Abstract : Leakage current and voltage in low-voltage power distribution systems can cause fires. This paper points out that leakage remains a significant cause of electrical fires and proposes technical preventative measures. Keywords : Leakage current, leakage current, leakage voltage, leakage protection, equipotential bonding. The main causes of electrical fires include: short circuits, overloads, poor contact, leakage, and ignition of flammable materials by lighting fixtures and electric heating appliances. In recent years, fires caused by leakage have been occurring frequently. Moreover, these fires are more insidious than those caused by short circuits, often starting silently and making it difficult to identify the true cause after they start (as they are masked by the illusion of a short circuit). Therefore, they are more dangerous. Fully understanding the fire hazards of leakage and strengthening technical preventative measures should be one of the important tasks of current electrical fire prevention work. I. Fire Hazards of Electrical Leakage When the insulation of electrical circuits or equipment is damaged, under certain environmental conditions, leakage current can occur to nearby materials (metal conduits for wiring, metal casings of electrical devices, damp wood, etc.). Just as a leaking water pipe causes localized dampness or water stains, leakage current can electrify localized materials, causing severe or fatal electric shocks, or generating sparks, arcs, overheating, and other fires. Currently, low-voltage power distribution systems often use grounding protection and overcurrent protection devices (fuses, etc.) to prevent severe leakage short circuits. 1. Problem Statement When electrical equipment experiences leakage current (i.e., a short circuit to the casing), the current forms a closed loop between the equipment casing, the protective grounding wire, and the neutral wire (earth). The leakage current is usually very large, causing the fuse to trip and cut off the power supply. It seems that this danger of leakage current can be avoided. However, due to the following reasons, overcurrent protection devices are not absolutely reliable. (1) The fuse specification may be artificially increased several times or replaced with copper wire, thus failing to provide overcurrent protection; (2) The electrical fault point may be at a sufficiently far end of the system, with a large fault circuit impedance, and the leakage short-circuit current is insufficient to trip the fuse; (3) If the electrical equipment capacity is large, the fuse will not trip if the rated current of the fuse element exceeds the leakage current; (4) The grounding device does not meet the requirements, resulting in a large grounding resistance, which leads to a small leakage short-circuit current that will not trip the fuse; (5) When using an overcurrent automatic protection switch, if the switch malfunctions or the tripping current is set too high, the automatic protection switch will not trip; (6) The connection of the protective neutral (grounding) wire terminals is not secure, resulting in excessive contact resistance, which limits the fault current and causes the fuse to not trip. The above phenomena are not uncommon in practice, and one or several may exist simultaneously and are not taken seriously. Therefore, once leakage occurs, it will persist, leading to electric shock or electrical fire accidents. Many leakage fire cases have also proven this point. This article will focus on introducing the fire hazards and prevention measures of this leakage. 2. Causes of fires caused by leakage current (1) Fire caused by leakage current. Under normal circumstances, the contact at the leakage fault point is not solid, and it is only partially connected, resulting in a large contact resistance, making it difficult for the overcurrent protection device to operate. At the same time, an electric arc will be generated at the fault point. It is estimated that the arc temperature of only 0.5A current can reach more than 2000℃, which is enough to ignite all combustibles. The wire diameter of the protective neutral wire or protective ground wire is easily overlooked. If it is too small, when a large leakage current passes through, the line temperature rises quickly, which can also cause a fire. Another situation is that a professor of the Department of Electrical Engineering at Akita University in Japan once conducted an experiment. In a humid environment, when a live bare wire comes into contact with wood, the leakage current flows through its surface cellulose, which will cause the wood to char and develop into a fire. This experiment reminds people that it is very dangerous for electrical lines to pass through combustibles without conduit protection. This kind of leakage danger exists in all power distribution systems. (2) Fire caused by poor connection at the terminals of the protective neutral wire or protective ground wire. A loose connection between the phase and neutral wire terminals will cause equipment malfunctions, which can be detected and addressed promptly. However, a loose connection between the protective neutral or ground wire terminals, resulting in excessive resistance, will allow the equipment to continue operating, but the fault location will be difficult to pinpoint. Once a leakage occurs, due to loose connections or corrosion at the fault point, high resistance can occur, causing localized overheating. High temperatures or electric arcs at the connection terminals can ignite surrounding flammable materials, or burn out electrical sockets, switches, etc., and ignite wooden bases. This is a relatively common form of leakage-induced fire. A case of this is the fire in the distribution panel of a restaurant in Huaibei City in July 1997. Before the fire, the restaurant had closed, and the load was at its lowest point; however, lighting was still functioning normally at the time of the fire. Investigation revealed that the main air switch in the distribution box was severely carbonized, and the protective neutral wire terminal had concave metal melting marks, consistent with a leakage. (3) Fire caused by leakage voltage: After a leakage current continues to occur, the current cannot dissipate and seeks another circuit with low impedance to ground. It will be conducted along the protective neutral wire (grounding wire), causing the metal casing of all connected electrical devices to carry a voltage to ground. At this time, it may arc to nearby low-potential metal components such as water pipes and gas pipes, becoming a source of fire. A sustaining voltage of only 20V can cause the arc to occur continuously, which can also ignite surrounding combustibles. If the arc is directed to a gas pipe, it may puncture the pipe wall, causing gas leakage and a fire. It should be noted that due to voltage conduction, the leakage point and the ignition point are not necessarily the same. 3. Factors causing leakage There are many factors that cause leakage. In summary, they mainly include the following: (1) The installation of low-voltage power distribution systems is mostly done by non-electrical professionals with varying levels of competence, making it difficult to guarantee quality. This is manifested in: wires being laid openly in humid or acid- or alkali-corrosive environments, and equipment being installed directly without protection; damage to the insulation layer by knives, pliers, hammers, etc. during wiring; and non-standard phenomena such as non-compliant wire joint connection quality and insulation wrapping quality. (2) Neglecting the inspection of electrical lines or equipment, resulting in insulation deterioration due to overload or long service life. (3) Using counterfeit or substandard electrical products. (4) External factors: water infiltration, squeezing, rodent bites, etc. II. Prevention measures for leakage fires 1. Strictly follow the operating procedures for low-voltage electrical devices in case of leakage fires. Non-electrical professionals are not allowed to work. Eliminate all kinds of human factors that can cause leakage. This will not be elaborated here. 2. Decoration and renovation projects should strictly adhere to the "Code for Fire Protection Design of Interior Decoration of Buildings." Flammable and combustible materials should be avoided or minimized, especially when electrical wiring passes through combustible materials. Metal conduits or flame-retardant hard plastic conduits should be used for protection. Due to the good insulation properties of plastic, it can effectively prevent leakage. When using metal conduits for wiring, care must be taken to prevent damage to the insulation layer. Electrical distribution devices (switches, sockets, distribution boxes, etc.) and electrical equipment should maintain a sufficient safe distance from combustible materials. If separation is impossible, proper heat insulation measures should be taken. 3. Installation of Residual Current Devices (RCDs): Current low-voltage power distribution systems with protective grounding and overcurrent protection devices are not entirely effective in preventing leakage fires. Therefore, a dedicated fire-resistant RCD should be installed at the main power inlet of the building. The international "Code for Design of Low-Voltage Power Distribution," implemented in 1996, also makes clear requirements in this regard. To prevent large-scale power outages, residual current devices (RCDs) should be installed in both the main power distribution box and user switch boxes. Their rated operating current and rated operating time should be appropriately matched to provide graded protection. 4. Protection Measures: The cross-sectional area of ​​the protective grounding wire and the protective earthing conductor must be calculated and verified using the short-circuit current test. The terminals must be reliably connected, with no looseness allowed, and the connection quality should be checked frequently. 5. Grounding Resistance Value should Meet Design Requirements: The protective grounding resistance value of electrical equipment should not exceed 4Ω. If the capacity of the electrical equipment is large and the fuse current is also large, the cross-sectional area of ​​the grounding wire should be increased or a grounding electrode should be connected in parallel to significantly reduce the grounding resistance value, increase the leakage short-circuit current, and facilitate the operation of the protection device. 6. Implementing Equipotential Bonding: Residual current devices (RCDs) only provide indirect contact protection for single-phase 220V lines. Furthermore, they are susceptible to malfunction due to factors such as component wear, poor contact, unstable quality, and short lifespan. Therefore, they cannot be considered a reliable protection measure on their own. Equipotential bonding is essential to effectively eliminate the generation of electric arcs and sparks between leaking electrical circuits or equipment and low-potential metal components, thus eliminating the possibility of fire caused by leakage voltage. Equipotential bonding refers to the measure of connecting the protective neutral bus to the building's main water pipe, main gas pipe, HVAC pipes, and other metal pipes or devices using wires to achieve the purpose of equalizing the potential within the building. This is especially important in flammable and explosive environments, where it plays an irreplaceable role.