contacts is called open distance. The opening distance is one of the factors to ensure the electrical isolation performance of MCB in the open state.

当开关电器用在高海拔地区时，只要海拔高度超过2000米，空气的击穿电压就会降低，并且由于空气稀薄因而电器的散热 能力也降低，此时就一定考虑让开关电器降容使用。所谓降容，指的就是降低它的额定电流值。

When the switchgear is used in high altitude areas, as long as the altitude is more than 2000 meters, the breakdown voltage of the air will be reduced, and because the air is thin, the heat dissipation capacity of the appliance will also be reduced. At this time, we must consider reducing the capacity of the switchgear. The so-called capacity reduction refers to the reduction of its rated current value.

对于断路器来说，它有三个重要参数与分断能力相对应。这三个参数分别是：额定短路分断能力、额定运行短路分断能力Ics、额定短路分断能力Icu。显见，对于开关而言，它不可能具备这三个参数，因而开关绝 不可能具有分断短路电流的能力。

For the circuit breaker, it has three important parameters corresponding to the breaking capacity. The three parameters are: rated short-circuit breaking capacity, rated operating short-circuit breaking capacity ICs, rated short-circuit breaking capacity ICU. Obviously, for the switch, it can not have these three parameters, so the switch can never have the ability to break the short-circuit current.

开关是被动元件。当线路中出现短路电流时，开关只能被动地承受短路电流的冲击；断路器是主动元件。当线路中出现短路电流时，断路器不但能承受短路电流的冲击，还能主动地分断短路电流。

The switch is a passive component. When short-circuit current appears in the circuit, the switch can only bear the impact of short-circuit current passively; Circuit breakers are active components. When short-circuit current occurs in the circuit, the circuit breaker can not only withstand the impact of short-circuit current, but also actively break the short-circuit current.