零序电流保护一般适合使用于TN接地系统。因为当发生一相接地时，对TN-S系统Id回路阻抗包括相线阻抗Z1，PE线阻抗ZPE和接触阻抗Zf，即Zs=Z1+ZPE+Zf；对于TN-C系统，Id回路阻抗包括相线阻抗Z1，PEN线阻抗ZPEN和接触电阻Zf，即ZS=Z1+ZPEN+Zf；对于TN-C-S系统，Id回路阻抗包括相线阻抗Z1，PEN线阻抗ZPEN，PE线阻抗ZPE和接触电阻Zf，即ZS=Z1+ZPEN+ZPE+Zf，产生的单相接地故障电流Id=220/ZS，明显大于无故障时的三相不平衡电流，只要整定合适，就可检测出发生接地故障时的零序电流，以切断故障回路。而对IT系统，一般均是使用对供电可靠性要求较高、对单相接地不必要立即切断供电回路、但需发出绝缘破坏监察信号、以维持继续供电一段时间的工矿企业内的不配出中性线的三相三线配电线路。当单相接地时，该故障线路上流过的零序电流是全系统非故障系统电容电流之和，因而容易检测出接地故障电流，故可用零序电流保护装置来监察相对地第一次接地故障。TT接地系统常应用于工农业、民用建筑的照明、动力混合供电的三相四线配电系统中，常发现三相不平衡电流较大，当发生一相接地时，Id回路阻抗包括相线阻抗Z1，PE线阻抗ZPE，负载侧接地电阻RA和电源侧接地电阻RB，接触阻抗Zf，即ZS=Z1+ZPE+RA+RB+Zf，接地故障电流Id=220/ZS，由于RA+RB>>Z1+ZPE+Zf，且RA+RB数值一般均较大，很明显TT系统的故障环路阻抗大，产生的单接故障电流Id，远远小于不平衡电流，很难检测出故障电流，故不适用于TT接地系统。

Zero sequence current protection is generally suitable for TN grounding system. Because when there is a ground, the Id loop impedance TN-S system including phase impedance Z1, PE line impedance ZPE and contact impedance Zf, Zs=Z1+ZPE+Zf; Id for TN-C system, including the phase loop impedance impedance Z1, PEN line impedance ZPEN and contact resistance Zf, ZS=Z1+ZPEN+Zf; Id for TN-C-S system, including phase loop impedance PEN line impedance impedance Z1, ZPEN, ZPE and PE line impedance contact resistance Zf, namely ZS=Z1+ZPEN+ZPE+Zf, ground fault current generated by the Id=220/ZS phase, significantly greater than the three-phase unbalanced current without fault, as long as the appropriate setting, can detect the occurrence of zero sequence current grounding fault, to cut off the fault circuit. The IT system is generally used for the power supply reliability, higher requirements of single-phase grounding unnecessary immediately cut off the power supply circuit, but need to send the monitoring signal, in order to maintain the insulation failure to supply power for a period of time in the industrial and mining enterprises deserve a three-phase neutral line three line distribution line. When the single-phase grounding fault line, the whole system is zero sequence current flowing through the capacitor current and non fault system, thus it is easy to detect the fault current, it can be used to monitor the zero sequence current protection device relative to ground fault first. TT grounding system often used in industry and agriculture, civil building lighting, power supply three-phase four wire distribution system, often found in three-phase unbalanced current is larger, when there is a ground, including the Id loop impedance phase impedance Z1, PE line impedance ZPE, load side grounding resistance RA and power supply side joint the resistance of RB, Zf or ZS=Z1+ZPE+RA+RB+Zf, contact impedance, grounding fault current of Id=220/ZS, due to RA+RB>>Z1+ZPE+Zf, and the RA+RB number are generally larger, fault loop impedance obviously TT system, the single fault current of Id, far less than the unbalanced current, it is difficult to fault detection, is not suitable for TT. System.