Voltage, like elevation, is meaningless without a reference point. While the standard reference for elevation is sea level, the standard reference for voltage is earth ground, considered to be zero.
Earth ground should not be confused with circuit ground. Consider a car's electrical system. The negative terminal of the car battery is connected to the steel frame of the car, making it the circuit ground. However, the car's frame is insulated from the Earth by its tires. This means that the car's frame can be energized to a voltage above that of earth ground without affecting the performance of the car's electrical system. The positive terminal of the battery is still 12 Volts higher than the frame of the car, and that is all that matters to the headlights, radio, cigarette lighter, and other systems. The positive terminal of the battery simply floats 12 Volts above whatever the frame is energized to.
Earth ground is important for the safe use of electricity. By ensuring the metal frame of your oven, toaster, computer, three phase motor, or electrical substation has a good connection to earth ground, you can be certain that touching the frame will not hurt you. This is the function of the ground prong on an electrical plug. It will always be at the same voltage potential that you are at, unless you were rubbing your feet on the carpet and have built up static electricity. But that's your fault for raising yourself above earth ground.
There are several ways to provide an earth ground to a building:
- Ground rod: An eight foot long copper rod buried in the ground.
- Ground ring: A loop of copper wire buried in a ring around the foundation of the building.
- Concrete encased: A network of copper encased in the concrete foundation of the building.
- Metal water pipe: Bonding the grounding conductor to a buried metal water pipe, as long as the bond is within five feet of where the pipe enters the building.
Never use a buried gas pipe as a ground!
Regardless of the method you use, NEC guidelines require that your system must have a ground resistance of 25 Ohms or less.