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GROUNDING OF ELECTRICAL

SYSTEMS AND EQUIPMENT

INTRODUCTION

Electrical grounding seems to be one of the most misunderstood subjects of the NationalElectrical Code (NEC). This is evidenced by one report which stated that the OccupationalSafety and Health Administration (OSHA) documented more than 20,000 violations ofgrounding in one year. Most of these violations came from loose, damaged, or missingexternal grounds. Just imagine the increased numbers of violations if the OSHA inspectoropened electrical equipment to check equipment ground connections or if they used test

equipment to verify proper grounding!The real problem is that most electrical equipment will work without a ground connec-

tion. For example, an electric drill will notwork if the hot or neutral wire is open however,if the ground wire is open, it will work properly. Even though the drill works properly with-out a ground connection, it is not safe to use.

This chapter addresses several issues dealing with general grounding and bonding, sys-tem grounding, and equipment grounding. Proper grounding is an issue that must be seri-ously considered for all electrical installations and equipment. Strict adherence to theOSHA requirements, NEC Article 250, as well as the numerous consensus standards andreference books, reduces the risk of electrical shock from contact with inadvertently ener-

gized equipment, enclosures, and structures.The information in this chapter is not intended to be a substitute for the NEC or OSHA

requirements. Nor is it intended to replace or be a substitute for any other standard quotedherein. For proper grounding of electrical systems and equipment, always comply with therequirements contained in the current standards and regulations.

ELECTRIC SHOCK HAZARD

Energized conductors and circuit components, installed within or on electrical equipment,are insulated from the equipments metal enclosure, to provide protection for personnelwho operate the equipment, from being exposed to dangerous voltages. When aging or mal-function causes the insulation to break down, the energized conductors within the equip-ment can make direct contact with the metal enclosure, thereby energizing it. Anyonemaking contact with energized equipment could be injured or killed.

Equipment grounding is a way of nullifying this shock hazard. An equipment groundforms a very low impedance path to ground. Personnel making contact with the energizedenclosure will be exposed to less voltage than they would otherwise.

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CHAPTER 4

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4.2 CHAPTER FOUR

Simply stated, proper bonding and grounding of electrical equipment will substantiallyreduce the risk of electrical shock by effectively eliminating a difference in potential.

GENERAL REQUIREMENTS FOR GROUNDING

AND BONDING

Definitions

The following terms are defined here due to their frequent use in this chapter. The definitionscontained in this chapter are taken verbatim from OSHA 29 CFR 1910.399. Corresponding

definitions can also be found in Article 100 of the current edition of the NEC.

Bonding (Bonded). The permanent joining of metallic parts to form an electrically con-ductive path that will ensure electrical continuity and the capacity to conduct safely any cur-rent likely to be imposed.

Bonding Jumper. A reliable conductor to ensure the required electrical conductivitybetween metal parts required to be electrically connected.

Bonding Jumper, Equipment. The connection between two or more portions of the

equipment grounding conductor.

Bonding Jumper, Main. The connection between the grounded circuit conductor and theequipment grounding conductor at the service.

Energized. Electrically connected to a source of potential difference.

Equipment. A general term including material, fittings, devices, appliances, fixtures,apparatus, and the like used as a part of, or in connection with, an electrical installation.

Ground. A conducting connection, whether intentional or accidental, between an electri-cal circuit or equipment and the earth, or to some conducting body that serves in place ofthe earth.

Grounded. Connected to earth or to some conducting body that serves in place of theearth.

Grounded, Effectively. Intentionally connected to earth through a ground connection orconnections of sufficiently low impedance and having sufficient current-carrying capacityto prevent the buildup of voltages that may result in undue hazards to connected equipment

or to persons.

Grounding Conductor. A conductor used to connect equipment or the grounded circuitof a wiring system to a grounding electrode or electrodes.

Grounding Conductor, Equipment. The conductor used to connect the non-current-carrying metal parts of equipment, raceways, and other enclosures to the system groundedconductor, the grounding electrode conductor, or both, at the service equipment or at thesource of a separately derived system.


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Grounding Electrode Conductor. The conductor used to connect the grounding elec-trode to the equipment grounding conductor, to the grounded conductor, or to both, of thecircuit at the service equipment or at the source of a separately derived system.

Grounding Electrode System. Two or more grounding electrodes that are effectivelybonded together shall be considered a single grounding electrode system.

Ground-Fault Circuit Interrupter. A device intended for the protection of personnel thatfunctions to de-energize a circuit or portion thereof within an established period of timewhen a current to ground exceeds some predetermined value that is less than that requiredto operate the overcurrent protective device of the supply circuit.

Ground-Fault Protection of Equipment. A system intended to provide protection ofequipment from damaging line-to-ground fault currents by operating to cause a discon-

necting means to open all ungrounded conductors of the faulted circuit. This protection isprovided at current levels less than those required to protect conductors from damagethrough the operation of a supply circuit overcurrent device.

Grounding of Electrical Systems

Grounded electrical systems are required to be connected to earth in such a way as to limitany voltages imposed by lightning, line surges, or unintentional contact with higher volt-age lines. Electrical systems are also grounded to stabilize the voltage to earth during nor-mal operation. If, for example, the neutral of a 120/240 V, wye-connected secondary of a

transformer were not grounded, instead of being 120 V to ground, the voltage could reachseveral hundred volts to ground. A wye-connected electrical system becomes very unstableif it is not properly grounded.

The following requirements are taken from OSHA 29 CFR 1910.304(f) and can also befound in the 2005 National Electrical Code (NEC) Section 250.20.

(f) Grounding. Paragraphs (f)(1) through (f)(7) of this section contain groundingrequirements for systems, circuits, and equipment.

(1) Systems to be grounded. The following systems, which supply premises wiring,shall be grounded:

(i) All 3-wire DC systems shall have their neutral conductor grounded.

(ii) Two-wire DC systems operating at over 50 volts through 300 volts between con-ductors shall be grounded unless:

(A) They supply only industrial equipment in limited areas and are equipped with aground detector; or

(B) They are rectifier-derived from an AC system complying with paragraphs (f)(1)(iii),(f)(1)(iv), and (f)(1)(v) of this section; or

(C) They are fire-protective signaling circuits having a maximum current of 0.030

amperes.(iii) AC circuits of less than 50 volts shall be grounded if they are installed as overheadconductors outside of buildings or if they are supplied by transformers and the trans-former primary supply system is ungrounded or exceeds 150 volts to ground.

(iv) AC systems of 50 volts to 1000 volts shall be grounded under any of the followingconditions, unless exempted by paragraph (f)(1)(v) of this section:

(A) If the system can be so grounded that the maximum voltage to ground on theungrounded conductors does not exceed 150 volts; (see Fig. 4.1)

GROUNDING OF ELECTRICAL SYSTEMS AND EQUIPMENT 4.3


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(B) If the system is nominally rated 480Y/277 volt, 3-phase, 4-wire in which the neu-tral is used as a circuit conductor; (see Fig. 4.2)

(C) If the system is nominally rated 240/120 volt, 3-phase, 4-wire in which the midpointof one phase is used as a circuit conductor; (see Fig. 4.3)

Or,

(D) If a service conductor is uninsulated.

(v) AC systems of 50 volts to 1000 volts are not required to be grounded under any ofthe following conditions:

(A) If the system is used exclusively to supply industrial electric furnaces for melting,refining, tempering, and the like.

(B) If the system is separately derived and is used exclusively for rectifiers supplying

only adjustable speed industrial drives.(C) If the system is separately derived and is supplied by a transformer that has a primaryvoltage rating less than 1000 volts, provided all of the following conditions are met:

{1} The system is used exclusively for control circuits,

4.4 CHAPTER FOUR

FIGURE 4.1 Voltage-to-ground less than 150 volts. (Courtesy of AVO Training Institute, Inc.)

FIGURE 4.2 480/277 volt systems. (Courtesy of AVO Training Institute, Inc.)


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{2} The conditions of maintenance and supervision assure that only qualified personswill service the installation,

{3} Continuity of control power is required, and

{4} Ground detectors are installed on the control system.

(D) If the system is an isolated power system that supplies circuits in health carefacilities.

(2) Conductors to be grounded. For AC premises wiring systems the identified con-ductor shall be grounded.

(4) Grounding path. The path to ground from circuits, equipment, and enclosures shallbe permanent and continuous.

(5) Supports, enclosures, and equipment to be grounded(i) Supports and enclo-sures for conductors. Metal cable trays, metal raceways, and metal enclosures for con-ductors shall be grounded, except that:

(7) Grounding of systems and circuits of 1000 volts and over (high voltage.)(i)General. If high voltage systems are grounded, they shall comply with all applicableprovisions of paragraphs (f)(1) through (f)(6) of this section as supplemented and mod-ified by this paragraph (f)(7).

(ii) Grounding of systems supplying portable or mobile equipment. [see1910.302(b)(3)] Systems supplying portable or mobile high voltage equipment, otherthan substations installed on a temporary basis, shall comply with the following:

(A) Portable and mobile high voltage equipment shall be supplied from a system hav-ing its neutral grounded through an impedance. If a delta-connected high voltage sys-tem is used to supply the equipment, a system neutral shall be derived.

(B) Exposed non-current-carrying metal parts of portable and mobile equipment shallbe connected by an equipment grounding conductor to the point at which the systemneutral impedance is grounded.

(C) Ground-fault detection and relaying shall be provided to automatically de-energizeany high voltage system component which has developed a ground fault. The continu-ity of the equipment grounding conductor shall be continuously monitored so as to de-energize automatically the high voltage feeder to the portable equipment upon loss ofcontinuity of the equipment grounding conductor.


FIGURE 4.3 Delta system with neutral. (Courtesy of AVO Training Institute, Inc.)


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(D) The grounding electrode to which the portable or mobile equipment system neu-tral impedance is connected shall be isolated from and separated in the ground by atleast 20 feet from any other system or equipment grounding electrode, and there shallbe no direct connection between the grounding electrodes, such as buried pipe, fence,

and so on.

Grounding of Electrical Equipment

OSHA 1910.304(f)(7)(iii) Grounding of equipment. All non-current-carrying metalparts of portable equipment and fixed equipment including their associated fences, hous-ings, enclosures, and supporting structures shall be grounded. However, equipment that isguarded by location and isolated from ground need not be grounded. Additionally, pole-

mounted distribution apparatus at a height exceeding 8 feet above ground or grade levelneed not be grounded.In 29 CFR 1910.303, General Requirements, OSHA states under (b) Examination,

installation, and use of equipment (1) Examination that Electrical equipment shall befree from recognized hazards that are likely to cause death or serious physical harm toemployees. This section continues with other factors which contribute to the practicalsafeguarding of employees using or likely to come in contact with the equipment. One ofthese other factors is proper grounding. If the non-current-carrying metal parts of elec-tric equipment are not properly grounded and these parts become energized, then anyemployee using or likely to come in contact with the equipment is at risk of an electri-

cal shock that may or may not be fatal. This is a risk that must not be taken. Proper ground-ing can effectively eliminate this shock hazard by providing a permanent and continuous lowimpedance path for ground-fault current to follow in order to clear the circuit protectivedevice(s).

Bonding of Electrically Conductive Materials and Other Equipment

Bonding is the permanent joining of metallic parts of materials and equipment. Whendifferent metal parts are not bonded together, a difference in potential could exist

between the metal parts. This creates an electrically hazardous condition between theparts. Anyone simultaneously coming into contact with the metal parts would be subjectto electrical shock, burns, or electrocution. When all conductive materials and parts ofequipment are permanently bonded together, there is only one piece of metal and nopotential difference exists between the parts. The metal parts must also be grounded toearth in order to be at earth potential. This minimizes the risk of touch-potential andstep-potential hazards when working on or around metal enclosures that could becomeenergized.

Other metallic equipment that is in contact with or adjacent to the electrical equipmentshould also be grounded to prevent a difference in potential in the event that a ground fault

occurs in the electrical equipment. This could include other piping as well as ducts in ven-tilation systems as shown in Fig. 4.4.

As discussed in the previous section, all non-current-carrying components of electricalequipment must be grounded. Equipment grounding conductors are installed and con-nected to the required terminal in the equipment to provide the low impedance path for faultcurrent to clear the circuit. All other metallic components of the equipment must be bondedto the grounded portion of the equipment in order to prevent a difference in potentialbetween the components. An example of this would be service enclosures. Figure 4.5 illus-trates bonding of service enclosures.

4.6 CHAPTER FOUR


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FIGURE 4.4 Bonding of other piping and duct systems. (Courtesy of AVOTraining Institute, Inc.)

FIGURE 4.5 Bonding service enclosures. (Courtesy of AVO Training Institute, Inc.)

4.7


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and other approved means. Solder must not be used as the sole connection. Solder is toosoft and has a very low melting point and, therefore, becomes a fuse in the grounding con-nection. Also, never use sheet metal screws to make connections between the groundingconductor and the enclosure. The use of sheet metal screws would provide limited metal-

to-metal contact between the screw and the enclosure and, therefore, may not provide therequired low-impedance connection.

Protection of Ground Clamps and Fittings

All grounding connections must be protected from physical damage either by location orby means of an enclosure made of wood, metal, or equivalent. Damaged grounding con-ductor connections can result in loss of continuity in the ground path, which will create a

potential shock hazard.

Clean Surfaces

If the grounding connection point is contaminated with paint or other such coatings, goodcontinuity may not be accomplished. All surfaces must be cleaned as needed to remove anysuch coatings or other contaminants that could interfere with the continuity of the ground-ing connection. As was stated earlier, the grounding system must create a sufficiently lowimpedance path in order for circuit protective devices to clear the circuit in the event of aground fault.

SYSTEM GROUNDING

Purposes of System Grounding

Power systems are grounded for one or more of the following reasons:

Control of the voltage to ground to within predictable limits.

To provide for current flow and allow detection and location of ground faults.

To reduce shock hazard to personnel.

It is not always possible to accomplish all these goals with a particular method of ground-ing. In some cases, the chosen method of grounding is a compromise between conflict-ing goals. Table 4.1 shows the advantages and disadvantages of the various groundingmethods.

Grounding Service-Supplied Alternating-Current Systems

The premises wiring system of a grounded electrical service must be grounded. Thismeans that each grounded service must have a grounding electrode conductor connectedto the grounding electrode and to the service equipment. The grounded service conductoris also connected to the grounding electrode conductor. OSHA provides this requirement in



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29 CFR 1910.304(3)(i), (ii), and (iii) (NEC Section 250.24 provides a more in-depthrequirement) as quoted below:

(3) Grounding connections. (i) For a grounded system, a grounding electrode conductor shallbe used to connect both the equipment grounding conductor and the grounded circuit conduc-tor to the grounding electrode (see Fig. 4.6). Both the equipment grounding conductor and thegrounding electrode conductor shall be connected to the grounded circuit conductor on the sup-ply side of the service disconnecting means, or on the supply side of the system disconnectingmeans or overcurrent devices if the system is separately derived.

(ii) For an ungrounded service-supplied system, the equipment grounding conductor shall beconnected to the grounding electrode conductor at the service equipment. For an ungrounded

4.10 CHAPTER FOUR

TABLE 4.1 Comparison of System Grounding Methods

Method of grounding Advantages Disadvantages

Ungroundedno intentional Little or no ground current. Possiblity of large transientconnection to ground overvoltages.

Plant does not need to trip Overvoltages on the unfaulted phases.

for a single ground fault. Hard to detect and locate ground

faults.

Possibility of ferroresonance.

High Resistanceground Limit transient Overvoltage on the unfaulted phases.

fault < 10 A. overvoltages to 250%. Surge arresters need to be rated for

Can usually keep plant the phase-to-phase voltage.

running through a single

ground fault. .Ground fault can be detected.

Low Resistanceground Limit transient overvoltages Overvoltage on the unfaulted

fault > 100 A. to 250%. phases.

Immediate and selective Surge arresters need to be rated

fault clearing is possible. phase-to-phase voltage.

Reactanceground fault at Prevent serious transient High ground fault currents that

least 25% and preferably overvoltages. must be cleared.

60% of the three-phase

fault

Xo< 10 X1 Limit generator phase toground fault to the

magnitude of the three-

phase fault.

Solidly groundedsolid Limits system overvoltages. Extreme ground fault magnitudes.

connection between Freedom from ferroresonance. Safety risks (arc blast and flash).

neutral and ground.

RoX1X0< 3X1 Easier detection, location, and Stray voltages due to ground

Note that ZoO selective tripping of ground faults (shock hazard).

faults. Mechanical and thermal stress

Can use phase to ground and damage during ground faults.rated surge arresters.

Ground fault must be detected and

cleared, even if the plant must take

an outage.

Source: Courtesy of AVO Training Institue, Inc.


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separately derived system, the equipment grounding conductor shall be connected to thegrounding electrode conductor at, or ahead of, the system disconnecting means or overcurrent

devices.(iii) On extensions of existing branch circuits, which do not have an equipment groundingconductor, grounding-type receptacles may be grounded to a grounded cold water pipe near theequipment.

An important point to make here is that the grounding and grounded conductors are onlyallowed to be connected together on the line-side of the service disconnecting means, they aregenerally not allowed to be connected on the load-side. This issue will be addressed in moredetail in the section titled Use of Grounded Circuit Conductor for Grounding Equipment.

Conductors to Be GroundedAlternating-Current Systems

The following conductors of an AC premises wiring system are required to be grounded:

1. One conductor of a single-phase, 2-wire system

2. The neutral conductor of a single-phase, 3-wire system

3. The common conductor of a multiphase system where one wire is common to all phases

4. One phase conductor of a multiphase system requiring a grounded phase

5. The neutral conductor of a multiphase system where one phase is used as the neutral

Main Bonding Jumper

In a grounded electrical system, an unspliced main bonding jumper is required to connectall grounding and grounded conductors to the service equipment enclosure. This connec-tion is made within the service equipment enclosure using either a ground bus, a screw,strap, or wire. Figure 4.7 further illustrates this requirement:


FIGURE 4.6 Grounding service conductor connection to grounding electrode. (Courtesy of AVO TrainingInstitute, Inc.)


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Grounding Electrode System

A grounding electrode system is made up of a grounding electrode(s), any required bondingjumpers between electrodes, and a grounding electrode conductor. All conductors and jumpersused in the grounding electrode system must be sized in accordance with NEC Section 250.66and connected as specified in Section 250.70. There may be several types of grounding elec-trodes available at each building or structure. All available electrodes must be bonded togetherand used to form a grounding electrode system. Available electrodes include metal under-

ground water pipe, the metal frame of a building, concrete-encased electrodes, and groundrings. Figure 4.8 illustrates several common types of grounding electrodes. Several of the elec-trodes mentioned above have specific requirements as noted in Fig. 4.9.

If only a water pipe is available for the grounding electrode, it must be supplementedby one of the other electrodes mentioned above or by a made electrode as illustrated inFig. 4.10.

Made electrodes, such as a ground rod, pipe, or plate, may also be utilized to supple-ment existing electrodes. Figure 4.11 illustrates the minimum requirements for a groundrod or pipe.

There are two alternate installation methods for ground rods or pipes that are permitted to

be utilized if vertical installation is not possible due to rock or other obstructions. Figure 4.12illustrates these alternate methods.

Figure 4.13 illustrates the minimum requirements for a ground plate.Where practicable, these made electrodes are required to be installed below the perma-

nent moisture level and be free from nonconductive coatings. If more than one electrode isused, they must be installed at least 6 ft from each other. A common practice is to place therods apart at a distance equal to the length of the rod. The best industry practice is to installthe rods at least 10 ft apart. This practice will minimize the risk of dissipation overlap in theevent of a ground fault.

4.12 CHAPTER FOUR

FIGURE 4.7 Location of the main bonding jumper. (Courtesy of AVO TrainingInstitute, Inc.)


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FIGURE 4.8 Types of Grounding Electrodes. (Courtesy of AVO Training Institute, Inc.)

FIGURE 4.9 Electrode connections. (Courtesy of AVO TrainingInstitute, Inc.)


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Grounding Electrode SystemResistance

Resistance of all grounding connections mustalso be addressed briefly. There are three com-ponents of resistance to consider: (1) ground-ing electrode resistance, (2) contact resistance

between the electrode and the soil, and (3) theresistance of the soil. These three componentsare illustrated in Figure 4.14.

The grounding electrode, as a general rule,has very low resistance. The contact of the elec-trode with the surrounding soil is also generallylow. The third component, soil, can very dramat-ically in resistance from one location to anotherdue to different types and conditions of the soil.For example, the resistivity of inorganic clay can

range from 1,0005,500 ohm/cm whereas gravelcan range from 60,000100,000 ohm/cm. Everytype and condition of soil will, as can be seen,vary tremendously. The soil type must beknown and grounding electrode testing must bedone periodically in order to know whether ornot a good grounding system is present. NECSection 250.56 specifies that the resistance toground for a made electrode must be 25 ohms orless. If this value cannot be obtained, then mul-

tiple electrodes must be used. As was mentionedearlier, make sure the electrodes are spaced farenough apart to prevent dissipation overlap.

Grounding Electrode Conductor

OSHA 29 CFR 1910.399 defines the grounding electrode conductor as: The conductor used toconnect the grounding electrode to the equipment grounding conductor and/or to the groundedconductor of the circuit at the service equipment or at the source of a separately derived system.

4.14 CHAPTER FOUR

FIGURE 4.10 Sole connections. (Courtesy of AVO TrainingInstitute, Inc.)

FIGURE 4.11 Minimum requirements for aground rod or pipe. (Courtesy of AVO TrainingInstitute, Inc.)


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Although copper is the preferred material for the grounding electrode conductor, alu-minum and copper-clad aluminum are also acceptable. There are, however, restrictions withthe latter two materials. The aluminum and copper-clad aluminum conductors are notallowed by code to be installed in direct contact with masonry or the earth, or where corro-sive conditions exist. If used, these materials must not be installed within 18 in of earth.Copper, on the other hand, is considered a corrosion resistant material and, therefore, is per-mitted to be installed in most locations.

NEC Section 250.64(C) requires the grounding electrode conductor to be installed con-tinuous without splice or joint. There are, however, two deviations from this rule. A splice canbe made using a listed irreversible compression-type connector or the exothermic weldingprocess. More on this subject will be addressed in the next section, Grounding ConductorConnections to Electrode.

The grounding electrode conductor must be sized by the requirements of NEC Section250.66. The conductor is sized based on the size of the largest service-entrance conductor


FIGURE 4.12 Alternate burial methods for ground rods. (Courtesy of AVO Training Institute, Inc.)

FIGURE 4.13 Plate electrode minimum requirements. (Courtesy of AVO Training Institute, Inc.)


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or equivalent area of parallel conductors. The size can also vary according to the type ofconnection used, as will be discussed in the next section. Other deviations to the valuespresented in NEC Table 250.66 is as follows:

1. Connections to made electrodes. The sole connection portion of the conductor is notrequired to be larger than No. 6 copper wire.

2. Connections to concrete-encased electrodes. The sole connection portion of the con-ductor is not required to be larger than No. 4 copper wire.

3. Connections to ground rings. The sole connection portion of the conductor is not

required to be larger than the conductor used for the ground ring. NEC Section250.52(A)(4) states not smaller than No. 2 copper wire.

Grounding Conductor Connection to Electrodes

NEC Section 250.68(B) states that the connection of the grounding electrode conductor mustbe made in a manner that will ensure a permanent and effective grounding path. Section250.70 provides the means of connection of the grounding electrode conductor to thegrounding electrode. These methods include exothermic welding, listed lugs, listed pressure

connectors, listed clamps, and other listed means of connection. Note the term listed forvarious connection methods. These are designed, manufactured, listed, and labeled for thepurpose and are the only ones permitted for use in grounding electrode systems. The codegoes on to state that connections made with solder must not be used. Solder has a very lowmelting point and, therefore, would disconnect the grounding connection in the event of aground-fault, because the solder would act like a fuse. The National Electrical Safety Code(NESC) states that joints in grounding conductors must be made and maintained so as not tomaterially increase the resistance of the grounding conductor. The NESC also states that theconnection must have appropriate mechanical and corrosion-resistant characteristics.

4.16 CHAPTER FOUR

FIGURE 4.14 Components of earth resistance in an earth electrode. (Courtesy of AVO TrainingInstitute, Inc.)


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IEEE Std. 80, Guide for Safety in AC Substation Grounding is an excellent resourcefor determining the minimum size conductor based on the type of connection used. TheOnderdonk AC equation is used to calculate fusing current of a conductor based on the typeof connection used. Connectors and splice connections must also meet the requirements of

IEEE Std. 837 in order to be acceptable. The following examples will further illustrate thispoint:

For this example, a shortened version of the Onderdonk equation will be used to deter-mine conductor size and to determine fusing current levels based on the type of connection.

where A= Cable size, circular milK= Connector factorS= Maximum fault time, secondsI= Maximum fault current, amperes

For copper conductors these equations use the following K values to represent con-nection temperature:

Conductor only1083C temperature ratingvalue of K = 6.96

Welded connections1083C temperature ratingvalue of K = 6.96

Irreversible compression-type connections1083C temperature ratingvalue of K = 6.96 Brazed connections450C temperature ratingvalue of K = 9.12

Pressure-type connections250C temperature rating value of K = 11.54

The following three examples, used to determine minimum conductor size, will use a20,000-A fault current with a protective device clearing time of 5 cycles (.083 second)using :

1. Pressure type connection:

A= 66,493 cm (minimum size conductor)

According to NEC Table 8, the size that corresponds to the minimum size conductorwould be a No. 1 AWG copper conductor (83,690 cm)

2. Brazed connection:

A = 52,549 cm (minimum size conductor)


3. Welded or irreversible compression-type connection:

A = 40,103 cm (minimum size conductor)

A 6.96 20,000= .083

A 9.12 20,000= .083

A= 11 54 20 000 083. , .

A KI S=

Conductor Size Fusing Current

A KI S IA

K S= =



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As can be seen by the previous examples, if a pressure type connection were used, it

would require a conductor approximately 60 percent larger than a connection made with awelded or irreversible compression-type connection. The brazed connection would requirea conductor approximately 31 percent larger than a welded or irreversible compression-type connection. For practical purposes, the welded or irreversible compression-type con-nection would be the preferred choice.

Another consideration would be the approximate fusing current of the conductor basedon the type of connection. This example will use a 4/0 (211,600 cm) conductor with a pro-tective device clearing time of 5 cycles (.083 second) using .

1. Pressure type connection:

I= 63,646 amperes fusing current

2. Welded or irreversible compression-type connection:

I= 105,528 amperes fussing current

As can be seen in the above examples the conductor utilizing the welded or irreversiblecompression-type connection will handle much more current before fusing than the pressure-type connection.

When reliability and cost are taken into consideration, it is plain to see that the welded,as well as the irreversible compression-type, connection are far superior to any other typeof connection.

Bonding

If only one statement were to be made about grounding and bonding, it would be that all non-current-carrying parts of electrical equipment and nonelectrical equipment, likely to becomeenergized, be effectively grounded and bonded together. By using this general philosophy,there will be a minimum (near zero) risk that the non-current-carrying parts of equipment couldbecome energized. This would greatly reduce the risk of electrical shock or electrocution ofany person likely to come into contact with the equipment. As discussed earlier, a general state-ment in OSHA 29 CFR 1910.303(b)(1)(vii) includes, other factors which contribute to the

practical safeguarding of employees using or likely to come in contact with the equipment.One of these other factors refers to grounding and bonding of equipment likely to becomeenergized.

NEC Section 250.90 states that bonding must be provided where necessary to ensureelectrical continuity. Bonding is also required to ensure the capacity to safely conduct anyfault current that is likely to be imposed on the equipment. This requirement applies to alltypes of equipment, systems, and structures. NEC Article 250, Part V. Bonding, must becomplied with in order to size the bonding jumper correctly. Table 4.2 illustrates furtherwhy Part V must be adhered to.

I= 211600

6 96 083. .

I= 211600

11 54 083. .

I A K S= /

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The key here is to know where the bonding jumper is and what it is being used for; itmakes a significant difference. If the wrong section or table is used, the bonding jumpermay not be of adequate current-carrying capacity to safely conduct any fault current likelyto be imposed on it.

EQUIPMENT GROUNDING

Equipment to be Grounded

Equipment to be grounded essentially means that all non-current-carrying metal parts ofelectrical equipment, whether fastened in place or portable, must be grounded. NEC Article250, Part VI, Equipment Grounding and Equipment Grounding Conductors very specifi-cally lays out the requirements for equipment grounding for electrical as well as nonelectri-cal equipment that could become energized. NEC Section 250.110 lists the requirements forequipment fastened in place (fixed) and has three exceptions. Exception No. 2 itemizes, dis-tribution apparatus, such as transformer and capacitor cases, mounted on wooden poles, at a

height exceeding 8 ft above the ground or grade level. This exception would protect the gen-eral public from possible contact with the ungrounded apparatus; however, it does not protectthe person working on the pole. With regard to these issues, OSHA 29 CFR 1910.269(l)(9)states:

Non-current-carrying metal parts. Non-current-carrying metal parts of equipment or devices,such as transformer cases and circuit breaker housings, shall be treated as energized at the high-est voltage to which they are exposed, unless the employer inspects the installation and deter-mines that these parts are grounded before work is performed.

The best practice is to always ground every case or enclosure that contains electricalequipment or conductors. When in doubt, ground it.

Grounding Cord- and Plug-Connected Equipment

NEC Section 250.114 states the same philosophy for cord- and plug-connected equipment,as does Sections 250.110 through 250.112 for fixed equipment. It directs that all exposed


TABLE 4.2 Reference Table for Sizing Bonding Jumpers

Paragraph Reference for sizing

Supply side of service Table 250.66Load side of service Table 250.122

Interior water pipe Table 250.66

Multiple occupancies Table 250.122

Multiple buildings-common service Section 250.122

Separately derived systems Table 250.66

Other metal piping Table 250.122

Structural steel Table 250.66


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non-current-carrying metal parts that are likely to become energized must be grounded. Akey point here is likely to become energized. Any metal housing or enclosure that con-tains electrical components is, at some time or another, likely to become energized. Properequipment grounding techniques will provide the sufficiently low impedance path required

to cause the overcurrent device to operate and clear the ground fault condition.In the material written about electrical safety-related work practices, OSHA has pro-

vided specific requirements for the use of portable electrical equipment and extensioncords. This requirement is found in OSHA 29 CFR 1910.334 where it states:

Use of equipment. (a) Portable electric equipment. (3) Grounding type equipment. (i) Aflexible cord used with grounding type equipment shall contain an equipment groundingconductor.

(ii) Attachment plugs and receptacles may not be connected or altered in a manner which would

prevent proper continuity of the equipment grounding conductor at the point where plugs areattached to receptacles. Additionally, these devices may not be altered to allow the grounding poleof a plug to be inserted into slots intended for connection to the current-carrying conductors.

(iii) Adapters which interrupt the continuity of the equipment grounding connection may notbe used.

OSHA makes it very clear that this type of equipment must be grounded. Note as well thestatement in (iii) concerning adapters. The adapters referred to here are used when a groundedplug is required to be used where an ungrounded receptacle exists. These adapters are UL

approved and can be used, but only if used properly, that is, the ground connection must beattached to a return ground path. The problem with adapters is that the ground connectiondevice is generally cut off or otherwise not used, thus defeating the ground continuity.

Equipment grounding conductors are required as stated previously, however, groundingalone does not give complete protection when using portable cord- and plug-connected,hand-held equipment and extension cords. The use of a ground-fault circuit interrupter(GFCI) when using portable equipment will provide additional safety for the user.Grounding does provide a path for ground fault current to flow to cause the overcurrentdevice to operate, however, it does not provide protection when current leakage occurs dueto moisture in a piece of equipment or when there is an undetected cut in the cord or crack

in the equipment case. The GFCI is designed to trip at 4 to 6 mA of current. To explain thisfurther, the following example is provided.

Given a 20-A molded case circuit breaker, the maximum load allowed by code is 16 A(80 percent of rating). At 16 A this circuit breaker will, under normal conditions, run indef-initely. Even at 20 A the circuit breaker will remain closed for several minutes to infinity.Taking this into consideration, the amount of current leakage from moisture in the equip-ment (a hand-held drill for example) will not trip this circuit breaker, however, a solidground fault will. Since the circuit breaker will not trip, a person contacting this piece ofequipment is exposed to a shock hazard. It takes approximately one-tenth of an ampere tocause ventricular fibrillation in most people, which is generally fatal. The point is that the

circuit breaker will not protect a person in this situation but a GFCI will.NEC Section 590.6 requires ground-fault protection for all temporary wiring installa-

tions for construction, remodeling, maintenance, repair, demolition of buildings, structures,equipment, or similar activities. In other words, any time an extension cord is used for oneof these activities, a GFCI is required. Although not specifically required here, and becausethe hazard risk is the same, all hand-held cord- and plug-connected portable electricalequipment should utilize a GFCI for the protection of the worker.

Figure 4.15 further illustrates how a GFCI works.Use a GFCI; it may save your life. Look at it this wayit is cheap life insurance.

4.20 CHAPTER FOUR


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Equipment Grounding Conductors

NEC Section 250.118 identifies several types of equipment grounding conductors. Thesetypes vary from an equipment grounding conductor run with circuit conductors or enclos-ing them. It can consist of one or more of the following:

1. A copper, aluminum, or copper-clad aluminum conductor.

2. Rigid metal conduit.

3. Intermediate metal conduit.4. Electrical metallic tubing.

5. Flexible metal conduit and fittings listed for grounding.

6. Liquid-tight flexible metal conduit and fittings listed for grounding.

7. Flexible metallic tubing, under conditions.

8. Type AC armored cable

9. Copper sheath of type MI cable.

10. Type MC cable metal sheath.

11. Cable trays meeting the requirements of NEC Section 392.3(C) and 392.7.

12. Cablebus framework permitted by NEC Section 370.3.

13. Other metal raceways listed for grounding that are electrically continuous.

14. Surface metal raceways listed for grounding.

The equipment grounding conductor can either be bare, covered, or insulated. Where theconductor is insulated it must be identified by a continuous outer finish of green, green withone or more yellow stripes, or bare. Equipment grounding conductors larger than No. 6 and


FIGURE 4.15 Internal diagram of a ground-fault circuit interrupter (GFCI). (Courtesy of AVO TrainingInstitute, Inc.)


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multiconductor cable, where one or more conductors are designated for use as an equipmentgrounding conductor, must be permanently identified as the equipment grounding conduc-tor by green stripping, coloring, or taping.

Sizing Equipment Grounding Conductors

Equipment grounding conductors are required to be sized not smaller than is stated in NECTable 250.122 and are not required to be larger than the supply conductors. Section250.122(B) states that if conductors are adjusted in size to compensate for voltage drop, theequipment grounding conductor must be adjusted proportionately according to circular milarea. The following example further illustrates this:

A circuit utilizing a No. 1 THW copper conductor that is protected by a 100-A circuitbreaker would use a No. 8 copper equipment grounding conductor (according to NECTable 250.122). Due to voltage drop, the ungrounded conductor size must be increased toa 1/0 conductor. In order to now adjust the size of the equipment grounding conductor pro-portionately by circular mil area, the following calculation must take place (values of cir-cular mil area are found in NEC Table 8):

givenNo. 1 conductor = 83,690 circular mil areaNo. 8 conductor = 16,510 circular mil area1/0 conductor = 105,600 circular mil area

This calculation determines that the minimum size of equipment grounding conductorrequires a 20,828 circular mil area. Referring again to NEC Table 8, it is noted that this does notcorrespond to a standard size conductor; therefore, the next larger conductor must be used. Inthis case, a No. 6 copper conductor must be used, which has a 26,240 circular mil area. This iswhat the NEC refers to when it says adjusted proportionately according to circular mil area.

There is an important note just below NEC Table 250.122 that is very often overlooked.It states, Where necessary to comply with Section 250.4(A)(5) or (B)(4), the equipmentgrounding conductor shall be sized larger than this table. Section 250.4(A)(5) states thatgrounding conductors shall be capable of safely carrying the maximum ground-fault cur-rent likely to be imposed on it. Too many times, equipment grounding conductors are sizedaccording to Table 250.122 without considering these other facts. In this case, the conduc-tor may not be able to safely carry the fault current. As was seen earlier, at a given value ofcurrent, the conductor will fuse (melt), the ground is now lost and the equipment case orenclosure would become energized, which would create a shock or electrocution hazard.

Use of Grounded Circuit Conductor for Grounding Equipment

NEC Section 250.24(A)(5) states that the grounded conductor (current-carrying neutral)and the grounding conductor (non-current-carrying) shall not be connected together on theload-side of the service disconnecting means (see Fig. 4.16).

Figure 4.17 further illustrates this point.The hazard of making this connection on the load-side is that a parallel return path has

now been established. The metallic raceway becomes a parallel return path with thegrounded conductor. The metallic raceway will generally have higher impedance than the

No.1 ungrounded conductor

No.8 grounded conductor

No.1/0 ungrounded conductor

answer from previous equation

= =

= =

83 690

16 510 5 07

105 600

5 0720 828

,

, .

,

.,

4.22 CHAPTER FOUR


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grounding conductor. As a result, the I R values will be different in the parallel returnpaths. A different voltage rise will occur and thus a difference in potential will existbetween the grounded conductor and the metallic raceway. A difference in potential mayresult in a shock hazard for anyone coming into contact with the metallic raceway. SeeFig. 4.18 for more on this issue.

Another problem with these parallel return paths is what is known as a ground loop.

Ground loops are dangerous because of the resonating pattern established with frequency.Lightning is generated along a diminishing sine wave or damper wave. There is a varyingfrequency along this damper wave. If the frequencies are in resonation with each other,lightning may be drawn to the ground loop. This would help in explaining why lightningstrikes some structures. Figure 4.19 further presents this phenomenon.

Another issue that must be addressed is proper grounding for two or more buildingswhere a common service is used. This issue deals with the grounding electrode, grounded


FIGURE 4.16 Neutral and ground to be separated on load side of service. (Courtesy of AVOTraining Institute, Inc.)

FIGURE 4.17 Grounding requirements at the service (supply side). (Courtesy of AVO TrainingInstitute, Inc.)


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4.24 CHAPTER FOUR

FIGURE 4.18 Ground loop. (Courtesy of AVO Training Institute, Inc.)

FIGURE 4.19 Lightning hazard. (Courtesy of AVO Training Institute, Inc.)


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(neutral) conductor, equipment grounding conductor, and bonding. NEC Section 250.50requires the grounding electrodes at each building to be bonded together to form thegrounding electrode system. Where no grounding electrode exists, one must be installed.There are, however, exceptions to this rule. Where the second building has only one branch

circuit and an equipment grounding conductor is installed with the supply conductors, thegrounding electrode at the main building is all that is required. Figure 4.20 illustrates thisin simple form.

Section 250.32(B)(1) requires an equipment grounding conductor to be run to the sepa-rate building where there is equipment that is required to be grounded. In this case, thegrounding and grounded conductors are prohibited from being connected together. Thegrounded conductor is considered to float in this case because it is not bonded to theequipment enclosure and grounding conductor. See Fig. 4.21 for further information on this.

Section 250.32(B)(2) states another situation that can be used. Where there is no equip-ment grounding conductor run with the supply conductors, the buildings are not bondedtogether through metal raceways or other piping, and there is no ground-fault equipmentinstalled at the common service, the grounded conductor from the main building must beconnected to the grounding electrode and bonded to the disconnecting means at the secondbuilding. This type of situation is essentially that same as individual services at each build-ing. Figure 4.22 illustrates this further.

Ferroresonance

Another very important safety issue associated with proper system grounding is ferroreso-nance. Ferroresonance is a rare phenomenon in power systems, but one that deserves men-tion. In response to a voltage transient, phase to ground fault, circuit breaker opening,equipment energization or de-energization, lightning induced overvoltages, or any numberof other sudden changes, the power system can take a sudden non-linear jump to a sustained


FIGURE 4.20 One branch circuit with equipment grounding conductor run, no electrode is required.(Courtesy of AVO Training Institute, Inc.)


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condition of severe harmonic distortion and high (several per-unit) overvoltages that canseverely damage power system equipment. Ferroresonance is an electrical resonancebetween the non-linear inductance of a transformer and system capacitance. Effectivegrounding decreases the chances of ferroresonance in three ways:

Placing a very low impedance in parallel with the system capacitance.

Fixing the electrical neutral of the system. Ferroresonance seems to require one pointin the system whose potential is not fixed, like the floating neutral of an ungroundedsystem.

Controlling and limiting system overvoltages that can initiate a ferroresonance.

For more information, consult any power system analysis text. An excellent reference is a

technical paper by P. Ferracci entitled Ferroresonance, Cahier Technique No. 190(ECT 190),March 1998. This paper can be obtained from the Internet at http://www.Schneiderelectric.com.tr/ftp/literature/Publications/ECT190.PDF.

4.26 CHAPTER FOUR

FIGURE 4.21 Equipment grounding conductor run to second building. (Courtesy of AVO TrainingInstitute, Inc.)

FIGURE 4.22 Common service with no equipment grounding conductor run. (Courtesy of AVOTraining Institute, Inc.)
http://www.schneiderelectric.com.tr/ftp/literature/Publications/ECT190.PDFhttp://www.schneiderelectric.com.tr/ftp/literature/Publications/ECT190.PDFhttp://www.schneiderelectric.com.tr/ftp/literature/Publications/ECT190.PDFhttp://www.schneiderelectric.com.tr/ftp/literature/Publications/ECT190.PDF


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SUMMARY

Proper grounding is an essential part of electrical safety. Without proper grounding, the

non-current-carrying metal components of electrical equipment have the risk of becomingenergized. Design and install a grounding system based on the requirements of the NationalElectrical Code, the IEEE Std. 142 (Green Book), and IEEE Std. 80, as well as any othernational standards that are relevant to the application.

Figure 4.23 provides an overall look at electrical system grounding requirements basedon the current edition of the National Electrical Code.

Note: Dont take a shortcut with grounding; it may cost someone his or her life. Thatsomeone might be you.


FIGURE 4.23 Service grounding and bonding requirements. (Courtesy of AVO Training Institute, Inc.)


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