Introduction Before the 1980s, China still used the Soviet model—zero-sequence protection for ground fault protection. This method detects zero-sequence current and can be used in all systems, including TN-C systems. However, the protection setting value must be greater than the sum of the three-phase unbalanced current, harmonic current, and normal leakage current flowing through the N and PEN lines, typically ranging from tens to hundreds of amperes. Such a large setting value can only protect the line insulation and cannot effectively prevent electric shock or electrical fires caused by grounding arcs. After the 1980s, residual current devices (RCDs) were adopted. These detect residual current, which is the algebraic sum of the instantaneous values ​​of the phase and neutral currents in the protected circuit (including the three-phase unbalanced current and harmonic current in the neutral line). This current is the normal leakage current and the ground fault current during a fault. Therefore, the RCD setting value, i.e., its rated operating current In, only needs to avoid the normal leakage current value, which is measured in milliamperes. Thus, the RCD can very sensitively disconnect the ground fault in the protected circuit and can also be used as backup protection against direct contact electric shock. This has been proven in the practical use of RCDs in my country over many years. However, in the further use of RCDs, its shortcomings should be noted. 1 [b]Limitations of RCD Function[/b][b] 1.1 RCDs cannot prevent electric shock accidents caused by fault voltages conducted from elsewhere[/b] RCDs have high sensitivity to ground fault currents and can interrupt fault currents in the milliampere range within tens of milliseconds. Even with a contact voltage as high as 220V, the highly sensitive RCD can quickly interrupt the current, preventing people from being shocked. This is well known. However, RCDs can only function for ground faults within their protection range and cannot prevent electric shock accidents caused by fault voltages conducted from elsewhere. Household B installed an RCD, while the adjacent household A installed a fuse (RD) for protection. During use, if household A arbitrarily increases the fuse cross-section and carelessly damages the insulation of electrical equipment, the fault current may not be able to melt the fuse in time to cut off the fault. In this case, the fault voltage will be conducted to household B's electrical equipment through the PE line. Since the RCD will not trip, household B will face a potential safety hazard of electric shock. Such an example would not exist under current urban electrical design standards. However, in rural areas of China, the author observed during investigations that due to significant differences in economic conditions and non-standard electrical design, this phenomenon is widespread and should be given serious attention. 1.2 Some locations and equipment are not suitable for installing RCDs. Large-capacity anti-interference filter capacitors are often installed on the power lines supplying certain data processing equipment. One end of the capacitor is grounded through the equipment casing and the PE line. Its ground capacitance current is: According to the above two formulas, when C is greater than 0.22µF, the normal operating capacitance current will exceed 15mA. An RCD with a rated operating current In of 30mA may malfunction because its rated non-operating current Ino = (1/2)In = 15mA. In reality, the initial charging current of the capacitor is much greater than this. That is, if an RCD is installed to prevent it from malfunctioning, the capacitance of the filter capacitor must be much smaller than 0.22µF, which is obviously unrealistic. Therefore, RCDs cannot be used to prevent electric shock in data processing equipment. The International Electrotechnical Commission has developed a special standard for the electrical safety of data processing equipment in IEC364-4-707. [b]1.3 Some places do not allow the installation of RCDs[/b] For example, RCDs are not allowed to be installed on the operating table of a hospital. Because the new type of operating table is a medical electrical device that uses electricity, its normal leakage current is only allowed to be 0.01mA, and the leakage current during a ground fault is only allowed to be 0.05mA. However, the sensitivity of the RCD is far from meeting this requirement. On the contrary, its possible malfunction could cause a power outage and a medical accident. There are also some electrical equipment and places where RCDs are not suitable for installation, which will not be described in detail here. [b]2 Selection and Installation of RCDs 2.1 Selection of RCDs[/b] Although the function of RCDs has certain limitations, its functional advantages cannot be ignored. In order to prevent electric shock and fire accidents caused by ground faults, all electrical devices should be placed under ground fault protection as required, except for equipment and lines where power failure would cause more serious consequences. Terminal socket circuits inevitably connect to some portable, handheld electrical equipment, which are most prone to grounding faults and pose the greatest risk of electric shock. To ensure electrical safety, regardless of the grounding system, terminal socket circuits should be equipped with an RCD (Regulator-Devices), specifically a high-sensitivity, standard-type RCD with a rated operating current In not exceeding 30mA and an operating time of no more than 0.04s at 0.5 times In current. This prevents both electric shock (including direct contact electric shock) and fires caused by arcing grounding faults. This type of RCD should also be installed for overcurrent protection of fixed equipment if it cannot cut off grounding faults within 5s. When the overcurrent protection on the building's main power supply line cannot cut off grounding faults within 5s, an S-type (selective) RCD with a slight delay should be installed to ensure selectivity with downstream RCDs. Its In value should be 100–500mA, and its operating time at 5 times In current should not exceed 0.15s. This S-type RCD is used to protect all electrical installations, but is mainly used to prevent fires caused by grounding faults, and also serves as a backup for socket circuit RCDs. [b]2.2 RCD Installation[/b] In a residential building, each household's distribution box socket circuit is equipped with a standard RCD, while the fire-resistant S-type RCD is only installed on the main power supply line for the entire building. For large electrical installations, an additional RCD can be added; its In value and disconnection time can be determined according to specific circumstances. For TT systems, due to the small grounding fault current, RCDs must be installed to prevent electrical accidents caused by grounding faults. IEC standards stipulate that if only one RCD is installed on an electrical installation in a TT system, this RCD must be installed on the main power supply line to ensure that the entire electrical installation is under its protection. For TN system electrical installations, if the overcurrent protection device on the main power supply line can disconnect the grounding fault occurring within the device within 5 seconds, an RCD does not need to be installed on the main power supply line. [b]3 Conclusion[/b] RCDs, with their highly sensitive operating performance, can serve as a backup measure for direct contact electric shock protection. For example, if a person accidentally touches the 220V live wire terminal of a broken lamp holder or plug, it can quickly cut off the power supply, preventing electric shock. However, this is only a backup measure when the insulation casing is damaged, not a formal protective measure. It should not be mistakenly assumed that electrical equipment can be ungrounded or that equipotential bonding is unnecessary after installing an RCD. In summary, RCDs are not perfect; they may fail to activate for various reasons and, like other protective electrical devices, are not entirely reliable. If grounding, especially equipotential bonding, is implemented, its function is to reduce the contact voltage, often preventing fatal electric shocks. Furthermore, if insulation damage causes the metal casing of electrical equipment to become energized, equipment grounding can provide a path for the fault current, and the RCD can cut off the fault before a person touches the energized casing, thus preventing a single electric shock. Therefore, neglecting grounding and equipotential bonding is very dangerous; both should be used in combination, complementing each other, to achieve the best protective effect.