In order to make the trunk circuit breaker reliably cut off the fault circuit, the action sensitivity of the circuit breaker release must be verified, i.e.
For long-distance single-phase medium circuit, zero sequence current protection can be adopted, and the cross-sectional area of neutral conductor can be increased, so as to increase the single-phase medium current short-circuit current.
Zero sequence current protection can be adopted, or the neutral line section can be increased to increase the short-circuit current, so as to improve the sensitivity coefficient.
The protection of the terminal distribution lines of high-rise buildings is reviewed in the past (1) protection of the terminal distribution lines of high-rise buildings (two): Hu Zhongkui sliding to the left, paying attention to the official account to hear our voices..
It can be seen that enlarging the cross-sectional area of PE line can amplify the single-phase short-circuit current and improve the sensitivity coefficient.
If 3×35 + 2x16mm2 cable is selected, the single-phase phase medium short-circuit current is 1.066ka and the sensitivity coefficient is KLZ = I dmin/I ZD = 1066 / 960 = 1.11 ＜ 1.3, not meeting the requirements.
For long-distance single-phase ground fault circuit, it may not be economical to increase the cross-sectional area of the conductor and then increase the single-phase ground fault current.
If 3×35 + 2x16mm2 cable is selected, the single-phase short-circuit current is 0.84ka, and the sensitivity coefficient is KLZ = 820 / 960 = 0.85 ＜ 1.3, which does not meet the requirements.
If the cross-sectional area of the cable is too small and the setting current of the switch is inappropriate, it may not be enough to make it act and fail to play a protective role.
When using low-voltage circuit breaker, the line short-circuit sensitivity is closely related to the line short-circuit current and the setting current of the circuit breaker.
If the setting value is too small, it cannot avoid the starting current of the fan, resulting in misoperation.
According to the previous calculation process, for 5x35mm2 cable, the single-phase grounding fault short-circuit current is 1.26ka, and the short-circuit sensitivity coefficient is, KLZ = I dmin/I ZD = 1260 / 960 = 1.31 ＞ 1.3, meeting the requirements of article 6.2.4 of code for design of low voltage distribution gb50054-2011.
KLZ = I dmin/I zd≥1.3，K LZ – action sensitivity coefficient of circuit breaker; I ZD – setting current of instantaneous overcurrent release of circuit breaker (a); Idmin – minimum current of expected short-circuit current of protected line (a), which is single-phase short-circuit current in TN system; If the line distance at the end of the equipment is long and the impedance is large, the single-phase short-circuit current is very small; If the setting value of the instantaneous overcurrent release of the circuit breaker is large, the short-circuit current may not be enough to promote the reliable action of the circuit breaker.
4.3 short circuit sensitivity coefficient verification.
In this case, residual current action protector can be added to protect the circuit.
For single-phase phase medium short-circuit current, it can not be protected by residual current action protector.
For 5x35mm2 cable, according to the previous calculation process, the single-phase phase medium short-circuit current is 1.59ka and the sensitivity coefficient is KLZ = I dmin/I ZD = 1590 / 960 = 1.66 ＞ 1.3, meeting the requirements.
If the conductor cross-sectional area of neutral line and protection line can be increased, the conductor resistance can be effectively reduced and the short-circuit current can be increased.
5 conclusion for long-distance power supply lines, due to large resistance and small single-phase short-circuit current, we must pay full attention to the verification of short-circuit sensitivity.