With the development of industrial equipment automation control technology, programmable logic controllers (PLCs) are increasingly widely used in industrial equipment control. The reliability of PLC control systems directly affects the safe production and economic operation of enterprises, and the system's anti-interference capability is crucial to the reliable operation of the entire system. This article details the types and sources of interference affecting PLC operation and proposes implementation strategies for anti-interference design. Among the various types of PLCs used in automation systems, some are centrally installed in the control room, while others are installed on the production site and various motor equipment. They are mostly located in the harsh electromagnetic environment created by high-voltage circuits and equipment. To improve the reliability of PLC control systems, on the one hand, PLC manufacturers need to improve the anti-interference capability of their equipment; on the other hand, application departments need to pay close attention to engineering design, installation, construction, and maintenance. Multi-party cooperation is essential to effectively solve problems and enhance the system's anti-interference performance. 1. Types of Electromagnetic Interference and Their Impact The interference sources affecting PLC control systems are similar to those affecting general industrial control equipment; they mostly originate from areas of drastic current or voltage change. These areas of rapid charge movement are the interference sources. Interference types are usually classified according to the cause of interference, noise interference mode, and noise waveform characteristics. Noise can be categorized in several ways. Based on its cause, noise can be classified as discharge noise, surge noise, high-frequency oscillation noise, etc. Based on its waveform and nature, it can be classified as continuous noise, intermittent noise, etc. Based on its interference mode, it can be classified as common-mode interference and differential-mode interference. Common-mode interference and differential-mode interference are commonly used classification methods. Common-mode interference is the potential difference between the signal and ground, mainly formed by the superposition of common-mode (same-direction) voltages induced on the signal line by grid interference, ground potential difference, and spatial electromagnetic radiation. Common-mode voltage can sometimes be large, especially when using power distribution equipment with poor isolation performance; the common-mode voltage of the transmitter output signal is generally high, sometimes exceeding 130V. Common-mode voltage can be converted into differential-mode voltage through asymmetrical circuits, affecting the measurement and control signal and causing component damage (this is the main reason for the high failure rate of some system I/O modules). This common-mode interference can be DC or AC. Differential-mode interference refers to the interference voltage acting between the two poles of a signal. It is mainly formed by the coupling induction of spatial electromagnetic fields between signals and by the conversion of common-mode interference by unbalanced circuits. This interference is superimposed on the signal and directly affects the measurement and control accuracy. 2. Main sources of electromagnetic interference 2.1 Radiated interference from space Radiated electromagnetic fields (EMI) are mainly generated by power networks, transient processes of electrical equipment, lightning, radio broadcasts, television, radar, high-frequency induction heating equipment, etc., and are usually called radiated interference. Its distribution is extremely complex. If the PLC system is placed in its radio frequency field, it will be subject to radiated interference. Its impact is mainly through two paths: one is direct radiation to the inside of the PLC, which induces interference through the circuit; the other is radiation to the PLC communication network, which induces interference through the communication lines. Radiated interference is related to the layout of field equipment and the magnitude of the electromagnetic field generated by the equipment, especially the frequency. It is generally protected by setting up shielded cables and local shielding of the PLC and high-voltage discharge components. 2.2 Interference from external leads of the system is mainly introduced through power supply and signal lines and is usually called conducted interference. This interference is more serious in industrial sites in China and mainly includes the following three types: The first type is interference from the power supply. Practice has shown that many PLC control system failures are caused by power supply interference. I encountered this during the commissioning of a project, and the problem was only resolved by replacing the PLC power supply with one offering higher isolation performance. The normal power supply for PLC systems is from the power grid. Due to the wide coverage of the power grid, it is subject to electromagnetic interference from all directions, inducing voltage and current on the lines. In particular, changes within the power grid, surges from switching operations, the start-up and shutdown of large power equipment, harmonics from AC/DC drives, and transient impacts from power grid short circuits are all transmitted to the primary side of the power supply through transmission lines. PLC power supplies typically use isolated power supplies, but their isolation is not ideal due to their structure and manufacturing processes. In reality, absolute isolation is impossible due to the presence of distributed parameters, especially distributed capacitance. The second type of interference comes from signal lines. Various signal transmission lines connected to the PLC control system, in addition to transmitting valid information, will always be subject to external interference signals. This interference mainly occurs through two pathways: one is power grid interference introduced through the power supply of transmitters or shared signal instruments, which is often overlooked; the other is interference induced by spatial electromagnetic radiation on the signal lines, i.e., external induced interference on the signal lines, which is often very serious. Interference introduced by signals can cause abnormal I/O signal operation and a significant reduction in measurement accuracy, and in severe cases, damage to components. For systems with poor isolation performance, it can also lead to mutual interference between signals, causing backflow on the common ground bus, resulting in changes in logic data, malfunctions, and system crashes. The number of I/O module damages caused by signal-introduced interference in PLC control systems is quite high, leading to numerous system failures. The third type of interference comes from a chaotic grounding system. Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Correct grounding can suppress the effects of electromagnetic interference and prevent the equipment from emitting interference; however, incorrect grounding can introduce serious interference signals, making the PLC system unable to function properly. The grounding wires of a PLC control system include system ground, shield ground, AC ground, and protective ground. The interference from a chaotic grounding system to the PLC system is mainly due to uneven potential distribution at various grounding points, creating ground potential differences between different grounding points, causing ground loop currents, and affecting normal system operation. For example, the cable shielding layer must be grounded at one point. If both ends A and B of the cable shielding layer are grounded, a ground potential difference exists, and current flows through the shielding layer. When abnormal conditions such as lightning strikes occur, the ground current will be even greater. Furthermore, the shielding layer, grounding wire, and earth may form a closed loop. Under the influence of a changing magnetic field, induced current will appear in the shielding layer, interfering with the signal loop through the coupling between the shielding layer and the core wire. If the system ground and other grounding treatments are inconsistent, the resulting ground loop current may create unequal potential distributions on the ground wire, affecting the normal operation of the logic and analog circuits within the PLC. The logic voltage interference tolerance of PLCs is low; interference from the logic ground potential distribution can easily affect the PLC's logic operations and data storage, causing data corruption, program crashes, or system freezes. The distribution of analog ground potential will lead to a decrease in measurement accuracy, causing serious distortion and malfunctions in signal measurement and control. 2.3 Interference from within the PLC system is mainly generated by mutual electromagnetic radiation between internal components and circuits, such as mutual radiation between logic circuits, mutual influence between analog and logic grounds, and mismatched use of components. This falls under the scope of electromagnetic compatibility (EMC) design within the PLC system by the manufacturer. It's quite complex and cannot be changed by the application department, so it doesn't need to be overly considered. However, it's crucial to choose systems with extensive application experience or proven track records. 3. Anti-interference Design To ensure the system is protected from or minimizes internal and external electromagnetic interference in industrial electromagnetic environments, three suppression measures must be implemented from the design stage: suppressing interference sources, cutting off or attenuating the propagation path of electromagnetic interference, and improving the anti-interference capability of the device and system. These three points constitute the basic principles of electromagnetic interference suppression. Anti-interference design for PLC control systems is a systematic project, requiring manufacturers to design and produce products with strong anti-interference capabilities. It also relies on the user department to comprehensively consider these aspects during engineering design, installation, and operation and maintenance, and to conduct integrated design based on specific circumstances to ensure the system's EMC and operational reliability. When designing anti-interference for specific projects, the following two aspects should be considered. 3.1 Equipment Selection When selecting equipment, the first priority should be choosing products with high anti-interference capabilities, including electromagnetic compatibility, especially resistance to external interference. This includes PLC systems employing floating ground technology and offering good isolation performance. Secondly, the anti-interference specifications provided by the manufacturer should be understood, such as common-mode rejection ratio, differential-mode rejection ratio, withstand voltage, and the permissible electric and magnetic field strengths and frequencies. Additionally, the application performance in similar work environments should be examined. When selecting imported products, it is important to note that my country uses a 220V high-resistance power grid, while Europe and the United States use a 110V low-resistance power grid. Due to the higher internal resistance of my country's power grid, greater zero-point potential drift, and larger ground potential variations, electromagnetic interference in industrial settings is at least four times higher than in Europe and the United States, requiring higher system anti-interference performance. PLC products that function normally abroad may not operate reliably in domestic industries. Therefore, when using foreign products, it is crucial to select them appropriately according to Chinese standards (GB/T13926). 3.2 Comprehensive anti-interference design mainly considers several suppression measures from outside the system, including: shielding the PLC system and external leads to prevent electromagnetic interference radiated from space; isolating and filtering external leads, especially power cables should be arranged in layers to prevent conducted electromagnetic interference introduced through external leads; correctly designing grounding points and grounding devices, and improving the grounding system. In addition, software methods must be used to further improve the system's safety and reliability. 4. Main Anti-interference Measures 4.1 Using high-performance power supplies to suppress interference introduced from the power grid. In a PLC control system, the power supply plays a crucial role. Power grid interference enters the PLC control system mainly through coupling with the PLC system's power supply (such as CPU power supply, I/O power supply, etc.), transmitter power supply, and instrument power supply that has a direct electrical connection to the PLC system. Currently, power supplies with good isolation performance are generally used for PLC system power supplies, while the power supplies for transmitters and instruments that have a direct electrical connection to the PLC system have not received sufficient attention. Although some isolation measures have been taken, they are generally insufficient, mainly because the isolation transformers used have large distributed parameters and poor interference suppression capabilities, leading to common-mode and differential-mode interference introduced through power supply coupling. Therefore, for the power supply of transmitters and shared signal instruments, power distribution units with small distributed capacitance and large suppression band (such as those using multiple isolation and shielding technologies and leakage inductance technology) should be selected to reduce interference in the PLC system. In addition, to ensure uninterrupted power supply from the power grid, an online uninterruptible power supply (UPS) can be used to improve the safety and reliability of the power supply. Moreover, UPS has strong interference isolation performance and is an ideal power supply for PLC control systems. 4.2 Correct selection and implementation of cable laying To reduce the radiated electromagnetic interference of power cables, especially the feeder cables of frequency converters, the author used copper tape armored shielded power cables in a certain project, which reduced the electromagnetic interference generated by the power lines. The project achieved satisfactory results after commissioning. Different types of signals should be transmitted by different cables. Signal cables should be laid in layers according to the type of signal transmitted. It is strictly forbidden to transmit power and signals simultaneously by different conductors of the same cable. Avoid laying signal lines and power cables close to each other in parallel to reduce electromagnetic interference. 4.3 Hardware Filtering and Software Anti-interference Measures Before signals are connected to the computer, a capacitor is connected in parallel between the signal line and ground to reduce common-mode interference; a filter is installed between the two poles of the signal to reduce differential-mode interference. Due to the complexity of electromagnetic interference, it is impossible to completely eliminate its effects. Therefore, in the software design and configuration of the PLC control system, anti-interference measures should also be implemented in the software to further improve the system's reliability. Some commonly used measures to improve the reliability of the software structure include: digital filtering and power frequency shaping sampling, which can effectively eliminate periodic interference; periodically correcting the reference point potential and using dynamic zero points to prevent potential drift; using information redundancy technology and designing corresponding software flag bits; using indirect jumps and setting software protection, etc. 4.4 Correctly Select Grounding Points and Improve the Grounding System. The purpose of grounding is usually twofold: safety and interference suppression. A sound grounding system is one of the important measures for PLC control systems to resist electromagnetic interference. There are three types of system grounding: floating ground, direct grounding, and capacitor grounding. For PLC control systems, which are high-speed, low-level control devices, direct grounding should be used. Due to the influence of distributed capacitance of signal cables and filtering of input devices, the signal exchange frequency between devices is generally lower than 1MHz. Therefore, the grounding wire of the PLC control system adopts single-point grounding and series single-point grounding methods. Centrally arranged PLC systems are suitable for parallel single-point grounding, with the central grounding point of each device's cabinet connected to a separate grounding wire. If the spacing between devices is large, a series single-point grounding method should be used, connecting the central grounding points of each device's cabinet with a large-section copper busbar (or insulated cable), and then directly connecting the grounding busbar to the grounding electrode. The grounding wire uses copper conductors with a cross-section greater than 22mm², and the main busbar uses copper busbars with a cross-section greater than 60mm². The grounding resistance of the grounding electrode should be less than 2Ω. The grounding electrode is best buried at a distance of 10-15m from the building, and the PLC system grounding point must be at least 10m away from the grounding point of high-voltage equipment. When the signal source is grounded, the shielding layer should be grounded on the signal side; when not grounded, it should be grounded on the PLC side. When there is a joint in the middle of the signal line, the shielding layer should be firmly connected and insulated, and multi-point grounding must be avoided. When connecting shielded twisted-pair cables for multiple measurement points to a multi-core twisted-pair shielded cable, each shield layer should be properly connected and insulated, and a single-point grounding should be selected at an appropriate grounding point. 5. Conclusion: Interference in PLC control systems is a highly complex issue. Therefore, anti-interference design should comprehensively consider various factors to effectively suppress interference. For some interference situations, specific analysis and targeted solutions are necessary to ensure the normal operation of the PLC control system and guarantee the safe and efficient operation of industrial equipment.