THE OFFICIAL TRADE JOURNAL OF BICSI

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1 ICT TODAY THE OFFICIAL TRADE JOURNAL OF BICSI January/February 2017 PLUS + + Information Security and ESS in the Age of the Internet of Things + Grounding System Design and Testing for Critical Facilities + Intelligent Power Management within High-Density Deployments Volume 38, Number 1

2 By Roy Whitten Grounding System Design and Testing for Critical Facilities Electrical grounding is critical to the protection and operation of sensitive electronics. This article will discuss the importance of grounding and its relationship to surge suppression and lightning protection. Proper ground testing is also critical, although a majority of this testing turns out to be invalid. The article will explain how to properly test and achieve valid results. Grounding is defined in the NFPA 70: National Electrical Code (NEC ) as a conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth. The unit of measurement for the performance of a grounding electrode is the ohm. Typically, it is said that a grounding electrode has so much resistance (measured in ohms) to earth. This is the resistance between the grounding electrode and remote earth. Remote earth is a term used to describe the resistance between one point in earth and an arbitrarily distant point (also in earth). The earth resistance between any two points typically increases proportional to the distance; however, there comes a point after which the earth resistance no longer increases appreciably. Any point greater than this minimum distance is therefore referred to as remote earth. When an electrical fault injects current into a grounding electrode, that current is dissipated into the surrounding soil. As this current passes through the electrode and the soil, it creates a voltage known as ground potential rise (GPR). This GPR will be its highest close to the electrode and will decrease with distance. At some distance from the electrode, the

3 To understand the importance of a low resistance grounding electrode, it is necessary to understand the relationship between the resistance to earth of the grounding electrode and the GPR to which the facility is subjected. FIGURE 1: An electrical current is injected through the soil between two outermost test probes, C1 and C2. GPR will be immeasurable; this is remote earth. For standard electrical applications, Article 250 of the NEC requires the resistance to earth of a single rod, pipe or plate electrode to be 25 ohms or less. This is a minimum safety standard meant to protect personnel and the structure. However, in critical facilities such as telecommunications, data, medical and precision manufacturing, the requirement is often 5 ohms and can be as low as 1 ohm. To understand the importance of a low resistance grounding electrode, it is necessary to understand the relationship between the resistance to earth of the grounding electrode and the GPR to which the facility is subjected. If a facility has a grounding electrode with 25 ohms to earth and an 18,000-ampere lightning event occurs, the equipment normally operating at voltages ranging from 120 to 480 volts (V) alternating current (ac) or 12 to 48 V direct current (dc) will be subjected to a GPR of 450,000 V. This means that the protective circuits must be able to safely and rapidly dissipate that 450,000 V fault without being damaged so that they can protect the equipment from future faults. Given the same conditions, with a 5-ohm grounding electrode the GPR would be limited to 90,000 V. Testing for Soil Resistivity The performance of the grounding electrode is directly proportional to the resistivity of the soil in which it is installed. Soil resistivity, measured in ohms per meter, can be defined as the soil s opposition to current flow. It is not possible to design a grounding electrode system with a predicted resistance to earth without knowing the soil resistivity. While there may be some basic generalities, it is impossible to accurately predict the soil resistivity by its physical description. One misconception is that clay will have a lower soil resistivity than rock or sand. However, depending on the amount of moisture and electrolytes in that moisture, sand is often less resistive than clay. Soil resistivity testing should be conducted for each grounding system design. This is where a known amount of constant electrical current is passed through the soil to determine the soil resistivity. The Wenner Four Point Test Method, a test referenced in IEEE 81: IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System, uses four equally spaced test probes and a specialized soil resistivity meter. An electrical current is injected through the soil between two outermost test probes, C1 and C2, as referenced in Figure 1. This current creates a GPR that is highest at probe C1 and lowest at C2. The voltage difference is measured between the two center probes, P1 and P2, and converted to resistance within the meter. This resistance, along with the depth and spacing between the test probes, is used in a mathematical formula to calculate soil resistivity. The Wenner method soil resistivity test allows for testing multiple depths of soil; the average

4 depth tested is equivalent to the spacing between the probes. If multiple tests are taken at different spacing, a multi-layered soil model can be generated. This multi-layered soil model can then be used to determine the length of grounding electrodes necessary to achieve the resistance goal most efficiently. While there are many types of grounding electrodes to choose from, the copper-clad steel-driven rod is usually the electrode of choice. It is relatively inexpensive and easily installed in most cases, and in many soil conditions, a low resistance to earth can be achieved. However, one of the drawbacks of this type of electrode is its susceptibility to the corrosiveness of the soil in which it is installed. As mentioned earlier, soil gets its conductivity from electrolytes and moisture. Usually, the better the soil is for grounding system performance, the more corrosive it is to metals. If asked, most users of the copper-clad steel rod will say that the copper cladding is there to make the driven rod conducts electricity better than a bare steel rod; and, in fact, copper is a better conductor than steel. In this instance, however, the non-ferrous copper cladding is present to protect the ferrous steel from the corrosiveness of the soil. There are certain requirements a copper clad steel rod has to meet to be listed by a nationally recognized testing laboratory. However, it is conceivable that during the installation process, the copper cladding could get damaged and the ferrous steel may come into direct contact with the corrosive soil. When considering the different types of grounding electrodes (e.g., FIGURE 2: In the FOP test, C1 and P1 are connected to the grounding electrode system. C2 is connected to a test probe installed approximately five times the distance of remote earth. stainless steel driven rods, copper plates, concrete encased [Ufer], building steel, cold water pipe, electrolytic [chemical] rods) there is no single type of electrode that is more useful than the others in all scenarios or soil types. This is why it is important to design sitespecific grounding electrode systems for all cases. While a company may have grounding standards that require certain minimums to be met, the type, number and spacing of electrodes should be adjusted to match the soil conditions. Measuring Resistivity Electrode spacing is usually a generic factor. It is common to see 10-foot vertical electrodes spaced 3 meters (m [10 feet (ft)]) apart. This may provide suitable results, but this spacing does not take into account the whole sphere of influence of a single electrode. When current is imposed onto an electrode, that current is dissipated into the surrounding soil. It is accepted that an electrode uses the soil effectively out a radial distance equal to its length. In other words, a 10-foot electrode uses the soil out to 3 m (10 ft) and a 20-foot electrode uses the soil out to 6 m (20 ft). This concept is commonly called the sphere of influence. If the electrodes are spaced equal to their depth, there will be an overlap of spheres of influence between the electrodes and the grounding system s performance will not be optimal. As stated in paragraph of NFPA 780 (2017): Standard for the Installation of Lightning Protection Systems, Where multiple connected ground rods are used, the separation between any two ground rods shall be at least the sum of their driven depths, where practicable. This way, the spheres of influence touch and do not overlap and the grounding electrode system takes full advantage of the volume of soil in which it is installed. Regardless of the grounding electrode system design or its makeup, care should be taken during installation and the performance of

5 FIGURE 3: True resistance of the grounding electrode system occurs at the point where the readings change the least from one to the next. a grounding electrode system should be checked on a regular basis to identify degradation. Upon completing the installation of the buried grounding electrode system and prior to that system being bonded to any other items or systems, the resistance to earth of the grounding electrode system should be tested using a specialized meter and the fall-ofpotential (FOP) test method in accordance with IEEE It is important to note that the grounding electrode system must be completely isolated from all items or systems in order to perform the FOP test. All of the meters designed for this purpose are constant current meters, and 100 percent of the current must pass through the grounding electrode system, into the soil, and return to the meter through a test probe. If an ac path exists, a portion of the current will bypass the grounding electrode system, and the meter s internal calculation will be incorrect. The FOP test uses the same meter as the Wenner soil resistivity test. C1 and P1 are connected to the grounding electrode system and C2 is connected to a test probe installed approximately five times the distance of remote earth. In the case of a single electrode, this distance can be defined as approximately ten times the depth of the electrode. In the case of a ground ring, this distance will be approximately ten times the distance between the two furthest points of the ring. A constant current is injected through the grounding electrode system through C1. This current returns to the meter through the soil and probe C2, creating a GPR that is highest at the electrode system and lowest at C2. A second probe, P2, is driven into the soil at varying intervals between the grounding electrode system and C2. These intervals are usually every ten percent of the total distance (Figure 2). The voltage difference is measured at each interval, converted to resistance within the meter and shown in ohms. As the distance increases between the grounding electrode system and probe P2, so will the resistance. When probe P2 reaches the point of remote earth, the increase in resistance from one reading to the next will diminish and then begin to increase again as the test intervals near the C2 probe. The resistance where the readings change the least from one to the next is the true resistance of the grounding electrode system, as depicted in Figure 3. It would be a mistake to assume it would be feasible to perform FOP tests on all sites. Many sites are inside urban areas and surrounded by asphalt and concrete. In many cases, one could not go three meters from the site in any direction without crossing a major thoroughfare or running into the wall of an adjacent building. In these cases, it may be possible to perform a clamp-on resistance to earth test. This test uses a specialized clamp-on meter to induce a voltage into the grounding electrode conductor. In simple terms, the meter uses the multigrounded neutral and all connected parallel grounding electrode systems as a known earth reference and indicates the resistance to earth of the grounding electrode system under test (Figure 4). Due to myriad performance requirements and pitfalls for both the FOP and clampon tests, it is highly recommended

6 that only well vetted and qualified personnel/companies be engaged for the purpose of performing these tests. Protecting Equipment The grounding electrode system is arguably the single most important site electrical protection system. All lightning protection systems, traditional or nontraditional, must be bonded to the grounding electrode system by down conductors, as follows: u ac surge protection circuits are usually connected between the phase and neutral conductors and the surge voltage is routed to the grounding electrode system through the main neutral to ground bonding jumper in the first service disconnect. u Data line surge protection components are installed between the signal conductor and ground. u Radio frequency (RF) cable protection is installed between the shield/center pin and ground. Without being effectively bonded to a properly designed and installed grounding electrode system, the aforementioned protections would be completely useless. It also must be stated that the best designed and installed grounding electrode system cannot provide complete site protection alone. If the constraints of a site make a low resistance to earth impractical, it is still possible to protect the equipment from the high GPR created by an electrical fault or FIGURE 4: The clamp-on resistance to earth test uses a specialized clampon meter to induce a voltage into the grounding electrode conductor. lightning. If all equipment is kept at the same electrical potential, fault current will not flow through the equipment components and damage should be averted. Picture a bald eagle perched atop a 360,000 V high-tension wire. Relative to the earth below, the eagle s potential is 360,000 V, yet it is unharmed. This is the same principle we use to protect the equipment during electrical faults. If the eagle was to spread its wings and touch the metallic highvoltage pole, the results would be catastrophic. This is what happens during an electrical fault when the equipment is interconnected with other systems such as utilities. If no surge protection device is installed, the GPR will equalize through these interconnections and the results will be just as catastrophic. Owners/ operators of critical facilities should always install proper grounding electrode, lightning protection, surge protection, RF protection and bonding systems as a total site protection package.t AUTHOR BIOGRAPHY: Roy Whitten is Senior Applications Engineer/Education Specialist, LPI Journeyman Installer at Lyncole Grounding Solutions. Since joining Lyncole Technical Services as Senior Applications Engineer in May of 1997, Roy has been responsible for developing testing procedures specific to the clamp-on resistance tester for testing grounding electrode systems. Roy has also been responsible for grounding system design calculations, manufacturing and quality assurance procedures, on-site testing, complete site grounding and bonding surveys, turnkey installations, specialized grounding courses, lightning damage evaluations, total site protection recommendations, project management/scheduling, and power quality surveys. He is the instructor for Lyncole s nationwide grounding courses, a member of the National Fire Protection Association (NFPA) and a Lightning Protection Institute (LPI) journeyman installer. Roy joined Lyncole Technical Services after a twenty-year career in the United States Marine Corps. He can be reached at rwhitten@lyncole.com.