Fall-of-Potential Method and the 62 % Rule Explained

Short answer. The fall-of-potential (FOP) method measures the impedance of a grounding system with respect to remote earth. A current is circulated between the grid and a distant current probe, and the voltage is measured between the grid and an intermediate potential probe. The 62 % rule is the theoretical position of that probe for uniform soil. IEEE Std 81-2012 (clause 8.2.2.4 and Annex C) explains the method and its limits.

The principle

The injected current leaves through the grid, crosses the soil and returns through the current probe. Close to the grid, the soil potential drops quickly with distance; far from it, it hardly changes. Between the two zones there is a point where the measured potential, divided by the current, equals the true impedance of the grid. Finding that point is the goal of the method.

So that the mutual resistances between electrodes do not distort the result, the current probe is placed far from the grid. The guide gives a typical distance of at least five times the largest dimension of the electrode under test. The potential probe is usually placed in the same direction as the current probe, but it can go in the opposite direction.

The 62 % rule

With the potential probe in the same direction as the current probe, it is common to place it at a distance X equal to 62 % of the distance to the current probe. That position is not empirical: it comes from theory. In Annex C, the guide shows that the measured resistance is the true impedance of the electrode plus a term that depends on the probe positions. The reading is exact when that term vanishes. With hemispherical electrodes in uniform soil, the potential at a distance is inversely proportional to it, and the condition has a positive root at x = 0.618 · D, where D is the distance to the current probe. This is the 61.8 % rule, attributed to Curdts.

Other results from the same annex:

• If the potential probe is placed beyond the current probe, the root is x = 1.618 · D.
• If it is placed behind the grid, on the side opposite the current probe, the equation has only complex roots: there is no exact position.
• If the electrode and the current probe are identical and D is large, the exact position is halfway, x = D/2.

What it depends on

Clause 8.2.2.4.1 summarizes three conditions for applying the 62 % rule:

1. A reasonably uniform soil.
2. Large distances between the grid and the reference electrodes, so that all of them can be taken as hemispherical.
3. That the electrode under test has no external connections to ground.

The origin from which the distances are measured must also be fixed. For a hemispherical electrode, it is its center. For large systems, some authors introduce the concept of an electrical center, which depends on how the current is distributed in the conductors and may not coincide with the geometric center of the grid. With a complex grid, or with a substation with interconnected shield wires and neutrals, that representation is very difficult to obtain.

When the soil has two layers, the exact position changes with the soil structure. The guide illustrates this in its figure 8, applicable to small systems, where the required position depends on the ratio of the resistivities and the thickness of the upper layer.

The impedance versus position curve

Since the exact position is hard to know, the guide proposes looking at the curve of readings as the potential probe is moved between the grid and the current probe. A practical way to know whether the probe is free of the influence of other electrodes is to take several readings at different positions. Two or three consecutive constant readings can be taken as the true resistance (flat slope method).

With the probe in the same direction as the current probe, the curve starts at zero near the grid and tends to infinity near the current probe. With the probes in opposite directions the curve is different: it approaches the former from below, but needs greater separation, and the variations due to non-uniform soil are larger. In an ideal situation, the curve has a clear inflection point that approaches the impedance of the electrode. With non-uniform soils, the curves may have no zero-slope sections.

When the value comes out much higher than expected

The test measures the resistance between a point on the grid and remote earth. If the value is much higher than expected, the test leads may be connected to a conductor that is poorly bonded to the grid. In that case the measurement is repeated at other points: if the values change noticeably, the grid may be damaged and an integrity test is advisable. That test can also be done before the fall-of-potential test.

Advantage and limit

The main advantage of the method is that the probes can have a resistance considerably higher than the earth under test without significantly affecting the measurement. The main limitation, according to the guide, is that it does not always deliver a true impedance value. An exact measurement is obtained only when the system can be represented as an equivalent hemisphere, with an electrical center from which to measure distances. Many isolated grids of simple geometry meet that. A large substation with shields and interconnected neutrals does not.

For an overview of what else the ground test measures, see the guide on the ground resistance test. For the design of the grid, the guide on the grounding grid. The free grounding resistance tool calculates the theoretical resistance of a grid, useful to compare with what was measured. When the grid is large and its electrical center is not known, the Tagg slope method corrects the probe position. With many grounds in parallel, the clamp meter or the computer-assisted ground multimeter are alternatives; for small electrodes, the two-point and three-point methods are enough. The errors that distort the curve are in Ground measurement errors.

We measure your grounding system

At Tiza Group we measure the impedance of grids and electrodes in substations, industrial plants and solar plants, in Colombia, or elsewhere on request, with the method your installation allows, and we deliver the result with the curve of readings and its 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

• Slope method for large grids (Tagg)
• Ground measurement errors: stray currents and coupling
• Measuring a ground with two or three points: when it works
• Calculated and measured ground resistance: why they differ
• 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: clauses 8.2.2.4 and 8.2.2.4.1 (figures 6, 7 and 8) and Annex C (C.1 to C.6).
• IEEE Std 80, IEEE Guide for Safety in AC Substation Grounding.

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