Dear Newsletter Member,
We have created a nice 8.5 x 11 poster
(example below) that I'm sure will get your attention and create a lot of discussion among
your peers. The poster attempts to demonstrate the importance of understanding the difference
between Bonding and Grounding.
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
- 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,
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
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
P.S. Please don't get all excited and
email me with all kinds of "theories" on why you "feel" the way you
do. Just give me a reference and page to a standard that covers this subject.
Download Printable Poster
- 257KB ] or [ JPG
300 DPI - 1.0 MB ] or [
600 DPI - 2.0 MB ] or [
1200 DPI - 6.0 MB ]