Potential Map and Integrity of an Installed Grid

Short answer

A built grounding grid is verified with four activities that IEEE Std 80-2013 describes in its clause 19: impedance measurement, the touch and step potential map, the integrity test of the conductors and bonds, and periodic reviews. The standard treats them in general terms and refers to IEEE Std 81 for the detail of the procedures. With them it is confirmed that what was built resembles what was designed and that it keeps doing so over time. The construction process is in grounding grid construction.

Impedance measurement

Carefully measuring the impedance of the installation as built can be desirable, above all when the soil resistivity measurements or the interpretation of the soil model were doubtful. It is not always practical: the grid may be connected to, or influenced by, other buried elements.

The standard highlights two aspects:

• Reactive component. Although we speak of resistance, when the value is below 0.5 Ω and the area is relatively large, the reactive component must be taken into account. With higher impedances it hardly matters.
• Fall-of-potential method. It is the universal one and works for every type of earth resistance measurement, but it presents many difficulties and sources of error on large grids, because of their size, configuration and soil heterogeneity. On very large grids, and especially if the shield wires and neutrals connected to the grid extend its zone of influence, the distances needed may be impractical, and the flat zone of the curve does not appear. In nonuniform soils, the location of the potential probe that gives the correct value can be very different from the usual 61.8 % rule: positions between 10 % and 90 % have been found.

The fall-of-potential method has an advantage: the current and potential electrodes can have a resistance much higher than that of the ground under test without significantly affecting the measurement.

With the measured value the maximum ground potential rise is recalculated. If it differs greatly from the calculated value, the precautions against transferred potentials must be reviewed. The measurement cannot directly check the calculated step and touch voltages, because those come from the resistivity; but a large difference between the calculated and measured resistance puts the resistance or the resistivity used in doubt. Each case is judged on its merits: investigate further, use larger safety factors or measure the hazardous voltages directly.

The procedure for this measurement is in the ground resistance test guide.

Touch and step potential map

Field tests of step and touch voltages, injecting current into the grid, help confirm that the substation is safe. Because of their cost, few utilities do them routinely. They are considered when:

• there are large differences between the calculated and the measured resistance,
• there are known anomalies in the soil resistivity that put the calculated voltages in doubt,
• the calculated values are near the tolerable limits and improving the ground would be difficult or costly.

In those cases it can be worth energizing the grid with a test current, preferably around 100 A, and measuring the potential gradients at chosen points of the substation and its perimeter. An EPRI project compared field measurements with a computer solution and found the method feasible and with good results.

To obtain the potentials in a real fault, the test values are multiplied by the ratio of the real ground fault current to the test current. The potential probe must make soil contact that represents a person’s foot. Because contact resistances are high, earth resistance instruments do not work; high-impedance voltmeters and test currents large enough to overcome residual currents in the soil are used.

There are two ways to present the result:

• Open-circuit touch voltage profiles and contour lines, with a high-impedance voltmeter, of the whole substation. With conservative values of the body and foot resistance and of the safe body current, the maximum safe value is determined and the hazardous voltages are identified.
• Measurement that simulates real contact, with an operator wearing rubber-soled gloves and boots with metal mesh contact surfaces; the measurement includes the contact resistance of the feet with the soil, and thus the effect of surface layers of stone or paving.

Integrity test

Solid-state relays, communication equipment, event recorders or the power supplies of the control house are often damaged by lightning or a fault when the grounding is deficient. The integrity test can be done after one of those events, when evaluating old grids and, sometimes, before approving a substation for operation. It is necessary to detect any open circuit or isolated structure or equipment.

The test consists of circulating current between two ground leads and measuring the voltage drop in the ground circuit. Each point is compared with the others: those farthest from the reference point should have increasing impedances, and points with abnormally high values reveal weak connections. The standard also mentions a reference value, 1.5 V per 50 ft with 300 A between ground leads, which does not apply if there are overhead metal structures that carry the test current from one point to another. The results are analyzed subjectively. A cable tracer can help locate a broken conductor.

Periodic reviews

Some utilities measure the substation earth resistance again periodically after the work is finished. It is also advisable to review the ground system from time to time because of system changes that affect the maximum ground current and because of substation expansions that affect the maximum current, the grid resistance or local potential differences. The standard suggests keeping a record of the total bus fault current used as the design basis and doing periodic reviews when the system short-circuit current grows.

How we do it at Tiza Group

At Tiza Group we measure and verify grounding systems in substations, industry and solar plants, with equipment from several manufacturers, and relate the results to the short-circuit study. Learn about our testing and commissioning service or write to us. Field measurements are done in Colombia, or elsewhere on request.

Related guides

• IEEE 80: Safety in Substation Grounding
• Grounding Grid Construction: Trench and Plow
• Ground Resistance Test: Fall-of-Potential and IEEE 81
• IEEE 81: How a Grounding System Is Measured
• Measuring Step and Touch Voltages in a Substation

References

• IEEE Std 80-2013, IEEE Guide for Safety in AC Substation Grounding, clauses 19.1 to 19.5.
• IEEE Std 81, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System.

Frequently asked questions