Attachment_10_-_Grounding_Standards.pdf

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Standard No. S24.809 December 01, 1989 (Revised 12/22/2000)

GROUNDING STANDARD

1.0 SCOPE

1.1 Scope. This document establishes standard electrical, electronic, and mechanical requirements for ground electronic equipment.

1.2 Application. The requirements of this standard shall apply to equipment developed or fabricated to Government specifications, including commercial production (original equipment manufacture (OEM)) items. When applied to OEM items, the requirement of this document shall serve as a basis of selection among comparably performing equipment.

1.3 Contracting Officer's Technical Representative. The Contracting Officer's Technical Representative (COTR) shall provide the final interpretation of any conflict between this standard and specific contract requirements.

1.4 Waivers. Any request for waiver of specific requirements of this standard shall be submitted in writing to the COTR and to the Contracting Officer. A request for waiver must include: a) identification of the paragraphs for which the waiver is requested; b) identification of the systems, equipment, or components for which the waiver is requested; and c) a discussion of rationale for granting the waiver, including impact on reliability, maintainability, schedule, and cost if the waiver is not granted.

2.0 APPLICABLE DOCUMENTS

2.1 Government documents. The following documents, of the issue in effect on the date of invitation for bid or request for proposal, form a part of this standard to the extent specified herein:

The requirements of the following NOAA/NESDIS standards shall apply to all hardware and software, as described in this document.

Standard No. S24.801 - Preparation of Operation and Maintenance Manuals.

Standard No. S24.802 - General Requirements for Ground Electronic Equipment

Standard No. S24.803 - Cable and Wire Identification.

Standard No. S24.804 - General Requirements for Training on Electronic and Computer Equipment

Standard No. S24.805 - Spare Parts ii

Standard No. S24.806 - Software Development, and Maintenance and User Documentation NOAA/NESDIS standards are available from: U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Environmental Satellite, Data, and Information Service, OSD3, Washington, D.C. 20233.

Military specifications and standards are available from: Commanding Officer, U.S. Naval Supply Center, 580l Tabor Avenue, Philadelphia, Pennsylvania 19120.

Federal Specifications and Standards are available from: The Superintendent of Documents, U.

S. Government Printing Office, Washington, D.C. 20402.

Federal Information Processing Standards Publications are available from: The Suprintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

ANSI Standards are available from: American National Standards Institute, Inc., 1430 Broadway, New York City, New York 10018.

NASA/GSFC specifications are available from: National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt Road, Greenbelt, Maryland 20771.

NFPA Standards are available from: National Fire Protection Association, Batterymarch Park, Quincy, Massachusetts 02269.

EIA Standards are available from: Electronics Industries Association, 2001 Eye Street, N.W., Washington, D.C. 20006.

The National Oceanic and Atmospheric Administration, National Environmental Satellite, Data, and Information Service (NOAA/NESDIS), operates satellite tracking and computer processing systems which require a comprehensive grounding standard to ensure personnel and equipment safety. This grounding standard provides equipment requirements, design and installation procedures, and test methods, suitable for all communications and data processing systems.

Specific requirements are defined for perimeter grounds, air terminals, down conductors, equipment rooms, antennas with integral electronic rooms, power grounds, and the interconnection of these subsystems throughout a widely spread site. The need for lightning and power fault protection, ground returns for signal and control cables, and radiation and interference suppression are addressed and integrated into a system for the entire facility.

Priorities for upgrading existing facilities are identified, post installation and maintenance test procedures are detailed, and guidelines are given for tracing and correcting problems that may arise during the upgrade process.

This document was developed by MITRE Corporation under NOAA/NESDIS Contract Number 50-DGNE-5-0089. It was reviewed and approved by a panel of twenty people concerned with the grounding of electronic equipment, from both within and outside of the NOAA/NESDIS organization, which was convened by NOAA/NESDIS on November 8, 1989.

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TABLE OF CONTENTS

SECTION PAGE

1 Introduction 1-1

1.1 Background 1-1

1.2 Scope 1-1

1.3 Document Organization 1-2

1.3.1 Section Organization 1-2

1.3.2 Verification Requirements 1-2

2 Reference Documents 2-1

2.1 Documents 2-1

2.2 Sources 2-1

2.3 Conflict Resolution 2-2

3 Basic Requirements 3-1

3.1 Application 3-1

3.2 Definition of Components Used in the Ground System 3-1

3.2.1 Vertical Ground Rods 3-1

3.2.2 Horizontal Ground Conductors 3-1

3.2.3 Perimeter Ground Cable 3-1

3.2.4 Ground Terminal Plate 3-2

3.2.5 Interconnections 3-2

3.2.6 Bonding 3-2

3.2.7 Air Terminals 3-3

3.2.8 Roof Conductors 3-3

3.2.9 Down Conductors 3-3

3.2.10 Chemical Treatment 3-3

3.2.11 Signal Reference Grid 3-4

3.2.12 Electrical Service Well 3-4

3.2.13 Cable Entrance Plate 3-5

3.3 Definition of Special Terms 3-5

3.3.1 Zone of Protection 3-5

3.3.2 Minimum Slope 3-6

3.3.3 Structure 3-6

3.3.4 Group of Structures 3-6

3.3.5 Distance Measurements 3-6

3.3.6 Conductor Sizes 3-6

3.3.7 Parallel Connections 3-7

4 Grounding System Composition 4-1 iv

4.1 Perimeter Counterpoise 4-1

4.1.1 Adjacent Structures 4-1

4.1.2 Resistance to True Ground 4-2

4.1.3 Achievement Methods 4-2

4.1.4 Selection Criteria 4-4

4.1.5 Structural Steel Columns and Other Building Metal 4-4

4.2 Ground Terminal Plate 4-5

4.2.1 Connection to the PCP 4-5

4.2.2 Connections to Other Cables 4-5

4.3 Lightning Ground System 4-5

4.3.1 Air Terminal Mounting 4-6

4.3.1.1 Roof Mounting 4-6

4.3.1.2 Towers and Masts 4-9

4.3.1.3 Antennas 4-9

4.3.1.4 Partial Protection 4-9

4.3.2 Roof Conductors 4-10

4.3.3 Down Conductors 4-10

4.3.3.1 Down Conductors for Building 4-11

4.3.3.2 Down Conductors for Towers and Masts 4-11

4.3.3.3 Down Conductors for Antennas 4-11

4.3.4 Waveguide, Coaxial and Shielded Cable Grounding 4-14

4.4 Equipment Room Ground System 4-15

4.4.1 Signal Reference Grid 4-15

4.4.2 Raised Floor as Reference Grid 4-15

4.4.3 Other Reference Grids 4-16

4.4.4 Spacing Variations 4-16

4.4.5 Entrance to Signal Reference Grid Area 4-17

4.4.6 Interconnection of Signal Reference Grids 4-17

4.4.7 Equipment Connections 4-19

4.4.8 Electronic Equipment Mounted in a Feed

Box or Vertex Box 4-19

4.4.9 Electronic Equipment Mounted Elsewhere

on an Antenna Structure 4-20

4.5 Power Ground System 4-21

4.5.1 Power Distribution Systems 4-21

4.5.2 Neutral Grounding 4-21

4.5.3 AC/DC Power System Grounds 4-21

4.5.4 Pipes, Tubes and Conduit 4-22

4.5.5 Cable Trays 4-22

4.5.6 Other Enclosures 4-23

4.5.7 Motors, Generators, Transformers, and Switchgear 4-23

5 Site Configurations 5-1

5.1 PCP Interconnections 5-1

5.2 Special Conditions for Irregular Buildings 5-2

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5.3 Underground Cables 5-2

5.3.1 Underground Pipes or Conduit 5-2

5.3.2 External Cable Troughs 5-3

5.3.3 Underground Cables with Shields 5-3

5.3.4 Buried Guard Wire 5-3

5.4 Grounding for Filters and Surge Arresters 5-4

5.5 Ground Return Paths 5-4

5.5.1 Signal and Control Return Paths 5-4

5.5.2 Power Distribution Ground Paths 5-4

5.6 Fence Grounding 5-5

5.6.1 Reinforcing Wire 5-5

5.6.2 Grounding 5-5

5.7 Permafrost Conditions 5-5

6 Implementation Guidelines 6-1

6.1 Priority 6-1

6.2 Ground System File Requirements 6-2

6.3 Contractor Documentation Requirements 6-3

7 Inspection and Test Procedures 7-1

7.1 Soil Resistivity Test 7-1

7.2 Grounding Resistance Test 7-1

7.3 Inspections 7-5

7.4 Test Verification Procedures 7-6

7.4.1 Inspection 7-6

7.4.2 Analysis 7-6

7.4.3 Demonstration 7-6

7.4.4 Test 7-7

7.4.5 Sample Test 7-7

8 Trouble Correction Guide 8-1

8.1 Conducted or Radiated Interference 8-1

8.1.1 Trace the Source of Interference 8-1

8.1.2 Characterize the Interfering Waveform 8-1

8.1.3 Determine the Coupling Mechanism 8-2

8.1.3.1 Conductive Coupling 8-2

8.1.3.2 Radiative Coupling 8-2

8.1.3.3 Inductive and Capacitive Coupling 8-2

8.1.4 Reduce the Interference 8-3

8.1.4.1 Conductive Coupling Reduction 8-3

8.1.4.2 Radiative Coupling Reduction 8-4

8.1.4.3 Inductive Coupling Reduction 8-4

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8.1.4.4 Capacitive Coupling Reduction 8-4

8.1.4.5 Transient Suppression Plate 8-5

8.2 Continued Lightning Damage 8-5

8.2.1 Interconnections 8-5

8.2.2 Desired Path Checks 8-5

8.2.3 Actual Path Checks 8-6

8.3 Multiple Shields 8-6

Appendix A Reference Equations and Procedures A-1

Appendix B List Of Abbreviations B-1

Distribution List DI-1 vii

LIST OF FIGURES

FIGURE PAGE

1 Example of Interconnections in a Group of Structures 4-3

2 Lightning Protection for Buildings 4-7

3 Lightning Protection for a Protrusion on a Flat Roof 4-8

4 Example Lightning Protection Layout for Large Building 4-12

5 Measurement of Soil Resistivity 7-2

6 Perimeter Counterpoise Resistance Test Setup 7-3

A1 Protection Zone For a Single Rod A-1

A2 The Low Point Between Lightning Rods A-2

A3 Protection Zone Between the Masts A-3

LIST OF TABLES

TABLE PAGE

1 Test Verification Table 7-8

A1 Properties of Standard Size Annealed Copper Wire A-5

SECTION 1

INTRODUCTION

1.1 BACKGROUND

The National Oceanic and Atmospheric Administration (NOAA), National Environmental Satellite, Data, and Information Service (NESDIS), operates and maintains several communication and data processing facilities, through which it provides a range of weather related information to users. Some of these facilities contain many large satellite tracking antennas, with related electronics, spread over many acres of land. All facilities contain computer and communication systems spread over several rooms. Over the years since Congress directed its formation in 1961, continued improvements in technology have required that NOAA/NESDIS obtain many additions and changes to the equipment used to fulfill it's mission.

This piece-by-piece acquisition of equipment has resulted in a grounding system at NOAA/NESDIS facilities that has also grown in a piece-by-piece fashion. This has resulted in varied installation practices and documentation procedures being applied to different parts of a facility and from one facility to another.

Recent lightning strikes at the Command and Data Acquisition (CDA) station at Wallops, Virginia, have highlighted the need for an overall grounding standard that would apply to all NOAA/NESDIS facilities to provide the maximum possible protection and safety for personnel and equipment.

1.2 SCOPE

This NOAA/NESDIS standard defines the requirements for the design, installation, modification, and testing of grounding systems at all NESDIS facilities. It applies to all facility construction and modification projects requiring the design or modification of, or the addition to, grounding systems within any NESDIS facility.

The grounding systems at existing facilities need not necessarily be modified for the sole purpose of complying with this standard. However, this document defines the ultimate grounding requirements to which all existing and any new NESDIS facilities must eventually comply. If problems exist at a facility which indicate a grounding system deficiency, if a normal facility upgrade includes work to the grounding system, or if new equipment is being installed, all work performed on the grounding system must comply with this standard, unless specific exemption is provided, in writing, by the Contracting Officer or his Technical Representative.

This document is based on the premise that safety is the prime requirement, for both personnel and equipment. No other requirement can be allowed to compromise it. Current technology, including fibre optics, balanced amplifiers and transmission lines, and waveguide theory as applied to ground planes, can provide solutions to interference problems that do not compromise safety.

1.3 DOCUMENT ORGANIZATION

1.3.1 Section Organization

The following sections of this document are organized in order of increasing complexity of the equipment and assemblies discussed in each section. Section 2 lists the references used in creating this document. Section 3 provides the requirements of the individual components to be used in the grounding systems and definitions of the special terms used herein. Section 4 shows how these components are to be assembled into the grounding subsystems and section 5 defines how those subsystems are to be interconnected throughout the facility. Section 6 provides guidelines for upgrading the existing facilities to comply with these requirements, section 7 defines the test procedures to be used after installation and to perform routine maintenance checks, and section 8 provides a trouble shooting guide to assist in handling problems that may arise during an upgrade process.

1.3.2 Verification Requirements

Many places in sections 3, 4 and 5 contain a number in brackets next to the word "shall". These are provided to identify the items for which some form of verification is required. Verification could take the form of inspection, analysis, demonstration, test, or sample test. Verification of individual requirements may be a part of the final acceptance tests or may be performed as a pre-test during the installation period, as identified in section 7.4. Each numbered shall is referenced in table 1, where the specific verification requirements for each are tabulated. Section 7.4 also provides more detailed descriptions of the verification requirements and defines the code letters used in table 1.

SECTION 2

REFERENCE DOCUMENTS

2.1 DOCUMENTS

The following documents provide theory, principles, and application steps and procedures in the areas of grounding, bonding, and shielding:

1. MIL-HDBK-419, Military Handbook dated 1-82: Grounding, Bonding, and

Shielding for Electronic Equipment and Facilities

2. MIL-STD-188-124A, Military Standard dated 2-84: Grounding, Bonding, and

Shielding for Common Long Haul/Tactical Communications Systems, including Ground Based Communications-Electronics Facilities and Equipments

3. Federal Information Processing Standards Publications (FIPS PUB) No. 94, National

Bureau of Standards dated 9-21-83: Guideline on Electrical Power for ADP Installations

4. National Fire Protection Association (NFPA) No. 70, National Electrical Code

5. NFPA No. 78, Lightning Protection Code

6. MIL-F-29046(TD), Flooring, Raised, General Specification For

Item 1 above is the primary reference for this document. Users requiring additional details or theory for the procedures and practices defined herein should consult that document, unless another document is specifically cited in this text. All the requirements in this standard are in agreement with reference 1, except for the following:

The spacing of air terminals on flat roofs, paragraph 1.3.2.1.2 of volume 2, is not self-consistent. This standard uses the equation derived for the spacing of air terminals and the coverage so provided, but not the specific spacing’s cited in parts "c" and "e" of the paragraph which are illustrated in figure 1-18.

2.2 SOURCES

Military Specifications, Standards, and Handbooks may be obtained from:

Naval Publications and Forms Center Attention Code 3064 5801 Tabor Ave.

Philadelphia, PA 19120-5099

Federal Information Processing Standards Publications (FIPS PUB) may be obtained from:

National Technical Information Service U.S. Department of Commerce Springfield, VA 22161

Publications of the National Fire Protection Association (NFPA) may be obtained from:

National Fire Protection Association, Inc.

Battery march Park Quincy, MA 02269

2.3 CONFLICT RESOLUTION

If any conflict is found between the requirements of this standard and the reference documents identified in section 2.1, the requirements of this standard take precedence. In the event of conflict between this document and any specification or other document into which it is incorporated by reference, the Contracting Officer's Technical Representative must decide the order of precedence.

SECTION 3

BASIC REQUIREMENTS

3.1 APPLICATION

A grounding system must be installed at each NESDIS facility that houses or is intended to house communications, computing, or other electronic equipment. All electrical and electronic systems must be attached to the grounding system with low impedance connections, in accordance with the requirements of the applicable paragraphs of this document. The ground system must also be connected to all metallic structures, provided they can be accessed in a reasonable manner, and without damage to their strength. This design standard is to be applied to all such facilities and systems.

3.2 DEFINITION OF COMPONENTS USED

The following paragraphs identify and define the component parts to be used in the grounding system.

3.2.1 Vertical Ground Rods

All ground rods shall [1] be copper clad steel and shall[2] be 3/4 inch or larger diameter, and with a length of 10 to 40 feet. They are to be driven vertically into undisturbed earth, or earth that has been compacted and settled. The tops of the ground rods shall [3] be between 1 and 2 feet below final grade level.

3.2.2 Horizontal Ground Conductors

Horizontal ground conductors shall[1] consist of No. 4/0 AWG bare copper cables, a minimum of 16 feet in length, and shall[2] be buried a minimum of two feet below final grade. They shall[3] be run perpendicular to an interconnecting cable, such as the perimeter ground cable, or radially from a single interconnection point. Horizontal ground conductors are permitted only when the use of vertical ground rods is not feasible due to geological conditions or other subterranean obstructions at the site.

3.2.3 Perimeter Ground Cable

The perimeter ground cable shall [1] be a No. 4/0 AWG bare copper cable, buried all the way around the perimeter of one or more structures and with its two ends fused together to completely encircle the structure(s). The actual route taken around a structure must be a compromise between the following needs:

1. To ensure that all precipitation soaks the soil around the cable, locate it 2 to 6 feet outside the drip line of the structure. This includes keeping to a minimum those parts of the cable that are routed under roads, walkways, or other areas covered by material water can not penetrate.

2. To minimize the impedance of the cable, eliminate as many bends as possible. When necessary, bends should be of the smallest angle and the largest radius that is practical.

3. To keep the cable length as short as possible.

The contractor shall [2] submit a diagram for the approval of the Contracting Officer's Technical Representative, showing the compromises proposed.

The perimeter ground cable shall [3] be buried between 1 and 2 feet below final grade level. It shall [4] be bonded to any incidental, buried, metal structures that are within 6 feet of the perimeter ground cable through a No. 1/0 AWG bare copper cable, which must be routed in the shortest possible path from the perimeter ground cable to the metal structure.

3.2.4 Ground Terminal Plate

The ground terminal plate shall [1] be solid copper and a minimum of 1/4 inch by 2 inches by 6 inches. This size shall [2] be increased, if necessary, to allow connection of all the cables that may be required in the specific application, plus at least 25 percent extra for future expansion.

Each plate shall [3] be installed in an electrical service well large enough to allow easy connection of additional cables and easy access for test purposes.

3.2.5 Interconnections

Unless otherwise specified in this document, all interconnections between the parts of the grounding system shall[1] be made by No. 4/0 AWG or larger, bare copper cable, buried between 1 and 2 feet below final grade level.

3.2.6 Bonding

Unless otherwise specified, all connections required by this standard shall [1] be made through a fusion process, such as cad-welds, brazing, or other process which causes the conductors to be melted and fused into a solid homogeneous mass. This is the only method that may be used to join dissimilar metals. Each such junction of dissimilar metals shall [2] be coated when necessary to prevent corrosion due to moisture.

When clamping is permitted, the connection shall[ 3] be easily accessible for inspection and test, by providing an electrical service well, if necessary, to allow access below grade level. If connection is required between a pipe carrying a flammable substance, such as oil or natural gas on which fusion can not safely be used, and the copper ground system; the pipe shall[4] be wrapped with a band of similar metal, that has been bonded previously to a copper cable, and clamped to the pipe. All clamped connections shall [5] be able to withstand a pull test of 200 lb minimum, except for clamps on waveguide. Conductive adhesive may also be used, provided the current capacity of the joint shall [6] be not less than the current capacity of the smallest of the conductors being joined.

3.2.7 Air Terminals

Air terminals shall[1] be made of solid copper or bronze, shall[2] be no less than 1/2 inch in diameter, a minimum of 2 feet in length, and extend at least 1 foot above the object being protected. Air terminals over 24 inches high shall [3] be rigidly supported at a point not less than one half their height.

3.2.8 Roof Conductors

Roof conductors shall [1] be routed around the perimeter of a flat roof or along any ridges on the roof. They shall [2] be No. 1/0 AWG copper cable, which shall [3] be insulated to prevent corrosion between dissimilar metals when installed on a metal roof. Roof conductors shall [4] be installed in a rectangular grid layout, inside the perimeter roof conductor, with a maximum spacing between any two parallel internal roof conductors, or between an internal roof conductor and a parallel portion of the perimeter roof conductor, equal to the spacing between the air terminals, along one axis of the rectangular grid, and three times the spacing between the air terminals, along the other axis of the rectangular grid. The roof conductors shall [5] be clamped or bonded at each crossover and to the perimeter roof conductor at each end and to the air terminal at that point. If necessary, due to a change in spacing between the air terminals for example, an internal roof conductor may be terminated in a bond to another internal roof conductor.

3.2.9 Down Conductors

The down conductors connect the roof conductors to the perimeter counterpoise. They shall[1] be No. 1/0 AWG copper cable, which shall[2] be insulated to prevent corrosion between dissimilar metals, if installed where contact is possible with metal siding or other metallic objects.

3.2.10 Chemical Treatment

When chemical treatment is required, magnesium sulphate, MgSO4, also known as epsom salts, shall [1] be used.

Chemical treatment of vertical ground rods is to be accomplished by digging a trench around the top of each vertical ground rod. The average dimensions for each trench shall [2] be an inner diameter of 18 inches, a depth of 12 inches and a width of 12 inches. A service well for inspection purposes shall [3] be placed around each ground rod treated with chemicals.

Chemical treatment of horizontal ground conductors is to be accomplished by digging a trench directly above each horizontal ground conductor. Each trench shall [4] average 6 inches deep, 1 foot wide, and 16 feet long. A service well shall [5] be placed at the center of the horizontal ground grid where the horizontal ground conductors are connected together to allow inspection of the conductors and the magnesium sulphate. The quantity of magnesium sulphate used shall [6] be sufficient to fill each trench at least 75 percent.

Users are cautioned that the introduction of large quantities of a chemical substance into the environment may require an impact statement. If necessary, copper sulphate, CuSO4, or calcium chloride, CaCl2, may be used provided their impact on the environment is less than that of the magnesium sulphate. Alternatively, a non-dispersive conductive gel may be used.

3.2.11 Signal Reference Grid

The signal reference grid is an approximation of an equipotential plane to provide a ground reference for all signal and control lines. A signal reference grid shall[1] be installed in all areas of any NESDIS facility containing more than two racks or cabinets of communications or computer equipment. The grid shall[2] consist of a No. 1/0 AWG bare copper cable routed around the perimeter of the area where the grid is required, with its two ends bonded together.

At intervals of not more than 2 feet, other conductors shall[3] be routed directly between opposite sides of the perimeter cable, fused to it, and connected to each other at each intersection. Non-metallic conduit shall[4] be used to route the cables through walls or partitions.

The signal reference grid may be located under a raised (computer) floor, in or on a floor, or suspended above the equipment. Under some conditions, a raised floor may be made part of the signal reference grid.

The 2 foot conductor spacing defined above provides an adequate equipotential plane for frequencies up to approximately 50 MHz, sine wave, or 10 Mbps, digital. For areas where higher frequencies are of interest, closer spacing may be necessary, i.e., so the size of the opening is equal to or less than one eighth of a wavelength at the highest frequency of interest.

However, due to the high relative difficulty in providing such closer spacing in an existing facility and the comparatively small area in which such frequencies are used, it is recommended that this not be done unless interference problems are shown to exist which involve such higher frequencies.

3.2.12 Electrical Service Well

An electrical service well is a hole in the ground to allow access to electrical connections for test and inspection as part of a long term maintenance plan. A well is required whenever a connection is made below grade level using a compression or clamping bond or as otherwise stated herein. The top of the well shall[1] be level with, or above grade, and the bottom at least 6 inches below the level of the object to be inspected. The sides may be round, square, hexagonal, or octagonal. It shall[2] be made of a non-conductive material, and shall[3] be able to support the stresses that may reasonably be expected to be placed on it, in its specific location. The well shall[4] have a removable cover of compatible strength. Cable entrance shall[5] be through holes in the wall of the well, large enough to accept a cable of at least 2 inches in diameter. Each well shall[6] have at least 8 such holes, or break-out areas, so cables may enter with minimum bending.

3.2.13 Cable Entrance Plate

The cable entrance plate is a metal plate, mounted in or on the wall of a building or antenna mounted equipment room, through which the waveguides, cables, metal pipes, and metal conduits which enter that building or equipment room must pass. When these items are run outside of equipment rooms, they will tend to pick-up voltages and currents from undesired sources. The purpose of the cable entrance plate is to keep these induced voltages or currents outside the equipment rooms, as much as is compatible with the construction style of the structure involved. If the construction style of the wall to be penetrated provides significant shielding of the internal space, the cable entrance plate shall[1] be made from a metal that is compatible with such shielding materials and be bonded to them, continuously if possible, all around the edges of the cable entrance plate. If the wall does not provide any significant shielding, the cable entrance plate is not required.

All waveguides, cables, metal pipes, and metal conduits that pass through a cable entrance plate shall[2] have their outer surfaces bonded to that plate, provided the outer surface is at a nominal ground potential. The bond shall[3] be continuous all around the perimeter of the item passing through the plate and shall[4] be made by welding, brazing, soldering, other exothermic process, by means of a feed-through connector, or as provided in section 4.3.4 of this standard. The procedure chosen for any specific case shall[5] be the one that does not damage the item passing through the plate and which will produce the longest lasting and lowest resistance bond for the particular materials to be bonded.

The cable entrance plate shall[6] be sized so that there is at least 6 inches of clearance between the edge of the plate and all of the items passing through it. The cable entrance plate may be part of a box, vault or other structure, used to facilitate installation and maintenance of cables where they enter a building.

3.3 DEFINITION OF SPECIAL TERMS

3.3.1 Zone of Protection

In this standard, the concept of a tall object protecting nearby, shorter objects from lightning discharges, is referred to as a zone of protection. Research has shown that the length (R) of the final step of the leader in a lightning strike (R) is generally 100 feet or more. It can therefore be shown that the zone of protection concept is valid only when the height of the "tall object" is equal to or less than R feet above the level of the earth's surface, or above an effective ground plane, which extends at least R feet out from the base of the tall object. Objects greater than 100 feet high will only provide a zone of protection equal to that of a 100 foot high object, if the final leader step is 100 feet. For the percentage of lightning strikes in which the final leader step is greater than 100 feet, a taller object will provide a correspondingly greater zone of protection, proportional to the actual leader step length. At this time, no data exists on the probability that a lightning strike may have a final leader length of any specified value greater than the minimum.

Therefore 100 feet is used throughout this standard.

The zone of protection provided by a single object, is defined as the space enclosed by a right circular cone with its axis coincident with the tall object and its apex at the top of the object. The sloping surface of the cone is formed by an arc with a radius of 100 feet, tangent to the earth's surface at one end, and passing through the top of the object at the other end. Where two or more protecting objects are used, the same concept is applied, however, in this instance, both ends of the arc defining the protected volume must terminate at the top ends of the protecting objects, or at the 100 foot point, whichever is lower. The basic equations derived from this concept are given in appendix A and a full explanation, including the derivation of these equations is published in MITRE Technical Report No. MTR-89W00206 entitled Calculating Lightning Protection Zones.

3.3.2 Minimum Slope

The term minimum slope is defined as the angle between the edge of the cone defined in 3.3.1 and a vertical line at the point where the cone touches the tip of the air terminal, mast, etc. If the height of the protecting object is H, and H is less than 100 feet, this slope is given by:

sin-1((100 - H)/100)

This provides a simple, conservative estimate of the zone of protection given by a single air terminal on its own. The derivation of this equation will also be given in the above report.

3.3.3 Structure

In this standard, a structure is any object which supports or contains equipment used by NESDIS for computing or communicating.

3.3.4 Group of Structures

A group of structures is any two or more structures that are located so the straight line distance of closest approach to each other is 20 feet or less, or when one or more structures is within, or partly within, the zone of protection of another.

3.3.5 Distance Measurements

Unless otherwise stated, all distances given in this document are to be measured along the route a cable takes or is intended to take, not the straight line distance.

3.3.6 Conductor Sizes

Whenever a conductor size is specified in this document, it means that it is the minimum size acceptable for the specific application. A larger size conductor is always acceptable.

3.3.7 Parallel Connections

Many of the requirements in this standard specify the installation of cables or wires that create parallel conducting paths, from item to item, and structure to structure, at a site. Unless otherwise stated herein, combining the requirements of several paragraphs into one physical wire or cable is not permitted. The needs of each individual requirement must be met by a unique conductor. For example, the required connections for the power ground system may not "share" the connections required for the equipment room ground system, nor may the requirement to connect the power system ground at a remote structure to the power system ground at the primary structure, "share" any conductor with the requirement to interconnect the perimeter counterpoise of each structure.

SECTION 4

GROUNDING SYSTEM COMPOSITION

The grounding system at each NESDIS facility must consist of one or more of each of the following parts:

1. Perimeter counterpoise

2. Ground terminal plate

3. Lightning ground system

4. Equipment room ground system

5. Power ground system

6. Interconnections between these ground systems

4.1 PERIMETER COUNTERPOISE

The perimeter counterpoise (PCP) shall[1] consist of a perimeter ground cable and two or more vertical ground rods. The ground rods shall[2] be evenly spaced along the perimeter ground cable at intervals of between 1.5 and 2 times their length. The tops of the ground rods shall[3] be directly under the perimeter ground cable and shall[4] be bonded directly to it. The ground rods shall[5] be long enough to maintain contact with damp or moist soil year round, or else a waiver must be obtained.

4.1.1 Adjacent Structures

Structures within 20 feet of each other at their nearest points shall[1] share the same PCP, preferably routed around the combined perimeter of the structures without any internal cross-connections. However, such internal connections are not prohibited, provided the outer-most cable meets the requirements of this standard. It is therefore not necessary to remove any of an existing PCP when it is extended to include a new adjacent structure.

Structures separated by more than 20 feet, but with one inside the zone of protection of another, may be included within the same PCP. Alternatively, they may each have their own PCP, joined to each other by two interconnecting cables. If more than two structures exist with overlapping zones of protection, each PCP shall[2] be joined to all adjacent PCP's.

The requirements for interconnecting the PCP's are depicted in figure 1 and defined as follows:

1. The points of connection on the smaller of the two structures shall[3] be at the extreme ends of that section of its PCP that is closest to the larger structure. If the direct path from such an end point is obstructed, the connection may be made at an internal point where a more direct routing is possible. See note 3 of figure 1.

2. The points at which the joining cables are attached to each PCP shall[4] be separated by at least the distance equal to the spacing between ground rods, or, if the spacing between the rods is greater than the length of the side closest to the other structure, the connection points shall[5] be within 1 foot of each corner of the PCP.

3. The cable shall[6] be routed in a straight line over the shortest possible distance, unless an increase of 10 percent or less in the cable length will allow the connection to be made to a ground rod, instead of the perimeter ground cable.

4. If the PCP of one structure is rounded, or at an angle to the other structure, the minimum length connection is only possible at the single point of closest approach.

In this case, the two points of attachment shall[7] be separated from each other by a minimum of 6 feet. This separation shall[8] be increased until the incremental increase in the separation is less than the incremental increase in the length of the connecting cable.

5. If structure A must be joined to structure B because of an overlapping cone of protection and Structure A must be joined to structure C because of other connecting cables, then structure B shall[9] be joined to structure C.

6. Structures separated by distances greater than defined above must be treated as separate structures or groups.

4.1.2 Resistance to True Ground

The resistance between the PCP for any structure, or group of structures, at a site, and true ground shall[1] be 5.0 ohm or less. This resistance must be measured, for each individual PCP in isolation, using the test procedures provided in paragraph 7.2.

4.1.3 Achievement Methods

The required resistance between a PCP and true ground may be achieved by the following methods, in order of preference:

1. Use rods longer than the 10 foot minimum, but not exceeding 40 feet in length.

2. Use the vertical rods at the minimum spacing.

3. Add chemical treatment to the rods.

4. Add vertical rods, outside the basic PCP, at the minimum spacing.

5. Use horizontal ground conductors, as well as, or instead of, the vertical ground rods.

6. Add chemical treatment to the horizontal ground conductors.

4.1.4 Selection Criteria

The following factors should be considered when determining the method to be used to achieve the required resistance to true ground.

1. The ground rod should make contact with the water table throughout the year.

2. At least 50 percent of the ground rod should be below the frost line.

3. Underground rock formations, if present, may limit the depth to which the rods can be driven.

4. If no other alternative provides the required resistance to true ground, the ground rod spacing may be either or both wider or narrower than the standard spacing.

4.1.5 Structural Steel Columns and Other Building Metal

When the PCP dissipates the energy in a lightning strike, all the conductors connected to it are raised to some high voltage, due to the finite impedance of the grounding system. Any item connected between one of these conductors and any other conductor normally at ground potential, but not connected to the same PCP, will have this high voltage applied across it. This will cause a hazard to both personnel and equipment.

The procedure that provides the greatest safety is for all structural steel, and any other conducting items in the structure, such as water pipes, ducts, etc., and to conducting items buried in the vicinity of the PCP, be made a part of the ground system by connecting it to the PCP. If this preferred procedure is followed, any metallic object that is at a nominal ground potential and comes within 10 feet of any PCP or the cables which interconnect them, shall[1] be connected to that PCP or interconnect cable. In an old or shared building, it is often difficult to be sure of the routing and interconnections made by all the metallic objects in and around the structure. This could result in some objects not being connected to the PCP due to their existence being unknown, a lack of access without interrupting operations, etc. Every effort should be made to determine the safest action in any specific set of conditions. If this preferred procedure is not followed, the reasons shall[2] be documented and approved by the COTR.

When it is decided to make these interconnections, the cable used shall[3] be a No. 1/0 AWG bare copper cable, [4] bonded at each end, and [5] routed between the metal object and the PCP in the shortest possible manner.

4.2 GROUND TERMINAL PLATE

A ground terminal plate must be clamped to the PCP wherever needed to allow connections to other parts of the ground system that must be removed for test purposes.

4.2.1 Connection to the PCP

When required, this connection shall[1] be made by clamping the plate directly to a ground rod, or by clamping the plate to a short length of No. 4/0 bare copper cable, which is bonded to the ground rod. The cable shall[2] be the minimum length necessary for access to disconnect and visually inspect the parts. That rod and the ground terminal plate shall[3] be in an electrical service well large enough to allow easy inspection and test, as well as the necessary access to make the connections to the ground terminal plate.

4.2.2 Connections to Other Cables

At facilities with many structures, e.g., the CDA stations, it is necessary to interconnect the PCP's of the various structures, or groups of structures, and also to be able to isolate them from each other, so that maintenance measurements may be made. All such connections to the PCP's must be made via one or more ground terminal plates.

At these sites, all down conductors shall[1] be bonded directly to the PCP, except if a down conductor is also an interconnect to a signal reference as required by subsection 4.4.6, item 4a.

All other cables that are routed to the PCP shall[2] be connected via the ground terminal plate.

These cables shall[3] be routed to a common ground terminal plate, when the additional length of cable required, above the shortest, most direct route, is not more than 10 percent of its length. If this routing would increase the length of any cable by more than 10 percent, an additional ground terminal plate shall[4] be installed. The location of all ground terminal plates shall[5] be such that the total number required is minimized.

At facilities with only one structure, or group of structures, e.g., Federal Building 4 at Suitland MD, no such disconnect is required and all connections may be made by bonding directly to the

PCP.

4.3 LIGHTNING GROUND SYSTEM

A lightning ground system must be installed on each structure that is not entirely within the zone of protection of another structure. Structures that are partly within such a zone of protection may still require some parts of the lightning ground system, as designated herein. The lightning ground system must provide as direct as possible a path to ground for all lightning induced currents.

The lightning ground system consists of air terminals, roof conductors, and down conductors.

The requirements that define how these components are incorporated into the system are given in the following paragraphs and illustrated with various examples in figures 1, 2 and 3.

4.3.1 Air Terminal Mounting

When not within the zone of protection of another structure, air terminals shall[1] be permanently and rigidly attached to the roof or upper walls of a building, or at the highest point(s) of a tower or antenna, so that the tips of the air terminals are at the desired height, or more, above the structure, or any other objects mounted on the top of such structure.

Mounting to structural metal is permitted, provided all structural metal and all conductive items within 6 feet of any structural metal are bonded together to form part of the lightning protection system and, for antennas, that any bearings are bridged by suitable jumpers, as defined in subsection 4.3.3.3. Air terminals shall[2] be mounted so they do not interfere with the operation of antennas or lighting fixtures, but protrude above all objects by the designed amount.

Examples of acceptable mounts for buildings are shown in figure 3.10-1 of NFPA 78, Lightning Protection Code.

If meeting all the requirements of section 4.3 would cause interference to, or degradation of, the normal operation of any equipment at a facility, an additional structure shall[3] be built to provide the required lightning protection without the undesired effects.

4.3.1.1 Roof Mounting

The preferred method of attachment to buildings is to use lag bolts into masonry or concrete.

Through bolts are also permitted. Either method must have at least 6 feet of clearance between all the conductive parts of the attachment device and any other conductive material that is not part of the lightning protection system.

For sloping roofs, air terminals shall[1] be placed as close as possible to, and in any case within 2 feet of, each end, or each corner, of the peak of the roof. This is the minimum requirement. The air terminal spacing along the peak of the roof shall[2] be such that the maximum distance between them is no greater than 28 feet if the air terminals project 1 foot above the level of the roof peak, 40 feet for a 2 foot projection, or 48 feet for a 3 foot projection. This will provide sufficient protection, provided the slope, from the tip of the air terminals to the building edge, including any protrusions (e.g., dormer windows, chimney, vent pipe), is less than the minimum slope. If it is greater than the minimum slope, the more accurate calculation given below must be made, and the locations modified to comply with the results.

On flat roofed structures, air terminals shall[3] be mounted at each corner, along each edge of the roof, and in a square pattern over the rest of the roof. Air terminals at the edge and corners of the roof shall[4] be mounted as close as possible to the outermost edge or corner, and in any case, within 2 feet of that edge or corner. The maximum length of the side of any such square pattern shall[5] be 20 feet if the air terminals project 1 foot above the level of the roof, 28 feet for a 2 foot projection, or 34 feet for a 3 foot projection. The minimum requirement shall[6] be two air terminals at diagonally opposite corners, projecting 1 foot above the roof line or anything mounted on the roof. This is adequate provided no wall of the structure is greater than 14 feet long. Figures 2 and 3 show some general examples of these requirements.

The above requirements are specific instances of the following general equation, which shall[7] be used whenever necessary to determine the protection range of any air terminal, provided only that H does not exceed 100 feet.

S = (200H - H2)1/2 Where:

S = The protection range, i.e. the horizontal distance from an air terminal to the point where protection is desired. This is normally the upper surface, such as the roof of a building, or the highest point of an antenna or support pole where protection is required from a direct lightning strike.

H = The height of the top of the air terminal above the roof or the highest surface on the protruding object.

If a square pattern of air terminals does not economically cover the roof area, a rectangular pattern may be used, provided the above equation is used to ensure the center of the rectangles are protected by the air terminal height and spacing to be used.

When there are other items on the roof which project above the roof surface, such as a stair well entrance, a chimney, or an air conditioner heat exchanger, the tops of these items shall[8] be the reference plane above which the air terminals must protrude, as defined above. The additional height that these air terminals are above the main roof surface may enable them to protect a larger area of the roof, adjacent to the roof mounted items. The protected area is defined by the above equation, applied to each surface in turn.

4.3.1.2 Towers and Masts

Air terminals on towers or masts shall[1] be mounted directly to the structural metal, or shall[2] be bonded to it by a No. 6 AWG bare copper cable in the shortest possible distance. Only one air terminal and one down conductor is required per tower or mast, unless otherwise recommended by the manufacturer, or unless more are needed to protect equipment mounted on the tower or mast. If the structural metal of the tower or mast is not used as the down conductor, as provided in subsection 4.3.3.2, the down conductor and the cable bonding the air terminal to the structural metal shall[3] be one continuous conductor.

4.3.1.3 Antennas

Air terminals shall[1] be mounted so that the tip of at least one air terminal is the highest object on the antenna, for any direction in which the antenna can be oriented.

4.3.1.4 Partial Protection

If one structure is partly, but not completely within the zone of protection of another structure, the need for some of the air terminals may be eliminated on the partly protected structure. An air terminal may be eliminated only if the area of protection it would have provided is totally covered by other air terminals. If any air terminals are required on a structure, the interconnecting roof and down conductor must still be provided, as specified below.

4.3.2 Roof Conductors

A continuous roof conductor shall[1] be routed around the perimeter of a flat roof and along the ridge on a ridged roof. There shall[2] be at least two current paths from each air terminal to ground. Any internal air terminals shall[3] be connected together by other roof conductors in a grid layout, with each end of the internal roof conductors bonded to the perimeter roof conductor. The maximum size of the grid shall[4] be equal to the spacing of the air terminals, in one dimension, and three times the air terminal spacing in the other dimension. The conductors shall[5] be bonded or clamped at all crossover points.

As a last resort, isolated air terminals may be connected to the grid, or a down conductor, by a dead end cable no more than 16 feet long. This should only be necessary on structures with an irregular outline, and such usage must be kept to the minimum possible.

All the conductors shall[6] be run in as straight a line as possible, on the surface of the roof.

There shall[7] be no turns of more than 90o, nor with less than an 8 inch radius at any point in any roof conductor run. If it is necessary to meet this requirement, the roof conductors shall[8] be run through walls, cornices, parapets, etc., using a non-conductive, non-flammable duct. All roof conductors shall[9] be securely fastened to the object on which they are placed, at intervals of not more then 5 feet. The fasteners may be attached by bolts, screws, nails, or adhesive and shall[10] withstand a 200 lb pull without loosening.