Many characteristic index low voltage circuit breaker, the circuit breaker of the type [the number of poles, current type, activation mode (direct human, remote human, overcurrent, undervoltage, reverse current), the rating of the main circuit, the control loop characteristics, auxiliary circuit characteristics, using categories and so on. The following figure shows a characteristic diagram low voltage circuit breaker protection.

protection features [: operating time t and the relationship between the overcurrent release operation current I. In order to make full use of electrical equipment and overload incident narrow range, automatic switch protection features must be selective, both to protect the characteristic should be segmented, generally can be divided into three sections:

• section ab : overload protection section. Its operation to the current time and the operation of inverse time relationship, the greater the overload factor, the shorter the operation time.
• section df: instantaneous action part. Fault current exceeds a current value corresponding to the point d, then the instantaneous overcurrent release operation.
• section ce: delay action part. Fault current greater than the value of the point c, the overcurrent release operation after a delay.

The object claimed, the protective properties of two-stage automatic switch are: The abdf formula (overload long and short circuit instantaneously) or abce formula (overload long and short short delay.