Plastic piping is not only widely used in new
installations, but also in repairing existing installations.
Even in a metal water piping system, maintenance and repairs
can introduce plastic pipe fittings that interrupt the
electrical continuity of the system. These fittings can also
significantly reduce the length of piping that can act
effectively as a grounding electrode. Water utilities do not
commonly install jumper wires around the fittings to maintain
electrical continuity. In fact, many utilities discourage the
connection of buried parts of metal water piping systems to
the electrical system. They say that electric current through
the pipe can hasten corrosion and affect the taste of the
water.
With the introduction of nonconductive plastic water piping
systems, the burden of providing a low-impedance connection to
earth at the service equipment falls on other types of
grounding electrodes. The research project described in this
article is being conducted to evaluate buried electrodes to be
sure they can handle the job.
Background
A project to evaluate grounding electrodes began a few years
ago in Clark County, Nev. (Las Vegas area), when questions
arose about the adequacy of certain types of grounding
electrodes. Subsequently, the National Fire Protection
Research Foundation (NFPRF) got involved, and a Technical
Advisory Committee (TAC) was organized to gather information
about the performance of grounding electrodes for building
wiring systems. Today, the TAC is composed of representatives
from NFPRF, NFPA, UL, electric utilities, industry
associations, electrical contractors, municipalities, the U.S.
Army, instrument manufacturers, grounding rod manufacturers,
the entertainment industry, and possibly more, as the list of
members grows. The organizations represented on the TAC are
sponsoring the long-term research project on grounding
electrodes. The project is being managed by the NFPRF.
The purpose of this grounding electrode research project is
to develop data to evaluate the corrosive effects of various
weather and soil conditions around the United States on
different styles of grounding electrodes, and how an electrode’s
ability to carry ground current is affected. The project
includes long-term testing of grounding electrodes buried in
the soil at a number of test sites. Presently, test sites are
located near Staunton, Va., and Las Vegas. This spring, the
third and fourth test sites are planned for installation in
Texas and on UL’s campus in Northbrook, Ill. Other sites are
proposed for New York, Montana, and central Florida (on the
Disney property). The testing involved will explore grounding
resistance and the electrical integrity of the electrodes over
time in various types of soil to determine whether moisture
content, pH levels and other soil conditions can corrode
grounding rods enough to degrade grounding paths to an
unacceptable extent.
Expected to take approximately 10 years or longer, the
research project consists of monthly measurements of
electrode-to-earth resistance; and measurements of soil
moisture content, soil pH, temperature, precipitation and
other environmental conditions. Some electrodes in the project
carry a small current from a dc power supply. At the end of
the testing period, the electrodes at each site will be
exhumed and weighed to determine weight loss due to corrosion.
The data will be sent to NFPRF for compilation and analysis.
NFPRF will issue a comprehensive report that will show how the
specific grounding electrodes fared at specific site
locations. Recipients of the report will then be able to make
judgements regarding the adequacy of specific electrodes in
certain soils, and whether some grounding electrode systems
may warrant further study. Changes in the codes covering
grounding methods could result from the data obtained from
this project.
Why are Electrical Systems
Grounded? Why is Grounding Important?
According to Article 250 of the National Electrical Code®
(1996 edition), systems and circuit conductors are grounded to
limit voltages on a system with respect to earth and items
that are in contact with the earth. The voltages can be caused
by lightning, line surges or unintentional contact with higher
voltage lines. Grounding the system also stabilizes the
voltage on the system with respect to ground during normal
operation. A low-impedance connection at the service equipment
between the grounded conductor of the electrical system and
earth can enhance the longevity of the electrical insulation
and reduce the risk of electric shock. Concerns regarding
electric shock include not only the effects of faults in
electrical insulation, but also the voltage that can appear
between "grounded" accessible parts under normal
conditions. These voltages are usually low, but can be
undesirable in areas where people have simultaneous access to
the earth and to equipment grounding conductors that are, in
turn, connected at the service equipment to the grounded
circuit conductor of the system.
An example of an electric shock scenario created in part
from a poor grounding connection to earth is shown in Figure
1. In the figure,
an electrical product is in contact with the earth through a
conductive part of the building structure, perhaps involving
its mounting means. A low-impedance fault occurs between a
live part inside the product to its enclosure. Fault current
flows as indicated by the dashed line in the figure,
but the impedance of the grounding electrode system limits the
magnitude of the fault current. The opening of the overcurrent
device is delayed, or the overcurrent device carries
insufficient current to operate.
A person in contact with the earth and any conductive part
connected to a grounding conductor of equipment plugged in or
permanently wired anywhere in the building while the fault
current flows could experience an electric shock, if the
voltage dropped across the grounding electrode system is high
enough. This situation is aggravated when the person is more
susceptible to the effects of voltage across the body (for
example, when the person is in a swimming pool or spa).
An overcurrent device, such as a circuit breaker, requires
a minimum of 110 percent of its rated current to trip and open
a circuit. For a 120-volt, 20-ampere circuit, for example, the
total impedance of the loop carrying the fault current,
according to Ohm’s Law, must not exceed 5.5 Ohms (i.e.,
120V/22A). The grounding electrode contributes in part to this
total value, and therefore, the resistance to earth of the
grounding electrode system alone might have to be
significantly lower than 5.5 Ohms.
What are Some Typical Grounding
Electrode Designs
Article 250 of the National Electrical Code covers
grounding. Grounding electrodes are described in Sections
250-81 and 250-83. Typical grounding electrode designs include
rod and pipe electrodes, grounding plates, chemically charged
electrodes, and concrete-encased electrodes.
What is The Scope of the Project
and What Types of Electrodes Will be Installed at the Test
Sites
The scope of the research project includes many types of
grounding electrodes permitted by the NEC According to
the scope, the project does not include water piping systems
and building steel, nor does it include the grounding
conductors and various equipment used to provide grounding
paths elsewhere in a premise’s wiring system. Since the
project focuses on building systems, it does not necessarily
cover all types of electrodes used by electric utilities.
The Northbrook, ILL.,
Site—Installation and Testing
The Northbrook, Ill., test site is similar to other test sites
in the NFPRF study. A 100' by 200' test site will be located
on the east side of the UL property. Within the test site, a
total of 63 grounding electrodes will be buried. There will be
two fields Ñ one passive (no electrical current applied), and
one much smaller, active field (electrical current of
approximately 5mA dc applied). The electrodes, installed
horizontally or vertically in the site, will represent designs
permitted by the NEC® in a number of configurations.
Fifty-seven electrodes will be buried in the passive field.
Some of these electrodes are specifically designed and
manufactured as grounding electrodes, while others are simply
various types of pipes and rods permitted by the NEC® for
grounding use. Electrodes include solid reinforcing bars,
metal rods, galvanized steel pipes, copper conductors,
concrete-encased electrodes, plates and chemical electrodes.
In the passive field, 30 electrodes will be vertically driven
or buried in augered holes up to 1-1/2 feet in diameter and
10-feet deep. The remaining 27 electrodes will be buried in
horizontal trenches 4-feet deep.
The chemical electrodes for installation in the passive
field are hollow copper tubes with small predrilled weep holes
along their lengths. The tubes are filled with salts. When the
salts come in contact with moisture, an electrolytic solution
is formed, promoting good electrical contact with the earth.
Some of these chemical electrodes will be encased in bentonite,
a material that also promotes good electrical contact with the
earth.
In the active field, a current of approximately 5mA dc will
flow through vertical electrodes of two types — 5/8"
copper-clad steel rods, and 3/4" trade-size galvanized
pipe. The current represents the dc component of the
electrical ground current in a hypothetical scenario.
Direct current flows through the grounding paths of
building wiring systems when products are used that rectify
the load current. Electrode deterioration/corrosion is caused
by electrolysis from direct current flowing through grounding
electrodes. Load currents flow through grounding paths between
the service equipment and the utility transformer because the
grounded circuit conductor is connected to earth at more than
one point. The NEC® requires one of these grounding points to
be connected at the service equipment of a building to limit
the voltages on the system with respect to earth. Utilities
ground systems along entire power distribution networks for
the same reason.
The importance of the active site is to illustrate how
direct current can accelerate the corrosion of grounding
electrodes, in contrast to the electrodes in the passive
fields that do not have dc current flowing through them. The
active site will also show the distribution of the corrosion
along the length of the electrode. If, for example, the
corrosion is concentrated near the point of connection to the
grounding electrode conductor, early failure of the grounding
electrode may result with minimal loss of material, leaving
most of the electrode in the soil, but disconnected from the
electrical system.
What Measurements Will be Used to
Evaluate an Electrode in the Test Site?
All electrodes will be weighed prior to burial and again at
the end of the project after they’ve been exhumed and
cleaned. This process will determine the amount of electrode
material lost to the earth by way of electrolysis and
corrosion. Weight retention is an important factor indicating
the ability of an electrode to provide a sound, low-impedance
connection to earth over time.
In addition, the electrode-to-ground resistance will be
periodically measured during the course of the project. The
probes for the instrumentation and each electrode in the
project will have buried leads terminating in a junction box
where resistance measurements can be taken. Records of
variable environmental conditions will be collected. Figure
2 illustrates the resistance measurement method.
How Are Grounding Electrodes
Installed at the Northbrook Site?
The installation equipment includes a backhoe and a power
auger. The backhoe is used to dig 12 trenches approximately
80-feet long for horizontal burial of 27 electrodes. Smaller
trenches will accommodate instrumentation leads. The power
auger is used to drill holes for vertically positioned
electrodes, except that driven ground rods are installed by
forcing them into the ground without the use of an excavated
hole. Following site excavation, the electrodes are placed in
the trenches and holes. A copper lead is connected to each
electrode, and terminates in a junction box where the
resistance measurements from electrode-to-earth will be taken
during the term of the study. Prior to backfilling, the leads
will be placed in plastic flexible conduit that serves as
mechanical protection when the trenches and holes are filled
with earth. The conduit also permits easier replacement of
damaged leads during the course of testing, if necessary. Figure
3 illustrates the trenching detail.
Summary
The results of this study will provide valuable information
for builders, designers, utilities, code authorities and
others who build, install, inspect or maintain electrical
systems. Future construction is likely to introduce more
plastic water pipe, and thus, new designs of grounding
electrodes for electrical systems may be required. These new
designs for grounding electrodes — a simple concept — must
respond to the complicated challenges of corrosion,
electrolysis and accelerated deterioration that have become
important problems to solve in our increasingly complicated
environment.
For more information on this project, call Peter Boden at
UL in Northbrook, Ill., at (847) 272-8800, ext. 42011; or
e-mail him at bodenp@ul.com. Call Walter Skuggevig at UL in
Melville, N.Y., at (516) 271-6200, ext. 22312; or e-mail
skuggevigw@ul.com. Or write Doug Brown at the National Fire
Protection Research Foundation, 1 Batterymarch Park, Quincy,
MA 02269; call (617) 984-7281; or e-mail dbrown@nfpa.org.
Note: [1] K. Michaels. "Earth Ground Resistance
Testing for Low-Voltage Power Systems," IEEE Trans. on
Industrial Applications, pp. 206Ð212, Vol. 31, No. 1,
Jan./Feb. 1995.
This article was reprinted by permission
of On the Mark.
Copyright © 1998 by On the Mark. Reprinted with
permission.
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