A ground, also called an earth, is a source of unlimited charge that remains at zero volts. It is the point from which all other potential differences in a circuit are measured, and can be used as the return path for current in a circuit.

Mains Power Ground

Each building will usually have a long metal rod driven into the soil that is connected to all the ground wires in the building. If a fault occur, the current is suppose to flow through the ground wires instead of the body, providing protection from electrocution. Ground wires usually have green or green-yellow insulation, and they only carries current when a fault occurs. Three-pin plugs have a ground wire connection but two-pin plugs do not.

Lightning Ground

Lightning rods are connected to the soil by thick copper strips that safely ground the massive current of a strike. Electrical transmission towers often have ground wires above the main wires to protect them from lightning strikes.

Static Charge Ground

Sensitive electronic circuits can be damaged by the high voltages in static discharges. Someone handling these circuits should be connected to the ground with a wrist band and floor mat to allow any charge buildup to dissipate through the ground and not the circuitry.

Static charge can also build up on large vehicles even when they are standing still. Before refueling, a vehicle should be attached to a ground wire to dissipate it's charge and prevent sparking that could start a fire.