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Service Maintenance Company, Inc. Electrical Contractors |
| Truth About Ground Rods | 9/15/2003 |
| Many individuals think that a ground rod at a metal pole for luminaires serves to
assist in clearing ground faults or some other useful purpose (which it
doesn't). For those of you that I offend with this poster and following
comments, please accept my apology, but facts are facts. Some people think a ground rod at metal poles provides: Lightning Protection. Nope, if lightning hits the pole, the lighting fixture is toast, with or without a ground rod. Protection for the Interior Wiring. A ground rod at the metal pole will NOT remove induced voltage on the conductors inside the pole. If you want to protect the interior wiring from induced voltage from the outside conductor, then you must provide proper transient voltage surge suppression (TVSS). Spend the money on surge protection, not a ground rod. Protection of the Concrete Pole Base. I don't think so, even though manufactures of grounding fitting say it does. Studies have shown that concrete encased rebar used as a grounding electrode "does not" cause any damage to the concrete foundation from lightning. Since the NEC permits the use of a concrete encased electrode (Ufer ground), then the logic follows that the concrete base is okay from damage from lightning. There is no study that shows that lightning damages concrete. Why do ground fitting companies say a ground rod connected to the concrete rebar will protect the concrete from lightning? Because they want you to believe them so that you'll buy their product. If you can, please forward this newsletter to those manufactures that claim that lightning will crack the concrete base. I can't wait to publish their response. Make the Installation Safe from Electric Shock. You would think so, but because the earth is a poor conductor of electricity, the voltage drop (touch potential) is very high at a very short distance [IEEE 142, Section 2.2.8]. If you are an engineer, you know that the ground resistance is distributed in rings of spheres of influence. According to IEEE Standard 142, IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems, about 52% of the total ground resistance is within 6 inches of the ground rod and 75% of the total ground resistance is within 3 ft of the ground rod [Table 9]. So the potential gradient (touch voltage) from the pole to the earth directly below the pole (6 in. from the ground rod) will be about 62V = 120V x 52%, and the potential gradient from the metal pole to the earth (3 ft from the pole) is about 90V = 120V x 75%. If the earth were a conductor, then the touch voltage would be zero volt, but then the entire earth would be energized! The 90 mA flowing through the person (in the poster) is more than sufficient to kill a person and it is calculated based on the touch voltage of 90V and a human having a resistance of 1,000 ohms (IEEE 80). The 4.8A current flowing through the earth (in the poster), via the ground rod is calculated on 120 line-to ground voltage with a ground resistance of 25 ohms. I believed all of the above "theories." As a matter of fact, I taught others to believe these as well. This is my way of making up for teaching so many things that I learned that was not true. "Learning is when we get the answers to our questions, but discoveries are made when we question the answers!" If you think I'm wrong (and I'm sure a few of you do), please provide me with a reference to a standard that supports your thoughts that a ground rod at a metal pole for outside luminaires should be installed because it serves some useful purpose (that justifies it cost). To help you out, visit http://www.joepower.com/ref/grounding.htm for a list of grounding standards. BONDING: The bottom line is that the metal pole must be bonded to an effective ground-fault path that provides a low impedance path back to the power supply for the purpose of clearing the fault [250.4(A)(5)]. Typically the effective ground-fault path will be a copper conductor sized in accordance with Table 250.122, because the earth cannot be use as an effective ground-fault path [250.4(A)(5)]. God Bless, Mike Holt |
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