The computer-based grounding multimeter is a modern version of fall of potential. It injects current pulses, measures voltage at six probes, filters the noise and estimates the impedance by least squares. It allows measuring without isolating the grid from other grounds and with the current probe closer in. It also measures step and touch voltages and the continuity between down conductors.
Short answer. The computer-based grounding multimeter applies the idea of fall of potential with digital measurement. It injects current pulses into the grid, measures the potential differences at six voltage probes, filters the noise and estimates the impedance through a statistical calculation. IEEE Std 81-2012 describes it in clause 8.2.2.7 and Annex E.3, and also uses it for touch voltages and to verify the continuity between down conductors.
What the setup looks like
The test uses one current return electrode and six electrodes that measure voltage. These are connected on two triaxial cables, each with three electrodes. The return electrode is placed at a distance of at least two times the largest dimension of the substation. The first voltage probe of each cable usually goes 15 m from the fence, and the distances of the others are set automatically from it. For greater accuracy, the probes are placed as far as possible from other grounded structures, such as pole grounds or pipes.
According to Annex E.3, the typical equipment has a computer and a variable-frequency current source (from 0 Hz to 2500 Hz) able to deliver between 2 A and 20 A. It adds a multichannel data acquisition system with its probes and cables.
What it does with the signal
The source injects continuous white-noise-type pulses, normally for 0.5 s, between the grid and the return electrode. The current and the soil potential differences at the six probes are measured. The program then:
- Filters the noise.
- Corrects the errors of the voltage and current transducers.
- Estimates the electrode impedance and a soil factor by solving a system of 2 by 6 equations with weighted least squares.
- Displays on screen the impedance, with magnitude and phase angle, versus frequency.
Before the test, the grid configuration and the probe locations are drawn with an editor in the program. The program then compares the modeled voltage gradients with the recorded ones and runs a statistical analysis to find the resistance that best fits the grid model to the test data. That is why the guide warns that an exact model of the grid and of the probe positions is essential.
What can be measured with it
The guide lists the options this type of equipment offers:
| Option | What it delivers |
|---|---|
| Ground impedance | Of an isolated or interconnected system: tower, pole or substation |
| Soil resistivity | With the same probes |
| Tower impedance | Of an isolated tower or pole, without disconnecting the shield wire or the neutral |
| Touch and step voltage | With the probes at the points of interest |
| Grid impedance | Of a substation grid without disconnecting shield wires or neutrals |
| Transferred voltage | With the same setup |
| Low impedance and continuity | Integrity of the grid conductors between down conductors |
In touch voltage mode, the current injection and the measurement of the six potential differences are the same; only the location of the probes changes, to the desired touch points. Each result consists of six touch voltages normalized to the substation fault current, which the user enters into the program before starting. To gain accuracy, the algorithm compensates for two critical variables. One is the proximity of the current return electrode. The other is the number of tower grounds connected to the grid or, more exactly, the total impedance of the other grounds bonded to it.
In continuity mode (clause 10.4), the current leads go between the reference down conductor and the down conductor under test, with a limiting resistor of about 50 Ω. The voltage between the two is measured with the voltage leads. The result is the complete characterization of the impedance (R + jX) between the two down conductors.
Advantages and limits according to the guide
Advantages (Annex E.3):
- The grid does not need to be isolated from other ground sources.
- The current electrode can be placed at a minimum of two times the grid diagonal, compared with the five times that traditional fall of potential requires.
- The data are less sensitive to interference, harmonics and induced voltages.
Limits:
- An exact model of the grid and the probes is essential for a good result.
- Metallic paths above or below ground can have an influence.
- Non-homogeneous soils moderate the accuracy.
Clause 8.2.2.7 adds that the method has limitations similar to those of fall of potential in other respects. The distances of the voltage and current probes are measured from an assumed electrical center, and for that reason the impedance of an interconnected system is not determined exactly. The measured value can vary with the location of the probes. In return, the method delivers an error range along with the impedance value, and maintains that the true value lies within that range.
When it is suitable
The method is useful when there is a lot of electrical noise, when the grid is connected to shield wires and neutrals that cannot be disconnected, or when there is no room to move the current probe so far away. For a rod or a small ground, the two-point and three-point methods or classic fall of potential may be enough. An example of equipment of this type, for a substation ground: Metrel MI 3290 ground analyzer. The other variants of impedance measurement are the two-point and three-point methods, the clamp-on measurement and the Tagg slope method. The errors the instrument must filter are in Ground measurement errors.
We measure your grounding system
At Tiza Group we measure the impedance of your grid, the step and touch voltages and the continuity between down conductors, in Colombia, or elsewhere on request, with the method your substation allows, and we deliver the interpreted result. See the testing and commissioning service or write to us through contact. In the quotation we recommend the test and the equipment your case needs.
Related guides
- Fall-of-potential method and the 62 % rule explained
- Clamp-on ground measurement, no stakes: limits
- Measuring step and touch voltages in a substation
- Ground measurement errors: stray currents and coupling
- Metrel MI 3290 ground analyzer: GP, GL, GF and GX sets
References
- IEEE Std 81-2012, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System: clauses 8.2.2.7 (computer-based multimeter), 9.4.4 (step and touch voltages), 10.4 (continuity) and Annex E.3.
- IEEE Std 80, IEEE Guide for Safety in AC Substation Grounding.
Frequently asked questions
What advantage does it have over classic fall of potential?
According to IEEE 81, the grid does not need to be isolated from other ground sources, the current electrode can be placed at a minimum of two times the grid diagonal instead of five, and the data are less sensitive to interference, harmonics and induced voltages.
Is the result of the computer-based multimeter exact?
It improves accuracy, but it has limits. It requires accurate modeling of the grid and the probe positions, and it is affected by metallic paths above or below ground and by non-homogeneous soils. The method delivers an error range along with the value and states that the true value lies within that range.
Does it only measure ground impedance?
No. Besides impedance, the guide lists soil resistivity, tower impedance without disconnecting the shield wire, touch, step and transferred voltages, grid impedance without disconnecting neutrals, and the continuity test of grid conductors.