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.
剩余電流，是指低壓配電線路中各相(含中性線)電流矢量和不為零的電流。通俗講當用電側發生了事故,電流從帶電體通過人體流到大地,使主電路進出線中的電流I相和I中的大小不相等,此時電流的瞬時矢量合成有效值稱為剩余電流，俗稱漏電。
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.
3.剩余電流的危害：
Risk of residual current:
（1）人身觸電。當電氣設備因絕緣損壞而使外殼帶電，而工作人員又接觸此外殼時，就會導致人身觸電事故。
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）燒損電氣設備，引起火災。長期存在的漏電電流，尤其是兩相經過電阻接地的漏電電流，在通過設備絕緣損壞處時將散發出大量的熱，使絕緣進一步損壞，甚至使可燃性材料（如非阻燃性橡套電纜）著火燃燒。
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）引起短路事故。據統計，約有30%的單相接地故障發展為短路。從而造成更大的電氣故障。長期存在的漏電電流及電火花使漏電處的絕緣進一步損壞，最后危及相間絕緣而造成短路。
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）引起電氣火災事故。電網發生單相接地或設備發生單相碰殼時，在接地點就會產生電火花，若此電火花具有足夠的能量，就可能點燃易燃易爆物品。
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.如何測剩余電流
How to measure residual current
在被測的三相導線路上與N線上各裝一個CT，或讓三相導線與N線一起穿過一個零序CT，得到三相導線與中性線N的電流矢量和IA*+IB*+IC*+IN*，當沒有發生單相接地故障時，無論三相負荷平衡與否，則此矢量和為零；當發生某一相接地故障時，故障電流中會通過保護PE及與地相關連的金屬構件回到電源，即IA*+IB*+IC*+IN*，此時數值為接地故障電流Id。
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系列剩余電流動作繼電器
ASJ series residual current operated relay
ASJ系列剩余電流動作繼電器可與低壓斷路器或低壓接觸器等組成組合式剩余電流保護裝置， 主要適用于交流 50Hz， 額定電壓 400V 及以下的 TT 和 TN 系統配電線路， 用來對電氣線路進行接地故障保護， 防止接地故障電流引起的設備損壞和電氣火災事故， 也可用來對人身觸電危險提供間接接觸保護。
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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