The two-point and three-point methods measure the resistance of a small electrode using other electrodes as auxiliaries. The two-point method suits a pass/fail test; the three-point method obtains the value by solving three measurements. Both fail if the auxiliaries have high resistance or are too close. Staged fault testing is another family: a real or reduced fault produced on purpose.
Short answer. In the two-point and three-point methods, the resistance of the electrode under test is measured against other electrodes used as auxiliaries. They work on small grounds, such as a rod, a pole ground or a small low-voltage substation. With large, complex electrodes, IEEE Std 81-2012 prefers other methods. Staged fault tests are different: they measure with real or reduced fault currents.
Two-point method
The resistance of the electrode under test is measured in series with an auxiliary electrode. The resistance of the auxiliary is assumed to be negligible compared with that of the electrode, so that the measured value represents that of the electrode under test.
A typical use is measuring the resistance of a driven rod with respect to a nearby house. The ground of a house usually has low impedance because of its connection to the neutral of the supply network, and it serves as an auxiliary with reasonable accuracy. The guide points out two limitations. The method can have large errors on low-resistance grounds, because the auxiliary is no longer negligible. And if the electrodes are very close, the mutual resistance between them also falsifies the value. Even so, it is useful where only a pass/fail test is needed.
Three-point method
Two auxiliary electrodes are used, with resistances r₂ and r₃. The electrode under test has resistance r₁. The resistance between each pair of electrodes is measured: r₁₂ = r₁ + r₂, r₁₃ = r₁ + r₃ and r₂₃ = r₂ + r₃. Solving the three equations, the resistance of the electrode under test is:
r₁ = (r₁₂ − r₂₃ + r₁₃) / 2
This is equation (8) of the guide. The result is obtained with three measurements and a calculation, without needing to know the resistance of the auxiliaries.
The limitations are clear. If the auxiliaries have a resistance considerably higher than that of the electrode under test, the errors of each measurement are magnified in the final result. The electrodes must be far enough apart that the mutual resistances are small: with insufficient distances, absurd results can appear, such as zero or negative resistances. When measuring a rod, the guide asks for the three electrodes to be spaced at least three times the depth of the rod under test, and assumes that the auxiliaries are driven to the same depth or less.
Why they suit only small grounds
Clause 6.2 explains that the two methods have opposite requirements for the auxiliary electrodes. In the two-point method, the error is reduced if the impedance of the test electrode is negligible compared with the ground being measured. In the three-point method, it is reduced if the impedance of the test electrodes is similar to that of the ground being measured. Those conditions limit the methods to grounds of small extent, such as those of a residential swimming pool, a pole or a small low-voltage distribution substation.
As the grounding system grows and becomes more complicated, the three-point method becomes harder to apply. If greater accuracy is needed, the guide prefers other methods, in particular fall of potential, in which the demands on the auxiliary electrodes are less critical.
| Method | Auxiliaries | Suitable for | Main risk |
|---|---|---|---|
| Two-point | One electrode, with negligible resistance | Pass/fail test; rod near an installation with a good ground | Large error on low-resistance grounds; mutual resistance if they are close |
| Three-point | Two electrodes, with resistance similar to that of the electrode under test | Rods and small grounds | Errors magnified with high-resistance auxiliaries; absurd results at short distances |
| Staged fault | A real or reduced fault in the system | Information specific to an installation | Complexity, risk, and the need for recorders and attenuators |
Staged fault tests
A staged fault test is a high-current test, at full or reduced voltage. It is used when specific information about a grounding installation is needed. The goal is to record the voltages between chosen points and the currents in specific circuits. The most practical way to record is to use existing instrumentation, such as the spare channels of fault recorders. Because the magnitudes are high, attenuation circuits are needed: current transformers, voltage transformers or voltage dividers. The voltages to be measured can be estimated beforehand with fall-of-potential tests.
The input impedance of the recording device must be much higher than the resistance being measured. The test circuit can include a high-resistance auxiliary electrode connected to the primary side of a voltage transformer. In that case the recorder is calibrated by inserting a variable voltage source between the transformer and the auxiliary electrode.
Clause 9.4.1 adds that measurements with the full current of a phase-to-ground fault give the most accurate data on step, touch and transferred voltages. That test is rarely done, unless there are other weightier reasons, such as determining circuit parameters, equipment behavior or protection characteristics. It requires special care and advance preparation to be done safely.
How to use this information
If you have a rod, a pole ground or a small ground, the two-point and three-point methods are a first check. For a substation grid or an extensive grounding system, the reference is fall of potential. See the guide on the ground resistance test and the one on grounding of distribution substations. For grounds with many parallel connections, see the clamp-on measurement. If the measured value does not match the design value, why calculated and measured resistance differ summarizes the causes.
We measure your grounding system
At Tiza Group we measure grounding systems of all sizes, from rods and equipment grounds to substation grids, in Colombia, or elsewhere on request, with the method each one allows. 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
- Measuring step and touch voltages in a substation
- Clamp-on ground measurement, no stakes: limits
- Fault recorder (DFR) and PMU: COMTRADE and synchrophasors
- 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 6.2, 8.2.2.1 (two-point), 8.2.2.2 (three-point, equation 8), 8.2.2.3 (staged fault tests) and 9.4.1.
- IEEE Std 80, IEEE Guide for Safety in AC Substation Grounding.
Frequently asked questions
Can I measure the ground of a rod with the two-point method?
Yes, if you have a very low-resistance auxiliary, such as the ground of a house connected to the network neutral. IEEE 81 presents it as a useful test when you only need to know whether the value passes or fails. With low-resistance grounds the error can be large.
Why can the three-point method give zero or negative resistance?
Because each measurement includes the mutual resistance between the electrodes. If they are too close, the system of three equations can give absurd results such as zero or negative resistances. For a rod, the guide asks for the three electrodes to be spaced at least three times the depth of the rod under test.
What is a staged fault test?
It is a high-current test, at full or reduced voltage, in which voltages between chosen points and currents in specific circuits are recorded. It is used when specific information about a grounding installation is needed, and it almost always makes use of the spare channels of a fault recorder.