Residual current devices (RCDs), commonly known locally as "electric shock protectors" or "life-saving devices," are widely believed to prevent electric shock injuries and fatalities. However, many factors can cause RCDs to fail to trip, rendering them ineffective. Therefore, thoroughly analyzing the reasons for RCD failure to trip and correcting misconceptions is crucial for preventing electric shock injuries and fatalities. 1. RCD in Abnormal Operating State, Failing to Provide Protection 1.1 Internal Fault or Damage to the RCD Although power departments have established relevant regulations for the management of RCDs, such as requiring rural electricians to periodically inspect and test them, the possibility of internal faults in RCDs (especially three-phase RCDs in rural power grids) cannot be avoided. If the fault is not detected and eliminated in time, the RCD will fail to provide the intended protection. To avoid unnecessary legal liability in the event of an accident on rural power grid lines, it is recommended that responsibility for rural power grid RCDs be assigned to specific individuals, with frequent inspections and proper record-keeping of operation and testing. 1.2 Deliberate Withdrawal of Residual Current Device (RCD) Due to various reasons, RCDs frequently trip or malfunction. Some users, for convenience, arbitrarily remove the RCDs from operation, rendering them ineffective. Besides the common method of disconnecting the RCD's input and output lines and directly reconnecting them, a more concealed and egregious method is to loosen the wiring screws on the secondary side of the RCD's internal current coil. 1.3 Incorrect Wiring of RCD Before installing an RCD, carefully review its nameplate and instruction manual, and adopt different wiring methods according to different site conditions. After installation, the following tests must be performed: (1) Open and close the switch three times under load; it should not malfunction; (2) Test with the test button three times; it should correctly disconnect; (3) Test each phase with a test resistor three times; it should operate correctly. Only after all the above tests are passed can the RCD be considered correctly wired. When installing a residual current device (RCD) in a TN system, special attention must be paid to strictly distinguishing between the neutral conductor and the protective conductor. The neutral conductor of a three-phase four-wire or four-pole RCD should be connected to the RCD. The neutral conductor passing through the RCD must not be used as a protective conductor, must not be repeatedly grounded, or connected to exposed conductive parts of equipment. The protective conductor must not be connected to the RCD. 2. The RCD is in normal operating condition but does not provide protection . 2.1 In a TT system, if the transformer neutral grounding wire is disconnected and a single-phase electric shock occurs, the RCD will not trip. When the transformer neutral grounding wire in a TT system is disconnected, the main RCD installed on the transformer load side will exhibit the following behavior: when the test trip button is pressed, the test trip is normal, but the RCD fails to trip during a short circuit to ground (i.e., a single-phase electric shock). This is because the test trip button can only detect whether the RCD itself is functioning correctly. After the transformer neutral grounding wire is disconnected, the short-circuit current cannot flow back to the distribution transformer through the protected line and the ground, thus preventing the RCD from tripping. The above situations require special attention from rural power management personnel, as many have long believed that as long as the residual current device (RCD) trips normally, it can provide protection. Especially given the serious problem of transformer grounding wire theft in recent years, it is even more important to strengthen inspections and adopt technical measures such as extending grounding rods. Even if the RCD trips normally, a grounding resistance test should still be conducted. 2.2 RCD not tripping during phase-to-neutral or phase-to-phase electric shock: When a person touches phase-to-neutral (or phase-to-phase), the human body's resistance is equivalent to a load. Although the person is standing on the ground, the electric shock current passing through the body is mostly shunt, forming a circuit through the phase-to-neutral (or phase-to-phase) conductor. Only a very small portion of the electric shock current returns to the distribution transformer via the ground. This current is insufficient to trip the RCD. GB6829-86 "Residual Current Operated Protective Devices" stipulates that "Residual current devices cannot protect against electric shock hazards caused by simultaneous contact with two wires of the protected circuit." 2.3 Over-tripping of the residual current device (RCD) and failure to operate: In the implementation of graded protection, if the RCD only has a current difference but no time difference, it is easy to cause over-tripping. The danger of over-tripping is that it expands the scope of the power outage. 2.4 Failure of the RCD to operate due to a blown neutral fuse in the switch before the RCD: If the RCD and the switch are installed together, and the power supply line enters the switch before the protection switch, when the neutral fuse in the switch blows, the RCD's own circuit will drive the power supply, preventing it from operating. At this time, if the phase fuses are not blown, although all electrical appliances have stopped working, the lines downstream of the switch remain energized, creating a "false" power outage. When users use electrical appliances or check for the "false" power outage, the RCD is prone to electric shock due to its failure to operate. To ensure the proper functioning of the residual current device (RCD), users of this wiring method are advised to remove the neutral fuse in the disconnector and replace it with a wire of the same specification. The RCD is not a "lifesaver"; many factors can cause it to fail to trip. Rural power workers and users must be aware of this to prevent electric shock accidents caused by momentary carelessness.