**General
Description**

The Smart Ground
Multimeter (SGM) is a computer controlled multi-function instrument
for grounding system measurements. It consists of hardware (Shown above) and software (WinSGM). For a detailed
description of the SGM hardware and accessories click on the **Components**
button. The WinSGM software and the various measurement functions are
described here.

**The
WinSGM Software**

The **WinSGM**
software controls the operation of the Smart Ground Multimeter. It is
capable of modeling the grounding system under study and correlates
the measurements obtained by the SGM hardware (shown above) for the
purpose of identifying the parameters of the grounding system. WinSGM
can be installed on any *Windows*™ personal computer. (Windows
98, NT, ME, and XP operating systems are supported). WinSGM has a comprehensive**
graphical user interface **that
allows setting of the measurement parameters, and provides detailed
reports of the measurement process and the analysis results.

The present version
of the WinIGS software provides 10 measurement functions:

1. Ground
Impedance Measurement

2. Soil Resistivity Measurement

3. Tower Ground Resistance Measurement

4. Touch Voltage Measurement

5. Step Voltage Measurement

6. Ground Mat Impedance Measurement

7. Transfer Voltage Measurement

8. Low Impedance/Continuity Measurement

9. Fall of Potential Method Measurement

10. Oscilloscope Function

These measurement
functions are described below.

**1.
Ground Impedance Measurement**

The ground impedance
measurement function can be applied to any existing grounding system,
of an energized or de-energized facility. It measures the ground impedance
of the system consisting of all interconnected grounding electrodes
as well as nearby grounds connected via shield and neutral wires. The
principle of operation of this function is illustrated in Figure 1.
The SGM generates an alternating polarity voltage across the black and
red terminals. The black terminal is connected to the ground under test
and the red terminal is connected to an auxiliary ground probe. As a
result, a current circulates between the ground under test and the auxiliary
electrode. The circulating current creates a potential distribution
on the soil. The SGM samples soil potential at six locations via six
voltage probes installed at six locations in the vicinity of the ground
under test. In addition, the SGM monitors the injected electric current.
The raw measurement data consists of the ground potential differences
(GPD) between the grounding system under test and the six voltage probes
due to the current injected by the SGM. From these data, the impedance
of the grounding system under test is extracted using estimation methods
and error correction techniques. The measured impedance of the grounding
system is obtained as a function of frequency. The estimated impedance
is reported in a plot of impedance magnitude and phase versus frequency.

**Figure
1**

Note that the measured ground impedance is the combination of the impedance
of the grounding system under test, in parallel with the impedance to
ground of all shield wires, neutral wires and other grounded metallic
structures connected to the grounding system under test. An estimation
algorithm "fits" the measurements to the model of the grounding
system, shield wires, neutrals, etc. No knowledge of the type and number
of shield wires, neutrals, etc. is required. The same estimation procedure
also provides statistical characterization of the measurement accuracy.
Specifically, the expected error of the measurement versus the confidence
level is computed and displayed. The statistical analysis is possible
because, a large number of data is acquired for every measurement (approximately
70,000 data points, using default settings).

**2.
Soil Resistivity Measurement**

The soil resistivity
function is based on an extension of the four-pin method. Specifically,
the SGM is capable of taking simultaneous measurements on nine probes
uniformly spaced along a line on the soil surface. The measurement arrangement
is illustrated in Figure 2. The measurements obtained from the nine
pins are processed by error correction and estimation algorithms in
order to construct a two-layer soil model. The resistivities of the
two soil layers and the upper soil layer thickness are reported. Soil
resistivity measurements are also characterized by an expected error
versus confidence level. For soil resistivity measurements the meter
displays the confidence level for a dynamically selected error bound
(based on the particular case). The results are displayed in both tabular
and graphical form.

**Figure
2 Standard Probe Arrangement for Soil Resistivity Measurements**

(Standard Arrangement at 10 feet Separation Distance)

Soil in general exhibits complex variations of resistivity even within
a relatively small area (i.e. several hundred feet). Yet, for design
purposes we need a soil model that is manageable. In addition, the soil
model must be measurable. The two-layer soil model meets both of these
requirements. More complex models can be constructed, for example comprising
three or more soil layers. However, it is practically difficult to accurately
measure the various layers with probes placed on the surface of the
earth. Extraction of a multi-layer soil model (more than two layers)
from these measurements is characterized by very high uncertainty. This
means that the results are not reliable. Boring samples can provide
this information if the number of samples is statistically significant.
This means that a large number of borings must be made for a meaningful
measurement, resulting in a relatively high measurement cost.

**3. Tower Ground Resistance Measurement**

The tower ground
resistance meter option measures the ground resistance of an energized
or de-energized transmission line tower. Shield wires may be connected
to the tower ground during measurements. A typical measurement arrangement
is illustrated in Figure 3. The SGM injects a current into the tower
ground and measures the ground potential differences (GPD) between the
tower ground and six voltage probes installed around the tower. These
data are used to compute the parallel combination of the tower ground
resistance and the impedance looking into the transmission line shield
wires. An identification algorithm determines the contribution of the
shield wires and eliminates this contribution from the measured total
impedance. The algorithm is based on the observation that the shield
wire impedance is predominantly reactive while the tower ground impedance
is resistive. Furthermore, both shield wire resistance and reactance
are functions of frequency. The algorithm takes advantage of this frequency
dependence to extract the tower ground resistance from the measurement
data. Note that no knowledge of the shield wire parameters is required.
The algorithm also provides statistical measures of the measurement
accuracy, which is expressed in terms of expected error versus confidence
level. Statistical analysis is possible since the meter acquires a large
number of data (about 70,000 voltage and current samples are processed).
The reported results consist of the tower ground impedance plotted as
a function of frequency and the measurement error versus confidence
level.

**Figure
3. SGM Setup for Tower Ground Resistance Measurements.**

For clarity of the illustration, only the blue voltage probes are shown.

**4. Touch Voltage Measurement**

This function measures
the actual touch voltage at a substation as a function of the fault
current. The measurement is performed at up to six points near a grounding
system. A typical measurement arrangement is illustrated in Figure 1.4.
A required input for this function is the fault current available at
the location of the grounding system. The data analysis takes into consideration
the effect of the proximity of the current return electrode on the measured
touch voltages. For this purpose, additional required input data are
the coordinates of the voltage probes, current return electrode and
the approximate size and shape of the grounding system.

**Figure
4. Typical Probe Arrangement for Touch Voltage Measurements**

**5. Step Voltage Measurement**

This function measures
the actual step voltage at a substation as a function of the fault current.
The measurement is performed at a user-selected point near a grounding
system. A typical measurement arrangement is illustrated in Figure 1.5.
Required input data are the fault current available at the location
of the grounding system, the coordinates of the voltage probes, current
return electrode, and the approximate size and shape of the grounding
system.

**Figure
5: Typical Probe Arrangement for Step Voltage Measurements**

**6. Ground Mat Impedance Measurement**

This function measures
the ground mat impedance without disconnecting the shield or neutral
wires which may be bonded to the ground mat. Setup and connections are
similar to the ground impedance measurements. A typical measurement
arrangement is illustrated in Figure 1.6. The measurement provides the
ground mat impedance as a function of frequency over a user selected
frequency range.

**Figure
6: Illustration of Probe Arrangement for Ground Mat Impedance Measurements**

**7. Transfer Voltage Measurement**

This function measures
the transfer voltage at a user-selected point or ground near the grounding
system under test as a function of the fault current. A typical measurement
arrangement is illustrated in Figure 1.7. Required input data are the
available fault current at the location of the grounding system, the
coordinates of the current return electrode, and the approximate size
and location of the grounding system.

**Figure
7: Illustration of Probe Arrangement for Transfer Voltage Measurements
to a Nearby Fence**

**8. Low Impedance/Continuity Measurement**

This function measures
the impedance between any two user selected points of a grounding system.
The measurement can be performed on energized grounding systems. Accurate
results can be obtained even in the presence of substantial external
electromagnetic noise. A typical measurement arrangement is illustrated
in Figure 1.8. This function requires a current limiting resistor inserted
in series with the red electrode. Note that the calibration and testing
kit can be used for this purpose.

**Figure
8 Illustration of SGM Setup for Low Impedance/Continuity Measurements
Using the Testing and Calibration Unit.**

**9. Fall of Potential Method Measurement**

This function allows
using the SGM to perform a ground impedance measurement using the fall-of-potential
method. The required setup for this function is the standard fall-of-potential
probe arrangement. Specifically, the current probe is placed at a distance
D from the center of the system under test. A typical measurement arrangement
is illustrated in Figure 1.9. The distance D must be at least four to
five times the longest dimension of the system under test. One or more
voltage probes are placed at a distance from the center of the system
under test equal to 0.62 times D. The meter displays the ratio of the
probe voltage (with respect to the grounding system under test over
the injected current.

**Figure
9. SGM Setup for Ground Impedance Measurement Using the Fall of Potential
Method**

**10. Oscilloscope Function**

The oscilloscope
function allows the SGM to be used as a general purpose six channel
waveform data acquisition system. This function can be used to measure
existing voltages on grounding systems due to induced voltages, imbalances,
kathodic protection equipment, etc. Therefore, this function can be
an excellent tool for investigating various electromagnetic interference
problems.

The utilization
of the oscilloscope function is quite simple. It involves the following
steps:

" Install the
voltage probes (yellow and green assemblies) and connect them to the
desired locations to be monitored.

" Connect the green terminal to the voltage reference point (typically
to ground).

" Setup and connect the Smart Ground Multimeter and computer via
the RS232 (serial port) cable.

" Turn on the SGM power switch, turn on the personal computer and
execute the SGM program.

At this point, the
existing voltages at the monitored points will be continuously displayed
on the PC screen. To store the displays, follow the instructions provided
in the section where this function is described in detail.

See Also Hood
Patterson & Dewar

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