The phenomenon of an appliance casing becoming live-often accompanied by the grounding wire itself becoming live-is primarily caused by electrical leakage, excessive grounding wire resistance, or a broken grounding wire. In such cases, the appliance casing may become electrified, posing a risk of electric shock. Standard household electrical wiring consists of three wires: the phase (live) wire, the neutral wire, and the grounding wire. The grounding wire typically has a cross-sectional area of 2.5 square millimeters and is color-coded with two colors (usually yellow and green). Wiring for plugs and sockets follows a specific convention: neutral on the left, phase on the right, and ground at the top. This ensures that the appliance casing connects to the grounding pin of the plug and subsequently to the grounding terminal of the socket, thereby establishing a reliable connection to the earth and ensuring electrical safety.
Grounding wires and neutral wires cannot be used interchangeably, as they serve distinct functions. The neutral wire connects directly to the transformer's neutral point, forming a closed circuit with the phase wire without significant voltage drop. Strict requirements apply to the grounding resistance of the neutral system: for transformers under 100 kVA, the grounding resistance must not exceed 10 ohms; for transformers over 100 kVA, it must not exceed 4 ohms. While the grounding wire and neutral wire are connected at the transformer, they remain separate thereafter; although repeated grounding occurs at the distribution box, the resistance requirement there is merely that it be below 30 ohms.
The neutral and phase wires share the same cross-sectional area and must carry the same current, whereas the grounding wire is typically 2.5 square millimeters and cannot handle high currents. Consequently, the grounding wire cannot substitute for the neutral wire. Users lacking a grounding wire cannot effectively ground their appliances; they should not attempt to install one themselves, as improper installation can create greater hazards. Furthermore, using the neutral wire as a grounding wire is strictly prohibited: if the three-phase current becomes unbalanced, the neutral wire becomes live, electrifying the appliance casing; if the neutral wire breaks, the appliance casing could be directly exposed to the phase current, creating an extremely dangerous situation.
For users without a grounding wire, installing a residual current device (RCD)-also known as a leakage protector-can ensure electrical safety. An RCD can rapidly cut off the power supply upon detecting leakage, thereby preventing electric shock accidents. Installing an RCD is an effective measure for ensuring safety in household electrical systems.