Why does my earth wire carry voltage when the main switch is off?

Finding voltage on an earth wire when you believe the power is off is understandably alarming, and it is right to take it seriously. Here is what is likely happening and what you should do. What the earth wire is supposed to do The earth wire in your home is a safety conductor. Under normal conditions it carries no voltage at all. Its job is to give fault current a safe path back to the supply so that a fuse blows or an RCD trips if something goes wrong. If it is showing voltage, something in the wiring is wrong. The most common reasons this happens 1. A neutral-to-earth fault somewhere in your installation. If the neutral conductor touches earth at any point other than the main earthing terminal, current can flow along the earth wires and create a voltage relative to true earth. 2. Backfeed from circuits not controlled by your main switch. Some installations have circuits, such as an alarm system, a solar PV inverter, or an electric vehicle charger, that have their own supplies which bypass the main consumer unit switch. These can energise earth conductors even when you think everything is off. 3. A borrowed neutral or cross-connection between circuits, sometimes left by a previous installer, can allow voltage to appear on conductors you expect to be dead. 4. An upstream fault on the DNO supply cable or meter tails. This is outside your control entirely. What you should check safely yourself Do not touch any exposed metal pipework, socket faceplates, or appliance casings until the fault is diagnosed. Do not try to trace the fault yourself by probing cables. You can safely look at whether any appliances or systems have their own independent power source, such as a solar inverter with a battery, and switch those off at their own isolators if you can do so without touching anything metallic that feels unusual. When to call an electrician Call a Part P registered electrician or a DNO emergency line if you believe the problem is on the supply side of your meter. This fault needs an insulation resistance test, an earth fault loop impedance measurement, and an RCD performance test. It is not something to leave. Voltage on an earth conductor means metalwork that should be safe to touch may not be, and that is a risk of electric shock or fire.