Ground Potential Rise & Equipotential Bonding

8/12/2019 Ground Potential Rise & Equipotential Bonding

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Earth potential rise

In electrical engineering, earth potential rise (EPR)also called ground potentialrise (GPR)occurs when a large currentflows to earth through an earth gridimpedance. The potential relative to a distant point on the Earth is highest at thepoint where current enters the ground, and declines with distance from the source.

Ground potential rise is a concern in the design of electrical substationsbecause thehigh potential may be a hazard to people or equipment.

The change of voltage over distance (potential gradient may be so high that aperson could be in!ured due to the voltage developed between two feet, or betweenthe ground on which the person is standing and a metal ob!ect. "ny conductingob!ect connected to the substation earth ground, such as telephone wires, rails,fences, or metallic piping, may also be energized at the ground potential in thesubstation. This transferred potential is a hazard to people and equipment outsidethe substation.

Contents

#hide$ % &auses

' tep and touch potentials

) *itigation

+ &alculations

tandards and regulations

- igh voltage protection of telecommunication circuits

/ ee also

0 1eferences

2 E3ternal lin4s

Causes

Earth potential rise (E51 is caused by electrical faults that occur at electricalsubstations, power plants, or high6voltage transmission lines.

hort6circuit current flows through the plant structure and equipment and into thegrounding electrode. The resistance of the Earth is finite, so current in!ected into theearth at the grounding electrode produces a potential rise with respect to a distantreference point.

The resulting potential rise can cause hazardous voltage, many hundreds of yards(metres away from the actual fault location.

*any factors determine the level of hazard, including7 available fault current, soiltype, soil moisture, temperature, underlying roc4 layers, and clearing time tointerrupt a fault.

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Earth potential rise is a safety issue in the coordination of power andtelecommunications services. An EPR event at a site such as an electrical

distribution substationmay epose personnel! users or structures toha"ardous voltages.

#tep and touch potentials

8tep potential8 is the voltage between the feet of a person standing near anenergized grounded ob!ect. It is equal to the difference in voltage, given by thevoltage distribution curve, between two points at different distances from the8electrode8. " person could be at ris4 of in!ury during a fault simply by standing nearthe grounding point.

8Touch potential8 is the voltage between the energized ob!ect and the feet of aperson in contact with the ob!ect. It is equal to the difference in voltage between theob!ect and a point some distance away. The touch potential could be nearly the fullvoltage across the grounded ob!ect if that ob!ect is grounded at a point remote fromthe place where the person is in contact with it. 9or e3ample, a crane that wasgrounded to the system neutral and that contacted an energized line would e3poseany person in contact with the crane or its uninsulated load line to a touch potentialnearly equal to the full fault voltage.

8*esh potential8 is a factor calculated when a grid of grounding conductors is

installed. *esh potential is the difference between the metallic ob!ect connected tothe grid, and the potential of the soil within the grid. It is significant because aperson may be standing inside the grid at a point with a large potential relative tothe grid itself.

$itigation

"n engineering analysis of the power system under fault conditions can be used todetermine whether or not hazardous step and touch voltages will develop. The resultof this analysis can show the need for protective measures and can guide theselection of appropriate precautions.

everal methods may be used to protect employees from hazardous ground6potential gradients, including equipotential zones, insulating equipment, andrestricted wor4 areas.

The creation of an equipotential zone will protect a wor4er standing within it fromhazardous step and touch potentials. uch a zone can be produced through the useof a metal mat connected to the grounded ob!ect. In some cases, a grounding gridcan be used to equalize the voltage within the grid. Equipotential zones will not,however, protect employees who are either wholly or partially outside the protected

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area. :onding conductive ob!ects in the immediate wor4 area can also be used tominimize the potential between the ob!ects and between each ob!ect and ground.(:onding an ob!ect outside the wor4 area can increase the touch potential to thatob!ect in some cases, however.

The use of insulating personal protective equipment, such as rubber gloves, can

protect employees handling grounded equipment and conductors from hazardoustouch potentials. The insulating equipment must be rated for the highest voltage thatcan be impressed on the grounded ob!ects under fault conditions (rather than for thefull system voltage.

1estricting employees from areas where hazardous step or touch potentials couldarise can protect employees not directly involved in the operation being performed.Employees on the ground in the vicinity of transmission structures should be 4ept ata distance where step voltages would be insufficient to cause in!ury. Employeesshould not handle grounded conductors or equipment li4ely to become energized tohazardous voltages unless the employees are within an equipotential zone or areprotected by insulating equipment.

In cases such as an electrical substation, it is common practice to cover the surfacewith a high6resistivity layer of crushed stone or asphalt. The surface layer provides ahigh resistance between feet and ground grid and is an effective method to reducethe step and touch potential hazard.

Calculations

In principle, the potential of the earth grid %gridcan be calculated using ;hm


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This case is a simplified systemB practical earthing systems are more comple3 than asingle rod, and the soil will have varying resistivity. It can, however, reliably be saidthat the resistance of a ground grid is inversely proportional to the area it coversBthis rule can be used to quic4ly assess the degree of difficulty for a particular site.5rograms running on des4top personal computers can model ground resistanceeffects and produce detailed calculations of ground potential rise, using various

techniques including the finite element method.

#tandards and regulations

The C ;ccupational afety and ealth "dministration (;" has designated E51as a 84nown hazard8 and has issued regulations governing the elimination of thishazard in the wor4 place. #%$

5rotection and isolation equipment is made to national and international standardsdescribed by IEEE, Dational Electrical &odes(C=&",9&&, and Telcordia.

IEEEtd. 0F6'FFF is a standard that addresses the calculation and mitigation of tep Touch 5otentials to acceptable levels.

igh voltage protection of telecommunication circuits

To protect wired communication and control circuits in sub stations, protectivedevices must be applied. igh voltage can damage equiment and present a dangerto personnel. Isolation devices prevent high voltages and currents from propagatingfrom the sub station towards the telephone company


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#*%&+G &G,%*-AGE PR*E$#&+ /&RE*E##01-&*&-2 C**CA-&+#

A#-RAC-

>ireless communications providers are using electric6utility transmission towers inhigh6voltage corridors throughout the world as sites for their e3uipment andantenna locations.-his collocation with high,power transmission linesoffers challenging engineering problems because of the effects of groundpotential rise (GPR).In the absence of actual test results, calculated G51 levelsmust be used in determining the engineering design necessary to properly isolatewire6line communications from damaging G51 effects. These calculated G51 levelscan be very largeB and if it were not for the ability to reduce these levels byimproving the grounding system, there would be limitations on use of wire6linecommunications serving some of these locations. =imiting the use of wire6linecommunications serving cell sites in high6voltage corridors would limit cost6effectiveengineering design. Dew methodologies for greatly improving small cell6sitegrounding systems are 4ey to reducing G51 in high6voltage corridors to levels thatcan be safely handled by isolation equipment.

&. &+-R41C-&+

Isolation equipment is readily available that will protect wire6line communicationsfacilities entering 5& locations within high6voltage corridors from a G51 as high asF 4H rms and 2F 4H surge. 5roperly installed, this isolation equipment will offermany years of maintenance6free, reliable protection from the effects of G51.

Those 5& locations within high6voltage corridors that have overhead groundconductors (;G& with no neutral will e3perience theoretical G51 levels under + 4Hpea4,provided thatthe 5& grounding system resistance is less than ohms. If aneutral is also present in the overhead, the theoretical G51 levels will be less than 'F

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4H pea4. This represents the vast ma!ority of the type of high6voltage corridors inuse today, and these magnitudes can easily be isolated with equipment available onthe mar4et.

The 5& locations within high6voltage corridors that have no ;G& and no neutral wille3perience much higher theoretical G51 levels, even with a 6ohm 5& grounding

system at the tower base. This is because all of the fault energy will pass downthrough the single tower into the ground. >orst case

theoretical G51 levels under these conditions could reach a ma3imum of 0 4H.Dote7 "ctual real6life G51 levels much over )F 4H pea4 asymmetrical may not occur,because earth ionization increases the earth conductivity if the current densitybecomes high enough.

;btaining less than ohms for a 5& grounding system in poor resistivity soils maybe very difficult at a cell site with a small grounding system. owever, significantgrounding improvement to these small grounding systems can be obtained withoute3pensive or elaborate grounding systems, as one of the authors has shown.

&&. GR1+4 P-E+-&A* RE (GPR)

Electrical damage from ground potential rise (G51 throughout the wireless industryhas an estimated cost in the many millions of dollars each year, but few engineers inthe industry are even aware of the phenomenon.

$ost times! the first sign that something is wrong comes right after athunderstorm or after a fault on the power line. #uddenly! the wire,line

service coming into your cell site has failed!and the delicate circuitry of

your communications e3uipment is damaged. -his is often misdiagnosed asan unavoidable maintenance problem! and much money is spent onrepairing e3uipment and replacing protective fuses and gas tubes C to say

nothing of potential lost revenue. &n the worst case! the safety of personnelwor5ing at the site may be seriously compromised.

&&&. #*%&+G -E $2#-ER2

In reality, this type of damage very well could be due to a phenomenon calledground potential rise (G51.

>hen a ground fault occurs at a power substation, some of the fault current willreturn to its source, namely the substation transformer, via the earth, through thesubstation


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6igure 7. 4evelopment of GPR from power system fault.

4efination

Ground Potential Rise or Earth Potential Rise (as defined by &EEE #tandard89,:999) is the maimum electrical potential that a (substation) grounding

grid may attain relative to a distant grounding point assumed to be atpotential of remote earth. -his voltage! GPR! is e3ual to the maimum grid

current times the grid resistance.

"s Figure 2shows, if your telecommunications lines coming into a cell site arecopper, and if these lines are not properly isolated, they provide a path for thevoltage impulse coming up from the grounding system, whether from lightning or apower fault as discussed earlier. Dormally, communications engineers loo4 upward

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for threats in the electrical environmentB but this one comes from below, from thevery grounding system that is part of the electrical protection scheme. This threat isreal and can compromise personnel safety and damage equipment.

6igure :. Communications location without isolation protection.

These G51 surge currents develop on the grounding system and are sent out ontoyour conductive copper communications lines bac4 to a remote ground, which in thiscase is the serving central office (&;. This is why ordinary surge protection devices

such as gas tubes are ineffective in protecting against G51.

owever, special high6voltage protection (H5 isolation devices & including isolationtransformers, optical couplers, and fiber optics & interrupt the conductive paths thatcarry the G51 currents (Figure 3). These devices provide an isolation gap rated atF 4H rms and 2F 4H for surges. The highest service reliability may actually be fromwire6line facilities using passive isolation equipment, i.e., isolation transformers."ctive isolation equipment using optical isolators requiring power will lower thereliability of a T% carrier or ?= service and needlessly e3pose maintenance

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personnel more frequently to possible harm.

6igure ;. Communications location with isolation protection.

&%. GE- -


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; "DIIEEE tandard 0F6%22/ 6 Guide for safety in "& substation grounding.

; D95" /F6%222 6 Dational Electrical &ode.

&ommunications protection engineers should not turn a blind eye to G51 damagebecause they believe special H5 devices are more e3pensive than gas tubes.

&onsider ongoing costs for continually replacing damaged equipment year after year."lso consider that the costs of labor for repairs and the lost revenue from downedcommunications lines can easily surpass the cost of G51 protection. "nd don


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6igure =. Conductive cement effectively enlarges the wire!

creating a conductive plate.

Figure 4shows a horizontal6strip configuration, or groundbed, and the formula forcalculating its resistance. The most common installation procedure follows7

%. ?ig a trench, )F in. deep, 'F in. wide, and as long as required to obtain thedesired resistance. (The length is a design calculation, discussed later. &enter a +Fstranded wire in the bottom of the trench.

'. 5our in the cement as a dry powder (it will later absorb moisture and harden bydragging an open bag of it down the trench. Cse one F6lb bag every %F ft. eap thecement up as shown.

).J=ift the wire slightly so it is completely covered by the cement for corrosionprotection. Tamp the cement with feet or a shovel toward the tapered edges.

+. &arefully shovel in a +6in. layer of soil and tamp it down.

. 5ush in the rest of the removed soil using construction equipment.

%&&. 4E#&G+&+G A R&'+-A* E*EC-R4E

The design procedure is as follows7

%. ?ecide upon the desired resistance of the electrode.

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'. *easure the soil resistivity with an earth tester.

). ?etermine the required length from the table, based on the desired resistance (or %F ohms and the soil resistivity.

-able 7. -able of lengths for >, and 79,ohm grounds.

1se the formula for intermediate values.

%&&&. GR1+4 R&+G

" typical pad6mounted wireless site has a buried ground ring around the pad, about' ft out from the pad, and another ring around the antenna. The formula given in9igure + appliesB however, the resistance thus obtained must be multiplied by %.%'to account for the reduced grounding efficiency of a square ring compared to astraight strip. 9or e3ample, if the two rings require %+ running feet (++ m !ust tosurround the pad and antenna, the table shows this would give about ohms in%,FFF ohm6cm soil (about %. times the average C.. soil resistivity. *ultiplying by%.%', the resistance would be about .- ohms. " still lower resistance could beachieved by e3tending radials from the four outer corners of the configuration.

&?. G&R4 -E GR&4

*eanwhile, bac4 at the substation, the source of the G51 from power faults, the G51can be reduced by lowering the resistance of the grounding grid. If conductivecement is used to surround grid wires on a %F6by6%F6ft spacing, the grid area can bereduced by %F or 'F percent, with a concomitant money saving and reduction in thee3tent of the critical )FF6H G51 contour. Cse IEEE td. 0F6%22/ data or E51I

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ubstation Grounding >or4station software and assume strip conductors of '6in.6by6%06in. cross section. 9or further information, refer to manufacturers< applicationnotes.

E3isting ground grids also can be improved by e3tending the grid area by %F or %percent and using conductive cement. In one application in high6resistivity soil, grid

resistance was reduced from %F ohms to ' ohms. In another, resistance was reducedfrom F.2- to F.' ohm. &onsolidated Edison and :oston Edison have used conductivecement to ground transmission towers and substations.

?. E$E44E4 GR1+4 R4

"bout FK of the resistance between a ground rod and remote earth is in a shellwithin the first - in. from the rod. If this shell is shorted out by encasing the rod in -in. of conductive cement, as shown in Figure 5, the resistance is halved. This is agood e3ample of how the resistance of any electrode can be decreased without

ma4ing the electrode longer. This is important wherever bedroc4 limits the length ofground rods or when property lines limit the length of a horizontal electrode.

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6igure >. -his embedded ground rod ta5es advantage of the fact that>9 percent of the earth resistance is within @ inches of the rod.

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Eplanation ,:

/hat &s Ground Potential Rise! Earth Potential Rise And /hat &s A Ground

Potential Rise #tudyGround 5otential 1ise (G51 or Earth 5otential 1ise is a phenomenon that occurs

when large amounts of electricity enter the earth. This is typically caused whensubstations or high6voltage towers fault, or when lightning stri4es occur (faultcurrent. >hen currents of large magnitude enter the earth from a groundingsystem, not only will the grounding system rise in electrical potential, but so will thesurrounding soil as well.

The voltages produced by a Ground 5otential 1ise or Earth 5otential 1ise event canbe hazardous to both personnel and equipment. "s described earlier, soil hasresistance 4now as soil resistivity which will allow an electrical potential gradient orvoltage drop to occur along the path of the fault current in the soil. The resultingpotential differences will cause currents to flow into any and all nearby groundedconductive bodies, including concrete, pipes, copper wires and people.

Ground Potential Rise #tudy

" Ground 5otential 1ise tudyis the process of automated timed andor continuousresistance6to ground measurement. These dedicated systems use the inducedfrequency test method to continuously monitor the performance of critical groundingsystems. ome models may also provide automated data reporting. These newmeters can measure resistance6to6ground and the current that flows on thegrounding systems that are in use. "nother benefit is that it does not requireinterruption of the electrical service to ta4e these measurements.

Ground Potential Rise (GPR) 4efinitions

Ground 5otential 1ise or Earth 5otential 1ise (as defined in IEEE tandard )-/ is theproduct of a ground electrode impedance, referenced to remote earth, and the

current that flows through that electrode impedance.

Ground Potential Rise or Earth Potential Rise (as defined by &EEE #tandard89,:999) is the maimum electrical potential that a (substation) grounding

grid may attain relative to a distant grounding point assumed to be atpotential of remote earth. -his voltage! GPR! is e3ual to the maimum grid

current times the grid resistance.

Ground 5otential 1ise or Earth 5otential 1ise events are a concern wherever electricalcurrents of large magnitude flow into the earth. This can be at a substation, high6voltage tower or pole, or a large transformer. In cases where an Earth 5otential 1iseevent may be of special concern, grounding precautions are required to ensurepersonnel and equipment safety. Electrical potentials in the earth drop abruptlyaround the perimeter of a grounding system, but do not drop to zero. In fact, in aperfectly homogeneous soil, soil potentials are inversely proportional to the distancefrom the center off the grounding system, once one has reached a distance that is asmall number of grounding system dimensions away.

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-he Earth Potential formula is as followsB

Earth 5otential L oil 1esistivity 3 &urrent (' 3 5I 3 ?istance

>here Earth 5otential is in volts, oil 1esistivity is in ohm6meters, &urrent is thecurrent flowing into the soil from the grounding system, in amperes, 5I is ).%+%2Mand ?istance is in meters.

5robably the most commonly noted Ground 5otential 1ise or Earth 5otential 1iseevent involves the death of cows in a field during a lightning stri4e. Imagine lightningstri4ing the center of an open field where cows are standing. The current in!ectedinto the earth flows radially away from the stri4e point, in all directions, creatingvoltage gradients on the surface of the earth, also in a radial direction. "ll the cowsfacing the lightning stri4e would have their fore hooves closer to the stri4e point thantheir rear hooves. This would result in a difference of potential between their foreand rear legs, causing current to flow through their bodies, including the heart area,and 4illing the cow. ;n the other hand, those cows with their flan4s turned towards

the lighting stri4e would have a greater chance of surviving, as the distance betweentheir fore legs and therefore the voltage applied between them, would be relativelysmall, resulting in a lesser current flow.

" Ground 5otential 1ise tudyis typically conducted on substations and high6voltagetowers to measure Earth 5otential. ubstations have relatively large groundingareas, especially when compared to high6voltage towers and poles. Towers and polesrepresent by far the most potentially dangerous and difficult Ground 5otential 1isesituations to handle and are often not protected, unless they are located in highe3posure areas or have equipment installed at ground level at which servicepersonnel might be required to wor4.

Ground Potential Rise #tudy

The primary purpose of a Ground 5otential 1ise tudy is to determine the level ofhazard associated with a given high6voltage location for personnel andor equipment.>hen the degree of hazard is identified, the appropriate precautions must be madeto ma4e the site safe. To do this, the engineer must identify what the minimumgrounding system for each location will be. The engineer must also ta4e intoconsideration all local and federal guidelines, including utility company and otherrequirements.

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9or e3ample7 *any utility companies require at a minimum that a simple ground ringbe installed at least %0 inches below ground and ) feet from the perimeter of allmetal ob!ects. This ground ring is also referred to as a counterpoise.

;nce the minimum grounding system is identified, the engineer can run a Ground5otential 1ise or Earth 5otential 1ise "nalysis and identify the e3tent of any electricalhazards.

Typically, items reported in a Ground 5otential 1ise tudywill include the following7the square footage, size and layout of the proposed grounding grid, resistance6to6ground of the proposed grounding system, the estimated fault current that wouldflow into the grounding system, Ground 5otential 1ise (in volts at the site, )FF Holt

5ea4 line, the N1 1atio, and the fault clearing time in seconds. tep and touchpotential voltages are usually computed as well, as these are the primary indicatorsof safety.

The grounding engineer needs three () pieces of information to properly conduct aGround 5otential 1ise tudy7

1. oil resistivity data from a oil 1esistivity Test'. ite drawings with the proposed construction). Electrical data from the power company

#oil Resistivity -est

The soil resistivity test data should include apparent resistivity readings at pin

spacings ranging from F. or % ft to as many as three grounding grid diagonals, ifpractical. Touch and step voltages represent the primary concern for personnelsafety. Cnderstanding the characteristics of the soil at depths ranging fromimmediately underfoot to one or more grid dimensions is required for a cost6effectiveand safe grounding system to be designed.

#ite 4rawings

The proposed site drawings should show the layout of the high6voltage tower orsubstation, and any additional construction for new equipment that may be occurring

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on the site, including fencing and gate radius. Incoming power and Telco runs shouldalso be included. In the case of high6voltage towers, the height and spacing of theconductors carried on the tower, and any overhead ground wires that may beinstalled on the tower, need to be detailed during the survey. This information isneeded to properly address all the touch and step voltage concerns that may occuron the site.

Electric 1tility 4ata

The electric utility company needs to provide electrical data regarding the tower orsubstation under consideration. This data should include the name of the substationor the number of the tower, the voltage level, the subtransient N1 ratio, and theclearing times. In the case of towers, the line names of the substations involved, theamount of current contributed by each substation in the event of a fault, and thetype and positions of the overhead ground wires, if any, with respect to the phaseconductors installed on each tower or pole. If overhead ground wires are present,tower or pole ground resistances along the line are of interest as well, be theymeasured, average or design values.

This information is important, as high6voltage towers have small ground area, yethandle very large amounts of electricity. nowing if a tower has an overhead groundwire is important, because the overhead wire will carry away a percentage of thecurrent, which will depend on the overhead ground wire type and ground resistancesof ad!acent towers, to other towers in the run, reducing the G51 event. "dditionally,towers with overhead ground wires tend to have shorter clearing times. The sameholds for substations7 overhead ground wires on transmission lines and neutral wireson distribution lines can significantly reduce the magnitude of fault current that flowsinto the substation grounding system during fault conditions.

The following information is required from the utility company7

%. 5hase6to6ground fault current contributed by each power line

circuit'. 9ault clearing time). =ine voltage+. ubtransient N1 ratio. The ma4etypenumber of overhead ground wires on each

towerpole line and position with respect to the phase conductors-. Ground wire continuity and bonding configuration bac4 to the

tower and substation/. The average distance from tower6to6tower and tower6to6

substation0. Typical towerpole ground resistance7 measured or design values

"s6built drawings are often acquired and are useful for towers with e3isting

grounding systems. They are also useful in the case of modifications and upgrades toe3isting substations, which will have e3tensive grounding systems already installed.

Personnel #afety during Ground Potential Rise or Earth Potential RiseEvents

The grounding engineer will be required to develop safety systems to protect anypersonnel wor4ing where Ground 5otential 1ise hazards are 4nown to e3ist. 9ederal=aw mandates that all 4nown hazards must be eliminated from the wor4 place for

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the safety of wor4ers. It is the engineerOs choice on which voluntary standards toapply in order to comply with the law. 9ederal law '2 &91 %2%F.'-2 specificallystates that tep and Touch 5otentialsmust be eliminated on transmission anddistribution lines that include any related communication equipment.

ubstations are always considered wor4places and tep and touch 5otentialsmust be

eliminated. Transmission and distribution towers or poles are not always consideredwor4 places and therefore are often e3empt from these requirements. Ta4e, fore3ample, a lonely tower on a mountain side or in the middle of the desert7 thesetowers are not typically considered wor4places. owever, any high6voltage tower orpole becomes a wor4place as soon as equipment is installed that is not related to theelectric utility company and requires outside vendors to support the new equipment.&ellular telecommunications, environmental monitoring, and microwave relayequipment are good e3amples of equipment that, when installed on a high6voltagetower, turns the tower into a wor4 place. This would ma4e the elimination of tepand Touch 5otentials required.

a"ardous %oltages

9ibrillation &urrent is the amount of electricity needed to cause cardiac arrest, fromwhich recovery will not spontaneously occur, in a person and is a value based onstatistics. IEEE td 0F6'FFF provides a method to determine the pertinent value of9ibrillation &urrent for a safety study, along with a good e3planation of how it isderived. *any different methods e3ist for calculating 9ibrillation &urrentB howeverthe F4g IEEE method is the most commonly used in Dorth "merica. The formulaused shows that the 9ibrillation &urrent level is inversely proportional to the squareroot of the fault durationB however, it must be increased by a correction factor,based on the subtransient N1 ratio, which can be quite large for shorter faultdurations. If personnel wor4ing at a site during fault conditions e3perience voltagesthat will cause a current less than the 9ibrillation &urrent to flow in their bodies, thenthey are considered safe. If a wor4er will e3perience a greater voltage than isacceptable, additional safety precautions must be ta4en.

The subtransient N1 ratio at the site of the fault is important in calculating theacceptable 9ibrillation &urrent and to determine the ma3imum allowable tep andTouch 5otentials that can occur at any given site.

9ault ?uration is a very necessary piece of data for properly calculating ma3imumallowable tep and Touch 5otentials. The 9ault ?uration is the amount of timerequired for the power company to shut off the current in the event of a fault.

Cltimately the engineer must determine two (' things7

%. The site6specific ma3imum allowable voltage that a person cansafely withstand

'. The actual voltages that will be e3perienced at the site during afault

Each site will have different levels of voltages for both of the above. Cnfortunately,we cannot simply say that a human being can withstand N6level of voltages and usethat value all the time, since this voltage is determined by the surface layerresistivity, the fault duration and the subtransient N1 ratio. "dditionally as each sitehas different fault durations and different soil conditions, it is critical that calculationsbe made for each and every possible fault location.

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Electrical bonding

Electrical bondingis the practice of intentionally electrically connecting all e3posedmetallic non6current carrying items in a room or building as protection from electric

shoc4. If a failure of electrical insulation occurs, all bonded metal ob!ects in the roomwill have substantially the same electrical potential, so that an occupant of the roomcannot touch two ob!ects with significantly different potentials. Even if the connectionto a distant earth ground is lost, the occupant will be protected from dangerouspotential differences.

Contents

% E3planation

' ow the earth protects

) Equipotential bonding

+ "ircraft electrical bonding ee also

- Dotes and references

Eplanation

In a building with electricityit is normal for safetyreasons to connect all metalob!ects such as pipestogether to the mainsearthto form an equipotential zone. Thisis done in the Cbecause many buildings are supplied with a single phasesupplycablewhere the neutral and earth conductors are combined. &lose to the electricitymeterthis conductor is divided into two, the earthterminal and the wire going to theneutralbusbarin the consumer unit. In the event of a brea4 in a neutral connectionthis earthterminal provided by the supply companywill be at a potential (relative tothe true earth which is the same as the live wire (phase wire coming to the home.

E3amples of articles that may be bonded include metallic water piping systems, gaspiping, airplanes, ducts for central heating and air conditioning systems, and e3posedmetal parts of buildings such as hand rails, stairs, ladders, platforms and floors.

" person touching the un6earthed metal casing of an electrical device, while also incontact with a metal ob!ect connected to remote earth, is e3posed to an electricshoc4hazard if the device has a fault. If all metal ob!ects are connected together, allthe metal ob!ects in the building will be at the same potential. It then will not be

possible to get a shoc4 by touching two ith all theconducting elements bonded, it is less li4ely that electric current will find a paththrough a swimmer. In concrete pools even the reinforcing bars of the concrete must

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be connected to the bonding system to ensure no dangerous potential gradients areproduced during a fault.

ow the earth protects

In a system with a grounded (earthed neutral, connecting all non6current6carrying

metallic parts of equipment to earth ground at the main service panel, will ensurethat current due to faults (such as a 8hot8 wire touching the frame or chassis of thedevice will be diverted to earth. In a TDsystem where there is a direct connectionfrom the installation earth to the transformer neutral, earthing will allow the branchcircuit over6current protection (a fuseor circuit brea4er to detect the fault rapidlyand interrupt the circuit.

In the case of a TTsystem where the impedanceis high due to the lac4 of directconnection to the transformerneutral, an 1&?(1esidual6&urrent ?evice, sometimes4nown as a 1esidual &urrent &ircuit :rea4er or Ground 9ault &ircuit Interruptermust be used to provide disconnection. 1&?s are also used in other situations whererapid disconnection of small earth faults (including a human touching a live wire by

accident, or damage is desired.

E3uipotential bonding

Equipotential bondinginvolves !oining together metalwor4 that is or may be earthedso that it is at the same potential(i.e., voltage everywhere. uch is commonly usedunder transformer ban4s by power companies and under large computerinstallations. E3act rules for electrical installations vary by country, locality, orsupplying power company.

Equipotential bonding is done from the ervice 5anel consumer unit(also 4nown as afuse bo3, brea4er bo3, or distribution board to incoming waterand gasservices. It

is also done in bathroomswhere all e3posed metal that leaves the bathroomincluding metalpipesand the earths of electrical circuits must be bonded together toensure that they are always at the same potential. Isolated metal ob!ects, includingmetal fittings fed by plastic pipe, are not required to be bonded. European and Dorth"merican practices differ hereB equipotential bonding in bathrooms is not required byDorth "merican codes, although it is required around swimming pools.

In "ustraliaand outh "frica, a house


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$ain onding or $ain E3uipotential onding of $etal #urfaces to give it! its

full title.

$ain e3uipotential conding conductors are re3uired to connect the followingmetallic parts to the main earthing terminal! where they are etraneous,

conductive,partsB

%. $etal water service pipes'. $etal gas installation pipes). ther metal service pipes (including oil and gas supply pipes) and

ducting+. $etal central heating and air conditioning system. Eposed metallic structural parts of the building-. *ighting protection systems

#upplementary onding or #upplementary E3uipotential onding in locations

of increased shoc5 ris5

&n a bathroom or shower room! local supplementary e3uipotential bonding is

re3uired to be provided connecting together the terminal of protectiveconductors of each circuit supplying Class & and Class && e3uipment in "ones

7! : or ; and etraneous,conductive,parts in these "ones including the

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followingB

%. $etal pipes supplying services and metallic waste pipes (e.g. water!gas)

'. $etal central heating pipes and air conditioning systems

). Accesible metal structural parts of the building (metal doorarchitraves! metal handrails! window frames and similar parts notconsidered to be etraneous,conductive,parts of the building)

+. $etal baths and metal shower basins

Documents

Ground Potential Rise & Equipotential Bonding