Grounding requires more
We need to raise awareness about the realities of grounding and its environmental effects. With the preference going to some form of nonmetallic piping, the cold water pipe grounding electrode has almost disappeared from new buildings. This article will discuss: By raising the awareness of these sometimes overlooked factors, everyone involved will benefit from have a better grounded system.
We need to raise awareness about the realities of grounding and its environmental effects. With the preference going to some form of nonmetallic piping, the cold water pipe grounding electrode has almost disappeared from new buildings.
This article will discuss:
Resistance to ground
Resistance to ground calculations
Temperature effects on resistance to ground
Reduction of resistance to ground with multiple ground rods.
By raising the awareness of these sometimes overlooked factors, everyone involved will benefit from have a better grounded system. The inspector will see more enhanced grounding being specified and designed and may question implementation all of the factors discussed above. The inspector and contractor will be involved in the building process sooner with Ufer grounds.
While Ufer grounds are not covered here, a short description is in order. The principle is simple: The Ufer ground takes advantage of concrete’s properties. Concrete absorbs moisture quickly and looses moisture slowly. The mineral properties of concrete (lime and others) and their inherent pH means concrete has a supply of ions to conduct current. The soil around concrete becomes “doped” by the concrete. As a result, the pH of the soil rises and reduces what would normally be 1,000-ohm meter soil conditions (hard to get a good ground). The moisture present, in combination with the “doped” soil, make a good conductor for electrical energy or lightning currents.
The contractor will be expected to do more testing and provide test results for a baseline for future reference for the high-tech facilities. The engineer will certainly add grounding requirements to the sensitive electronic high-tech projects based on information listed in the factors above.
RESISTANCE TO GROUND
The National Electrical Code (NEC) states that resistance to ground is for safety, but is not necessarily efficient, convenient, or adequate for good service of future expansion of electrical use. For the most part, everyone agrees grounding must meet the NEC. The NEC establishes a presumably acceptable level of resistance to ground as 25 ohms or less.
IEEE Standard 1100-2005, “Recommended Practice for Powering and Grounding Electronic Equipment,” indicates that in special applications like data processing, telephone switches, and medical modules like MRI, CT, and other sophisticated medical equipment, 5 ohms or less is required by the manufacturer’s written recommendations for the values of resistance to ground.
Acceptable grounding electrodes are plates, rods, pipes, concrete-encased electrode, metallic underground water pipe, and the building’s steel. Economics almost always plays into the design for the best value for the best results. In trying to compare ground rod and ground plates, I made phone calls for pricing grounding plates. In my area, no one could even remember selling a grounding plate electrode. IEEE Standard 142 indicates that ground plates can be buried either horizontal or vertical on edge and is the preferred method because a minimum of excavation is required. For plates of 10 to 20 sq. ft, the optimum burial depth is about 8 ft. However, IEEE Standard 1100 Chapter 9, “Telecommunications, Information Technology, and Distributed Computing,” list examples of approved grounds, and the ground plate is not listed.
Where there is insufficient real estate to work with, or under conditions of unusually high ground resistivity, deep grounds may be required. Long copper pipe-type ground rods, sometimes tens or hundreds of feet long in bored holes are not unheard of, but are rare. In mountaintop locations, for example, in order to achieve the target ground resistance value, it may be more economical to bore a deep ground than to spread out a shallow ground system over rocky terrain or steep slopes.
Generally speaking, deeper ground rods are more effective than shallow rods, so a 20-ft. rod is preferred to a 10-ft. rod, and so on. Figure 1 shows that resistance falls quickly as rod length increases, due to more stable temperatures and increased moisture at lower depths.
Electrode spacing is also important. The general rule of thumb is that multiple rods should be spaced apart at least twice the length of one rod. That is, two 10-ft. rods should be placed no closer than 20 ft. apart. From the graph for a grounding rod, one can see that if the single ground rod is 8 ft. long, then its resistance to ground is an unknown value. When two 10-ft.-long ground rods are stacked and driven one on top of the other, then the resistance may be about 20 ohms. This assumes the soil resistivity matches the chart.
IEEE Standard 142-1991 Table 10, “Resistivity of Soils and Resistance of Single Ground Rods,” shows the different soil types and the different soil associated resistivities. This is shown in Table 1 on page 21.
The range is staggering, from 1,000 ohms cm for inorganic clays of high plasticity to 250,000 ohms cm for poorly graded gravels, gravel sand mixtures, little or not fines. The table shows for the same two extremes a single 5/8-in. by 10-ft. ground rod resistance ranges from 3 to 750 ohms. The NEC does not address directly any value of soil resistivity.
RESISTANCE TO GROUND CALCULATIONS
The International Assn. of Electrical Inspectors (IAEI) “Soares Book on Grounding” indicates the theoretical resistance to ground can be calculated based on a general resistance formula, where resistance equals the resistivity of the earth times the quotient of the length of the conducting path divided by the cross-sectional area of the path. Refer to IEEE Standard 142 Table 13 formulas for the calculations of resistance to ground, where you will note each formula is based on the installed ground rods configuration. Because the earth’s resistivity is neither uniform nor consistent, a simple and direct method of measuring earth resistance is needed.
The NEC does address locating the electrodes in moist locations away from the building. The typical specified distance from the building is beyond the drip line or beyond the gutters. IEEE Standard 142-1991 Table 11, “Effect of Moisture Content on Soil Resistivity,” shows with a low moisture content of 2% by weight sandy loam soil resistivity of approximately 185,000 ohms. At 24% moisture content the same soil resistivity is near 7,000 ohms. This is shown in Table 2 at the right.
TEMPERATURE EFFECTS ON RESISTANCE TO GROUND
IEEE Standard 142-1991 Table 12, “Effect of Temperature on Soil Resistivity,” shows the relationship of soil resistivity and soil temperature. At -5 C, the resistivity can be as high as 70,000 ohms cm. At 50 C, the resistivity is shown for the same soil to be 4,000 ohms cm. See Table 3 on page 22.
The resistivity will be seasonal; winter is the worst. Most contractors will provide a second electrode/ground rod as allowed by code in lieu of doing insulation resistance testing. Master type specifications appear to be based upon what is allowable by the NEC.
REDUCING RESISTANCE TO GROUND WITH MULTIPLE GROUND RODS
IEEE Standard 142-1991 Table 14, “Multiplying Factors for Multiple Rods,” shows how many additional ground rods are required to reduce the resistance to ground. The section and table shows that with two rods the equivalent resistance is approximated with the starting resistance divided by number of rods used and multiplied by the factor in the table. See Table 4 on page 22.
Using the 750 ohms from above in properly graded gravel as a starting point, with one ground rod and adding 23 more results in the following: (750/24) x 2.16 = 67.5 ohms. Working from the desired results of 25 ohms = (277/24 x 2.16). Best application of using 24 ground rods would probably be in a ground ring around the building.
With the growth of high-tech facilities with sensitive electronics, one will see the 5-ohm resistance value as the new target for grounding system. The engineer will need the soil resistivity numbers for the facility. More complex or enhanced grounding systems will be designed. Contractors will have more challenges and may be required to include test equipment in his or her arsenal of tools. Inspectors will inspect many of the grounding components as they are installed. Owners will benefit by having a good grounding system because their computers will work better, health care equipment will work better, and telecommunication systems will work better.
|Soil Description||Group Symbol*||Average Resitivity (ohms cm)||Resistance of 5/8 in x 10 ft. rod|
|* The terminology used in these descriptions is from the United Soil Classifications and is a standard method of describing soils in a geotechnical or geophysical report. |
† These soils classification resistivity results are highly influenced by the presence of moisture.
|Well graded gravel, gravel-sand mixtures, little or no fines||GW||60,000—100,000||180-300|
|Poorly graded gravels, gravel-sand mixtures, little or no fines||GP||100,000-250,000||300-750|
|Clay gravel, poorly graded gravel, sand-clay mixtures||GC||20,000-40,000||60-120|
|Silty sands, poorly graded sand-silts mixtures||SM||10,000-50,000||30-150|
|Clay sands, poorly graded sand- clay mixtures||SC||5,000-20,000||15-60|
|Silty or clay fine sands with slight plasticity||ML||3,000-8,000||9-24|
|Fine sandy or silty soils, elastic silts||MH||8,000-30,000||24-90|
|Gravelly clays, sandy clays, silty clays, lean clays||CL||2,500-6,000†||17-18†|
|Inorganic clays of high plasticity||CH||1,000-5,500†||3-16†|
|Moisture Content (% by weight)||Top Soil||Resistivity (ohms cm) Sandy Loam||Red Clay|
|2||no data||185,000||no data|
|4||no data||60,000||no data|
|Temperature (C)||Resistivity (ohms-cm)|
|Number of Rods||Factor|
|Ostendorf is Senior Electrical Engineer, Senior Associate, ME Group, Omaha.|
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