The attention given to the design and installation of a facility ground system is a key element in the day-to- day reliability of the plant. A well-designed and -installed ground network is invisible to the engineering staff. A marginal ground system, however, will cause problems on a regular basis. Grounding schemes can range from simple to complex, but any system serves three primary purposes:
• Provides for operator safety
• Protects electronic equipment from damage caused by transient disturbances
• Diverts stray radio frequency energy from sensitive audio, video, control, and computer equipment
Most engineers view grounding mainly as a method to protect equipment from damage or malfunction. However, the most important element is operator safety. The 120 or 208 Vac line current that powers most equipment can be dangerous — even deadly — if handled improperly. Grounding of equipment and structures provides protection against wiring errors or faults that could endanger human life.
Proper grounding is basic to protection against ac line disturbances. This applies whether the source of the disturbance is lightning, power-system switching activities, or faults in the distribution network. Proper grounding is also a key element in preventing radio frequency interference in transmission or computer equipment. A facility with a poor ground system can experience RFI problems on a regular basis. Implementing an effective ground network is not an easy task. It requires planning, quality components, and skilled installers. It is not inexpensive. However, proper grounding is an investment that will pay dividends for the life of the facility.
Any ground system consists of two key elements: (1) the earth-to-grounding electrode interface outside the facility, and (2) the ac power and signal-wiring systems inside the facility.
Terms and Codes
A facility can be defined as something that is built, installed, or established to serve a particular purpose . A facility is usually thought of as a single building or group of buildings. The National Electrical Code (NEC) uses the term premises to refer to a facility when it defines premises wiring as the interior and exterior (facility) wiring, such as power, lighting, control, and signal systems. Premises wiring includes the service and all permanent and temporary wiring between the service and the load equipment. Premises wiring does not include wiring internal to any load equipment.
The Need for Grounding
The Institute of Electrical and Electronics Engineers (IEEE) defines grounding as a conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth or to some conducting body of relatively large extent that serves in place of the earth. It is used for establishing and maintaining the potential of the earth (or of the conducting body) or approximately that potential, on conductors connected to it, and for conducting ground current to and from the earth (or the conducting body) . Based on this definition, the reasons for grounding can be identified as:
• Personnel safety by limiting potentials between all noncurrent-carrying metal parts of an electrical distribution system
• Personnel safety and control of electrostatic discharge (ESD) by limiting potentials between all noncurrent-carrying metal parts of an electrical distribution system and the Earth
• Fault isolation and equipment safety by providing a low-impedance fault return path to the power source to facilitate the operation of overcurrent devices during a ground fault
The IEEE definition makes an important distinction between ground and earth. Earth refers to mother earth, and ground refers to the equipment grounding system, which includes equipment grounding conductors, metallic raceways, cable armor, enclosures, cabinets, frames, building steel, and all other noncurrent-carrying metal parts of the electrical distribution system.
There are other reasons for grounding not implicit in the IEEE definition. Overvoltage control has long been a benefit of proper power-system grounding and is described in IEEE Standard 142, also known as the Green Book . With the increasing use of electronic computer systems, noise control has become associated with the subject of grounding and is described in IEEE Standard 1100, the Emerald Book. .
Personnel safety is achieved by interconnecting all noncurrent-carrying metal parts of an electrical distribution system and then connecting the interconnected metal parts to the earth . This process of inter- connecting metal parts is called equipment grounding and is illustrated in Figure 18.1, where the equipment grounding conductor is used to interconnect the metal enclosures. Equipment grounding insures that there is no difference of potential, and thus no shock hazard, between noncurrent-carrying metal parts anywhere in the electrical distribution system. Connecting the equipment grounding system to earth insures that there is no difference of potential between the earth and the equipment grounding system. It also prevents static charge buildup.
System grounding, which is also illustrated in Figure 18.1, is the process of intentionally connecting one of the current-carrying conductors of the electrical distribution system to ground . The figure shows the neutral conductor intentionally connected to ground and the earth. This conductor is called the grounded conductor because it is intentionally grounded. The purpose of system grounding is overvoltage control and equipment safety through fault isolation. An ungrounded system is subject to serious overvoltages under conditions such as intermittent ground faults, resonant conditions, and contact with higher voltage systems. Fault isolation is achieved by providing a low-impedance return path from the load back to the source, which will ensure operation of overcurrent devices in the event of a ground fault. The system ground connection makes this possible by connecting the equipment grounding system to the low side of the voltage source. Methods of system grounding include solidly grounded, ungrounded, and impedance- grounded.
Solidly grounded means that an intentional zero-impedance connection is made between a cur- rent-carrying conductor and ground. The single-phase (1φ) system shown in Figure 18.1 is solidly grounded. A solidly grounded, three-phase, four-wire, wye system is illustrated in Figure 18.2. The neutral is connected directly to ground with no impedance installed in the neutral circuit. The NEC permits this connection to be made at the service entrance only . The advantages of a solidly grounded wye system include reduced magnitude of transient overvoltages, improved fault protection, and faster location of ground faults. There is one disadvantage of the solidly grounded wye system. For low-level arcing ground faults, the application of sensitive, properly coordinated, ground-fault protection (GFP) devices is necessary to prevent equipment damage from arcing ground faults. The NEC requires arcing ground-fault protection at 480 Y/277 V services, and a maximum sensitivity limit of 1200 A is permitted. Severe damage is less frequent at the lower voltage 208 V systems, where the arc may be self-extinguishing.
Ungrounded means that there is no intentional connection between a current-carrying conductor and ground. However, charging capacitance will create unintentional capacitive coupling from each phase to ground, making the system essentially a capacitance-grounded system. A three-phase, three-wire system from an ungrounded delta source is illustrated in Figure 18.3. The most important advantage of an ungrounded system is that an accidental ground fault in one phase does not require immediate removal of power. This allows for continuity of service, which made the ungrounded delta system popular in the past. However, ungrounded systems have serious disadvantages. Because there is no fixed sys- tem ground point, it is difficult to locate the first ground fault and to sense the magnitude of fault current. As a result, the fault is often permitted to remain on the system for an extended period of time. If a second fault should occur before the first one is removed, and the second fault is on a different phase, the result will be a double line-to-ground fault, causing serious arcing damage. Another problem with the ungrounded delta system is the occurrence of high transient overvoltages from phase-to-ground. Tran- sient overvoltages can be caused by intermittent ground faults, with overvoltages capable of reaching a
phase-to-ground voltage of from six to eight times the phase-to-neutral voltage. Sustained overvoltages can ultimately result in insulation failure and thus moreground faults Impedance-grounded means that an intentional impedance connection is made between a current- carrying conductor and ground. The high-resistance grounded wye system, illustrated in Figure 18.4, is an alternative to solidly grounded and ungrounded systems. High-resistance grounding will limit ground-fault current to a few amperes, thus removing the potential for arcing damage inherent in solidly grounded systems. The ground reference point is fixed, and relaying methods can locate first faults before damages from second faults occur. Internally generated transient overvoltages are reduced because the neutral-to-ground resistor dissipates any charge that may build up on the system-charging capacitances.
Table 18.1 compares the three most common methods of system grounding. There is no one best system grounding method for all applications. In choosing among the various options, the designer must consider the requirements for safety, continuity of service, and cost. Generally, low-voltage systems should be operated solidly grounded. For applications involving continuous processes in industrial plants or where shutdown might create a hazard, a high-resistance grounded wye system, or a solidly grounded wye system with an alternate power supply, may be used. The high-resistance grounded wye system combines many of the advantages of the ungrounded-delta system and the solidly grounded wye system. IEEE Standard 142 recommends that medium-voltage systems less than 15 kV be low-resistance
grounded to limit ground fault damage yet permit sufficient current for detection and isolation of ground-faults. Standard 142 also recommends that medium-voltage systems over 15 kV be solidly grounded. Solid grounding should include sensitive ground-fault relaying in accordance with the NEC.
The Grounding Electrode
The process of connecting the grounding system to earth is called earthing and consists of immersing a metal electrode or system of electrodes into the earth . The conductor that connects the grounding system to earth is called the grounding electrode conductor. The function of the grounding electrode conductor is to keep the entire grounding system at earth potential (i.e., voltage equalization during lightning and other transients) rather than for conducting ground-fault current. Therefore, the NEC allows reduced sizing requirements for the grounding electrode conductor when connected to made electrodes.
The basic measure of effectiveness of an earth electrode system is called earth electrode resistance. Earth electrode resistance is the resistance, in ohms, between the point of connection and a distant point on the earth called remote earth. Remote earth, about 25 ft from the driven electrode, is the point where earth electrode resistance does not increase appreciably when this distance is increased. Earth electrode resistance consists of the sum of the resistance of the metal electrode (negligible) plus the contact resistance between the electrode and the soil (negligible) plus the soil resistance itself. Thus, for all practical purposes, earth electrode resistance equals the soil resistance. The soil resistance is nonlinear, with most of the earth resistance contained within several feet of the electrode. Furthermore, current flows only through the electrolyte portion of the soil, not the soil itself. Thus, soil resistance varies as the electrolyte content (moisture and salts) of the soil varies. Without electrolyte, soil resistance would be infinite.
Soil resistance is a function of soil resistivity. A 1-cubic-meter sample of soil with a resistivity ρ of 1 ohm-meter will present a resistance R of 1 ohm between opposite faces. A broad variation of soil resistivity occurs as a function of soil types, and soil resistivity can be estimated or measured directly. Soil resistivity is usually measured by injecting a known current into a given volume of soil and measuring the resulting voltage drop. When soil resistivity is known, the earth electrode resistance of any given configuration (single rod, multiple rods, or ground ring) can be determined by using standard equations developed by Sunde , Schwarz , and others.
Earth resistance values should be as low as practicable, but are a function of the application. The NEC approves the use of a single made electrode if the earth resistance does not exceed 25 Ω. IEEE Standard 1100 reports that the very low earth resistance values specified for computer systems in the past are not necessary. Methods of reducing earth resistance values include the use of multiple electrodes in parallel, the use of ground rings, increased ground rod lengths, installation of ground rods to the permanent water level, increased area of coverage of ground rings, and the use of concrete-encased electrodes, ground wells, and electrolytic electrodes.
Earth electrodes may be made electrodes, natural electrodes, or special-purpose electrodes . Made electrodes include driven rods, buried conductors, ground mats, buried plates, and ground rings. The electrode selected is a function of the type of soil and the available depth. Driven electrodes are used where bedrock is 10 ft or more below the surface. Mats or buried conductors are used for lesser depths. Buried plates are not widely used because of the higher cost when compared to rods. Ground rings employ equally spaced driven electrodes interconnected with buried conductors. Ground rings are used around large buildings, around small unit substations, and in areas having high soil resistivity.
Natural electrodes include buried water pipe electrodes and concrete-encased electrodes. The NEC lists underground metal water piping, available on the premises and not less than 10 ft in length, as part of a preferred grounding electrode system. Because the use of plastic pipe in new water systems will impair the effectiveness of water pipe electrodes, the NEC requires that metal underground water piping be supplemented by an additional approved electrode. Concrete below ground level is a good electrical conductor. Thus, metal electrodes encased in such concrete will function as excellent grounding electrodes. The application of concrete-encased electrodes is covered in IEEE Standard 142.