Abstract: Subcommittee 17D of the International Electrotechnical Commission (IEC) drafted a technical document entitled "Supplement 1 to IEC 439 on Insulation Coordination Requirements," formally introducing the issue of insulation coordination into low-voltage switchgear and controlgear assemblies. In China's current situation, insulation coordination remains a significant issue in both high- and low-voltage electrical products. Furthermore, the formal application of the concept of insulation coordination in low-voltage switchgear and controlgear assemblies is a relatively recent development, only occurring within the last two years. Therefore, properly addressing and resolving insulation coordination issues in products is crucial. Keywords: Low-voltage switchgear, insulation coordination, insulation materials I. The Introduction of Insulation Coordination Issues in Low-Voltage Switchgear Assemblies Insulation coordination is a critical issue related to the safety of electrical equipment products and has always received considerable attention. Insulation coordination was first applied to high-voltage electrical products. In 1987, Subcommittee 17D of the International Electrotechnical Commission (IEC) drafted a technical document entitled "Supplement 1 to IEC 439 on Insulation Coordination Requirements," formally introducing the issue of insulation coordination into low-voltage switchgear and controlgear assemblies. In the current situation in China, insulation coordination remains a significant issue in high- and low-voltage electrical products. Statistics show that 50%-60% of accidents in China's electrical products are caused by insulation systems. Furthermore, the concept of insulation coordination has only been formally adopted in low-voltage switchgear and control equipment in the last two years. Therefore, properly handling and resolving insulation coordination issues in products is crucial. II. Basic Principles of Insulation Coordination Insulation coordination refers to selecting the electrical insulation characteristics of equipment based on its operating conditions and surrounding environment. Insulation coordination can only be achieved when the equipment design is based on the intensity of forces it will withstand during its expected lifespan. Insulation coordination issues arise not only from external factors but also from the equipment itself, involving various aspects that require comprehensive consideration. The key points can be divided into three parts: first, the operating conditions of the equipment; second, the operating environment of the equipment; and third, the selection of insulation materials. (I) Operating Conditions of the Equipment The operating conditions of the equipment mainly refer to the voltage, electric field, and frequency used by the equipment. 1. The Relationship between Insulation Coordination and Voltage. In considering the relationship between insulation coordination and voltage, it is necessary to consider the voltage that may occur in the system, the voltage generated by the equipment, the required continuous voltage operation level, and the risk of personal safety and accidents. (1) Classification of voltage and overvoltage, waveform. a) Continuous power frequency voltage, voltage with constant r, m, s b) Temporary overvoltage, power frequency overvoltage with a relatively long duration c) Transient overvoltage, overvoltage with a duration of a few milliseconds or less, usually highly damped oscillation or non-oscillation. Slow wave front overvoltage: a transient overvoltage, usually unidirectional, with a peak time of 20μs - Fast wave front overvoltage: a transient overvoltage, usually unidirectional, with a peak time of 0.1μs - Steep wave front overvoltage: a transient overvoltage, usually unidirectional, with a peak time of Tf≤0.1μs, a total duration of <3ms, and with superimposed oscillation, with an oscillation frequency of 30kHz d) Combined (temporary, slow wave front, fast wave front, steep wave front) overvoltage. According to the above overvoltage types, the standard voltage waveform can be described. (2) The relationship between long-term AC or DC voltage and insulation coordination should consider the rated voltage, rated insulation voltage, and actual operating voltage. During normal and long-term operation of the system, the rated insulation voltage and actual operating voltage should be considered. In addition to meeting the requirements of the standard, the actual situation of China's power grid should also be taken into account. Given that the quality of China's power grid is not high at present, the actual operating voltage that may occur is more important for insulation coordination when designing products. (3) The relationship between transient overvoltage and insulation coordination is related to the conditions of controlled overvoltage in the electrical system. There are various forms of overvoltage in the system and equipment. The influence of various overvoltages should be fully considered. In low-voltage power systems, overvoltages may be affected by various variable factors. Therefore, the overvoltage in the system is evaluated by statistical methods, reflecting a concept of probability of occurrence, and the need for protection and control can be determined by probability statistics. 2. Overvoltage categories of equipment According to the operating conditions of the equipment and the required long-term continuous voltage operation level, the overvoltage categories of low-voltage power grid equipment are directly divided into Class IV. Overvoltage category IV equipment is used at the power supply end of a power distribution system, such as meters and upstream current protection devices. Overvoltage category III equipment is installed in power distribution systems for tasks requiring special safety and suitability, such as switching devices in power distribution systems. Overvoltage category II equipment is energy-consuming equipment powered by a power distribution system, such as loads for household and similar purposes. Overvoltage category I equipment is connected to devices that limit transient overvoltages to a relatively low level, such as electronic circuits with overvoltage protection. For equipment not directly powered by a low-voltage mains grid, the highest voltage that the system equipment may experience and severe combinations of various conditions must be considered. When equipment needs to operate at a higher overvoltage category, and the equipment's own permissible overvoltage category is insufficient, measures need to be taken to reduce the overvoltage at that point. The following methods can be used. a) Overvoltage protection devices b) Transformers with isolation windings c) Multi-branch circuit power distribution systems that disperse and transfer surge overvoltage energy d) Capacitors that can absorb surge overvoltage energy e) Damping devices that can absorb surge overvoltage energy 3. Electric field and frequency (I) Electric field conditions are divided into uniform electric fields and non-uniform electric fields. In low-voltage switchgear, it is generally considered to be in a non-uniform electric field condition. Regarding the frequency issue, it is currently under consideration. It is generally believed that low frequencies have little impact on insulation coordination, but high frequencies still have an impact, especially on insulation materials. (II) Relationship between insulation coordination and environmental conditions The macroscopic environment in which the equipment is located affects insulation coordination. From the current practical application and standard requirements, air pressure changes only consider the air pressure changes caused by altitude. Daily air pressure changes have been ignored, as have temperature and humidity factors. However, if there are more precise requirements, these factors should still be considered. From a microscopic perspective, the macroscopic environment determines the microscopic environment, but the microscopic environment may be better or worse than the macroscopic environment. Different protection levels of the outer casing, heating, ventilation, and dust can all affect the microscopic environment. The microscopic environment is clearly defined in relevant standards, which provides a basis for product design. (III) Insulation Coordination and Insulation Materials The problem of insulation materials is quite complex. Unlike gases, it is an insulating medium that cannot be restored once damaged. Even an accidental overvoltage event can cause permanent damage. During long-term use, insulation materials will encounter various situations, such as discharge accidents. Furthermore, due to various factors accumulated over a long period of time, such as thermal stress, temperature, and mechanical impact stress, the aging process of the insulation material itself will be accelerated. For insulation materials, due to the diversity of varieties, although there are many indicators for measuring the characteristics of insulation materials, they are not uniform. This brings certain difficulties to the selection and use of insulation materials. This is why, currently, internationally, other characteristics of insulation materials, such as thermal stress, mechanical properties, and partial discharge indicators, are not considered. The effects of stress on insulating materials have begun to be discussed in IEC publications, providing some qualitative guidance for practical applications. However, quantitative guidance is not yet available. Currently, the most commonly used quantitative indicators for insulating materials in low-voltage electrical products are the Comparative Tracking Index (CTI), which is divided into three groups and four categories, and the Tracking Resistance Index (PTI). The CTI provides a quantitative comparison based on the leakage traces formed when a water-contaminated liquid droplet falls onto the surface of the insulating material.