In the national standard GB50054-95 "Low-voltage power distribution design specification" Article 4.4.10 clearly pointed out two methods of ground fault protection, zero-sequence current protection and residual current protection (also known as leakage current protection). The basic working principle of these two current protections is the same, based on Kirchhoff's current law, but the requirements for use range and installation are different.
Zero sequence: Zero sequence current protection is generally suitable for use in TN grounding systems. Because when a phase is grounded, the Id loop impedance to the TN-S system includes phase line impedance Z1, PE line impedance ZPE and contact impedance Zf, ie Zs=Z1+ZPE+Zf; for TN-C system, Id loop impedance Including phase line impedance Z1, PEN line impedance ZPEN and contact resistance Zf, ie ZS=Z1+ZPEN+Zf; for TN-CS system, Id loop impedance includes phase line impedance Z1, PEN line impedance ZPEN, PE line impedance ZPE and contact The resistance Zf, ie ZS=Z1+ZPEN+ZPE+Zf, produces a single-phase ground fault current Id=220/ZS, which is significantly larger than the three-phase unbalanced current when there is no fault. As long as the setting is appropriate, the ground fault can be detected. The zero sequence current at the time to cut off the fault loop. For IT systems, it is generally used in industrial and mining enterprises that require high reliability of power supply, do not need to immediately cut off the power supply circuit for single-phase grounding, but need to issue insulation damage monitoring signals to maintain power supply for a period of time. Three-phase three-wire distribution line of the sex line. When the single-phase is grounded, the zero-sequence current flowing through the faulty line is the sum of the capacitor currents of the non-faulty system of the whole system, so that the ground fault current is easily detected, so the zero-sequence current protection device can be used to monitor the first ground fault. . During installation, the A, B, and C three-phase wires pass through a zero sequence C.T, and the low voltage can also be installed on the neutral line N with a zero sequence C.T.
Remaining: Residual current transformer protection action The setting current can be from mA level to A level, and has a high operational sensitivity. Therefore, the residual current protection device can be applied to the TT, IT, TN-S TN-C-S system grounding system. During installation, the A, B, and C three-phase wires pass through a remaining C.T together with the N line.