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Grounding

Electricity is simply the movement of electrons, whether it is the power to operate a toaster or a lightning bolt from the sky. It is the movement of electrons to earth ground. It is important to remember this. The electrons are all going to the same place. Earth ground. (There are exceptions like chemically produced electricity from a car battery that returns electrons to the positive post, but we will deal with that elsewhere)

The electricity we use to run equipment like refrigerators and lights must use a completed circuit. The utility power plant that makes the electricity with heat from burning coal or nuclear fission actually pumps electrons through the wires just like water through a pipe. The circuit I refer to, means the electrons delivered to your equipment must be returned to the power plant for the system to work.

Equipment that operates on 240 volts single phase or any three-phase voltage returns electrons on the same wires that bring it. This is possible because we use alternating current. But, equipment that uses 120 volts single phase must have a dedicated conductor for the spent electrons to follow back to the power plant.

Utility companies use a simple expedient. Rather than run a wire back to the power plant for the return flow, they run the used electrons into the earth at the place where the power is used. In the electrical distribution system within your building, this wire is called the common or neutral. The NEC (NFPA’s National Electric Code) allows the neutral to be connected to the utility ground wire at the first device past the meter. This usually means the ground and neutral are tied together at the main disconnect switch. This is the only place the neutral and ground can be tied. From this point forward, the neutral must be isolated from the ground and insulated just like a load-carrying conductor.

The ground wire that runs throughout your electrical distribution system is connected to an earth ground provided by the electric company and is only there in case of a malfunction. It is not to be used as a neutral. If, for example, a "hot" wire (load carrying conductor) rubs against a conduit fitting or appliance frame until it wears through the insulation, the grounded metal will provide a short to earth ground through this ground wire. That will create a load sufficient to blow the fuse or breaker on that circuit. If you replace the fuse or reset the breaker, it will just blow again until you fix the problem. The ground wire also carries any maverick voltages created when equipment runs back to earth ground and thus prevents shock.

The utility company provides the earth ground at the power pole or transformer. On the power pole they wind a copper ground wire on the bottom of the pole before they set the pole. This provides a pretty good earth ground. If they use a pad-mounted transformer, they will often drive a ground rod into the earth near the pad. The utility ground wire that runs through your building returns electrons to these ground points in the earth.

Other things in your building can be earth grounds. Water pipes are often metal and pass through the earth. These make reasonably good grounds. Recently, however, plastic pipe has made this type grounding impossible. I have seen iron gas pipe used as ground, but you should never allow this because of the possibility of fire. Often ground rods or field ground networks are placed around a large property to provide convenient grounding points. These extra grounding points sometimes allow lightning to do extra damage.

When a high voltage transient enters your facility through utility service (electric, telephone, or cable) or through a direct lightning hit, a huge volume of electrons are delivered to your equipment and have to find their way on to ground. This creates a potential or voltage difference between the source lightning hit or transient and anything connected to earth.

The damage actually comes from one of two likely directions:

1. As the extra electrons flow to earth ground from the normal power source inside your equipment, the high voltage potential does the damage.
2. If you have grounds other than the ground provided by the electric company at the power pole or transformer, you may have created a field ground under your property. When this condition exists, and a direct lightning hit occurs on your facility or on the utility’s wires, it will create a potential between the grounds. For example: If lightning hits the utility, it will energize the ground with extra electrons where the electric company grounded the transformer. This, however, may be 500 hundred feet or more from your facility where you have a field ground created by ground rods or water pipes. Because of the distance between these two grounds a potential will exist between them until the charge has time to equalize or dissipate into the earth. Since electricity flows more slowly through earth than it does through wire, until the potential dissipates, you may incur damage to your equipment as electrons flow ground to ground inside your equipment.

7/18/99