The Tagg slope method is a variant of fall of potential for large grids. It lets you measure distances from a convenient point, such as the edge of the grid, without knowing its electrical center. With three readings it calculates a slope variation coefficient, and a table indicates where to place the potential probe.
Short answer. The slope method was described by G. F. Tagg and is included in IEEE Std 81-2012 (clause 8.2.2.4, table 5). It solves a problem of fall of potential on large grids. The 62 % rule needs to know where to measure distances from, and on an extensive grid that point, the electrical center, is hard to locate. With three readings and a table, the method lets you measure from a convenient point, such as the edge of the grid.
The problem it solves
In fall of potential, the current probe is placed far from the grid and the potential probe at an intermediate position. For a small grid, those distances are measured from its center. For a large or complex grid, the guide warns that the electrical center may not coincide with the geometric center. The electrical center is the point where most of the test current flows, and it depends on how that current is distributed through the conductors.
Tagg proposed measuring the distances from a convenient point, such as the edge of the electrode system, and introducing the error distances into the fall-of-potential equation. The assumptions are the same as before: soil of uniform resistivity and an electrode system that can be represented as an equivalent hemispherical electrode.
How it works, in general terms
The method relies on three resistance readings, R₁, R₂ and R₃, taken with the potential probe at increasing fractions (0.2, 0.4 and 0.6) of the distance to the current probe. With them the slope variation coefficient is calculated:
μ = (R₃ − R₂) / (R₂ − R₁)
That coefficient describes how steep the curve of readings is. Table 5 of the guide assigns to each value of μ a position of the potential probe, expressed as a fraction of the distance to the current probe (PP_T / CP). With the probe at that position, the measurement that gives the grid resistance is taken. It is a final fall-of-potential reading, but with the location corrected by the real shape of the curve.
How to read the table
Table 5 covers values of μ between 0.40 and 1.59. The higher the μ, the smaller the distance fraction the table indicates:
| μ | Potential probe position (fraction of the distance to the current probe) |
|---|---|
| 0.40 | 0.643 |
| 0.57 | 0.618 |
| 1.00 | 0.542 |
| 1.59 | 0.341 |
In the table, the value 0.618 of the 62 % rule corresponds to μ = 0.57. For other values of μ, the probe position moves away from that fixed rule, and that is the correction the method provides.
Why the origin point matters
In fall of potential, all distances are measured from an origin. For a hemispherical electrode, the origin is the center of the hemisphere. For large systems, some authors introduce the concept of an electrical center. The guide specifies that it depends mainly on the current density profile in and around the grid conductors, unlike the geometric center. An error in the origin shifts the position of both probes and, with it, the reading. The slope method avoids that problem: it starts from a fixed, convenient point and lets the readings themselves indicate where the probe should be.
How it compares with the other variants
| Variant | What it needs to know | Where it is described |
|---|---|---|
| 62 % rule | The center of the grid, to measure from it | Clause 8.2.2.4 and Annex C |
| Flat slope | Nothing: the potential probe is moved until a section of constant readings is found | Clause 8.2.2.4 |
| Tagg slope method | A convenient starting point, such as the edge of the grid | Clause 8.2.2.4 and table 5 |
| Computer-assisted multimeter | The grid configuration and the probe locations, for its model | Clause 8.2.2.7 and Annex E.3 |
With any of them, the guide gives the same underlying advice: place the current probe at a distance where all the mutual resistances are sufficiently small and the curve of readings flattens.
What to know before using it
The method does not remove the assumptions of fall of potential: uniform soil, no buried metallic objects nearby and no significant external connections of the grid. Nor does it make the errors of the measurement itself disappear: stray currents, coupling between cables and probe resistance. The guide on the ground resistance test summarizes the factors that can falsify the result.
If the soil has layers with large differences in resistivity, the result must be compared with another method.
When it is suitable
The slope method is an option when the grid is large, there is no obvious center to measure from, and you want to reduce the uncertainty of the potential probe position. For small electrodes in uniform soil, the 62 % rule is the theoretical reference. For heavily interconnected systems, with shield wires and neutrals, the guide also describes clamp methods and the computer-assisted ground multimeter. If the site is noisy or has many interconnected grounds, see the computer-assisted ground multimeter and the clamp-on measurement. To compare the measured value with the design value, the free grounding resistance tool calculates the theoretical resistance of a grid, and why the calculated and measured values differ explains the differences.
We measure your grounding system
At Tiza Group we measure the impedance of large grids in substations and plants, in Colombia, or elsewhere on request, with the method your installation allows, and we deliver the readings, the curve and their interpretation. 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
- Ground measurement errors: stray currents and coupling
- Computer-assisted ground multimeter
- Clamp-on ground measurement, no stakes: limits
- Ground resistance: what the test measures and IEEE 81
References
- IEEE Std 81-2012, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System: clause 8.2.2.4 (slope method) and table 5 (slope method coefficients).
- Tagg, G. F., slope method, cited in the bibliography of IEEE Std 81-2012.
Frequently asked questions
What is the slope method for if the 62 % rule already exists?
The 62 % rule requires knowing the point from which the distances are measured, which in a large grid is its electrical center, hard to locate. The slope method lets you measure from a convenient point, such as the edge of the grid, and build the error distance into the fall-of-potential equation.
Does the slope method work in any soil?
No. It keeps the assumptions of fall of potential: soil of uniform resistivity and a grid that can be represented as an equivalent hemispherical electrode. If the soil has very different layers or there are significant external connections, the result may drift away from the true impedance.
What does the slope variation coefficient indicate?
It is the ratio between the difference of the third and second readings and the difference of the second and first readings. It describes how steep the curve of readings is, and the table in the guide converts it into the position of the potential probe as a fraction of the distance to the current probe.