According to IEC947 low voltage circuit breaker, it is necessary to ensure that the breaker with the specified isolation distance between contacts can be used as isolation switch. This article emphasizes that not all circuit breakers can be used as isolators, especially MCCB. Because of its compact structure, it is difficult to meet the requirements of the isolator creepage distance and air gap. Therefore, if the sample of manufacturer indicates that the breaker has isolation function, we can use the breaker as isolator in design, or cancel all releases and use it as an isolation switch separately.