Attch 5 MIL-HDBK-419Apdf.pdf

MIL-HDBK-419A

29 DECEMBER 1987

SUPERSEDING

MIL-HDBK-419

21 JANUARY 1982

MILITARY HANDBOOK

GROUNDING, BONDING, AND SHIELDING

FOR

ELECTRONIC EQUIPMENTS AND FACILITIES

VOLUME II OF 2 VOLUMES

APPLICATIONS

AMSC N/A EMCS/SLHC/TCTS

DISTRIBUTION STATEMENT A. Approved for public release; distribution is unlimited

DEPARTMENT OF DEFENSE

WASHINGTON DC 20301

MIL-HDBK-419A

GROUNDING, BONDING, AND SHIELDING FOR ELECTRONIC EQUIPMENTS AND FACILITIES

1. This standardization handbook was developed by the Department of Defense in accordance with established procedure.

2. This publication was approved on 29 December 1987 for printing and inclusion in the military standardization handbook series. Vertical lines and asterisks are not used in this revision to identify changes with respect to the previous issue due to the extensiveness of the changes.

3. This document provides basic and application information on grounding, bonding, and shielding practices recommended for electronic equipment. It will provide valuable information and guidance to personnel concerned with the preparation of specifications and the procurement of electrical and electronic equipment for the Defense Communications System. The handbook is not intended to be referenced in purchase specifications except for informational purposes, nor shall it supersede any specification requirements.

4. Every effort has been made to reflect the latest information on the interrelation of considerations of electrochemistry, metallurgy, electromagnetic, and atmospheric physics. It is the intent to review this handbook periodically to insure its completeness and currency. Users of this document are encouraged to report any errors discovered and any recommendations for changes or inclusions to: Commander, 1842 EEG/EEITE, Scott AFB IL 62225-6348.

5. Copies of Federal and Military Standards, Specifications and associated documents (including this handbook) listed in the Department of Defense Index of Specifications and Standards (DODISS) should be obtained from the DOD Single Stock Point: Commanding Officer, Naval Publications and Forms Center, 5801

Tabor Avenue, Philadelphia PA 19120. Single copies may be obtained on an emergency basis by calling

(AUTOVON) 442-3321 or Area Code (215)-697-3321. Copies of industry association documents should be obtained from the sponsor. Copies of all other listed documents should be obtained from the contracting activity or as directed by the contracting officer.

PREFACE

This volume is one of a two-volume series which sets forth the grounding, bonding, and shielding applications for communications electronics (C-E) equipments and facilities. Grounding, bonding, and shielding are complex subjects about which in the past there has existed a good deal of misunderstanding. The subjects themselves are interrelated and involve considerations of a wide range of topics from electrochemistry and metallurgy to electromagnetic field theory and atmospheric physics. These two volumes reduce these varied considerations into a usable set of principles and practices which can be used by all concerned with, and responsible for, the safety and effective operation of complex C-E systems. Where possible, the principles are reduced to specific steps. Because of the large number of interrelated factors, specific steps cannot be set forth for every possible situation. However, once the requirements and constraints of a given situation are defined, the appropriate steps for solution of the problem can be formulated utilizing the principles set forth.

Both volumes (Volume I, Basic Theory and Volume II, Applications) implement the Grounding, Bonding, and

Shielding requirements of MIL-STD-188-124A which is mandatory for use within the Department of Defense.

The purpose of this standard equipment by reducing noise strikes.

This handbook emphasizes the is to ensure the optimum performance of ground-based telecommunications and providing adequate protection against power system faults and lightning necessity for including considerations of grounding, bonding, and shielding in all phases of design, construction, operation, and maintenance of electronic equipment and facilities. Volume I, Basic Theory, develops the principles of personnel protection, fault protection, lightning protection, interference reduction, and EMP protection for C-E facilities. In addition, the basic theories of earth connections, signal grounding, electromagnetic shielding, and electrical bonding are presented. The subjects are not covered independently, rather they are considered from the standpoint of how they influence the design of the earth electrode subsystem of a facility, the selection of ground reference networks for equipments and structures, shielding requirements, facility and equipment bonding practices, etc. Volume I also provides the basic background of theory and principles that explain the technical basis for the recoin m ended practices and procedures, illustrates the necessity for care and thoroughness in implementation of grounding, bonding, and shielding; and provides supplemental information to assist in the solution of those problems and situations not specifically addressed.

In Volume II, Applications, the principles and theories, including RED/BLACK protection, are reduced to the practical steps and procedures which are to be followed in structural and facility development, electronic engineering, and in equipment development. These applications should assure personnel, equipment and structural safety, minimize electromagnetic interference (EMI) problems in the final operating system; and minimize susceptibility to and generation of undesirable emanations. The emphasis in Volume II goes beyond development to assembly and construction, to installation and checkout, and to maintenance for long term use.

Four appendices are provided as common elements in both volumes. Appendix A is a glossary of selected words and terms as they are used herein. If not defined in the glossary, usage is in accordance with Federal Standard

1037, Glossary of Telecommunication Terms. Appendix B is a supplemental bibliography containing selected references intended to supply the user with additional material. Appendix C contains the table of contents for the other volume. Appendix D contains the index for the two-volume set.

Paragraph

1.1

1.2

1.2.1

1.2.1.1

1.2.1.2

1.2.1.3

1.2.1.4

1.2.2

1.2.2.1

1.2.2.2

1.2.2.3

1.2.3

1.2.4

1.3

1.3.1

1.3.2

1.3.2.1

1.3.2.1.1

1.3.2.1.2

1.3.2.2

1.3.2.2.1

1.3.2.2.2

1.3.2.3

1.3.3

1.3.3.1

1.3.3.2

1.3.3.3

1.3.3.4

1.3.3.5

1.3.3.5.1

1.3.3.5.2

1.3.3.5.3

1.3.3.5.4

1.3.3.5.5

1.3.3.5.6

1.3.3.5.7

1.3.3.5.8

MIL-HDBK-419A

TABLE OF CONTENTS

CHAPTER 1- NEW FACILITIES DESIGN CRITERIA

INTRODUCTION

EARTH ELECTRODE SUBSYSTEM

Determination of Site Parameters

Soil Resistivity

Geological Effects

Physical Features

Local Climate

Design Procedure

Selection of Electrode Configuration

Calculation of Earth Resistance

Alternate Configurations

Design Guidelines

Installation Practices

LIGHTNING PROTECTION FOR STRUCTURES

Principles of Protection

Integral Protection System

Air Terminals

Size and Materials

Location

Grounding Conductors

Roof Conductors

Down Conductors

Fasteners

Separately Mounted Protection Systems

Mast Type

Overhead Ground Wire Type

Waveguide Installation and Grounding

Cable Installation and Grounding

Lightning-Generated Transient Surge Protection

Transient Source and Equipment Damage

Minimizing Damage

Susceptible Components

Frequency of Transient Occurrence

Transient Definition, AC Service Conductors

Methods for Transient Protection on AC Service Conductors

Use of Ferrous Metal Conduit

Use of Overhead Guard Wires

Page

1-1

1-2

1-2

1-2

1-5

1-5

1-5

1-6

1-6

1-9

1-9

1-14

1-22

1-23

1-23

1-23

1-24

1-24

1-24

1-33

1-33

1-37

1-39

1-40

1-40

1-41

1-42

1-49

1-49

1-49

1-50

1-50

1-51

1-51

1-56

1-56

1-56 i

1.3.3.5.9

1.3.3.5.10

1.3.3.5.11

1.3.3.5.12

1.3.3.5.13

1.3.3.5.14

1.3.3.5.15

1.3.3.5.16

1.3.3.5.17

1.3.3.5.18

1.3.3.5.19

1.3.3.5.20

1.3.3.5.21

1.3.3.5.22

1.3.3.5.23

1.3.3.5.24

1.3.3.5.25

1.3.3.5.26

1.3.3.5.27

1.3.3.5.28

1.3.3.5.29

1.3.3.6

1.3.3.7

1.3.3.7.1

1.3.3.7.2

1.3.3.7.3

1.3.3.7.4

1.3.3.7.5

1.3.3.7.6

1.3.3.7.7

1.3.3.7.8

1.3.3.8

1.3.3.9

1.3.3.10

MIL-HDBK-419A

TABLE OF CONTENTS (Continued)

CHAPTER 1- NEW FACILITIES DESIGN CRITERIA

Protection of Underground Cables

Buried Guard Wire

Secondary AC Surge Arrester

Surge Arrester Installation

Operating Characteristics of Surge Arresters

Desirable Operating Characteristics for Transient Suppressors

Characteristics of Different Types of Surge Arresters

Transient Protection for Externally Exposed Equipment Lines

Frequency of Transient Occurrence

Amplitudes and Waveforms of Occurring Transients

Equipment Withstand Levels

Protection Methods Against Transients

Enclosing Cable Runs in Ferrous Metal Conduit

Transient Suppression

Types of Available Transient Suppression

Operating Characteristics of Transient Suppressors

Transient Suppressor Packaging Design

Coaxial Cable Shield Connection Through an Entrance Plate

Grounding of Unused Wires

Transient Suppression for RF Coaxial Lines

Equipment-Level Transient Suppression

Lightning Generated Transient Protection Evaluation

Transient Protection

Protection Requirement

Transient Defini t ion

Determination of Equipment Damage (Withstand) Levels

Determination of Need for Transient Protection

Minimizing Transient Damage

AC Power Input

Power Supply Transient Suppression

Landline Transient Suppression

C o r r o s i o n C o n t r o l

J o i n t s

Physical Protection

1.4 FAULT PROTECTIVE SUBSYSTEM

1.4.1 Purpose

1.4.2 Equipment Fault Protection Subsystem Composition

ii

Page

1-57

1-57

1-59

1-59

1-60

1-67

1-67

1-73

1-73

1-73

1-74

1-74

1-74

1-75

1-77

1-77

1-78

1-78

1-78

1-79

1-79

1-79

1-80

1-80

1-81

1-82

1-83

1-83

1-84

1-89

1-89

1-98

1-99

1-99

1-99

1-99

1-100

TABLE OF CONTENTS (Continued)

CHAPTER 1- NEW FACILITIES DESIGN CRITERIA

Paragraph

1.4.3

1.4.4

1.4.5

1.4.5.1

1.4.5.2

1.4.5.3

1.4.5.4

1.4.5.5

1.4.6

1.4.7

1.4.8

1.4.9

1.4.9.1

1.4.9.2

1.4.9.3

1.5

1.5.1

1.5.1.1

Configuration of the Equipment Fault Protection Subsystem

P i p e s a n d T u b e s

Electrical Supporting Structures

Metal Conduit

C a b l e T r a y s

Enclosures

C a b l e A r m o r

Rotating Machinery

Power Distribution Systems

Standby AC Generators

Equipment Fault Protection Subsystems for Transportable Equipment

MIL-STD-188-124A and NEC Compliance Evaluation

Measurements

MIL-STD-188-124A and NEC Compliance Inspection

Correction of Deficiencies

SIGNAL REFERENCE SUBSYSTEM FOR NEW FACILITIES

Higher Frequency Network

Multipoint Ground System

1.5.1.1.1 Types of Equipotential Planes

1.5.1 .1.1.1 Copper Grid Embedded in Concrete

1.5.1.1.1.2 Equipotential Plane Under Floor Tile or Carpet

1.5.1.1.1.3 Overhead Equipotential Plane

1.5.1.1.1.4 Raised (Computer) Flooring

1.5.1.1.1.4.1 Bolted-Grid (Stringer) or Rigid Grid System

1.5.1.1.1.4.2 Drop-In or Removable Grid System

1.5.1.1.1.4.3 Free-Standing, Pedestal-Only or Stringerless System

1.5.1.1.1.5 Ground Risers

1.5.1.1.1.6 Equipment Cabinet Grounding

1.5.2 Lower Frequency Signal Reference Network

1.6 GROUNDING PHILOSOPHY FOR EQUIPMENTS PROCESSING NATIONAL SECURITY

RELATED INFORMATION (RED/BLACK EQUIPMENTS)

1.7 BONDING PRACTICES

1.7.1 Application Guidelines

1.7.2 Surface Preparation

1.7.3 Bond Protection Code

1.7.3.1 Jumper Fasteners

1.7.4 Typical Bonds

iii

Page

1-100

1-102

1-102

1-103

1-103

1-103

1-103

1-103

1-104

1-104

1-104

1-105

1-105

1-105

1-111

1-113

1-113

1-118

1-120

1-120

1-120

1-120

1-120

1-125

1-131

1-131

1-131

1-131

1-131

1-134

1-140

1-140

1-142

1-143

1-147

1-148

CHAPTER 1- NEW FACILITIES DESIGN CRITERIA

Paragraph

1.7.4.1

1.7.4.2

1.7.4.3

1.8

1.8.1

1.8.2

1.83

1.8.4

1.9

1.9.1

1.9.2

1.10

1.10.1

1.10.2

1.10.3

1.10.4

1.11 l.11.1

Cable Trays . . .

Tubing and Conduit

Other Examples . .

SHIELDING

Establishing Requirements

D e s i g n G u i d e l i n e s

Select ion of Mater ia ls

Construction Guidelines

COMMON-MODE NOISE AND INSTRUMENTATION

Design Practices

Instrumentation Considerations

EMP PROTECTION

Earth Connection

EMP Shield Applications

EMP Bonding Practices

Construction Guidelines

MILITARY MOBILE FACILITIES

General Tactical Grounding Requirements

1.11.l.l

1.11.1.1.1

1.11.1.1.1.1

1.11.1.1.1.2

1.11.1.1.1.3

1.11.1.1.1.4

1.11.1.1.1.5

1.11.l.l.2

1.11.1.1.2.1

1.11.1.1.2.2

1.11.1.1.2.3

1.11.1.1.3

1.11.1.1.3.1

1.11.1.1.3.2

1.11.1.1.4

1.11.1.1.5

1.11.1.2

1.11.1.3

1.11.1.3.1

Facility Ground System

Earth Ground

General Earth Electrode Subsystem Requirements

Earth Electrode Subsystem Types

Soil Resistance

Ground Rod Resistance

Ground Resistance She's

Power Ground

Three-Phase Power Distribution System

Single-Phase Power Distribution System

DC Power System (2-Wire)

Lightning/EMP Protection Subsystem

Air Terminals

Terminal Protection Devices

Signal Reference Subsystem

Fault Protection Subsystem

Measuring Ground Resistance in Tactical Environments

Reducing Ground Resistance in Tactical Environments

Existing Facilities iv

Page

1-148

1-149

1-149

1-154

1-154

1-159

1-160

1-162

1-164

1-171

1-172

1-172

1-172

1-173

1-173

1-176

1-177

1-177

1-177

1-177

1-177

1-177

1-178

1-178

1-178

1-178

1-178

1-179

1-179

1-179

1-179

1-182

1-182

1-182

1-182

1-182

1-182

1.11.1.3.2

1.11.1.3.3

1.11.1.3.4

1.11.2

1.11.2.1

1.11.2.1.1

1.11.2.1.2

1.11.2.2

1.11.2.2.1

1.11.2.2.2

MIL-HDBK-419A

TABLE OF CONTENTS (Continued)

CHAPTER 1- NEW FACILITIES DESIGN CRITERIA

1.11.2.2.2.1

1.11.2.2.2.1.1

1.11.2.2.2.1.2

1.11.2.2.2.1.3

1.11.2.3

1.11.2.3.1

1.11.2.3.2

1.11.2.4

1.11.2.4.1

1.11.2.4.2

1.11.2.5

1.11.2.5.1

1.11.2.5.2

1.11.2.5.2.1

1.11.2.5.2.2

1.11.2.5.2.3

1.11.2.5.2.4

1.11.2.5.2.5

1.11.2.5.2.5.1

1.11.2.5.2.5.2

1.11.2.5.2.5.3

Multiple Electrode System

Earth Electrode Subsystem

Chemical Treatment

Detailed Tactical Grounding Requirements

Introduction

Training

Testing

Stand-Alone Equipment

General Description

Grounding Procedure

Low Resistance Grounds

Existing Facilities

Earth Electrode Subsystem, Single Ground Rod

Earth Electrode Subsystem, Multiple Ground Rods

Stand-Alone Shelter

General Description

Interconnection of Subsystems

Collocated Military Mobile Equipments

General Description

Grounding Procedure

Collocated Shelters

General Description

Grounding Procedure

Power Ground

Signal Ground

Fault Protection Subsystem

Lightning/EMP Protection

Collocated Shelters Greater than 8 Meters Apart

Ground Resistance Difference of Less than 150 Ohms

Ground Resistance Difference of Greater than 150 Ohms

Earth Electrode Subsystem

1.11.2.5.2.5.3.1 Power Ground

1.11.2.5.2.5.3.2 Signal Ground

1.11.2.5.2.5.3.3 Safety/Equipment Ground (Green Wire)

1.11.2.5.2.5.3.4 Lightning/EMP Protection

1.11.2.6 Fixed Prefabricated Shelters

1.11.2.6.1 General Description

Page

1-185

1-185

1-185

1-185

1-185

1-186

1-186

1-186

1-186

1-186

1-186

1-186

1-187

1-187

1-187

1-187

1-187

1-187

1-187

1-188

1-188

1-188

1-188

1-188

1-188

1-188

1-189

1-189

1-189

1-189

1-190

1-190

1-190

1-190

1-190

1-190

1-190 v

1.11.2.6.2 Electrical

1.12

1.12.1

1.12.2

1.12.3

1.13

1.13.1

1.13.2

1.13.3

1.13.4

1.13.5

1.14

2.1

2.2

2.2.1

2.2.2

2.2.2.1

2.2.2.2

2.2.2.2.1

2.2.2.2.2

2.2.2.2.3

2.2.2.2.4

2.2.2.2.5

2.2.2.2.6

2.2.2.2.7

2.2.2.3

2.2.2.3.1

2.2.2.3.2

2.2.2.3.3

2.2.2.4

FENCES. . .

Introduction .

Grounding . .

Installation . .

MIL-HDBK-419A

TABLE OF CONTENTS (Continued)

CHAPTER 1- NEW FACILITIES DESIGN CRITERIA

Connection

INSPECTION AND TEST PROCEDURES FOR A NEW FACILITY

Earth Electrode Subsystem

Lightning Protection Network

Signal Reference and Fault Protection Subsystems

Bonds and Bonding

Facility Checkout Form

Part I Earth Electrode Subsystem

Part II Lightning Protection Network

Part III Facility Ground System

Part IV Bonding

R E F E R E N C E S

CHAPTER 2 - EXISTING FACILITIES

INTRODUCTION

U P G R A D I N G

Drawings

Facility Survey

Survey Steps

Inspection Procedure

Earth Electrode Subsystem

B o n d s a n d B o n d i n g

Lightning Protection Network

S a f e t y G r o u n d i n g

Signal Grounding Practices

Ground System Noise Survey

Shielding

Test Procedures

B o n d R e s i s t a n c e

Ground System Noise Current

Differential Noise Voltage

Survey Form

Page

1-190

1-191

1-191

1-191

1-191

1-193

1-193

1-194

1-194

1-195

1-196

1-196

1-197

1-198

1-199

1-200

2-1

2-1

2-4

2-4

2-4

2-5

2-5

2-10

2-13

2-16

2-17

2-17

2-19

2-19

2-19

2-20

2-22

2-24 vi

CHAPTER 2- EXISTING FACILITIES

Paragraph

2.2.3

2.2.4

2.2.5

2.3

2.3.1

2.3.2

2.3.2.1

2.3.2.2

2.3.2.3

2.3.2.4

2.3.2.5

2.3.2.6

2.3.3

2.3.4

2.4

2.4.1

2.4.2

2.4.3

2.4.4

2.4.5

2.4.6

3.1

3.2

3.2.1

3.2.1.1

3.2.1.1.1

3.2.1.1.2

3.2.1.1.3

3.2.1.1.4

3.2.1.1.5

3.2.1.1.6

3.2.1.1.7

3.2.1.2

Guidelines for Upgrading

Expansion of Existing Facilities

Expansion of Existing Facilities for Higher-Frequency Grounds

MAINTENANCE

Schedules and Records

Maintenance Procedures

Earth Electrode Subsystem

Lightning Protection Subsystem

Bonding

Fault Protection Subsystem (Safety Ground)

Signal Reference Subsystem (Signal Grounding)

Shielding

Facility Maintenance Report

Performance Evaluation Program

GROUNDING CONSIDERATIONS FOR CLASSIFIED INFORMATION PROCESSORS

(RED/BLACK EQUIPMENTS) INSTALLED PRIOR TO THIS HANDBOOK

Introduction

Existing Facilities

Protection Grounds

Signal Reference Subsystem

Signal Filter Ground

Grounding Precautions

CHAPTER 3 - EQUIPMENT DESIGN CRITERIA

INTRODUCTION

GROUNDING PROCEDURES

Signal Grounds

Lower Frequency Equipment

Signal Ground Network Configuration

Signal Ground Terminals

Color Code

Cabinet Bus Bar

Isolation

Signal Interfacing

Signal Grounding

Higher Frequency Equipment

Page

2-33

2-35

2-35

2-36

2-36

2-36

2-39

2-41

2-42

2-43

2-44

2-45

2-46

2-54

2-59

2-59

2-59

2-59

2-60

2-61

2-61

3-1

3-1

3-1

3-2

3-2

3-2

3-3

3-3

3-4

3-4

3-6

3-12 vii

3.2.1.2.1

3.2.1.2.2

3.2.1.3

3.2.2

3.2.3

3.3

3.4

3.4.1

3.4.2

3.4.3

3.4.3.1

3.4.3.2

3.5

3.5.1

3.5.2

3.5.2.1

3.5.2.1.1

3.5.2.1.2

3.5.2.1.3

3.5.2.2

3.5.2.3

3.5.2.3.1

3.5.2.3.2

3.5.2.3.3

3.6

3.7

3.7.1

3.7.2

3.7.3

3.7.4

3.7.5

3.7.6

3.7.7

3.7.8

3.7.9

3.7.10

3.7.11

3.8

MIL-HDBK-419A

TABLE OF CONTENTS (Continued)

CHAPTER 3 - EQUIPMENT DESIGN CRITERIA

Signal Interfaces

Cable Connectors

Equipments Containing Both Lower and Higher Frequency Circuits .

Fault Protection

Cabinet Grounding

BONDING PRACTICES

SHIELDING GUIDELINES

Parts Selection

Layout and Construction

Equipment Enclosures

Seams

Penetrations and Apertures

COMMON-MODE NOISE CONTROL AND INSTRUMENTATION GROUNDING

Common-Mode Noise Control

Instrumentation Grounding

Analog Systems

Grounded Transducers . .

Ungrounded Transducers .

Amplifiers

Digital Data Systems . . .

Recording Devices

Magnetic Tape Recorders

Strip Chart Recorders . .

X-Y Plotters

EMP CONSIDERATIONS

EQUIPMENT INSPECTION AND TEST PROCEDURES

Lower Frequency Equipments

Higher Frequency Equipments

Hybrid Equipments

Installed Equipments

Fault Protection Subsystem

Bonding

Shielding

Instrumentation System

EMP Design

Other Observations

Inspection Form

REFERENCES

Page

3-13

3-13

3-14

3-14

3-16

3-18

3-25

3-25

3-25

3-27

3-27

3-28

3-34

3-34

3-34

3-35

3-35

3-38

3-38

3-40

3-40

3-40

3-42

3-42

3-42

3-43

3-43

3-47

3-47

3-47

3-47

3-49

3-49

3-50

3-50

3-50

3-50

3-54 viii

CHAPTER 4 - NOTES

Paragraph Page

4.1 SUBJECT TERM (KEY WORD) LISTING . . . . . . . . . . . . . . . . . . . . . . . 4-1

APPENDICES

A GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-1

B SUPPLEMENTAL BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . B-1

BI SUBJECT CROSS REFERENCE . . . . . . . . . . . . . . . . . . . . . . . . . . . B-1

BII LISTINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-2

C TABLE OF CONTENTS FOR VOLUME I . . . . . . . . . . . . . . . . . . . . . . . C-1

D INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-1 ix

LIST OF FIGURES

Figure

1-1

1-2

1-3

1-4

1-5

1-6

1-7

1-8

1-9

1-10

1-11

1-12

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1-27

1-28A

1-28B

1-29

1-30

1-31

1-32

1-33

1-34

1-35

1-36

1-37

Measurement of Soil Resistivity

Resistivity Determination of a Small Site

Minimum Earth Electrode Subsystem Configuration for Rectangular Shaped Facility

Nomograph for Determining the Resistance to Earth of a Single Ground Rod

Effective Resistance of Ground Rods When Arranged in a Straight Line or a Large Circle

Graph of Multiple-Rod Resistance Ratio

Electrode Configuration for Irregular Shaped Facility

Electrode Configuration for Adjacent Structures

Electrode Configuration for Closely Spaced Structures

Grounding System for Typical Radar Installation

Details of Ground Rod/Earth Electrode Subsystem Installation

Concrete Grounding Well

Typical Grounding Well Installation

Connections to Earth Electrode Subsystem

Grounding Practices for Lightning Protection

Location of Air Terminals for Common Roof Types

Location of Air Terminals on Gently Sloping Roofs

Air Terminal Placement on Flat-Roofed Structures

Graphical Method for Determining Need for Additional Air Terminals

Field Expedient Technique for Determining the Protection of Prominent Dormers

Illustration of Method for Determining the Protection of Flat Surfaces as Provided by Air Terminals

Criteria for Dead End Coverage

Recommended Construction Practices for Integral Lightning Protection Systems

The Protected Zone Provided by Two Vertical Masts

Overhead Ground Wire Lightning Protection System

Waveguide Entry Plate Detail

Grounding Detail for Elliptical Waveguide (Front View)

Grounding Detail for Elliptical Waveguide (Side View)

Heat Shrink Grounding

Ground Strap Detail for Elliptical Waveguide

Strap Cutting Detail for Elliptical Waveguide

Typical Communication Cable Entry Installation

Ground Strap Detail

Grounding Steps for Cables

Mean Number of Thunderstorm Days per Year for the United States

Lightning Protection for Underground Service

Secondary AC Surge Arrester Installation, Grounded Service

Secondary AC Surge Arrester Installation, Ungrounded Service

Page

1-3

1-4

1-10

1-11

1-12

1-13

1-16

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1-25

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1-37

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1-48

1-53

1-58

1-61

1-62 x

LIST OF FIGURES (Continued)

Figure Page

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1-64

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1-68

1-69

1-70

1-71

Typical Operating Curve for Two Series of Gas-filled Spark Gap Arresters

With Nonlinear Series Resistor

Typical Arrester Operating Curves, ZNR and SAS

Typical Transient Suppressor Installation, Facility and Equipment Level

Typical Configuration for Protection of Equipment from Conducted Powerline Surges and Transients (Neutral Grounded)

Typical Configuration for Protection of Equipment from Conducted Powerline Surges and Transients (Ungrounded)

Typical Configuration for Protection of Equipment from Conducted

Landline Transients

Transient Suppression for Coaxial Lines (DC to 3 MHz)

Transient Suppression for Twinaxial Lines (DC to 3 MHz)

Typical Equipment Fault Protection Subsystem

Method for Determining the Existence of Improper Neutral Ground Connections

(Deleted)

Typical Equipotential Ground Plane for Multi-Deck Building

Typical Building Floor Plan (Top View)

Typical Multi-Deck Building Plan (Side View)

Elements of the Facility Ground System (With Grid)

Typical Equipotential Ground Plane for New Construction Higher or Hybrid

Frequencies Facilities Installation

Ground Connector for Equipotential Plane in Concrete

Typical Ground Connectors

Example of Cable to Bar Ground Connectors

(Deleted)

Rigid Grid Floor System Details

Example of Rigid Grid to Pedestal Bolted Connection

Example of Rigid-Grid to Pedestal Clamped Connection

Example of Unacceptable Grid to Pedestal Bonding

Example of Drop-In Grid Floor Construction

Example of Pedestal Only Floor Construction

Typical Equipment Cabinet Grounding Detail

Typical RED/BLACK Signal Reference Subsystem

Typical RED Signal, Shield Ground, Bus Distribution System

Typical Intermediate Distribution Frame (Shield Ground Bus in Distribution Frames)

Typical Intermediate Distribution Frame (Data Concentrator Frame Installation)

Facility Power and AC Ground Distribution

Order of Assembly for Bolted Connection

Bonding of Cable Trays

1-69

1-72

1-76

1-86

1-87

1-92

1-95

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1-101

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1-148 xi

Figure P a g e

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7-13

Single-Point Signal Ground (for Lower Frequencies)

Single-Point Ground Bus System Using Separate Risers (Lower Frequency) . . .

Single-Point Ground Bus System Using a Common Bus

Use of Single-Point Ground Configuration to Minimize Effect of Facility Ground

Currents

Multipoint Ground Configuration

Use of Structural Steel in Multiple-Point Grounding

Recommended Signal Coupling Practice for Lower Frequency Equipment

Ground Network Used as Signal Return (Practice Not Generally Recommended) .

Idealized Energy Transfer Loop

Energy Transfer Loop With Noise Sources in Ground System

Equivalent Circuit of Non-Ideal Energy Transfer Loop

Practical Combinations of Source-Load Pairs

Coupling Between Circuits Caused by Common Return Path Impedance

Conductive Coupling of Extraneous Noise into Equipment Interconnecting Cables

Magnetic Field Surrounding a Current-Carrying Conductor

Illustration of Inductive Coupling

Illustration of Capacitive Coupling

Equivalent Circuit of Network in Figure 6-9

Characteristic Voltage Transfer Curve for Capacitive Coupling

Electric Field Patterns in the Vicinity of a Radiating Dipole

Illustration of Conductively-Coupled Corn men-Mode Noise

Common-Mode Noise in Unbalanced Systems

Common-Mode Noise in Balanced Systems

Effects of Poor Bonding on the Performance of a Power Line Filter

Current Flow Through Direct Bonds

Nature of Contact Between Bond Members

Resistance of a Test Bond as a Function of Fastener Torque

Typical Exothermic Connections

Typical Bond Configuration Which Can be Implemented With the Exothermic Process

Nomograph for Torque on Bolts

Bonding Path Established by Rivets

An Improperly Riveted Seam

Inductive Reactance of Wire and Strap Bond Jumpers

Relative Inductive Reactance Versus Length-to-Width Ratio of Flat Straps

Frequency Variation of the Impedance of Simple Conductors

Equivalent Circuit for Bonding Strap

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5-30

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7-24 xii

Figure Page

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Typical Bonding Deficiencies

Typical Bonding Deficiencies

Severely Damaged Down Conductor

Method for Determining the Existence of Improper Neutral Ground Connections

Measurement of Stray Current Level in Safety Ground Conductor

Typical Bond Resistance and Stray Current Measurement Locations in an Electronic Facility

Bond Resistance Measurement Technique

Test Setup for Stray Current Measurements

Oscilloscope Connections for Measuring Voltage Levels on Ground Systems

Example of Equipotential or Multipoint Grounding

Major Discrepancy Report Form

Typical Multiple Area Ground Distribution

Typical Signal, Shield Ground, Bus Distribution System for Single-Point Ground

Crypto Equipment Grounds

Typical Facility Ground System

Grounding in Lower Frequency Equipment

Lower Frequency Signal Ground Bus Bar Installation in Rack or Cabinet

Use of Balanced Lines to Avoid Ground Loops

Effect of an Unbalanced Cable on the Single-Point Ground

Effect of Arbitrarily Grounding the Source End of Unbalanced Equipment

Interconnecting Cables

Method of Grounding the Individual Shields on Long Lower Frequency Shield Cables

Grounding of Overall Cable Shields to Connectors

Grounding of Overall Cable Shields to Penetrated Walls

Establishment of Shield Continuity Between Higher Frequency Equipments

Grounding Practices in Equipments Containing Both Higher Frequency and

Lower Frequency Circuits

Typical Equipment Cabinet Grounding Detail

Acceptable and Unacceptable Uses of Bonding Jumpers . . . , Bonding of Subassemblies to Equipment Chassis

Bonding of Equipment to Mounting Surface

Typical Method of Bonding Equipment Flanges to Frame or Rack

Bonding of Rack-Mounted Equipments Employing Dagger Pins. . . . . . , Recommended Practices for Effective Bonding in Cabinets

Method of Bonding Across Hinges

Bonding of Connector to Mounting Surface

Method of Making Permanent Seam Using a Gasket

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Figure

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Mounting of Gasket on Hinged Side of Equipment Doors and Panels

Illustration of Proper and Improper Shield Penetration

Use of Cylindrical Waveguide - Below-Cutoff for Control Shaft Shield Penetration

Method of Mounting Wire Screen Over a Large Aperture

Acceptable Methods of Shielding Panel - Mounted Meters

Grounding Practices for Single-Ended Amplifiers

Grounding Practices for Differential Amplifiers

Method of Grounding Bridge Transducers

Use of Isolated Differential Amplifier With Balanced Bridge Transducer

Recommended Grounding Practices for Floating Traducers

Grounding for Single Channel Strip Chart Recorder

Resistive Isolation of Data Channels

Single Ground Terminal Isolation Resistance Test for an Individual Equipment

Signal Ground Terminal Isolation Resistance Test for an Equipment Assembly

Measurement of Connector Bonding Resistance

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LIST OF TABLES

Table Page

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1-30

3-1

Relative Advantages and Disadvantages of the Principal Types of Earth Electrodes

Minimum Requirements for Roof and Down Conductors on Structures Not Greater than

75 Feet (23 Meters) in Height

Minimum Requirements for Roof and Down Conductors on Structures Greater than

75 Feet (23 Meters) in Height

Solid Copper Wire - Weight, Breaking Strength, DC Resistance

Frequency of Transient Occurrences

Parameter for Direct Lightning Strike Current

Peak Currents from Direct Lightning Strikes

Transient Surge Amplitudes

Transient Occurrence, High-Incident Lightning Areas

Transient Occurrence, Low-Incident Lightning Areas

Generalized Characteristics for Surge Arresters by Type

Typical Maximum Clamp Voltage for Gap Arresters

ZNR Type Devices (Molded Case Type) Typical Characteristics

High Energy ZNR Surge Arrester Typical Characteristics

Test Results for Parallel-Connected ZNR

Transients Projected to Occur on Externally Exposed Line in High-Lightning

Incident Area Over 10-Year Period

Transient Surges, Line-to-Ground

Transient Surges, Line-to-Line

Transient Surges Projected to Occur in 10-Year Period on Externally-Exposed

Landlines

Grounding Electrode Conductor Size

Equipment Grounding Conductor Size Requirement

Size of Equipment Ground Cables

Minimum Torque Requirements for Bolted Bonds

Compatible Groups of Common Metals

Bond Protection Requirements

Protective Finishes for Bond Members

Metal Connections for Aluminum and Copper Jumpers

Attenuation Correction Factors for Reinforcing Steel

Relative Conductivity and Relative Permeability of Common Metals

Soil Resisitivity (Ohm-m)

1-8

1-34

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Frequency Properties of Standard Sizes of Honeycomb . . . . . . . . . . . . . . . . . 3-30 xv/xvi

CHAPTER 1

NEW FACILITIES DESIGN CRITERIA

1.1 INTRODUCTION.

This chapter presents the design, installation practices, test and acceptance procedures associated with the incorporation of effective grounding, bonding, and shielding for a new facility.* The major elements of the facility covered are the (1) earth electrode subsystem, (2) fault protection subsystem, (3) lightning protection subsystem, (4) signal reference subsystem, (5) bonding, and (6) shielding. Design and construction steps for these six elements are contained in the following sections:

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

1.10

1.11

1.12

1.13

Earth Electrode Subsystem

Lightning Protection for Structures

Fault Protection Subsystem

Signal Reference Subsystem for New Facilities

Grounding, Philosophy for Equipments Processing National Security Related Information

Bonding Practices

Shielding

Common-Mode Noise and Instrumentation

EMP Protection

Military Mobile Facilities

Fences

Inspection and Test Procedures for a New Facility

Secure transmission facility requirements are covered in Section 1.6. Supplemental measures which are needed to be incorporated in a facility to help reduce common-mode and instrumentation noise problems are presented in Section 1.9, Common-Mode Noise and Instrumentation. The special construction practices recommended to reduce facility vulnerability to the electromagnetic pulse (EMP) threat are contained in Section 1.10, EMP

Protection. Tactical grounding requirements are presented in Section 1.11, Military Mobile Facilities.

Inspection and Test Procedures for a New Facility, provided in Section 1.13, should be utilized in verifying that recommended practices and procedures are properly implemented and to help establish a perform ante baseline against which future measurements can be compared.

To obtain optimum performance of electronic equipment and personnel safety while providing adequate protection against power system faults, EMP, and lightning strikes, thorough consideration must be given to the grounding system for the building; to the bonds needed and the method of their implementation and to the shielding needed throughout the building for personnel safety and equipment interference control. For a new facility, the requirements in each of these areas are defined and appropriate design steps set forth to assure that the necessary measures are incorporated into the final structure and equipment installation.

*A new facility is considered to be one of new construction or an existing one that will undergo major renovation or major equipment reconfiguration. The project engineer shall determine the grounding requirements whenever minor equipment reconfigurations are accomplished in existing facilities. Refer to

Chapter 2 for additional information.

1-1

1.2 EARTH ELECTRODE SUBSYSTEM. The earth electrode subsystem establishes the electrical connection between the facility and earth. This connection is necessary for lightning protection, useful in power fault protection, and and in the minimization of noise. The system should be tailored to reflect the characteristics of the site and the requirements of the facility.* It must be properly installed and steps must be taken to assure that it continues to provide a low resistance connection throughout the life of the structure. To achieve these objectives, first determine the electrical and physical properties of the site, design an earth electrode subsystem appropriate for the site, install the subsystem in accordance with the recommended procedures, and finally, measure the earth resistance of the subsystem to verify that it meets the recoin mended goals or design specifications.

1.2.1 Determination of Site Parameters (Site Survey). Before beginning the design, conduct a survey of the site where the earth electrode subsystem is to be installed. Through this survey, determine the resistivity of the soil, identify significant geological features, gather information on architectural and landscape features which may influence the design of the subsystem, and review local climate effects. (If possible, conduct this survey in advance of the final site selection in order to avoid particularly troublesome e locations.)

1.2.1.1 Soil Resistivity. As the first step of the site survey, measure the resistivity of the soil at several points over the area of the planned facility. For even the smallest facility, the effective facility area in so far as the electrode subsystem is concerned is assumed to be at least 15 meters by 15 meters (50 feet by 50 feet).

For larger facilities, the facility areas are assumed to extend at least 6 meters (20 feet) beyond the basic building or structural outline, i.e., the ground floor plan, substation grid, tower footing, transformer housing, etc. It is necessary that the soil resistivity be known over the area encircled or covered by the earth electrode subsystem.

a. A single soil resistivity measurement is made using the four-probe method (see Volume I, Section

2.4) in the following manner:

(1) At a location near the center of the site, insert the four short probes supplied with the earth resistance test set into the soil in a straight line as illustrated in Figure 1-1. A convenient probe spacing of 6 to 9 meters (20 to 30 feet) is recommended as a start. If probes are not supplied with the test set or if they have been lost or misplaced, four metal (steel, copper, or aluminum) rods, 1/4 to 3/8 inch in diameter and 12 to

18 inches in length, may be used. Drill and tap No. 6-32, 8-32, or 10-24 screws, according to rod size and securely fasten the test set leads to the rods. Clamps may also be used for connecting the leads to the probes.

*The relationship between the performance of an electronic system and the resistance of the earth ground is unclear. The value of 10 ohms earth electrode resistance recommended in Section 1.2.2.la represents a carefully considered compromise between overall fault and lightning protection requirements and the estimated relative cost of achieving the resistance in typical situations. In locations characterized by high soil resistivities, to achieve 10 ohms could be very expensive. In such locations, examine all elements of the site, consider the requirements of the planned facility, and then choose the best compromise based on soil conditions, relative costs, etc.

1-2

(2) Following the manufacturer's instruction, obtain a resistance reading, R, with the test set.

(3) Convert the probe spacing, A, to centimeters. (See Page xvi for metric conversion factors.)

(4) Compute resistivity from

= 6.28AR (in ohm-cm)

EXAMPLE: Assume that a resistance of 2 ohms is measured with probe spacings of 20 feet.

Convert 20 feet to centimeters: 20 ft x 30.5 cm/ft = 610 cm

Calculate resistivity = 6.28 x 610 x 2 = 7662 ohm -cm

A = ELECTRODE SPACING

B = DEPTH OF PENETRATION < A/20

D = DEPTH AT WHICH RESISTIVITY IS DETERMINED = A

(l -1)

Figure 1-1. Measurement of Soil Resistivity

1-3

b. The reading obtained indicates the average resistivity of the soil in the immediate vicinity of the test area. A resistivity profile of the site requires that the above procedure be repeated at many sample locations over the region being mapped. For small sites up to 2500 square feet (232 square meters), make at least one measurement at the center of the site and at each of the four corners of a 50-foot square as shown in

Figure 1-2. Drive a stake or marker at the locations shown. Position the potential and current probes in a straight line with the stake or marker centered between the probes. Make a resistance measurement at each location and calculate the resistivity as in step a-4 above. Take the average of the five readings as the resistivity for the soil at the site. If possible, soil measurements should be made during average/normal weather conditions. Measurements should never be made immediately after a rain or storm.

c. For larger sites, make measurements every 100 to 150 feet (30 to 45 meters) over the site area.

Include in the site area the locations of support elements such as transformer banks, towers, engine-generator buildings, etc. Choose a sufficient number of test points to give an indication of the relative uniformity of the soil composition throughout the area. Be particularly alert for the presence of localized areas of very high or very low resistivity soils.

NOTE: NOT DRAWN TO SCALE.

Figure 1-2. Resistivity Determination of a Small Site

1-4

1.2.1.2 Geological Effects.

a. Identify the significant geological features of the site. Specifically, attempt to establish:

(1) the distribution of major soil types (see Volume l, Section 2.3.2 ) to include the locations of sand and gravel deposits,

(2) major rock formations,

(3) the presence of water sources to include underground streams, and

(4) the depth of the water table.

Utilize test borings, on site inspections, studies of local maps, and interviews companies, well drillers, and other local personnel to obtain the desired information.

with local construction

b. Evaluate the information provided by these sources for indications of particularly troublesome (or particularly helpful) characteristics that may influence the design or installation of the earth electrode subsystem of the facility.

1.2.1.3 Physical Features. Locate and identify those other physical features that will influence the general placement of the earth electrode subsystem, the location of test and access points, physical protection requirements, and the cost of materials and installation. For example, indicate on the general site plan:

a.

b.

c.

d.

the planned physical layout of the building or structure, locations of paved roads and parking lots, drainage, both natural and man-made, and the location of buried metal objects such as pipes and tanks.

1.2.1.4 Local Climate.

a. Review local climatic conditions and determine the annual amount and seasonal distribution of rainfall, the relative incidence of lightning, and the depth of freezing (frost line) typical of the area. Obtain the rainfall and frost line information from the local weather service; project the relative lightning incidence from the isokeraunic maps given in Volume I, Section 3.4, Figures 3-2 and 3-3.

b. Record the data and make it a part of the facility files for the site. Immediately, however, use this information to aid in the design of the earth electrode subsystem for the facility to be constructed at the site.

1-5

1.2.2 Design Procedure.

1.2.2.1 Selection of Electrode Configuration. Determine what type of earth electrode subsystem is most appropriate for the facility (complex, building, structure, transformer bank, substation, etc). The directed configuration is a ring ground outlined in paragraph 5.1.1.1.3 of MIL-STD-188-124A. If this configuration cannot be employed, alternate configurations meeting these requirements are described in Section 1.2.2.3 of this volume.

a. Establish the primary functional requirements to be met by the earth electrode subsystem. For example.

Lightning. For a facility located in an area of high lightning incidence or a high degree of exposure to lightning, or both, (see Volume 1, Section 3.4) the earth electrode subsystem must safely dissipate the lightning energy without melting conductors or overheating the soil (see Volume I, Section 2.8.2.2). Also, the subsystem must minimize step voltages in areas where personnel are present.

Impulse Properties and RF Impedance Characteristics. If the antenna counterpoise must serve as an earth electrode subsystem, it must have low rf impedance properties.

Mobility. Mobile facilities or temporary transportable facilities will generally not justify the installation of an extensive fixed electrode subsystem. For such facilities, install only a basic system capable of providing the minimum acceptable lightning and personnel fault protection (see Section 1.11).

Resistance. At fixed C-E facilities, the earth electrode subsystem should exhibit a resistance to earth of 10 ohms or less. If 10 ohms is not economically feasible by the ring ground, alternate methods should be considered. Paragraph 5.1.1.1.3.2 of MIL-STD-188-124A refers. Resistance measurements using the fall-of-potential method shall be accomplished in 3-month increments for 12 months following installation.

Measurements shall be conducted in 21-month intervals after the first year.

b. Evaluate local conditions.

Soil resistivity. Is soil resistivity low (< 5000 ohm-cm), average (5000 to 20,000 ohm -cm), or high

(> 20,000 ohm-cm)? The higher the soil resistivity, the more complex (and expensive) will be the electrode subsystem necessary to achieve 10 ohms resistance.

Moisture content. Is the water table near the surface or far below grade, and is it subject to large seasonal variations? Design the earth electrode subsystem so that it makes and maintains contact with soil that stays damp or moist year round if at all possible. Penetration of the permanent water table is highly desirable.

Frost line. How deeply does the frost line extend, even during coldest periods? The resistivity of soil rises greatly (see Volume I, Section 2.3.3) as the soil temperature drops below 32° F. Thus for maximum stability of electrode resistance, the subsystem should penetrate far enough into the soil so that contact is always maintained with unfrozen soil. The earthing techniques described in this chapter are not directly applicable to permafrost. In permafrost, fault protection must be provided through the use of metallic returns accompanying the power conductors to insure the existence of a return path to the transformer or generator.

1-6

Personnel protection in permafrost requires an even greater emphasis on the bonding of all metal objects subject to human contact and to the power system neutral and is described in Volume I, Chapter 2. Because of the high resistance of permafrost, stray earth currents can be expected to be minimal with consequently reduced concern with inter-facility power frequency noise problems (see Volume I, Section 2.1.3). In the event that earth-current related noise problems exist, the common-mode rejection techniques described in Volume I, Section 6.4 should be applied.

Rock Formations. Are major rock formations near the surface and are they large enough to influence the design and layout of the earth electrode subsystem? In regions of shallow bedrock, vertical ground rods may not be usable and horizontal grids, wires, or plates must be used. Large rock outcropping or subsurface boulders may force the alternate routing of conductors or the placement of rods. There is no need to incur the expense of drilling holes in rock to insert rods or lay wires because the resistivity of rock is so high that generally the rods or wires would be ineffective.

Architectural layout. Design the earth electrode subsystem so that it will not be materially influenced by the weather shielding effects of parapets and overhangs. Lightning down conductor placement and routing will frequently be influenced by architectural considerations. Design the earth electrode subsystem to accommodate such considerations by providing convenient connection points near the down conductors.

Route the interconnecting cable of the earth electrode subsystem near down conductors to avoid long extensions between the down conductor and the effective grounding point. Configure the earth electrode subsystem such that convenient connections are possible between the earth electrode subsystem and grounding conductors of the power and signal ground systems inside the facility.

Landscape features. Preferably locate ground subsystem conductors under sodded areas or those otherwise covered with vegetation. Locate conductors to take maximum advantage of the wetting effects of runoff or drainage water from the roof, parking lots, etc. Try to avoid placing major portions of this earth electrode subsystem under extensive paved areas such as roads and parking lots.

c. Considering the relative advantages and disadvantages given in Table 1-1, choose a basic type of electrode most appropriate for meeting the functional requirements of the facility at the site under construction.

d. Estimate the relative costs to meet the objectives with the different types of configurations.

Include the cost of materials, installation costs, and relative maintenance and upgrading costs.

1-7

Table 1-1

Relative Advantages and Disadvantages of the Principal Types of Earth Electrodes

Type Advantages Disadvantages

Ring Ground Straightforward design. Easy to install Not useful where large rock

(particularly around an existing facility). formations are near surface.

Hardware readily available. Can be extended to reach water table.

Horizontal Bare Can achieve low resistance where rock

Wires (Radials) formations prevent use of vertical rods.

Low impulse impedance. Good rf counterpoise when laid in star pattern.

Horizontal Grid Minimum surface potential gradient.

(Bare Wire) Straight for ward installation if done before construction. Can achieve low resistance contact in areas where rock formations prevent use of vertical rods.

Can be combined with vertical rods to stabilize resistance fluctuations.

Vertical Rods Straightforward design. Easiest to install (particularly around an existing facility). Hardware readily available.

Can be extended to reach water table.

Plates

Incidental

Electrodes

(Utility pipes, building foundations, buried tanks)

Can achieve low resistance contact in limited area.

Can exhibit very low resistance.

Subject to resistance fluctuations with soil drying.

Subject to resistance fluctuations with soil drying if vertical rods not used.

High impulse impedance. Not useful where large rock formations are near surface. Step voltage on earth surface can be excessive under high fault currents or during direct lightning strike.

Most difficult to install.

Little or no control over future alterations. Must be employed with other made electrodes.

1-8

1.2.2.2 Calculation of Earth Resistance.

Once the most appropriate configuration is chosen for the facility, calculate the resistance to earth for the configuration. If the calculated resistance meets the design goal (or requirement), complete the design to include all necessary interconnections. To illustrate this design procedure, assume that a 100 ft x 160 ft rectangular configuration like that shown in Figure 1-3 is initially chosen. Further, assume that the soil resistivity measurements made during the site survey showed an average resistivity of 10,000 ohm-cm for the area. In addition, the site survey indicated that all rock formations are at depths greater than 10 feet; the water table never drops more than 5 feet below grade; and the frost line extends only to 1 foot below grade.

Therefore, 10-foot ground rods are initially selected for evaluation. (The minimum rod diameter required in

MIL-STD-188-124A, para 5.1.1.1.4 is 3/4 inch.)

a. Determine the resistance of one of the ground rods from Figure 1-4. First, place a straight edge between the point marked 3/4 on line “d” and the point marked 10 on line Indicate on line “q” where the straightedge crosses. Next , p lace the s t ra ightedge be tween the poin t jus t marked on “q” and the

10,000 ohm-cm point on the vertical line labeled “Resistivity.” Read the resistance as 32 ohms at the point where the straightedge crosses the vertical line labeled “Resistance.”

b. Assume an initial spacing of 20 feet or twice the rod length (see Volume I, Section 2.6.2) between rods. Figure 1-3 shows that 26 rods are required to encircle the structure. Use Figure 1-5 to determine the relative lowering of the resistance of one rod that is produced by 26 rods in parallel. (The answer is about 5.5 percent.) Thus the resistance of the 26 rods in 10,000 ohm-cm soil is

R = 32 x 0.055 = 1.76 ohms.

Figure 1-5 primarily applies to ground rods laid out in a straight line or around the perimeter…