An earth wire is a safety conductor that protects you from electric shock by giving fault current a low-resistance path to the ground, instead of through your body. It’s the green-and-yellow (or bare copper) wire connected to the metal parts of appliances and wiring systems, and its entire job is to trigger a fuse or circuit breaker the instant something goes wrong.
How an Earth Wire Protects You
Inside any appliance with a metal body, such as a washing machine, oven, or kettle, wires carry electricity at mains voltage. If one of those live wires comes loose and touches the metal casing, the casing itself becomes electrified. Without an earth wire, you wouldn’t know anything was wrong until you touched the appliance. Your body would then complete the circuit between the live casing and the ground beneath your feet, and current would flow through you.
The human body has relatively high resistance, especially with dry skin (around 100,000 ohms). That means the current flowing through you in a fault would be small enough that it wouldn’t blow the fuse. The fuse stays intact, the appliance stays “on,” and you receive a sustained shock. With wet skin, your resistance drops to roughly 100 ohms, making the same scenario far more dangerous.
An earth wire changes this completely. When the live wire contacts the metal casing, current immediately rushes through the earth wire to ground instead of waiting for you to complete the circuit. Because the earth wire has extremely low resistance, this creates a massive surge of current, well above the fuse’s rating. The fuse melts and breaks the circuit (or the circuit breaker trips), cutting power to the appliance in a fraction of a second. By the time anyone touches the casing, the live supply is already disconnected.
What Triggers the Fuse or Breaker
The earth wire doesn’t just absorb dangerous current. It actively forces the protective device to respond. When a live wire shorts to a grounded metal casing, the full mains voltage is applied across a very low resistance path. This produces an extremely high fault current that trips the breaker or blows the fuse almost instantly.
Circuit breakers use two mechanisms to detect faults. For moderate overcurrents, a bimetallic strip inside the breaker heats up and bends, releasing a spring-loaded switch. For large surges from a short circuit, a small electromagnet (wire loops around a piece of iron) pulls the strip down immediately, tripping the breaker without waiting for heat to build up. Fuses work more simply: a thin metal filament melts when the current exceeds its rating, physically breaking the circuit.
For this system to work, the earth path must have low enough resistance to generate a fault current that exceeds the breaker’s trip threshold. In UK regulations (BS 7671), the total earth fault loop impedance for a typical 32-amp circuit with a Type B breaker must not exceed 1.37 ohms. In practice, electricians test against an even lower value of about 1.1 ohms to account for the wire heating up under load. If the resistance is too high, the fault current won’t be large enough and the breaker won’t trip quickly, which defeats the purpose entirely.
What It Looks Like
Earth wires are made of copper, chosen for its excellent conductivity. In residential wiring, the earth conductor is often bare copper with no insulation, or it’s covered in a green-and-yellow striped sleeve. The color coding is consistent across most of the world:
- UK and Europe (IEC standard): green and yellow striped
- United States: bare copper, green, or green with a yellow stripe
- Older UK installations: also green and yellow (this didn’t change in the 2006 wiring color update that affected live and neutral)
If you open a plug on a UK appliance, the earth wire connects to the top pin, which is the longest of the three. Inside the walls, earth conductors run alongside the live and neutral wires in every circuit, connecting back to the consumer unit (fuse box) and ultimately to a grounding point.
How Your Home Connects to the Ground
The earth wire inside your home needs to connect to the actual ground (the planet) to complete its safety function. There are a few ways this happens, and which one your home uses depends on how your electricity supply is set up.
In a TN system, which is the most common in the UK, the earth path runs back through the supply cable to a grounding point at the electricity supplier’s transformer. Your home’s earth conductors connect to a terminal provided by the supplier, and the fault current returns through the supply network. This creates a reliable, low-resistance loop. In a TT system, more common in rural areas, your home has its own earth rod driven into the ground outside the property. Because soil resistance can be higher and less predictable, TT systems typically need a residual current device (RCD) for additional protection.
In the US, the ground wire is required to be bonded back to the electrical neutral at the service panel. Simply connecting a ground wire to a rod in the earth isn’t enough on its own to produce a fault current high enough to trip a breaker. The bonding ensures that fault current has a complete, low-resistance loop back through the supply transformer, which acts as the driving force behind the current flow.
Which Appliances Need an Earth Wire
Not every appliance has an earth wire, and the difference comes down to how the appliance is built. Electrical equipment falls into two main categories.
Class I appliances have metal parts on the outside that you can touch, such as a toaster with a metal body, a refrigerator, or a washing machine. These require an earth wire because if internal insulation fails, the metal casing could become live. The earth connection ensures that any fault current flows safely to ground and trips the breaker. You’ll recognize Class I appliances by their three-pin plug.
Class II appliances, sometimes called “double insulated,” are built with two layers of insulation or reinforced insulation so that no metal parts are ever accessible to the user. Think of a phone charger, a plastic-bodied hair dryer, or a laptop power supply. These don’t need an earth wire because even if the first layer of insulation fails, the second layer still prevents current from reaching you. Class II appliances carry a small square-within-a-square symbol on their label and use a two-pin plug (or a three-pin plug with the earth pin present only for mechanical reasons, not connected to anything inside).
What Happens When the Earth Wire Fails
A broken or disconnected earth wire doesn’t cause any obvious symptoms. Your appliance continues to work normally because the earth wire carries no current during regular operation. It only matters during a fault, which is exactly what makes a missing earth connection so dangerous. You have no warning that the safety net is gone until something goes wrong.
If a live-to-casing fault occurs with no earth connection, the metal body of the appliance sits at mains voltage, silently waiting for someone to touch it. The current through a human body is typically too small to trip a standard fuse or breaker, so the circuit stays energized. This is why periodic testing of earth continuity is part of routine electrical inspections and portable appliance testing.
RCDs provide a second layer of defense. These devices monitor the balance of current flowing out on the live wire and returning on the neutral. If even a small amount of current (typically 30 milliamps) leaks to earth through any path, including through a person, the RCD trips within about 40 milliseconds. RCDs protect against scenarios where the earth wire alone might not be enough, such as someone touching a live wire directly. They do not replace the earth wire but work alongside it.