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SOLVINGillGH-VOLTAGEPROBLEMSIN WIRELESS/UTILITY COLLOCATIONS
Clayton HallmarkGrounding Systems LLCChagrin Falls, OH 44023
Ernest M Duckworth, Jr., P.E.Positron Industries, Inc.
Sed alia, CO 80135
ABSTRACT
Wireless communications providers are using electric-utility
transmission towers in high-voltagecorridors throughout the world
as sites for their equipment and antenna locations. This
collocation withhigh-power transmission lines offers challenging
engineering problems because of the effects of groundpotential rise
(GPR). In the absence of actual test results, calculated GPR levels
must be used indetermining the engineering design necessary to
properly isolate wire-line communications fromdamaging GPR effects.
These calculated GPR levels can be very large; and if it were not
for the abilityto reduce these levels by improving the grounding
system, there would be limitations on use of
wire-linecommunications serving some of these locations. Limiting
the use of wire-line communications servingcell sites in
high-voltage corridors would limit cost-effective engineering
design. New methodologies forgreatly improving small cell-site
grounding systems are key to reducing GPR in high-voltage corridors
tolevels that can be safely handled by isolation equipment.
I. INTRODUCTION
Isolation equipment is readily available that will protect
wire-line communications facilities enteringPCS locations within
high-voltage corridors from a GPR as high as 50 kV rms and 90 kV
surge. Properlyinstalled, this isolation equipment will offer many
years of maintenance-free, reliable protection from theeffects of
GPR
Those PCS locations within high-voltage corridors that have
overhead ground conductors (OGC) withno neutral will experience
theoretical GPR levels under 45 kV peak, provided that the PCS
groundingsystem resistance is less than 5 ohms. If a neutral is
also present in the overhead, the theoretical GPRlevels will be
less than 20 kV peak. This represents the vast majority of the type
of high-voltagecorridors in use today, and these magnitudes can
easily be isolated with equipment available on themarket.
The PCS locations within high-voltage corridors that have no OGC
and no neutral will experiencemuch higher theoretical GPR levels,
even with a 5-ohm PCS grounding system at the tower base. This
isbecause all of the fault energy will pass down through the single
tower into the ground. Worst case
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theoretical GPR levels under these conditions could reach a
maximum of85 kV. Note: Actual real-lifeGPR levels much over 30 kV
peak asymmetrical may not occur, because earth ionization increases
theearth conductivity if the current density becomes high
enough.
Obtaining less than 5 ohms for a PCS grounding system in poor
resistivity soils may be very difficultat a cell site with a small
grounding system. However, significant grounding improvement to
these smallgrounding systems can be obtained without expensive or
elaborate grounding systems, as one of theauthors has shown.
II. GROUND POTENTIAL RISE (GPR)\ffi
l".'1'"
Electrical damage from ground potential rise (GPR) throughout
the wireless industry has an estimatedcost in the many millions of
dollars each year, but few engineers in the industry are even aware
of thephenomenon.
Most times, the first sign that something is wrong comes right
after a thunderstorm or after a fault onthe power line. Suddenly,
the wire-line service coming into your cell site has failed, and
the delicatecircuitry of your communications equipment is damaged.
This is often misdiagnosed as an unavoidablemaintenance problem,
and much money is spent on repairing equipment and replacing
protective fusesand gas tubes -to say nothing of potential lost
revenue. In the worst case, the safety of personnelworking at the
site may be seriously compromised.
ill. SOLVING THE MYSTERY
In reality, this type of damage very well could be due to a
phenomenon called ground potential rise(GPR).
When a ground fault occurs at a power substation, some of the
fault current will return to its source,namely the substation
transformer, via the earth, through the substation's ground grid
(Figure 1). Thisground grid has its own characteristic impedance.
Following Ohm's law, a current passing through animpedance will
result in a voltage. This increase in the potential of the
grounding system, referenced toremote earth, is called ground
potential rise (GPR).
Figure 1. Development ofGPR from power system fault.
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As Figure 2 shows, if your telecommunications lines coming into
a cell site are copper, and if theselines are not properly
isolated, they provide a path for the voltage impulse coming up
from the groundingsystem, whether from lightning or a power fault
as discussed earlier. Nonnally, communicationsengineers look upward
for threats in the electrical environment; but this one comes from
below, from thevery grounding system that is part of the electrical
protection scheme. This threat is real and cancompromise personnel
safety and damage equipment.
Figure 2. Communications location without isolation
protection.
These GPR surge currents develop on the grounding system and are
sent out onto your conductivecopper communications lines back to a
remote ground, which in this case is the serving central
office(CO). This is why ordinary surge protection devices such as
gas tubes are ineffective in protectingagainst GPR.
However, special high-voltage protection (HVP) isolation devices
-including isolationtransfonners, optical couplers, and fiber
optics -interrupt the conductive paths that carry the GPRcurrents
(Figure 3). These devices provide an isolation gap rated at 50kV
fillS and 90 kv for surges. The;highest service reliability may
actually be from wire-line facilities using passive isolation
equipment, i.e.,isolation transfonners. Active isolation equipment
using optical isolators requiring power will lower thereliability
of a TI carrier or HDSL service and needlessly expose maintenance
personnel more frequentlyto possible hanD.
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Figure 3. Communications location with isolation protection.
IV. GET TO KNOW THE STANDARDS
Follow existing national codes and IEEE standard installation
procedures while using HVP devices.The most important standards
include:
.ANSI/IEEE Standard 487-1992 -Guide for the protection of
wire-line communication facilitiesserving electric power
stations.
.ANSI/IEEE Standard 367-1996 -Recommended practice for
detennining the electric power stationground potential and induced
voltage from a power fault.
.ANSI/IEEE Standard 80-1997 -Guide for safety in AC substation
grounding.
.NFP A 70-1999 -National Electrical Code.
Communications protection engineers should not turn a blind eye
to GPR damage because theybelieve special HVP devices are more
expensive than gas tubes. Consider ongoing costs for
continuallyreplacing damaged equipment year after year. Also
consider that the costs of labor for repairs and thelost revenue
from downed communications lines can easily surpass the cost of GPR
protection. Anddon't forget personnel safety and liability issues:
employees working in, on, or around equipmentconnected to a remote
ground potential are at a safety risk if standards and codes are
not followed.
Properly protected GPR locations, designed and maintained by
trained employees, will reduce overallcosts, improve productivity,
and increase circuit reliability over any time period.
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V. MODERN GROUNDING TECHNOLOGIES
As can be seen in Figure 2 above, the GPR voltage can be reduced
by bringing the resistance at thecommunications location (cell
site) to a low level with respect to the remote ground location,
effectivelyshorting out the GPR. The trend among wireless service
providers is to specify a resistance to remoteearth of 5 ohms or
less. A low ground resistance produces the following benefits:
.Reduces touch and step potentials, which are dangerous to
personnel
.Reduces voltages across insulators that can cause current
flashover across the insulators
.Reduces the likelihood of sideflashing, or arcing through air,
between exposed and groundedstructures and components
.Diverts lightning current around concrete tower foundations,
which can be exploded by the current
.Facilitates the discharge to ground of currents intercepted by
protectors and arresters
.Keeps GPR within the specifications of HVP isolators.
Modem lightning research has led to improved understanding of
the lightning threat. It has shown thewaveshape and magnitude of
lightning strokes. This has shown the importance of low-impedance
as wellas low-resistance grounds, since the destructive voltages
developed by fast transients such as lightningdepend more on the
inductive-reactance component of the impedance than on the
resistance. However, itis usually easier to calculate, predict, and
measure the resistance of an electrode than the inductive
andcapacitive reactances. Fortunately, if we design and install an
electrode for a low resistance such as 5ohms, it also tends to have
low reactances.
VI. THINK LATERAL
When a really low resistance is required, the best advice for
the grounding designer is to "thinklateral," especially when the
soil is highly resistive or too thin to allow driving rods. A flat
electrode ofsignificant lateral extent, at a shallow depth of only
30 in., may be the best or only option. It resemblesa buried plate,
which provides a highly capacitive electrode. The resistance of an
electrode is inverselyproportional to the capacitance. In fact, the
fonnula for resistance of any earth electrode is based on
thecapacitance between the buried electrode and its hypothetical
image above the earth.
Conductive cement such as EarthLink from Grounding Systems
provides an easy, economical way todesign and install extensive
electrodes. The cement is employed as a backfill material around
commonlyused metallic electrodes such as driven rods and buried
wires (counterpoise) and rings, increasing theircross-sectional
area by a factor of 100 or 200 (for a 4/0 wire) or even more. In
many adverse groundingsituations, the conductive electrode may be
the only economic and practical method of obtaining 5 ohms.
Cement can be used to augment almost any kind of electrode, and
the results are easy to calculate andpredict and are permanent.
Cement is well known to contractors to protect buried metal from
corrosion.Not just any kind will do for grounding, however.
Conductive cements have over 200 times lowerresistivity than
ordinary cement -low enough that standard formulas can be used for
calculating theresistance of electrodes made with them, just as if
the electrodes were made of metal.
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Figure 4. Conductive cement effectively enlarges the wire,
creating a conductive plate.
Figure 4 shows a horizontal-strip configuration, or groundbed,
and the formula for calculating itsresistance. The most common
installation procedure follows:
1. Dig a trench, 30 in. deep, 20 in. wide, and as long as
required to obtain the desired resistance. (Thelength is a design
calculation, discussed later.) Center a 4/0 stranded wire in the
bottom of the trench.
2. Pour in the cement as a dry powder (it will later absorb
moisture and harden) by dragging an open bagof it down the trench.
Use one 50-lb bag every 10ft. Heap the cement up as shown.
3.'Lift the wire slightly so it is completely covered by the
cement for corrosion protection. Tamp thecement with feet or a
shovel toward the tapered edges.
4. Carefully shovel in a 4-in. layer of soil and tamp it
down.
5. Push in the rest of the removed soil using construction
equipment.
VB. DESIGNING A HORIZONTAL ELECTRODE
The design procedure is as follows:
1. Decide upon the desired resistance of the electrode.
2. Measure the soil resistivity with an earth tester.
3. Detennine the required length from the table, based on the
desired resistance (5 or 10 ohms) and thesoil resistivity.
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Length for5-0hmGround
Length for10-0hmGround
SoilResistivity
5000 Q-cm7000 Q-cm
10,000 Q-cm15,000 Q-cm20,000 Q-cm30,000 Q-cm50,000 Q-cm
100,000 Q-cm
10m (33 ft)16 m (52.5 ft)26 m (85 ft)44 m (144 ft)63 m (207
ft)105 m (344 ft)194 m (636 ft)440 m (1444 ft)
3 m (9.8 ft)6 m (20 ft)10 m (33 ft)18 m (59 ft26 m (85 ft)44 m
(144 ft)84 m (276 ft)194 m (636 ft)
Table 1. Table of lengths for 5- and IO-ohm grounds. Use the
formula for intermediate values.
vIn. GROUND RING
A typical pad-mounted wireless site has a buried ground ring
around the pad, about 2 ft out from thepad, and another ring around
the antenna. The formula given in Figure 4 applies; however,
theresistance thus obtained must be multiplied by 1.12 to account
for the reduced grounding efficiency of asquare ring compared to a
straight strip. For example, if the two rings require 145 running
feet (44 m)just to surround the pad and antenna, the table shows
this would give about 5 ohms in 15,000 ohm-cmsoil (about 1.5 times
the average U.S. soil resistivity). Multiplying by 1.12, the
resistance would beabout 5.6 ohms. A still lower resistance could
be achieved by extending radials from the four outercorners of the
configuration.
IX. GIRD THE GRID
Meanwhile, back at the substation, the source of the GPR from
power faults, the GPR can be reduced bylowering the resistance of
the grounding grid. If conductive cement is used to surround grid
wires on a10-by-10-ft spacing, the grid area can be reduced by 10
or 20 percent, with a concomitant money savingand reduction in the
extent of the critical 300-V GPR contour. Use IEEE Std. 80-1997
data or EPRISubstation Grounding Workstation software and assume
strip conductors of2-in.-by-18-in. cross section.For further
information, refer to manufacturers' application notes.
Existing ground grids also can be improved by extending the grid
area by 10 or 15 percent and usingconductive cement. In one
application in high-resistivity soil, grid resistance was reduced
from 10 ohmsto 2 ohms. In another, resistance was reduced from 0.96
to 0.2 ohm. Consolidated Edison and BostonEdison have used
conductive cement to ground transmission towers and
substations.
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X. EMBEDDED GROUND ROD
About 50% of the resistance between a ground rod and remote
earth is in a shell within the first 6 in.from the rod. If this
shell is shorted out by encasing the rod in 6 in. of conductive
cement, as shown inFigure 5, the resistance is halved. This is a
good example of how the resistance of any electrode can bedecreased
without making the electrode longer. This is important wherever
bedrock limits the length ofground rods or when property lines
limit the length of a horizontal electrode.
CONNECTINGWIRE
R=.E!-[ In ',-In '.] +..!.. [ In 4L -1 -In r.2nL 2nL 7 FT, IN
12-IN. DIA
AUGERED HOLE(6 -IN. RADIUS)
POURED CYLINDER OFCONDUCTIVE CEMENT
NOTE:Po (CEMENT)=20 C1-cmp,=SOIL RESISTIVITY INr.= RADIUS OF ROD
= O.79cmr,= RADIUS OF CEMENT = 15.23cmL = LENGTH = 244cm
~
1ONE FOOT OF
8-FT. RODDRIVEN INTO SOil
NOT DRAWN TO SCALE
~
d = 1.6 cm for 5/8 in. rod
Figure 5. This embedded ground rod takes advantage of the fact
that 50 percent of the earth resistance iswithin 6 inches of the
rod.
REFERENCES
Positron Industries, Inc., Teleline Isolator Product Guide,
Montreal, Quebec, Canada, 1999.Grounding Systems Co., Application
Note TD-l, Ground Grid Improvements and Extensions, ChagrinFalls,
OR, 1999.C. L. Hallmark, Horizontal Strip Electrodes for Lowering
Impedance to Ground, INTELEC 97Proceedings, Sec. 17-2, pages
368-375.Gilbert Sharick, Grounding and Bonding, Vol. 13 of abc
TeleTraining Basic Series, abc TeleTraining,Geneva, IL, 1999.