“There is a misconception that all grounding improvements mean better system performance,” says Jim Burke, executive advisor, InfraSource Technology. “In fact, better grounding certainly helps some things. But, it can have very little effect on other issues.” The following are some effects of grounding on a 4-wire multigrounded system.

• Overvoltages (swells). Swells are steady-state overvoltages caused by faults on adjacent phases. The duration of these overvoltages depends on the protection practices used by the utility. Swells can result in power-quality problems as well as arrester failure. Some grounding considerations regarding the magnitude of swells are as follows:

• Ground-rod spacing, footing resistance and soil resistivity. Adding to the number of grounds per mile does not have a significant effect on reducing swells. This is especially true where there are many equipment grounds on the system, which is usually the case. Also, a soil resistivity change from 100 V-m to 1000 V-m impacts footing resistance and causes virtually no change in the magnitude of swells.

• Neutral conductors. Larger neutral conductor sizes will reduce swells, and, broken neutrals will play a major role in the effectiveness of the grounding system. Thus, neutral continuity and size are more important factors than the grounding.

• Substation grounding. Again, this is a factor that has little effect on swells caused by faults out on the feeder. Substation grounding-impedance values of 0.5 Ω, 1.0 Ω, 2.0 Ω or 3.0 Ω show little difference in their effect on swell magnitudes.

• Electromagnetic fields (EMF). Unbalanced load current flows in the ground and the neutral wire. The current flowing in the ground creates most of the magnetic field associated with EMF. Current in the neutral tends to reduce this field. Studies show that for typical conditions approximately 50% of the return current flows in the earth and the other 50% in the neutral. A case can be made that poor grounding forces more current in the neutral and thereby reduces the EMF.

• Fault levels. Studies show that ground-rod footing resistance slightly affects fault-current levels for faults “close in” to the substation. Ground-rod-footing resistance also has little effect for faults more than 4 or 5 miles (6 to 8 km) from the substation. Since close-in fault magnitudes are almost always sufficient to operate protection properly, footing resistance in this area is usually not an issue. And, since fault magnitudes farther from the substation are not seriously affected by ground-rod impedance, this is not a reason to improve grounding.

• Stray voltage. Utility-caused stray voltage is the result of the return current (or unbalanced 3-phase current) returning via the neutral wire and the ground, and producing a voltage that is passed to the customer premises via the distribution transformer connection. The flow of current in these cases is very complex and depends on many factors: distance from substation, number of grounds, footing resistance and neutral size, to name a few. While good ground-footing resistances near the affected customer are sometimes important, the problem is more affected by the magnitude of the return current and the size of the neutral conductor. Reducing the ground-footing resistance near the customer has many times proved in-effective for this reason. Because this situation is so complex, its successful solution usually requires analysis.

• Arrester grounding for line protection is not as critical as most engineers believe. Studies show that where arresters are put on every phase and every tower or pole, ground-rod resistance variation between 0 Ω and 250 Ω had little effect on flashover rates. For shield-wire applications, ground-rod resistance becomes very important.

Studies indicate that investment in larger neutral conductors probably have more beneficial effects, in regard to distribution-system performance than simply driving more and better grounds. While good grounding is desired, it may not be the panacea, for the distribution engineer, as is sometimes suggested.