The earth, or ground in America, in electrical terms, carries no current, and it is this that electricity will make a dash for when it is allowed to escape from its secure home in an electric cable or flex. This is because one side of the electrical supply, the neutral, is intentionally connected to earth.
If someone touches a live conductor then a current will flow through the person, their shoes, the floor, the wall, via earth and back to the supply transformer via one or more earth connections of the transformer neutral. The person has completed the electrical circuit.
Should any fault develop in an electrical system the electricity will always head for earth, taking the easiest route there. The electrical appliances and supplies in the home are of a much higher potential and if any of these become available to touch and are electrically charged at a different voltage to the earth, the possibility of an electric shock exists when the current passes through the connection between the charged parts and earth.
If, for instance, a person comes into contact with a conductive part that is at a potential difference to earthed metalwork and that metalwork, then a very serious shock can result. In order to eliminate this possibility all electrical earths of circuits supplying equipment in the bathroom and all extraneous conductive parts are bonded together.
In this way, even if a potential does develop, such as during an earth fault on one of the electrical circuits, all the conductive parts that someone could touch will be at substantially the same voltage. No dangerous shock current can then flow.
On its way to earth, leaking current may pass through walls, floors or anything capable of carrying it. This is made much easier when the connecting substance is wet. Water is an excellent conductor of electricity which is why special care must be taken in the bathroom.
source:here
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