Selection and Setting of Overcurrent Release
(1) The selection of rated current for overcurrent release The rated current IN · OR of overcurrent release should not be less than the calculated current IC of the line, that is, IN · OR ≥ IC
(2) Setting of the operating current of the overcurrent release
① Setting of instantaneous overcurrent release action current
The operating current IOP (0) of the instantaneous overcurrent release should avoid the peak current IPK of the line, that is, IOP (0) ≥ Krel · IPK. In the formula, Krel is the reliability coefficient. For circuit breakers with an action time of more than 0.02s, such as DW, ME type, etc., Krel=1.35; For circuit breakers with an action time of less than 0.02s, such as DZ type, Krel=2-2.5. Determine the setting value of the operating current based on the calculated value of the operating current IOP (0) and the technical data of the instantaneous overcurrent release.

② The set operating current IOP (S) of the short delay overcurrent release should avoid the short-term peak current IPK that occurs in the line, that is, IOP (S) ≥ Krel · IPK. In the formula, Krel is the reliability coefficient, taken as 1.2. The action time of the short delay release does not exceed 1 second in a period, usually divided into three levels of 0.2, 0.4, and 0.6 seconds. However, the short delay time in some new products is also different. For example, the DW40 circuit breaker has a fixed time characteristic of four levels of 0.1, 0.2, 0.3, and 0.4 seconds. When the ME series circuit breaker uses a semi conductor overcurrent release, its short delay range is 30-270ms, graded, with each level of 30 or 60ms. The action time can be determined according to the protection requirements.

③ Setting of operating current and operating time for long delay overcurrent release
The action current IOP (1) only needs to avoid the maximum load in the line to calculate the current IC, that is:
In the equation where IOP (1) ≥ Krel · I, Krel is the reliability coefficient, taken as 1.1. The long delay overcurrent release is used for overload protection, and its action time is an inverse time characteristic. The general action time is 1-2 hours.
④ Requirements for the coordination between overcurrent release and distribution lines
In the formula IOP ≤ KOL · Ial, Ial is the allowable current carrying capacity of the insulated wire or cable, and KOL is the allowable short-term overload coefficient of the wire or cable. For instantaneous and short delay overcurrent releases, KOL=4.5, and for long delay overcurrent releases, KOL=1. KOL=0.8 for distribution lines in areas with explosive gases. When the above requirements cannot be met, the tripping current of the release can be selected or the cross-sectional area of the distribution line wire can be increased.

2. Selection and setting of thermal release for low-voltage circuit breakers
① The rated current of the thermal release should not be less than the maximum calculated load current ICIN of the line R ≥ IC
② Setting of the operating current of the thermal release
The action current should be set based on the maximum calculated load current of the line
IOP TR ≥ Krel IC
3. Selection of undervoltage release and shunt release. The undervoltage release is mainly used for undervoltage or undervoltage protection, and can act when the voltage drops to (0.35-0.7) UN. The shunt release is mainly used for the opening operation of circuit breakers, and can reliably operate at (0.85-1.1) UN.