Abstract: This paper describes the causes and harms of residual current and designs a set of intelligent earth leakage circuit breakers . The hardware structure and software design are given. The test results show that: The intelligent circuit breaker reliable quality, stable performance, fully meet the requirements of national standards. Electric grounding fault arcing short circuit to ground is an important cause of electrical fire. Arc-to-ground short circuit has a great resistance and voltage drop, which limits the fault current, so that over-current protection can not move or can not act in time to cut off the power supply, and several hundred milliamps of leakage arc generated by the local high temperature up to 2000 ℃ above, enough to ignite the surrounding combustible and cause a fire. Moreover, the distribution of electrical equipment in all corners of the building poses a wide range of hazards. Without monitoring and prevention of system leakage, it poses a threat to personal and property safety and poses a great fire hazard. Intelligent leakage circuit breakers can accurately monitor the electrical circuit fault and abnormal conditions, can effectively prevent the common cause of earth leakage due to electrical leakage caused by the building electrical fire accident. In order to ensure the safety of people's lives and property, it is necessary to install an intelligent earth leakage circuit breaker at the power inlet and the trunk of a building. 1 Remaining current causes and protection principle Let the three-phase four-wire lead through a zero-sequence current transformer CT together, and also install a zero-sequence current transformer CT on the neutral line N to detect the sum of the three-phase current vectors, that is, the residual current, As shown in Figure 1, according to the circuit principle, it can be seen that when there is no leakage or ground fault in the circuit and the three-phase load is completely balanced, the vector sum of the instantaneous current in the primary side is zero, ie Ia + Ib + Ic + IN = , The vector sum of the magnetic flux generated in the current transformer is equal to zero. At this moment, the induced current IL = 0 in the secondary coil. When the circuit to be protected when there is insulation failure, the load side of the ground leakage current, zero sequence current transformer vector and is not zero, that is Ia + Ib + Ic + IN ≠0, in the current transformer magnetic flux Is not equal to zero. At this moment, the induced current will be generated in the secondary winding of the zero-sequence current transformer, that is, the residual current IL ≠0. Leakage circuit breakers mainly by the zero sequence current transformer CT, leakage detection circuit, release components. When the circuit to be protected has electric leakage or human body electric shock, as long as the electric leakage current or the electric shock current reaches the leakage current value, the secondary winding of the zero-sequence current transformer outputs a signal which is amplified by the integrated circuit amplifier and sent to the CPU. The CPU outputs the driving signal Leakage trip action circuit breaker trip, thus cutting off the power supply to play the role of leakage and electric shock protection. 2 circuit breaker controller design 2.1 system basic functions Intelligent residual current circuit breakers set the residual current, short circuit, overload, overvoltage and undervoltage (phase loss) and other electrical fault monitoring, analysis, alarm and control in one, with the following main features: 1) Residual current detection and protection function, when detected leakage occurs, the residual current IL ≠0, the signal through the microcontroller after sampling fast judge, when the residual current reaches the setting action value, the drive thyristor, connected to the electromagnetic Release power supply, electromagnetic release pull-pull, so that circuit breakers trip, so as to achieve leakage protection. 2) Protecting the operating current, the breaking time can be adjusted: When used as the total protection in the station, the residual current can be set to 300 ~ 1 000 mA and the breaking time can be set to 0.6 s. As the second protection, The current can be set to 200 mA and the breaking time can be set to 0.3 s. This setting avoids the occurrence of large-scale blackouts caused by over-level trip. 3) Can identify the sudden change of residual current and slowly change the residual current, thereby identifying equipment leakage and live electric shock. Gradient and Mutation Leakage Separately identified for China's rural low-voltage power grid characteristics, has been widely used, is an effective rural power technology safety measures. 4) With over-current delay, over-current short-delay and short-circuit protection of instantaneous protection three components, the composition of the required leakage protection circuit breaker characteristics. Intelligent setting of leakage current, over-current delay, over-current short delay and over-current instantaneous setting and early warning value. 5) Display and save the line address, fault type, fault occurrence time and leakage current, three-phase current value of the fault occurred point. Can record up to 200 historical faults, long-term preservation, until the order to delete. 6) The use of RS485 bus communication technology, you can use the bus and host form master-slave monitoring system to achieve user networking, on a computer can be 1 to 250 intelligent remote control circuit breakers online at any time to check the safety of electricity users Situation, at any time connected to or disconnected from the user power supply lines, and various parameters of the circuit breaker can be remotely set. 2.2 overall hardware design Intelligent leakage circuit breaker mainly by the power circuit, microcontroller PIC24FJ64, three-phase AC voltage and current detection circuit, the remaining current detection circuit, serial communication interface circuit, human-machine interface circuit and alarm and other parts of the composition. Its main working principle: the current transformer and linear optical isolation obtained from the three-phase current, leakage and voltage signal conditioning, the input to the microcontroller A / D conversion, the microcontroller to analyze their samples, the output of the corresponding Display and alarm signals. The results of its analysis can also be transmitted to the host computer through the RS485 bus. 2.2.1 microcontroller circuit Microcontroller selection PIC24FJ64, which is designed by Micrchip company an improved Harvard architecture high-performance CPU, is the core of intelligent circuit breakers, which complete the intelligent circuit breaker of various control functions, including three-phase voltage, three-phase current and leakage Current sampling, data processing, alarm output, communication with the host computer, LCD display and buttons and other functions. Microchip's development, development and production of microcontroller technology has the following advantages: 1) Harvard bus architecture; 2) RISC technology; 3) simple addressing scheme; 4) 6) Very low power consumption; 7) PIC16F877 chip has A / D, MSSP, USART serial bus ports and so on, and has the external circuit is simple, easy to develop, available C language programming, program confidentiality and so on. 2.2