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250.56
Resistance of Ground Rod Electrode
What's
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Top 50 NEC Rules
250.56
Resistance of Ground Rod Electrode
By Mike Holt
When the resistance of a single ground rod is more than
25 ohms, an additional electrode is required to augment the first
ground
rod electrode, and it must be installed not less than 6 feet away.
However, no more than two ground rods are required -- even if the
total resistance of the two parallel ground rods exceeds 25 ohms.
You can measure the resistance of a grounding electrode with a
ground
resistance clamp meter or a 3-point fall of potential ground resistance
tester. Let's review the concepts behind each test method.
- Ground clamp meter. The ground resistance clamp meter
measures the resistance of the grounding (earthing) system by injecting
a high-frequency signal via the grounded neutral conductor to the
utility ground, and then measuring the strength of the return signal
through the earth to the grounding electrode being measured.
- Fall of potential ground resistance meter. The 3-point fall
of potential ground resistance meter determines the ground resistance
by
using Ohm's Law: R = E / I. This meter divides the voltage difference
between the electrode to be measured and a driven potential test stake
(P) by the current flowing between the electrode to be measured and a
driven current stake (C). The test stakes are typically made of
1/4-inch diameter steel rods, 24 inches long, driven two-thirds of
their
length into the earth.
The distance and alignment between the potential and current test
stakes, and the electrode, is extremely important to the validity of
the
ground resistance measurements. For an 8-foot ground rod, the accepted
practice is to space the current test stake (C) 80 feet from the
electrode to be measured.
The potential test stake (P) is positioned in a straight line between
the electrode to be measured and the current test stake (C). The
potential test stake should be located at approximately 62% of the
distance that the current test stake (C) is located from the electrode.
Since the current test stake (C) is located 80 feet from the grounding
(earthing) electrode, the potential test stake (P) will be about 50
feet
from the electrode to be measured.
The 3-point fall of potential meter can only be used to measure one
electrode at a time. Two electrodes bonded together cannot be measured
until they have been separated. The total resistance for two separate
electrodes is calculated as if they were two resistors in parallel. For
example, if the ground resistance of each electrode were 50 ohms, the
total resistance of the two electrodes bonded together is about 25
ohms.
If the electrode to be measured is connected to the electric utility
ground via the grounded neutral service conductor, the ohmmeter will
give an erroneous reading. To measure the ground resistance of
electrodes that aren't isolated from the electric utility (such as at
industrial facilities, commercial buildings, cell phone sites,
broadcast
antennas, data centers, and telephone central offices), a clamp-on
ground resistance tester would better serve the purpose.
The resistance of the grounding electrode can be lowered by bonding
multiple grounding (earthing) electrodes that are properly spaced apart
or by chemically treating the earth around the grounding (earthing)
electrode. There are many readily available commercial products for
this
purpose.
One final note on soil resistivity. The earth's ground resistance is
directly impacted by the soil's resistivity, which varies throughout
the
world. Soil resistivity is influenced by the soil's electrolytes,
which
consist of moisture, minerals, and dissolved salts. Because soil
resistivity changes with moisture content, the resistance of any
grounding (earthing) system will vary with the seasons of the year.
Since moisture becomes more stable at greater distances below the
surface of the earth, grounding (earthing) systems appear to be more
effective if the grounding electrode can reach the water table. In
addition, having the grounding electrode below the frost line helps to
ensure less deviation in the system's resistance year round.
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Code Challenge
What's Wrong Here?
By Joe Tedesco
Think you know how this installation violates the
NEC?
Visit EC&M's Web
site to see the answer.
Hint: Bad bend or lack of protection?
Code Q&A
By Mike Holt
Q. What are the NEC requirements for using
electrical metallic tubing (EMT) to physically protect NM cable?
Visit EC&M's
Web site to see the answer.
Code Quiz
By Steven Owen
Q. When sizing an equipment grounding conductor
for a solar photovoltaic system, which also has ground-fault protection
for equipment, what is the minimum size equipment grounding conductor
required for a circuit that is protected by a circuit breaker rated at
30A?
A) The ground-fault protection equipment is all that is
required.
An equipment grounding conductor is not required.
B) The minimum size of the equipment grounding conductor shall
be
based on 125% of the photovoltaic-originated short-circuit currents in
that circuit.
C) A 10 AWG conductor, as noted in Table 250.122.
D) An 8 AWG conductor, as noted in Table 250.66.
Visit EC&M's
Web site for the answer and explanation.
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