Ground (electricity)From Wikipedia, the free encyclopedia

A typical earthing electrode (left of gray pipe), consisting of a conductive rod driven into the ground, at a home

in Australia.

Most electrical codes specify that the insulation on protective earthing conductors must be a distinctive color (or

color combination) not used for any other purpose.

In electrical engineering, ground or earth is the reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the Earth.

Electrical circuits may be connected to ground (earth) for several reasons. In mains powered equipment, exposed metal parts are connected to ground to prevent user contact with dangerous voltage if electrical insulation fails. Connections to ground limit the build-up of static electricity when handling flammable products or electrostatic-sensitive devices. In some telegraph and power transmissioncircuits, the earth itself can be used as one conductor of the circuit, saving the cost of installing a separate return conductor (seesingle-wire earth return).

For measurement purposes, the Earth serves as a (reasonably) constant potential reference against which other potentials can be measured. An electrical ground system should have an appropriate current-carrying capability to serve as an adequate zero-voltage reference level. In electronic circuit theory, a "ground" is usually idealized as an infinite source or sink for charge, which can absorb an unlimited amount of current without changing its potential. Where a real ground connection has a significant resistance, the approximation of zero potential is no longer valid. Stray voltages or earth potential rise effects will occur, which may create noise in signals or if large enough will produce an electric shock hazard.

The use of the term ground (or earth) is so common in electrical and electronics applications that circuits in portable electronic devices such as cell phones and media players as well as circuits in vehicles may be spoken of as having a "ground" connection without any actual connection to the Earth, despite "common" being a more appropriate term for such a connection. This is usually a large conductor attached to one side of the power supply (such as the "ground plane" on a printed circuit board) which serves as the common return path for current from many different components in the circuit.

History[edit]

Long-distance electromagnetic telegraph systems from 1820 onwards[citation needed] used two or more wires to carry the signal and return currents. It was then discovered, probably by the German scientist Carl August Steinheil in 1836–1837,[1] that the ground could be used as the return path to complete the circuit, making the return wire unnecessary. However, there were problems with this system, exemplified by the transcontinental telegraph line constructed in 1861 by the Western Union Company between Saint Joseph, Missouri, and Sacramento, California. During dry weather, the ground connection often developed a high resistance, requiring water to be poured on the ground rod to enable the telegraph to work or phones to ring.

Later, when telephony began to replace telegraphy, it was found that the currents in the earth induced by power systems, electrical railways, other telephone and telegraph circuits, and natural sources including lightning caused unacceptable interference to the audio signals, and the two-wire or 'metallic circuit' system was reintroduced around 1883.[2]

§Radio communications[edit]

An electrical connection to earth can be used as a reference potential for radio frequency signals for certain kinds of antennas. The part directly in contact with the earth - the "earth electrode" - can be as simple as a metal rod or stake driven into the earth, or a connection to buried metal water piping (the pipe must be conductive). Because high frequency signals can flow to earth due to capacitative effects, capacitance to ground is an important factor in effectiveness of signal grounds. Because of this, a complex system of buried rods and wires can be effective. An ideal signal ground maintains a fixed potential (zero) regardless of how much electric current flows into ground or out of ground. Lowimpedance at the signal frequency of the electrode-to-earth connection determines its quality, and that quality is improved by increasing the surface area of the electrode in contact with the earth, increasing the depth to which it is driven, using several connected ground rods, increasing the moisture content of the soil, improving the conductive mineral content of the soil, and increasing the land area covered by the ground system.

Some types of transmitting antenna systems in the VLF, LF, MF and lower SW range must have a good ground to operate efficiently. For example, a vertical monopole antennarequires a ground plane that often consists of an interconnected network of wires running radially away from the base of the antenna for a distance about equal to the height of the antenna. Sometimes a counterpoise is used as a ground plane, supported above the ground.

§Building wiring installations[edit]

See also: Earthing system

Electrical power distribution systems are often connected to ground to limit the voltage that can appear on distribution circuits. A distribution system insulated from ground may attain a high potential due to transient voltages caused by arcing, static electricity, or accidental contact with higher potential circuits. A ground connection of the system dissipates such potentials and limits the rise in voltage of the grounded system.

In a mains electricity (AC power) wiring installation, the term ground conductor typically refers to three different conductors or conductor systems as listed below.

Equipment earthing conductors provide an electrical connection between non-current-carrying metallic parts of equipment and the earth. According to the U.S. National Electrical Code (NEC), the reason for doing this is to limit the voltage imposed by lightning, line surges, and contact with higher voltage lines. The equipment earthing conductor is usually also used as the equipment bonding conductor (see below).

Equipment bonding conductors provide a low impedance path between non-current-carrying metallic parts of equipment and one of the conductors of that electrical system's source, so that if a part becomes energized for any reason, such as a frayed or damaged conductor, a short circuit will occur and operate a circuit breaker or fuse to disconnect the faulted circuit. The earth itself has no role in this fault-clearing process[3] since current must return to its source; however, the sources are very frequently connected to earth.[4] (see Kirchhoff's circuit laws). By bonding (interconnecting) all exposed non-current carrying metal objects together, they should remain near the same potential thus reducing the chance of a shock. This is especially important in bathrooms where one may be in contact with several different metallic systems such as supply and drain pipes and appliance frames. The equipment bonding conductor is usually also used as the equipment earthing conductor (see above).

Metal water pipe used as grounding electrode

A grounding electrode conductor (GEC) connects one leg of an electrical system to one or more earth electrodes. This is called "system grounding" and most systems are required to be grounded. The U.S. NEC and the UK's BS 7671 list systems that are required to be grounded. The grounding electrode conductor connects the leg of the electrical system that is the "neutral wire" to the grounding electrode(s).[5] The grounding electrode conductor is also usually bonded to pipework and structural steel in larger structures. According to the NEC, the purpose of earthing an electrical system is to limit the voltage to earth imposed by lightning events and contact with higher voltage lines, and also to stabilize the voltage to earth during normal operation. In the past, water supply pipes were often used as grounding electrodes, but this was banned where plastic pipes are popular. This type of ground applies to radio antennas and to lightning protection systems.

Permanently installed electrical equipment usually also has permanently connected grounding conductors. Portable electrical devices with metal cases may have them connected to earth ground by a pin in the interconnecting plug (see Domestic AC power plugs and sockets). The size of power ground conductors is usually regulated by local or national wiring regulations.

§Earthing systems[edit]

In electricity supply systems, an earthing (grounding) system defines the electrical potential of the conductors relative to that of the Earth's conductive surface. The choice of earthing system has implications for the safety and electromagnetic compatibility of the power supply. Regulations for earthing systems vary considerably between different countries.

A functional earth connection serves a purpose other than providing protection against electrical shock. In contrast to a protective earth connection, a functional earth connection may carry a current during the normal operation of a device. Functional earth connections may be required by devices such as surge suppression and electromagnetic-compatibility filters, some types of antennas and various measurement instruments. Generally the protective earth is also used as a functional earth, though this requires care in some situations.

§Impedance grounding[edit]

Distribution power systems may be solidly grounded, with one circuit conductor directly connected to an earth grounding electrode system. Alternatively, some amount of electrical impedance may be connected between the distribution system and ground, to limit the current that can flow to earth. The impedance may be a resistor, or an inductor (coil). In a high-impedance grounded system, the fault current is limited to a few amperes (exact values depend on the voltage class of the system); a low-impedance grounded system will permit several hundred amperes to flow on a fault. A large solidly-grounded distribution system may have thousands of amperes of ground fault current.

In a polyphase AC system, an artificial neutral grounding system may be used. Although no phase conductor is directly connected to ground, a specially constructed transformer (a "zig zag"

transformer) blocks the power frequency current from flowing to earth, but allows any leakage or transient current to flow to ground.

Low-resistance grounding systems use a neutral grounding resistor (NGR) to limit the fault current to 25 A or greater. Low resistance grounding systems will have a time rating (say, 10 seconds) that indicates how long the resistor can carry the fault current before overheating. A ground fault protection relay must trip the breaker to protect the circuit before overheating of the resistor occurs.

High-resistance grounding (HRG) systems use an NGR to limit the fault current to 25 A or less. They have a continuous rating, and are designed to operate with a single-ground fault. This means that the system will not immediately trip on the first ground fault. If a second ground fault occurs, a ground fault protection relay must trip the breaker to protect the circuit. On an HRG system, a sensing resistor is used to continuously monitor system continuity. If an open-circuit is detected (e.g., due to a broken weld on the NGR), the monitoring device will sense voltage through the sensing resistor and trip the breaker. Without a sensing resistor, the system could continue to operate without ground protection (since an open circuit condition would mask the ground fault) and transient overvoltages could occur.[6]

§Ungrounded systems[edit]

Where the danger of electric shock is high, special ungrounded power systems may be used to minimize possible leakage current to ground. Examples of such installations include patient care areas in hospitals, where medical equipment is directly connected to a patient and must not permit any power-line current to pass into the patient's body. Medical systems include monitoring devices to warn of any increase of leakage current. On wet construction sites or in shipyards, isolation transformers may be provided so that a fault in a power tool or its cable does not expose users to shock hazard.

Circuits used to feed sensitive audio/video production equipment or measurement instruments may be fed from an isolated ungrounded technical power system to limit the injection of noise from the power system.

§Power transmission[edit]

In single-wire earth return (SWER) AC electrical distribution systems, costs are saved by using just a single high voltage conductor for the power grid, while routing the AC return current through the earth. This system is mostly used in rural areas where large earth currents will not otherwise cause hazards.

Some high-voltage direct-current (HVDC) power transmission systems use the ground as second conductor. This is especially common in schemes with submarine cables, as sea water is a good conductor. Buried grounding electrodes are used to make the connection to the earth. The site of

these electrodes must be chosen carefully to prevent electrochemical corrosion on underground structures.

A particular concern in design of electrical substations is earth potential rise. When very large fault currents are injected into the earth, the area around the point of injection may rise to a high potential with respect to distant points. This is due to the limited finite conductivity of the layers of soil in the earth. The gradient of the voltage (changing voltage within a distance) may be so high that two points on the ground may be at significantly different potentials, creating a hazard to anyone standing on the ground in the area. Pipes, rails, or communication wires entering a substation may see different ground potentials inside and outside the substation, creating a dangerous touch voltage.

§Electronics[edit]

Signalground

Chassisground

Earthground

Ground symbols[7]

Signal grounds serve as return paths for signals and power (at extra low voltages, less than about 50 V) within equipment, and on the signal interconnections between equipment. Many electronic designs feature a single return that acts as a reference for all signals. Power and signal grounds often get connected, usually through the metal case of the equipment. Designers of printed circuit boardsmust take care in the layout of electronic systems so that high-power or rapidly-switching currents in one part of a system do not inject noise into low-level sensitive parts of a system due to some common impedance in the grounding traces of the layout.

§Circuit ground versus earth[edit]

Voltage is a differential quantity. To measure the voltage of a single point, a reference point must be selected to measure against. This common reference point is called "ground" and considered to have zero voltage. This signal ground may not be connected to a power ground. A system where the system ground is not connected to another circuit or to earth (though there may still be AC coupling) is often referred to as a floating ground.

§Separating low signal ground from a noisy ground[edit]

In television stations, recording studios, and other installations where sound quality is critical, a special signal ground known as a "technical ground" (or "technical earth", "special earth" and "audio

earth") is often installed, to prevent ground loops. This is basically the same thing as an AC power ground, but no general appliance ground wires are allowed any connection to it, as they may carry electrical interference. For example only audio equipment is connected to the technical ground in a recording studio.[8] In most cases, the studio's metal equipment racks are all joined together with heavy copper cables (or flattened copper tubing or busbars) and similar connections are made to the technical ground. Great care is taken that no general chassis grounded appliances are placed on the racks, as a single AC ground connection to the technical ground will destroy its effectiveness. For particularly demanding applications, the main technical ground may consist of a heavy copper pipe, if necessary fitted by drilling through several concrete floors, such that all technical grounds may be connected by the shortest possible path to a grounding rod in the basement.

§Lightning protection systems[edit]

Busbars are used for ground conductors in high-current circuits.

Lightning protection systems are designed to mitigate the effects of lightning through connection to extensive grounding systems that provide a large surface area connection to earth. The large area is required to dissipate the high current of a lightning strike without damaging the system conductors by excess heat. Since lightning strikes are pulses of energy with very high frequency components, grounding systems for lighting protection tend to use short straight runs of conductors to reduce the self-inductance and skin effect.

§Bonding[edit]

Main article: Electrical bonding

Strictly speaking, the terms grounding or earthing are meant to refer to an electrical connection to ground/earth. Bonding is the practice of intentionally electrically connecting metallic items not designed to carry electricity. This brings all the bonded items to the same electrical potential as a protection from electrical shock. The bonded items can then be connected to ground to bring them to earth potential.[9]

§Ground (earth) mat[edit]

Main article: Ground mat

In an electrical substation a ground (earth) mat is a mesh of conductive material installed at places where a person would stand to operate a switch or other apparatus; it is bonded to the local supporting metal structure and to the handle of the switchgear, so that the operator will not be exposed to a high differential voltage due to a fault in the substation.

In the vicinity of electrostatic sensitive devices, a ground (earth) mat or grounding (earthing) mat is used to ground static electricity generated by people and moving equipment. [10]There are two types used in static control: Static Dissipative Mats, and Conductive Mats.

A static dissipative mat that rests on a conductive surface (commonly the case in military facilities) are typically made of 3 layers (3-ply) with static dissipative vinyl layers surrounding a conductive substrate which is electrically attached to ground (earth). For commercial uses, static dissipative rubber mats are traditionally used that are made of 2 layers (2-ply) with a tough solder resistant top static dissipative layer that makes them last longer than the vinyl mats, and a conductive rubber bottom. Conductive mats are made of carbon and used only on floors for the purpose of drawing static electricity to ground as quickly as possible. Normally conductive mats are made with cushioning for standing and are referred to as "anti-fatigue" mats.

3 ply static dissipative vinyl grounding mat shown at macro scale

For a static dissipative mat to be reliably grounded it must be attached to a path to ground. Normally, both the mat and the wrist strap are connected to ground by using a common point ground system (CPGS).[11]

In computer repair shops and electronics manufacturing workers must be grounded before working on devices sensitive to voltages capable of being generated by humans. For that reason static dissipative mats can be and are also used on production assembly floors as "floor runner" along the assembly line to draw static generated by people walking up and down.

§Isolation[edit]

See also: Galvanic isolation

Isolation is a mechanism that defeats grounding. It is frequently used with low-power consumer devices, and when electronics engineers, hobbyists, or repairmen are working on circuits that would normally be operated using the power line voltage. Isolation can be accomplished by simply placing a "1:1 wire ratio" transformer with an equal number of turns between the device and the regular power service, but applies to any type of transformer using two or more coils electrically insulated from each other.

For an isolated device, touching a single powered conductor does not cause a severe shock, because there is no path back to the other conductor through the ground. However, shocks and electrocution may still occur if both poles of the transformer are contacted by bare skin. Previously it was suggested that repairmen "work with one hand behind their back" to avoid touching two parts of the device under test at the same time, thereby preventing a circuit from crossing through the chest and interrupting cardiac rhythms/ causingcardiac arrest.

Generally every AC power line transformer acts as an isolation transformer, and every step up or down has the potential to form an isolated circuit. However, this isolation would prevent failed devices from blowing fuses when shorted to their ground conductor. The isolation that could be created by each transformer is defeated by always having one leg of the transformers grounded, on both sides of the input and output transformer coils. Power lines also typically ground one specific wire at every pole, to ensure current equalization from pole to pole if a short to ground is occurring.

In the past, grounded appliances have been designed with internal isolation to a degree that allowed the simple disconnection of ground by cheater plugs without apparent problem (a dangerous practice, since the safety of the resulting floating equipment relies on the insulation in its power transformer). Modern appliances however often includepower entry modules which are designed with deliberate capacitive coupling between the AC power lines and chassis, to suppress electromagnetic interference. This results in a significant leakage current from the power lines to ground. If the ground is disconnected by a cheater plug or by accident, the resulting leakage current can cause mild shocks, even without any fault in the equipment.[12] Even small leakage currents are a significant concern in medical settings, as the accidental disconnection of ground can introduce these currents into sensitive parts of the human body. As a result, medical power supplies are designed to have low capacitance.[13]

Class II appliances and power supplies (such as cell phone chargers) do not provide any ground connection, and are designed to isolate the output from input. Safety is ensured by double-insulation, so that two failures of insulation are required to cause a shock.

http://en.wikipedia.org/wiki/Ground_%28electricity%29

Section 6: System Grounding Bill Brown, P.E., Square D Engineering Services Introduction The topic of system grounding is extremely important, as it affects the susceptibility of the system to voltage transients, determines the types of loads the system can accommodate, and helps to determine the system protection requirements. The system grounding arrangement is determined by the grounding of the power source. For commercial and industrial systems, the types of power sources generally fall into four broad categories: A Utility Service – The system grounding is usually determined by the secondary winding configuration of the upstream utility substation transformer. B Generator – The system grounding is determined by the stator winding configuration. C Transformer – The system grounding on the system fed by the transformer is determined by the transformer secondary winding configuration. D Static Power Converter – For devices such as rectifiers and inverters, the system grounding is determined by the grounding of the output stage of the converter. Categories A to D fall under the NEC definition for a “separately-derived system.” The recognition of a separatelyderived system is important when applying NEC requirements to system grounding, as discussed below. All of the power sources mentioned above except “D” are magnetically-operated devices with windings. To understand the system voltage relationships with respect to system grounding, it must be recognized that there are two common ways of connecting device windings: wye and delta. These two arrangements, with their system voltage relationships, are shown in figure 6-1. As can be seen from the figure, in the wye-connected arrangement there are four terminals, with the phase-to-neutral voltage for each phase set by the winding voltage and the resulting phase-to-phase voltage set by the vector relationships between the voltages. The delta configuration has only three terminals, with the phase-to-phase voltage set by the winding voltages and the neutral terminal not defined. Neither of these arrangements is inherently associated with any particular system grounding arrangement, although some arrangements more commonly use one arrangement vs. the other for reasons that will be explained further below. Figure 6-1: Wye and delta winding configurations and system voltage relationships 2 Solidly-grounded systems The solidly-grounded system is the most common system arrangement, and one of the most versatile. The most commonly-used configuration is the solidly-grounded wye, because it will support single-phase phase-toneutral loads. The solidly-grounded wye system arrangement can be shown by considering the neutral terminal from the wye system arrangement in figure 6-1 to be grounded. This is shown in figure 6-2: Several points regarding figure 6-2 can be noted. First, the system voltage with respect to ground is fixed by the phase-to-neutral winding voltage. Because parts of the power system, such as equipment frames, are grounded, and the rest of the environment essentially is at ground potential also, this has big implications for the system. It means that the line-to-ground insulation level of equipment need only be as large as the phase-to-neutral voltage, which is 57.7% of the phase-to-phase voltage. It also means that the system is less susceptible to phase-to-ground voltage transients. Second, the system is suitable for supplying line-to-neutral loads. The operation of a single-phase load connected between one phase

and neutral will be the same on any phase since the phase voltage magnitudes are equal. This system arrangement is very common, both at the utilization level as 480 Y/277 V and 208 Y/120 V, and also on most utility distribution systems. While the solidly-grounded wye system is by far the most common solidly-grounded system, the wye arrangement is not the only arrangement that can be configured as a solidly grounded system. The delta system can also be grounded, as shown in figure 6-3. Compared with the solidly-grounded wye system of figure 6-2 this system grounding arrangement has a number of disadvantages. The phase-to-ground voltages are not equal, and therefore the system is not suitable for single-phase loads. And, without proper identification of the phases there is the risk of shock since one conductor, the B-phase, is grounded and could be mis-identified. This arrangement is no longer in common use, although a few facilities where this arrangement is used still exist. The delta arrangement can be configured in another manner, however, that does have merits as a solidlygrounded system. This arrangement is shown in figure 6-4. While the arrangement of figure 6-4 may not appear at first glance to have merit, it can be seen that this system is suitable both for three-phase and single-phase loads, so long as the single-phase and three-phase load cables are kept separate from each other. This is commonly Figure 6-2: Solidly-Grounded Wye System arrangement and voltage relationships Figure 6-3: Corner-Grounded Delta System arrangement and voltage relationships 3 used for small services which require both 240 VAC three-phase and 120/240 VAC single-phase. Note that the phase A voltage to ground is 173% of the phase B and C voltages to ground. This arrangement requires the BC winding to have a center tap. A common characteristic of all three solidly-grounded system shown here, and of solidly-grounded systems in general, is that a short-circuit to ground will cause a large amount of short-circuit current to flow. This condition is known as a ground fault and is illustrated in figure 6-5. As can be seen from figure 6-5, the voltage on the faulted phase is depressed, and a large current flows in the faulted phase since the phase and fault impedance are small. The voltage and current on the other two phases are not affected. The fact that a solidly-grounded system will support a large ground fault current is an important characteristic of this type of system grounding and does affect the system design. Statistically, 90-95% of all system short-circuits are ground faults so this is an important topic. The practices used in ground-fault protection are described in a later section of this guide. The occurrence of a ground fault on a solidly-grounded system necessitates the removal of the fault as quickly as possible. This is the major disadvantage of the solidly-grounded system as compared to other types of system grounding. A solidly-grounded system is very effective at reducing the possibility of line-to-ground voltage transients. However, to do this the system must be effectively grounded. One measure of the effectiveness of the system grounding is the ratio of the available ground-fault current to the available three-phase fault current. For effectively-grounded systems this ratio is usually at least 60% [2]. Most utility systems which supply service for commercial and industrial systems are solidly grounded. Typical utility practice is to ground the neutral at many points, usually at every line pole, creating a multi-grounded neutral system. Because a separate grounding conductor is not run with the utility line, the resistance of the earth limits the circulating ground currents that can be caused by this type of grounding. Because separate grounding conductors are used inside a commercial or industrial facility, multi-grounded neutrals not preferred for power systems in these facilities due to the possibility of circulating ground currents. As will be explained later in this Figure 6-4: Center-Tap-Grounded Delta System arrangement and voltage relationships Figure 6-5: Solidly-Grounded System with a ground fault on phase A 4 section, multi-grounded neutrals in NEC jurisdictions, such as commercial or industrial

facilities, are actually prohibited in most cases by the NEC [1]. Instead, a single point of grounding is preferred for this type of system, creating a uni-grounded or single-point grounded system. In general, the solidly-grounded system is the most popular, is required where single-phase phase-to-neutral loads must be supplied, and has the most stable phase-to-ground voltage characteristics. However, the large ground fault currents this type of system can support, and the equipment that this necessitates, are a disadvantage and can be hindrance to system reliability. Ungrounded systems This system grounding arrangement is at the other end of the spectrum from solidly-grounded systems. An ungrounded system is a system where there is no intentional connection of the system to ground. The term “ungrounded system” is actually a misnomer, since every system is grounded through its inherent charging capacitance to ground. To illustrate this point and its effect on the system voltages to ground, the delta winding configuration introduced in figure 6-3 is re-drawn in figure 6-6 to show these system capacitances. If all of the system voltages in figure 6-6 are multiplied by √3 and all of the phase angles are shifted by 30˚ (both are reasonable operations since the voltage magnitudes and phase angles for the phase-to-phase voltage were arbitrarily chosen), the results are the same voltage relationships as shown in figure 6-4 for the solidly-grounded wye system. The differences between the ungrounded delta system and the solidly-grounded wye system, then, are that there is no intentional connection to ground, and that there is no phase-to-neutral driving voltage on the ungrounded delta system. This becomes important when the effects of a ground fault are considered. The lack of a grounded system neutral also makes this type of system unsuitable for single-phase phase-to-neutral loads. In figure 6-7, the effects of a single phase to ground fault are shown. The equations in figure 6-7 are not immediately practical for use, however if the fault impedance is assumed to be zero and the system capacitive charging impedance is assumed to be much larger than the phase impedances, these equations reduce into a workable form. Figure 6-8 shows the resulting equations, and shows the current and voltage phase relationships. As can be seen from figure 6-8, the net result of a ground fault on one phase of an ungrounded delta system is a change in the system phase-to-ground voltages. The phase-to-ground voltage on the faulted phase is zero, and the phase-to-ground voltage on the unfaulted phases are 173% of their nominal values. This has implications for power equipment – the phase-to-ground voltage rating for equipment on an ungrounded system must be at least equal the phase-to-phase voltage rating. This also has implications for the methods used for ground detection, as explained later in this guide. Figure 6-6: Ungrounded Delta System winding arrangement and voltage relationships 5 The ground currents with one phase is faulted to ground are essentially negligible. Because of this fact, from an operational standpoint ungrounded systems have the advantage of being able to remain in service if one phase is faulted to ground. However, suitable ground detection must be provided to alarm this condition (and is required in most cases by the NEC [1] as described below). In some older facilities, it has been reported that this type of system has remained in place for 40 years or more with one phase grounded! This condition is not dangerous in and of itself (other than due to the increased phase-to-ground voltage on the unfaulted phases), however if a ground fault occurs on one of the ungrounded phases the result is a phase-to-phase fault with its characteristic large fault current magnitude. Another important consideration for an ungrounded system is its susceptibility to large transient overvoltages. These can result from a resonant or near-resonant condition during ground faults, or from arcing [2]. A resonant ground fault condition occurs when the inductive reactance of the ground-fault path approximately equals the Figure 6-7: Ungrounded Delta System with a ground-fault on one phase Figure 6-8:

Ungrounded Delta System – simplified ground fault voltage and current relationships 6 system capacitive reactance to ground. Arcing introduces the phenomenon of current-chopping, which can cause excessive overvoltages due to the system capacitance to ground. The ground detection mentioned above can be accomplished through the use of voltage transformers connected in wye-broken delta, as illustrated in figure 6-9. In figure 6-9, three ground detection lights “LTA,” “LTB” and “LTC” are connected so that they indicate the A, B and C phase-to-ground voltages, respectively. A master ground detection light “LTM” indicates a ground fault on any phase. With no ground fault on the system “LTA,” “LTB” and “LTB” will glow dimly. If a ground fault occurs on one phase, the light for that phase will be extinguished and “LTM” will glow brightly along with the lights for the other two phases. Control relays may be substituted for the lights if necessary. Resistor “R” is connected across the broken-delta voltage transformer secondaries to minimize the possibility of ferroresonance. Most ground detection schemes for ungrounded systems use this system or a variant thereof. Note that the ground detection per figure 6-10 indicates on which phase the ground fault occurs, but not where in the system the ground fault occurs. This, along with the disadvantages of ungrounded systems due to susceptibility to voltage transients, was the main impetus for the development of other ground system arrangements. Modern power systems are rarely ungrounded due to the advent of high-resistance grounded systems as discussed below. However, older ungrounded systems are occasionally encountered. Figure 6-9: A Ground Detection method for ungrounded systems B A C VT VT VT LT A LT B LT C LT M GROUND FAULT LOCATION LTA LTB LTC LTM PHASE A PHASE B PHASE C NONE DIM DIM DIM OFF OFF BRIGHT BRIGHT BRIGHT BRIGHT OFF BRIGHT BRIGHT BRIGHT BRIGHT DIM BRIGHT R High-resistance grounded systems One ground arrangement that has gained in popularity in recent years is the high-resistance grounding arrangement. For low voltage systems, this arrangement typically consists of a wye winding arrangement with the neutral connected to ground through a resistor. The resistor is sized to allow 1-10 A to flow continuously if a ground fault occurs. This arrangement is illustrated in figure 6-10. Figure 6-10: High-Resistance Grounded System with no ground fault present 7 The resistor is sized to be less than or equal to the magnitude of the system charging capacitance to ground. If the resistor is thus sized, the high-resistance grounded system is usually not susceptible to the large transient overvoltages that an ungrounded system can experience. The ground resistor is usually provided with taps to allow field adjustment of the resistance during commissioning. If no ground fault current is present, the phasor diagram for the system is the same as for a solidly-grounded wye system, as shown in figure 6-10. However, if a ground fault occurs on one phase the system response is as shown in figure 6-11. As can be seen from figure 6-11, the ground fault current is limited by the grounding resistor. If the approximation is made that ZA and ZF are very small compared to the ground resistor resistance value R, which is a good approximation if the fault is a bolted ground fault, then the ground fault current is approximately equal to the phase-to-neutral voltage of the faulted phase divided by R. The faulted phase voltage to ground in that case would be zero and the unfaulted phase voltages to ground would be 173% of their values without a ground fault present. This is the same phenomenon exhibited by the ungrounded system arrangement, except that the ground fault current is larger and approximately in-phase with the phase-to-neutral voltage on the faulted phase. The limitation of the ground fault current to such a low level, along with the absence of a solidly-grounded system neutral, has the effect of making this system ground arrangement unsuitable for single-phase line-toneutral loads. The ground fault current is not large enough to force its removal by taking the system off-line. Therefore, the high-

resistance grounded system has the same operational advantage in this respect as the ungrounded system. However, in addition to the improved voltage transient response as discussed above, the high-resistance grounded system has the advantage of allowing the location of a ground fault to be tracked. A typical ground detection system for a high-resistance grounded system is illustrated in figure 6-12. The ground resistor is shown with a tap between two resistor sections R1 and R2. When a ground fault occurs, relay 59 (the ANSI standard for an overvoltage relay, as discussed later in this guide) detects the increased voltage across the resistor. It sends a signal to the control circuitry to initiate a ground fault alarm by energizing the “alarm” indicator. When the operator turns the pulse control selector to the “ON” position, the control circuit causes pulsing contact P to close and re-open approximately once per second. When P closes R2 is shorted and the “pulse” indicator is energized. R1 and R2 are sized so that approximately 5-7 times the resistor continuous ground fault current flows when R2 is shorted. The result is a pulsing ground fault current that can be detected using a clamp-on ammeter (an analog ammeter is most convenient). By tracing the circuit with the ammeter, the ground fault location can be determined. Once the ground fault has been removed from the system pressing the “alarm reset” button will de-energize the “alarm” indicator. This type of system is known as a pulsing ground detection system and is very effective in locating ground faults, but is generally more expensive than the ungrounded system ground fault indicator in figure 6-10. Figure 6-11: High-Resistance Grounded System with a ground fault on one phase 8 For medium voltage systems, high-resistance grounding is usually implemented using a low voltage resistor and a neutral transformer, as shown in figure 6-13. Reactance grounding In industrial and commercial facilities, reactance grounding is commonly used in the neutrals of generators. In most generators, solid grounding may permit the level of ground-fault current available from the generator to exceed the three-phase value for which its windings are braced [2]. For these cases, grounding of the generator neutral through an air-core reactance is the standard solution for lowering the ground fault level. This reactance ideally limits the ground-fault current to the three-phase available fault current and will allow the system to operate with phase-to-neutral loads. Low-resistance grounded systems By sizing the resistor in figure in 6-11 such that a higher ground fault current, typically 200-800 A, flows during a ground fault a low-resistance grounded system is created. The ground fault current is limited, but is of high enough magnitude to require its removal from the system as quickly as possible. The low-resistance grounding arrangement is typically used in medium voltage systems which have only 3-wire loads, such as motors, where limiting damage to the equipment during a ground fault is important enough to include the resistor but it is acceptable to take the system offline for a ground fault. The low-resistance grounding arrangement is generally less expensive than the high-resistance grounding arrangement but more expensive than a solidly grounded system arrangement. Creating an artificial neutral in an ungrounded system In some cases it is required to create a neutral reference for an ungrounded system. Most instances involve existing ungrounded systems which are being upgraded to high-resistance grounding. The existence of multiple transformers and/or delta-wound generators may make the replacement of this equipment economically unfeasible. Figure 6-12: Pulsing Ground Detection System Figure 6-13: Medium Voltage implementation for high-resistance grounding 9 The solution is a grounding transformer. Although several different configurations exist, by far the most popular in commercial and industrial system is the zig-zag transformer arrangement. It uses transformers connected as shown in figure 6-14: The zig-zag transformer will only pass ground current. Its typical implementation on an ungrounded system, in order to convert the system to a high-resistance

grounded system, is shown in figure 6-15. The zig-zag transformer distributes the ground current IG equally between the three phases. For all practical purposes the system, from a grounding standpoint, behaves as a high-resistance grounded system. The solidly-grounded and low-resistance grounded systems can also be implemented by using a grounding transformer, depending upon the amount of impedance connected in the neutral. NEC system grounding requirements The National Electrical Code [1] does place constraints on system grounding. While this guide is not intended to be a definitive guide to all NEC requirements, several points from the NEC must be mentioned and are based upon the basic principles stated above. As a starting point, several key terms from the NEC need to be defined: Ground: A conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth or to some body that serves in place of the earth. Grounded: Connected to earth or to some body that serves in place of the earth. Figure 6-14: Zig-Zag grounding transformer arrangement Figure 6-15: Zig-Zag grounding transformer implementation 10 Effectively Grounded: Intentionally connected to earth through a ground connection or connections of sufficiently low impedance and having sufficient current-carrying capacity to prevent the buildup of voltages that may result in undue hazards to connected equipment or to persons. Grounded Conductor: A system or circuit conductor that is intentionally grounded. Solidly Grounded: Connected to ground without inserting any resistor or impedance device. Grounding Conductor: A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes. Equipment Grounding Conductor: The conductor used to connect the non-current-carrying metal parts of equipment, raceways and other enclosures to the system grounded conductor, grounding electrode conductor, or both, at the service equipment or at the source of a separately-derived system. Main Bonding Jumper: The connection between the grounded circuit conductor and the equipment grounding conductor at the service. System Bonding Jumper: The connection between the grounded circuit conductor and the equipment grounding conductor at a separately-derived system. Grounding Electrode: The conductor used to connect the grounding electrode(s) to the equipment grounding conductor, to the grounded conductor, or to both, at the service, at each building or structure where supplied by a feeder(s) or branch circuit(s), or at the source of a separately-derived system. Grounding Electrode Conductor: The conductor used to connect the grounding electrode(s) to the equipment grounding conductor, to the grounded conductor, or to both, at the service, at each building or structure where supplied by a feeder(s) or branch circuit(s), or at the source of a separately-derived system. Ground Fault: An unintentional, electrically conducting connection between an ungrounded conductor of an electrical circuit and the normally non–current-carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or earth. Ground Fault Current Path: An electrically conductive path from the point of a ground fault on a wiring system through normally non–current-carrying conductors, equipment, or the earth to the electrical supply source. Effective Ground-Fault Current Path: An intentionally constructed, permanent, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground fault detectors on high-impedance grounded systems. Ground-Fault Circuit Interrupter: A device intended for the protection of personnel that functions to de-energize a circuit or portion thereof within an established period of time when a current to ground exceeds the values established for a Class A device. FPN: Class A ground-fault circuit interrupters trip when the current to ground has a value in the range of 4 mA to 6 mA. For further

information, see UL 943, Standard for Ground-Fault Circuit Interrupters. Ground Fault Protection of Equipment: A system intended to provide protection of equipment from damaging line-to-ground fault currents by operating to cause a disconnecting means to open all ungrounded conductors of the faulted circuit. This protection is provided at current levels less than those required to protect conductors from damage through the operation of a supply circuit overcurrent device. Qualified Person: One who has the skills and knowledge related to the construction and operation of the electrical equipment and installations and has received safety training on the hazards involved.

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