Zero sequence current protection is generally suitable for TN ground system. For a TN-S system, the ID loop impedance includes the phase line impedance Z1, the Pe Line impedance Zpe and the contact impedance ZF, namely zs = Z1 + Zpe + Zf; for a TN-C system, the ID loop impedance includes the phase line impedance Z1, the line impedance of PEN ZPEN and the contact resistance ZF, namely zs = Z1 + Zpen + ZF; for the TN-C-S system, the impedance of ID loop includes the Line impedance Z 1, the line impedance of PEN ZPEN, the line impedance of Pe Zpe and the contact resistance ZF, namely zs = Z1 + Zpen + ZPE + ZF, the single-phase-to-ground fault current ID = 220/ZS is obviously larger than the three-phase unbalanced current without fault.

If the setting is proper, the zero-sequence current can be detected to cut off the fault circuit. And for IT systems, in general, three-phase three-wire distribution lines which require high reliability of power supply, do not need to cut off power supply circuit immediately for single-phase grounding, but need to send out insulation damage monitoring signal to maintain power supply for a period of time.