Your home's earthing system explained

Inside every appliance with a metal case, and every metal light fitting, there's a chance a live wire could come loose and touch the metal. Without an earth, that case would sit at mains voltage until someone touched it. With an earth, the fault current has an easy path back through the earth wire, enough current flows to trip the breaker or safety switch, and the power goes off.

That's the job in one sentence: give fault current a low-resistance path so the protection operates quickly. The earth pin on a plug and the green-and-yellow wire in a power point are the visible ends of it. The rest runs back through the cables to the switchboard and out to the main earth.

Earthing and safety switches work together. Energy Safe Victoria describes a safety switch as turning the power off within 0.03 of a second when it detects a problem that may put people at risk. A sound earth means a fault on a metal appliance shows up as leakage the safety switch can see straight away, rather than waiting for a person to complete the circuit.

Inside the main switchboard is an earth bar that collects the earth wires from every circuit. From it, a main earthing conductor runs to an earth electrode, usually a metal stake driven into the ground close to the meter box or switchboard. You may see a green or green-and-yellow cable running down the wall into the ground, sometimes to a small cover or a label.

Australian homes use what's called a multiple earthed neutral, or MEN, system. At the main switchboard, the earth and neutral are linked, and the network's neutral is also earthed along the street. So a fault inside the home returns mostly through the wiring rather than through the soil, which is what makes the fault current large enough to trip protection quickly.

Because of that link, the earth also matters when something goes wrong with the neutral, such as a loose or broken neutral in the supply. That's one cause of tingles from taps and metal fittings. If you feel a tingle from a tap or appliance, stop using it, keep others away, let your distributor know and have a licensed electrician check it straight away.

Earthing deals with metal that's part of the electrical installation. Bonding deals with other metal that could end up in a fault path, most often metal water pipes. A bonding conductor connects the pipework to the main earth, so the pipes and the earthed parts of the installation sit at the same voltage. If a fault occurs, there's no voltage difference between a tap and a metal appliance for a person to bridge.

In some older homes, the metal water service itself was used as part of the earth. That worked when every pipe was copper or galvanised steel. It works less well now that sections of pipe are often replaced with plastic, which doesn't conduct.

A plumber replacing a length of copper with plastic, a new water meter, or a burst pipe fixed with a plastic joiner can break the electrical path that bonding or an older earth relied on. Nothing looks different and the taps work as before. The earth simply isn't there any more on part of the system.

Outside, the earth stake is easy to disturb. Concreting a path beside the house, laying pavers, building a deck or moving the meter box can bury, cut or loosen the stake and its connection. If you're landscaping near the meter box, find out where the earth runs and tell whoever is digging.

Inside, a renovation that replaces circuits or moves the switchboard changes the earth path too. That's why electricians test the earth whenever they work on the board, rather than assuming the old connection is still good.

Energy Safe Victoria lists the main earthing system among examples of prescribed electrical work, alongside consumer mains, the main switchboard and the meter box. Prescribed work has to be inspected by a Licensed Electrical Inspector, and power can't go back onto a new main switchboard until that inspection is done.

On a switchboard or mains job, the electrician tests the resistance of the main earth, checks the connection at the electrode, confirms the MEN link and bonding, and tests earth continuity on each circuit. Earths are then checked where they're used: power points, metal light fittings and hard-wired appliances.

Surge protection depends on it too. Energy Safe Victoria explains that a surge diverter captures voltage spikes that would otherwise pass into your equipment, and it does that by giving them a short path to earth. A poor earth leaves the spike looking for another way through, which is why our guide to surge protection puts the earth first.

Most earthing problems are silent. There's no buzzing or smell, and everything works normally until there's a fault. A few situations are worth acting on.

Don't dig around the meter box looking for the stake, and never disconnect or move the earth cable. Energy Safe Victoria says it's illegal, and potentially fatal, for unlicensed people to do electrical work.