The Smart Ground Meter Measurement Functions

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.

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