Introduction Electromagnetic compatibility (EMC) refers to the ability of a device or system to operate in its electromagnetic environment without causing unacceptable electromagnetic interference to any object in that environment. Residual current devices (RCDs), as protection devices for power supply lines at the end of the power grid (below 400 V), must meet this requirement. The national EMC standard GB/T17626.5—1999 requires 3C certification before they can be put into operation on the power grid. Figure 1 shows a block diagram of an intelligent residual current device system designed using a P87LPC767 microcontroller. EMC design was considered simultaneously in circuit design, software design, and PCB board design. Residual current devices are low-voltage electrical equipment with no high-power, high-frequency circuits inside. Their electromagnetic radiation is weak, and the electromagnetic interference they generate has little impact on other equipment; therefore, this is not the focus of EMC design. The focus of EMC design for residual current devices is their ability to maintain stable operation when subjected to electromagnetic interference from other equipment, i.e., their anti-interference capability. The interference experienced by residual current devices (RCDs) mainly originates from the power grid itself. This includes unipolar surges (impacts) caused by sudden line breaks or lightning transient overvoltages, and transient disturbances in signal parameters caused by lightning, grounding faults, or switching inductive equipment. These two aspects are key areas and challenges in electromagnetic compatibility (EMC) design. The following section introduces the EMC design methods for this intelligent residual current device from several perspectives. 1. Anti-interference Design of the Microcontroller System This residual current device uses a P87LPC767 microcontroller. Important tasks such as residual current sampling, detection, calculation, display, action criteria, and execution of protection actions are all performed by the microcontroller. The anti-interference capability of the microcontroller system directly determines the overall anti-interference capability of the device. However, the microcontroller's clock signal, reset circuit, interrupt signal, and sampling signal are easily affected by electromagnetic interference. Therefore, eliminating or suppressing the impact of electromagnetic interference signals on the microcontroller is crucial. The design is implemented in the following aspects: The microcontroller's power supply is designed separately from the power supplies of other system circuits to avoid interference from other circuits. Sufficient margin is provided in the microcontroller's power supply design to prevent power fluctuations from affecting its operation. When designing the PCB board, capacitors and transient voltage suppressors (TVS) are connected to the microcontroller's power pins, as shown in Figure 2. The energy stored in the 100 μF electrolytic capacitor is released during power fluctuations (reduction), maintaining power stability; the 0.1 μF high-frequency capacitor absorbs high-frequency interference on the power supply; the TVS absorbs transient surge power, clamping the voltage between its terminals to a predetermined value, thus maintaining the stability of the microcontroller's power supply. The reset circuit uses the P87LPC767 microcontroller's internal power-on reset circuit to avoid direct interference signals affecting the reset circuit. As long as the microcontroller's power supply is stable, malfunctions caused by false resets will not occur. Interrupt and other I/O port capacitors are used for filtering to reduce the impact of interference. In terms of system software design, the watchdog timer inside the P87LPC767 is enabled to prevent the PC from going out of control due to interference, causing program erratic behavior and entering an "infinite loop." A software trap is set in the non-program area of ​​the program storage space. When interference causes the operating system to go out of control and enter the non-program area, the boot instructions are redirected to a program specifically designed to handle program errors, bringing the program back on track. 2. Elimination of High-Frequency Interference in the System – “Sampling Monitoring” Method High-frequency interference signals generated by transient disturbances in signal parameters due to lightning, grounding faults, or switching inductive equipment mainly enter the system through the system's incoming power supply. The anti-interference capability in this regard is primarily verified through the “Electrical Fast Transient Burst Immunity Test.” According to the testing standards for low-voltage electrical products, a burst pulse with a frequency of 2500 Hz and an amplitude of 4000 V is superimposed at the power input terminal during the test. Differential mode and common mode tests, positive polarity and negative polarity tests are performed on the phase line and ground line, respectively. Each test lasts for 1 minute. The waveform of the burst pulse is shown in Figure 3. If the protector does not malfunction during the "Electrical Fast Transient Burst Immunity Test," its anti-interference capability is considered合格 (qualified). This system uses a "sampling monitoring" method to monitor high-frequency interference on the power supply, providing corresponding flags, which are then processed by the software program to prevent malfunctions. The detailed principle is as follows: To ensure the real-time performance of residual current measurement, the protector uses a digital sampling method to measure the residual current, converting the residual current signal into the waveform shown in Figure 4, and sending it to the A/D converter inside the P87LPC767 microcontroller for sampling and measurement. The digital sampling method has a digital filtering effect, suppressing interference in the signal channel. 100 sampling points are set for each cycle, with the sampling time interval generated by timer T0. The power supply signal of a certain phase (e.g., phase A) is converted into a square wave signal through a step-down and shaping circuit as shown in Figure 5. This square wave is used as the trigger signal for microcontroller interrupt 0. Under normal circumstances, the frequency of the power supply signal is approximately 50Hz, and the residual current is sampled 100 times during the two interrupts. When the power supply has high-frequency interference signals, the converted square wave signal is also synchronously interfered with, and high-frequency pulses are superimposed on the frequency, which will affect the interrupt. The time between the two interrupt triggers is shortened, and the number of samplings of the residual current during the two interrupts will also be less than 100. By using the microcontroller interrupt to monitor the residual current sampling process, it is possible to determine whether the system is affected by high-frequency interference. Appropriate software processing can be taken to avoid the malfunction of the protector. The monitoring and sampling software program is as follows: } The system main program judges bgrc1. If it is effective, interference software processing is performed; if it is ineffective, the measurement process is not affected by high-frequency interference from the power supply, and normal calculation processing is performed. 3 Other anti-interference measures The unipolar surge caused by circuit breaking or lightning transient overvoltage is also a key point of electromagnetic compatibility design of the protector. Adding a zinc oxide varistor between the live and neutral wires of the power line, when the voltage across the varistor is lower than the nominal voltage, its resistance is almost infinite. After slightly exceeding the rated value, the resistance value drops sharply, with a response time in the nanosecond range. The varistor can significantly attenuate surge interference and reduce the impact on other circuits. When designing the PCB board for the residual current device, various factors that might affect the device's performance were comprehensively considered to improve the PCB's ability to suppress interference. High-voltage circuitry was concentrated at one edge of the circuit board and kept at an appropriate distance from low-voltage components. Analog signal processing and digital processing circuits were separated. Since the microcontroller's internal A/D converter cannot be completely separated, the analog and digital grounds were routed to share a common ground at a single point. System power and ground lines were thickened, and blank areas were copper-clad with a mesh structure as part of the digital ground, reducing analog signal interference to the digital processing circuit. The microcontroller's clock circuit was placed as close as possible to the chip pins and maintained adequate space from other components and PCB traces to minimize the impact of high-frequency radiation on the system. Conclusion Due to the correct and effective hardware and software measures, the protector successfully passed product type testing, obtained 3C certification, and operated reliably without malfunctions. The microcontroller's interrupts were used to monitor power supply interference and subsequently the AC signal sampling process. Practice has proven that the combination of software and hardware is an effective method for eliminating high-frequency interference in AC sampling systems.