Volume_7-_FS_Lighting_amd_Grounding_Standards.doc

Volume 7 – Lightning Protection And Grounding Standards

USDA Forest Service

Chief Information Office

Radio System Technical Standards

VOLUME 7

LIGHTNING PROTECTION AND GROUNDING STANDARDS

Prepared for USDA Forest Service

FINAL Version 2.0.2 13 February 2008

Prepared by:

Radio Modernization Program Team

DOCUMENT HISTORY

RELEASE

LABEL

Approved by RMP Team 14 February 2008 RMP Team:

Brad Mitchell, Team Lead

Debra Marohn, Tech Writer Dave Johnson

Don J. Schnee

Duane Kierstead Gary B. Paustian Jim Byrd

Kary Mavencamp

Mark Hooper

Tommie J. Cluff

REviewers

TABLE OF CONTENTS

11.0 Introduction

22.0 Industry Standards

33.0 Grounding Guidelines

33.1 Performance Objectives

33.1.1 25 Ohms

33.1.2 5 Ohms

33.1.3 “Best Attainable”

43.2 External Protective System

43.2.1 Air Terminals

43.2.2 Main System Conductors

63.3 Earth Grounding System

73.2.3 Ground Radials

73.2.4 Ground Electrodes

93.2.5 Ground Enhancement Materials

93.3 Antenna Tower and Transmission Line Protection

93.3.1 Antenna Tower

103.3.2 Antennas

103.3.3 External Ground Buses

123.3.4 Single Point of Entry

123.4 Internal Protective System

123.4.1 Internal Conductors

153.4.2 Main Ground Bus

163.4.3 Connections

163.5 Power Service Protection

163.5.1 DC (Photovoltaic) Service

173.5.2 AC (Commercial) Service

183.5.3 Power Back-up Systems

183.6 Miscellaneous Protection

183.6.1 Fuel Tanks

193.6.2 Fences, Mounting Frames, Other External Objects

193.6.3 Stoves, Metal Cabinets, Other Internal Objects

204.0 System Performance Testing

204.1 Clamp-On Meter Method

204.2 Fall-Of-Potential Method

21Appendix A—Lightning Protection Concepts

23Appendix B—Soil Resistivity Measurements

24Appendix C—System Performance Testing (Clamp-On Meter Method)

28Appendix D—System Performance Testing (Fall-Of-Potential Method)

30Appendix E—Glossary of Terms

34Appendix F—References

Section 1

1.0 Introduction

Volume 7 establishes technical standards and uniform direction for service area technicians and radio system installation teams that install grounding systems for Forest Service radio communications equipment enclosures, including but not limited to structures, equipment housing, and lookout facilities.

It must be recognized that certain conditions such as a direct lightning strike cannot be prevented. It is the intent of this volume to provide uniform standards for installing equipment in order to minimize and possibly eliminate any damage that may result from such phenomena. The degree of lightning protection implemented at an electronics site will inversely affect the amount of damage sustained during lightning strikes or power surges.

The implementation of these modernization standards will minimize induced noise and static in radio communications equipment enclosures and provide a high level of personnel and radio system lightning protection, thereby providing effective lightning protection systems and reliable radio communications systems across the Forest Service.

Section 2

2.0 Industry Standards

The following general guidelines from industry standards as well as existing Forest Service standards have been adopted by the Information Solutions Organization:

1. The Forest Service will use the Motorola R56 Standards and Guidelines for Communications Sites, where applicable to the Forest Service, and when practical to do so. Not every item from the R56 manual applies to Forest Service needs. The R56 manual was originally intended for use by Motorola employees and contractors, although Motorola recognizes that the manual is used by system designers and technicians outside the company (R56, 1-1). The R56 standards were derived from specifications written by standards organizations including the National Fire Protection Association (NFPA), Underwriters Laboratories (UL), and the Occupational Safety and Health Administration (OSHA). Private grounding and lightning protection companies, such as Lyncole Industries, have provided input to the standards. Private companies may be consulted to aid the Forest Service in ground system design, planning, and testing.

2. In addition to the Motorola R56 standards, Forest Service guidelines were derived from National Electrical Code (NFPA 70) and National Fire Protection Association (NFPA 780). It is recommended that all personnel responsible for the installation, testing, and maintenance of Forest Service grounding and lightning protection systems reference the Motorola R56, NEC, and NFPA 780 standards for more detail on the subjects contained in this document.

Section 3

3.0 Grounding Guidelines

This section contains general guidelines and requirements for Forest Service communications site electrode grounding systems.

3.1 Performance Objectives

Conductivity-to-ground is the primary goal for an effective grounding system. “Zero ohms” resistance can be considered the ultimate performance objective; however, a more realistic performance rule-of-thumb for Forest Service grounding systems is “best attainable”. The Forest Service grounding system performance objectives are as follows:

3.1.1 25 Ohms

25 Ohms is considered the standard level of protection. This is the recommended objective for all “Type A” radio sites (R56, 6.2.1). It is also the minimum objective for the National Electrical Code (NEC Article 250). Type A radio sites are those sites that, in the event of failure, will not critically disable the main radio system. In well designed systems that allow for coverage overlap, a stand-alone repeater site typically falls into this classification. A system test will be performed to check actual system resistance (see Section 4, 2).

3.1.2 5 Ohms

5 Ohms is considered the high level of protection. This is the recommended objective for all “Type B” radio sites (R56, 6.2.2). Type B radio sites are those sites that, in the event of failure, will critically disable the main radio system. Radio systems must be designed to avoid a critical single point of failure. However, where such conditions do exist, the highest level of lightning protection possible must be applied—especially in lightning-prone areas (see Appendix A). For Type B sites, a soil resistivity measurement (see Appendix B) will be performed and the results applied to the ground system design plan for the Type B site.

3.1.3 “Best Attainable”

The “rule-of-thumb” for all Forest Service radio sites is to achieve the best attainable level of protection. Adherence to the grounding guidelines herein will ensure that this objective is met.

3.2 External Protective System

3.2.1 Air Terminals

Air Terminals will be ½ inch, solid copper. The tip of the air terminal will not be less than 10 inches above the object or area it is to protect. Tip may be sharp or blunt. On flat roofs, air terminals will be placed on each corner. On pitched roofs, air terminals will be placed at the ridge ends. If separation between terminals exceeds 50 feet, additional terminals are required (NFPA 780, Section 4.8). Air terminals are to interconnect with the main system conductors with exothermic (e.g., CadweldTM or ThermaweldTM) or high-compression connectors.

3.2.2 Main System Conductors

3.2.2.1 Upper Ground Ring (Rooftop)

For all runs not exceeding 75 feet, rooftop conductors will be, at a minimum, #2 American Wire Gauge (AWG) stranded or solid copper. For all runs exceeding 75 feet, rooftop conductors will be, at a minimum, #2/0 AWG copper. Roof conductors will be coursed along ridges of gable, gambrel, and hip roofs; around the perimeter of flat roofs; behind or on top of parapets; and across flat or gently sloping roof areas as required (NFPA 780, Section 4.9.7). Upper ground conductors are to interconnect with air terminals and down conductors with exothermic or high-compression connectors. No bend will have a radius less than 8 inches or turn less than 90 degrees. See Figure 3-1:

FIGURE 3-1. MINIMUM BENDING RADIUS FOR GROUNDING CONDUCTORS

3.2.2.2 Down Conductors

For all runs not exceeding 75 feet, down conductors will be, at a minimum, #2 AWG stranded or solid copper. For all runs exceeding 75 feet, down conductors will be, at a minimum, #2/0 AWG copper. Down conductors will run vertically along the outer corners of the building and remain flush with the building/shelter until they are below grade, when they will sweep outward and connect with the buried ground ring. Down conductors will interconnect with rooftop and buried ground rings with exothermic or high-compression connectors. No sweep or bend will have a radius less than 8 inches or turn less than 90 degrees.

3.2.2.3 Buried Ground Ring

Ground ring conductors will be buried at least 18 inches below ground level whenever possible. In conditions where such levels are not possible, ring conductors will be buried as deep as conditions will permit. Ground enhancement material is used to encase the ground ring on all sides before back-filling with native soil. For all runs not exceeding 75 feet, ground ring conductors will be, at a minimum, #2 AWG stranded or solid copper. For all runs exceeding 75 feet, ground ring conductors will be, at a minimum, #2/0 AWG. A ground ring will encircle the main communications building/shelter, outside the drip line of the roof. A second ring will encircle the antenna tower whenever possible. If the tower is less than 6 feet of building/shelter, then the building ground ring may be extended to enclose the tower to form a single ground ring. No turn will have a radius less than 8 inches or bend less than 90 degrees.

The external grounding system may consist of, but is not limited to, the components shown in Figure 3-2:

FIGURE 3-2. TYPICAL TYPE B EXTERNAL GROUNDING ELECTRODE SYSTEM

3.3 Earth Grounding System

Many Forest Service sites are located on high mountaintops, where soil conditions are exceptionally poor for providing sufficient lightning protection. For stone mountaintops, the following procedures are recommended (R56, 6.4.11.6):

· Consult an engineering firm specializing in grounding electrode system design.

· Use down conductors (radials) to a lower area where there is usable soil.

See Figure 3-3.

· Install the standard design system as required for a dedicated communications site with components buried as deep as the soil will allow and encase all components with a ground enhancing material.

· Install ground plates instead of ground rods and encase the plates with ground enhancing material.

· Install tower radial extensions and/or radial extensions from the building throughout the property. Install radials and rods as specified in this document or to a depth allowed by the soil.

· Install horizontal electrolytic rods.

· Use concrete encased electrodes as part of the building construction whenever possible.

· Install copper strap radials on the surface of the rocks in all directions from the tower. The copper straps may be covered with top soil and/or ground enhancing material. Each copper strap radial will be a different length to prevent ringing of the tower during a lightning strike.

· The earth-grounding component of the lightning protection system will consist of ground radials (if needed) and grounding electrodes.

FIGURE 3-3. EXAMPLE OF SOILLESS GROUND SYSTEM

3.2.3 Ground Radials

For all runs not exceeding 75 feet, ground radial conductors will be, at a minimum, #2 AWG stranded or solid copper. For all runs exceeding 75 feet, ground radial conductors will be, at a minimum, #2/0 AWG. Radial conductors will interconnect with down conductors and/or ground rings with exothermic or high-compression connectors.

3.2.4 Ground Electrodes

Grounding electrodes are placed into the earth to maintain electrical equipment at the potential of the earth and to dissipate over-voltages into the earth. Grounding electrodes may be ground rods, metal plates, concrete encased conductors (Ufer grounds), ground rings, electrolytic (XITTM) ground rods, the metal frame of a building or structure, or metal underground pipes (NFPA 70, Article 250 (c) and NFPA 780, Section 3).

· Ground rods are the first choice of ground electrodes and will be a minimum of 8 feet in length and 5/8 inches in diameter, solid copper or copper clad steel. Under extreme rocky soil conditions, they may be buried at an angle no greater than 45 degrees from vertical. They will never be any closer to one another than two times the rod length, as at closer distances they tend to act as a single ground electrode. See Figure 3-4:

FIGURE 3-4. TYPICAL SINGLE GROUND ROD INSTALLATION

· Electrolytic Ground Rods, such as XITTM rods, are recommended for “Type B” sites where the performance objective is 5 Ohms and soil conditions allow. Electrolytic ground rods are self-contained with electrolytic material that seeps through weep-holes in the casing. These rods improve conductivity over time. XITTM rods are standard 8 feet in length and are available in straight or L-shaped configurations. See Figure 3-5:

FIGURE 3-5. ELECTROLYTIC GROUND RODS

· Metal grounding plates will be used when soil conditions render the use of ground rods impractical. Plates will be 24 inch by 24 inch, .032 inch thick copper. A ground plate will expose not less than 2 feet of the plate below the earth’s surface. The plate will be completely encased in ground enhancement material before being covered with native soil (NFPA 780, Section 3-13.1.5).

3.2.5 Ground Enhancement Materials

Ground enhancement material will be used to improve the grounding electrode system and to reduce corrosive effects from very acidic soil conditions. Material will be used that is packaged specifically for the purpose of ground enhancement, such as BentoniteTM or GEMTM. This material will be mixed with water like concrete and will be used in accordance with the manufacturer’s recommendations.

3.3 Antenna Tower and Transmission Line Protection

3.3.1 Antenna Tower

· Antenna masts and metal structures will be grounded according to industry standards (NFPA 70, Articles 810-15 and 810-21, ANSI T1.313-1997, and ANSI/EIA/TIA 222-f). The tower ground ring will be bonded to the building ground ring with at least two conductors of minimum #2 AWG stranded or solid copper. All connections to the tower will be exothermically bonded or as required by manufacturer’s recommendations.

· Antenna masts will be encircled by a ground ring containing at least 3 equally-spaced ground rods. The optimum of 16 feet (using 8 foot rods) requires a tower ground ring with a diameter of at least 18 feet (R56, 6.4.5.1).

· Each leg of a self-supporting tower will be exothermically bonded to the ground ring using minimum #2 AWG stranded or solid copper. See Figure 3-5:

FIGURE 3-5. SELF-SUPPORTING TOWER LEG GROUNDING

· Guyed towers will be bonded to the ground ring using a minimum of 3 equally-spaced conductors using minimum #2 AWG stranded or solid copper, exothermically welded to each leg of the tower or as recommended by the manufacturer. Ground rods will be installed at each guy anchor point (R56, 6.4.5.1). Unless encased in concrete, the anchor itself may serve as the ground rod. If the anchor is encased in concrete, ground rods will be placed at least 2 feet away. Each guy wire must be bonded to the anchor ground rod using stainless steel clamps. See Figure 3-6:

FIGURE 3-6. TOWER GUY WIRE GROUNDING

3.3.2 Antennas

Antennas are considered to be at ground potential when they are connected to grounded radio equipment. If positioned at a high level, they tend to act as strike termination devices or lightning rods. To avoid this characteristic, antennas will be positioned so that they fall inside the “zone of protection” (see Appendix A, Figure A-1). The tower itself or a well-placed air terminal is a more suitable strike termination point than an antenna. To help prevent antenna strikes, the technician will follow proper isolation procedures of the power ground per Section 3.6 and to avoid tower reflection, the technician will try placing the antenna lower on the tower and use stand-offs.

3.3.3 External Ground Buses

The high-frequency characteristic of lightning causes the current to flow along the outer portion of the transmission line—the so-called “skin effect”. Without proper protection, a considerable amount of current will flow directly through the radio equipment. The objective of lightning protection is to redirect this current to earth ground before it enters the building, as follows:

· Use tower ground bus (TGB) and antenna line ground kit per industry standards (R56, 6.4.6). Antenna line ground kits will be installed per manufacturer’s specifications.

· Mount external ground bus (EGB) on building with standoffs just below the single point of entry (bulkhead panel). The EGB serves as a common connection point for multiple grounding conductors outside the building. The EGB will be connected to the external ground ring using a minimum #2 AWG stranded or solid copper conductor. Because copper straps offer less impedance than #2 AWG cable, solid copper straps (minimum 26 gauge, 2–3 inches wide) may also be added to further enhance conductivity to the ground ring. See Figures 3-7 and 3-8.

FIGURE 3-7. LOCATION OF TRANSMISSION LINE GROUNDING KITS

FIGURE 3-8. TRANSMISSION LINE GROUNDING AT BUILDING ENTRY POINT

3.3.4 Single Point of Entry

A single point of entry (bulkhead) will be used for all antenna feed lines as well as power, internal ground conductors, telephone and other cables. Antenna feed lines will be protected using antenna line surge suppression devices (SSDs), such as the Polyphaser model IS-12VDC or IS-24VDC. Antenna line surge suppressors must be installed per manufacturer’s specifications and grounded to the main protective system to be effective. Entry bulkhead panel may serve as the main ground bus (MGB) if designed to do so (see Section 3.5.1).

3.4 Internal Protective System

3.4.1 Internal Conductors

3.4.1.1 Basic Internal Ground Conductor Layout

An internal ground conductor (internal perimeter conductor) of minimum #6 AWG green jacketed stranded copper will be used to follow the inside perimeter of the shelter at, or just below the ceiling level, to serve as a convenient connection point for all equipment racks, ancillary, and power systems. The internal perimeter ground conductor and other internal ground conductors will be connected to the MGB. Sub-systems connect into respective sub-system ground bus (SSGB) before being connected to the MGB. Connections to the MGB will be made with compression lugs or exothermic connection. All internal conductors will maintain a bend radius of greater than 8 inches and less than 90 degrees. See Figure 3-9.

FIGURE 3-9. TYPICAL SINGLE-POINT INTERNAL GROUNDING SYSTEM

3.4.1.2 Radio Equipment Connections

Individual conductor runs of minimum #6 AWG green jacketed copper will be used to bond each equipment rack to internal ground ring. It is recommended that equipment racks be insulated from concrete floor to avoid ground loops (R56, 11.6.1). If racks are to be bolted to the concrete floor, follow insulated mounting procedure per R56, 11.6.1.3. Do not allow equipment racks to touch. Do not connect the racks together with cable. Each radio equipment chassis should have independent runs to a rack ground bus (RGB) located along one vertical edge of the rack or at the top of the rack. Never “daisy chain” the radio chassis (or other metallic objects) together. See Figure 3-10.

FIGURE 3-10. TYPICAL INSTALLATION OF RGB

BONDED TO GROUND BUS CONDUCTOR

3.4.2 Main Ground Bus

The MGB will serve as the single ground point within the communications shelter, equipment building, or room to which all communications equipment, ancillary support equipment, antenna transmission lines, transmission line surge suppression devices (SSDs), and utility grounds are connected. The MGB will be 2 inch by 12 inch by 1/4 inch solid copper, mounted within 2 feet of the single point of entry into the shelter. The MGB will interconnect to the external ground system using minimum #2 AWG stranded or solid copper. See Figure 3-11.

FIGURE 3-11. EXAMPLE OF MASTER GROUND BAR

CONNECTION CONFIGURATION

The MGB will be visually divided into four sections from left to right, with sections labeled P, A, N, and I, from left to right. Connections to the MGB will be made to the appropriate section, as described below. Making connections to the MGB in this manner ensures that any surges introduced onto the MGB are disseminated to ground as directly as possible to minimize any part of that surge from reaching the equipment.

Section P stands for Producers, meaning these connections have the potential for introducing a surge onto the ground bus. Examples of these types of connections are lightning arrestor grounds, cable grounds, radio equipment rack grounds, and grounds from back up generators.

Section A stands for Absorbers, meaning any connection in this section will provide a path for absorbing any surge on the bus. Examples of these types of connections are connection to the external ground electrode system, and connection to water pipes.

Section N stands for Non-Isolated Equipment Frame grounds. Examples of these connections are non-radio equipment racks, battery cases/racks, and solar controllers.

Section I stands for Isolated Equipment Frame grounds such as digital phone switches and other sensitive equipment. This concept is described in more detail in NFPA 70, Article 800, and the Rural Utilities Service Bulletin 1751F-810, Sectio n 4.

3.4.3 Connections

3.4.3.1 Exothermic Bonding

Exothermic bonding (e.g., CadweldTM or ThermaweldTM) will be used on all external and internal connections whenever possible. Only personnel trained in exothermic welding may attempt this bonding method. Manufacturer’s warnings and safety requirements must be followed and heavy clothing, work shoes or boots, gloves, and safety glasses must be worn when performing exothermic welding. Exothermic welding will not be performed unless another person capable of rendering first aid is present. A suitable fire extinguisher will be close by with an attendant during the process.

3.4.3.2 High Compression Fittings

High compression fittings may be used as a secondary alternative to exothermic bonding. The technician will follow tool manufacturer’s specifications for proper fittings and applications for use.

3.4.3.3 Mechanical Fittings

Mechanical fittings are typically used to bond guy wires, propane tanks, internal equipment and metal objects that are impractical by other means or that need to be easily removed from the site. If mechanical connections are used to attach grounding conductors to bus bars, the cable lug will be connected to the cable with exothermic or high-compression fittings. Mechanical fittings will be torqued to manufacturer’s specifications.

3.5 Power Service Protection

3.5.1 DC (Photovoltaic) Service

The technician will follow the following procedures for DC (photovoltaic) service:

· Bond the metal frame of solar panel array to nearest the conductor of the exterior ground protection system using minimum #2 AWG stranded or solid copper conductor. Use a solar panel lightning protection unit (LPU), such as Specialty Concepts, Inc. model LPU-50-10, which uses a gas discharge arrester and metal oxide varistors (MOVs) to route lightning current to ground instead of through equipment or batteries. DC power distribution systems should be protected with Silicon Avalanche Suppressor Diodes (SASDs) because of their fast response time (1-5ns); Transtector CB Series DC circuit breakers utilize SASD protection.

· Bond metal-clad batteries, such as GNB, to the internal perimeter ground conductor using a minimum #6 AWG stranded green jacketed copper conductor. Insulate all batteries from direct contact with concrete flooring whenever possible. Isolated DC power systems should operate with a “floating ground”. The negative terminal of the battery array should not be connected to the lightning protection system.

3.5.2 AC (Commercial) Service

The technician will follow the following procedures for AC (commercial) service:

· Bond the commercial AC neutral of the service feed to the lightning protection system. The bond may be made at either the service panel neutral (inside the building) or at the meter panel (outside the building).

· The AC Service will be installed per manufacturer’s specifications using a suitable AC power surge protection device that utilizes SASDs, such as an Apex Imax 120V AC surge protector, between equipment and service panel.

· Because of the difficulty in isolating the ground system from the AC power service without powering down the site, use the clamp-on meter method when testing the ground resistance of an AC powered site. See Appendix C for the relative bonding of the AC neutral in order to place the clamp-on meter at the correct point.

3.5.3 Power Back-up Systems

The technician will follow the following procedures for power back-up systems:

· Bond the outside generator, within 6 feet of building/shelter, to the external ground ring using a minimum #2 AWG stranded or solid copper conductor. If the generator is further than 6 feet from building/shelter, the generator will require a separate ground electrode. See Figure 3-12:

FIGURE 3-12. EXTERNAL GENERATOR GROUNDING

· Bond inside generators to the internal perimeter ground ring per Section 3.7, 3.

3.6 Miscellaneous Protection

3.6.1 Fuel Tanks

The technician will ground propane fuel tanks to the building ground ring or ground radial if passing within 4 feet of the tank, using a mechanical connection to one leg or mounting bolt to minimum #2 AWG stranded or solid copper extended outward away from building. Surface will be cleaned and prepared for a metal-to-metal contact. A suitable conductive lubricant will be used and torqued to manufacturer’s specifications. The metallic fuel line will not be bonded or considered as a part of the lightning protection system.

3.6.2 Fences, Mounting Frames, Other External Objects

The technician will ground fences and other large metallic objects to the external ground system using a single #2 AWG stranded or solid copper conductor. Connection will be made using exothermic or high-compression fittings in the manner shown in Figure 3-13:

FIGURE 3-13. FENCE AND GATE GROUNDING METHOD

3.6.3 Stoves, Metal Cabinets, Other Internal Objects

The technician will ground all stoves, cabinets, and other large metallic objects to the internal perimeter conductor using exothermic or high-compression fittings, where possible. Any paint will be scraped off and surface will be prepared for clean, metal-to-metal contact. Minimum #6 AWG stranded green jacketed copper conductors will be used. As a rule, end-runs will be made to all metallic objects to avoid ground loops.

Section 4

4.0 System Performance Testing

The technician will perform ground system performance tests using one of two common methods:

4.1 Clamp-On Meter Method

The clamp-on meter method is recommended for 25 Ohm, standard performance sites and/or sites where isolation from AC power is difficult. The clamp-on meter test is relatively simple to perform and can be done without disabling the radio equipment, but is subject to error and somewhat less reliable than the 3-point fall-of-potential method. See Appendix C.

4.2 Fall-Of-Potential Method

The fall-of-potential method is recommended for high-performance (5 Ohm) sites and DC powered sites with isolated ground systems. The 3-point fall-of-potential method (not to be confused with the 4-point soil resistivity measurement) is a bit more difficult, but more reliable than the clamp-on test. See Appendix D.

Appendix A

Lightning Protection Concepts

The zone of protection can be best demonstrated by the rolling sphere model. The concept is that an imaginary rolling sphere with a 150-foot radius moving over a structural area represents all the places that lightning will strike in that area. Every place the sphere could come into contact represents a strike spot; everywhere inside the strike spots represent the zone of protection. Air terminals and strike termination devices are connected to the lightning protection conductors, which route the majority of the strike current directly into the earth-grounding electrodes and not the equipment. Well-designed lightning protection systems place protected equipment inside the areas within the zone of protection. See Figure A-1.

Figure A-1 ZONE OF PROTECTION DEPICTING ROLLING SPHERE

To get a relative idea as to the lightning risks for a given geographic area in the lower 48 states, the lightning flash density map shows the average number of lightning flashes per square kilometer per year during a 10-year period from 1989–1998. See Figure A-2.

FIGURE A-2. 1989–1998 AVERAGE U.S. LIGHTNING FLASH DENSITY FLASHES PER SQUARE KILOMETER PER YEAR

Appendix B

Soil Resistivity Measurements

Soil resistivity is a value of both resistance and a cubic area of soil. It is given in ohm-meters or ohm-centimeters and symbolized by “”. By decreasing the resistance or increasing the area of a volume of soil one can achieve an overall lower resistivity. The lower the value of , the better.

The soil resistivity test should be performed on Class B communications sites requiring a high performance (5 Ohm) ground system resistance. The soil resistivity found for the site is entered into a calculation to determine the depth, number and relative separation of ground electrodes to help achieve a 5 Ohm performance objective. The test is performed with a 4-terminal ground resistance tester using the Wenner 4-point method per R56, 4.3. (This Forest Service guideline does not provide the procedure for soil testing.) For more information, Biddle Instrument’s “Getting Down To Earth” manual for soil resistivity measurement is a recommended reference guide. The same ground resistance tester can be used to perform the “fall-of-potential” method of ground system resistance measurement. See Appendix D.

Appendix C

System Performance Testing (Clamp-On Meter Method)

The clamp-on meter ground test is an alternative to the more complicated fall-of-potential method. The clamp-on test is best suited for sites where a true isolated ground is difficult to obtain (commercially powered AC) and/or where equipment needs to remain powered up. Following proper procedures and measuring at correct location will produce accurate results. If a reading of a few ohms or less is obtained, a ground loop point is most likely being measured.

For measurement of single grounding electrode systems, place the clamp-on meter at the point where all legs of the electrode system are measured at the same time so the meter only sees one electrode and is not measuring a ground loop. See Figure C-1.

FIGURE C-1. CORRECT OHMETER

For multi-bonded/multi-grounding electrode systems and neutral-ground, bond is made at the service panel and then the clamp-on meter measurement is made at this point. See Figure C-2.

FIGURE C-2. CLAMP-ON OHMETER PLACEMENT FOR MULTI-BONDED/

MULTI-GROUNDING ELECTRODE SYSTEM (NEUTRAL-GROUND BOND AT THE SERVICE PANEL)

If the neutral-ground bond is made at the meter panel, then the clamp-on meter measurement must be made outside the building at that point. See Figure C-3.

FIGURE C-3. CLAMP-ON OHMETER PLACEMENT MULTI-BONDED/

MULTI-GROUNDED ELECTRODE SYSTEM (NEUTRAL-GROUND BOND AT

METER BOX)

Appendix D

System Performance Testing (Fall-Of-Potential Method)

In order to determine the ground resistance of a lightning protection system, it is necessary to remove it from any other ground connection. This can prove to be a virtually impossible task, necessitating certain assumptions. In reality, ground resistance-measuring equipment works at low frequencies relative to the lightning discharge. The resistance it computes is therefore often affected by the resistance of power-system ground electrodes or a similar ground medium that can be several thousand feet from the structure being protected. The ground resistance to be used to calculate lightning conductor potentials when a high-frequency lightning discharge strikes a building must be based on the grounds in the immediate area of the building—not the remote ones that ground measuring equipment probably monitors.

If the building is small and the lightning protection system can be disconnected totally from any other grounding network, its resistance can be measured by the three-point technique described below. If the building is large or cannot be disconnected totally from any other grounding network, then the ground resistance of the individually isolated lightning protection rods should be measured by the three-point technique described below and this resistance multiplied by a factor depending on the number of ground rods.

The principle of ground resistance measurement is shown below in Figure D-1. L is the lightning ground rod or ground rod system, P is a test probe, and A is an auxiliary current probe. M is the standard AC measuring equipment for three-point technique ground resistance measurements. Convenient distances for LP and LA are 23 meters (75 feet) and 36 meters (120 feet), respectively. In general, P should be at 62 percent of the distance from L to A. If 36 meters (120 feet) is not convenient, it could be increased significantly [or reduced to no less than 15 meters (50 feet)], provided LP is increased proportionately.

FIGURE D-1. MEASUREMENT OF GROUND RESISTANCE

A current, I, is passed through the electrode or electrodes to be tested, L, and through an auxiliary probe, A. The distance, LA, is long compared to the electrode length. The voltage, V, between L and P is measured by the test equipment, which also monitors I and calculates the ground resistance, R, as V/I. Alternating current is used to avoid errors due to electrolytic factors in the soil and to remove effects due to stray currents.

Three-point ground resistance-measuring equipment using these principles is relatively inexpensive and allows direct reading of R.

Variations in soil resistivity due to temperature and moisture fluctuations can affect ground resistance measurements.

If the building ground is complex in nature, the resistance of single ground rods can be measured and certain assumptions made. The average single ground rod resistance, Rm, must be multiplied by a factor depending on the number of lightning protection rods, n, spaced at least 10.7 meters (35 feet) apart.

The total system ground resistance, R, can be calculated from the formula:

Where:

R = total system ground resistance

Rm = average single ground rod resistance n = number of lightning protection ground rods

Note: A test junction, where test points L, P, and A appear, can be installed at the site for testing convenience and consistency.

Appendix E

Glossary of Terms

Air Terminal – A strike termination device that acts as a receptor for routing lightning flashes to the lightning protection system. Also called a “lightning rod”.

American Wire Gauge (AWG) – A standard that is used to describe the size of a wire. The larger the number, the smaller the wire. See also Conductor.

Bonding – An electrical connection between an electrically conductive object and a component of a lightning protection system that is intended to significantly reduce potential differences created by lightning currents.

Building – For the purpose of communications site planning, a permanent structure capable of regular human occupancy built on a foundation that contains communications equipment and related ancillary support systems, and may contain other unrelated equipment and/or facilities.

Bus Bar – The metal bar used to carry electrical currents to supply electrical circuits or to divert such currents to ground. See also Main Ground Bar (MGB).

Cable – A conductor formed of a number of wires stranded together.

CadweldTM – A brand of exothermic weld.

Compression Lug – A grounding connector made by using a specifically made, high compression tool for connection of a ground conductor to a bus bar.

Concrete Encased Electrode – A grounding system comprised of reinforcing rods that are contained in the concrete walls and footings. Electrical connections are made directly to the metal reinforcing bars. See also Ufer Ground.

Conductor – A substance that offers little resistance to electrical current flow. Copper wire is the most common form of conductor.

Driven Rod – A metallic stake used for electrical grounding applications. Usually made of copper clad steel, but can be made of solid copper or galvanized steel.

Electrolytic Ground Rod – A grounding electrode made of hollow pipe with earth salts that are environmentally friendly to the earth. See also XITTM Rod.

Exothermic Weld – A permanent electrical connection using powdered metallurgy technology that creates molecular bonds between conductors. See also CadweldTM and ThermoweldTM.

External Ground Bus Bar (EGB) – A ground bus bar that provides a bonding point for multiple grounding conductors (such as all coaxial connections) and connection to the grounding electrode system.

Grounded – Connected to the earth or some conducting body that serves in place of the earth. Connection could be intentional or unintentional.

Ground Electrode – Also called “ground terminal”. A driven rod, electrolytic rod or copper plate which serves as the primary connection point between an electrical or lightning protection system and the earth.

Ground Ring – A buried length of conductor, usually around a structure or tower. Sometimes called a “counterpoise”.

Halo – Another name for “internal perimeter conductor”, which is a ring conductor installed within a structure, just below the ceiling to facilitate the interconnection of equipment.

Lightning Protection System – A complete system of strike termination devices, conductors, ground terminals, interconnecting conductors, surge suppression devices, and other connectors or fittings required to complete the system.

Main Ground Bar (MGB) – A metal bar, usually mounted in a dominant area of a structure, to which all other bus bars, grounding electrodes, and grounding conductors are interconnected or bonded. See also Bus Bar.

Mechanical Clamp – A device used to secure a conductor to a grounding element, usually consisting of an oval of metal with a bolt that is mechanically tightened onto the conductor. Mechanical clamps are not considered very reliable, as the clamp can become corroded or come loose over time.

Metal Oxide Varistor (MOV) – The most common type of varistor used to prevent circuits against excessive transient voltages.

Motorola R56 Quality Standards – A manual published by Motorola, Inc. which provides Motorola’s internal guidelines for the installation of communications equipment, infrastructure, and facilities.

National Electrical Code (NEC) – A governing code in the United States that contains safety guidelines for all types of electrical installations. It is updated every three years.

National Fire Protection Association (NFPA) – An organization that publishes standards such as the National Electrical Code (NEC) (NFPA 70) and the Lightning Protection Standards (NFPA 780).

Rack Ground Bus Bar (RGB) – A single bus bar to which all equipment ground wires are bonded within a single equipment rack.

Shelter – A permanent structure built on a foundation that contains communications equipment and related ancillary support systems, and may contain other unrelated equipment and/or facilities. A shelter will be suitable for permanent or temporary human occupancy during equipment installation, maintenance and use.

Side Flash – An electrical spark caused by differences of potential, which occur between conductive metal bodies or between conductive metal bodies and a component of the lightning protection system or ground.

Silicon Avalanche Suppressor Diode (SASD) – A fast-acting surge protector which features a narrow voltage clamping range.

Spark Gap – Any short air space between two conductors that are electrically insulated from or remotely electrically connected to each other.

Strike Termination Device – A component of a lightning protection system that intercepts lightning flashes and connects them to a path to ground. Strike termination devices including air terminals, metal masts, permanent metal parts of structures, and overhead ground wires are installed in lightning protection systems.

Striking Distance – The distance over which the final breakdown of the lighting stroke occurs.

Sub-System Ground Bus Bar (SSGB) – A single bus bar to which all equipment ground wires are bonded within a single room or portion of a communication site.

Surge Protection Device (SPD) – A device that will protect equipment from damage caused by surges, spikes, and other over-voltages. See also Transient Voltage Surge Suppression (TVSS).

ThermoweldTM – A brand of exothermic weld.

Tower Ground Bus Bar (TGBB) – A bus bar located at the bottom of a communications tower to which the tower’s down conductors and the transmission line’s ground kits are bonded. The tower ground bus bar is grounded to the external site grounding ring.

Transient Voltage Surge Protector (TVSS) – A surge protective device listed for connection on the load side of the main over-current protection in circuits not exceeding 600 volts rms. See also Surge Protection Device (SPD).

Ufer Ground – A grounding system comprised of reinforcing rods that are contained in the concrete walls and footings. Electrical connections are made directly to the metal reinforcing bars. See also Concrete Encased Electrode.

Underwriter’s Laboratory (UL) – A non-profit organization that was established by the insurance industry to test devices, materials, and systems for safety. If an item passes UL set standards, the device is marked as “UL Approved”.

XITTM Rod – A brand of electrolytic ground rod. See also Electrolytic Ground Rod.

Zone of Protection – The space adjacent to a lightning protection system that is substantially immune to direct lightning flashes.

Appendix F—References

Publication: “Standards and Guidelines for Communications Sites”, Motorola, Inc., PN 68P81089E50-A, version 3/1/00 – UP.

Publication: “NFPA 780 Standard for the Installation of Lightning Protection Systems”, National Fire Protection Association, 2004 edition.

Publication: “NFPA 70 National Electrical Code”, National Fire Protection Association, 1999 and 2001 editions.

Training documentation: “Grounding System Design and Testing”, Lyncole Industries, March 2006.

Internal documentation: “United States Department of Agriculture Rural Utilities Service Bulletin 1751F-810”.

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