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Temporary Grounding and Bonding Techniques
Safe Practice Guide
ihsa.ca
1
Infrastructure Health & Safety Association
Safe Practice Guide
Temporary Grounding and Bonding Techniques
Foreword
This Guide designates the practices that should befollowed by the member firms of the InfrastructureHealth & Safety Association (IHSA) when involved inde-energizing isolated electrical circuits or apparatus.This Guide is not designed as a training manual, butcontains information, best practices and generalrecommendations deemed appropriate to perform ajob in a responsible and safe manner.
The contents of this Safe Practice Guide, including alladvice, recommendations and procedures, areprovided as a service by the Infrastructure Health &Safety Association. No representation of any kind ismade to any persons whatsoever with regard to theaccuracy, completeness or sufficiency of the informa-tion contained herein. Any and all use of or reliance onthis Safe Practice Guide and the information containedherein is solely and entirely at the user's risk. The useralso acknowledges that the safe practices describedherein may not satisfy all requirements of Ontario law.
The Infrastructure Health & Safety Association wishesto express its appreciation to those who assisted inthe preparation of this Guide.
All rights reserved. This publication may not be reproduced,in whole or in part, without the express written permission of
the copyright owner.
12/05
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TABLE OF CONTENTS
INTRODUCTION 5
PURPOSE 6
DEFINITIONS 6
SECTION IGENERAL
100 Safe Execution of Work 8
101 Competent Personnel 8
102 Job Planning 8
103 Teamwork 8
104 Work Methods 9
SECTION IIAPPLYING TEMPORARY GROUNDS
ON OVERHEAD LINES
200 Hazards of Isolated Lines 12
201 Grounding Systems for Overhead Lines– General 13
202 Preparation for Temporary Groundingof Overhead Lines 19
203 Installation of Temporary Grounds on anOverhead Star (Wye) Connected Circuit 20
204 Installation of Temporary Grounds on anOverhead Delta Connected Circuit 22
205 Rationale for Personal Grounding Using theGrounding Cluster Bar 23
206 Temporary Personal Grounding Applicationsin Urban Areas – Multi-Grounded SystemNeutral 26
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207 Temporary Personal Grounding ApplicationsIn Rural Areas – Non Multi-Grounded SystemNeutral 28
208 Ground Probes 29
209 Preparation for Temporary Personal Groundingof Overhead Lines 30
210 Installing Temporary Personal Grounds onOverhead Circuits 31
211 Personal Grounding for Work From an AerialDevice 37
SECTION IIITEMPORARY GROUNDING OF VEHICLES
300 Vehicle Grounds – General 40
301 Vehicle Ground Equipment 43
302 Complete Isolation VersusTemporary Grounding of Vehicles 45
SECTION IVGROUND GRADIENT CONTROL MATS
400 General 48401 Step Potential 48
402 Touch Potential 48
403 Description of a Ground Gradient Control Mat 49
404 Operation of Air Break Switches 50
SECTION V
TEMPORARY GROUNDING METHODSFOR CONDUCTOR STRINGING OPERATIONS
500 General Precautions 54
501 Procedures for Setting up Fence Barriers 60
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SECTION VITEMPORARY GROUNDING OFUNDERGROUND EQUIPMENT
600 General Precautions 64
601 Temporary Grounding System for UndergroundApplications 64
602 Considerations Prior to Undertaking Work 64
603 Procedure for De-energizing anUnderground Cable at an Elbow ConnectedPadmount Transformer 65
604 Procedure for De-energizing an UndergroundCable at an Arc Strangler Switchgear (NX) 69
SECTION VII
TEMPORARY GROUNDING WITHIN SUBSTATIONS
700 General Precautions 72
701 Suitable Temporary Grounding Systems forSubstation Applications 72
702 Temporary Grounding Procedures forSubstations 74
SECTION VIIICONCLUSION
800 Dangerous Misconceptions 78
801 Conclusions 78
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Safe Practice Guide
TEMPORARY GROUNDINGAND BONDING TECHNIQUES
INTRODUCTION
It has been generally accepted that when temporarygrounds were applied to isolated equipment, it wouldbecome de-energized, thus ensuring electrical hazardswould be totally eliminated for anyone who might makecontact with the equipment.
Through extensive laboratory testing, this has beenproven to be a false perception.
Current will seek any and every available path to ground.The lower the resistance to the travel of electricity, themore current will flow. This is not to say that a person inparallel with a properly installed grounding system willnot also be a simultaneous path for current to flow.
A person in contact with even the best grounded equip-ment may be subjected to a lethal amount of current,should that equipment become energized.
Current will only flow where there is a difference ofpotential (voltage). Therefore, if we can create a workzone where all equipment is at or close to the samepotential, we can eliminate or substantially reducecurrent flow. This work zone is called an equipotentialzone.
PURPOSE
Effective temporary grounding techniques must utilize acombination of grounding and bonding; grounding toclear accidental re-energization and minimize potential;bonding to ensure workers are not subjected to hazard-ous potential differences during energized situations.
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The effective application of temporary grounds will:
1. Provide positive proof of isolation.
2. Eliminate/control induction.
3. Provide a low resistance path for current to groundto ensure rapid isolation, should re-energizationoccur.
4. Provide a work zone at or near ground potentia (zerovolts) for the duration of the de-energized work.
DEFINITIONS
Bonding
Making a mechanically-secure electrical connectionbetween two or more objects to ensure they are at thesame potential.
Grounding
Metallically connecting a piece of equipment to ground(earth) potential.
Induction
Voltage produced on a conductive object that is sub-jected to a changing magnetic field.
Multi-Grounded System Neutral
A system neutral found in areas of mid to high loaddensity, where it is grounded (connected to earth) atfrequent intervals.
This ensures the neutral has strong reference toground and is at a potential (voltage) which is at, orvery near, ground potential.
Non Multi-Grounded System Neutral
A system neutral usually found in rural areas of lowload density, where it is infrequently grounded. It mayoften be at a potential higher than ground potential.
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SECTION I
GENERAL
100 SAFE EXECUTION OF WORK
101 COMPETENT PERSONNEL
102 JOB PLANNING
103 TEAMWORK
104 WORK METHODS
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SECTION I
GENERAL
100 SAFE EXECUTION OF WORK
The safe execution of temporary grounding andbonding techniques requires:
- competent personnel
- job planning
- teamwork and communication
- approved work methods and procedures
- approved testing, grounding and bonding equipment
101 COMPETENT PERSONNEL
Workers involved with temporary grounding andbonding applications must have been previouslyinstructed, or be under instruction from a competentperson, in the implementation of proper live linetechniques.
102 JOB PLANNING
As in all other phases of line work, job planning is ofprime importance so that work may be performed safelyand efficiently.
103 TEAMWORK
The best teams are made up of people who will workcompatibly with one another. Good communication isessential while work is being performed.
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104 WORK METHODS
1. Prior to the application of the temporary groundingsystem, serious consideration must be given to thefollowing:
- proper identification of equipment using up-to-date operating diagrams,
- testing for isolation,
- personal protective equipment (PPE) require-ments, and
- adherence to all work procedure requirements.
2. In addition to good temporary grounding andbonding practices, in situations such as conductorstringing, worker and public safety will be greatlyenhanced by completely barricading certain piecesof equipment at ground level through the use ofplastic fencing, rope netting, barricades, etc. Theproper use of this equipment will help eliminate thepossibility of someone entering an area, where theymay be subjected to a situation such as a dangerousparallel path to ground.
3. Because of the high number of possible applicationswith regard to overhead and underground configura-tions, no attempt is being made to give detailedinstructions regarding individual situations. Thereare, however, many fundamental safe work practicesand concepts which can be applied in all temporarygrounding applications as reflected in the Purposesection of this Safe Practice Guide.
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SECTION II
APPLYING TEMPORARY GROUNDSON OVERHEAD LINES
200 HAZARDS OF ISOLATED LINES
201 GROUNDING SYSTEMS FOR OVERHEADLINES – GENERAL
202 PREPARATION FOR TEMPORARYGROUNDING OF OVERHEAD LINES
203 INSTALLING TEMPORARY GROUNDS ONAN OVERHEAD STAR (WYE) CONNECTEDCIRCUIT
204 INSTALLING TEMPORARY GROUNDSON AN OVERHEAD DELTA CONNECTEDCIRCUIT
205 RATIONALE FOR PERSONAL GROUNDINGUSING THE GROUNDING CLUSTER BAR
206 TEMPORARY PERSONAL GROUNDINGAPPLICATIONS IN URBAN AREAS –MULTI-GROUNDED SYSTEM NEUTRAL
207 TEMPORARY PERSONAL GROUNDINGAPPLICATIONS IN RURAL AREAS – NONMULTI-GROUNDED SYSTEM NEUTRAL
208 GROUND PROBES
209 PREPARATION FOR TEMPORARY PERSONALGROUNDING OF OVERHEAD LINES
210 INSTALLING TEMPORARY PERSONALGROUNDS ON OVERHEAD CIRCUITS
211 PERSONAL GROUNDING FOR WORKFROM AN AERIAL DEVICE
206 TEMPORARY PERSONAL GROUNDING
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SECTION II
APPLYING TEMPORARY PERSONAL GROUNDSON OVERHEAD LINES
200 HAZARDS OF ISOLATED LINES
Good grounding practices eliminate possible hazardswhich could exist on isolated overhead circuits orapparatus. Some of these hazards are:
(a) INDUCTION: There is a very real possibility ofvoltage being induced from energized circuits on thesame structure, parallelling structures or circuitscrossing over or under the isolated system.
(b) ACCIDENTAL ENERGIZATION: Inadvertent opera-tion of switchgear may energize circuits or apparatusin the work area. Accidents on adjacent circuits atcrossovers or underbuilds could result in energizedlines coming in contact with isolated lines. Someexamples are: customer generation, vehicles hittingpoles, trees falling, conductors making contactduring stringing operations, other crews working inthe area, etc. (See Figure #1)
(c) WIND: Dust particles suspended in the air inconditions of high humidity, fog, etc., could becomea conducting medium. Wind blowing over longtransmission and distribution lines has an electro-static generating effect. Water flowing in a river
Figure #1
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passing beneath acircuit or alongside itcan also be a sourceof electrostatic charg-ing of the conductors.
(d) LIGHTNING: Althoughthe work area may befree of electricalstorms, lightningstriking another partof the system couldresult in transientvoltages, which wouldmake the work areavery dangerous unlessproperly grounded. When electrical storms can beseen or heard, all work on overhead lines shouldbe suspended immediately. (See Figure #2)
These situations all call for adequate groundingprocedures to help ensure that a safe work area isestablished and maintained. Proper groundingprocedures are effective only if the current carryingcapacity of the temporary grounding system is suffi-cient to carry the available fault current safely ground,thereby activating overcurrent devices (such as linefuses, reclosers, breakers) to isolate the offendingcircuit.
201 GROUNDING SYSTEMS FOROVERHEAD LINES – GENERAL
A satisfactory temporary grounding system must beeasy to apply; meet the requirements of all field applica-tion conditions; require minimum preparation, time, andeffort for installation; carry the fault current available;and accept a wide range of conductor sizes andconfigurations.
Figure #2
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1. ADEQUATE CAPACITY CLAMPSClamps should be chosen for their fault currentcarrying capacity as well as their mechanicalstrength and size to fit the conductor (cable or bus).The clamp must have adequate electrical capacity towithstand the maximum short circuit current avail-able for the full time duration over which that currentmay flow. One example of such a clamp hasmanufacturer ratings as follows:
ELECTRICAL CAPACITY OF AN APPROVEDTEMPORARY GROUNDING SYSTEM
Continuous Current Fault Current
15 Cycles 30 Cycles
I/O Extra Flexible Copper Grounding Cable
250 amps 21,000 amps 15,000 amps
4/O Extra Flexible Copper Grounding Cable
400 amps 43,000 amps 30,000 amps
Grounding Cluster Bar (See Figure #3)
400 amps 40,000 amps 30,000 amps
Single ("Duck Bill") Clamp (See Figure #4)
400 amps 35,000 amps 25,000 amps
Flat Face Copper Clamp (See Figure #5)*
400 amps 25,000 amps 25,000 amps
* There is a set screw which is incorporated into the
fixed jaw of the flat face copper clamp. Tests have
shown that tightening this screw when applying
temporary grounds can interfere with the overall
surface contact.
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Figure #4Single Clamp
Figure #3Grounding Cluster Bar
NOTE: In any grounding system there may beweak links. Using the aforementionedexample, the 1/0 extra flexible coppergrounding cable could be considered theweak link as its fault current capacity isless than that of the clamp's. On the otherhand, if using the same clamps with 4/0extra flexible grounding cable, theclamps becomethe weaker link.
It is imperative to know thefault currents available atall work locations on yourelectrical system, and toselect the appropriatetemporary groundingsystem to cover eachapplication.
Most approved groundingclamps are designed for Figure #5
Flat Face CopperClamp
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the attachment of ground clamp support studs. (SeeFigure #6) Using the support studs as intendedhelps to maintain control of the grounding leadsduring installation.
2. ADEQUATE CAPACITY CABLESThere are two major considerations in selecting thecables. The terminal should be such that there is agood mechanical and electrical connection betweenthe clamp and cable. Low resistance is the key. It isprovided through the use of machined ferruleswhich, when crimped properly, provide good electri-cal contact and mechanically strong connections.This ensures the cable will withstand the severemechanical forces of short circuit current, and thatthe current will readily transfer from the clamps tothe cables. (See Figure #7)
One manufacturer promotes a different type ofcompression ferrule for use with its type of clamp. Itconnects the cable strands with a threaded studwhich screws into a tapped boss on the clamp. A
Figure #6
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nut increases the contact pressure and furthersecures the assembly. This device is used in con-junction with a strain relief sleeve which reinforcesthe cable at this termination. (See Figure #8)
Generally, a temporary grounding system with 1/Ogrounding cables is adequate for distribution sys-tems, while 4/O grounding cables are selected fortransmission and substation applications.
Pressure typeterminal
Section "A" to be crimpedagainst the cable strands
Section "B" to be crimped over thecable insulation, which will serve as astress relief to help minimize fatigue atthe flexing point
Figure #7
Threaded studterminal
Section "A" to be crimped againstthe cable strands
Section "B" to be crimped over the cableinsulation, which will serve as a stressrelief to help minimize fatigue at the flexingpoint
Figure #8
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The manufacturers' current rating capacity is basedon new equipment used in ideal conditions. Thisis rarely the case in field applications.
Recognizing the extreme service a temporarygrounding system would experience in an inadvert-ent energization situation, helps us to understandthe need for regular maintenance and continuousinspections.
One cracked grounding clamp, corroded connec-tion, frayed cable or loose connection could resultin catastrophic failure at a critical instant.
3. TEMPORARY GROUNDING SYSTEMSThe trend in temporary grounding arrangements isto individual jumpers. This is due to varyingconfigurations in construction, different applica-tions, and the weight of a combined system. Thejumper sets are comprised of four or five lengths ofcable, depending upon the construction styles. Anacceptable grounding system may be comprised ofthe following lengths of extra flexible coppergrounding cable:
- two - 1.8 m (6 ft.) lengths
- one length of cable capable of connecting agrounding cluster bar to the nearest phaseconductor
- one length of cable capable of connecting thegrounding cluster bar to the system neutral
NOTE: In rural areas, a ground probe will be acomponent in the temporary groundingsystem. In this case, an additional lengthof cable will be necessary to connect thetemporary ground probe and the systemneutral (or in the case of a Delta con-nected system, a grounding cluster bar).
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202 PREPARATION FOR TEMPORARYGROUNDING OF OVERHEAD LINES
1. It is fundamental that before considering anyconductor or equipment as "dead" or de-energized,it must be properly grounded. Refer to the currentElectrical Utility Safety Rules (EUSR) and the UtilityWork Protection Code (UWPC) for specific rulespertaining to grounding.
2. PICK A GOOD LOCATIONIf at all possible, temporary grounding devicesshould be installed at the pole on which work isbeing performed. However, they should be placedwell outside the immediate work area. The violentmovement of the grounding cables under high faultcurrent conditions could cause serious injury. Toprevent accidental contact with live equipment suchas transformers, switches, reclosers, etc., plan thelocation of temporary grounds carefully.
3. TESTING FOR POTENTIALPerform an approved test for potential on the iso-lated circuit after receiving confirmation that isola-tion of the circuit has been completed. Isolationmay or may not have been completed, or contactmay have been made between the isolated circuitand another energized circuit in the area. Failure toperform an approved test for potential, could resultin the worker attempting to install grounds on anenergized circuit.
NOTE: Teasing the isolated conductor with themetallic end of an approved live line tool,or the end of a ground jumper about to beinstalled is not an approved test forpotential.
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4. CLEANING THE CONDUCTOR AND EQUIPMENTIt is very important to clean the conductor usingapproved methods before installing temporarygrounding equipment. The surfaces of conductorsare normally contaminated or corroded. In somecases, equipment or metal structures are coated withpaint. This high resistance surface contaminationmust be eliminated with an approved conductorcleaning wire brush or a filing process, to ensurepositive contact with the surface of the groundingclamps. Consideration might also be given to theuse of clamps with serrated jaws to penetrate theresidual corrosion or paint following the cleaningattempt. Serrated jaws, however, should not beused on aluminum conductors unless a “split sleeve”or “stirrup” has been installed prior to applying thegrounding clamp.
5. MINIMIZE CABLE SLACKShorter grounding cables offer lower resistance.During fault currents, tremendous forces result inunpredictable, severe and dangerous cable move-ment, if there is excessive slack in the groundingcable. Long leads should be lashed at some inter-mediate point to reduce the possible hazard topersonnel, and prevent the dislocation of groundingconnections, should a fault current develop.
203 INSTALLING TEMPORARY GROUNDS ON ANOVERHEAD STAR (WYE) CONNECTED CIRCUIT
Historically, the trend for temporary grounding hasbeen to install grounding jumpers between the primaryconductors and the system neutral; either on both sidesof the worksite or between any source of energy andthe worksite. The following installation illustrates thismethod.
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1. In the following procedure, it is assumed that thenecessary steps have been taken, as discussed inSection 202, Preparation for Temporary Groundingof Overhead Lines, and the work involves thetemporary grounding of a three phase circuit ofpost-type construction. In this scenario, the workerwill be working from an aerial bucket device. Thisis a live line tool procedure.
(a) If work is taking place in a rural area, a groundprobe must be installed as deeply as possibleinto the ground. A suitable lead is connected to itand the system neutral.
(b) If there is live underbuild on the pole, it firstwould be protected with cover-up material of theappropriate rating.
(c) At this point, one of the short jumper cables ofthe set is connected to the system neutral. Theclamp with the support stud is used for thispurpose. (See Figure #9)
Figure #9 Figure #10
(d) Jumper from the system neutral to the nearestphase. (See Figure #10)
(e) Jumper that phase to the next nearest phase,using the proper clamp first. (See Figure #11)
SystemNeutral
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(f) Follow the same procedure for the last phase.(See Figure #12)
By following this procedure, the worker guards againstaccidental contact with ungrounded conductors.
204 INSTALLING TEMPORARY GROUNDS ON ANOVERHEAD DELTA CONNECTED CIRCUIT
Temporary Personal GroundingMost delta connected primary systems utilize a ground-ing bank for ground fault indication and relaying pur-poses. This constantly monitors the circuit andimmediately detects any current flowing to ground. Inthe event of a relatively small amount of current flowingto ground, the circuit breaker should automatically trip.With no neutral in the area, the only option for tempo-rary personal grounding is to utilize a ground probe.The ground probe should be approximately 9 m (30 ft.)from the base of the pole, whereby ground personnelcan work in the clear and avoid possible groundgradients, which expose workers to step and touchpotentials.
Figure #11 Figure #12
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205 RATIONALE FOR PERSONAL GROUNDINGUSING THE GROUNDING CLUSTER BAR
For many years the electrical industry has used severalmethods of applying temporary grounds to protect itswork force. The most widely used methods are:
WORKING GROUNDS – temporary grounding jump-ers, connecting the three primary conductors and thesystem neutral, are installed between any source ofenergy and the work site.
BRACKET GROUNDS – working grounds installed onboth sides of the work site are used where the line couldbe energized from either direction.
PERSONAL PROTECTIVE GROUNDS – in addition tothe historical method of grounding; working groundsinstalled on one side of the work site, with a groundingcluster bar on the pole below the worker's feet and ajumper from the cluster bar to the system neutral.
Investigations by government and private organizationshave shown that in most cases, personal protectivegrounding, using the grounding cluster bar duringconstruction and maintenance of transmission lines,provides the most effective worker protection. TheInstitute of Electrical and Electronic Engineers (IEEE)Study No. 80-1986 summarized the following data:
- the human limit of electrical perception is onemilliampere (ma)
- the so called "let go" current is 9 ma- exposure to 100 ma for a duration of three seconds,
could cause ventricular fibrillation of the heart(Longer duration or higher current flow couldcause internal burning and lessen the likelihoodof survival.)
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Laboratory tests were conducted, using a 911 ohmcarbon resistor to simulate a human, to learn thecurrent flow over a worker during the re-energization ofa 7.2 kV single phase circuit for 14 to18 cycles. Avail-able fault current ranged from 4,200 to 5,700 amps.
In the worst case scenario of the testing sequence, theresistance of the pole was jumpered out, simulating afreshly treated pole, wet pole or concrete pole. Thevoltage through the resistor, in the situation shown inFigure #13, was recorded at 1,745 volts (1.88 amps).In the situation shown in Figure #14, the readings were19.5 volts (21 ma).
SummaryIdentical electrical conditions were brought undercontrol by applying a grounding cluster bar to create anequipotential work zone, as seen in Figure #14.
A worker working from a treated, damp or concrete polewithout an equipotential work zone, and contacting aphase of a circuit that was grounded with the bestgrounding equipment available, would have experi-enced a potentially lethal electrical shock.
A worker performing the same work within an equi-potential work zone, as depicted in Figure #14, wouldhave experienced a mild electrical shock.
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Primary
System
Neutral
1
2 3
Figure #13
Primary
System Neutral
Figure #14
Equipotential Work Zone
Primary
Grounding Cluster Bar System Neutral
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206 APPLICATIONS IN URBAN AREAS –MULTI-GROUNDED SYSTEM NEUTRAL
In urban areas with Wye connected systems, only themulti-grounded system neutral should be used fortemporary grounding procedures. Ground probesshould not be used in urban areas for the followingreasons:
(a) In some cases, it is not feasible to utilize groundprobes. Most often the work location is on a pavedor concrete surface.
(b) Prior to installing objects into the ground in anyarea, the locations of all underground servicesshould be identified. Consequently, this would betoo time consuming when involved with temporarygrounding procedures, especially if the job calls forfrequent movement of equipment.
(c) A ground probe could create a false sense ofsecurity. Low resistance to earth is not guaranteedfor many reasons, including whether the soil is wetor dry, loose or hard-packed, sand or clay. WithWye connected systems, currents in excess of thecurrent ratings of system protection devices, (e.g.,reclosers, breakers, fuses, etc.), are required to tripthe circuit. Should an accidental contact be made,the full available short circuit current may flowthrough the grounding system without tripping thecircuit.
As current flows to the earth through the groundprobe, a heating and drying out process swiftlyraises ground resistance, thereby lowering anychance of enough current flowing to the earth to tripthe circuit. Therefore, it is important that, prior touse, ground rods and temporary ground probeswhen required, are meggered to a resistance of 25ohms or less.
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(d) The use of a ground probe could result in groundgradients which would be dangerous to both thegeneral public and workers. When applying tempo-rary personal grounds, three phase jumperingprovides a short circuiting effect and assists inshort circuiting a re-energization from a threephase source. However, should this circuit be-come energized by induction, or from a singlephase source, the most effective method of drain-ing/controlling induction or tripping the offendingelectrical source is to provide a direct low resist-ance path to the system neutral.
NOTE: You can't drain off electric or electromag-netic induction, as it's always present –you can only control it by converting itfrom high voltage to current with theapplication of temporary grounds.
NOTE: In urban areas with delta connected sys-tems and no system neutral available,the only option for temporary groundingis to utilize a ground probe. The groundprobe should be approximately 9 m (30ft.) from the base of the pole to avoidpossible ground gradients. The probeshould be meggered and a resistance of25 ohms or less should be obtained.Prior to installing ground probes, identifythe location of all underground services.
In conclusion, in urban areas with Wye connectedsystems, only the multi-grounded system neutralshould be used for temporary grounding procedures.If good grounding practices are followed, the rapidisolation of equipment will be facilitated by providing alow resistance path (multi-ground system neutral)through which enough current can flow to operate thecircuit overcurrent protection devices. (See Figure #14)
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Grounding
Cluster Bar
9 m(30 ft.)
25 OHM’s
Wye Configuration
Equipotential
Work Zone Primary
NonMulti-groundedNeutral
207 TEMPORARY PERSONAL GROUNDINGAPPLICATIONS IN RURAL AREAS –NON MULTI-GROUNDED SYSTEM NEUTRAL
Both the non multi-grounded system neutral and theground probe should be used in the grounding systemin rural areas. In rural areas, a ground probe shouldbe properly installed before attempting any type oftemporary grounding procedure. A resistance of 25ohms or less should be obtained with the earth/groundprobe contact. A suitable lead would then be con-nected to the ground probe and to the non multi-grounded system neutral. At this point, follow thesame sequence as would be followed when applyingtemporary personal grounds on a Wye connectedsystem. (See Figure #15)
Figure #15
Wye Configuration
25 OHMS
EquipotentialWork Zone
GroundingCluster Bar
9 m(30 ft.)
Primary
NonMulti-groundedNeutral
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The logic for utilizing the non multi-grounded systemneutral in tandem with a ground probe in rural areas is:
(a) The non multi-grounded system neutral could con-ceivably carry voltage due to unbalanced loadingconditions on the circuit involved, especially whencombined with inadequate or deteriorated ground-ing points often found in rural areas.
(b) The conductor of the non multi-grounded systemneutral could be burned off, broken, or section-alized, preventing a return path to the systemprotection (e.g., line fuses, oil circuit reclosers).
(c) Induction from the phases can raise potential onthe non multi-grounded system neutral above thatof ground potential due to a lack of groundingpoints, and often coupled with deteriorated ground-ing apparatus or conditions. The ground connec-tion from the ground probe to the system neutralshould always be made using rubber gloves and agrip-all stick.
(d) The non multi-grounded system neutral is some-times installed on a crossarm along with theenergized primary phases. A mistake in identifica-tion, when installing the grounding lead, wouldcreate a hazardous situation. Positive identificationshould be made with the use of an approvedpotential indicator.
208 GROUND PROBES
1. An attempt should be made to install temporaryground probes in moist earth and as deep aspossible to ensure the lowest resistance.
2. Guy wires should never be used as a means ofgrounding because of corrosion and non-electricaldesigned connection to the anchor rod. Availablefault currents could completely burn off the guy wire
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209 PREPARATION FOR TEMPORARY PERSONALGROUNDING OF OVERHEAD LINES
1. TESTING FOR POTENTIALPerform an approved test for potential on theisolated circuit after receiving confirmation thatisolation of the circuit has been completed. Isola-tion may or may not have been completed, orcontact may have been made between the isolatedcircuit and another energized circuit in the area.Failure to perform an approved test for potential,could result in the worker attempting to installgrounds on an energized circuit.
NOTE: Teasing the isolated conductor with themetallic end of an approved live line tool,
allowing the structure to topple over, should anunexpected energization occur.
3. Anchor rods should not be used as a substitute fora driven ground rod, as corrosion could causethese to be in a seriously deteriorated state, even tothe point of total isolation from earth.
4. Grounding to street signs, steel fence posts, etc.,provides a false sense of security and could createdangerous ground gradients, should an unexpectedenergization occur, in addition to creating a hazardfor workers and the public.
5. Individual transformer grounds or lightning arrestergrounds are not heavy enough to carry all of theavailable fault current, and should, therefore, not beused for temporary grounding. Lack of currentcarrying capacity, and poor connections, couldcause these ground wires to burn off at the pointwhere the grounding clamp of the temporarygrounding equipment is applied.
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or the end of a grounding jumper about tobe installed, is not an approved test forpotential.
2. CLEANING THE CONDUCTOR AND EQUIPMENTIt is very important to clean the conductor by ap-proved methods before installing temporarygrounding equipment. The surfaces of conductorsare normally contaminated or corroded. In somecases, equipment or metal structures are coatedwith paint. This high resistance surface contami-nation must be eliminated with an approvedconductor cleaning wire brush or a filing process,to ensure positive contact with the surface of thegrounding clamps. Consideration might also begiven to the use of clamps with serrated jaws topenetrate the residual corrosion or paint followingthe cleaning attempt. Serrated jaws, however,should not be used on aluminum conductorsunless a “split sleeve” or “stirrup” has beeninstalled prior to applying the grounding clamp.
3. MINIMIZE CABLE SLACKShorter grounding cables offer lower resistance.During fault currents tremendous forces result inunpredictable, severe and dangerous cable move-ment, if there is excessive slack in the groundingcable. Long leads should be lashed at someintermediate point to reduce the possible hazard topersonnel, and prevent the dislocation of groundingconnections, should a fault current develop.
210 INSTALLING TEMPORARY PERSONALGROUNDS ON OVERHEAD CIRCUITS
NOTE: In the following scenarios, potential testsshould be taken to prove isolation before
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beginning the bonding and grounding proce-dure.
In each of the scenarios presented, linemen workingfrom a structure have created an equipotential workzone around themselves. The key device in thissystem is the grounding cluster bar which is alwayspositioned lower on the structure than the worker'sfeet. In some instances it may be above the systemneutral and in other instances it may have to bepositioned beneath the system neutral. The groundingcluster bar should not be placed more than 2.4 m (8 ft.)beneath the worker's feet.
The grounding cluster bar is a device securely fastenedto the surface of a pole or structure. This is necessarybecause an electrical surge over the pole will remainpredominantly on the outer surface. Therefore, it isparamount that the grounding cluster bar be as tight aspractical against the outer surface of the pole, andcompletely around the pole's circumference.
1. On a Multi-Grounded System, position the clusterbar and secure it tightly to the structure, then install abonding lead from the cluster bar to the systemneutral to equalize potential. (Application Sequence#1 in Figure #16)
The next step is to connect the grounding cableleading from the cluster bar to the nearest phasewire. (Application Sequence #2 in Figure #16) Withthese connections completed, the remaining phasescan be grounded in the usual fashion. (ApplicationSequences #3 and #4 in Figure #16)
(b) On a Non Multi-Grounded System, a ground probeis driven deep into the earth approximately 9 m (30ft.) from the structure and meggered. A cluster bar ispositioned and secured tightly to the pole.
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Grounding
Cluster Bar
Equipotential
Work Zone
Primary
Neutral
1
2
34
The next step is to connect the grounding cableleading from the ground probe to the cluster bar.(Application Sequence #1 in Figure #17) A bondinglead is then connected from the cluster bar to thesystem neutral. (Application Sequence #2 in Figure#17) This is done to ensure that any potential on
Figure #16
Primary
Neutral
Grounding
Cluster Bar
Application Sequence
Cluster bar positioned below worker's feet:
1. Install bonding lead from cluster bar to system neutral.
2. Install grounding lead from cluster bar to nearest phase.
3. Install grounding lead from nearest phase to secondphase.
4. Install grounding lead from second phase to thirdphase.
43
2
1
Equipotential Work Zone
34
the neutral has been reduced to a level as near tozero as practical. At this point, connect the ground-ing cable leading from the cluster bar to the nearestphase wire. (Application Sequence #3 in Figure#17)
Grounding
Cluster Bar
25 OHM’s
Equipotential
Work Zone
Primary
Neutral
1
2
3
45
9 m(30 ft.)
Figure #17
Primary
Neutral
Grounding
Cluster Bar
9 m (30 ft.)
25 ohms
5 4
3
2
1
Application Sequence
Install and megger ground probe; connect a longgrounding lead to probe; install groundingcluster bar:
1. Install grounding lead from ground probe tocluster bar.
2. Install bonding lead from cluster bar to systemneutral.
3. Install grounding lead from cluster bar tonearest phase.
4. Install grounding lead from nearest phase tosecond phase.
5. Install grounding lead from second phase tothird phase.
Equipotential Work Zone
35
With these connections completed, the remainingphases can be grounded in the usual fashion.(Application Sequences #4 and #5 in Figure #17)
3. On a Delta Connected System, a ground probe isdriven deep into the earth approximately 9 m (30 ft.)
Grounding
Cluster Bar
25 OHM’s
Equipotential
Work Zone
Primary
1
2
3
4
9 m(30 ft.)
Figure #18
Application Sequence
Install ground probe and connect longgrounding lead to probe. Install groundingcluster bar:
1. Install grounding lead from ground probeto cluster bar.
2. Install grounding lead from cluster bar tonearest phase.
3. Install grounding lead from nearest phaseto second phase.
4. Install grounding lead from second phaseto third phase.
Equipotential Work Zone
Primary
Grounding
Cluster Bar
9 m (30 ft.)
25 ohms
2
43
1
36
Grounding
Cluster Bar
Maximum Distance 90 m (300 ft.)
Equipotential
Work Zone
Primary
SystemNeutral
Typical temporarygrounding system
installed at anadjacent structure
2
13
Typical use ofgrounding cluster bar
to create an equalpotential work zonewhen the installation
of full temporarygrounding system
from the structure, and meggered.
The next step is to install the grounding cluster barand connect the grounding cable from the groundprobe to the cluster bar.
(Application Sequence #1 in Figure #18) At thispoint connect the ground-ing cable leading from thecluster bar to the nearest phase. (Application Se-quence #2 in Figure #18) With these connectionscompleted the remaining phases can be groundedin the usual fashion. (Application Sequences #3and #4 in Figure #18)
4. Pick a Good LocationIf at all possible, temporary grounding devicesshould be installed at the pole where work is beingperformed. However, they should be placed welloutside the immediate work area. Serious injurycould result from violent movement of the ground-ing cables under high fault current conditions.Likewise, to prevent accidental contact, exercisegood judgement with regard to the use of tempo-
Typical temporarygrounding system
installed at anadjacent structure.
Equipotential work zone
Groundingcluster bar
Maximum distance 90 m (300 ft.)
Primary
Figure #19
Typical use of groundingcluster bar to create an
equipotential work zone whenthe installation of a full
temporary grounding systemcould pose a hazard.
37
rary grounds on poles where there is energizedequipment, such as transformers, switches,reclosers, etc.
At these locations the installation of a groundingcluster bar with the usual configuration to the primaryphases will create the equipotential work areaneeded to protect the worker. However, in this case,the grounding system can be situated on anadjacent structure with the same effect, if theneutral and pole are bonded. (See Figure #19.)
211 PERSONAL GROUNDING FOR WORKFROM AN AERIAL DEVICE
A worker working from an insulated aerial device willencounter many tasks to be performed on de-energizedoverhead circuits. While positioned in an aerial deviceout of reach of other potentials, the worker is actually inan equipotential zone. However, workers are seldomin a position where other potentials aren't within reach.Other potentials would be any structure, neutral, otherphases, trees, etc.
For this reason (use the same principles as dis-cussed in Section 210 using a grounding cluster bar),personal grounding affords the optimum protectionagainst electrical shock in the event of re-energization.For example, a worker making simultaneous contactwith a grounded conductor and the supporting struc-ture would be in a position to receive a severe electri-cal shock if there were to be a re-energization, unlessa grounding cluster bar had been used to create anequipotential work zone. (See Figure #20)
38
Grounding
Cluster Bar
Equipotential
Work Zone
Primary
System
Neutral
1
2
34
Figure #20
Equipotential Work Zone
Primary
Grounding Cluster Bar
System Neutral
39
SECTION III
TEMPORARY GROUNDING OF VEHICLES
300 VEHICLE GROUNDS – GENERAL
301 VEHICLE GROUND EQUIPMENT
302 COMPLETE ISOLATION VERSUSTEMPORARY GROUNDING OF VEHICLES
40
SECTION III
TEMPORARY GROUNDING OF VEHICLES
300 VEHICLE GROUNDS – GENERAL
1. When used in proximity to energized overheadconductors, it is imperative that all radial boomderricks (RBDs), aerial bucket devices (with lowermetal booms) and aerial devices that have lowerboom inserts which are shunted and/or are beingmonitored for current leakage, be grounded to thesystem neutral of Wye connected systems in urbanareas, and a combination of system neutral andground probes in rural areas. Just as important isthe fact that all workers and members of the generalpublic must be kept clear of utility vehicles duringthese operations, with the exception of workers whomay be operating the controls of a radial boomderrick.
No harm would come to an operator as long as he/she remains on the operator’s platform, in theoperator's seat, or on a ground gradient control matbonded to the truck frame. The operator would beunharmed because he/she would be within theequipotential zone similar to that discussed inSection II.
2. Even though the vehicle ground has been connectedto the multi-grounded system neutral, a worker ormember of the general public standing on theground, while in contact with the truck body, wouldcreate a parallel path to ground should the metallicportion of the boom make contact with overheadenergized equipment. That person could besubjected to lethal currents. (See Figure #21)
3. If hold-off protection was in effect and the vehicle
41
was properlygrounded to themulti-groundedsystem, the nonmulti-groundedsystem or a properlyinstalled groundprobe (Delta con-nected system), thevehicle would onlybecome energizedmomentarily until thesystem protection activated. Once the offendingcircuit became isolated it would remain isolated,thereby returning the work area to a relatively safecondition. During the instant of energization,anyone touching the vehicle while in contact withanother potential (another vehicle, earth, structure,etc.) could receive a lethal electrical shock.
4. Remember, even though a hold-off is in effect and atruck ground is used, the workers and generalpublic should be kept clear of the vehicle when theboom is in proximity to energized overhead equip-ment. Work area protection should be used toseparate the vehicle from the people on the ground.Barricading the vehicle is an option to keep person-nel away.
5. In rural areas where a Wye connected systemexists, the non multi-grounded neutral should beused the same as in urban areas where the neutralis classified as a multi-grounded neutral. Inaddition, a properly installed ground probe shouldbe used to help ensure rapid isolation should aninadvertent energization or contact occur. Theground probe should be positioned approximately
Figure #21
42
Grounding
Cluster Bar
Equipotential
Work Zone
Equipotential Zone
t th
Primary
System
Neutral
9 m (30ft.) fromthe workarea.
6. Vehiclesworkingin proxim-ity toDeltacon-nectedcircuitsshould begroundedusing agroundprobedrivendeep intothe earth.
The ground probe should be positioned approxi-mately 9 m (30 ft.) from the work area and con-nected to the grounding cluster bar.
7. If the grounded boom of an RBD or similar hoistingdevice enters an equipotential work zone, it shouldbe at the same potential as the equipotential workzone. This is done by direct metallic electricalconnection of the truck ground to the temporarypersonal grounding system (using the truck groundto connect directly to the grounding cluster bar).When more than one vehicle is used and a personcould touch two vehicles at the same time, thevehicles should be bonded together.
8. The same concept applies whenever the structureis guyed. Guy wires entering an equipotential zonecould defeat the safety of the zone by introducing
Equipotential Work Zone
Primary
System Neutral
Grounding Cluster Bar
Equipotential zone common to the equipotentialwork zone at pole top
Figure #22
43
another potential. Guy wires must also be bondedto the grounding system.
NOTE: To not ground in this manner could causea total breakdown of the equipotentialwork zone. (See Figure #22)
301 VEHICLE GROUND EQUIPMENT
1. All vehicle grounds should be comprised of at least1/0 extra flexible copper cable, and be capable ofbeing attached to the overhead system neutralusing a grip-all stick. One end of the cable shouldhave the appropriate current carrying capacitygrounding clamp. The other end of the cableshould be solidly and permanently connected to thevehicle frame using an appropriate compressionlug fitting, attached via an appropriate currentcarrying clamp to a vehicle mounted parking stud,or via a reel type vehicle ground.
2. All components of the vehicle, such as the chassisutility box and boom assembly, should be bondedtogether.
3. Reel type vehicle grounds provide a good means ofstoring the cable whennot in use, and ease ofhandling when required.This arrangement offersgood conductivity whenthe moving contact(arbour) is inspected andcleaned on a regularbasis. Conductivity isfurther established bywelding the metal reelstand to the bin structureof the line vehicle. (SeeFigure #23) The reel Figure #23
44
stand frame to be bonded to the vehicle chassis.
All conductors carrying current, produce an electro-magnetic field (EMF). The field size and intensityare directly related to the voltage level and theamount of current flowing.
The configuration of the conductor is extremelyimportant in its capability to carry current. The mostefficient configuration is straight and the leastefficient is coiled.
A great deal of effort goes into reducing resistancein grounding systems to the lowest levels practica-ble. This is accomplished by using appropriateground clamps, heavy copper grounding cables,and maintaining clean, low resistance connections.
These efforts to achieve low resistance can bedefeated by leaving grounding cable stored on thereel. When the ground cable is in use, the circularconfiguration causes inductance which impedesthe flow of current. This dramatically impedes theflow of electrons. The more turns, the moreimpedance. This electrical principle is known asLenz's Law.
All excess cable must be pulled off any groundingreel during use, in order for the resistance toremain as low as practicable. This will permit thegrounding system to function at its greatest faultcurrent capability. In turn, the offending re-energization will be interrupted in fewer cycles ascircuit protection devices operate.
NOTE: The reel should be completely emptied toprevent any inductive reaction shouldcurrent flow. Once all the cable is offthe reel drum, check the connection forfraying and/or corrosion.
45
4. Another methodfor storing thegroundingcable when it isnot in use is tosimply utilizehangers
attached to the
rear of the bin
structure.
Some believethat the vehicleground should always be at the rear of the vehicle,especially with RBD units, as a constant reminderto the operator to ground the vehicle when theboom is used in proximity to energized apparatus.(See Figure #24)
302 COMPLETE ISOLATION VERSUSTEMPORARY GROUNDING OF VEHICLES
1. This is in reference to line vehicles being used inurban areas, in proximity to energized overheadequipment, by non-electrical utility organizationssuch as line clearing firms and telecommunica-tions companies.
Throughout this Guide it has been stated thatground probes for use with temporary groundsshould not be used in urban areas, and are not aseffective as using the system neutral. In addition,personnel from non-electrical utility organizationsare not qualified and should not attempt to makeconnections to the system neutral. Therefore, lineclearing aerial devices specifically, should beequipped with lower boom insulated inserts. If not
Figure #24
46
so equipped, the only approach would be to ropeoff or barricade the entire vehicle when the boom ofthe unit is used aloft in proximity to energizedequipment. This would protect workers and thegeneral public on the ground from any possiblehazard created by a boom contact. Highly visiblewarning signs would be affixed to the barriers,barricades, etc. (See Figure #25)
2. The operator of an RBD vehicle standing on theoperator's platform, sitting in the operator's seat, orstanding on a ground gradient control mat bondedto the chassis, would not be harmed should acontact occur. This is due to the fact that the workeris within an equipotential zone.
Figure #25
47
SECTION IV
GROUND GRADIENT CONTROL MATS
400 GENERAL
401 STEP POTENTIAL
402 TOUCH POTENTIAL
403 DESCRIPTION OF A GROUNDGRADIENT CONTROL MAT
404 OPERATION OF AIR BREAKSWITCHES
48
SECTION IV
GROUND GRADIENT CONTROL MATS
400 GENERAL
The ground gradient control mat is used to create anequipotential zone. It brings the surface of the earth,where workers are standing, to the same potential asthe equipment on which work is being done. Shouldan accidental energization occur, or should there be aninduced potential, the ground gradient control matprotects against step and touch potentials.
To accomplish its purpose, the ground gradient controlmat must be correctly sized and positioned, so workcan be performed without workers stepping off the mat.
Ground gradient control mats are commonly used forstringing conductor or messenger wire, and for operat-ing air break or load break switches.
401 STEP POTENTIAL
Step potential is defined as the potential differencebetween two points in a ground gradient area. Aground gradient area is produced where voltage entersthe earth.
Due to the variation in the earth's resistance, a personwalking or standing in an area where a ground gradi-ent is produced could have a potential differencebetween their feet. Therefore, current would flowacross that potential difference. (See Figure #26)
402 TOUCH POTENTIAL
Touch potential is defined as the potential differencebetween the point where a person is standing and a
49
point that person could normally reach. If a persontouched a conductor, equipment, or a ground probe atthe same time it was energized, that person would besubjected to touch potential due to being a parallel paththrough which the current can flow. (See Figure #26)
403 DESCRIPTION OF A GROUND GRADIENTCONTROL MAT
A common ground gradient control mat is a grid of metal(galvanized steel; or high flex copper braid strategicallypositioned on fabric) arranged in such a manner that,workers standing or walking on the mat will always bebridging the grid with their feet. To accomplish this, thesteel mat typically consists of a minimum of No. 10gauge (0.1350) galvanized steel wire, constructed in a5 cm (2 in.) square mesh, 1.5 m by 6 m (5 ft. by 20 ft.),or as required (i.e. circumstances dictate the lengthand width of the mat).
The latter would be a typical size for a conductor ormessenger stringing set up. In fact, most stringingoperations require two or three such mats bondedtogether. Each mat has a bonding cable threaded
Figure #26
step
touch step
50
around its perimeter (1/0 copper). Typically, a portableground gradient control mat for switching operationswould be approximately 1.2 m x 1.5 m (4 ft. x 5 ft.), andhave an attached bonding lead and clamp. (SeeFigure #27)
404 OPERATION OF AIR BREAK SWITCHES
A portable ground gradient control mat should be usedto stand on while operating air break switches. In theevent of mechanical failure or a flashover to themetallic frame of the switch, the operating rod, handleand the earth could become energized. The mat isnormally found mounted in a weatherproof container atremote switch locations, or carried on a truck for usewhen required. The extra flexible copper lead, equip-ped with an approved type grounding clamp, is con-nected to either the switch operating rod close to the
Figure #27
51
handle, or to the flexible ground strap or cable attachedto the operating rod.
To be sure that no potential difference can existbetween hands and feet during the operation, followthis procedure:
(a) Place the mat in position on the earth
(b) Wear rubber gloves
(c) Stand on the mat and connect the mat lead to theoperating rod or flexible ground strap
(d) Keeping both feet on the mat, perform switchingoperations wearing rubber gloves
(e) Visually check the blades of the air break switch toensure proper operation, lock the handle, and tagas necessary
(f) Disconnect the extra flexible grounding lead
(g) Step off the mat and remove the rubber gloves
(h) Store the ground gradient control mat
52
53
SECTION V
TEMPORARY GROUNDING METHODS FORCONDUCTOR STRINGING OPERATIONS
500 GENERAL PRECAUTIONS
501 PROCEDURES FOR SETTING UPFENCE BARRIERS
54
SECTION V
TEMPORARY GROUNDING METHODS FORCONDUCTOR STRINGING OPERATIONS
500 GENERAL PRECAUTIONS
Considerable emphasis is placed on isolation tech-niques and grounding procedures when using largehydraulically-driven tension machines. However, thereis a tendency not to take the same precautions wheninvolved with routine stringing operations using smalltension brakes or reel brakes, in conjunction with reeltrailers – even though the stringing may be done in thearea of energized equipment. The same precautionsshould apply to routine stringing operations nearenergized equipment as apply to major stringing jobsusing large hydraulically-driven tension machines.
The grounding/bonding of tensioning machines, pullingmachines, ground gradient mats, conductors, andtravellers is to create an equipotential work zone. Thisis a very important component in providing a safe workzone for crew members and the general public. Everyeffort taken during preparation to eliminate a potentialdifference throughout the project will help prevent injuryshould something go wrong.
Sometimes, through equipment failure, loss of control,missed communication, oversights or misjudgements,the conductor being strung contacts something that isenergized. Equipment may be damaged, power inter-rupted, and the project delayed. However, if thisunplanned event causes no personal injuries thegrounding/bonding has worked as designed.
All grounding/bonding connections should be re-garded the same as making electrical connections.The lower the resistance and the more direct path to
55
the system protection (fuses, reclosers, etc.), the morerapid the interruption. Therefore, the preferred connec-tions would always be to the system neutral, whenavailable.
In locations where a system neutral is not available, aseries of ground probes with 25 ohms or less resistanceis the next best choice. In rural areas, a combination ofground probes and the non multi-grounded systemneutral is necessary.
A ground gradient mat should be used for the place-ment of the tension machine.
NOTE: The mat should be large enough to carry out all work without stepping off the mat.
At the tension (pay out) end, work includes operatingthe machine, changing reels and splicing conductors.A space of 2.4 to 3 m (8 to 10 ft.) is necessary to spliceconductors behind a tension machine, without steppingoff the mat.
In most instances, two or three separate mats will needto be positioned to adequately encompass the equip-ment placed upon it. Each mat used should bebonded to a common bus to ensure an equipotentialwork zone is created.
Bonding cable of 1/0 bare, braided or stranded copperis threaded around the perimeter of the mat, then themat and lead (bonding cable) are connected togetherwith an appropriate connector, approximately every 0.9m (3 ft.).
At an appropriate location, an extra flex lead, equippedwith an approved type grounding clamp should beconnected from the bonding lead to the system neutral.This lead should be a minimum of 1/0 extra flexcopper, and should be treated as a possible energizedconductor.
56
NOTE: Ground rods are required when workingwith delta connected circuits. At eachcorner of the mat, ground rods would bedriven and connected to the bonding cable.Where practical, sufficient ground rodsshould be driven to obtain a megger read-ing of 25 ohms or less.
Grounding/Bonding During Stringing OperationsTo achieve the goal of establishing a safe workenvironment, the following setup would be consid-ered as necessary. (See Figure #28)
At the ReelThis is the first of a series of grounds to be applied.
Even though there are several types of tensionmachines in use, a standard method is used toground the conductor on the tension stringing reels.
On the large hydraulic tension machines, a bonding
Conductor Travelling Ground
1/0 HighFlex Copper
Connect Tail ofConductor toReel Stand
Connect toSystem NeutralWhenever Available
Connect toDriven GroundRods when noNeutral isavailable
Neutral
1
2
3
4
Each section ofGrounding Matconnects to CommonGrounding Point
Figure #28
Connect tosystem neutralwheneveravailable
Neutral
Connectto drivenground rodwhen noneutral isavailable
ConductorTravellingground
Connect tailof conductorto reel stand
Each section ofgrounding matconnects tocommongrounding point
1/0 extraflex copper
57
lead is connected from the tail of the conductor (pro-jecting through the reel) to the ground lug provided onthe drive arm of the tensioner. Internally, on the driveshaft, a collector ring provides an electrical path,through a set of brushes and extra flex copper, to anexternal ground lug on the tensioner.
NOTE: This is the only opportunity to ground coveredconductor during the stringing procedure.
Ahead of the Reel (Travelling Ground)This is the second opportunity to ground the conductor.It maintains a high integrity connection to system pro-tection throughout the entire run. To help ensure this:
a) the full capacity leads and clamps should bethoroughly inspected and adequately tightened;
b) the entire circumference of the wire is involved;
c) the mechanism is spring loaded to accommodateall irregularities in the conductor.
This ground will ensure continuity with theequipotential zone around the tensioner. It will alsoensure the conductor is grounded as it passes upthrough any underbuilt circuits. It is also moveable andremains on the conductor tail as the conductor is cutand lowered down through any underbuilt circuits.
The travelling ground is connected to the tensionmachine using a 1/0 extra flex copper lead attached toa common grounding point. (See Figure #29)
The ground gradient mat(s) are also connected to thecommon grounding point. Another 1/0 lead is con-nected to either the system neutral or to groundprobes, as discussed earlier.
First and Last TravellerThis is the third and last point in the run to ground theconductor. The conductor's angle of deflection at thesetravellers allows for greater surface contact between
58
the conductor andgrounded travellers.
Pressure and increasedcontact area betweenthese travellers and theconductor is desirable toprovide a good path toground. These may bethe only travellers in therun that are able to begrounded.
When the conductor iscut after dead-ending,the grounded travellercontinues to providesome contact withground.
NOTE: Travellers with protective coatings on thesheaves are not designed to be grounded.
General Rule: Fifth Traveller GroundingThis grounding will provide additional paths to groundthroughout the run.
In circumstances where induction could be present,these multiple grounds will help ensure continualdraining of induced voltage. Should an inadvertentcontact occur, these grounds will help isolate theoffending circuit more rapidly. This is also the rationalefor grounding both sides of traversing energizedcircuits.
All workers should understand when grounding anyapparatus they are making electrical connections. Thesame care is to be taken as if the device was beingconnected to an energized medium to carry current.
Figure #29
59
The fault current during a short circuit could rise to tensof thousands of amps. Any underrated, loose, orcorroded connections will fail, some with catastrophicresults.
The more paths to ground the better. The better theconnections, the more rapid the protection system willoperate; thereby providing a safer work environment.(See Figure #30)
Figure #30
When full puller/tensioner machines are not used,other types of tension devices are used, as shown inFigure #31. Regardless of the type of tensioning orpulling device used, the grounding procedure shouldbe adequate to protect the workers and the general
Figure #31
PullerGrounded
25 OHM’s
25OHM’s
TravellerGrounded
ConductorGrounded
TravellerGrounded Equipotential Work Zone
TravellerGrounded(Every 5th Structure)
Pulling End
Last Traveller 1st Traveller
Tension End
Puller grounded
25 ohms
Pulling End
Last Traveller First Traveller
TravellerGrounded
Traveller Grounded(every 5th structure)
TravellerGrounded Equipotential Work Zone
25ohms
Tension End
ConductorGrounded
Vertical pivoting action
Horizontal pivoting
action
60
public. Equipotential work zones are always theobjective when grounding systems are being installed.
501 PROCEDURES FOR SETTING UPFENCE BARRIERS
1. Nonconductive barriers should be installed aroundthe perimeter of the ground mat, to prevent person-nel from straying on and off the mat except at acontrolled location. This controlled location is a 0.9m (3 ft.) opening for entry/exit. The barrier systemwill also remind personnel that they should nothand tools into and out of the zone when stringingis in progress.
2. Approximately 0.9 to 1.2 m (3 to 4 ft.) outside thisbarrier, another barrier (rope, tape, barricades, etc.)should be installed around the enclosure. “DangerLive Apparatus” signs should be hung on this barrier.
3. At the entry/exit point, a piece of plywood 0.9 m by1.8 m by 1.3 cm (3 ft. by 6 ft. by ½ in.), covered by anonconductive rubber or plastic mat, should beplaced so that one end is on the ground mat and theother is clear of the barrier around the enclosure.This is to protect personnel from step potentialswhen entering or leaving the enclosure. No person-nel may enter or leave the enclosure when stringingis in progress. (See Figure #32)
61
Figure #32
Second physicalbarrier
Ground matwire fabric
First physicalbarrier
1/0 strandedcopper bondinglead, threadedaroundperimeter of matand connectedto systemneutral
Plywood covered with rubber mats, used to provideentranceway in and out of enclosure
Equipmentrequiringground
matprotection
Work areainsidesecondphysicalbarrier
Setting up ground gradient mat area for stringing
62
63
SECTION VI
TEMPORARY GROUNDING OFUNDERGROUND EQUIPMENT
600 GENERAL PRECAUTIONS
601 TEMPORARY GROUNDING SYSTEMFOR UNDERGROUND APPLICATIONS
602 CONSIDERATIONS PRIOR TOUNDERTAKING WORK
603 PROCEDURE FOR DE-ENERGIZINGAN UNDERGROUND CABLE AT ANELBOW CONNECTED PADMOUNTTRANSFORMER
604 PROCEDURE FOR DE-ENERGIZINGAN UNDERGROUND CABLE AT ARCSTRANGLER SWITCHGEAR (NX)
64
SECTION VI
TEMPORARY GROUNDING OFUNDERGROUND EQUIPMENT
600 GENERAL PRECAUTIONS
To be considered de-energized and safe to work on(for underground equipment), it must first be positivelyidentified, checked as being isolated, then grounded andtagged. Through good engineering and design of theseinstallations, and utilization of proper procedures, toolsand equipment, underground plant can be de-ener-gized in the true sense and work can be performedsafely.
601 TEMPORARY GROUNDING SYSTEMFOR UNDERGROUND APPLICATIONS
There is quite a variety of grounding applications, dueto the many types of underground equipment andinstallations.
Most manufacturers or suppliers of undergroundequipment can identify the specific type of groundingcomponents required for their particular design.
In some cases, equipment, similar to that used foroverhead applications, will work fine on undergroundinstallations. In other cases, unique equipment isrequired for specific designs.
All else considered, the ground sets, by whateverdesign or application, should meet the ground faultcurrent capacities of your system.
602 CONSIDERATIONS PRIOR TOUNDERTAKING WORK
Just as in temporary grounding of overhead equip-
65
ment, other good safe work practices must accompanytemporary grounding procedures of undergroundequipment. If applicable to the job at hand, considera-tion must be given to the following:
- Remove obstacles, snow, etc. and be aware ofchildren and pets prior to opening vaults or doors ofenclosures having energized apparatus.
- Do not lay tools and equipment on top of padmountenclosures.
- Always follow the IHSA lock to lock rubber gloverule.
- If a door is part of the underground installation orenclosure, make sure that it will not close acciden-tally while work is in progress.
- Verify switchgear and cable nomenclature withoperating diagrams in accordance with the Applica-tion for Work Protection and Switching OperationsForms (UWPC).
- Arrange hold-off protection in accordance with yourstandard operating procedures.
- Always wear the appropriate personal protectiveequipment including clothing, head protection, andeye protection.
603 PROCEDURES FOR DE-ENERGIZING ANUNDERGROUND CABLE AT AN ELBOWCONNECTED PADMOUNT TRANSFORMER
In general, the same principles and proceduresdiscussed under this heading, apply to isolating andde-energizing an underground cable at a three-wayjunction point in an underground vault.
In the following procedures, the necessary steps asoutlined in Section 602, will apply and special empha-
66
sis will be placed on thefact that it is a live line tooloperation. The cablesinvolved are terminatedwith loadbreak elbows,which are quite easy todistinguish by their bellshape. (See Figure #33)
(a) Verify nomenclature ofthe cable, switchgear,or transformer locationwith the operating dia-gram, in conjunctionwith the SwitchingOperations Order.
(b) Check the installation forloose wall brackets orparking stands, obviousheating of the elbowsemi-conductive exterioror junctions, and for anyother components thatmight fail during opera-tion.
(c) Install a feed-throughdevice on the attachedparking stand. It should be cleaned and lubricatedprior to installation. (See Figure #34)
NOTE: Use a grip-all live line tool during alloperations.
(d) Remove the loadbreak elbow of the cable inquestion and install it on one of the bushings of thefeed-through device.
(e) Install a dead-end receptacle on the vacant ener-
Figure #34
Figure #33
67
gized bushing of the padmounttransformer. (See Figure #35)
(f) Using an approved test point indica-tor or full potential indicator, verifyisolation of the cable in question.
(g) Prepare to install the groundingelbow. (See Figure #36) A propergrounding clamp secured to the otherend of this device is installed to thesystem neutral at the padmount
transformer location. Thegrounding elbow is theninstalled on the vacantbushing of the feed-through device. (SeeFigure #37)
(h) Tag the de-energized apparatusas per your SwitchingOperations Form.
Figure #35
Feed-throughdevice installedon parkingstand.
Dead-endreceptacleinstalled onvacanttransformerbushing.
Loadbreak elbowtransferred to feed-through device. (Cableis now de-energized.)
Grounding elbowinstalled on feed-throughdevice. Other end ofgrounding elbowconnected to systemneutral at this location.
Figure #37
Figure #36
68
The cable in question at this point is de-energized. Ifthe situ-ation was such that it was a loop systemutilizing padmount transformers, the opposite end ofthe cable would have been previously isolated.
NOTE: Under no circumstances should a non-loadbreak elbow or dead-end receptaclebe removed from an energized bushing pluginsert at a transformer installation, switch-ing installation, stand-off plug, feed-throughdevice or junction point. Minimal clearancesand ionization of air will result in a flashoverbetween the pin contact and conductiveshield of the elbow, or between the semi-conductive body of the bushing pluginsert and the socket contact. (See Figure#38)
A cable spiking tool shall be used, as the last step, toguard against the possibility of cutting into mis-identified cable. Approved safe work practices and therecommended manufacturer procedures should beused when using these spiking tools.
Figure #38
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Figure #39
604 PROCEDURE FOR DE-ENERGIZING ANUNDERGROUND CABLE AT ARC STRANGLERSWITCHGEAR (NX)
The necessary steps outlined in Section 602,Considerations Prior to Undertaking Work, should befollowed prior to beginning the operation. The pro-cedure is as follows:
(a) Verify switchgear and cable nomenclature withoperating diagrams.
(b) Using an approved live line tool, open the desig-nated switching apparatus under the direction ofthe work protection holder, entering times asoperations are completed.
Do not use a grip-all stick to operate theswitchgear, as damage will result to the pulling eye.Use an approved live line tool like the one shown inFigure #39.
(c) Using the sameapproved live linetool, remove the arcstrangler (NX)assembly. (SeeFigure #40)
(d) Use an approved fullpotential indicatingdevice to test forpotential at the lowerhinge assembly.
(e) When isolation hasbeen established, anapproved temporaryground clamp (NX),first attached to a low resistance ground, may thenbe installed with a grip-all stick to the lower hinge
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Figure #41
assembly of the switchgear. The cable at thislocation is now de-energized. (See Figure #41)
(f) Tag the apparatus as per the Work Protection Coderequirements.
Figure #40
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SECTION VII
TEMPORARY GROUNDINGWITHIN SUBSTATIONS
700 GENERAL PRECAUTIONS
701 SUITABLE TEMPORARY GROUNDINGSYSTEMS FOR SUBSTATIONAPPLICATIONS
702 TEMPORARY GROUNDINGPROCEDURES FOR SUBSTATIONS
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SECTION VII
TEMPORARY GROUNDINGWITHIN SUBSTATIONS
700 GENERAL PRECAUTIONS
Improved design of modern substations, along withmore advanced technology and equipment, hashelped to eliminate many of the inherent hazardswhich were sometimes found in older substations.
However, the importance of proper grounding proce-dures cannot be over emphasized, regardless of thetype of substation.
Of equal concern is the available fault current at thesubstation, and the ability of temporary groundingsystems to provide necessary protection for workers.
701 SUITABLE TEMPORARY GROUNDINGSYSTEMS FOR SUBSTATION APPLICATIONS
In most cases, the selection of adequate temporarygrounding systems would be on the advice of areputable manufacturer. The electromechanicalspecifications of certain grounding equipment shouldbe researched and verified so that it will meet therequirements of its intended use.
To some extent, the jumper-type grounding systemsreferred to in Section II would apply. Considerationshould be given to the size of the conductor or buswork, and whether it is tubular or flat. (A popular clampused in substation applications is shown in Figure#42)
It can swivel so that it can be connected readily indifficult areas such as vertical bus work, or on aconductor at the end of a string of dead-end insulators.
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Another feature ofthis clamp is itsinterchangeablejaws, which can beadapted for eitherflat or tubular buswork.
During the construc-tion of modern sub-stations, permanentstirrup arrangements are often installed on the largesize tubular bus. Grounding clamps large enough toadapt to this bus would not be practical and, as aresult, the stirrups allow for the use of regular sizegrounding clamps. (See Figure #43)
Figure #42
Figure #43
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Figure #44
Support studs are sometimes permanently installedon large size tubular bus work. Grounding sets,comprised of two cables per phase, with specializedclamps, are attached to these studs when temporarygrounding is required.
As previously mentioned, 1/0 copper extra flex cableshould be the minimum size for temporary groundingsets. In many cases, the requirements for substationapplications could be, for example, 4/0 cable orequivalent parallel cables.
702 TEMPORARY GROUNDING PROCEDURESFOR SUBSTATIONS
Basically, the same temporary grounding techniquesapply to substation situations as those discussed inthe grounding of overhead lines. However, due to thehigh fault current available atsubstations, heavier dutyequipment may be required.Depending on the weight ofthe cable involved, con-sideration may have to begiven to the use of specialtools and techniques forhoisting the temporarygrounding equipment intoplace. (See Figure #44)
The same considerations aspreviously mentioned in thisGuide are equally importantin substation applications.These include picking theproper location for temporarygrounding, proper testing for
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potential prior to applying grounds, cleaning theconductor, and minimizing cable slack.
In substations, where numerous sets of temporarygrounds could be applied at any given time, groundingdevices should be identified to ensure all groundshave been removed before service is restored.
Refer to the current EUSR for "Work on IsolatedCircuits" and "Use of Temporary Grounds."
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77
SECTION VIII
CONCLUSION
800 DANGEROUS MISCONCEPTIONS
801 CONCLUSIONS
78
SECTION VIII
CONCLUSION
800 DANGEROUS MISCONCEPTIONS
Do not be misled by some of these common beliefs:
1. Electricity only takes the path of least resistance toground.
2. If vehicles and equipment are grounded, they aresafe to touch in any circumstance.
3. Grounds placed between a worker and all taggedopen points guaranteeing isolation, act as barricadesor insulators, blocking any voltage rise or currentflow at the worker's location.
4. Working near a ground rod connected to a set ofgrounds does not present any particular hazard.
5. Inductive voltages build up over time, therefore,grounds can sometimes be removed by hand.
6. If you are standing on a wood pole, you are safesince wood poles are nonconductive.
7. Induction is eliminated once grounds are applied.
801 CONCLUSIONS
1. Induction is an ever present danger to utility per-sonnel and the public, and can result in seriousinjury or even a fatality.
2. Any vehicle, piece of equipment, or ground probe,connected to an isolated line, can subject the bodyto severe step and touch potentials under givenconditions.
3. Induction on isolated equipment cannot be elimi-nated, but it can be controlled.
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4. Phase to neutral voltage increases as you moveaway from a temporary grounding site.
5. Creating an equipotential work zone greatly de-creases the potential for hazardous current flowthrough the body.
6. An equipotential work zone can only be establishedthrough the combined efforts of grounding andbonding.
T 905-625-0100�•�T 1-800-263-5024�•�F [email protected]�•�ihsa.ca
© Copyright 2011. All Rights ReservedInfrastructure Health & Safety Association
SPG16
• Bare Hand Live Line Techniques
• Conductor Stringing• Entry and Work in a
Confi ned Space• Excavating with
Hydrovacs in the Vicinity of Underground Electrical Plant
• High Voltage Rubber Techniques up to 36 kV
• Hydraulics
• Ladder Safety • Line Clearing Operations• Live Line Tool Techniques • Low Voltage Applications • Pole Handling• Ropes, Rigging and
Slinging Hardware• Temporary Grounding
and Bonding Techniques• Underground Electrical
Systems
Available Safe Practice Guides
