What does earthing actually do?

The third conductor in the installation, green and yellow. It takes no part in running any appliance and carries nothing at all in normal operation.

What happens without it

Picture a washing machine whose insulation has failed, so the live conductor touches the metal casing.

With earthing: the fault current flows straight to earth, the breaker or the RCD disconnects the circuit, and nobody notices anything except that the machine stopped.

Without earthing: nothing happens at all. The machine runs normally and the voltage on the casing waits. The first person to touch it becomes the path to ground.

That is the whole difference, and it is invisible until the day it matters.

Why the RCD needs it too

An RCD reacts to the difference between outgoing and returning current. If a casing goes live and there is no earth, nothing flows anywhere, so there is no difference and the RCD sits still.

It will still trip once a person touches the casing, and that is genuinely worth having. But it acts after contact rather than before it, which is why earthing and an RCD are not alternatives.

A wire is not proof

The green and yellow conductor may be present and connected to nothing. That is the single most common finding in flats where somebody has previously tidied up the wiring.

It cannot be checked with a test screwdriver, because an unconnected earth and a working one both read as dead. It is proven by measurement: loop impedance, electrode resistance, and continuity through each circuit, written down as numbers. What that involves is on the earthing page, and the symptoms of a bad one in signs of poor grounding.

What is never acceptable

Bonding earth to a water pipe, which used to be practice and stops working the moment a neighbour swaps a section for plastic. And joining earth to neutral inside a socket, which works until the neutral fails and then puts the full voltage on every appliance casing at once.