1.Overview Residual current relay can be combined with external residual current transformer to detect and evaluate the ground fault current. They can also be combined with protective devices to realize the disconnection of circuits and the protection of lines and workers. 2.Definition and risk of residual current Residual current is the current vector sum of each phase (including the neutral ) in the low-voltage distribution that not be zero. Generally, when there is an accident on the power side, the current flows from the charged body to the earth through the human body,it will cause that the current phase I in the main circuit not equal to the size in phase I. At this time, the value of instantaneous current vector synthesis is called residual current, commonly known as leakage. TN System 
3.Risk of residual current: （1）Physical electric shock. When shell of the electrical equipment is electrified due to insulation damage, and the workers touch the shell again, it will lead to physical electric shock accident. （2）Burn electrical equipment and cause fire. The long-term leakage current, especially the leakage current of two-phase grounded through resistance, will release a lot of heat when flow through the damaged part of the equipment insulation, further damaging the insulation, and even making combustible materials (such as non flame retardant rubber sheathed cable) burn on fire. （3） Cause short circuit accident. According to statistics, about 30% of single-phase grounded faults develop into short circuit which will give rise to heavier electrical failure. The long-term leakage current and spark further damage the insulation at the leakage point, and finally endanger the interphase insulation and cause short circuit. （4）Cause electrical fire accident. When the single-phase grounding occurs in the power grid or the single-phase collision occurs in the equipment, the electric spark will be generated at the grounding point. If the electric spark has enough energy, it may ignite flammable and explosive materials. 4.How to measure residual current Installing a CT on each of the three-phase conducting lines to be tested and N line or let the three-phase conductor and the n-line pass through a zero sequence CT together to get the current vector sum IA * + IB * + IC * + IN * . when there is no single-phase grounded fault, whether the three-phase load is balanced or not, the vector sum is zero; when there is a phase grounded fault, the fault current will flow through the protection PE and the metal components connected to the earth to the power supply, at this time, the value is the ground fault current Id. 
5.ASJ series residual current operated relay ASJ series residual current action relay can be combined with low voltage circuit breaker or low voltage contactor to form a combined residual current protection device.It is mainly used for TT and TN system distribution lines with AC 50Hz and rated voltage of 400V and below.It is used to protect the electrical circuit from ground faults, prevent equipment damage caused by ground fault current and electrical fire accidents, and can also be used to provide indirect contact protection against personal electric shock hazard 
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