A ground line is an electrical wire used to reduce the risk of electric shock. By nature, electricity always follows the fastest path to the earth. If a person were to touch an ungrounded wire, the electricity would pass through the body to reach the ground, resulting in injury or death. If a ground line is present, it offers a short path with low-resistance, which allows excess electricity to reach the earth more quickly than it would if passing through a person.
To understand how a ground line is used, it is helpful to understand how most home wiring systems are setup. Standard electrical systems carry a pair of wires, including one hot wire and one neutral. Electricity travels from the panel through the hot wire to power the home, then passes back through the neutral wire to complete the loop. If a surge of power passes through the system, it will pass into the panel through the neutral wire and trip the circuit breaker. This minimizes the risk of fire or electric shock to the home's occupants.
In the event that the neutral wire is cut or fails, however, electricity will have no effective path back to the panel. It will either damage the home's electrical system or pass through individuals who may be using an electronic device in the home. A ground line acts as a backup for the neutral wire in this type of scenario. It provides the excess electrical energy with a low-resistance path back to the panel, where it simply trips the circuit breaker.
Many appliances also have their own ground line to protect users and extend the life of the appliance. In a refrigerator for example, a built-in ground line is tied in to the metal casing of the unit. If the neutral wire fails, or a surge of electricity passes into the unit, the ground unit prevents this surge from blowing the refrigerator's motor. It also protects users who may be standing close to the unit when an electrical surge occurs. This electricity passes through the ground wire back into the home's electrical system rather than passing through the individual in an effort to reach the earth.
A ground line may also be used as part of a lightning protection system. If a lightning strike hits a building, it can overwhelm the electrical system, destroying appliances or even causing electrical fires. A ground line is a wire connected to a point high on the building. It extends to the earth, then connects to metal spikes or rods that are buried underground. When lightning strikes the home, it simply passes down the ground wire then down into the earth.