What is an earth fault?

Glossary · Wiring, circuits and faults

How an earth fault works

Normally, current flows out along the live conductor and back along the neutral. In an earth fault, the live conductor touches something connected to earth, or the metal casing of an appliance becomes live, and current finds a path to earth that it was never meant to take.

Electrical Safety First explains the danger. If there is a fault in your installation you could get an electric shock if you touch a live metal part, because the electricity may use your body as a path from the live part to earth.^{[1]Electrical Safety First, Electrical Safety Advice for Earthing and Bonding}

How the protection responds

Earthing protects you by providing a path, called a protective conductor, for fault current to flow to earth. It also ensures that whatever protective device is in place, a fuse, circuit-breaker or RCD, switches off the current to the circuit with the fault.^{[1]Electrical Safety First, Electrical Safety Advice for Earthing and Bonding} Electrical Safety First gives the example of a cooker fault: the fault current flows to earth through the earthing conductors, the protective device in the consumer unit switches off the supply and the cooker is then safe to touch.^{[1]Electrical Safety First, Electrical Safety Advice for Earthing and Bonding}

The IET describes this as automatic disconnection of supply. A fault to earthed exposed metal results in operation of a circuit-breaker, fuse or RCD, disconnecting the circuit and protecting the user and the installation.^{[2]IET, Mythbusters #4 Double insulated cables} In its wording, earthing is provided to connect exposed metal parts to the main earthing terminal so that fault current has a path to earth.^{[6]IET, BS 7671 FAQs - Earthing and Bonding} Our guide to protective bonding covers the related method for pipes and other metal.

The role of the RCD

A residual current device watches for current going where it should not. NICEIC says RCDs immediately switch off the electricity if they detect a fault and are designed to protect people from electric shocks and earth faults.^{[4]NICEIC, Consumer units and fuse boxes: installation and replacement} The IET says RCDs are rated in milliamps, so they are thousands of times more sensitive than circuit-breakers or fuses, and that they will also provide some protection from fires arising from insulation faults.^{[3]IET, Consumer units - safe fuse-box upgrades for modern homes}

Some homes cannot rely on a fuse or breaker for this. On a TT supply the earth path has higher resistance, so fault current is too small to operate a fuse or breaker in time, and an RCD must be used for shock protection.^{[5]IET, TT earthing considerations} Our guide to earthing systems explains why, and earth fault loop impedance covers how the path is measured.

What you can check yourself

The IET says RCDs have a test button marked T or Test, and that householders should test them at a maximum interval of six months. The device should switch off when you press it, and you can then reset it to restore supply. If it does not switch off, seek expert advice from a registered electrical contractor.^{[3]IET, Consumer units - safe fuse-box upgrades for modern homes} Our guide to the RCD trip test shows the routine.

If an RCD trips on its own, the IET describes unplugging every appliance fully from the affected circuit, resetting, and adding appliances back one at a time to find a faulty one. If it will not reset with everything unplugged, call a registered electrician.^{[3]IET, Consumer units - safe fuse-box upgrades for modern homes} The IET also says repeated resetting on a fault can damage the device.^{[3]IET, Consumer units - safe fuse-box upgrades for modern homes}

When to call an engineer

If you get a shock, if an RCD will not reset, or if you smell burning or see scorching, stop using the circuit and call a registered electrician. Do not open the consumer unit or any accessory. See electric shock for what to do after one.

Example

Hamza in Seacroft notices the RCD in his hallway keeps tripping when he runs the washing machine. He presses the test button, which switches the RCD off as it should. He unplugs everything on that circuit, resets once and plugs the machine in alone. The RCD trips again straight away, so he leaves the machine unplugged, does not use it and books an electrician. Hamza did not open the machine or the consumer unit.

What people get wrong

Many people think an earth fault means the house will blow up, or that earthing only matters if something dramatic happens. Earthing matters most when nothing seems wrong, because it is the unseen path that turns a fault into a tripped switch and not a shock.

The second mistake is treating a trip as a nuisance and resetting until it holds. The IET warns that repeated resetting can damage the device. Another is assuming bonding and earthing are the same. The IET says they are different concepts, with earthing giving fault current a path to earth and bonding keeping metal parts at the same potential.^{[6]IET, BS 7671 FAQs - Earthing and Bonding}

The bottom line

An earth fault sends current to earth along a path it was never meant to take, often through exposed metal and possibly through a person. Earthing gives the fault a safe path, and a fuse, breaker or RCD switches off the supply. Test your RCD about every six months, unplug and reset once after a trip and call a registered electrician if it keeps tripping or you see warning signs.