Grounding is probably the source of the greatest confusion in the understanding of electrical power distribution. The National Electric Code® does little to clear this fog of misunderstanding.

Purposes of Grounding

• "To limit voltages due to lightning, line surges or unintentional contact with higher voltage lines..."

These three sources of dangerous overvoltage are provided with an alternative path around the electrical system of your home or workplace by intentionally connecting the system to the earth.

fig.1 Ben's Big Idea

In practice this proviso of the code only minimizes damage from such occurances. The NEC® provides only minimal standards for lightning protection. If lightning is a common occurance in your area additional provisions must be made to protect your property.

• "To stabililize voltages..."

This is a difficult concept. There are many sources of electricity. Every transformer can be considered a separate source. If there were not a common reference point for all these voltage sources it would be extremely difficult to calculate their relationships to each other.

The earth of course is the most omnipresent conductive surface, and so it was adopted in the very beginnings of the National Electric Code® as a nearly universal standard for all electric systems(there are a few exceptions where ungrounded systems are permitted).

• 3) "To provide a path in order to facilitate the operation of overcurrent devices..."

From a day to day point of view this last purpose of grounding is the most important one to understand.

Home Grounding System

If you have ever opened an electrical outlet box, you probably discovered that there were three wires attached to the receptacle. One black, one white and one bare copper or green. Both the white wire(the grounded conductor) and the green/bare wire(the equipment grounding conductor) are electrically tied together at the main electrical service entrance. The white wire is designated to carry current as it returns to the source(utility company's transformer).

The Equipment Grounding Conductors under normal conditions carry NO current. The only time they carry current is under abnormal conditions when an electrical appliance or piece of electrical equipment is faulty and has become a potential shock or fire hazard. Under a fault condition the grounding conductor that is connected to the outer shell or sheet metal of the equipment or appliance must be able to provide a very low resistance path back to the source of the power(utility company's transformer) so that enough current will flow causing a breaker or fuse to open the circuit and automatically disconnect the hazard from the system.

It is NOT the purpose of this equipment grounding system to send current through the ground. Sending equipment fault currents through the earth can be a fatal misunderstanding of how a grounding system works. For the most part, the only time you intentially send current into the earth is during a lightning strike or line surge due to a nearby lightning strike.

For the breaker or fuse to operate the path of an equipment fault must be much more direct than sending it through the earth back to the transformer. In a romex(non-metallic sheathed cable) system there is a bare conductor. In a conduit/raceway system the metal of the conduit or raceway provides the path.

fig.2

It might help with understanding the grounding system to think of it as having two components, each with distinct functions. The below ground portion(cold water, ground rod etc) and the above ground equipment grounding system.