Introduction : Many factors should be considered when selecting a residual current device (RCD). The first and foremost is correctly selecting the RCD's operating current. Keywords : Residual current device, residual current, operating current, starting leakage current. In high-risk locations such as bathrooms, swimming pools, and tunnels, high-sensitivity, fast-acting RCDs (operating current not exceeding 10mA) should be selected. If, in the event of an electric shock, someone can be quickly rescued from the power source, the RCD's operating current can exceed the let-go current; for fast-acting devices, the operating current can be selected based on the ventricular fibrillation current. If it is a front-stage protection (the main protection before the branch protection), the operating current can exceed the ventricular fibrillation current. If someone cannot be quickly rescued from the power source, the RCD's operating current should not exceed the let-go current. In situations where electric shock may lead to serious secondary accidents, a fast-acting RCD with an operating current of 6mA should be selected. To protect children or patients, fast-acting residual current devices (RCDs) with an operating current of 10mA or less should be used. For Class I handheld power tools, fast-acting RCDs with an operating current of 10-30mA should be installed, depending on the level of hazard in the workplace. The possibility of false tripping should also be considered when selecting the operating current. The protector should be able to avoid tripping due to leakage current imbalances in the circuit; it should also be able to withstand electromagnetic interference that may occur at the installation location. The actual manufacturing conditions of the protector should also be considered when selecting the operating current. For example, because it is difficult to achieve an operating current of less than 40mA for purely electromagnetic products, excessively sensitive electromagnetic RCDs should not be pursued. In the case of multi-level protection, the selection of the operating current should also consider the selectivity required for multi-level protection; the main protection should preferably be equipped with a RCD with lower sensitivity or a slight delay. Medium-sensitivity RCDs are recommended for RCD alarm devices used to prevent leakage fires. Their operating current can be selected within the range of 25-1000mA. Electrical equipment connected to outdoor overhead lines should be equipped with impulse voltage-non-operating RCDs. For electric motors, the protector should be able to withstand the starting leakage current of the motor (up to 15mA for a 100kW motor) without tripping. The protector should have good balance characteristics to avoid malfunction under the impact of a stall current several times the rated current. For motors that cannot be stopped, a leakage alarm method should be used instead of a leakage cut-off method. For lighting circuits, a graded protection method should be adopted according to the magnitude and distribution of the leakage current. High-sensitivity protectors should be selected for branch lines, and medium-sensitivity protectors for main lines. In high-risk locations such as construction sites and metal structures, Class I portable or mobile equipment should be equipped with high-sensitivity leakage protection devices. The insulation resistance of electric heating equipment fluctuates within a wide range with temperature changes. For example, the insulation resistance of polyethylene insulation material at 60℃ is only a fraction of that at 20℃. Therefore, the operating current of the protector should be selected according to the hot leakage condition. For welding machines, the normal operation of the protector should be considered to be unaffected by the short-term inrush current of welding, rapid current changes, and power supply voltage fluctuations. For high-frequency welding machines, the protector should also have good electromagnetic interference resistance. For equipment with nonlinear components that generate high-order harmonics, and for equipment with rectifier components, a protector with a zero-sequence current transformer connected to a filter capacitor on the secondary side should be used, and the transformer core should be made of a soft magnetic material with low residual magnetism. The number of poles of the leakage current protection device should be selected according to the characteristics of the circuit. A two-pole protector is used for single-phase circuits, a three-pole protector can be used for three-phase circuits or three-phase equipment with only three-phase loads, and a four-pole protector must be used for three-phase four-wire circuits for both power and lighting, and three-phase lighting circuits. The rated voltage, rated current, breaking capacity, and other performance indicators of the leakage current switch should be adapted to the circuit conditions. The type of leakage current protection device should be adapted to the power supply line, power supply method, system grounding type, and characteristics of the electrical equipment. Residual Current Protection Device Installation and Operation I. Residual Current Protection Device Installation The protection type and installation method of the leakage current protection device should be adapted to the environmental and usage conditions. Residual current devices (RCDs) should be installed on all Class I portable electrical equipment and hand-held power tools with metal casings, electrical equipment installed in harsh environments such as damp or highly corrosive places, electrical construction machinery and equipment on construction sites, temporary electrical equipment, sockets in hotel rooms, sockets in civil buildings with a high risk of electric shock, underwater lighting equipment in swimming pools or bathhouses, power lines and electrical equipment installed underwater, and electrical medical equipment in hospitals that comes into direct contact with the human body (except for those in thoracic operating rooms). For power supplies for passageway lighting and emergency lighting in public places, fire-fighting elevators and electrical equipment ensuring public safety, power supplies for fire-fighting equipment (such as fire alarm devices, fire pumps, fire escape lighting, etc.), power supplies for burglar alarms, and power supplies for other places or electrical installations where sudden power outages are not permitted, immediately cutting off the power in case of leakage would cause accidents or significant economic losses. In these cases, residual current alarm devices that do not disconnect the power supply should be installed. From the perspective of preventing electric shock, electrical equipment powered by safe voltage, electrical equipment with double or reinforced insulation structures used under general environmental conditions, electrical equipment powered by isolation transformers, electrical equipment used in locations with ungrounded local equipotential bonding measures, and other electrical equipment without leakage or electric shock hazards do not require residual current devices (RCDs). The installation of RCDs should comply with the manufacturer's product instructions. The leakage current of electrical circuits and equipment equipped with RCDs must be controlled within permissible limits. The rated non-operating current of the selected RCD should not be less than twice the maximum normal leakage current of the electrical circuit and equipment. When the leakage current of an electrical circuit or equipment exceeds the permissible value, the well-insulated electrical circuit or equipment must be replaced. When electrical equipment is equipped with a highly sensitive RCD, the grounding resistance of the individual grounding device can be appropriately relaxed, but the expected contact voltage should be limited to within permissible limits. The insulation resistance of motors and other electrical equipment equipped with RCDs during normal operation should not be less than 0.5 MΩ. Before installing an RCD, its casing, nameplate, terminals, test button, certificate of conformity, etc., should be carefully inspected for integrity. Residual current devices (RCDs) used to prevent electric shock accidents can only serve as supplementary protection. Adding an RCD must not negate or abandon existing safety measures. When installing an RCD with short-circuit protection, sufficient arc distance must be maintained in the direction of arc emission. RCDs should not be installed in locations with significant mechanical vibration or strong alternating magnetic fields. The installation of RCDs should consider the hazards of water, dust, etc., and take necessary protective measures. After installing an RCD, the basic anti-electric shock measures for low-voltage power lines and electrical equipment should not be removed in principle; only appropriate adjustments are permitted within a certain range. II. Wiring of Residual Current Devices The wiring of residual current devices must be correct. Incorrect wiring may cause the RCD to malfunction or fail to operate. Before wiring, the input and output terminals, phase wires and neutral wires of the RCD must be clearly distinguished; reverse or incorrect connections are prohibited. If the input and output terminals are connected incorrectly, the electronic circuitry of an electronic RCD may malfunction due to lack of power. The external connection of the control circuit of a combined residual current device (RCD) should use copper wire with a cross-sectional area of ​​not less than 1.5 mm², and the connecting wire should not be too long. The circuit on the load side of the RCD must be independent; that is, the circuit on the load side (including phase wire and neutral wire) must not be connected to the grounding device, the protective neutral wire, or other electrical circuits. In the protective neutral grounding circuit, the neutral wire should be separated; the neutral wire must pass through the protector, and the protective neutral wire must not pass through the protector. In other words, the neutral wire on the load side of the protector can only be the neutral wire, not the protective neutral wire. It should be noted that the protective conductor of the equipment downstream of the RCD must not be connected to the neutral wire downstream of the protector. Otherwise, the leakage current will return through the protector when the equipment leaks current, and the protector will refuse to operate. III. False Triggering and Refusal to Operate False tripping refers to the operation of the RCD when no expected electric shock or leakage has occurred in the line or equipment; refusal to operate refers to the RCD refusing to operate when an expected electric shock or leakage has occurred in the line or equipment. False tripping and failure to trip are among the main problems affecting the normal operation and full function of leakage current protection devices. 1. False tripping The causes of false tripping are multifaceted. There are reasons from the line and reasons from the protector itself. The main causes of false tripping and their analysis are as follows: (1) Wiring errors For example, in a TN system, if the N line does not pass through the protector together with the phase line, the protector will malfunction once the three phases are unbalanced; if the neutral line behind the protector is connected to other neutral lines or grounded, or if the phase line behind the protector is connected to the same phase line of other branches, or if the load is connected across the power supply side and the load side of the protector, the protector may also malfunction when the load is connected. (2) Deterioration of insulation If the insulation between one or two phases behind the protector to ground is damaged, or the insulation to ground is asymmetrically reduced, an unbalanced leakage current will be generated, leading to false tripping of the protector. (3) When the impact overvoltage quickly disconnects the low-voltage inductive load, an impact overvoltage of 20 times the rated voltage may be generated. The impact overvoltage will generate a large unbalanced impact leakage current, causing the fast-acting leakage protection device to malfunction. (4) When closing asynchronously, the first phase to close may generate a sufficiently large leakage current, causing the protector to malfunction. (5) When starting large equipment, the locked rotor current of large equipment is very large. If the balance characteristics of the zero-sequence current transformer in the protector are not good, the leakage flux of the primary line of the transformer may cause malfunction during startup. (6) When the ambient temperature, relative humidity, mechanical vibration, etc. deviate from the operating conditions, the malfunction may be caused when they exceed the design conditions of the protector. (7) Poor quality of the protector: Poor quality of parts or poor assembly will reduce the reliability and stability of the protector and lead to malfunction. (8) Additional magnetic field If the shielding is not good, or there is a conductor with a large current flowing nearby, or there are magnetic components or large magnetic conductors, additional magnetic flux may be generated in the core of the transformer, leading to false operation. 2. Failure to operate Failure to operate is less common than false operation, but the danger caused by failure to operate is greater than that of false operation. The main reasons for failure to operate and the analysis are as follows: (1) Wiring error If the protective wire (PE wire) on the outer shell of the electrical equipment is connected to the protector, it will cause the equipment to fail to operate when leakage occurs. (2) Improper selection of operating current The operating current of the protector is too large or the setting is too large, which will cause the protector to fail to operate. (3) Poor product quality The secondary circuit of the transformer is open, the tripping element is stuck, and other quality defects can cause the protector to fail to operate. (4) The insulation impedance of the line is reduced or the line is too long Because some of the electric shock current does not flow along the insulation impedance of the distribution network working ground or the front of the protector, but flows through the protector and returns to the power source along the insulation impedance of the back of the protector, it will cause the protector to fail to operate. IV. Use and Maintenance: All parts of the residual current device (RCD) casing, its components, and connecting terminals should be kept clean and undamaged during operation. Connections should be secure, and terminals should not show discoloration. The RCD operating handle should be flexible and reliable. After installation, the RCD's operating characteristics should be checked by operating the test button. It should only be put into use after confirming normal operation. The button should also be tested periodically during use to verify its reliability. To prevent damage to the test resistor, testing should not be too frequent. The maximum temperature of the bakelite components of the RCD casing during operation should not exceed 65°C, and the maximum temperature of the metal components should not exceed 55°C. The insulation resistance of all parts of the primary circuit of the protection device should not be less than 1.5 MΩ. If the RCD suddenly trips during operation, the cause must be identified and the fault rectified before resetting the circuit.