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.

當開關電器用在高海拔地區時，只要海拔高度超過2000米，空氣的擊穿電壓就會降低，并且由于空氣稀薄因而電器的散熱 能力也降低，此時就一定考慮讓開關電器降容使用。所謂降容，指的就是降低它的額定電流值。

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.

對于斷路器來說，它有三個重要參數與分斷能力相對應。這三個參數分別是：額定短路分斷能力、額定運行短路分斷能力Ics、額定短路分斷能力Icu。顯見，對于開關而言，它不可能具備這三個參數，因而開關絕 不可能具有分斷短路電流的能力。

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.

開關是被動元件。當線路中出現短路電流時，開關只能被動地承受短路電流的沖擊；斷路器是主動元件。當線路中出現短路電流時，斷路器不但能承受短路電流的沖擊，還能主動地分斷短路電流。

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.