MIL-HDBK-419A.pdf

MIL-HDBK-419A

2 9 D E C E M B E R 1 9 8 7

SUPERSEDING

MIL-HDBK419

2 1 J A N U A R Y 1 9 8 2

MILITARY HANDBOOK

GROUNDING, BONDING, AND SHIELDING

FOR

ELECTRONIC EQUIPMENTS AND FACILITIES

VOLUME 1 OF 2 VOLUMES

BASIC THEORY

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 recoin mended 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 theory 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 atlmspheric 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 is to ensure the optimum performance of ground-based telecommunications equipment by reducing noise and providing adequate protection against power system faults and lightning strikes.

This handbook emphasizes the necessity for including considerations of grounding, bonding, and shielding in all phases of design, construction, operation, and maintenance of electronic equipment and facilities. Volume 1, 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 recommended 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

1.3

1.4

1.5

1.5.1

1.5.2

1.5.3

1.6

1.7

2.1

2.1.1

2.1.2

2.1.3

2.1.4

2.2

2.2.1

2.2.2

2.2.2.1

2.2.2.2

2.2.3

2.3

2.3.1

2.3.2

2.3.3

2.4

2.4.1

2.4.2

2.4.2.1

2.4.2.2

2.5

2.5.1

2.5.2

2.5.3

MIL-HDBK-419A

TABLE OF CONTENTS

CHAPTER 1- FACILITY GROUND SYSTEM

GENERAL

APPLICATION

DEFINITIONS

REFERENCED DOCUMENTS

DESCRIPTION

Facility Ground System

Grounding and Power Distribution Systems

Electrical Noise in Communications Systems

BONDING, SHIELDING, AND GROUNDING RELATIONSHIP

GROUNDING SAFETY PRACTICES

CHAPTER 2- EARTHING AND EARTH ELECTRODE SUBSYSTEM

OBJECTIVES

Lightning Discharge

Fault Protection

Noise Reduction

Summary of Requirements

RESISTANCE REQUIREMENTS

General

Resistance to Earth

National Electrical Code Requirements

Department of Defense Communications Electronics Requirements

Lightning Require meats

SOIL RESISTIVITY

G e n e r a l

Typical Resistivity Ranges

Environmental Effects

MEASUREMENT OF SOIL RESISTIVITY

G e n e r a l

Measurement Techniques

One-Electrode Method

F o u r - T e r m i n a l M e t h o d

TYPES OF EARTH ELECTRODE SUBSYSTEMS

General

Ground Rods

Buried Horizontal Conductors

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1-1

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1-2

1-2

1-3

1-4

1-5

1-5

2-1

2-1

2-2

2-2

2-2

2-5

2-5

2-5

2-5

2-5

2-5

2-7

2-7

2-7

2-7

2-8

2-8

2-8

2-8

2-13

2-15

2-15

2-15

2-15 i

TABLE OF CONTENTS (Continued)

Paragraph

2.5.4

2.5.5

2.5.6

2.5.7

2.5.8

2.5.9

2.6

2.6.1

2.6.1.1

2.6.1.2

2.6.2

2.6.2.1

2.6.2.2

2.6.2.3

2.6.2.4

2.6.3

2.6.4

2.6.4.1

2.6.4.2

2.6.4.3

2.7

2.7.1

2.7.2

2.7.2.1

2.7.2.2

2.7.2.3

2.7.3

2.8

2.8.1

2.8.1.1

2.8.1.2

2.8.1.2.1

2.8.1.2.2

2.8.1.2.3

2.8.1.3

2.8.2

2.8.2.1

2.8.2.2

2.8.2.3

Grids

Plates

Metal Frameworks of Buildings . . .

Water Pipes

Incidental Metals

Well Casings

RESISTANCE PROPERTIES

Simple Isolated Electrodes

Driven Rod

Other Commonly Used Electrodes .

Resistance of Multiple Electrodes . .

Two Vertical Rods in Parallel . . .

Square Array of Vertical Rods . .

Horizontal Grid (Mesh)

Vertical Rods Connected by a Grid

Transient Impedance of Electrodes .

Effects of Nonhomogeneous (Layered) Earth

Hemispherical Electrode

Vertical Rod

Grids

MEASUREMENT OF RESISTANCE-TO-EARTH

Introduction

Fall-of-Potential Method

Probe Spacing

Extensive Electrode Subsystems

Test Equipments

Three-Point (Triangulation) Method

OTHER CONSIDERATIONS

Surface Voltages Above Earth Electrodes .

Step Voltage Safety Limit

Step Voltages for Practical Electrodes . .

Flush Vertical Rod

Buried Vertical Rod

Buried Horizontal Grid

Minimizing Step Voltage

Heating of Electrodes

Steady State Current

Transient Current

Minimum Electrode Size

OF ELECTRODES.

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2-59 ii

Paragraph

2.9 ELECTRODE ENHANCEMENT

2.9.1 Introduction

2.9.2 Water Retention

2.9.3 Chemical Salting

2.9.4 Electrode Encasement

2.9.5 Salting Methods

2.10 CATHODIC PROTECTION

2.10.1 Introduction

2.10.2 Protection Techniques

2.10.3 Sacrifical Anodes

2.10.4 Corrosive Atmospheres

2.11 GROUNDING IN ARCTIC REGIONS

2.11.1 Soil Resistivity

2.11.2 Improving Electrical Grounding in Frozen Soils

2.11.2.1 Electrode Resistance

2.11.2.2 Installation and Measurement Methods . . .

2.11.2.2.1 Electrode Installation

2.11.2.2.2 Backfill

2.12

3.1

3.2

3.3

3.4

3.5

3.5.1

3.5.2

3.6

3.6.1

3.6.2

3.6.3

3.6.3.1

3.6.3.2

3.6.3.3

3.6.3.4

3.7

3.8

3.8.1

REFERENCES

CHAPTER 3- LIGHTNING PROTECTION SUBSYSTEM

THE PHENOMENON OF LIGHTNING

DEVELOPMENT OF A LIGHTNING FLASH

INFLUENCE OF STRUCTURE HEIGHT

STRIKE LIKELIHOOD

ATTRACTIVE AREA

Structures Less Than 100 Meters High

Cone of Protection

LIGHTNING EFFECTS

Flash Parameters

Mechanical and Thermal Effects

Electrical Effects

Conductor Impedance Effects

Induced Voltage Effects

Capacitively-Coupled Voltage

Earth Resistance

BASIC PROTECTION REQUIREMENTS

DETERMINING THE NEED FOR PROTECTION

Strike Likelihood iii

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2-63

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2-65

2-65

2-66

2-66

2-66

2-70

2-71

2-7 1

2-71

2-71

2-75

3-1

3-3

3-3

3-4

3-10

3-10

3-11

3-13

3-13

3-15

3-17

3-17

3-18

3-21

3-21

3-25

3-26

Paragraph Page

3.8.2 Type of Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-26

3.8.3 Criticalness to System Mission . . . . . . . . . . . . . . . . . . . . . . . . . . 3-27

3.9 APPLICABLE CODES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-27

3.10 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-28

CHAPTER 4 - FAULT PROTECTION SUBSYSTEM

4.1 FAULT PROTECTION

4.1.1 Power System Faults

4.1.2 Ground-Fault-Circuit-Interrupter (GFCl) .

4.2 EARTH CONNECTION

4.3 AC POWER LINE GROUND

4.4 TEST EQUIPMENT

4.5 REFERENCES

5.1

5.2

5.2.1

5.2.2

5.2.2.1

5.2.2.2

5.2.2.3

5.2.2.4

5.2.3

5.2.4

5.2.4.1

5.2.4.2

5.2.4.3

5.2.4.4

5.3

5.3.1

5.3.2

5.3.3

5.3.3.1

5.3.3.2

5.3.4

CHAPTER 5- GROUNDING OF SIGNAL REFERENCE SUBSYSTEM

INTRODUCTION

CONDUCTOR CONSIDERATIONS.

Direct Current Resistance . . .

Alternating Current Impedance .

Skin Effect

AC Resistance

Reactance

Proximity Effect

Resistance Properties vs Impedance Properties

Effects of Geometry

Stranded Cables

Rectangular Conductors

Tubular Conductors

Structural Steel Members

SIGNAL REFERENCE SUBSYSTEM NETWORK CONFIGURATIONS

Floating Ground

Single-Point Ground (for Lower Frequencies)

Multipoint Ground (for Higher Frequencies)

Equipotential Plane

Types of Equipotential Planes

Floating System

4-1

4-1

4-3

4-3

4-3

4-5

4-6

5-1

5-1

5-1

5-1

5-3

5-5

5-7

5-10

5-10

5-12

5-13

5-13

5-13

5-15

5-15

5-15

5-19

5-24

5-26

5-27

5-28 iv

Paragraph

5.4

5.4.1

5.4.2

5.4.3

5.5

6.1

6.2

6.2.1

6.2.2

6.2.2.1

6.2.2.2

6.2.2.3

6.2.2.4

6.3

6.3.1

6.3.2

6.4

6.4.1

6.4.1.1

6.4.1.2

6.4.1.3

6.4.1.4

6.4.1.5

6.4.2

6.5

6.6

7.1

7.2

7.3

7.4

7.4.1

7.4.1.1

7.4.1.2

7.4.1.3

SITE APPLICATIONS

Lower Frequency Network

Higher Frequency Network

Frequency Limits

REFERENCES

CHAPTER 6- INTERFERENCE COUPLING AND REDUCTION

INTRODUCTION

COUPLING MECHANISMS

Conductive Coupling

Free-Space Coupling

Near-Field Coupling

Inductive Coupling

Capacitive Coupling

Far-Field Coupling

COMMON-MODE NOISE

Basic Theory of Common-Mode Coupling . .

Differential Amplifier

MINIMIZATION TECHNIQUES

Reduction of Coupling

Reference Plane Impedance Minimization

Spatial Separation

Reduction of Circuit Loop Area

Shielding

Balanced Lines

Alternate Methods

FACILITY AND EQUIPMENT REQUIREMENTS

REFERENCES

CHAPTER 7 - BONDING

DEFINITION OF BONDING

PURPOSES OF BONDING

RESISTANCE CRITERIA

DIRECT BONDS

Contact Resistance

Surface Contaminants

Surface Hardness

Contact Pressure

Page

5-28

5-29

5-30

5-31

5-32

6-1

6-5

6-5

6-6

6-6

6-8

6-11

6-14

6-17

6-19

6-23

6-23

6-23

6-23

6-24

6-24

6-24

6-24

6-24

6-25

6-25

7-1

7-1

7-3

7-4

7-6

7-7

7-7

7-7 v

7.4.1.4

7.4.2

7.4.2.1

7.4.2.2

7.4.2.3

7.4.2.4

7.4.2.5

7.4.2.6

7.4.2.7

7.5

7.5.1

7.5.2

7.5.2.1

7.5.2.2

7.5.2.3

7.6

7.6.1

7.6.2

7.6.3

7.6.4

7.7

7.8

7.8.1

7.8.1.1

7.8.1.2

7.8.2

7.8.3

7.9

7.10

7.11

Bond Area .

Direct Bonding

Welding . .

Brazing . .

Soft Solder.

Bolts . . .

R i v e t s . .

Techniques

Conductive Adhesive

Comparison of Techniques

INDIRECT BONDS . .

Resistance

Frequency Effects .

Skin Effect

Bond Reactance .

Stray Capacitance

SURFACE PREPARATION

Solid Materials

Organic Compounds . .

Platings and Inorganic Finishes

Corrosion By-Products

COMPLETION OF THE BOND .

BOND CORROSION

Chemical Basis of Corrosion .

Electrochemical Series . .

Galvanic Series

Relative Area of Anodic Member

Protective Coatings

WORKMANSHIP

SUMMARY OF GUIDELINES . . .

REFERENCES

CHAPTER 8- SHIELDING

8.1 FUNCTION OF AN ELECTROMAGNETIC SHIELD

8.2 BASIC SHIELDING THEORY

8.2.1 Oppositely Induced Fields

8.2.2 Transmission Line Analogy

8.2.3 Nonuniform Shielding

Page

7-8

7-10

7-10

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7-14

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7-16

7-19

7-19

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7-19

7-23

7-25

7-26

7-26

7-29

7-29

7-29

7-30

7-30

7-31

7-31

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7-34

7-34

7-36

7-37

8-1

8-2

8-2

8-2

8-4 vi

P a r a g r a p h

Paragraph

8.3

8.3.1

8.3.2

8.3.2.1

8.3.2.2

8.3.2.3

8.3.3

8.3.4

8.3.4.1

8.3.4.2

8.4

8.4.1

8.4.2

8.4.3

8.5

8.5.1

8.5.2

8.5.3

8.5.3.1

8.5.3.2

. 8.6

8.7

8.7.1

8.7.2

8.8

8.8.1

8.8.2

8.9

8.9.1

8.9.2

8.9.3

8.10

8.10.1

8.10.2

8.11

8.12

8.12.1

8.12.2

8.12.3

SHIELDING EFFECTIVENESS OF CONTINUOUS SINGLE-THICKNESS SHIELDS

A b s o r p t i o n L o s s

Reflection Loss

Low Impedance Field

P lane Wave Fie ld

H i g h I m p e d a n c e F i e l d

Re-Reflection Correction Factor

Total Shielding Effectiveness

Measured Data

S u m m a r y

SHIELDING EFFECTIVENESS OF OTHER SHIELDS

Multiple Solid Shields

Coatings and Thin-Film Shields

Screens and Perforated Metal Shields

SHIELD DISCONTINUITY EFFECTS (APERTURES)

Seams Without Gaskets

Seams With Gaskets

Penetration Holes

Waveguide-Below-Cutoff

Screen and Conducting Glass

SELECTION OF SHIELDING MATERIALS

USE OF CONVENTIONAL BUILDING MATERIALS

C o n c r e t e

Reinforcing Steel (Rebar)

CABLE AN D CONNECTOR SHIELDING

Cable Shields

Terminations and Connectors

SHIELDED ENCLOSURES (SCREEN ROOMS)

Remountable (Modular) Enclosures

Custom Built Rooms

Foil Room Liners

TESTING OF SHIELDS

Low Impedance Magnetic Field Testing Using Small Loops

Additional Test Methods

PERSONNEL PROTECTION SHIELDS

DETERMINATION OF SHIELDING REQUIREMENTS

Equipment Disturbances

Electromagnetic Environmental Survey

Equipment EMI Properties

Page

8-4

8-5

8-6

8-10

8-13

8-15

8-19

8-19

8-27

8-27

8-31

8-31

8-32

8-33

8-41

8-42

8-45

8-45

8-50

8-52

8-53

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8-56

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8-59

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8-70

8-71

8-72

8-73

8-74

8-74

8-74

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8-76

8-77 vii

Paragraph Page

8.13

8.13.1

8.13.2

8.13.3

8.13.4

8.14

9.1

9.1.1

9.1.2

9.2

9.3

9.4

9.5

9.6

10.1

10.2

10.2.1

10.2.1.1

10.2.1.2

10.2.1.3

10.2.2

10.2.3

10.2.4

10.3

10.3.1

10.3.1.1

10.3.1.2

10.3.1.3

10.3.2

10.3.2.1

10.3.2.2

10.3.2.3

SYSTEM DESIGN CONSIDERATIONS

Size

Layout

Signal Properties

Cost

REFERENCES

CHAPTER 9-PERSONNEL PROTECTION

ELECTRIC SHOCK

Levels of Electric Shock

Shock Prevention

STATIC ELECTRICITY

RADIO FREQUENCY (RF) RADIATION HAZARDS

LASER HAZARDS

X-RAY RADIATION

REFERENCES

CHAPTER 10- NUCLEAR EMP

INTRODUCTION

EMP GENERATION

High-Altitude EMP (HEMP)

Early-Time HEMP

Late-Time HEMP (MHDEMP)

Intermediate-Time HEMP

Surface-Burst EMP

0ther EMP Phenomena

Comparison With Lightning

HEMP INTERACTION WITH SYSTEMS

Current in Long Lines

Long Overhead Lines

Long Buried Lines

Vertical Structures

HEMP Interaction With Local Structure

Shields

Penetrating Conductors

Apertures

EFFECTS

. 8-77

. 8-78

. 8-78

. 8-78

. 8-78

. 8-79

9-1

9-1

9-3

9-3

9-5

9-5

9-6

9-6

10-1

10-1

10-1

10-1

10-3

10-3

10-3

10-4

10-5

10-5

10-6

10-6

10-7

10-9

10-9

10-9

10-10

10-11 v i i i

Paragraph

10.4 PROTECTION AGAINST HEMP

10.4.1

10.4.1.1

10.4.1.2

10.4.1.3

10.4.2

10.4.2.1

10.4.2.2

10.4.2.3

10.4.2.3.1

10.4.2.3.2

10.4.2.3.3

10.4.2.3.4

10.4.2.4

10.4.2.4.1

10.4.2.4.2

10.4.2.4.2.1

10.4.2.4.2.2

10.4.2.4.2.3

HEMP Barrier

Shield

Penetrating Conductors

Apertures

Allocation of Protection

Amount of Protection Needed

Where Protection is Applied

Terminal Protection Devices

Spark Gaps and Gas Tubes

Metal-Oxide Varistors . .

Semiconductors

Filters

Waveguide Penetration of Facility Shield

Introduction

In-Line Waveguide Attachment

Sleeve and Bellows Attachment

Braided Wire Sleeve

Stuffing Tube for Waveguide

10.5 REFERENCES

CHAPTER 11-NOTES

11.1 SUBJECT TERM (KEY WORD) LISTING

APPENDICES

Page

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10-25

11-1

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 II . . . . . . . . . . . . . . . . . . . . . . . C-1

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

LIST OF FIGURES

Figure

2-1

2-2

2-3

2-4

2-5

2-6

2-7

2-8

2-9

2-10

2-11

2-12

2-13

2-14

2-15

2-16

2-17

2-18

2-19

2-20

2-21

2-22

2-23

2-24

2-25

2-26

2-27

2-28

2-29

2-30

2-31

2-32

2-33

Voltage Differentials Arising from Unequal Earth Electrode Resistances and Unequal

Stray Currents

Voltage Differentials Between Structures Resulting from Stray Ground Currents

Typical Variations in Soil Resistivity as a Function of Moisture, Temperature, and

Salt Content

Current Flow From a Hemisphere in Uniform Earth

Idealized Method for Determining Soil Resistivity

Effect of Rod Length Upon Resistance

Effect of Rod Diameter Upon Resistance

Earth Resistance to Shell Surrounding a Vertical Earth Electrode

Resistance of Buried Horizontal Conductors

Resistance of Buried Circular Plates

Ground Rods in Parallel

Ratio of the Actual Resistance of a Rod Array to the Ideal Resistance of N Rods in Parallel

Transient Impedance of an Earth Electrode Subsystem as a Function of the Number of Radial Wires

Current Distribution in Nonuniform Soil

Fall-of-Potential Method for Measuring the Resistance of Earth Electrodes

Effect of Electrode Spacing on Voltage Measurement

Resistance Variations as Function of Potential Probe Position in Fall-of-Potential

Method

Earth Resistance Curves for a Large Electrode Subsystem

Earth Resistance Curve Applicable to Large Earth Electrode Subsystems

Intersection Curves for Figure 2-18

Triangulation Method of Treasuring the Resistance of an Earth Electrode

Variation of Surface Potential Produced by a Current Flowing Into an Isolated

Ground Rod

Surface Potential Variation Along a Grid

Effect of Chemical Treatment on Resistance of Ground Rods

Seasonal Resistance Variations of Treated and Untreated Ground Rods

Trench Method of Soil Treatment

Alternate Method of Chemical Treatment of Ground Rod

Relative Depths of Unconsolidated Materials, Subarctic Alaska

Typical Sections Through Ground Containing Permafrost

Illustration Showing Approximate Variations in Substructure

Installation of an Electrode During the Process of Backfilling

Apparent Resistivity for Two Soils at Various Moisture and Soil Contents

Configuration of Nearly Horizontal Electrodes Placed in the Thawed Active Layer x

Page

2-3

2-4

2-9

2-11

2-14

2-18

2-18

2-20

2-24

2-25

2-26

2-28

2-31

2-34

2-37

2-38

2-41

2-44

2-45

2-47

2-48

2-52

2-54

2-61

2-61

2-64

2-64

2-67

2-68

2-69

2-72

2-73

2-73

LIST OF FIGURES (Continued)

Figure Page

2-34 Resistance-to-Ground Curves for an Electrode Driven Into Ice-Rich Silt

2-35 Resistance-to-Ground Curves for an Electrode Surrounded by Backfill of Saturated Silt. . .

2-36 Resistance-to-Ground Curves for an Electrode Surrounded by Water Saturated

Salt-Soil Backfill

2-37 Resistance-to-Ground Curves for an Electrode Surrounded by Water Saturated

Salt-Soil Backfill

2-38 Resistance-to-Ground Curves for Electrodes Placed in Holes Modified by Spring

Changes

3-1

3-2

3-3

3-4

3-5

3-6

3-7

3-8

3-9

3-10

3-11

3-12

3-13

Charge Distribution in a Thundercloud

Mean Number of Thunderstorm Days Per Year for the United States

Worldwide Isokeraunic Map

Attractive Area of a Rectangular Structure

Effective Height of a Structure

Zones of Protection Established by a Vertical Mast and a Horizontal Wire

Some Commonly Used Lightning Shielding Angles

Illustration of Processes and Currents Which Occur During a Lightning Flash to Ground . . .

Inductive Coupling of Lightning Energy to Nearby Circuits

Normalized Voltage Induced in a Single-Turn Loop by Lightning Currents

Capacitive Coupling of Lightning Energy

Coupling of Lightning Energy Through an Interconnected Facility

Step-Voltage Hazards Caused by Lightning-Induced Voltage Gradients in the Earth

4-1 Grounding for Fault Protection

4-2 Single-Phase 115/230 Volt AC Power Ground Connections

4-3 Three-Phase 120/208 Volt AC Power System Ground Connections

4-4 Connections for a Three-Phase "Zig-Zag" Grounding Transformer

5-1

5-2

5-3

5-4

5-5

5-6

5-7

5-8

5-9

5-1o

5-11

Surface Resistance and Skin Depth for Common Metals

Resistance Ratio of Isolated Round Wires

Nomograph for the Determination of Skin Effect Correction Factor

Low Frequency Self Inductance Versus Length for 1/0 AWG Straight Copper ‘Wire

Self Inductance of Straight Round Wire at High Frequencies

Resistance Ratio of Rectangular Conductors

Resistance Versus Length for Various Sizes of Copper Tubing

AC Resistance Versus Frequency for Copper Tubing

Resistance Ratio of Nonmagnetic Tubular Conductors

Inductance Versus Frequency for Various Sizes of Copper Tubing

Floating Signal Ground xi

2-73

2-74

2-74

2-74

2-74

3-2

3-5

3-6

3-12

3-12

3-14

3-14

3-15

3-19

3-20

3-22

3-23

3-24

4-2

4-4

4-5

4-6

5-4

5-6

5-8

5-9

5-9

5-14

5-14

5-16

5-17

5-18

5-19

Figure Page

1-72

1-73

1-74

1-75

1-76

1-77

1-78

1-79

1-80

1-81

1-82

1-83

1-84

1-85

1-86

1-87

1-88

1-89

1-90

1-91

1-92

1-93

1-94

1-95

1-96

1-97

1-98

1-99

1-100

1-101

Bonding of Equipment Cabinets to Cable Tray

Bonding to Flexible Cable and Conduit

Bonding to Rigid Conduit

Connection of Bonding Jumpers to Flat Surface

Bolted Bond Between Flatcars

Bracket Installation (Rivet or Weld)

Use of Bonding Straps for Structural Steel Interconnections

Direct Bonding of Structural Elements

Connection of Earth Electrode Riser to Structural Column

Measured Electromagnetic Shielding Effectiveness of a Typical Building at 6 Feet

Inside Outer Wall

Measured Electromagnetic Shielding Effectiveness of a Typical Building at 45 Feet

Inside Outer Wall

Shielding Effectiveness of Rebars

Shielding Effectiveness of a Grid as a Function of Wire Diameter, Wire Spacing, and

Wavelength

Shield Absorption Loss Nomograph

Nomograph for Determining Magnetic Field Reflection Loss

Nomograph for Determining Electric Field Reflection Loss

Nomograph for Determining Plane Wave Reflection Loss

Shielding Effectiveness of Aluminum Foil Shielded Room

Shielding Effectiveness of Copper Foil Shielded Room

Formation of Permanent Overlap Seam

Good Corner Seam Design

Pressure Drop Through Various Materials Used to Shield Ventilation Openings

Typical Single-Point Entry for Exterior Penetrations (Top View)

Entry Plate Showing Rigid Cable, Conduit, and Pipe Penetrations

Effect of Rod Length on Ground Resistance

Grounding of 120/208V 3-Phase, 4-Wire Wye Power Distribution System

Grounding of Single-Phase, 3-Wire 110/220V Power System

Grounding of 28 VDC 2-Wire DC Power System

Connecting Ground Subsystems for Collocated Shelters Greater than 20 Feet Apart

Method of Grounding a Fence

1-148

1-149

1-149

1-150

1-151

1-151

1-152

1-153

1-153

1-155

1-155

1-156

1-158

1-161

1-165

1-166

1-167

1-168

1-168

1-169

1-169

1-170

1-174

1-175

1-180

1-181

1-183

1-184

1-189

1-192

2-1 Transmitter Building . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2

2-2 Communication Center/Receiver Building Expansion. . . . . . . . . . . . . . . . . . 2-3

2-3 Earth Resistance Measurement at a Typical Facility . . . . . . . . . . . . . . . . . . 2-7

2-4 Resistance Measurement Worksheet . . . . . . . . . . . . . . . . . . . . . . . . 2-8

2-5 Sample of a Completed Resistance Measurement Work Sheet . . . . . . . . . . . . . . 2-9 xii

Figure Page

7-14 True Equivalent Circuit of a Bonded System

7-15 Measured Bonding Effectiveness of a 9-1/2 Inch Bonding Strap

7-16 Measured Bonding Effectiveness of 2-3/8 Inch Bonding Strap

7-17 Basic Diagram of the Corrosion Process

7-18 Anode-to-Cathode Size at Dissimilar Junctions

7-19 Techniques for Protecting Bonds Between Dissimilar Metals

8-1

8-2

8-3

8-4

8-5

8-6

8-7

8-8

8-9

8-10

8-11

8-12

8-13

8-14

8-15

8-16

8-17

8-18

8-19

8-20

8-21

8-22

8-23

8-24

8-25

8-26

8-27

8-28

8-29

Electromagnetic Transmission Through a Slot

Transmission Line Model of Shielding

Absorption Loss for One Millimeter Shields

Wave Impedance Versus Distance from Source

Reflection Loss for Iron, Copper, and Aluminum With a Low Impedance Source . .

Universal Reflection Loss Curve for a Low Impedance Source

Plane Wave Reflection Loss for Iron, Copper, and Aluminum (r>2 )

Universal Reflection Loss Curve for Plane Waves

Universal Reflection Loss Curve for High Impedance Field

Reflection Losses for Iron, Copper, and Aluminum With a High Impedance Source .

Graph of Correction Term (C) for Copper in a Magnetic Field

Absorption Loss and Multiple Reflection Correction Term When r = 1

Theoretical Attenuation of Thin Copper Foil

Theoretical Attenuation of Thin Iron Sheet

Measured Shielding Effectiveness of High Permeability Metals

Measured Shielding Effectiveness of High Permeability Material as a Function of Measurement Loop Spacing

Measured Shielding Effectiveness of Two Sheets of High Permeability Metal . . .

Measured and Calculated Shielding Effectiveness of Copper Screens to Low

Impedance Fields

Shielding Effectiveness of a Perforated Metal Sheet as a Function of Hole Size . .

Shielding Effectiveness of a Perforated Metal Sheet as a Function of Hole Spacing.

Slot Radiation (Leakage)

Shielding Effectiveness Degradation Caused by Surface Finishes on Aluminum . . .

Influence of Screw Spacing on Shielding Effectiveness . . . . , Shielding Effectiveness of AMPB-65 Overlap as a Function of Screw Spacing Along

Two Rows, l.5 Inches Apart

Shielding Effectiveness of an AMPB-65 Joint as a Function of Overlap

Typical Mounting Techniques for RF Gaskets

Enlarged View of Knitted Wire Mesh

Shielding Effectiveness of Conductive Glass to High Impedance Waves

Shielding Effectiveness of Conductive Glass to Plane Waves

7-24

7-27

7-28

7-30

7-35

7-35

8-3

8-4

8-9

8-10

8-12

8-13

8-14

8-15

8-16

8-17

8-22

8-22

8-26

8-26

8-29

8-29

8-32

8-37

8-40

8-40

8-43

8-44

8-46

8-46

8-47

8-49

8-50

8-54

8-55 xiii

Figure Page

8-30

8-31

8-32

8-33

8-34

8-35

8-36

8-37

8-38

8-39

8-40

10-1

10-2

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

Light Transmission Versus Surface Resistance for Conductive Glass

Shielding Effectiveness of Some Building Materials

Center Area Attenuation of Induced Voltage by 15 Foot High Single-Course

Reinforcing Steel Room

Surface Transfer Impedance

Shielding Effectiveness of Various Types of RF Cables as a Function of Frequency

Connector for Shield Within a Shield

RF-Shielded Connector

Effectiveness of Circumferential Spring Fingers for Improving the Shielding of a

Connector

Use of Finger Stock for Door Bonding

Coaxial Loop Arrangement for Measuring Shield Effectiveness

Coplanar Loop Arrangement for Measuring Shield Effectiveness

EMP From High Altitude Bursts

Schematic Representation of High-Altitude EMP Generation

Surface-Burst Geometry Showing Compton Electrons and Net Current Density, Jc net

Short-Circuit Current Induced at the End of a Semi-Infinite Above-Ground Wire

By an Expodential Pulse

The Normalized Current Waveform for Various Valves of the Depth Parameter p

(Expodential Pulse)

Short Circuit Current Induced at the Base of a Vertical Riser by a Vertically

Polarized Incident Wave

Shield to Exclude Electromagnetic Fields

Electromagnetic Penetration Through Small Apertures

Shielding Integrity Near Interference - Carrying External Conductors

Magnetic Field Penetration of Apertures

Exclusion of Waveguide Current From Interior of Facility

Waveguide Feedthroughs

Bellows With Slitted Sleeve Waveguide Attachment

Braided Wire Sleeve Clamped to Waveguide

Stuffing Tube for Waveguide

8-55

8-57

8-58

8-62

8-62

8-65

8-65

8-66

8-69

8-75

8-75

. . . . 10-2

. . . . 10-2

. . . . 10-4

. . . . 10-7

. . . . 10-8

. . . . 10-9

. . . . 10-11

. . . . 10-12

. . . . 1o-1-1

. . . . 10-16

. . . . 10-19

. . . . 10-20

. . . . 10-22

. . . . 10-23

. . . . 10-24 xiv

LIST OF TABLES

Table Page

2-1

2-2

2-3

2-4

2-5

2-6

2-7

2-8

2-9

2-10

Facility Ground System: Purposes, Requirements, and Design Factors

Approximate Soil Resistivity

Resistivity Values of Earthing Medium

Resistance Distribution for Vertical Electrodes

Simple Isolated Electrodes

Resistance Accuracy Versus Probe C2 Spacing

Step Voltages for a Buried Vertical Ground Rod

Methods of Reducing Step Voltage Hazards

Effect of Moisture Content on Earth Resistivity

Effect of Temperature on Earth Resistivity

3-1 Range of Values for Lightning Parameters

5-1 Properties of Annealed Copper Wire

5-2 Parameters of Conductor Materials

5-3 DC Parameters of Some Standard Cables

5-4 Sixty-Hertz Characteristics of Standard Cables

5-5 One-Megahertz Characteristics of Standard Cables

5-6 Impedance Comparisons Between #12 AWG and 1/0 AWG

7-1 DC Resistance of Direct Bonds Between Selected Metals

7-2 Ratings of Selected Bonding Techniques

7-3 Calculated Inductance of a 6 Inch (15.2 cm) Rectangular Strap

7-4 Calculated Inductance (µH) of 0.05 Inch (1.27 mm) Thick Straps

7-5 Calculated Inductance (µH) of Standard Size Cable

7-6 Standard Electromotive Series

7-7 Galvanic Series of Common Metals and Alloys in Seawater

8-1

8-2

8-3

8-4

8-5

8-6

8-7

8-8

8-9

8-10

Electrical Properties of Shielding Materials at 150 kHz

Absorption Loss, A, of 1 mm Metal Sheet

Coefficients for Magnetic Field Reflection Loss

Calculated Reflection Loss in dB of Metal Sheet, Both Faces

Coefficients for Evaluation of Re-Reflection Correction Term, C

Correction Term C in dB for Single Metal Sheet

Calculated Values of Shielding Effectiveness

Measured Shielding Effectiveness in dB for Solid-Sheet Materials . . . , Summary of Formulas for Shielding Effectiveness

Magnetic Material Characteristics xv

2-6

2-9

2-10

2-21

2-22

2-43

2-50

2-56

2-66

2-66

3-16

5-2

5-3

5-11

5-11

5-12

5-12

7-8

7-18

7-20

7-20

7-21

7-32

7-33

8-7

8-8

8-11

8-18

8-20

8-21

8-23

8-28

8-30

8-31

Table

8-11

8-12

8-13

8-14

8-15

8-16

8-17

8-18

8-19

9-1

10-1

MIL-HDBK-419A

LIST OF TABLES (Continued)

Calculated Values of Copper Thin-Film Shielding Effectiveness in dB Against

Plane-Wave Energy

Effectiveness of Non-Solid Materials Against Low Impedance and Plane-Waves

Effectiveness of Non-Solid Shielding Materials Against High Impedance Waves

Comparison of Measured and Calculated Values of Shielding Effectiveness for

No. 22, 15 mil Copper Screens

Characteristics of Conductive Gasketing Materials

Shielding Effectiveness of Hexagonal Honeycomb Made of Steel With 1/8-Inch

Openings l/2-Inch Long

Comparison of Cable Shields

Connector Application Summary

Characteristics of Commercially Available Shielded Enclosures

Page

8-33

8-38

8-39

8-41

8-48

8-51

8-60

8-64

8-67

Summary of the Effects of Shock . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2

Shielding by Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-10 xvi xvii/xviii

CHAPTER 1

FACILITY GROUND SYSTEM

1.1 GENERAL.

1.1.1 This handbook addresses the practical considerations for engineering of grounding systems, subsystems, and other components of ground networks. Electrical noise reduction is discussed as it relates to the proper installation of ground systems. Power distribution systems are covered to the degree necessary to understand the interrelationships between grounding, power distribution, and electrical noise reduction.

1.1.2 The information provided in this handbook primarily concerns grounding, bonding, and shielding of fixed plant telecommunications-electronics facilities; however, it also provides basic guidance in the grounding of deployed transportable communications/electronics equipment.

1.1.3 Grounding, bonding, and shielding are approached from a total system concept, which comprises four basic subsystems in accordance with current Department of Defense (DOD) guidance. These subsystems are as follows:

a.

b.

c.

d.

An earth electrode subsystem.

A

A

A lightning protection subsystem.

fault protection subsystem.

signal reference subsystem.

1.2 APPLICATION. This handbook provides technical information for the engineering and installation of military communications systems related to the background and practical aspects of installation practices applicable to grounding, bonding, and shielding. It also provides the latest concepts on communications systems grounding, bonding, and shielding installation practices as a reference for military communications installation personnel.

1.3 DEFINITIONS. A glossary of unique terms used in this handbook is provided in Appendix A. All other terms and definitions used in this handbook conform to those contained in Joint Chiefs of Staff Publication No.

1. (JCS Pub 1), FED-STD-1037, MIL-STD-463, and the Institute of Electrical and Electronics Engineers (IEEE) dictionary.

1.4 REFERENCED DOCUMENTS. Publications related to the subject material covered in the text of this handbook are listed in Appendix B. The list includes publications referenced in the text and those documents that generally pertain to subjects contained in the handbook but are not necessarily addressed specifically.

1-1

1.5 DESCRIPTION.

system must receive maintenance is aided

MIL-HDBK-419A

The ground system serves three primary functions which are listed below. A good ground periodic inspection and maintenance to retain its effectiveness. Continued or periodic through adequate design, choice of materials, and proper installation techniques to ensure that ground Subsystems resist deterioration or inadvertent destruction and thus require minimal repair to retain their effectiveness throughout the life of the facility.

a. Personnel safety. Personnel safety is provided by low-impedance grounding and bonding between equipment, metallic objects, piping, and other conductive objects, so that currents due to faults or lightning do not result in voltages sufficient to cause a shock hazard.

b. Equipment and facility protection. Equipment and facility protection is provided by low-impedance grounding and bonding between electrical services, protective devices, equipment, and other conductive objects, so that faults or lightning currents do not result in hazardous voltages within the facility. Also, the proper operation of overcurrent protective devices is frequently dependent upon low-impedance fault current paths.

c. Electrical noise reduction. Electrical noise reduction is accomplished on communication circuits by ensuring that (1) minimum voltage potentials exist between communications-electronics equipments, (2) the impedance between signal ground points throughout the facility to earth is minimal, and (3) that interference from noise sources is minimized.

1.5.1 Facility Ground System. All telecommunications and electronic facilities are inherently related to earth by capacitive coupling, accidental contact, and intentional connection. Therefore, ground must be looked at from a total system viewpoint, with various subsystems comprising the total facility ground system. The facility ground system forms a direct path of known low impedance between earth and the various power, communications, and other equipments that effectively extends in approximation of ground reference throughout the facility. The facility ground system is composed of an earth electrode subsystem, lightning protection subsystem, fault protection subsystem, and signal reference subsystem.

a. Earth electrode subsystem. The earth electrode subsystem consists of a network of earth electrode rods, plates, mats, or grids and their interconnecting conductors. The extensions into the building are used as the principal ground point for connection to equipment ground subsystems serving the facility. Ground reference is established by electrodes in the earth at the site or installation. The earth electrode subsystem includes the following: (1) a system of buried, driven rods interconnected with bare wire that normally form, a ring around the building; or (2) metallic pipe systems, i.e., water, gas, fuel, etc., that have no insulation joints;

or (3) a ground plane of horizontal buried wires. Metallic pipe systems shall not be used as the sole earth electrode subsystem. Resistance to ground should be obtained from the appropriate authority if available or determined by testing. For EMP considerations, see Chapter 10.

b. Lightning protection subsystem. The lightning protection subsystem provides a nondestructive path to ground for lightning energy contacting or induced in facility structures. To effectively protect a building, mast, tower, or similar self-supporting objects from lightning damage, an air terminal (lightning rod) of adequate mechanical strength and electrical conductivity to withstand the stroke impingement must he provided. An air terminal will intercept coverings of the structure, and prevent it the discharge to keep it from penetrating the nonconductive outer from passing through devices likely to be damaged or destroyed. A

1-2 low-impedance path from the air terminal to earth must also be provided. These requirements are met by either (1) an integral system of air terminals, roof conductors, and down conductors securely interconnected to provide the shortest practicable path to earth; or (2) a separately mounted shielding system, such as a metal mast or wires (which act as air terminals) and down conductors to the earth electrode subsystem.

c. Fault protection subsystem. The fault protection subsystem ensures that personnel are protected from shock hazard and equipment is protected from damage or destruction resulting from faults that may develop in the electrical system. It includes deliberately engineered grounding conductors (green wires) which are provided throughout the power distribution system to afford electrical paths of sufficient capacity, so that protective devices such as fuses and circuit breakers installed in the phase or hot leads can operate promptly.

If at all possible the equipment fault protection conductors should be physically separate from signal reference grounds except at the earth electrode subsystem. The equipment fault protection subsystem provides grounding of conduits for signal conductors and all other structural metallic elements as well as the cabinets or racks of equipment.

d. Signal reference subsystem. The signal reference subsystem establishes a common reference for

C-E equipments, thereby also minimizing voltage differences between equipments. This in turn reduces the current flow between equipments and also minimizes or eliminates noise voltages on signal paths or circuits.

Within a piece of equipment, the signal reference subsystem may be a bus bar or conductor that serves as a reference for some or all of the signal circuits in the equipment. Between equipments, the signal reference subsystem will be a network consisting of a number of interconnected conductors. Whether serving a collection of circuits within an equipment or serving several equipments within a facility, the signal reference network will in the vast majority of cases be a multiple point/ equipotential plane but could also, in some cases, be a single point depending on the equipment design, the facility, and the frequencies involved.

1.5.2 Grounding and Power Distribution Systems. For safety reasons, both the MIL-STD-188-124A and the

National Electrical Code (NEC) require the electrical power systems and equipments be intentionally grounded;

therefore, the facility ground system is directly affected by the proper installation and maintenance of the power distribution systems. The intentional grounding of electrical power systems minimizes the magnitude and duration of overvoltage on an electrical circuit, thereby reducing the probability of personnel injury, insulation failure, or fire and consequent system, equipment, or building damage.

a. Alternating currents in the facility ground system are primarily caused as a result of improper ac wiring, simple mistakes in the ac power distribution system installation, or as a result of power faults. To provide the desired safety to personnel and reduce equipment damage, all 3-phase wye wiring to either fixed or transportable communication facilities shall be accomplished by the 5-wire or conductor distribution system consisting of three phase or “hot” leads, one neutral lead and one grounding (green) conductor. A single building receiving power from a single source requires the ac neutral be grounded to the earth electrode subsystem on the source side of the first service disconnect or service entrance panel as well to a ground terminal at the power source (transformer, generator, etc.). This neutral shall not be grounded at any point within the building or on the load side of the service entrance panel. The grounding of all C-E equipment within the building is accomplished via the grounding (green) conductor which is bonded to the neutral bus in the source side of the service entrance panel and, in turn, grounded to the earth electrode subsystem. In addition to the three phase or "hot" leads and the neutral (grounded) conductor, a fifth wire is employed to interconnect the facility earth electrode subsystem with the ground terminal at the power source.

1-3

To eliminate or reduce undesired noise or hum, multiple facilities supplied from a single source shall ground the neutral only at the power source and not to the earth electrode subsystem at the service entrance point. Care should be taken to ensure the neutral is not grounded on the load side of the first disconnect service or at any point within the building. The grounding (green) conductor in this case is not bonded to the neutral bus in the service disconnect panel. It is, however, bonded to the facility earth electrode subsystem at the service entrance panel. The fifth wire shall be employed to interconnect the earth electrode subsystem with the ground terminal at the power source.

The secondary power distribution wiring for a 240 volt single phase system consists of two phase or “hot” leads, a neutral (grounded) and a grounding (green) conductor while the three conductor secondary power distribution system is comprised of one phase, one neutral, and one grounding lead. In both cases, the neutral shall not be grounded on the load side of the first service disconnect. It shall, however, be grounded to the ground terminal at the power source and to the earth electrode subsystem if one power source supplies power only to a single building.

The ac wiring sequence (phase, neutral, and equipment fault protection) must be correct all the way from the main incoming ac power source to the last ac load, with no reversals between leads and no interconnection between neutral and ground leads. Multiple ac neutral grounds and reversals between the ac neutral and the fault protection subsystem will generally result in ac currents in all ground conductors to varying degrees. The

NEC recognizes and allows the removal or relocation of grounds on the green wire which cause circulating currents. (Paragraph 250-21(b) of the NEC refers.) Alternating current line filters also cause seine aC currents in the ground system when distributed in various areas of the facility, this is due to some ac current passing through capacitors in the ac line filters when the lines are filtered to ground. Power line filters should not induce more than 30 milliamperes of current to the fault protection subsystem.

b. Dc power equipment has been found to be a significant electrical noise source that can be minimized through proper configuration of the facility, the physical and electrical isolation of the dc power equipment from communications equipment, and filtering of the output. Certain communications equipment with inverter or switching type power supplies also cause electrical noise on the dc supply leads and the ac input power leads.

This noise can be minimized by the use of decentralizing filters at or in the equipment. The location, number, and termination of the dc reference ground leads are also important elements in providing adequate protection for dc systems and, at the same time, minimizing electrical noise and dc currents in the ground system.

1.5.3 Electrical Noise in Communications Systems. Interference-causing signals are associated with time-varying, repetitive electromagnetic fields and are directly related to rates of change of currents with time. A current-changing source generates either periodic signals, impulse signals, or a signal that varies randomly with time. To cause interference, a potentially interfering signal must be transferred from the point of generation to the location of the susceptible device. The transfer of noise may occur over one or several paths. There are several modes of signal transfer (i.e., radiation, conduction, and inductive and capacitive coupling).

1-4

1.6 BONDING, SHIELDING, AND GROUNDING RELATIONSHIP.

a. The simple grounding of elements of a communications facility is only one of several measures necessary to achieve a desired level of protection and electrical noise suppression. To provide a low-impedance path for (1) the flow of ac electrical current to/from the equipment and (2) the achievement of an effective grounding system, various conductors, electrodes, equipment, and other metallic objects must be joined or bonded together. Each of these bonds should be made so that the mechanical and electrical properties of the path are determined by the connected members and not by the interconnection junction. Further, the joint must maintain its properties over an extended period of time, to prevent progressive degradation of the degree of performance initially established by the interconnection. Bonding is concerned with those techniques and procedures necessary to achieve a mechanically strong, low-impedance interconnection between metal objects and to prevent the path thus established from subsequent deterioration through corrosion or mechanical looseness.

b. The ability of an electrical shield to drain off induced electrical charges and to carry sufficient out-of-phase current to cancel the effects of an interfering field is dependent upon the shielding material and the manner in which it is installed. Shielding of sensitive electrical circuits is an essential protective measure to obtain reliable operation in a cluttered electromagnetic environment. Solid, mesh, foil, or stranded coverings of lead, aluminum, copper, iron, and other metals are used in communications facilities, equipment, and conductors to obtain shielding. These shields are not fully effective unless proper bonding and grounding techniques are employed during installation. Shielding effectiveness of an equipment or subassembly enclosure depends upon such considerations as the frequency of the interfering signal, the characteristics of the shielding material, and the number and shapes of irregularities (openings) in the shield.

1.7 GROUNDING SAFETY PRACTICES.

a. It is essential that all personnel working with Cormmunications-Electronics (C-E) equipment and supporting systems and facilities strictly observe the rules, procedures, and precautions applicable to the safe installation, operation, and repair of equipment and facilities. All personnel must be constantly alert to the potential hazards and dangers presented and take all measures possible to reduce or eliminate accidents.

b. Safety precautions in the form of precisely worded and illustrated danger or warning signs shall be prominently posted in conspicuous places, to prevent personnel from making accidental contact with high-voltage sources such as power lines, antennas, power supplies, or other places where uninsulated contacts present the danger of electrical shock or short circuits. Signs shall also warn of the dangers of all forms of radiation hazards, acids, and chemical inhalation, plus all other potential sources of personnel danger. Power cutoff features built into the equipment must be used in strict adherence to the intended use.

c. During the installation of equipment, warning tags are used to note the existence of potential danger when individual circuits or stages are being checked out. The tags should contain appropriate information to alert all personnel of the dangers involved and specific restrictions as to the use of the equipment. The equipment being installed shall be appropriately tagged in accordance with the directives of the local safety officer, equipment manufacturer, or other responsible agent.

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d. Installation personnel, when working with equipment having high-voltage devices, must ensure that the devices are grounded and that the high-voltage circuits have been disconnected or turned off. Do not rely solely on the presence of interlock switches for protection from electrical shock.

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CHAPTER 2

EARTH ELECTRODE SUBSYSTEM

2.1 OBJECTIVES.

Earth grounding is defined as the process by which an electrical connection is made to the earth. The earth electrode subsystem is that network of interconnected rods, wires, pipes, or other configuration of metals which establishes electrical contact between the elements of the facility and the earth. This system should achieve the following objectives:

a. Provide a path to earth for the discharge of lightning strokes in a manner that protects the structure, its occupants, and the equipment inside.

b. Restrict the step-and-touch potential gradient in areas accessible to persons to a level below the hazardous threshold even under lightning discharge or power fault conditions.

c. Assist in the control of noise in signal and control circuits by minimizing voltage differentials between the signal reference subsystems of separate facilities.

2.1.1 Lightning Discharge. A lightning flash is characterized by one or more strokes with typical peak current amplitudes of 20 kA or higher. In the immediate vicinity of the point of entrance of the stroke current into the earth, hazardous voltage gradients can exist along the earth’s surface.