Abstract : Residual current devices (RCDs) are currently effective protective electrical appliances to prevent electric shock and electrical fires caused by grounding faults. However, improper application often prevents them from functioning properly. This article aims to offer some preliminary thoughts on this issue. Keywords : Residual current device, safety, high voltage 1. Protection against electric shock only requires the installation of an RCD with an operating current of 30mA. According to the International Electrotechnical Commission standard IEC 4.79 (Effects of current through the human body), when the AC 50Hz current passing through the human body does not exceed 30mA, the human body will not die from ventricular fibrillation. This is not directly related to the degree of moisture on the body or the level of contact voltage. Therefore, international electrotechnical standards stipulate the use of RCDs with an operating current of no more than 30mA in all provisions for protection against electric shock. Accordingly, RCDs with an operating current of 30mA can be installed in high-risk places such as hospital operating rooms and bathrooms to protect against electric shock. For rural electricity use, it is unnecessary to install RCDs with higher sensitivity, such as 10mA RCDs. Because a 10mA RCD and a 30mA RCD are equally effective in preventing electric shock, both can prevent death from ventricular fibrillation. However, a 10mA RCD is expensive and not suitable for widespread use, while its rated non-operating current is only 5mA. Rural low-voltage power grid equipment, often located outdoors and in damp environments, has a relatively large normal leakage current, making it prone to malfunction. Frequent malfunctions leading to power outages often result in the RCD being short-circuited or removed, leaving the line without ground fault protection and causing dangerous consequences. Only handheld and portable electrical equipment requires a 30mA high-sensitivity RCD. Handheld and portable electrical equipment pose a greater risk of electric shock. This is because these devices are frequently moved during use, making insulation prone to damage and grounding faults. The muscles in the hand holding the equipment contract due to electricity, making it difficult to shake off the electrified equipment. Prolonged exposure to electricity can easily lead to ventricular fibrillation and death. Fixed-installation equipment is less prone to grounding faults due to the inability of a person's hand to grip the equipment casing. In the event of an electric shock, the hand can easily break free from contact with the live equipment casing. Regardless of whether a 30mA RCD is installed, an electric shock from fixed equipment can cause a person to lose their balance and fall, but will not result in death due to ventricular fibrillation. Therefore, handheld and portable equipment must be equipped with a 30mA instantaneous RCD, while this requirement does not apply to fixed equipment such as chandeliers and fixed indoor water pumps. International electrical standards differentiate between the two to avoid the indiscriminate installation of 30mA instantaneous RCDs, saving unnecessary investment and reducing power outages caused by improper installation. 3 Commonly Used Two-Stage Residual Current Protection Most short circuits are grounding faults, i.e., short circuits between a phase wire and the earth, the casing of electrical equipment, or metal pipe structures. Grounding faults can cause electric shock accidents and are more likely to cause electrical fires than phase-to-phase or single-phase short circuits. my country's "Low-Voltage Power Distribution Design Code" (GB50054-95) stipulates that all power distribution lines should have ground fault protection, and RCDs are the most effective ground fault protection devices. When an arcing ground fault causes a fire, due to the small arc current, circuit breakers and fuses often cannot cut off the power supply before the fire occurs, while RCDs can immediately trip and cut off the power supply. Therefore, in addition to installing 30mA instantaneous RCDs on the terminal lines of handheld and mobile equipment, circuit breakers with a slight time delay leakage protection function should also be installed on the main power supply line, as shown in Figure 1. It is mainly used to prevent electrical fires caused by ground faults and line-to-ground potential rise accidents, with no dead zone in the protection range. The operation of RCD1 and RCD2 in the figure should be selective to avoid cascading tripping and expanding the power outage area. Selectivity cannot be provided by the magnitude of the RCD's operating current. If the operating current difference between RCD1 and RCD2 is 2 to 3 times, but both operate instantaneously, when a ground fault with a fault current of tens of amperes occurs at the end of the line, the fault current exceeds a hundred times the operating current. Since both RCDs operate instantaneously, selectivity cannot be guaranteed. Therefore, the selectivity between different levels of RCDs can only be guaranteed by the difference in operating time. That is, the operation of RCD2 in Figure 1 should have an appropriate delay. For example, the operating time t1 of RCD1 shown in the figure is ≤0.04s, and the operating time t2 of RCD2 is 0.3s. 4. Technical Requirements for Circuit Breakers with Time-Delay Residual Current Protection The wiring diagram of a circuit breaker with time-delay residual current protection installed on the power supply main line is shown in Figure 2. As can be seen from the figure, this type of circuit breaker simply adds a 1:1 zero-sequence current transformer and a trip unit to the lower end of the original circuit breaker used for short-circuit and overload protection. When a ground fault occurs in the protected circuit, the current transformer detects the residual current (commonly known as leakage current), and the trip unit causes the circuit breaker to trip. my country's "Low-Voltage Power Distribution Design Code" stipulates that the operating current of this level of RCD should not exceed 500mA for safety, as the energy of an arc below 500mA is insufficient to ignite a fire. However, when the normal leakage current of the line is large, it can be taken as greater than 500mA to avoid unnecessary tripping and power outages. The leakage current tripping delay of this circuit breaker is generally taken as about 0.3s. Because there is a relatively long process from the occurrence of a grounding arc to the ignition of nearby flammable materials, this delay of about 0.3s effectively prevents fires, does not expand the power outage area, and does not cause overheating and burnout of the protected line. This level of protection cannot use ordinary leakage current protection devices, nor can it use leakage current protection devices combining leakage current relays and contactors, because the metallic ground fault current on the power supply main line can be in the thousands of amperes, and contactors and ordinary RCDs with a breaking capacity of 300A are insufficient to interrupt such a large current. Many manufacturers in China produce molded case circuit breakers with time-delay leakage current protection. Their rated current ranges from 100 to 400A, the leakage current operating current is 30mA to 2A, the time delay is 0.2 to 0.8s, and the short-circuit current breaking capacity is 3 to 6.5kA, which meets the aforementioned general requirements. 5. Application of Three-Stage Leakage Protection When the power supply range and mains current are large, sometimes three-stage leakage protection is required, i.e., adding an RCD3 before RCD2 in Figure 2, as shown in Figure 3. It consists of separate zero-sequence current transformers, leakage relays, and circuit breakers (or signalers). The transformer ratio is also 1:1. The circuit current it passes through is limited by the diameter of the transformer's through-hole through which the four conductors of the circuit pass. The current detected by the leakage relay is the residual current on the primary side, and its operating current and time delay are adjustable. Currently, China produces zero-sequence current transformers for residual current protection (RCD) equipped with residual current relays. Their through-hole diameter is 25–100 mm, the corresponding circuit current is 100–800 A, and the operating current of the RCD is 50 mA–3 A with a delay of 0.2–2 seconds. These transformers are also suitable for adding RCD protection to existing lines. For RCD protection on power lines with large power supply areas, it is often undesirable to trip immediately in the event of an arcing ground fault within the protected area to avoid large-scale power outages. In this case, the RCD can be used to generate a signal to locate the faulty circuit and partially disconnect the power supply. If a large short-circuit current of a metallic short circuit occurs in the circuit, the electromagnetic trip unit in the circuit breaker will trip to disconnect the power supply and protect the line. 6. Inspection of Residual Current Protection Currently, during construction acceptance, pressing the RCD test button or simulating a ground fault is commonly used to check whether the RCD can operate. These two methods are not entirely reliable. The former only indicates that the RCD itself can operate, but it cannot prove whether the installation is correct or guarantee that it will definitely operate in the event of a ground fault; while the latter is only a qualitative test rather than a quantitative test. With the development of electrical technology in my country, my country has produced instruments that can measure the operating current and operating time of the RCD, as well as the normal leakage current of lines and equipment. The results obtained using such instruments will be more reliable and accurate.