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MILHDBK419A
Measure the resistance to earth of the earth electrode subsystem using the fallofpotential
f.
method. (For a detailed description of the principles of this technique see Volume I, Section 2.7.2.)
(1) Connect the terminals marked C1 and P1 of the test set together and connect them to the
electrode under test.
(2) Determine the maximum dimension of the earth electrode subsystem. For a circle, this
dimension is the diameter; for a square or rectangle, the dimension is along the cornertocorner diagonal; for
other shapes, select the longest lateral distance across the system. Locate the current probe, C2, of the earth
tester, at a distance equal to or greater than five times the longest dimension from the point of connection to
the electrode under test. Earth testers are typically supplied with approximately 30 meters (100 feet) of
interconnecting wire. For all but the simplest earth electrode subsystem, 30 meters (100 feet) is insufficient
spacing of probe C2. The 30meter (100foot) spacing may be used for one or tworod systems where the rods
are not more than 6 meters (20 feet) long nor spaced more than 6 meters (20 feet) apart. Thus, in most cases,
additional wire will be needed for connecting the test instrument to the C2 probe and the the potential probe,
P2. For distances up to 300 meters (1000 feet), insulated No. 16 AWG or No. 18 AWG wire may be used.
(3) Position the C2 probe along a line which maximizes the distance from the electrode under test
and from other buried metals such as utility pipes, power and signal cables, fuel tanks, sewers, etc. In choosing
the direction for placement of the C2 probe, examine the configuration of the earth electrode subsystem for
the facility and determine the location of all such buried metals. Then locate the probe as far as possible from
all these metals as illustrated in Figure 23.
(4) Next position the potential probe P2 along a straight line between the C2 probe and the point
of connection to the electrode under test. Insert both P2 and C2 probes into the soil to a depth of between
10 cm (4 inches) and 30 cm (12 inches). (In the event that the probes supplied with the tester by the
manufacturer are not available, make a set of probes following the instructions given in Chapter 1, Section
1.2.1.1.) Take the first resistance reading, following the instructions supplied with the tester, with probe P 2
located about 20 percent of the distance X between the electrode and C2 Repeat the measurement at the 0.4,
0.5, 0.6, 0.7, 0.8, and 0.9X points. Record the C2 distance and the resistance measured at each P2 location on
the worksheet shown in Figure 24. (Figure 25 is an example of a typical worksheet that has been completed.)
(5) The resistance of 0.5X and 0.7X should be within 10 percent of the value at 0.6X. If these
readings do not fall within 10 percent of the reading at 0.6X, either measure along a line in a different direction
or move the position of C2 farther away from the electrode under test. If locating C2 equal to greater than
five times the largest dimension is impractical, perform a series of tests for at least five C2 locations out to
the maximum practical distance. The resistance of the subsystem must then be determined graphically, using
the method for extensive electrode subsystems described in Volume I, Section 2.7.2.2. (An illustrative example
is also shown in that section.)
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