|Truth About Ground Rods
|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.
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
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
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
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
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
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
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