A Guide for Proper Earth Grounding Procedures for use with Tactical Systems

8/10/2019 A Guide for Proper Earth Grounding Procedures for use with Tactical Systems

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CECOM TR-96-2

Earth Grounding Pamphlet A Guide to Proper Earth Grounding Methods and

Procedures fo r use with Tactical Systems.

. . . r \^- ^ v .-

Andrew Burbelo John M . Tobias CECOM Safety Office

M ay 1996

DISTRIBUTION S TAT E M E N T Approved fo r public release;

distribution is unlimited.

CECOM U.S. Army Communications-Electronics Command CECOM Safety Office ATTN: AMSEL-SF-SEP Fort Monmouth, New Jersey 7703-5024


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NOTICES

Disclaimers

The findings i n this report are no t to be construed as an official Department of the Army position, unless so designated by other authorized documents.

The citation of trade names and names of manufacturers i n this report is no t to be construed as official Government endorsement or approval of commercial

products or services referenced herein.


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REPORT DOCUMENTATION PAGE ftvm Approved

OMB No . 704-0188

_ ,jn ^TIT^ ii# o o r p e r rwpome. i n c l u d i n g the time tor reviewing i n s t r u c t i o n sarching e x i s t i n g o*"**^"' 2w 2SSS2j7n?SSSnq coH*ci.on o t ,ntormK,n. er^eommerm reg.rt.ng m burteneWf or .mo the r *9*tnenna a n d m ,frL ,n _>tl_ ."^^;^_ . ..^ ,* *..,,*. n wMmnM MMdiiutmn S e r v i c e i . Directorate for Information O p e r a t i o n s a n d Reoom. 2 1 5 Jettenon Z~?SZ^Jvjaoi om to. twuana om O u r O n t . to wawtinc . Moiarm StrvKt. Oirtnoro for Inform* Opwioom n d JMoro. 21 S Jrftmon lZX I w ?,Sg5i MMTiS a OH Of M,^Sm .no Bodo. P.Pefwo MUM. P T O K (OTWnre). WMtngum. D C 20.

1. AGENCY US E ONLY Xeaw *r>W 2. R E P O RT DATE

May 996

4. ITLE AND SUBTITLE

EARTH G R O U N D I N G PA M P H L E T

3. R E P O RT TYPE ND D AT E S C O V E R E D

Technical Report

6. A U T H O R f S )

Andrew Burbe lo a nd J o h n Tob ia s

7. PERFORMING O R G A N I Z AT I O N N A M E ( S ) A N D ADDRESS(ES)

U S Army Communica t ions -E lec t ron ic s C o m m a n d CECOM) C E C O M S a f e t y Off i ce ATTN: M S E L - S F - S E P Fort M o n m o u t h , N 7 7 0 3 - 5 0 2 4

9. S P O N S O R I N G /MONITORING AGENCY NAME(S) A ND A D D R E S S E S )

11 . SUPPLEMENTARY N O T E S

5. UNDING N U M B E R S

8. P E R F O R M I N G ORGANIZATION R E P O RT N U M B E R

CECOM-TR-96-2

10. SPONSORING/MONITORING A G E N C Y R E P O RT NUMBER

12a . DISTRIBUTION/AVAILABILITY STATEMENT

Approved or ublic elease; istribution s nlimited.

i?h. nisTpmirno CODE

13 . ABSTRACT Maximum 200 words

Thi s p a m p h l e t i s a gu ide t o prope r ea r th ground ing m e t h o d s a nd procedure s fo r u s e wi th t a c t i ca l sys t ems . t desc r ibes d i ff e ren t ea r th ground ing sys t ems a nd prov ides g u i d a n c e on th e prope r methods fo r t h e i r i n s t a l l a t i o n . a r th ground ing he lps t o pro tec t pe r sonne l a n d equ ipmen t f rom e l ec t r i ca l f a u l t s , power su rges , a nd o the r su rges a nd t r a n s i e n t s . ar th ground ing a l s o he lps r educe c i r c u i t n o i s e a nd o the r t r a n s m i s s i o n i n t e r f e r ence t ha t c a n degrade c o m m u n i c a t i o n s - e l e c t r o n i c s sys t em pe r fo rmance . h e i n s t r u c t i o n s con ta ined i n t h i s p a m p h l e t , no t of t en f o u n d i n t e chn ica l m a n u a l s , w i l l he lp i n se t t i ng u p e f f e c t i v e a nd sa fe e a r t h ground ing s y s t e m s fo r t a c t i c a l equ ipmen t a n d she l t e r s .

14 . UBJECT T E R M S

Grounding; electrical grounding; lightning; lightning protection; urges; azards; oils

17 . SECURITY CLASSIFICATION O F R E P O RT

Unclassified NS N S4O-01-280-S500

18. SECURITY CLASSIFICATION OF HI S PAGE

U n c l a s s i f i e d

19 . SECURITY CLASSIFICATION O F ABSTRACT

Unclassified

15. N U M B E R O F PAGES

46 16 . PRICE CODE

20. LIMITATION O F ABSTRACT

UL

Standard Form 298 Rev. -89) fmcnDW Or AN S t d . Z3-l 2 9 8 - 1 0 2


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C O N T E N T S

1 . he Earth Grounding System 2. arth Grounding System Key Components 3. election of an Earth Grounding Electrode

4. round Rod Installation

5. urface Wire Ground Kit (SWGK) Installation

6. round Plate Installation

0

7. rounding to Underground Objects

1

8. nstalling Horizontal Conductors and Rods 2 9. mproving Soil Conductivity 2 10. ip s fo r Grounding in Poor Soil Conditions and Environments

3

11. ollocating Shelters and Stand-alone Equipment

6

12. ow Good is a Good Earth Ground Reference

7

13. ightning Protection

8

14. ightning Protection Systems: onsiderations, Examples and Applications

1

Appendixes:

A. rounding Systems and th e Confusion about Grounding

9

B. dditional Documentation

2

C. he Threat

3

D. azards due to Step Potentials

4

E. arth Grounding Test

5

F. ersonnel Protection from Lightning

7

G. efinitions

8

H. hecklist

9


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N E E D M O R E I N F O R M AT I O N ?

In addition to th e information in this pamphlet , more information is available in th e documents listed in Appendix B. n particular, see:

FM 1-487-4 Kr IL-HDBK-419A KT FPA 70 (National Electrical Code) > * NFPA 780 Lightning Protection Code)

For help, contact:

- our local Safety Office - our CECOM Logistics Assistance Representative w he CECOM Safety Office

You can call th e CECOM Safety office at DSN 992-0084 or (908) 532-0084, or write to :

Commander

CECOM Safety Office ATTN: AMSEL-SF-SEP Fort Monmouth, NJ 7703-5024 email: [email protected]


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SAFE EARTH GROUNDING OF COMMUNICATIONS-ELECTRONICS

EQUIPMENT IN THE FIELD

This pamphlet describes different earth grounding systems and provides guidance on th e proper methods fo r their installation. arth grounding helps to protect personnel and equipment from electrical faults, power surges, and other surges and transients. arth grounding also helps reduce circuit noise and other transmission interference that can degrade Communications-Electronics (C-E) system performance. ften, little training or information is provided in technical

manuals to address th e proper methods of

earth grounding system

installation. hese instructions will help you to se t up an effective and safe earth grounding system fo r your equipment or shelter.

1. HE EARTH GROUNDING SYSTEM A n earth grounding system helps keep th e electrical potential on noncurrent- carrying metal surfaces at a similar level as that of th e surrounding earth. arth grounding also provides a preferred discharge path fo r externally generated electrical surges due to power switching, faults, lightning, etc. his earth ground reference is established by firmly connecting a suitably sized wire between th e equipment (generator, C-E system, shelter, etc.) and running it to a buried metal electrode (ground rod, water pipe, plates, etc.) which is in contact

with moist subsoil or reaches into th e underground water table. more detailed discussion about th e concept of grounding is provided in Appendix A. r ^

It is important to note that earth grounding is only a part of the total ground system. qually important to the earth grounding system is the need to inter- bond all equipment and power supply enclosures through th e equipment grounding conductor (green wire), as well as th e need to bond th e power supply


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neutral circuit conductor to earth (known as system grounding; refer to Appendix A). owever, since th e primary area of soldier involvement is in th e area of earth grounding shelters, generators, and equipment using rods and straps, this will be th e area of focus. or more information on th e additional requirements associated with equipment grounding conductors and system grounding, refer to th e list of sources detailed in Appendix B.

2. E A RT H G R O U N D I N G SYSTEM KEY C O M P O N E N T S

The earth grounding system consists of three ke y elements which must al l be properly incorporated. hese elements are th e earth grounding conductor, th e connection point, and th e earth grounding electrode. lack of attention to any of th e three elements can create a weak link which could lead to a failure. (Refer to Appendix C fo r a discussion of "the threat" to earth grounding systems)

connection oint

earth grounding conducto

fl T

f ..;/ earth grounding electrode

ke y elements of an earth grounding system

2.1 Ear th Grounding Conductors :

The purpose of th e earth grounding conductor is to provide a lo w impedance

path between th e equipment noncurrent-carrying metal parts (enclosure) and th e earth. he term "low impedance" is used, which covers both resistance (which is independent of signal frequency) and reactance (resistance that changes with frequency). he importance of this can be best illustrated as follows: say that a properly sized earth grounding wire is provided which has a number of loops and sharp bends. his grounding path m ay show a lo w resistance reading using an ohmmeter, and m ay indeed be suitable fo r DC or 60 Hz (low frequency) related events. owever, th e loops and sharp bends will substantially increase th e path's impedance during higher frequency events such as lightning related transients (over 100,000 Hz) and signal noise. his example points out th e reason w hy in some cases equipment grounding-related problems occur although a grounding wire is provided. long with loops and sharp bends, other


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factors that affect path impedance include wire size, length, shape and surface area.

It's important to consider th e following when choosing and installing th e earth grounding wire:

1) he Earth Grounding Conductor must be as large as possible: at least 6 AW G . his gives it th e mechanical strength necessary to withstand th e severe mechanical forces subjected during lightning strikes, as well as th e everyday wear and tear (being kicked or stepped on, setup and teardown, etc.). ince high frequency currents tend to pass over the surface of wires as opposed to through them, wires such as flat straps that have larger surface areas than round wires can lessen path impedance.

2) he wire should be copper or copper clad aluminum. oatings on th e wire (such as nickel) or insulation are acceptable and help to reduce corrosion. Steel or stainless steel cables can be used only temporarily (systems frequently on the move) and where frequently inspected fo r corrosion.

3) he wire should be continuous between the equipment and earth grounding electrode; do not splice. nsure that th e grounding wire is not damaged or heavily corroded. f crimped or brazed connectors are provided at th e wire ends, check fo r damage or looseness.

4) un th e conductor as straight and short as possible. arth Grounding Conductors should always ru n in a downward direction; do not ru n th e conductor up and over obstacles.

5) inimize any twists, loops, or sharp bends. ny kinks should be straightened out. ncorrected, such conditions will increase th e conductor's impedance and cause failures under certain conditions.

2.2 onnections: Frequently, connection points can be th e source of problems. hough grounding connections can look okay, they m ay fail if loose or corroded. t's important to consider th e following when connecting th e earth grounding wire to th e equipment or earth grounding electrode:

1) lamp or bolt th e connections tightly to prevent loosening over time. se a lockwasher where nuts or bolts are used. o not overtighten th e connection to th e point where th e conductor strands are damaged.


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3ROUH0 tO O CU

examples of suitable clamping devices

2) hen attaching, ensure that th e bonding surfaces are free of paint, corrosion, grease, or dirt.

3) rotect th e connection points from corrosion and inspect regularly. 4) onnecting dissimilar metals, especially copper & aluminum or copper &

galvanized parts (zinc plated surfaces or washers) can cause corrosion at th e bonding point due to galvanic action. void connecting such dissimilar metals together. therwise, frequently inspect and clean as necessary.

5) ever twist or tie a ground wire around a ground rod. f a bolt is not provided and a clamp is not available, th e ground strap should be bound to th e ground ro d with at least 24 tightly wound turns of stripped telephone wire or other bare wire. his connection should then be taped to block out moisture. See figure below.

GROUND STRAP


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2.3 arth Ground Electrode: Various types of electrodes are used to establish an interface with th e earth ground. he most commonly used electrode is th e ground rod, which is available in various lengths and configurations. nder certain conditions, a good earth ground can be achieved by connecting to existing objects such as a buried metal pipe or th e steel frame of a building. here poor soil conditions exist, other methods and combinations of methods are required, which will be discussed later. he following is a description of some of th e typical earth grounding electrodes that are used:

I) round Rod - T w o are available in th e Army inventory: th e 8-foot ground rod, N SN 5975-00-596-5324, or th e 6-foot ground rod, N SN 5975-00-224- 5260. he 6-foot ro d is being phased out and replaced by th e 8-foot rod. ou m ay continue using a 6-foot ro d as long as it is serviceable. he 6-foot ro d comes in a single section, whereas th e 8-foot ro d is a three-section rod. round rods can be installed using a sledge hammer or in th e case of th e sectional rod, a driving hammer.

I M U M B S C H E W

V

0

V

6-foot and 8-foot ground rods

W

U

W

2) rounding plates - Though more difficult to install, plates ca n achieve very low earth resistance values. he plates must have at least 2 square feet of surface contact with ground (i.e., front and back of a t x t plate). uch a plate would have a 50% larger surface area than that of th e 8 ft ground ro d listed above. lates should be a minimum of 1/4 inch thick if iron or steel, or 1/16 inch thick if nonferrous.


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grounding plate

3) urface Wire Grounding Kit (SWGK) - This grounding system was designed primarily fo r use with systems requiring high mobility/quick installation and teardown operation. t consists of 15 ten-inch stakes installed in a circular pattern and interconnected with a 3/16-inch steel cable he SWGK is available in th e Army inventory under th e official nomenclature of Grounding Kit, MK-2551 /U , N S N : 5820-01-263-1760.

to r IE B . A B U 5 j) ITH 73 AM P O P P E R CLIPS

a typical installation of the Surface

Wire rounding Kit

4) ater plumbing - Traditionally, this was th e preferred grounding electrode at fixed sites since th e resistance to earth of th e extensive water piping system w as quite low. owever, much plumbing is being changed over to or being coated with nonconductive materials, and therefore is no longer as reliable.

5) uried tanks - Just as with water plumbing, buried tanks ca n provide an effective earth grounding terminal. owever, tanks are often coated with or made of nonconductive materials which ca n reduce grounding effectiveness.


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6) orizontal conductors - Ideally, horizontal conductors should be at least 2 A WG and of copper material. ever use aluminum as th e earth grounding electrode.

7) etal framework of buildings - often, a nearby steel frame building can serve as a suitable arth ground. epending on th e size of th e metal building and th e type of footings, a very low resistance to earth m ay exist. o ensure a suitable ground, al l elements of th e framework must be bonded well, especially between th e steel frame and th e footing steel reinforcement bars.

8) rids - these consist of buried copper cables that form a network of squares over an entire area. his grounding system, though effective, is the least practical from a tactical standpoint, and will not be addressed.

3. E L E C T I O N O F A N E A RT H G R O U N D I N G E L E C T R O D E

3.1 mportant considerations: The choice of th e best system to install will typically depend on th e soil conditions (clay, loam, sand, rock, etc.) and the location's climate (tropics,

deserts, mountains, arctic, etc.).

ifferent types of soil have different electrical characteristics. ere is a brief summary of soil types, ground qualities, and suggested types of earth grounding electrodes.

Type of Soil Quality of Ground Suggested Ear th Grounding Electrode

Wet, organic soil Very good Ground rod, SWGK Clay, loam, or shale Good Ground rod, SWGK or

plate Clay, loam, or shale mixed with gravel or sand

Poor Buried pipes, building frame, or other metal object or a ground plate

or several

ground

rods

electrically connected together

Gravel, sand, or stone Very Poor Same as above *

* Under poor or very poor conditions, you'll have to take special steps to establish and maintain electrical conductivity explained in section 9.

3.2 enera l recommendat ions and precautions:

1) Never use metal natural gas lines fo r earth grounding. ever use buried tanks containing flammable liquids fo r earth grounding.


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2) hen choosing a location fo r the earth grounding electrode, keep it away from locations having normal pedestrian traffic. During an electrical storm or a fault condition, dangerous voltages induced at and near th e earth grounding terminal could be hazardous to personnel (see Appendix D).

3) luminum should not be used as an earth grounding electrode. 4) rior to connecting th e earth grounding system, ensure that power to th e

system is off. his is not necessary if th e system is initially self-powered, such as with an onboard generator.

5) ever lay system power cables or signal cables over th e location of the earth grounding electrode, grounding conductors, or SWGK cable. uring a fault condition, step potentials at and near these components m ay be induced on collocated signal and power cables.

6) nstall th e earth grounding electrode at locations where ground will receive rain water (i.e., outside th e drip line of shelters, etc.).

7) here air conditioning units are operated near grounding electrodes, earth grounding resistance can be further improved by routing th e air conditioner's

condensation

water to th e grounding electrode

location

via a tube or hose.

8) ear gloves to protect hands when handling or inspecting grounding electrodes, cables, and connections. ear safety goggles when driving ground rods to protect from flying metal chips.

4. R O U N D R O D INSTALLATION Take th e following steps when installing a ground rod:

a. lean th e ro d to remove al l grease, oi l or paint. ear a safety glove to protect your hand from sharp metal fragments.

b. ig a hole at least 2 inches deep and 36 inches across. c. rive th e ro d through to th e moist subsoil. he ground ro d should be installed straight if possible, but can be installed up to an angle of 45 degrees. Allow about 3 inches of th e ro d to protrude above th e bottom of th e hole. hen installing a multiple section ground rod, ensure that to p section is tight against the lower ro d to prevent damage to th e coupling sleeve threads. imilarly, th e driving bolt should be tight against th e lower rod. e careful not to hit th e threaded end of th e ro d section with th e hammer, or damage th e threads in any other way. hen possible, use a driving hammer when installing a multiple section ground rod.


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f OAMNOKM IUTVOm A 0 A 1 H S T TWOBOUNO 00

TO T SECTION MUOTK TOHTMAMST I W l O W E f t wcnoN

fl) ROUND R O D @ RIVING R O D OUPLING SLEEVE R O U N D RODCLAMP

ground rod components and assembly

d. fter th e ground ro d is in place, connect th e ro d to th e equipment or shelter using a ground strap or other suitable earth grounding conductor. onnect the ground strap to th e ground ro d using th e provided terminal screw. f your ground ro d has no terminal screw or if it is missing or broken, connect th e ground strap with a tight-fitting clamp, or as outlined in section 2.2.

e. fter one end of th e ground strap or conductor is connected to th e ground rod, connect th e other end to th e ground lu g on th e shelter or equipment. eep th e strap or wire as short and straight as possible. ake sure that there are no loops or knots in th e ground strap or conductor. ee that al l connections ar e clean and tight.

f. ill th e hole with water, and le t it soak in . hen fill th e hole with soil. dd water as often as needed to keep th e soil moist around th e ground rod. heck th e grounding conductor and connections every day and keep them clean an d tight.

5. U R FA C E W I R E G R O U N D I N G KIT ( S W G K ) INSTALLATION In many cases, th e SWGK has proven to have a lower resistance to earth than that of a grounding rod. owever, there is concern that th e SWGK m ay have greater step potentials than a properly installed ground rod. or this reason, the SWGK is primarily approved fo r use with vehicle mounted systems, antenna masts, and trailers. urther instructions are available in TM 1-5820-1118- 12&P.

a. emove th e S W G K from th e bag and lay it in a circular or U shaped pattern around the equipment without overlapping th e cable. nspect the


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components and make sure that they are clean and not damaged. harp bends and kinks must be avoided in th e cable. n important step here is to maximize th e spacing between th e individual stakes. ear gloves when handling th e steel cable as loose strands can cause cuts. ttach th e connector end of th e cable to th e equipment grounding lug.

b. egin with th e stake closest to th e grounding stud. ull th e cable taught. Twist stake 30 to 45 degrees. rive stake until to p is flush with th e ground. Continue until all stakes are driven into th e ground. riving th e stakes fully into ground and slightly twisting them helps to ensure a tight connection between th e stakes and cable.

c. ttach th e tw o jumper cables. onnect one from th e front bumper of th e vehicle to th e center of the cable, and th e other from th e rear bumper to th e end of th e grounding cable. hese cables ar e provided to improve SWGK survivability and performance during high current events associated with lightning. t is not necessary to scrape paint to improve clip contact in this case since, under high current conditions, th e paint will vaporize. owever, th e grounding lug end must still be attached to a stud or other location free of paint.

6. R O U N D PLATE INSTALLATION

Where soil conditions are poor or very dry, a ground plate m ay work better than a ground rod.

a. elect any clean, bare metal plate or sheet that's at least 2 square feet of surface contact with ground as described in section 2.3(2). ote that th e larger th e plate is , th e lower th e resistance will be to ground; this is important to consider where grounding in poor soil (some standards recommend a 3-foot by 3-foot plate). elect a metal bolt, nut and lock washer and drill a hole in th e center of th e plate just large enough fo r th e bolt. asten an appropriate earth grounding conductor to th e plate. ake sure th e connection is clean and tight. b. ig a hole so that th e plate can be buried vertically at 5 feet below th e surface. hough some procedures recommend installing th e plate horizontally, vertical installation ensures good soil contact on both sides of th e plates. our a mixture of water and salt into th e soil around th e plate to further increase soil conductivity if necessary. ee section 9 fo r information on improving soil conductivity.

c. onnect th e other end of th e grounding conductor to th e equipment, keeping the path as straight as possible.

1 0


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S A LT WAT E R

GROUND S T R A P

grounding plate installation

7. G R O U N D I N G T O U N D E R G R O U N D O B J E C T S

Underground metal objects (buried metal pipes, steel frame building footings, metal poles, storage tanks) provide excellent grounding IF:

(1 ) hey are entirely buried at least tw o and a half feet below th e earth's surface.

(2 ) hey do not contain gasoline or other flammable liquids or gas.

(3 ) t can be assured that th e metal object is continuous and not coated with or made of nonconductive materials.

a. onnect th e earth grounding conductor to th e underground metal object with a solid connection. o not wrap or tie it on. void sharp bends. b.

se water plumbing

only if it can be

assured that at

least 0

feet of

continuous metal plumbing is in contact with the earth. he connection to the plumbing should be made within 5 feet of th e oint where it enters th e ground, and should bypass any water metering equipment. ever use natural gas plumbing as a grounding electrode.

c. arth grounding quality m ay have to be verified through testing. efer to Appendix E describing a field ground test that can be conducted using standard items available in th e field.

1 1


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8. NSTALLING HORIZONTAL CONDUCTORS A N D RODS Where bedrock is near th e surface of th e earth, horizontal ground rods and conductors can be installed. efer to section 0.4 fo r tips on selecting locations fo r earth grounding.

a. ig a trench as deep as possible; at least tw o and a half feet deep. b. nstall th e ground ro d horizontally or at least ten feet of 2 A WG cable in th e trench. our a mixture of water and salt into th e soil around th e conductor/rod to further increase soil conductivity if necessary (see section 9 fo r improving soil conductivity). ill and pack th e trench with dirt. c. f th e soil has a poor quality, such as dry, sandy, or with gravel, a second ground ro d or 20 feet of 2 AWG cable should be used. ince th e length of a cable directly increases impedance, it is better to ru n tw o 0-foot conductors in separate directions rather than one 20-foot conductor.

d. here th e soil depth does not permit a trench deeper than a foot, a loop conductor should be installed in a trench around th e perimeter of th e equipment. Alternatively, th e SWGK should be used if available.

9. M P R O V I N G SOIL C O N D U C T I V I T Y A t sites where th e soil quality is very poor or very dry, special steps need to be taken to enhance th e soil conductivity near th e earth grounding electrode. he first option is to drive th e ro d deep enough to reach th e moist subsoil, if possible. se a multiple-section ro d assembly, possibly using an added section. If there is no moist subsoil or if th e soil condition won't allow fo r deep penetration, drive th e ro d as deeply as you can and apply salt and water.

Where salt water is required, use ound of salt per gallon of water. ig a shallow trench at a distance of 18 inches around th e ground ro d so that th e liquid does not ru n off. o replace th e salt that leaches into th e soil, mix salt with th e water at least once a week fo r th e first four weeks of use. fter th e fourth week of use, add th e mixture of salt and water at least once a month (Special measures apply to grounding in th e desert; se e paragraph 7.a). Optionally, salt can be placed in th e trench and then covered, which will leach into th e soil whenever it rains. ro m th e standpoint of effectiveness and anti- corrosion qualities, th e salt types listed below rank as follows:

1) agnesium Sulfate 2) opper Sulfate 3) alcium Chloride

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4) odium Chloride 5) otassium Nitrate

treatment of soil

Prior to using any such chemicals in th e soil, you should verify that there are no environmental restrictions against using salt fo r such applications fo r th e region or area.

10. IP S FO R G R O U N D I N G IN P O O R SOIL C O N D I T I O N S A ND E N V I R O N M E N T S

One of the

most

important

factors that must be stressed

is that fo r any of

the areas listed below, advanced thought must be given to th e placement of equipment from an earth grounding perspective. lthough mission requirements may predetermine areas fo r setup, locating equipment on one side of a camp as opposed to th e other, or even moving th e equipment a dozen feet can sometimes drastically improve earth grounding installation and effectiveness. here grounding resistance is poor, additional emphasis must be placed on interbonding equipment enclosures to limit hazardous voltages developed between such equipment enclosures which can be contacted by personnel (see section 1).

When your site does not have fine to p soil, clay, loam, or shale, you must

1 3


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compensate fo r poor electrical conductivity by taking th e following applicable steps.

10.1 se of multiple earth grounding electrodes When a single ground ro d provides a poor ground, drive in additional rods spaced ideally 4 ro d lengths apart, but as a minimum at 2 ro d lengths apart. he more rods you use, th e better will be th e electrical connection to th e earth. Install them around th e perimeter of your shelter or equipment. onnect al l rods together and connect th e closest ro d to th e shelter's power entry panel using th e heaviest wire you can find, preferably #6 A WG or larger bare copper cable. f possible, connect th e other perimeter rods to other points on the shelter to further enhance th e grounding. reat th e soil with a mixture of salt and water as explained in section 9.

If a perimeter installation is impractical, you m ay install th e rods in a star- ground configuration. onnect th e center ro d to th e shelter and treat th e soil as explained in section 9.

TOOL- APART HMMOM

t FT GROUND BOO 13 FT . GRODNO STRAPS

K5iE

typical ground rod configurations

10.2 oor soils mixed with sand or gravel Us e of multiple ground rods, ground plates, or other electrodes as specified in

14


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sections 4 to 8 is recommended. here frequent on-the-move operations make it difficult to install and retrieve a ground rod, use th e SWGK. o further enhance soil conductivity, a salt water mixture should be applied as discussed in section 9.

10.3 esert

In desert environments, it is important fo r th e ground ro d to reach th e water table below. ne possibility is to install a sectional ground ro d using added extensions to reach deeper onto th e soil.

n

addition,

soil conductivity can

be

substantially improved by keeping th e soil moist and by adding a salt water mixture as specified in section 9.

Since sand is easy to excavate, use of grounding plates can be practical. he ground plates should be spaced at least 0 feet apart. lternatively, a number of ground rods can be installed in parallel.

10.4 Mounta inous terrain The soil in mountainous areas is often only a fe w inches thick. herefore, it is especially difficult and often impossible to penetrate to moist soil or a water table in the mountains. o properly ground your equipment, you must carefully select a site where a ground ro d can be installed. ites near a stream bed will have higher levels of soil moisture and should be considered fo r grounding locations. lso try to choose a site which does not have rock outcroppings, which are indicative of shallow soil.

Where soil is shallow, earth grounding electrodes will have to be installed horizontally as detailed in section 8. he deeper th e electrode can be buried, the better th e grounding protection. s trenches become shallower, step voltages at th e ground surface will be greater, therefore increasing th e potential hazard to personnel.

The location of personnel traffic areas needs to be considered when locating the

earth

grounding electrode,

since step voltages during a lightning event could be

very high. ersonnel tents should be kept as fa r away from earth grounding electrodes as possible. etal enclosures offer th e best protection to personnel and, if possible, should be considered fo r sleeping quarters as opposed to tents (refer to Appendix F addressing personnel protection during electrical storms).

Grounding effectiveness can be substantially improved through th e use of a salt- water mixture. n addition, salt can be placed along th e trench just above the electrode, which will leach into th e soil whenever it rains.

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10.5 ropics

Install th e ground ro d or SWGK as described in sections 4 and 5. ecause of th e constant high humidity, extra measures need to be taken to ensure that th e strap connection at th e equipment is clean and dry to prevent corrosion. over th e connection of th e ground with waterproof tape and check it every day.

10.6 Arctic

Cold substantially increases soil resistance and also, earth grounding impedance. Hence, it is important to drive ground rods or install ground plates below th e area's frost line, if possible. ry to ground to an extension of a buried metal object such as an underground pipe or a building frame. f no buried object is available, drive in several ground rods as deeply as possible. pace them at least tw o ro d lengths from each other. lternatively, a trench can be dug to insert a horizontal ro d or conductor, as discussed in section 8. he trench should be filled with salt water which will improve conductivity when frozen.

11. O L L O C AT I N G SHELTERS A ND S TA N D - A L O N E E Q U I P M E N T During thunderstorms, lightning flashover or arcing can occur between tw o or

more unconnected or poorly connected metal structures that are located close to each other. lashover between objects ca n cause damage to th e objects and cause lethal voltage on th e ground in th e vicinity of these objects. o avoid lightning flashover, separate equipment shelters, antenna masts, and other metal structures at least 6 feet from each other. therwise, th e objects must be bonded together using a heavy copper cable, at least 6 AW G . he best w ay to achieve this is by interconnecting each shelter's ground ro d (electrode) with th e heavy copper cable along th e ground. he cable lengths should be as short and as straight as practical.

Equipment and shelters located within armslength of each other (6-8 feet) must be interbonded to eliminate any hazardous voltages that m ay develop between such enclosures should a fault occur in one equipment. ersonnel can sustain much worse injuries when contacting tw o metal surfaces at different potentials with bare hands (a low resistance path provided across th e chest) than by contacting a surface energized to ground while wearing boots (a high resistance path to earth).

Furthermore, where electronic equipment shelters are located within 25 feet of each other, it is recommended that the individual earth grounding electrodes be interconnected using a heavy copper conductor (6 A WG minimum) ru n along th e ground. his helps to create an earth grounding electrode system having an overall lower grounding resistance, and results in other benefits as well.

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interconnection of earth grounding electrodes for collocated equipment

It's important to note that th e Equipment Grounding Conductor (the green wire you usually see running with your power lines) is sized to handle primarily power related faults. lthough th e Equipment Grounding Conductor will help to discharge some of th e current from lightning related events, its small relative size and the way it's ru n (twists, turns, kinks, etc.) can create a higher impedance path that will fail during a lightning event. or this reason, collocated shelters should be interconnected using an independent bonding conductor as described above. he Equipment Grounding Conductor should never be relied upon fo r such purposes.

12 . O W G O O D IS A G O O D E A RT H G R O U N D R E F E R E N C E ? When it comes to earth grounding installation, th e greater th e effort that is applied - th e greater th e level of safety that is obtained. t is this greater level of safety that can make th e difference between a soldier receiving a shock or being electrocuted. he Army Safety Management Information System lists a number of accidents where improper grounding contributed to fatalities. n th e other hand, reports also exist where soldiers walked away unharmed because proper and effective grounding limited th e hazardous voltages contacted by th e soldier during th e fault condition.

It is difficult to assign a specific resistance value that must always be achieved. Various codes and handbooks recommend anywhere from 0 to 25 ohms resistance. IL-HDBK-419, Handbook of Grounding, Bonding, and Shielding recommends an earth grounding resistance of 10 ohms fo r tactical, mobile systems. owever, experience has shown that this requirement is often difficult to achieve in a high-mobility, tactical environment with only one or tw o ground rods installed. hen making the decision as to how extensive th e earth grounding electrode system should be, consideration needs to be given to the specific mission scenario, location, and environment. s a goal, one should strive to achieve th e 0 ohm grounding resistance when possible. he following table lists tradeoffs that should be considered when making a decision.

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Install an extensive earth grounding system

Install th e minimum earth grounding system

Mission requires equipment to stay in one place fo r a longer time (days, weeks)

Equipment must move frequently (daily)

Equipment is very complex, is collocated with other systems an d interconnected to many other systems

System is primarily stand-alone

Extremely expensive equipment or mission critical (uninterrupted operation required)

Lower cost equipment or having a lower mission priority (can be interrupted)

Equipment susceptible to noise, may be subjected to electromagnetic interference (electronic warfare)

Equipment typically no t affected by electronic noise or no t subjected to electronic warfare

Equipment communications range needs improvement

Equipment communications range is suitable

Soil ha s very poor electrical quality Soil has very good electrical quality Site area ha s a high lightning risk Site area has a lo w lightning risk

Although th e equipment operator should always strive to implement an effective

earth grounding system, there will be situations where either du e to mission or

poor soil conditions, a good earth ground reference cannot be achieved. When this becomes he case, al l equipment enclosures, as a minimum, must be suitably bonded to each other an d to th e power supply through equipment grounding conductors an d other bonding straps. single point bond should also be made at th e power source between th e equipment grounding conductor an d neutral conductor. his will help to limit shock hazards between equipment during a fault condition (refer to appendix A ).

Note that th e decision covering th e extent of th e earth grounding system does not bypass th e requirements fo r proper earth grounding conductor installation or connections.

13 . IGHTNING P R O T E C T I O N Tw o aspects need to be considered, personnel protection an d equipment protection. ppendix F contains some guidance fo r personal protection during thunderstorms.

13.1 Basics of Operat ion

To understand how th e lightning protection system operates, we'll examine how lightning interacts with th e ground an d how it attaches to objects on th e ground. ost lightning that reaches th e ground (over 90 ) is negatively charged. I t begins to intercept th e ground by lowering a stepped leader; a

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precursor to the actual lightning discharge. This leader progresses in steps toward the ground and is comprised of electric charge. t completes this process in a length of time measured in tens of milliseconds. elow the leader is a region of very high electric field. lthough the leader is very faintly luminous, chances are it will not be visible; the only indication you might have is noticing your hair standing on end. t is also possible that there will be no warning of an approaching leader. his is a consequence of the high electric field developed under the leader. s the leader approaches the earth, the high electric field under the stepped leader induces objects on th e ground to emit leaders of opposite polarity charge. ince opposite charges attract, these leaders eventually connect and th e actual current discharge associated with lightning begins. The process is illustrated in the figure below.

IN > I'C Mil . CADK K

I.IOI1TN NT. s H (I

Metallic objects electrically connected to the ground that are comparatively sharp emit the leader better than other objects such as trees and people. Taller objects generally have an advantage since they are closer to the stepped leader and begin to emit their ow n leader sooner. his is th e primary principle of operation of a conventional lightning protection system.

This method of lightning protection is a "collector-diverter" concept consisting of three basic subsystems. he air terminal system forms the uppermost component. This m ay be a collection of metal terminals, sometimes called lightning rods, or an overhead wire or possibly just a metallic component of a structure. ll are bonded together by heavy wire or similar conductors. he bottom section is the earth electrode system, most often provided by a ground rod. t is responsible fo r effective charge injection into the earth; away from structures. The system that connects the earth electrode and th e air terminal is the downconductor system. ownconductors consist of heavy (usually copper wire) of approximately #2 A W G or larger, depending on th e application. It is possible to us e smaller conductors, but th e risk of failure increases (risk of failure is approximately 0-15% or #6 A W G wire).

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Contrary to some beliefs, he lightning protection system does no t prevent lightning. Rather, t works by collecting th e lightning stroke an d diverting the electrical energy, n a controlled fashion, to earth. The air terminal is designed to intercept the lightning leader propagating from the cloud by providing an upward propagating leader. preferred path is then established with th e downconductor and earth electrode system. he lightning current is then dissipated into the earth.

The concept of a protected zone" s often attached to lightning protection systems. In our context, we can define this as the area in which th e lightning event has a high probability of being intercepted by the lightning protection system. For the purposes of tactical systems, th e zone protected forms a cone from th e tip of the air terminal down to a distance on th e ground equal to th e height of the air terminal. his approximation is not valid fo r structuresexceeding 50 feet in height.

The lightning protection system in its simplest form consists of a single air terminal, downconductor and ground electrode (ground rod). This is often seen in tactical antenna mast applications. Another common field installation is use of an overhead wire connected to ground rods. n this case th e air terminal and th e downconductor are really th e same. ther configurations are possible.

13.2 W h e n Do I Install Lightning Protection? Obviously if the structure or system has been hit by lightning more than once, it is probably a strong candidate for lightning protection. s you can se e from the explanation above, t is entirely possible (and probable) that lightning m ay strike twice in th e same place. Otherwise examine the risk associated with operating the system in your particular location. Use th e table on the next page to determine the risk and what to do. Assign higher scores to the statements that apply.

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FA C TO R

System is critical to mission, uninterrupted operation is required.

Location is on high ground, nprotected by terrain features or trees.

A lightning strike to this location will cause injury to personnel (e.g., he system or structure is frequently occupied).

SCORE

(disagree - agree)

Construction of system/structure is mainly nonmetallic.

Structure/system is taller than other nearby structures.

Structure/system houses flammables, gases, or ammunition.

There are more than 4 thunderstorms with lightning every month.

A dd the results. f the number is 8 or greater, ightning protection is a priority fo r that location. f greater than 4 but less than 8, th e location is a strong candidate. If between 0 and 4 there is risk, ut it is comparatively low. Scores between 7 and 0 indicate minimal risk.

14. IGHTNING P R O T E C T I O N S Y S T E M S - C O N S I D E R AT I O N S E X A M P L E S A ND A P P L I C AT I O N S

The first consideration in lightning protection fo r tactical systems is terrain. n general, f systems can be placed equal in height or lower than their surroundings, hi s will somewhat lower the risk of being directly struck by lightning. A good example is the placement of antenna masts. If th e antenna mast is deployed in a heavy forest, th e probability of the mast getting directly struck will be considerably lower as there is a higher probability of th e lightning striking trees.

The following examples of lightning protection systems fo r mobile, tactical systems are provided fo r guidance.

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14.1 lightning Protective Mast

A field expedient used successfully is deploying a mast or wooden pole with a lightri ? orotect'on system. his provides a circular protective zone on the ground equal to the height of th e mast. ecall that since the protective zone is conical, tructures near th e mast that extend outside of the protected zone are at risk of getting struck. his is best used to protect a small area where there m ay be several vehicles or shelters. he ground ro d or other electrode system must be placed close to th e mast as the downconductor m ay not be ru n along

th e

ground.

In

this

sense

it

is

not

suited

to places

with

high

personnel

traffic.

FIELD EXPEDIENT LIGHTNING PROTECTION M A S T:

A IR T E R M I N A L

CLAMP & D O W N

CONDUCTOR C O N N E C T I O N

S E E DETAIL #1

M A S T

G R O U N D R O D

INSTALL IA W I N S T R U C T I O N S

DOWN CONDUCTOR - US E #2 A WG COPPER

S U G G E S T E D MATERIALS:

M A S T: OE-254 (AB-1244) M A S T O R WOODEN TELEPHONE P O L E . WARNING: D O N O T US E M A S T S U N T E S T E D FO R WINDLOADING.

G U Y WIRES S H O U L D M A K E MINIMUM 45 D E G R E E A N G L E WITH M A S T - INSTALL IA W T M INSTRUCTIONS.

AIR T E R M I N A L : O R 2 SECTIONS O F GROUND R O D , N S N 5975- 00-596-2324

G R O U N D R O D : NS N 5975-00-596-2324 O R 5820-01-263-1760 O R E Q U I VA L E N T.

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DETAIL

HOSE-TYPE CLAMPS

EXCESSIVE MAST WIDTH MAY REQUIRE ALTERNATE CLAMPING METHOD. IF THICK W O O D E N POLES AR E USED, DRILL HOLES INTO THE AIR TERMINALS AND ATTACH WITH BOLTS.

AIR TERMINAL

SAFETY CORD (USE 550 CORD) CONNECTS AIR TERMINAL AND MAST IN CASE O F INADVERTENT SEPARATION.

HOSE-TYPE CLAMPS

DOWN CONDUCTOR

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14.2 verhead Wire Another successful lightning protection system is th e overhead wire. his system protects a larger area. It s protected zone is shaped like a tent with th e peak at the overhead wire. Note that the wire must be six to eight feet above the protected structure due to th e danger of flashover. aking the wire to o long will increase th e chance of flashover due to inductive loading on the wire. A realistic maximum wire length is 60 to 80 feet.

FIELD O V E R H E A D WIRE:

S EE DETAIL #2 & #3 O V E R H E A D WIRE (SEE DOWN C O N D U C TO R IN S U G G E S T E D MATERIALS)

G UY WIRES

G R O U N D RO D

DETAIL # 2 NAIL METAL PLATE TO

TO P OF MAST

MAXIMIZE ANGLE OF DOWN CONDUCTOR WITH MAST. AT LEAST 45 D E G R E E S - NO SHARP BENDS

DOWN CONDUCTOR

CUT NOTCH IN W O O D

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DETAIL #3 ALTERNATE METHODS OF FASTENING

NAIL OR S C R E W H O S E CLAMP TO MAST

CAN ALSO FASTEN TO TO P OF MAST.

O R: O O P O NE CLAMP INSIDE THE O T H E R .

SIDE VIEW:

O V E R H E D WIRE DOWN ONDU TOR

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14.3 asic Structural Installation It is possible to install air terminals directly to a structure. In this case, the air termi' al ^.ust bt bonded together. here must not be any dead ends" n th e down conductor. un each end down as illustrated to the earth electrode. f the width of the roof exceeds 50 feet, air terminals need to be installed on th e roof perimeter. o no t use these guidelines fo r structures exceeding 50 feet in height. o not use these guidelines fo r large structures or complex roof structures. Careful engineering of protection systems must be accomplished in

these

cases.

Bonding of all-metal

roofs

to ground

is

also

a good

method

to

protect against lightning, but there is a risk of burn-through if the metal is less than 3/16-inch thick.

S T R U C T U R A L P R O T E C T I O N :

A IR TERMINALS 2 FT . HEIGHT M A XI M U M ; AIR TERMINAL S PA C I N G 20 FT . M A X I M U M ; M U S T B E WITHIN 2 FT . O F R O O F E D G E .

D O W N CONDUCTOR .INSTALL O N OPPOSITE CORNERS O F STRUCTURE.

B E N D S M U S T B E G R A D U A L ; MINIMUM 8 INCH B E N D RADIUS

D O W N CONDUCTOR M U S T N O T B E N D B A C K U N D E R E AV E S :

I N C O R R E C T: >r C -I CORRECT COMMERCIAL C O N N E C T O R B L O C K S AR E USUA LLY U S E D T O MOUNT AIR TERMINALS T O STRUCTURES, B UT APPROPRIATE E X P E D I E N T S C O U L D B E U S E D .

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14.4 ignal Entry Protection A loop" or sharp bend can be made in signal coaxial cables and th e outer conductor connected to ground at that point. Lightning current is most likely to flashover at that point in th e conductor. f the downconductor is connected to ground as straight as possible at th e connection point, ightning will find this as a preferred path to ground, diverting th e bulk of the current from the signal entry. heck if the system has surge protection before doing this; a better arrangement may be built into your system. his arrangement m ay also be used in the case where a stand-alone radio is connected to an antenna mast. If y ou don't want to damage the coax, use a connector at the loop point and bond the connector to ground. ond the ground system used here to th e main grounding system of the shelter or structure protected, f applicable.

EXPEDIENT SIGNAL ENTRY PROTECTION:

COAX (EXAGGERATED FO R CLARITY)

MAKE "LOOP" N SIGNAL COAX

G R O U N D R O D

WRAP A R E A WITH ELECTRICAL TAPE

WARNING: O NO T CRUSH WIRE BRAID O N COAX. DAMAGE TO BRAID MAY RESULT IN SIGNAL LOSS O R DEGRADATION.

SHELTER SIGNAL

ENTRY PANEL

STRIP OUTER INSULATION FROM COAX

HOSE-TYPE CLAMP

C O P P E R WIRE TO GROUND, #6 AWG MINIMUM

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14.5 luid/Gas Storage Tanks Permanent /Semipermanent Installat ion) Ground tanks to an earth electrode system and bond to nearby metal objects. This will minimize the possibility of flashover or arcing which has th e potential to ignite fumes from flammable liquids or residual gases.

14.6 u m m a r y Major principles and simple examples of basic lightning protection fo r field applications have been discussed. These cover th e major portion of field applications. If more complex applications arise ASK FO R TECHNICAL ADVICE (see page ii). he examples given are valid within the restrictions of th e drawings but may not be applicable to large complex systems or structures.

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APPENDIX A

Grounding Systems A nd T he Confusion Abou t Grounding.

From a safety standpoint, th e functions of th e grounding system can be placed i n tw o functional categories. ne function is to provide a low impedance path between all equipment and power supply noncurrent-carrying metal parts (enclosures, etc.), as well as provide a bond between these parts and th e power

supply neutral

conductor back at th e power supply.

onding of

enclosures

is

carried out through the us e of an Equipment Grounding Conductor (the green wire provided with power cables), and bonding between th e grounding an d neutral conductor is provided through the Main Bonding Jumper. s you ca n se e in th e figure below, the lo w impedance path provided by these conductors would permit sufficient current to flow back to the power supply to open an y circuit breaker protection in th e event of a ground fault. he equipment grounding conductor also helps to keep th e voltage developed between collocated equipment enclosures down during a fault. n these cases, the Equipment Grounding Conductor acts more like a bonding conductor, in that its main function is to bond noncurrent-carrying parts back to th e power supply rather than bond them to earth.

s: nainbondfngjui p e r

ground feilt- sr

rqilpacnt gmilit Mftdvcttr

B a r t h grounding conductor

tank criiodt dtrfrtdi

low impedance path provided by the EG C during a fault

The second function involves keeping the equipment enclosures or other noncurrent-carrying metal parts at earth potential, to provick a prefetreu

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discharge path to earth fo r surges, and provide a ground reference plane for communication? signals. s seen in th e previous figure, this is provided by th e Earth Grounding Conductor and the Earth Grounding Electrode. he Earth Grounding system protects against hazardous voltages between enclosures and earth ground developed as a result of power surges, lightning, and other faults.

Confusion stems from th e fact that in many cases, th e Equipment Grounding Conductor not only bonds th e equipment back to th e power supply, but also

earth-grounds th e equipment through th e earth grounding conductor provided back at th e power supply. oth functions can be suitably carried out by th e Equipment Grounding Conductor fo r simpler equipment such as appliances, tools, and individual equipment. owever, as systems become larger, more complicated, more sensitive to EMP, have more power sources or are interconnected via numerous control and signal cables, faults can occur just about anywhere which could follow unpredictable paths over which there is no control (see figure below). or complex systems, such as shelters, additional equipment grounding electrodes and conductors should be installed to ensure that under fault conditions, hazardous voltages cannot be developed on enclosures relative to earth ground.

l ightning

discharge

electronic warfare

agatfai | discharge

pa tck pane) located io tent reawted eqalpmeat

complex systems can have multiple sources and paths for faults

It is important to stress that earth grounding is only part of th e overall grounding system, and is not a substitute fo r th e Equipment Grounding Conductor or the Main Bonding Jumper. ince power faults always follow a path back to th e

30


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power source, and since th e earth is a poor conductor of electricity, sufficient current will no t flow back to the power supply to clear any circuit breaker protection (see figure below). low impedance path must be provided back to th e power source via th e Equipment Grounding Conductor.

rqatpMn

Krovnd faali. IS

Mrth ra*()lmf rkrtrndf

AAAA-

m

Eirlfc RtsHfntt

firth

L nh 4 voll i t

4vt to trth rubtmrt

Soil provides a poor impedance path; this energizes the equipment s surfe ace

During th e development of equipment, the system designer is heavily involved in ensuring that equipment grounding conductors are incorporated an d suitably sized; th e designer does not get involved with the particulars of Earth Grounding. owever, th e tables turn in the field in that th e soldier's involvement with Equipment Grounding Conductors is very limited (making sure th e green wire is connected at th e generator, etc.), and much work is placed on installing earth grounding conductors and electrodes, and interbonding equipment. s a result, this pamphlet concentrates on proper earth grounding procedures which are of primary concern, to th e soldier. dditional information on equipment grounding conductors and system/neutral grounding is available in documents listed in Appendix B.

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A P P E N D I X B

Addi t ional Domrmentation and References

CECOM TR-93-1 Lightning Protection System Design

CECOM TR-94-8 ngineering Application Notes: Grounding Kit, MK-2551 A/U (Surface Wire Grounding Kit).

FM 1-487-4 nstallation Practices: Communications Systems Grounding, Bonding, and Shielding. M1L-HDBK-419 rounding, Bonding, and Shielding fo r Electronic

Equipments and Facilities.

NFPA 70 ational Electrical Code - Article 250 Grounding - Article 550 Mobile Homes, Manufactured Homes, and Mobile

Home Parks - Article 525

NFPA 780 nstallation of Lightning Protection Systems TB 43-0125 nstallation of CE equipment: Hookup of Electrical Cables to

Mobile Generator Sets on Fielded Equipment to meet Electrical Safety Standards.

TM 1-5820-1118-12&P perator's and Unit Maintenance Manual Including Repair Parts and Special Tools List fo r Grounding Kit, MK-2551 A /U

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APPENDIX C

The Threat

The worst event that an earth grounding system ca n sustain is a direct lightning strike. ore often, equipment is subjected to and damaged by th e strong electromagnetic fields created by a nearby lightning strike, or th e brief electrical surge that ca n pass through signal. y installing the system with th e anticipation of th e "worst case event, it can be assured that th e earth grounding system will function suitably fo r th e other cases as well. dditionally, n>: n y of the factors that can lead to grounding system failure during a lightning strike can also be th e source of problems for minor events such as system noise and communications effectiveness.

In order to get a true appreciation fo r what lightning is an d what it can do, let us take a brief look. ightning is an atmospheric event by which an electrical discharge occurs between the earth and sky. single strike can be divided into four components. he first is characterized by an extremely short pulse that averages around 5,000 amps, but can go as high as 200,000 amps. his first pulse has an extremely fast rise time, from zero to peak current in microseconds, making it a very high frequency event. he next component is a very short transitional

period

of

a fe w thousand amps that

leads to the third component

that's characterized by a steady current of a fe w hundred amps fo r up to 0.75 seconds. he last component is similar to th e first component with about half th e current peak. or the average lightning strike, these components are repeated 3 or 4 times, and have been measured up to 26 times in one event

N ow let's translate these components and events into what they do to the grounding system. uring th e first "pulse component," extremely high electromagnetic fields are developed. hese fields ca n create strong mechanical effects on th e conductors and bonding connections causing twisting, pulling, squeezing, and snapping. ny loose connections ca n be pulled apart quite easily at this point. The following tw o components are primarily responsible fo r heating. uring these tw o components, enough current ca n be passed to raise th e temperature of a wire a fe w hundred degrees. ny loose, corroded, or other connections having some resistance would melt at this point. ote how th e first component can cause connections to loosen, which would then melt during th e next heating component." hen considering th e sheer magnitude of th e event that occurs and that it's repeated a number of times, it becomes very clear that any single weak link in th e grounding system could cause it to fail.

33


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A P P E N D I X E

Earth Grounding Test

I t is possible to conduct a quick test to make a rough measurement of th e earth grounding electrode's resistance. ou will need the following equipment:

- multimeter (two can be used) - four lengths of wire, 2 A WG or larger, up to 70 feet long. - 2 short grounding electrodes (copper or steel with /2 - to 3/4-inch diameter

and 2 feet long; a single section of a sectional ground ro d works fine). - at least four wire clamps (these can be th e clip type or other type). our

terminals m ay also be needed to connect th e wire to the multimeter if th e multimeter doesn't have test clips.

- a hammer to drive th e ground electrodes. - an automobile or truck type battery, charged. - a tape measure.

Setup: ee figures below Place all of th e probes in a straight line with respect to the grounding electrode being tested. lace th e current probe about 60 feet away and the potential probe at 0.62 t imes this distance from th e ground ro d under test (about 36 feet). rive in th e current and potential probes about V feet. onnect th e wire as in the figure.

Procedure:

I) lace current probe, th e multimeter and th e battery in series with th e ground under test. t is important to place the probes in a straight line with respect to th e ground ro d under test. ecord the current

reading. isconnect the battery.

S T E P 1 S E T MULTIMETER FO R C U R R E N T READING S E T UP CIRCUIT AS S H O W N TAKE C U R R E N T READING D I S C O N N E C T B AT T E RY

CURRENT PROBE

EARTH

60 FEET

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2) lace th e multimeter in series with th e potential probe and th e ground rod. Set th e multimeter fo r a voltage reading.

3) onnect th e current probe and battery in series with th e ground under test. Take the voltage reading with th e multimeter. isconnect th e battery.

STEP 2 SET MULTIMETER FOR VOLTAGE READING SETUP CIRCUIT IN DIAGRAM - CONNECT BATTERY LAST

TAKE VOLTAGE READING DISCONNECT BATTERY

+

CURRENT o ROBE

P R O B E

GROUND

UNDER TEST

4) ivide the voltage [volts] by the current [amperes] yielding resistance. his is the resistance of th e ground under test. Make sure th e units are correct; th e multimeter most likely will read in milliamperes, which is one thousandth of an ampere).

SPECIAL CIRCUMSTANCES A SINGLE B AT T E RY MA Y N O T B E SUFFICIENT WHEN M E A S U R I N G HIGH

R E S I S TA N C E TYPE SOILS DRY, SANDY, ROCKY, ETC) . F T H E

MULTIMETER R E A D I N G S AR E NOT S TA B L E , U S E TWO BAKERIES N

S E R I E S AS S H O W N B E L O W (NOTE P R O P E R POLARITY):

C U R R E N T

P R O B E

EARTH f

Jumper A

0 - + - +

batl bat2

$1.5 FEET 60 FEET

CAUTION

G R O U N D I

UNDER TESlB

D O NO T E X C E E D 30 VOLTS TO TA L (Vtolal = Vbatl + Vbat2)

T O F U RT H E R REDUCE T H E P O T E N T I A L FO R S H O C K , CONNECT J U M P E R A

LAST B E F O R E T H E TEST, AN D DISCONNECT J U M P E R A FIRST AFTER T H E

T E S T. THIS WILL UMIT T H E VOLTAGE B E T W E E N AN Y ACCESSIBLE PARTS

AND G R O U N D T O T H E VOLTAGE RATING O F A SINGLE BATTERY.

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APPENDIX F

Personnel Protection From Lightning.

The following is an extract of guidance developed at Fort Gordon on Personal Safety During Thunderstorms.

1 . o not go outdoors or remain out during thunderstorms unless it is necessary. eek shelter as follows:

a. wellings or other buildings protected against lightning, like shelters. b. nderground shelters such as subways, tunnels, or caves. c. arge metal frame buildings. d. arge unprotected buildings. e. nclosed automobiles, buses, and other vehicles with metal tops & bodies. f. nclosed metal trains and street cars. g. nclosed metal boats or ships. h. oats which are protected against lightning. i. ity streets which m ay be shielded by nearby buildings.

2. f possible, avoid th e following places which offer little or no protection from lightning:

a. mall, unprotected buildings. b. ents and temporary shelters. c. utomobiles with nonmetal to p or open. d. railers which ar e nonmetal or open.

3. ertain locations are extremely hazardous during thunderstorms and should be avoided if at al l possible. pproaching thunderstorms should be anticipated and th e following locations avoided when storms ar e in the immediate vicinity:

a. illtops and ridges. b. reas on to p of buildings. c. pen fields, parking lots, athletic fields, golf courses, tennis courts, etc. d. wimming pools, lakes and seashores. e. ear wire fences, clotheslines, overhead wires, and railroad tracks. f. nder isolated trees. g. void use of or contact with electrical appliances and plumbing fixtures

near earth grounding systems. lso avoid using telephones; most lightning injuries occur from using phones during electrical storms.

4. f you ar e hopelessly isolated in an exposed area and you begin to feel your hair stand end on end (indicating that lightning is about to strike in th e immediate area), drop to your knees and bend forward, putting your hands on your knees. o not lie flat on th e ground or place your hands on th e ground.

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A P P E N D I X G

Definitions: he National Electrical Code (NEC) creates some confusion by using the word grounding " to address equipment bonding, supply neutral grounding, and earth grounding. n order to clarify this, th e following terms and definitions have been used in this pamphlet .

Bonding - th e permanent joining of metallic parts to form an electrically conductive path that will ensure electrical continuity and th e capacity to conduct

safely any current likely to be imposed.

Equipment Grounding Conductor - this conductor is used to tie al l noncurrent- carrying parts or an equipment (enclosure, etc.) together and back to th e power source. nlike th e grounded (neutral) conductor, it is intended to carry current only during a fault condition.

Earth Grounding - A conductive connection between an electrical circuit or equipment and the earth (the N EC calls this Grounding).

Earth Grounding Conductor - this conductor is used to tie th e equipment/system noncurrent-carrying metal parts to th e earth grounding electrode (the N EC calls

this

th e

grounding electrode conductor).

Earth Grounding Electrode - this is th e actual element that contacts and makes a good reference to earth, and usually consists of a ground rod, buried plates, wires, pipes, etc. (the NEC calls this th e grounding electrode).

Ground Fault - a fault condition where an ungrounded conductor (a hot wire, such as th e black power line) shorts to a noncurrent-carrying metal part, such as the equipment enclosure. n contrast, a short circuit covers a fault between tw o current carrying conductors, such as th e hot (black) and neutral (white) wires.

Grounded Conductor - this is a current carrying conductor which is referenced to ground by bonding it to earth ground at th e power source or main service

panel (also known as th e neutral, or white wire).

Impedance - a combination of resistance and reactance. hereas the resistance does no t change with the current frequency, reactance increases with increasing frequency.

Main Bonding Jumper - this is a conductor that is used to tie th e power supply grounded conductor (neutral) to th e supply enclosure, equipment grounding conductors, and earth grounding conductor.

Neutral Conductor - This conductor often is also th e grounded conductor (with some exceptions) and is colored white or natural grey.

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APPENDIX H

This checklist details the principal factors required for a durable and effective grounding/lightning protective system. ot ll of the items require compliance, depending on the particular situation. ather, he grounding/lightning protection system should be reviewed in light of this checklist to ensure that no particular area ha s been left unaddressed.

Preparation:

G Has consideration been given to th e placement of equipment or shelters from an earth grounding perspective (local soil quality, etc.; se e section 0)?

Has consideration been given to how extensive th e earth grounding electrode system should be (refer to section 2)? D Has th e soil condition been considered in selecting the type and quantity of earth grounding electrodes (refer to section 3.1)?

Earth Grounding Conductor:

G s the grounding wire suitably sized and of a corrosion-resistant material? D A re loops, inks, and sharp bends avoided? D A re grounding conductors routed horizontally or down toward the ground rod? avoid going up and over objects) D A re splices in the conductor avoided? D A re the grounding wires free of heavy corrosion and damage, specially at the ends?

Bonding Points:

G A re ground wires not twisted around ground rods, masts, tc.? 0 A re wires rigidly clamped (not twisted or solely taped)? G A re clamps/bonding points secured and tightened to avoid loosening with time? G A re grounding/bonding connections free of paint or any signs of corrosion? G A re dissimilar materials avoided at th e bonding point?

Earth Grounding Electrode:

G A re ground rods or other electrodes suitably installed? G A re ground rods or other electrodes free of paint? G s the use of aluminum fo r electrodes avoided? G Where possible, re electrodes installed away from areas where outcroppings or rock are evident (signifying poor soil conditions)? G Where soil is shallow, are ground rods/wires buried horizontally? G f used, re grounding plates suitably sized? G Where the SWGK is used, are th e twojumper cables installed?

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L I re electrodes installed where th e ground will receive rain water or other source of moisture? D re grounding electrodes installed away from locations with normal pedest; ian traffic or personnel tents? D A re power and signal cables no t laid across or near earth grounding conductors or cables? D Where the earth grounding resistance is questionable (poor soil, buried plumbing that m ay be coated, etc.), has a test been conducted (Appendix E). Where soil quality is poor, are multiple grounding electrodes installed and

interconnected? I l Where soil quality is poor, s conductivity enhanced through the use of salt?

Grounding/Bonding Dead Meta l Objects/Structures:

D A re all electronics equipment shelters, generators, etc. properly earth grounded? G Where used, re al l CONEXes or other metal shelters normally occupied by personnel grounded (including observation towers, bathrooms, nd kitchens)? D A re al l electronics equipment shelters, CONEXes, nd other metal shelters located within 6 feet of each other bonded together? D re metal roofs which are provided to protect electronic equipment, generator, etc. roperly grounded as well as bonded to nearby equipment? G A re generators, water tanks, O L tanks, nd other metal objects grounded as well as bonded to any other metal objects/structures within 6 feet?

Lightning Protective Mas t s (LPM):

G s all equipment requiring protection located within the LP M cone of protection? as much as is reasonably possible) [] s th e air terminal at to p of the LP M durable? I .l s a suitable grounding conductor provided from th e air terminal to ground? U s th e LPM suitably grounded via a dedicated ground rod, or otherwise grounded to an existing ground ro d system? [] f any separate grounding electrodes are located near the LP M grounding electrode,

re

they

interconnected

to

the

LP M grounding electrode?

[] A re all bonding connections secure? clamped, not taped or twisted) Other:

G re personnel informed to remain within appropriate shelters when possible during electrical storms (see Appendix F)? G re lightning protection masts and related grounding inspected following electrical storms to ensure that damage was not sustained? G f tents are used to house personnel, re they located away from ground rods an d antenna masts (tents provide poor protection; especially where personnel lie across the ground)?

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A Guide for Proper Earth Grounding Procedures for use with Tactical Systems

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