The residual current sensor is also called the leakage current transformer. Similar to the principle of zero-sequence current transformers, they are based on Kirchhoff's current law, but the requirements for use and installation are different.
The zero-sequence current transformer is generally suitable for the TN grounding system, and the residual current sensor protection action setting current can be from mA level to A level, which has a relatively high operational sensitivity, so the residual current protection device for TT, IT, TN-STN -CS system grounding system is available. During installation, the A, B, and C three-phase wires pass through a residual current sensor together with the N line.
The low voltage system is a neutral point direct grounding system. A single-phase ground fault must have zero-sequence current flowing through the earth. Zero-sequence current transformers are not mainly measuring small currents, while residual current sensor mainly measure small currents. Therefore, its motion sensitivity is higher than that of the zero-sequence current transformer.https://www.ctsensorducer.com/