CSE Pure Power - Summer 2008 - (Page 20)

❯❯ PURE POWER // SUMMER 2008 20 grounding story Figure 1 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. 30 20 10 0 0 20 40 60 80 Depth of rod — ft 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. Economwww.purepowermagazine.com ics 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. 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 100 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. Resistance — ohms SOIL RESISTIVITY 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. GROUND DEPTH 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 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 http://www.purepowermagazine.com

Table of Contents Feed for the Digital Edition of CSE Pure Power - Summer 2008

CSE Pure Power - Summer 2008 Contents In the News Industry Roundup Risk Assessments for COPS Grounding Requires More Power Systems to Protect Healthcare Important Changes Coming in NFPA 70E A Look at Arc-Resistant Switchgear Agencies and Associations New Products Ad Index

CSE Pure Power - Summer 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.