1. Introduction The electrical environment at construction sites is generally poor, with numerous safety hazards in the equipment and wiring used. Construction sites are characterized by high mobility, repetitiveness, and temporary nature, and the varying skill levels of electrical personnel and even management staff. The mandatory adoption of the TN-S three-phase five-wire power supply system at construction sites aims to ensure electrical safety and strengthen power management. Residual current devices (RCDs) at all levels are the most critical protective equipment in a TN-S power supply system. However, due to the unique characteristics of construction sites, frequent tripping of these RCDs is a common problem. This not only severely impacts normal construction operations but also fails to effectively guarantee electrical safety. Based on experience and management of construction site electricity, the following analysis addresses the reasons for frequent tripping of RCDs at construction sites. 2. Reasons for Frequent Tripping of Residual Current Devices (RCDs) at Construction Sites 2.1 Unreasonable Layout of RCDs According to the "Technical Specification for Temporary Power Supply Safety at Construction Sites" (JCJ46-88), residual current devices (RCDs) should be installed in the main distribution box and switch box of temporary power supply, forming a three-level power distribution and two-level residual current protection mode. Due to the special characteristics of construction sites, such as poor electrician skills, wiring errors, non-electrician wiring, damaged lines, damaged RCDs in switch boxes, some electrical appliances not passing through switch boxes, and poor site management, as well as the unavoidable malfunctions and failures of RCDs themselves, coupled with the fact that RCDs are not arranged according to the actual conditions of the construction site, the main RCD trips frequently, resulting in a large power outage area. During peak construction periods, the frequent tripping of the main RCD not only seriously affects the normal construction of the site but also keeps the electricians busy and even helpless when dealing with faults. To address this situation, in addition to strengthening site management, it is necessary to rationally arrange the RCDs from a technical perspective based on the actual conditions of the construction site. In large construction sites such as residential building construction sites and industrial projects, it is necessary to divide the entire site into several small residual current protection (RCD) zones based on specialties or different construction teams. Each protection zone should have secondary RCD protection, and tertiary RCD protection may be necessary in some cases. This improves the sensitivity of the secondary or tertiary RCDs within each protection zone, increases the operating rate of the RCDs in case of fault leakage within the protection zone, and reduces the tripping of the main RCD. A reasonable layout also promotes independent management by individual construction teams and facilitates centralized management by the project management team. In this way, the RCD on the main power supply line to the construction site can primarily serve as the main protection against electrical fire hazards and short circuits, while also acting as backup protection for each small RCD zone. Its rated residual current operating current can be selected between 200 and 500 mA depending on the size of the construction site, and the rated residual current operating time can be selected between 0.2 and 0.3 seconds. This significantly reduces the impact of surge voltage, current, and electromagnetic interference on the main RCD, improving its selectivity and reliability. If the management of residual current devices (RCDs) at construction sites can be strengthened to ensure that the secondary RCDs within each RCD protection range are in an effective protection state, the frequent tripping probability of the main RCD at the construction site can be greatly reduced. 2.2 In cases where there is no effective secondary or tertiary RCD within the protection range, the final-stage RCD in the switch box is the main protection for the electrical equipment. If the final-stage RCD is not installed, is damaged, or is improperly selected, it may cause the upper-stage RCD to trip frequently. For example, some lighting sections at construction sites are quite chaotic and have many problems: the lighting wires at construction sites are often re-laid as the construction location changes, with many instances of haphazard wiring and poor wire insulation, resulting in frequent leakage; although the lighting wires in the site offices are relatively fixed, they are generally fixed low, making them easy for people to touch, and also include some socket circuits. In many cases, RCDs are not installed, especially when lighting is needed at dusk, which often causes the main RCD to trip frequently. There are many mobile devices on the construction site, such as vibratory rods, electric drills, small cutting machines, rammers, and small welding machines. These are used randomly and sometimes are not connected to the switch box, which increases the probability of the main leakage current protection device tripping frequently. Only by forming an effective two- or three-level leakage protection mode in each protection range can the frequent tripping of the leakage current protection device be effectively reduced. 2.3 The leakage current protection device itself has certain limitations (1) The current leakage current protection devices, whether electromagnetic or electronic, all use magnetic induction voltage transformers to pick up the leakage current in the main circuit of the electrical equipment. It is impossible for the three-phase or three-phase four-wire to be arranged completely evenly in the magnetic ring. There are many two-phase or single-phase loads such as welding machines on the construction site. The three-phase current cannot be completely balanced and may even be very different. Under high current or high overvoltage, a certain electromotive force will be induced in the magnetic ring with high permeability. When this electromotive force is large enough, it will cause the leakage current protection device to trip. Furthermore, since the leakage current protection device with a larger rated current uses a relatively larger magnetic ring, the leakage flux generated is also relatively large. Moreover, the leakage current has to overcome the magnetization force of the magnetic ring itself, resulting in the leakage current protection device with a larger rated current, lower sensitivity, and a higher false tripping or failure to trip rate. (2) There is an uncertain operating region between the rated leakage current and the rated leakage non-operating current of the leakage protection device. When the leakage current of the leakage protection device fluctuates in this region, it may cause the leakage protection device to trip irregularly. 2.4 Unreasonable selection of leakage protection device (1) The leakage protection device used in the switch box has a rated leakage current exceeding 30mA or more than twice the rated current of the electrical equipment, or a leakage protection device with a time delay is selected. Due to the increase in the rated leakage current or the decrease in protection sensitivity, when a leakage fault occurs, the last-stage leakage protection device does not operate, but the upper-stage leakage protection device may operate. (2) Some randomly used loads do not have dedicated switch boxes, such as Class I and II electric hammers, electric drills, small cutting machines and other hand-held power tools. When a residual current device (RCD) with a large rated current is connected to it, the final RCD may fail to operate or trip simultaneously with the previous RCD in the event of leakage or fault. (3) There are many welding machines on the construction site. The RCD for welding machines is selected according to the rated current of the welding machine. The large current when the welding machine starts welding may cause the RCD to trip, which is the reason why some welding machine RCDs trip. For this type of electrical equipment, electromagnetic RCDs that are not very sensitive to surge overvoltage and overcurrent should generally be selected; or electronic RCDs with a rated current 1.5-2 times greater than that of the welding machine should be selected. However, as the final stage of RCD protection, the rated leakage operating current should not exceed 30mA. (4) Tower cranes are large construction equipment on construction sites and have multiple motors. Although Y-Δ starting and rotor circuit series resistance starting are used to reduce the starting current, there will still be a large starting current. Y-Δ starting and motor speed change will randomly generate a certain overvoltage. The tower crane distribution box and power lines are located at high altitudes and are exposed to sun and rain all year round, so the insulation is inevitably damaged, resulting in a corresponding increase in leakage current. These factors may cause the tower crane's leakage current protection device to trip frequently. When considering the use of electronic leakage current protection devices, its rated current should be appropriately increased by 1.5-2 times to reduce the sensitivity of the leakage current protection device itself and reduce the probability of frequent tripping. (5) The rated leakage current and rated non-operating leakage current of the upper-level leakage current protection device of the final stage leakage protection device are selected too small, without considering that there may be a relatively large normal leakage current on the distribution line after the leakage current protection device. Generally, the rated leakage current of the upper-level leakage current protection device is selected to be about twice the rated leakage current of the lower-level device. For example, for the previous level of leakage protection, when the protection range is small, the rated leakage operating current of the previous level leakage protection device can be selected as 50mA or 75mA; when the protection range is large or there are many single-phase or double-phase loads such as welding machines after the previous level leakage protection device, it should be considered that when the wiring of many single-phase and double-phase loads is unbalanced, there may be a relatively large leakage current, and the rated leakage operating current of the previous level leakage protection device can be selected as 75mA or 100mA. When conditions permit, this level of leakage protection device should have a delay of 0.2s, which can improve the selectivity of the operation of the last level and the previous level leakage protection device within the leakage protection range. 2.5 There are problems with the wiring of the leakage protection device (1) A single-phase load is used, but the neutral line does not pass through the leakage protection device. (2) After the neutral line passes through the leakage protection device, it is directly grounded or grounded through electrical equipment, etc., and the leakage protection device will trip; the neutral line has poor insulation to the ground or poor grounding, and is only partially connected, which causes the leakage protection device to trip irregularly and the fault is difficult to find. (3) After the neutral wire passes through the residual current device (RCD), it is connected to the neutral wires of other RCDs or to other neutral wires without RCDs. (4) When using a three-phase four-wire or four-pole electronic RCD for three-phase or two-phase loads, if the neutral wire is not connected to the RCD or is connected but loosely, the RCD control circuit will not have power and will fail to operate. Once a leakage accident occurs, it will cause the upstream RCD to operate. (5) Three-phase loads such as motors generally do not have a neutral wire connected and use a four-core cable. One core should be connected to the PEN protective conductor and the motor casing. However, in some cases, this PEN protective conductor is connected to the PE neutral wire. In fact, the neutral wire is grounded through the motor casing. The system can operate normally when there is only a three-phase load or a two-phase load but the three phases are balanced. When there is a single-phase load or the load is unbalanced and the neutral point is offset, the upstream RCD will trip. If the neutral wire resistance is large, it may cause the RCD to trip irregularly, making it difficult to find the fault. (6) The load after the leakage current protection device is not evenly distributed. Most of the welding machines on the construction site use AC 380V power supply. The vector sum of the leakage current to ground of the primary line of the welding machine after the leakage current protection device is not zero. For the upper-level leakage protection of the final protection, if the wiring of multiple welding machines is extremely unbalanced, it will increase the leakage current through it, and at the same time raise the neutral line to ground potential, increasing the probability of neutral line leakage and increasing the probability of the upper-level protection of the welding machine tripping. When leakage faults or leakage current increases in electrical equipment and lines, the upper-level leakage protection will trip before the final-level leakage protection of the welding machine or both leakage protections will trip at the same time. (7) The neutral line is broken or has poor contact, causing the neutral point potential to deviate from zero potential, increasing the probability of neutral line leakage and causing other faults. 2.6 Leakage Current in Electrical Equipment and Wiring: The operating environment of electrical equipment at construction sites is relatively harsh, maintenance and repair are limited, and the quality varies, with good and bad insulation. Some equipment has a relatively large leakage current. The same applies to wiring. Some lines use poor-quality insulated wires, are not laid according to regulations, and have poorly wrapped joints, such as direct burial of wires and cables crossing roads without protective conduits. This causes the final-stage leakage current protection device (RCD) to trip. If the final-stage RCD is damaged or disconnected, it will cause frequent tripping of the upstream RCD. 3. Conclusion In summary, frequent tripping of residual current devices (RCDs) is the result of a combination of factors at the construction site. The most important thing is to rationally arrange RCDs, narrow the protection range of secondary or tertiary RCDs, and correctly select RCDs and wiring to ensure that each secondary or tertiary RCD within its range is in an effective protection state. On the other hand, it is also necessary to strengthen the management of temporary power use at the construction site and improve the professional skills of power users through training. This will both meet the safety requirements of power use at the construction site and reduce the frequent tripping of RCDs, creating better power supply conditions for normal construction. References 1. Pan Yi. Lecture on Electromagnetic Residual Current Protection Devices. Electrical Engineering Technology Magazine, 1999 2. Lian Lizhi. Lecture on the Application of Residual Current Operated Protection Devices. Electrical Engineering Technology Magazine, 2002 3. Wang Houyu. Lecture on Ground Fault Protection of Low-Voltage Power Distribution Systems. Electric World, 2002-2003