Single-Point Grounds --What ?

Single-Point Grounds --What ? By John Cavallaro Carolina Powe & Light Question: How many ground rods does it take to ground a Power System? Answer: It depends on what type of grounding you want to do! Power System Grounding (or Earthing, if you live in the Caribbean) is a science of itself. In fact, there are two IEEE books that deal specifically with this subject: the Green book, IEEE Std 142-1991, and the Emerald book, IEEE Std 1100-1992. But I am not going to spit out theories on Grounding for this Month’s column. For most people’s needs, there are two types of grounding that need to be understood: Safety and Signal. Safety Grounding keeps electrical energy AWAY FROM the operator or person. Signal Grounding provides a reference point FOR a data signal. Let’s talk about Signal Grounding. Every signal, whether power or data or voice or video must be referenced to something. That means, it must have a point that the signal can call HOME. A signal ground, the starting point, is usually a wire that connects to the Logic Ground in the equipment. It eventually connects to the electrical power ground plane of the equipment. It is designed to handle just enough ‘power’ to provide the signal reference. One cannot and should not expect a signal ground to provide a reference for a 208 Volt, 50 amp power line. Put 50 amps on that wire and you will see the 4th of July fireworks, as well as, destroy the equipment that is attached to it. Are you still with me? Good! A Safety Ground means just that: A wire, or wires, that will prevent the electrical energy from harming someone. The energy is moved away from the person. Visually, an electrical safety ground wire has its own color. . .Guess what color? . . Remember the Green and Emerald books? . . Right! Green!! It can also be Green with a Yellow stripe running through it. This is very important. Never, never, never should a ground wire be any other color but green or green with yellow. If you have no choice but to use a different colored wire, (say you are on a deserted island and that is all you can find) then it is allowable for you to wrap a small piece of green tape around each end of the wire. I have seen it where a small wrap of green wire is placed every 1-2 feet. Either way, this works. So, how many ground rods does it take to ground a power system? If you are talking about Safety Grounding then: One. It provides the safety ground for all energies occurring within a building’s electrical wiring system. If you are talking about Signal Grounding, then: Several. The signal wires circulating through a data center are referenced to the computer internal grounds, which carry very little current. For more information on Grounding and on power quality in the facility, give John Cavallaro, P.E. a call at 919-870-3115.