Tech Tips
Ground Grid Integrity
by Jeff Jowett
Megger
T
he testing of grounding electrodes…grids, meshes, and the like…is most
often considered in terms of a resistance test. That is to say, the resistance
of the surrounding environment to current flow to some arbitrary or
carefully plotted point typically referred to as remote earth or infinite earth. The
ground grid is intended to serve the dual purpose of carrying currents into the
earth without exceeding the operating tolerances of any protected equipment
while assuring that personnel in the vicinity are not exposed to electric shock as
would result from excessive step or touch potentials. Resistance tests indicate
the overall capability of the grid in this regard: its electrical relationship to its
environment. But there remains the question of the internal condition of the
grid itself.
Out of sight, out of mind? Buried under ground, the grounding electrode
doesn’t call attention by mere visual inspection, which is the first step in most
electrical maintenance. Though they may seem inert, grounding electrodes are
subject to their own unique set of stresses, just like other electrical equipment.
Fault clearance and lightning protection can severely damage a grid or mesh,
separating individual elements, interrupting continuity, and introducing high
resistance across bonds. But in the meantime, the electrode may have cleared the
fault perfectly well, leaving no obvious indication that it has been compromised.
A subsequent event may not be afforded the same level of protection.
Furthermore, a less dramatic but more persistent force of deterioration is
the incessant process of corrosion and weather. Freezing and expansion exert
pressures that can break apart a grid. Ironically, the best grounding soils are also
the most corrosive. Low resistivity soil that facilitates the flow of fault current
also promotes electrolytic current that eats away at the metallic structure of a
grounding electrode. Use of dissimilar metals hastens the process. Rods have been
known to last as little as two years, with typically a risk of at least some corrosion
effects being present after four.1 A standard ground resistance test just looks at
voltage drop across the surrounding soil and gives no measure of the physical
condition of the electrode itself.
www.netaworld.org The grounding electrode typically is
carrying only noise, but must be able
to accommodate worst case conditions
of high current flow when called on
line during an event. Therefore, to test
grid integrity, the tester must be able
to produce high current. A grid tester
works similarly to a ground tester in
that it supplies current and measures
voltage drop across the test item. In
this case, the test item is the grid,
whereas in a ground test, it includes
the surrounding soil. It is dissimilar in
that the grid tester typically employs
an industry standard of 300 amperes,
whereas a ground tester operates on
the milliampere level. Rather than
calculating and displaying resistance,
the grid tester evaluates the change in
current flow.
Test equipment consists of a variable current source requiring on the
order of a 10.5 kVA capability, operated from a 50 ampere, 240 Vac source.
Test leads can range anywhere from
10 to 100 feet of 2/0 welding cable. A
reference ground is first established,
preferably a transformer neutral. The
leads are connected, one to the test
ground and the other to the reference
ground below any bonding connections (Fig. 1). The tester is then energized and adjusted to pass 300 amperes
via the reference ground through the
grid under test for a duration of three
minutes.
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C
A
D
B
Measurements after Test Hook-Up
Figure 1
A clamp-on ammeter is then used to measure current
at critical points around this system: through the reference
ground above (A) and below (B) the attachment of the test
lead and on the opposite side of the system through the test
ground both above (C) and below (D) the lead connection.
Current readings are recorded, and the tester indicates
voltage drop across the system. Voltage drop of the leads
themselves is also measured. This is done by disconnecting
from the test item, shorting the leads together, and passing
300 amperes for three minutes, noting the voltage. This value
is then subtracted from the voltage drop taken during the
test to isolate voltage drop across the grid from the lead
contribution. For an indication of acceptable continuity, a
value of no more than 1.5 volts per 50 feet of straight line
ground path should be measured. The straight line ground
path is the distance between the two lead connections.
Though valuable, this method is not rigorously precise,
and so a redundant system of evaluation exists based on
current return. For single driven electrodes, at least 200
amperes should return to the source via the ground path.
For mats and grids, at least half of the current must return
via the ground path. If not, it indicates a potentially bad
connection and should be dug up for repair.
What is the method that makes this procedure successful? What happened to Kirchhoff ’s laws? Kirchhoff ’s first
law states that the sum of the currents flowing from a point
in a circuit equals the current flowing to that point, i.e.,
current is a precisely measurable quantity that doesn’t just
“disappear into thin air.” Operation of the high-current grid
tester is based on an application of Kirchhoff ’s first law to
NETA WORLD Fall 2008
account for all the current that is injected into the system.
By injecting a substantial amount of current, it becomes
comparatively easy to note its division along stress lines. It
is expected that most of the current will follow the shortest, straightest path (least resistance) between the two test
points. The ammeter readings indicate to what extent this
is occurring. Discontinuity or high resistance connections
anywhere between the test points will divert proportionate
amounts of current through the rest of the system.
Substations are multiply-bonded into a Faraday cage
configuration, and other facilities with complex or extensive
grids are also typically connected to the electrical system
at multiple points. Therefore, it cannot be presumed that
all current is flowing in a particular path. Current flowing
from the tester must first be measured for any diversion into
the system (point A, Fig. 1), and to determine the amount
flowing into the grid (B). This value is then compared to
the amount returning through the test ground (D), and
that which is diverted through parallel paths into the rest
of the system (C).
To illustrate, an example of an acceptable test is shown
in Fig. 2. Pretest conditions indicate typical values of current flowing on the system. The distance between the test
connections is measured, and the voltage drop across the
leads is taken from the tester. Performance of the test then
indicates 270 amperes flowing into the grid, with some
diversion through the reference ground back into the electrical system. The ground connection being measured then
shows 280 amperes returning (test current enhanced by
some “noise” on system). Since this is a grid, the industry
standard calls for at least a 150 ampere return, so this is well
exceeded. Voltage drop across the test was measured at 7.9,
but as 7.5 of this was lead resistance, only 0.4 volt is across
the tested path. This falls within the allowance for 1.5 volts
per 50 feet (1.5/50 x 15 = 0.45). The tested ground path
passes both criteria with acceptable values.
A failed test is outlined in Fig. 3. Here, only a negligible
amount of current returns through the tested ground connection, while 280 amperes flow through building structure
via a parallel connection. Voltage drop calculates to 8.1 (15.6
– 7.5), which fails the requisite criterion (100 feet allows 2
x 1.5 = 3 volts). If the test setup were switched to the other
leg of the structure, results would be essentially reversed, so
the ground connection on the left would have to be dug up
and inspected for a fault in continuity.
Similarly, ground cables, clamps and ferrules can be
tested prior to installation using the same equipment and
parameters. Cable manufacturer’s specifications should
provide proper voltage drop. For instance, 300 amperes
on 100 feet of cable yields 30,000 ampere-feet. For 4/0
bare copper, the voltage drop should be 4.1 volts. For a 10
foot section, therefore, the voltage drop would be 0.41. If
manufacturer’s guidelines are not available, the following
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1.
2.
3.
4.
Measurements before Test Hook-Up
Hook-Up and Connections
Figure 2
Figure 3
Measurements before Test Hook-Up
Current in transformer neutral (reference) = 82 amperes
Current in the post ground wire = 6 amperes
Distance between reference ground and post ground = 15 feet
Voltage drop in test leads with 300 amperes = 7.5 volts
Measurements after Test Hook-Up and with 300 Amperes Flowing
in the Test Circuit (i.e., test set meter reading)
1.
2.
3.
4.
5.
Current flow in reference ground to grid = 270 amperes
Current flow in reference ground to T(X) = 50 amperes
Current flow from grid to post ground = 280 amperes
Current flow from the structure to post ground = 1 ampere
Voltage reading at the test set meter = 7.9 volts
formula can be used to get an approximation of voltage
drop, bearing in mind that manufacturer’s specifications
are always preferable:
V = (2 x I x L x R)/1000 where,
I = test current
L = length
R = resistance per 1000 feet
Specific code requirements are not in effect, but standards exist that provide guidelines for grid testing. Notably,
NFPA70E-1983, Part I, Chapter 2, Section F, Item 4 outlines low-impedance continuity, and Part III, Chapter I,
Section B, Item 1 calls for continuous maintenance. OSHA
has adopted this as a safety requirement, and IEEE 81
references testing of grid structure. By this method, each
ground connection around a substation or other facility can
be tested. Faults are not precisely pinpointed, but by isolating
www.netaworld.org 1.
2.
3.
4.
Measurement before Test Hook-Up
Current in transformer neutral (reference) = 82 amperes
Current in frame ground = none
Distance between reference ground and frame ground = 100 feet
Voltage drop of the test leads with 300 amperes = 7.5 volts
Measurement after Test Hook-Up and with 300 Amperes Flowing
in the Test Circuit (i.e., test set meter reading)
1.
2.
3.
4.
5.
Current flow in reference ground to grid = 270 amperes
Current flow in reference ground to T(X) = 50 amperes
Current in frame ground from grid = 2.5 amperes
Current flow in frame ground from the structure = 280 amperes
Voltage reading at the test set meter = 15.6 volts
a faulty current path, the work of excavation and repair is
markedly reduced.
Lyncole XIT Grounding
Electrical Equipment Testing and Maintenance, A. S. Gill, Prentice
Hall
1
Jeffrey R. Jowett is Senior Applications Engineer for Megger in Valley
Forge, Pennsylvania, serving the manufacturing lines of Biddle®, Megger®, and Multi-Amp® for electrical test and measurement instrumentation. He holds a BS in Biology and Chemistry from Ursinus College.
He was employed for 22 years with James G. Biddle Co. which became
Biddle Instruments and is now Megger.
Fall 2008 NETA WORLD