$28.95 U.S. Practical Guide To Electrical Grounding - Practical Guide To...Practical Guide To...

PracticalGuide ToElectricalGrounding


An

Publication W. Keith SwitzerW. Keith Switzer

$28.95 U.S.

First Printing, First Edition, August 1999First Printing, First Edition, August 1999

G157LT99 Grounding Book COVER 9/10/1999 2:40 PM Page 1

Electrical Protection Products34600 Solon RoadSolon, Ohio 44139

W. Keith Switzer, Senior Staff EngineerPhone: (440) 248-0100Fax: (800) 677-8131E-mail: [email protected]

Library Of Congress CatalogCard Number: 99-72910

Copyright © 1999 ERICO, Inc.

All rights reserved. No part of this work covered by thecopyright hereon may be reproduced or used in any form orby any means – graphic, electronic, or mechanical,including photocopying, recording, taping, or informationstorage and retrieval systems – without written permissionof ERICO, Inc.

Practical Guide to Electrical Grounding

Grounding Book 4/14/99 10/5/99 6:01 PM Page IFC1 (Black plate)



An

PublicationFirst Printing, First Edition, August 1999

W. Keith SwitzerW. Keith Switzer

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Table of Contents

Chapter Description Page

1 Building and Service Entrance Grounding - 1The grounding of buildings and facilities where people work.

Building GroundingGround ResistanceElectrical Service GroundingUfer Grounding

2 Building Lightning Protection - A critical extension of grounding. 21

3 Building Interior Bonding and Grounding - The bonding and 47grounding of building steel, electrical panels and other powersystems equipment.

IntroductionBondingGroundingGround Bars & Ground Bus

4 Transients & Other High Frequency Bonding and “Grounding” Requirements 65The bonding and grounding of electronic systems.

5 Selection of Components Used in Grounding 79Grounding ConductorsConnectorsGrounding Electrodes

6 Special Grounding Situations - Areas not covered elsewhere 89AirportsCorrosion and Cathodic ProtectionRadio Antenna GroundingStatic GroundingWire MeshFences and Gates

7 Application of Surge Protection Devices 113

Definitions 119

References and Bibliography 121

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WHY DO WE NEED ANOTHERBOOK ON GROUNDING?

This book is designed for the contractor who finds thatinstalling grounding systems, which are in compliance withall relevant codes and standards, is a complex andsomewhat mystifying assignment. While in larger facilities,the design of a proper grounding system is certainlycomplex and should be left to a qualified engineer, theeveryday grounding installations and applications coveredin this text are well within the scope of the qualifiedcontractor. In most facilities, a thoughtful contractor canfollow the guidelines and techniques in this book and bereasonably ensured that he has done a competent and codecompliant job. This book is not written for the casualcontractor who was in the painting business last week. It isfor the electrical contractor who intends to be in businessnext week, next year, and in the years to come. Design andinstallation of electrical grounding systems is one of themost important aspects of any electrical distributionsystem, yet it is all too often misunderstood andsubsequently installed improperly. Some detailedknowledge of the facility is needed, and the contractor whointends to do the job correctly must make the investment intime and tools - or hire someone to do these things for him.Guesswork won’t do! The subject is too serious andcomplex for that kind of approach. We hope you find ourrecommended approaches helpful and cost-effective.

Article 250 of the National Electrical Code (NEC) containsthe general requirements for grounding and bonding ofelectrical installations in residential, commercial andindustrial establishments. Many people often confuse orintermix the terms grounding, earthing and bonding. To usesimple terms:

Grounding is connecting to a common point which isconnected back to the electrical source. It may or may notbe connected to earth. An example where it is not connectedto earth is the grounding of the electrical system inside anairplane.

Earthing is a common term used outside the US and is theconnection of the equipment and facilities grounds toMother Earth. This is a must in a lightning protection systemsince earth is one of the terminals in a lightning stroke.

Bonding is the permanent joining of metallic parts to forman electrically conductive path that will ensure electricalcontinuity and the capacity to conduct safely any currentlikely to be imposed. A comprehensive review of groundingand bonding requirements contained in the NEC appears inChapter 3 of this text.

NEC is a copyright of NFPA.

WHY GROUND?

There are several important reasons why a groundingsystem should be installed. But the most important reasonis to protect people! Secondary reasons include protectionof structures and equipment from unintentional contactwith energized electrical lines. The grounding system mustensure maximum safety from electrical system faultsand lightning.

A good grounding system must receive periodic inspectionand maintenance, if needed, to retain its effectiveness.Continued or periodic maintenance is aided throughadequate design, choice of materials and proper installationtechniques to ensure that the grounding system resistsdeterioration or inadvertent destruction. Therefore, minimalrepair is needed to retain effectiveness throughout the life ofthe structure.

The grounding system serves three primary functionswhich are listed below.

Personnel Safety. Personnel safety is provided by lowimpedance grounding and bonding between metallicequipment, chassis, piping, and other conductive objects sothat currents, due to faults or lightning, do not result involtages sufficient to cause a shock hazard. Propergrounding facilitates the operation of the overcurrentprotective device protecting the circuit.

Equipment and Building Protection. Equipment andbuilding protection is provided by low impedancegrounding and bonding between electrical services,protective devices, equipment and other conductive objectsso that faults or lightning currents do not result in hazardousvoltages within the building. Also, the proper operation ofovercurrent protective devices is frequently dependent uponlow impedance fault current paths.

Electrical Noise Reduction. Proper grounding aids inelectrical noise reduction and ensures:

1. The impedance between the signal ground pointsthroughout the building is minimized.

2. The voltage potentials between interconnectedequipment are minimized.

3. That the effects of electrical and magnetic fieldcoupling are minimized.

Another function of the grounding system is to provide areference for circuit conductors to stabilize their voltage toground during normal operation. The earth itself is not

Preface iii

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Practical Guide to Electrical Grounding

essential to provide a reference function. Another suitableconductive body may be used instead.

The function of a grounding electrode system and a groundterminal is to provide a system of conductors which ensureselectrical contact with the earth. Two Fine Print Notes(FPN) that appear in Section 250-1 of the NEC provide agood summary of the reasons for grounding systems andcircuit conductors and the conductive materials whichenclose electrical conductors and equipment.

TYPES OF GROUNDING

As noted above, grounding and bonding are not the same.In addition, not all grounding is the same. Each chapter orsection in this book will describe one or more of the varioustypes of grounding and bonding that are widely used in theelectrical industry. Topics of primary interest are:

• Power System Grounding Including The “Service Entrance”

• Bonding

• Grounding Electrical Equipment

• Lightning Protection

• Protection Of Electronic Equipment (Shielding Is Not Discussed)

Grounding is a very complex subject. The proper instal-lation of grounding systems requires knowledge of soilcharacteristics, grounding conductor materials andcompositions and grounding connections and terminations.A complete guide to proper grounding is often part ofnational and international standards. For example, IEEEStd 80, Guide for Safety in AC Substation Grounding, is acomprehensive and complex standard for only oneparticular grounding application. This standard is neededfor proper substation design in an electric powertransmission facility or the power feed to a very largefactory. Smaller facilities can use these design guides also,but such an approach may be too costly. This book takes“conservative” shortcuts that allow the design of thegrounding system to proceed without undue design effort.We emphasize that the approaches in this book, in orderto be conservative and correct, may trade a small increasein grounding components in order to avoid a largeengineering expense. Remember that any electrical instal-lation is, and properly should be, subject to a review by theauthority having jurisdiction over the electrical installation.Electrical design and installation personnel are encouragedto discuss and review the electrical installation with theauthority having jurisdiction PRIOR to beginning any workon the project.

Designers of electrical grounding systems also should findthis a handy guide because we have included extensivereferences to the National Electrical Code (NEC)(NFPA70), ANSI and IEEE Standards as well as otherNFPA Standards. It is not the purpose to be a guide to theNEC but we will not make recommendations that disagreewith it. Keep in mind that Section 90-1 (c) of the NECstates that the Code is not intended to be used as a designspecification. Still, it is difficult to imagine how personneldesign and construct electrical systems in the USA withoutreferencing the NEC. Also keep in mind that the NECcontains minimum requirements only. In some cases,minimum standards are not sufficient or efficient for thedesign project. For example, existing standards do not coverthe need to maintain the operational reliability of modernelectronic equipment - especially telecommunications andinformation technology (computer-based) systems. We willcover these situations in this book. Where no standardsexist, the ERICO engineering staff can make recommen-dations based on more than 58 years of successfulexperience.

While written primarily for readers in the U.S. and Canada,readers from other nations also will find this work usefulbecause it concentrates on cost-effective, proven solutions.This book is written around U.S. standards with referencesto Canadian Standards. The standards in your country maybe different. We welcome your comments. ERICO operatesin 23 countries around the globe. We are familiar with mostcommonly referred standards. If you contact us, we will tryto assist you in any way.

A fundamental fact is that electricity ALWAYS flows backto its source. Some designers and installers who accept anduse this fact in their designs of power systems, seem toforget it when designing and installing grounding systems.Our job is to ensure that electricity, including faults,lightning and electronic noise, return to their source with amaximum of safety to people while maintaining thereliability of equipment. This means that we must be sure toroute the current back to its source with a minimum voltagedrop. In many individual situations there are no specificNEC requirements to accomplish this so we will let theoryand experience be our guide.

ERICO is publishing this book as a service to ourcustomers and other industry professionals who realize thatgrounding, bonding, lightning protection and overvoltageprotection are an integral part of a modern electrical design.We have referenced many of our products in the midst of acomprehensive technical paper. We acknowledge that thereare other good products one could use. ERICO’s 70 plusyears of experience in designing and manufacturingbonding and grounding products has led us to what we feel

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Preface

are some of the best, long lasting and cost effective productsavailable. Here we combine these with our knowledge ofmethods to assist the industry professional in soundchoices. It is most often an electrician or electrical workerwho is affected by poorly designed ground systems.

All of the drawings (non shaded versions) in this book areavailable in AutoCAD® .DWG files. These are availablethrough the ERICO CAD-Club™. Please write forinformation on this no-cost shareware program. Weencourage you to join.

This book is designed to be useful immediately. We know,however, that no work is ever really complete. We lookforward to your comments (both favorable and not-so-favorable) and suggestions so that future editions may beimproved.

ABOUT THE AUTHOR

The primary author of this book is Keith Switzer, whohas over 40 years of technical and managerial experiencein the electrical industry. He has a BSME degree fromPennsylvania State University. Switzer joined ERICO,Inc. in 1958 and has worked in various engineeringdepartments. He is currently Senior Staff Engineer in theElectrical/Electronic Engineering Section at the ERICOheadquarters in Solon, Ohio.

Switzer is a member of IEEE Power Engineering Society,Substations Committee, Working Groups D7 (Std 80,IEEE Guide for Safety in AC Substation Grounding), D9(Std 837, IEEE Standard for Qualifying PermanentConnectors Used in Substation Grounding), D4 (Std1246, IEEE Guide for Temporary Protective GroundingSystems Used in Substations), and E5 (Std 998, DirectLightning Stroke Shielding of Substations.)

He is a member of the Technical Advisory Committee(TAC) of the National Electrical Grounding ResearchProject (NEGRP), investigating the long-term reliabilityof various electrodes in various types of soils. He is alsoa member of the USNG/IEC TAG reviewing proposedIEC standards. Switzer is also a member of AmericanSociety of Mechanical Engineers (ASME), Armed ForcesCommunications and Electronics Association (AFCEA),Insulated Conductors Committee (ICC), InternationalAssociation of Electrical Inspectors (IAEI), NationalAssociation of Corrosion Engineers (NACE), NationalElectrical Manufacturers Associate (NEMA), and Societyof American Military Engineers (SAME).

Many thanks to Michael Callanan, Frank Fiederlein,Warren Lewis, Dick Singer and Dr. A.J (Tony) Surtees fortheir input to this book.

DISCLAIMER

While the staff of ERICO and the outside contributors tothis book have taken great pains to make sure ourrecommendations, pictures and list of references areaccurate and complete, we may have missed something. Wedo not assume responsibility for the consequential effects ofthese errors or omissions. The designer is still completelyresponsible for his own work regarding fitness of the designand adherence to applicable laws and codes. In the samemanner, the contractor is responsible for following thedesign and for the installation in a workmanlike manner.

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vi Practical Guide to Electrical Grounding

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Chapter 1Building and ServiceEntrance Grounding

Building GroundingGround Resistance

Electrical Service GroundingUfer Grounding

1Chapter 1: Building and Service Entrance Grounding

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2 Practical Guide to Electrical Grounding


3

BUILDING GROUNDING —AN OVERVIEW

Despite the electrical designers’ best efforts, electricalground faults, short circuits, lightning and other transientscan and often do occur in building electrical distributionsystems. ERICO believes that, besides attempting tominimize the occurrence of these faults, designers andinstallers of electrical grounding systems should designthese systems to clear these faults in the quickest possiblemanner. This requires that the grounding system beconstructed to achieve the lowest practical impedance.Many factors determine the overall impedance of thegrounding system. Building components, such as structuralsteel and interior piping systems, can be used to create aneffective grounding system. The manner in which thesecomponents are installed and interconnected can have adramatic effect on the overall effectiveness of the groundingsystem. One of the primary factors that can increase theimpedance of the grounding system is the type and mannerin which the electrical connections to the grounding systemare made. ERICO has a complete line of connectors whichcan be used to make grounding connections withoutaffecting the integrity of the grounding system. Contractorsand others who install these systems cannot underestimatethe importance of ensuring that each grounding connectionis made in a manner that is efficient and effective.

Interconnected electronic equipment, such as telecommuni-cation systems and computer systems, also require a low-impedance grounding system. Specific bonding andgrounding techniques are available and are covered inChapter 4, which will help to enhance the operation of thissensitive electronic equipment.

Designers and installers of these systems will do well toinclude all aspects of facilities protection in the initialdesign. The figure below includes the major subsystems offacilities grounding. Any omission of these subsystems bydesign personnel is risky at best. Later additions and/ormodifications to the system can be very costly.

With these thoughts in mind, let’s look at the componentsof the building grounding system and see how theseindividual components impact the overall effectiveness ofthe grounding system.

GROUND RESISTANCE

While many factors come into play in determining theoverall effectiveness of the grounding system, the resistanceof the earth itself (earth resistivity) can significantly impactthe overall impedance of the grounding system. Severalfactors, such as moisture content, mineral content, soil type,soil contaminants, etc., determine the overall resistivity ofthe earth. In general, the higher the soil moisture content,the lower the soil’s resistivity. Systems designed for areaswhich typically have very dry soil and arid climates mayneed to use enhancement materials or other means toachieve lower soil resistivity. ERICO has products availablewhich help to reduce earth resistivity and maintain a lowsystem impedance. See the discussion on GEM™ on page14.

Ground resistance is usually measured using an instrumentoften called an earth resistance tester. This instrumentincludes a voltage source, an ohmmeter to measureresistance directly and switches to change the instrument’sresistance range. Installers of grounding systems may berequired to measure or otherwise determine the groundresistance of the system they have installed. The NationalElectric Code (NEC), Section 250-84, requires that a singleelectrode consisting of rod, pipe, or plate that does not havea resistance to ground of 25 ohms or less shall beaugmented by one additional electrode of the type listed inSection 250-81 or 250-83. Multiple electrodes shouldalways be installed so that they are more than six feet (1.8m) apart. Spacing greater than six feet will increase the rodefficiency. Proper spacing of the electrodes ensures that themaximum amount of fault current can be safely dischargedinto the earth.

To properly design a grounding system, the earth resistivitymust be measured. Several methods can be used to measureearth resistivity: the four-point method, the variationin-depth method (three-point method) and the two-pointmethod. The most accurate method and the one that ERICOrecommends is the four-point method. The details ofmaking these measurements and the set-up for themeasurements are included with the testing equipment.

Chapter 1: Building and Service Entrance Grounding

Fault Protection Subsystem

Lightning Protection Subsystem

Signal Reference Subsystem

Earth Electrode Subsystem


BUILDING GROUNDING

Electrical design and installation professionals need toconsider several different building grounding systems forany building or structure on which they may work. Buildinggrounding components can be broken down into severalsubdivisions:

• The building exterior grounds• The electrical service grounding• The building interior bonding• Equipment grounding and bonding• Lightning protection

This chapter will look at the first two items. Lightningprotection will be covered in Chapter 2, interior bondingand grounding will be covered in Chapter 3 and equipmentgrounding and bonding in Chapter 4.

BUILDING EXTERIOR GROUNDS

It is important to keep in mind that the requirementscontained in the NEC constitute minimum electrical instal-lation requirements. For many types of installations, therequirements listed in Article 250 of the NEC do not go farenough. These minimum requirements cannot ensure thatthe equipment operated in these buildings will perform in asatisfactory manner. For these reasons electrical designpersonnel often will require additional groundingcomponents. One of the most common of these consists ofa copper conductor that is directly buried in the earth andinstalled around the perimeter of the building. The steelbuilding columns are bonded to this conductor to completethe grounding system.

The columns around the perimeter of the building areexcellent grounding electrodes and provide a good path intothe earth for any fault currents that may be imposed on thesystem. The electrical designer, based on the size and usageof the building, will determine whether every column orjust some of the columns are bonded. ERICO recommendsthat at least one column every 50 feet shall be connected tothe above described ground ring. (Fig. 1-1)

When grounding large buildings, and all multiple buildingfacilities, perimeter grounding provides an equipotentialground for all the buildings and equipment within thebuilding that are bonded to the perimeter ground. Thepurpose of this perimeter grounding is to ensure that anequipotential plane is created for all components that areconnected to the perimeter ground system. The size of theground ring will depend upon the size of the electricalservice but is seldom less than 1/0 AWG copper. In some

cases, an electrical design requires ground rods to beinstalled in addition to the perimeter ground ring. The useof ground rods helps to minimize the effects of dry orfrozen soil on the overall impedance of the perimeterground system. This is because the ground rods can reachdeeper into the earth where the soil moisture content maybe higher or the soil may not have frozen. ERICO offers acomplete line of ground rods from 1/2 inch to 1 inch indiameter to meet the needs of the designer and installer. Itis recommended that the ground ring and ground rods becopper or copperbonded steel and installed at least 24 inchfrom the foundation footer and 18 inch outside the roof dripline. This location will allow for the greatest use of thewater coming off of the roof to maintain a good soilmoisture content.

Although less common than in the past, “triad” ground rodarrangements (rods placed in a triangular configuration) aresometimes specified, usually at the corners of the buildingor structure. Figure 1-2 shows possible conductor/groundrod configurations. Triad arrangements are notrecommended unless the spacing between the ground rodsis equal to or greater than the individual ground rod length.Three rods in a straight line spaced at least equal to thelength of the individual ground rods are more efficient andresult in a lower overall system impedance.

Installers of these perimeter ground systems need toprovide a “water stop” for each grounding conductor thatpasses through a foundation wall. This is especiallyimportant when the grounding conductor passes throughthe foundation wall at a point that is below the water table.The water stop ensures that moisture will not enter thebuilding by following the conductor strands and seepinginto the building. A CADWELD Type SS (splice) in theunspliced conductor and imbedded into the concrete wallprovides the required water stop (Fig. 1-3).


3'-0"

Grade

Typical InstallationWeld At Column Base.

First Floor

2'-0

"

Typical Down Conductor

Fig. 1-1


5

When “inspection wells” are required to expose points fromwhich to measure system resistance, several methods areavailable. Inspection wells are usually placed over a groundrod. If the grounding conductors do not have to be discon-nected from the rod, the conductors can be welded to therod, and a plastic pipe, Figure 1-4, a clay pipe, Figure 1-5,or a commercial box, ERICO T416B, Figure 1-6, can beplaced over the rod.

The plastic pipe also works well when an existingconnection must be repeatedly checked, since it can becustom made in the field to be installed over an existingconnection. If the conductors must be removed from the rodto enable resistance measurements to be made, either abolted connector or lug may be used (Fig. 1-7).


Bare CopperGround Cable

CADWELD Type TAConnection

CopperbondedGround RodTypical for 3

CADWELDType GTConnection


Scheme 1


CADWELDType GTConnection


CADWELD Type TAConnection

Scheme 2


CADWELDType GRConnection

CADWELDType TAConnection

Scheme 3 At Building Corner

®

“Triad” Ground Rod DetailsFig. 1-2

Fig. 1-4

CADWELDType SSConnection

Where a stranded conductor enters a buildingthrough a concrete wall below grade, a waterstopmay be made on the cable by installing a CADWELD Type SS on the conductor where itwill be buried inside the wall.

Cut Slots to MatchConductor Sizeand Configuration

PVC Pipe withScrew End Cap

Fig. 1-5

CADWELDType GTGround RodConnection

1'' Dia. Lift Hole atCenter of Cover

5/16'' Hot DipGalvanized SteelCover

Grade

Ground GridConductor

12'' Dia.x 24''VitrifiedClay Pipe

Ground Rod

Fig. 1-6

GRD TEST

Fig. 1-3

Fig. 1-7Disconnect for attaching 1 to 8 1” wide lugs.

1-3/4

1-1/4

Rod

CADWELD Connection

B542C0031/4X3X6-1/2"Copper

CADWELD Type Gl Lugs

9/16" D.


When the required resistance is not achieved using theusual grounding layouts, ERICO prefabricated wire meshcan be added to lower the overall grounding impedance(Fig. 1-8). ERICO offers a complete line of prefabricatedwire mesh products in sizes ranging from No. 6 to No. 12AWG solid conductors. Another method which can be usedto lower the grounding system impedance is groundenhancement materials. These materials can be addedaround ground rods or other conductors to enhance systemperformance. See the discussion on GEM™ on page 14 andsee Fig. 1-9, Fig. 1-10 and Fig. 1-35.

The National Electrical Safety Code (NESC) recommendsthat where fences are required to be grounded, such

grounding shall be designed to limit touch, step andtransferred voltages in accordance with industry practice.The NESC requires that the grounding connection be madeeither to the grounding system of the enclosed equipment orto a separate ground. In addition, the NESC in Section 92E,lists six separate requirements for fences:

1. Where gates are installed, the fence shall begrounded at each side of the gate or similar opening(Fig. 1-11).


CADWELD Connection

CADWELD Connection (Typical)

Ground bushing

Copper Ground Conductorin 1 Inch Conduit

Grounding BushingCADWELD Connection (Typical)

ERICO Pipe Bonding Strap(Locate Within 5 Feet ofPipeEntrance Into Building)

3 Inch or LargerMetal Cold WaterPipe, 10 Linear FeetMinimum Undergroungin Direct Contact withEarth and Electrically Continuousto Bonding Connection inAccessible Location

Neutral Bus

Grounding Bushingwith Bonding ConductorSame Size as GroundingElectrode Conductor

Copper GroundConductor in Conduit

Equipment Ground Bus

EnclosureCADWELD Connection (Typical)

Concrete Pad

CADWELDConnection

AsphaltPavement

CopperBondedGround Rod

Conduit Grounding Bushing

ERICO GEMGroundEnhancementMaterial

Fig. 1-10

Fig. 1-8

Fig. 1-9

ERICO GEM

Ground Rod


7

2. If a conducting gate is used, a buried bondingjumper must be installed across the opening(Fig. 1-11).

3. Where gates are installed, they shall be bonded tothe fence, grounding conductor or other bondingjumper (Fig. 1-12).

4. If the fence posts consist of a conducting material,the grounding conductor must be connected to thefence posts with a suitable connecting means(Fig. 1-13).

5. If the fence contains sections of barbed wire, thebarbed wire must also be bonded to the fence,grounding conductor or other bonding jumper(Fig. 1-14).

6. If the fence posts consist of a nonconductingmaterial, a bonding connection shall be made to thefence mesh strands and barbed wire strands at eachgrounding conductor point (Fig. 1-14).

ERICO offers a complete line of welded connectionssuitable for use with various shaped fence posts.(Fig. 1-15). Any fence around a substation on the propertyshould be grounded and tied into the substation groundsystem. If a facility fence meets the substation fence, it isrecommended to isolate the two fences to prevent any faultin the substation from being transferred throughout thefacility using the fence as the conductor (Fig. 1-16). Forfurther discussion on fence grounding, see Chapter 6.


ERICO Flexible Jumper With CADWELD Connections.

Fig. 1-12

Fig. 1-11

x xx

xx xx x x xx xx x x x x

Insulated section offence supported on suitablepost type insulatorssee detail "A"(6 per insulated section)

10"-0" Barbed Wire (Typ.)

Detail A

2"

Bottom of fence must be above grade.(Typ for insulated fence sections)

Grade

Typical Perimeter Fence Isolation SectionFig. 1-16

Fig. 1-15

CADWELD Type VS ConnectionFig. 1-13

Split Bolt ConnectorsFig. 1-14


Other items that are located on the outside of the buildingthat should be considered are lighting fixture standards, pullbox covers and rails. Handhole, manhole and pull boxcovers, if conductive, should be bonded to the groundingsystem using a flexible grounding conductor (Fig. 1-17).The NEC Section 370-40 (d) requires that a means beprovided in each metal box for the connection of anequipment grounding conductor. Metal covers for pullboxes, junction boxes or conduit bodies shall also begrounded if they are exposed and likely to becomeenergized. The NEC in Section 410-15 (b) Exception,permits metal poles, less than 20 feet (6.4 m) in height to beinstalled without handholes if the pole is provided with ahinged base. Both parts of the hinged base are required tobe bonded to ensure the required low impedanceconnection. Lighting standards in parking lots and otherareas where the public may contact them should begrounded (Fig. 1-18). Keep in mind that the earth cannotserve as the sole equipment grounding conductor. Lightstandards which are grounded by the use of a separateground rod must also be grounded with an equipmentgrounding conductor to ensure that the overcurrentprotective device will operate. Rails or sidings intohazardous locations such as grain storage facilities,ammunition dumps, etc., should also be properly bondedand grounded (Fig. 1-19). Designers and installers must notforget that distant lightning strikes can travel through therails for many miles. In northern climates suitable bondingjumpers should be applied across slip joints on water pipesto enable thawing currents to be applied without burningthe joint gasket (Fig. 1-20).


CADWELD Connection

3/16 Bronze Flexible Cable

Connect To Ground

Pull Box Cover GroundingFig. 1-17

Bare Copper Conductor

CADWELD Type GR or GTTo Copperbonded Rod Finished Grade

CADWELD Type RD To All Vertical Rebars At Or Near Unstressed End Of Rebars

Copperbonded Ground RodDriven 10 Feet

Light Pole GroundingFig. 1-18

—A

CADWELD TypeST Connection

CADWELD TypeTP Connection

CADWELD TypeST Connection

CADWELD TypeTP Connection

Far Rail Near Rail

Bare Stranded Copper WireTo Main Ground Grid Section A

1/16" x 1" x 20 " Copper Bond CADWELDConnection

Slip Joint Ductile Iron Pipe600 AMP BondBond P/N: CAA817A16Welder P/N: CACHA-AEC- "Pipe Size"W/M: CA32XF19

Water Pipe BondingFig. 1-20

Rail Siding GroundingFig. 1-19


9

Grounding conductors shall be protected against physicaldamage wherever they are accessible (Fig. 1-21).Grounding conductors installed as separate conductors inmetal raceways always must be bonded at both ends toensure that current flow is not choked off by the inductiveelement of the circuit. See page 15 for a discussion of howto accomplish the required bonding.

ELECTRICAL SERVICE GROUNDING

Article 230 of the NEC contains the requirements forinstalling electrical services for buildings and dwellings.Contractors, however, should keep in mind that localauthorities, including local electrical utilities, often haverequirements which supersede or augment the NEC.Contractors should contact the local authorities anddetermine if requirements for electrical services exist whichdiffer from the NEC.

The requirements for grounding electrical services arecontained in Article 250 of the NEC. Section 250-23(a)requires that a grounded electrical system, which supplies abuilding or structure, shall have at each service a groundingelectrode conductor connected to the grounding electrodesystem. In addition, the grounding electrode conductorshall also be connected to the grounded service conductor.This connection may occur at any accessible point from theload end of the service drop or service lateral to thegrounded conductor (neutral) terminal block in the servicedisconnecting means. (Fig. 1-22 and Fig. 1-23) Keep inmind that the service disconnecting means is often the heartof the electrical system. This is frequently the point atwhich the required grounding connections occur(Fig. 1-24).


Plastic ConduitProtection

Fig. 1-21

To ElectricalService

A B C

Service EquipmentEnclosure

Grounded CircuitConductor

MBJ250-53(b)

GroundedNeutral Bar

EGC250-50(a)

To BranchCircuit Load

GroundingElectrodeConductor250-92

GroundedElectrode250-81

Fig. 1-24Grounding Of AC Power Per NEC 250-23

Fig. 1-22

Grounding Of AC Power Per NEC 250-23 Exception 5Fig. 1-23

GROUND

Phase Conductors

MainBondingJumper

GroundingElectrodeConductor

Conductor Sizeper NEC 250-94

Electrode SystemNEC 250-81 or 83May consist of:

Water PipeStructural SteelRing GroundConcrete ElectrodeRod or Pipe

E

PHAS

NEUTRAL

ServiceEntranceCabinet

Grounded Conductor (Neutral)Power

CompanyTransformer

GROUND

Phase Conductors

MainBondingJumper


Conductor Sizeper NEC 250-94

Electrode SystemNEC 250-81 or 83May consist of:

Water PipeStructural SteelRing GroundConcrete ElectrodeRod or Pipe

E

PHAS

NEUTRAL

ServiceEntranceCabinet

Grounded Conductor (Neutral)Power

CompanyTransformer


The grounding electrode system is designed to providemultiple electrical paths into the earth. As stated in thePreface, grounding of electrical systems helps to ensurepersonnel safety, provide equipment and buildingprotection and achieve electrical noise reduction. Section250-81 requires that four components, if available, bebonded together to form the grounding electrode system.Notice the words “if available.” Contractors are not giventhe choice of which components they want to bondtogether. If they are available, all four must be used.(Fig. 1-25)

The first component is the metal underground water pipe.Metal water piping that is in direct contact with the earth for10 feet or more must be part of the grounding electrodesystem. Contractors should be aware that, with theincreasing presence of plastic in water piping systems,these systems may not be suitable as grounding electrodes.Note, however, that under the bonding requirements ofSection 250-80 (a) all interior metal water piping shall bebonded to the service equipment enclosure or otherpermissible attachment points as listed in the section. Whenconnecting the grounding electrode conductor to the metalwater pipe, use a UL listed clamp or other listed means tomake the connection. Ground clamps shall be listed for thematerials of which the metal water pipe is constructed andnot more than one grounding electrode conductor shall be

connected to each clamp unless the clamp is listed formultiple connections (Fig. 1-26). One final considerationwhen connecting the metal water piping to the groundingelectrode system: the point of connection must be locatedwithin the first 5 feet of the point of entrance of the metalwater pipe into the building. This is to ensure thatdownstream alterations of the piping system, such as theinstallation of plastic fittings, doesn’t result in isolation ofthe grounding electrode system. The NEC does not permitmetal water piping beyond the first 5 feet into the buildingto be used as part of the grounding electrode system or as aconductor to interconnect parts of the grounding electrodesystem. Contractors should be aware that, because of theuncertainty of the metal water pipe construction, the metalwater pipe is the only grounding electrode which must besupplemented by an additional electrode. If the otherelectrodes are not available, a “made” electrode will need tobe installed by the contractor to supplement the metal waterpiping. Made electrodes are discussed on page 14.

The second component of the grounding electrode systemis the metal frame of the building. If the metal frame of thebuilding is effectively grounded, meaning it is intentionallyconnected to the earth by means of a low-impedanceground connection, it must be bonded to the groundingelectrode system. Once again the connection of thegrounding electrode conductor to the building steel must beaccomplished by the use of exothermic welding(CADWELD), listed lugs, listed pressure connectors, listedclamps or other listed means. See Section 250-115. If thebuilding steel is dirty or contains nonconductive coatings,contractors are required by the NEC to remove coatings,such as paint, lacquer and enamel, from contact surfaces toensure good electrical continuity. See Section 250-118.ERICO has a full line of horizontal and vertical cable tosteel or cast iron connections that can meet any installationrequirements (Fig. 1-27).


Fig. 1-26

StructuralSteelNEC 250-81 (b)

Water Supply (Street Side)

Ring Ground, NEC 250-81 (d) Rod/Pipe ElectrodeNEC 250-83 (c)

WaterMeter

Bonding JumperNEC 250-80 (a)

Grounding ElectrodeConductor, NEC 250-94

Metal UndergroundWater Pipe, NEC 250-81 (a)(Must Be Supplimented)

To AC Service EntranceGrounded Conductor (Neutral)

Water Supply (House Side)

Typical ElectrodesFig. 1-25


11

The third component of the grounding electrode system isconcrete-encased electrodes. These are usually referred toas “rebar,” which is short for reinforcing bar (Fig. 1-28 and1-29). Rebar is used to add strength to poured concreteinstallations and by its nature tends to be an excellentgrounding electrode. This is because the rebar issurrounded by concrete which has a lower resistivity thanthe earth. This, coupled with the fact that concrete absorbsmoisture from the surrounding earth, makes the concrete-encased electrode an excellent grounding electrode. See thediscussion on page 17 on “Ufer” grounding. The NECrequires that the concrete-encased electrode be covered byat least 2 inch (50 mm) of concrete and consist of at least 20feet (6.4 m) of reinforcing bars of not less than 1/2 inch indiameter (No. 4 rebar) located near the bottom of a concretefooting or foundation. Contractors should look closely atthe material used for the reinforcing bars. The rebar is oftencovered with a nonconductive coating, such as epoxy,which do not make them suitable for grounding electrodes.The NEC also permits at least 20 feet (6.4 m) of barecopper, not smaller than No. 4 AWG, to be used as asubstitute for the rebar for a grounding electrode.(Fig. 1-30) Connections of the grounding electrode arecritical to maintaining the integrity of the groundingsystem. Section 250-115 requires that where the grounding


HA

HB

VN

HA

HS

HC

HT

VS

VS

VF

VB

VG

VT

VV

Fig. 1-27

Fig. 1-29

Copper Wire As Concrete Encased Electrode

Fig. 1-30

FinishedSurface

ConcreteFoundation

Foundation Rebar(See Note Below)

CADWELD Type RR or RD

Foundation Rebar Ground ConnectionFig. 1-28


electrode conductor is connected to buried electrodes theclamp or fitting must be listed for direct soil burial.CADWELD offers the best solution for contractors tryingto meet the NEC requirements for connecting to rebar.CADWELD offers a full line of connections in variousconfigurations for welding of grounding conductors toreinforcing bars (Figure 1-31). Contractors should selectthe point of attachment for such connections by locating theweld away from areas of maximum tensile stress, such asnear the free end of the bar in a lap splice, to avoid harmfulstresses in the rebar. Note, where rebar mat is required to bebonded, bar to bar bonds should be made with a copperconductor jumper (Fig. 1-32). CADWELD connectionscannot be used to make direct rebar to rebar electricalconnections.

If a foundation with rebar is used as part of the groundingelectrode system, it is recommended that the anchor boltsbe bonded to the main rebars and a conductor extendedfrom the rebar to an outside electrode to minimize possibledamage to the foundation. See (Figure 1-33) and thediscussion on “Ufer” grounding on page 17.

The last component of the grounding electrode system is aground ring. The NEC requires that if a ground ring isavailable it shall be bonded to the grounding electrodesystem. A ground ring should consist of at least 20 feet (6.4m) of No. 2 AWG bare copper or larger which encircles thebuilding. The ground ring should be in direct contact withthe earth at a depth below the earth surface of at least 2 1/2feet (0.75 m). Contractors should note that while the groundring is frequently not “available,” it is becoming more andmore prevalent as a supplemental grounding systemcomponent, especially when highly sensitive electronicequipment is installed within the building. As noted above,the connection to the ground ring will more than likely be adirect burial connection so the ground clamps or fittingsmust be listed for direct soil burial. ERICO has a full line ofcable-to-cable connections that can meet any installation orapplication requirement (Fig. 1-34).


Fig. 1-33

RCRR

RD RJ

See Detail "A"

Detail "A Type RRCadweld Connection

Fig. 1-31

Fig. 1-34

PC XB

PT

SS

PH

TA

PG

PG

PG

XA

Fig. 1-32



Soil Backfill

Soil

4"

GEM1"

4"

30"Trench

GEM 1"

GEM

Ground Conductor

Soil Backfill

6"GEM packed

around Ground Rod

3" or Larger

6" shorter thanGround Rod

Augered Hole

Ground Rod6"

12"

1

2

3

4

5

1

2

3

4

6GEM Trench Installation

GEM Ground Rod Installation

Fig. 1-35



Section 250-83 contains requirements for other (frequentlyreferred to as “made”) electrodes. These electrodes can beused to supplement the grounding electrode system or areto be used when none of the grounding electrodes coveredpreviously are available. Local metal piping systems, suchas water wells, can be used but metal underground gaspiping systems shall not be used as the grounding electrode.The most common made electrodes consist of rod, pipe orplates. Ground rods can be constructed of iron or steel, of atleast 5/8 inch in diameter. Nonferrous ground rods, such ascopperbonded steel or stainless steel can also be used,provided they are not less than 1/2 inch in diameter and arelisted. Design life of the facility being protected should beconsidered when choosing ground rod material. Galvanizedsteel ground rods are often used for grounding structuressuch as a telephone booth with an anticipated service of 10years or less. A UL Listed copperbonded steel ground rodwith a copper thickness of 10 mils (0.25 mm) will last 30years or more in most soils. A 13 mil (0.33 mm) copperthickness copperbonded steel rod will last 40 years or morein most soils. Unusual soil conditions demand additionalconsiderations. Contractors should be aware of the manyfactors that influence the impedance of grounding systemsthat utilize ground rods. The dimension of the ground roddoes have some affect on its resistance. Typically, the largerthe diameter of the ground rod, the lower its resistance, butto a very minor extent. A far more important factor indetermining the resistance of the ground rod is the depth towhich it is driven. Usually, the deeper the ground rod isdriven, the lower its resistance. Another very important andfrequently unknown factor is the resistivity of the soilwhere the ground rod is driven. As stated above, the higherthe moisture content of the soil, the lower its resistivity.ERICO GEMTM, Ground Enhancement Material, is theanswer in situations where reducing earthing resistance andmaintaining low resistance permanently is required. GEMreduces the resistance of the electrode to the earth andperforms in all soil conditions. GEM can be used aroundground rods in an augured hole or installed in a trench aspermitted by Section 250-83 (c) (3), of the NEC. See Figure1-35 (Page 13). As with all of the grounding electrodes, theconnection is critical to maintaining the integrity of thegrounding system. While listed clamps or fittings arepermitted, exothermic welding provides the best solution tothe contractor needs. ERICO offers a complete line of cableto ground rod connections, including the CADWELDONE-SHOT® connection, which can be used for both plainor threaded copperbonded and galvanized steel or stainlesssteel rods. See (Figures 1-36 and 1-37).

CADWELD Ground Rod ConnectionsFig. 1-37

GR GT

NX NT

GR

GB

GT

GY

ND

NC

CADWELD One-Shot® ConnectionsFig. 1-36


15

Also fitting into this category are chemical type groundelectrodes consisting of a copper tube filled with salts.Moisture entering the tube slowly dissolves the salts, whichthen leach into the surrounding earth thru holes in the tube.(Fig. 1-38) This lowers the earth resistivity in the areaaround the electrode, which reduces the electroderesistance.

For maximum efficiency, we recommend back-filling theelectrode with bentonite for the lower 1 to 2 feet and thenERICO GEM to the level marked on the electrode.Alternatively, the electrode can be back-filled only withbentonite for a less efficient installation or only with earthfor an even lower efficient installation. Long term (over fiveyears) tests comparing 10-foot chemical type electrodesback-filled with bentonite to 8-foot copper bonded rodsback-filled with ERICO GEM indicated that the two arenearly equal with the GEM back-filled rod slightly better.

The chemical ground electrode system is available fromERICO. Chemical electrodes are available in both verticaland horizontal configurations. All ERICO chemicalelectrodes are provided with a pigtail welded to the electrodeusing the CADWELD process. Standard pigtail sizes include4/0 AWG and #2 AWG tinned solid copper conductors.

The NEC requires that the ground rods be installed suchthat at least 8 feet (2.5 m) of length is in contact with theearth. If rock is encountered, the ground rod can be drivenat an angle, not to exceed 45° from vertical, or buried in atrench which is at least 2 1/2 feet (0.75 m) below the earth.The point of connection of the grounding electrodeconductor shall be below or flush with grade unless it issuitably protected against physical damage.

The remaining type of “made” electrode permitted by theNEC is the plate electrode. Section 250-83 (d) permits plateelectrodes that offer at least 2 square feet (0.19 sq. m) ofsurface area which is in contact with the earth to be used.The plates may be constructed of iron or steel of at least 1/4inch (6.4 mm) in thickness or other nonferrous materials ofat least 0.06 inch (1.5 mm) in thickness. ERICO providescopper plate electrodes with CADWELD pigtails that meetthe requirements of the NEC. CADWELD horizontal andvertical steel surface connections can be used to connect thegrounding electrode conductor to the plate electrodes.Wherever possible, the plates should be installed below thepermanent moisture or frost line. As with all electrodeconnections, any nonconductive coatings shall be removedbefore making the connection. Recent testing indicates thatplate electrodes are the least-efficient type of groundingelectrode for power system grounding. Plate electrodes do,however, provide large surface area for capacitive coupling(high frequency) required in lightning protection.

No matter which grounding electrode or electrodes are usedthe NEC requires that the grounding electrode conductor,which connects to these electrodes, be suitably protected.Section 250-92 (a) of the Code permits the groundingelectrode conductor (GEC) to be securely fastened directlyto the surface of a building or structure. A No. 4 AWG orsmaller copper or aluminum GEC, which is exposed tosevere physical damage must be protected. While there isno definition provided for “severe”, it is safe to assume thatlocations subject to vehicular traffic, forklifts or lawnmowers would be such locations. A No. 6 AWG GEC thatis free from exposure to physical damage can be installedon the surface of a building or structure without anymechanical protection. Smaller conductors shall beinstalled in rigid metal conduit, intermediate metal conduit,rigid nonmetallic conduit, electrical metallic tubing orcable armor.

Installers of electrical systems should be aware that Section250-92 (b) of the NEC requires that any metal enclosures orraceways for the grounding electrode conductor shall beelectrically continuous from the electrical equipment to thegrounding electrode. If the metal enclosures are notelectrically continuous they shall be made so by bonding


GEM GroundEnhancementMaterial

ChemicalRod

GroundWell

Downconductor

Bentonite

CADWELDConnection

Fig. 1-38



each end of the enclosure or raceway to the groundingconductor. IEEE paper No. 54 and other studies have shownthat, in cases where such bonding is omitted, the impedanceof the conductor is approximately doubled. Bonding inthese cases is essentially to ensure proper operation of thegrounding electrode system. Bonding can be accomplishedby connecting each end of the GEC enclosure or raceway tothe electrical equipment enclosure and the actual electrode.Section 250-79 (d) requires that the size of the bondingjumper for GEC raceways or enclosures be the same size orlarger than the enclosed grounding electrode conductor(Fig. 1-39). Another possible solution to protecting thegrounding electrode conductor from physical damage is touse a nonmetallic raceway. Such raceways are permittedand, because they are constructed of nonmetallic materials,they do not require bonding (Fig. 1-40).

Occasionally during construction, a grounding conductormay be damaged where it is stubbed through the concrete.Installers should note that ERICO features a full line ofCADWELD connections that can be used to repair theconductor without any loss of capacity in the conductor.Repair splices are available for both horizontal and verticalconductors. A minimum amount of concrete may need to bechipped away in order to make the splice (Fig. 1-41).Installers may also encounter applications where the GECneeds to be extended to a new service location or for amodification to the electrical distribution system. Section250-81 Exception No. 1 permits the GEC to be splicedby means of irreversible compression-type connectorslisted for this use or by the exothermic welding process.CADWELD offers a complete line of connectionssuitable for splicing the full range of groundingelectrode conductors.

All of these components, when installed, comprise thegrounding electrode system for the building or structureserved. All of these must be bonded together and when theyare installed where multiple grounding systems are present,such as lightning protection systems, they shall be installedat a point which is not less than 6 ft (1.8 m) from any otherelectrode of another grounding system. Section 250-54requires that when an AC system is connected to agrounding electrode system, as described above, the sameelectrode shall be used to ground conductor enclosures andequipment in or on that building. Separate groundingelectrode systems are not permitted within the samebuilding. In the event that a building is supplied by two ormore services as permitted by Section 230-2 Exceptions,the same grounding electrode system shall be used. Two ormore electrodes which are bonded together are considered

a single grounding electrode system.

Contractors must understand that these groundingconnections are critical to the overall electrical power distri-bution system and they must take great care when theymake these connections.

Bonding Jumper,2/0 or Larger

ElectricalServicePanelboard


Main BondingJumper

NeutralGround Bus

Building Steel

CADWELD Connections

Metal Raceway

2/0 GEC

Fig. 1-39

Fig. 1-40

SupportStrap

Wall orColumn (Typ.)

Max. 6"

Bare Copper Ground Wire#6 AWG and Larger Note 2

3/4" Schedule 80-PVCConduit

Support Strap (Typ)

Grade or Floor8'

-0"

Min

imum


CONCRETE ENCASED ELECTRODES,“UFER GROUNDING”

Herb Ufer reported on probably the first use of concrete-encased electrodes at a bomb storage facility at Davis-Monthan AFB in Tucson, Arizona which he inspected earlyin World War II. The grounding system was to protectagainst both static electricity and lightning. He laterreinspected the installation and made further tests, provingthat concrete-encased electrodes provide a lower and moreconsistent resistance than driven ground rods, especially inarid regions. Due to this early usage, the use of a wire or rodin the concrete foundation of a structure is often referred toas a “Ufer ground.”

The concrete electrode, however, was never tested underhigh fault conditions until 1977 when Dick and Holliday ofthe Blackburn Corp. published an IEEE paper discussinghigh-current tests on concrete-encased electrodes. Theyconcurred with the previous tests that concrete-encasedelectrodes do provide a low resistance ground, both beforeand after high current faults. But they also found that a highcurrent fault (500 to 2600 amperes) usually caused damageto the concrete - from minor damage to completedestruction.

In a 1975 survey of 1414 transmission towers, a largeelectrical utility found 90 fractured foundations that weregrounded using the Ufer method. They believed thefractures were the result of lightning strikes on the staticwires. Verbal reports have discussed leakage currentscausing disintegration of the concrete (which turns topowder) if a break in the metallic path occurs within thecurrent path in the concrete. This could also be the case ifthe anchor bolts were not connected to the rebar cage inthe foundation.

Based on the above and other reports, the latest edition(1986) of IEEE Std 80 (substation grounding guide)discusses both the merits and problems of the Ufer ground.The document also points out that it is practicallyimpossible to isolate the rebar from the grounding system.

The lower resistance of the Ufer grounding system can beexplained by both the large diameter or cross section of theconcrete as compared to a ground rod and the lowerresistivity of the concrete as compared to the earth.Concrete is hygroscopic (absorbs moisture from thesurrounding earth). This aids in lowering the resistance,even in arid regions.


3-1/2"

3-1/2"

3-1/2"

CADWELD

1" Min.

Repair Splices Without Current Derating With CadweldOnly 1 Inch of Conductor Need Be Exposed From Concrete

3-1/2"

Typical Horizontal Repair Splice

Spliced Cable

Weld Collar

Broken Cable Stub

Horizontal Splice Vertical Splice Conductor Mold Weld Mold Weld Weld

Size P/N Metal P/N Metal Collar*

1/0 SSR2C001 #45 SVR2C001 #90 B3452C2/0 SSR2G001 65 SVR2G001 90 B3452G4/0 SSR2Q005 90 SVR2Q001 115 B3452Q250 SSR2V002 115 SVR2V001 150 B3452V350 SSR3D002 150 SVR3D001 200 B3453D500 SSR3Q003 200 SVR3Q001 250 B3453Q

*One required per weld, horizontal or vertical splice.L160 handle clamp required for above molds.Contact factory for other sizes.

Fig. 1-41



The damage to the concrete can be explained due to itsnon-homogeneous character and moisture content. Duringa fault, one path from the rebar to the outside soil throughthe concrete will have a lower resistance than any other. Thefault current following this path will cause heating andvaporization of the water (moisture). The expansion, asthe water turns to steam, can cause the concrete to crackor spill.

The Ufer grounding system is an excellent method for lowfault currents (housing, light commercial, etc.), especiallyin arid regions where driven rods are less effective. Butwhen high current faults are possible, including lightning,care must be exercised in designing the system, especiallysince it is impossible to isolate the foundations from the restof the grounding system.

We recommend that the current path into the foundationmust be connected (wire ties between rebars as a minimum)and a metallic path should be provided from the rebar tothe earth. This metallic path should be connected toan external ground electrode. See Figure 1-42, “Ufer”ground detail.

Bare Copper:Size Per N E C

CADWELDTo OtherAvailableElectrodes

Foundation NearElectricalService Entrance

Finished Grade

Rebar MeetingRequirementsof N E C 250-81

CADWELD To Rebar

“UFER “ Ground DetailFig. 1-42


Practical Guide to Electrical Grounding20


21Chapter 2: Building Lightning Protection

Chapter 2Building Lightning

ProtectionA Critical Extension Of

Grounding


LIGHTNING - AN OVERVIEW

Lightning is an electrical discharge within clouds, fromcloud to cloud, or from cloud to the earth. Lightningprotection systems are required to safeguard againstdamage or injury caused by lightning or by currentsinduced in the earth from lightning.

Clouds can be charged with ten to hundreds of millions ofvolts in relation to earth. The charge can be either negativeor positive, although negative charged clouds account for98% of lightning strikes to earth. The earth beneath acharged cloud becomes charged to the opposite polarity. Asa negatively charged cloud passes, the excess of electrons inthe cloud repels the negative electrons in the earth, causingthe earth’s surface below the cloud to become positivelycharged. Conversely, a positively charged cloud causes theearth below to be negatively charged. While only about 2%of the lightning strikes to earth originate from positivelycharged clouds, these strikes usually have higher currentsthan those from negatively charged clouds. Lightningprotection systems must be designed to handle maximumcurrents.

The air between cloud and earth is the dielectric, orinsulating medium, that prevents flash over. When thevoltage withstand capability of the air is exceeded, the airbecomes ionized. Conduction of the discharge takes placein a series of discrete steps. First, a low current leader of

about 100 amperes extends down from the cloud, jumpingin a series of zigzag steps, about 100 to 150 feet (30 to 45m) each, toward the earth. As the leader or leaders (theremay be more than one) near the earth, a streamer ofopposite polarity rises from the earth or from some objecton the earth. When the two meet, a return stroke of veryhigh current follows the ionized path to the cloud, resultingin the bright flash called lightning. One or more returnstrokes make up the flash. Lightning current, ranging fromthousands to hundreds of thousands of amperes, heats theair which expands with explosive force, and createspressures that can exceed 10 atmospheres. This expansioncauses thunder, and can be powerful enough to damagebuildings.

The National Weather Services of the NationalAtmospheric Administration (NAA) keeps records ofthunderstorm activity. This data is plotted on maps showinglines of equal numbers of thunderstorm days (days in whichthere was at least one occurrence of thunder is heard). Suchisokeraunic charts show a wide geographic variation ofthunderstorm activity, from more than 90 days per year incentral Florida to less than 5 on the West Coast. (Fig. 2-1)Such charts cannot predict the lightning activity at anylocation, but make it possible to judge the extentof exposure and the potential benefits of a lightningprotection system. However, the overriding concerns inprotection must be the protection of people and thereliability of equipment.


16

5

5

30

40

10

5

10

10

50

5050

50 50

50

50

5050

50

40

40

4060

60

60

70

70

40

5

2030 40 40

50

30

49

9

5

1020

20

30

30

40 30

30

3030

30

30

20

20

20

3040

40

50

60

70

80

80

70

60

607080

90

90

807060

30

4050

20

Isokeraunic MapFig. 2-1

This isokeraunic map shows mean annual number of days with thunderstorms in the United States. The highest frequency is encountered in south central Florida.


New detection devices have been installed around the U.S.which count the total number of lightning strokes reachingthe earth. This data results in precise occurrence of the totalstrokes for a particular period of time for any particular arearather than thunderstorm days per year.

Lightning is the nemesis of communication stations, signalcircuits, tall structures and other buildings housingelectronic equipment. In addition to direct strike problems,modern electronics and circuitry are also highly susceptibleto damage from lightning surges and transients. These mayarrive via power, telecommunications and signal lines, eventhough the lightning strike may be some distance from thebuilding or installation.

LIGHTNING PROTECTIONLightning protection systems offer protection against bothdirect and indirect effects of lightning. The direct effects areburning, blasting, fires and electrocution. The indirecteffects are the mis-operation of control or other electronicequipment due to electrical transients.

The major purpose of lightning protection systems is toconduct the high current lightning discharges safely into theearth. A well-designed system will minimize voltagedifferences between areas of a building or facility andafford maximum protection to people. Direct or electro-magnetically induced voltages can affect power, signal anddata cables and cause significant voltage changes in thegrounding system. A well-designed grounding, bondingand surge voltage protection system can control andminimize these effects.

Since Ben Franklin and other early studiers of lightning,there have been two camps of thought regarding theperformance of direct strike lightning protection systems.Some believe that a pointed lightning rod or air terminalwill help prevent lightning from striking in the immediatevicinity because it will help reduce the difference inpotential between earth and cloud by "bleeding off" chargeand therefore reducing the chance of a direct strike. Othersbelieve that air terminals can be attractors of lightning byoffering a more electrically attractive path for a developingdirect strike than those other points on the surface of theearth that would be competing for it. These two thought"camps" form the two ends of a continuum upon which youcan place just about any of the direct strike lightningprotection theories. The continuum could be represented asshown below.

ACTIVE ATTRACTION SYSTEMS

On the left we have systems that are designed to attract thelightning strike. The theory behind this practice is to attractthe lightning to a known and preferred point thereforeprotecting nearby non-preferred points. The most commonway this is done is to have an air terminal that initiates astreamer that will intercept the lightning down stroke leaderwith a pre-ionized path that will be the most attractive forthe main lightning energy to follow.

PASSIVE NEUTRAL SYSTEMS

The middle of the continuum represents the conventional ortraditional theory of direct strike protection. Conductorsare positioned on a structure in the places where lightningis most likely to strike should a strike occur. We havelabeled these systems as neutral since the air terminal orstrike termination devices themselves aren’t considered tobe any more attractive or unattractive to the lightning strokethen the surrounding structure. They are positioned wherethey should be the first conductor in any path that thelightning strike takes to the structure.

ACTIVE PREVENTION SYSTEMS

The right third of the continuum is where we find thesystems that are designed to prevent the propagation of adirect stroke of lightning in the area where they arepositioned. There are two theories as to how preventativepower occurs. The first is the “bleed off” theory mentionedpreviously. The second is that the sharp points on theprevention devices form a corona cloud above them thatmakes the device an unattractive path to the lightning stroke.

There are some commonalities in these three approaches.Each system’s design requires the following:

1. The air terminal or strike termination device mustbe positioned so that it is the highest point onthe structure.

2. The lightning protection system must be solidly andpermanently grounded. Poor or high resistanceconnections to ground is the leading cause of light-ning system failure for each one of these systems.

To go further in our comparison, we must separate theprevention systems from the other two. Obviously, if youare counting on preventing a lightning stroke from arrivingnear you, you don’t have to worry about how to deal withthe lightning current once you have it on your lightningprotection system. None of these systems claims to protectagainst 100% of the possibility of a lightning stroke arrivingnear you. A compromise must be made between protectionand economics.


Active Passive Active

Attraction

Early Streamer Emission Streamer DelayFranklin/Faraday Cage

Neutral Prevention

Dynasphere Spline BallsBlunt Ended Rods Sharp Pointed Rods



There is general agreement that the best theoreticallightning system is a solid faraday cage around whatever itis that is being protected. An airplane is an example of this.But even in the case of the airplane, there are incidentsreported of damage from direct lightning strokes. On theground, a complete faraday cage solidly tied to ground is anattractive protection scheme, but is expensive toaccomplish. If it is a general area, and not a structure thatyou are trying to protect, the faraday cage approach is veryimpractical.

This book will dwell basically on the passive “FranklinRod” theory for lightning protection. While lightningcannot be prevented, it is possible to design a lightningprotection system that will prevent injury to people anddamage to installations in the majority of lightning strikes.Standards and codes for passive lightning protectionmaterials and installations that ensure safety and minimizedamage and fire hazards in the great majority of lightningstrikes are published by Underwriters Laboratory (UL96 &96A), the National Fire Protection Association (NFPA 780)and the Lightning Protection Institute (LPI-175). Protectionfor 100% of the lightning strikes is usually cost prohibitive.

Meeting the codes and standards does not necessarilyprovide protection to sensitive electronic equipment anddata interconnections. These can be damaged or affected byvoltage levels below those that will harm people or startfires. A well-designed lightning system exceeds theminimum code requirements, providing not only safety topeople and protection against fire, but also providingprotection for equipment and the integrity of data andoperations. Manmade structures of steel, concrete or woodare relatively good conductors compared to the path oflightning through the ionized air. The impedance of astructure is so low compared to that of the lightning paththat the structure has virtually no effect on the magnitude ofthe stroke. As a result, lightning can be considered aconstant current source. The current may divide amongseveral paths to earth, along the outside walls, sides andinterior of a structure, reducing the voltage drop to ground.Better protection is provided by multiple paths to ground,including the many paths through the steel buildingstructure. All structural metal items must be bonded. Boltedjoints in steel columns are usually adequately bonded as areproperly lapped and tied or mechanical rebar splices.

Effective lightning protection involves the integration ofseveral concepts and components. In general, lightningprotection can be indexed as follows:

1. Capture the lightning strike on purpose designedlightning terminals at preferred points.

2. Conduct the strike to ground safely through purposedesigned down conductors.

3. Dissipate the lightning energy into the ground withminimum rise in ground potential.

4. Eliminate ground loops and differentials by creatinga low impedance, equipotential ground system.

5. Protect equipment from surges and transients onincoming power lines to prevent equipment damageand costly operational downtime (See Chapter 7).

6. Protect equipment from surges and transients onincoming telecommunications and signal lines toprevent equipment damage and costly operationaldowntime (See Chapters 4 and 7).

My thanks to Dr. A. J. (Tony) Surtees, Manager - FacilityElectrical Protection, North / South America, ERICO, Inc.who greatly assisted in the following section.

A NEW APPROACH TO LIGHTNINGPROTECTION

The overall purpose of a lightning protection system is toprotect a facility and it's inhabitants from the damage of adirect or nearby lightning strike. Since ERICO believes thattrying to prevent a lightning strike is unreliable, the bestway to protect is to shunt the lightning energy “around” thevital components/inhabitants of the facility and dissipatethat energy into the earth where it wants to go anyway. Thefirst step in that process is to make sure that lightning, whenapproaching the facility, is attracted to the striketermination devices that have been installed on the structurefor that purpose. The role of a lightning strike terminationsystem is to effectively launch an upward leader at theappropriate time so that it, more so than any othercompeting feature on the structure, becomes the preferredattachment point for the approaching down leader(lightning strike).

As the down leader approaches the ground, the ambientelectric field rapidly escalates to the point where any pointon the structures projecting into this field begin to cause airbreakdown and launch upward streamer currents. If theambient field into which such streamers are emitted is highenough, the partially ionized streamer will convert to a fullyionized up-leader. The ability of the air termination tolaunch a sustainable up-leader that will be preferred overany other point on the structure, determines it’seffectiveness as an imminent lightning attachment point.

The Franklin Rod or conventional approach to lightningprotection has served the industry well, but since its


inception over 200 years ago, the nature and scope oflighting protection has changed considerably. Lightningprotection then was principally a defense against fire.Wooden buildings, when struck by lightning, would oftenburn. Barns and churches were the main facilities seekingthis protection due to their height. Today, fire is still aconcern, but not always the main concern. A modernfacility of almost any kind contains electronic equipmentand microprocessors. Facility owners are concerned aboutavoiding downtime, data loss, personnel injury &equipment damage as well as fire.

The materials used to construct facilities have changeddramatically also. Steel columns and the steel in reinforcedconcrete compete as low impedance conductors for lightningenergy. The myriad of electrical/electronic equipment andconductors that crisscross every level of the facility are at riskjust by being near conductors energized from nearbylightning strikes. The lightning codes of the past don’tadequately address these issues. Bonding of downconductorsto electrical apparatus within 3 to 6 feet is required and canadd substantial wiring to a facility if there are a lot ofdownconductors. Further, the need for lightning protectionfor these electrically sophisticated facilities is growing.

The amount of knowledge about lightning has increaseddramatically also. Information about the behavior ofleaders, the changing of electrical fields leading up to astrike, the effects of impedance of various competingdownconductors, and diagnostic equipment has allincreased dramatically. This gives today's designers oflightning protection systems a large advantage over those ofjust 20 years ago.

These technological advances and market demands formore cost effective lightning protection systems haveprompted many new and novel approaches to lightning pro-tection. One such system is the ERICO System 3000™. Thissystem’s components are Dynasphere™ Controlled LeaderTriggering (CLT) air terminals typically used with Ericore™

low impedance, insulated downconductor. This systemenables the facility owner to use fewer air terminals withfewer downconductors. The result is:

• fewer conductors to bond to nearby electrical apparatus.

• the ability to run downconductors down through themiddle of a building.

• less congestion on the roof of a building (this isespecially important when reroofing).

• a safer building roof for workers.

• the ability to protect open spaces as well as buildings.

• an overall more cost effective lightningprotection system.

The Dynasphere CLT is a passive terminal, which requiresno external power source, relying solely on the energycontained in the approaching leader for its dynamicoperation. This remarkable terminal has the ability toconcentrate only that electric field which occurs in themillisecond time slots as the leader charge approaches theground. The principle of operation of this terminal relies onthe capacitive coupling of the outer sphere of the terminal tothe approaching leader charge. This in turn raises the voltageof the spherical surface to produce a field concentrationacross the insulated air gap between the outer sphere andgrounded central finial. As the leader continues to approach,the voltage on the sphere rises until a point is reached wherethe air gap between the central finial and outer surfacebreaks down. This breakdown creates local photo-ionizationand the release of excess free ions. These then accelerateunder the intensified field to initiate an avalanche conditionand the formation of an up streamer begins.

The DYNASPHERE CLT is designed to ensure that it onlylaunches an up-streamer when it has sensed that the electricfield ahead of it is high enough to ensure propagation. Thisis unlike the way in which many other so called EarlyStreamer Emission terminals operate. It was developedthrough research and test equipment that wasn't available toearlier designers, but also developed by building on thewealth of knowledge created by those that came before us.

Fig. 2-2 Dynasphere™ Controlled Leader Emission(CLT) Air Terminal


Spark Gap

Corona DrainImpedance

InsulatedAluminumSphere

FRP SupportMast

ConductorTermination


27

Calculation of the Protective Coverageoffered by an air terminal

Collection Volume Design Method

A more efficient air terminal demands a new designphilosophy and discipline. ERICO has developed analternative design method matched to the performance ofthe System 3000™ lightning protection system. This methodis based on the work of Dr. A. J. Eriksson, the notedlightning researcher. A detailed description can be found inthe Australian Lightning Protection StandardsNZS/AS1768-1991, section A8.

The Collection Volume method provides an empirical andquantitative method based on design parameters such as, thestructure height, field intensification of structural projections,leader charge, site height and relative propagation velocitiesof the intercepting leaders. The model can be developed forthree dimension structures and offers a more rigorousapproach to lightning protection design.

Table 1 (Table A1 NZS/AS1768-1991)Distribution of the Main Characteristics of the

Lightning Flash to Ground

Table 1 (taken from NZS/AS1768-1991) illustrates thestatistical distribution of lightning parameters. Item 3 in thetable can be used in determining the statistical levels ofprotection. Using the equation below, protection levelsdirectly relating to peak current discharge, I, and thecorresponding leader charge, Q, are derived:

I = 10.6 Q0.7

where I is measured in kA and Q in coulombs. From Table2 a discharge having a peak current of 5kA wouldcorrespond to a leader charge of approximately 0.5coulombs. Further calculation and extrapolation from Table1 are shown in Table 2.

Table 2 - Statistical probability of a down-leaderexceeding the peak current indicated

Figure 3 shows a downward leader approaching an isolatedground point. A striking distance hemisphere is set up aboutthis point. The radius is dependent on the charge on theleader head and corresponds to the distance where theelectric field strength will exceed critical value. That is, thefield strength becomes adequate to launch an interceptingupward leader.

Fig. 2-3 Spherical Surface withStriking Distance radius about point A

The striking distance hemisphere reveals that lightningleaders with weak electric charge approach much closer tothe ground point before achieving the critical conditions forinitiation of the upward leader. The higher the magnitude ofcharge, the greater the distance between leader and groundpoint when critical conditions are achieved. For designpurposes a hemisphere radius can be selected which relatesto a desired level of protection. The Collection Volumemethod takes into account the relative velocities of theupward and downward leaders. Not all leaders that enter astriking distance hemisphere will proceed to interception.Leaders entering the outer periphery of the hemispheres arelikely to continue their downward movement and tointercept a different upward leader (issuing from an

Chapter 2: Building Lightning Protection

Leader Peak Percent ProtectionCharge Current (I) Exceeding Level

(Q) Value

0.5C 6.5kA 98% High0.9C 10kA 93% Medium1.5C 16kA 85% Standard

Item Lightning Percentage of events having UnitCharacteristic value of characteristic

99 90 75 50 25 10 1

1 Number of 1 1 2 3 5 7 12componentstrokes

2 Time Interval 10 25 35 55 90 150 400 msbetweenstrokes

3 First stroke 5 12 20 30 50 80 130 kAcurrent Imax

4 Subsequent 3 6 10 15 20 30 40 kAstroke peakcurrent Imax

5 First stroke 6 10 15 25 30 40 70 GA/sbetweenstrokes (dI/dt)max

6 Subsequent 6 15 25 45 80 100 200 GA/sstroke (dI/dt)max

7 Total charge 1 3 6 15 40 70 200 Cdelivered

8 Continuing 6 10 20 30 40 70 100 Ccurrent charge

9 Continuing 30 50 80 100 150 200 400 Acurrent Imax

10 Overall duration 50 100 250 400 600 900 1500 msof flash

11 Action integral 102 3x102 103 5x103 3x104 105 5x105 A2.s

SphericalSurface

Ground

LightningLeader

B

C

StrikingDistance


alternative structure or feature on the ground). This leads tothe development of a limiting parabola. The enclosedvolume is known as the Collection Volume. A downwardprogressing leader entering this volume is assured ofinterception. Figure 4 shows how the velocity parabolalimits the size of the Collection Volume.

Fig. 2-4 Collection Volume formed by equal-probability locus and spherical surface

Designing with Collection Volumes using statisticallyderived lightning parameters as in Table 2 will providedesigners with better risk analysis. Magnitudes ofCollection Volumes are determined according to peakcurrent. That is, if the designer desires a high level ofprotection (peak current 6.5kA), 98% of all lightningexceeds this value. Discharges of greater magnitude willhave larger Collection Volumes that create greater overlapin the capture area of air terminals. A design based onlightning with small peak current can be considered conser-vative. The design performed to 98% High level does notmean that all lightning less than that level will miss an airtermination. There is simply a statistical chance somelightning may not intercept with an upward leaderemanating from within the Collection Volume.

The Collection Volume model assumes all points on thestructure are potential strike points, and as such exhibitnatural Collection Volumes.

ERICO, Inc. has developed a computer program thatevaluates the corresponding electric field intensity at eachstage and compares the electric field intensification ofcompeting points (building corners and edges, antennae,equipment, masts etc). The program then evaluates whichpoint will first generate the upward moving leader whichmeets the downward leader. The main discharge returnstroke follows the upward/downward leader path. Anattractive radius for each relevant point can then be calculated.

The larger collection volumes of enhanced air terminalsmeans that fewer such terminals are required on a structure.They should be positioned such that their collectionvolumes overlap the natural small Collection Volumes ofthe structure projections.

This method is visually more attractive and convenient toapply by consultants in lightning protection design. Figure5 shows the Collection Volume Concept when applied to astructure.

Fig. 2-5 The Collection Volume Design Concept

The design discipline employed in lightning protectiondesign is critical to reliable systems. Erico’s system hasbeen tested and has been used in the accomplishment ofover 7000 successful installations around the globe over thepast 15 years. Many of these installations are on high riskstructures in some of the most active lightningenvironments on the planet.

LIGHTNING PROTECTION COMPONENTS

A lightning protection system is comprised of a chain ofcomponents properly specified and properly installed toprovide a safe path to ground for the lightning current. Thelightning protection system provides an uninterruptedconductive (low impedance) path to earth. Lightning doesnot always strike the highest point. The rolling ball theoryof determining what is protected from lightning strikes,described below, is widely accepted as a sound approach tosizing and positioning air terminals on the top of structures,and for tall structures, on the sides of the structure.

Properly designed lightning protection systems based onexisting standards ensure adequate conducting and surgediverting paths which have been proven safe for people,structures and equipment in the great majority of cases.Other systems exist which are not covered in standards.These systems, which claim to prevent lightning strikes,must be considered carefully before installation.


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