electrical system grounding- static grounding and lightning protection...

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DEPARTMENT OF THE AI R FORCEHEADQUARTERS UNI TED STATES AI R FORCE

WASHI NGTON DC20330- 5000

SUBJECT: Engi neer i ng Techni cal Let t er ( ETL) 90- 6:

El ec t r i cal Syst em Gr oundi ng, St at i c Gr oundi ng and Li ght ni ngPr ot ec t i on

See Di s t r i but i on Li s t

1. At t ac hed f or y our i nf or mat i on and ac t i on ar e c r i t er i a f or t he des i gn ofel ec t r i c al gr oundi ng, s t at i c pr ot ec t i on and l i ght ni ng pr ot ec t i on s ys t ems f orAi r For c e f ac i l i t i es.

2. Pur pose. Thi s ETL:

a. Spec i f i es and c l ar i f i es bas i c r equi r ement s f or el ec t r i c algr oundi ng, s t at i c and l i ght ni ng pr ot ec t i on c r i t er i a f or t he des i gn of s af e

and ef f ec t i v e pr ot ec t i on s ys t ems f or Ai r For c e f ac i l i t i es .

b. Super sedes t hose por t i ons of r e f er enced publ i cat i ons onl y asspec i f i c al l y st at ed.

c . I s aut hor i zed i n accor dance wi t h AFR 8- 7 , Ai r For ce Engi neer i ngTechni cal Let t er s (ETLs) , and i s t o be i mpl ement ed accor di ngl y . Wai ver s f orexpl os i ves f ac i l i t i es wi l l be pr ocessed i n accor dance wi t h AFR 127- 100. Al lot her wa i ver s wi l l be pr ocessed i n accor dance wi t h t he pr ocedur es es t abl i shedby t he Model I ns t a l l at i on Pr ogr am.

3. Ef f ec t i v e Dat e. Thi s ETL i s ef f ec t i v e:

a. For a l l pr oj ec t s whi ch have not r eached t he 35 percent p r el i mi nar ydes i gn s t age as of t he dat e of t hi s l et t er .

b. For p r o j ec t s , r egar dl es s of t hei r d es i gn s t age, wi t h pr opos edsyst ems whi ch ar e unsaf e or i nadequat e f or accept abl e equi pment oper at i on asdet er mi ned by t he Maj or Command ( MAJCOM) .

4. Thi s i s a coor di nat ed HQ USAF/ LEED and HQ AFESC/ DEM l et t er . Our poi ntof cont act i s Mr . Fer nandez at DSN 297- 4083.

CHARLES L. PEARCE, Col onel , USAF 3 At chChi ef , I ns t al l at i on Dev el opment Di v i s i on 1. Di s t r i but i on Li s t

Di r ec t or at e of Engi neer i ng and Ser v i ces 2. Des i gn Cr i t er i a3. I ndex



DI STRI BUTI ON LI ST

DI STRI BUTI ON

ACTI ON OFFI CES

HQ 11TH AF/ DEE/ DEMELMENDORF AFB, AK 995506- 5001

HQ AFCC/ DEE/ DEOSCOTT AFB, I L 622250- 6001

HQ AFLC/ DEE/ DEMWRI GHT- PATTERSON AFB, OH 45433- 5001

HQ AFRES/ DEE/ DEMROBI NS AFB, GA 31098- 5000

HQ AFSC/ DEE/ DEM

ANDREWS AFB, DC 20334- 5000

HQ ATC/ DEE/ DEMRANDOLPH AFB, TX 78148- 5001

HQ AU/ DEE/ DEM

HQ ESC/ LEEE/ LEEPSAN ANTONI O, TX 78243- 5001

HQ PACAF/ DEE/ DEMHI CKAM AFB, HI 96853- 5001

HQ MAC/ DEE/ DEMSCOTT AFB, I L 62225- 5000

HQ SAC/ DEE/ DEMOFFUTT AFB, NE 68113- 5001

HQ AFSPACECMD/ DEE/ DEMPETERSON AFB, CO 80914- 5001

HQ TAC/ DEE/ DEMLANGLEY AFB, VA 23665- 5001

HQ USAFE/ DEE/ DEM

APO NY 09012- 5001

HQ USAFE/ DERRAF RUI SLI P ADM, UKAPO NY 09241- 5000

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HQ AFESC/ DEMMTYNDALL AFB, FL 32403- 6001

1100 ABG/ DEE/ DEMBOLLI NG APB, DC 20332- 5000

USAF RGN CI VI L ENGR - EASTERN REGI ON/ ROE77 FORSYTH ST. , SUI TE 291ATLANTA, GA 30335- 6801

USAF RGN CI VI L ENGR - CENTRAL REGI ON/ ROE1114 COMMERCE ST, ROOM 207DALLAS, TX 75242- 0216

USAF RGN CI VI L ENGR - WESTERN REGI ON/ ROE630 SANSOME ST, ROOM 1316SAN FRANCI SCO, CA 94111- 2278

AFRCE- BMS/ DEE

NORTON AFB, CA 92409- 6448

AFI T/ DEEWPAFB, OH 45433- 6583

HQ USAFA/ DEMACOLORADO SPRI NGS, CO 80840- 5671

I NFO OFFI CES

AAFES/ EN- CE

ANGSC/ DEE/ DEO

ANDREWS AFB, MD 20331- 6008

NAVFAC CODE 04/ 05

CEEC- ET/ ES

HQ AFCOMS/ DEEKELLY AFB, TX 78241- 6290

DEFENSE MEDI CAL FACI LI TI ES OFFI CE6 SKYLI NE PLACE 109 LEESBURG PI KEFALLS CHURCH, VA 22041

HQ USAF/ LEYS/ SCTXWASHI NGTON, DC 20330- 5000

SSD/ DELOS ANGELES AFB, CA 90009- 2260

At ch 1 ( 2 of 3)



AD/ DEEGLI N AFB, FL 32542- 5000

HQ AFOMS/ SGSFBROOKS AFB, TX 78235- 5000

HQ AFI SC/ SEGNORTON AFB, CA 92409- 7001

1842 EEG/ EELSCOTT AFB, I L 62225- 6001

1839 EI G/ EI EEKEESLER AFB, MS

HQ DNAATTN: RAEE6801 TELEGRAPH RDALEXANDRI A, VA 22310- 3398

HQ DCA/ DTJ( B410)WASHI NGTON, DC 20305- 2200

SAF/ FMABDWASHI NGTON, DC 20330- 5000

SAF/ AQQSWASHI NGTON, DC 20330- 1000

HQ AGMC/ MLMNEWARK AFB, OH 43055- 5000

WL/ NTCABLDG 909KI RTLAND AFB, NM 87117- 6008

EI D/ EI WTI NKER AFB, OK 73145- 5000

AFCSC/ SRVTSAN ANTONI O, TX 78243- 5000

HQ AFI SC/ SEWVNORTON AFB, CA 92409- 7001

At ch 1 ( 3 of 3)



ETL 90- 63 OCTOBER 1990

ENGI NEERI NG TECHNI CAL LETTER

ELECTRI CAL SYSTEM GROUNDI NG,STATI C GROUNDI NG

AND LI GHTNI NG PROTECTI ON

Depar t ment of t he Ai r For ceUni t ed St at es of Amer i ca

DI RECTORATE of ENGI NEERI NG AND SERVI CES

I NSTALLATI ON DEVELOPMENT DI VI SI ONENGI NEERI NG BRANCH

DI STRI BUTI ON STATEMENT: APPROVED FOR PUBLI C RELEASE DI STRI BUTI ON I S UNLI MI TED



Tabl e of Cont ent sPar a No. Ti t l e Page No.

Par t 1 - Gener al

1 I nt r oduct i on . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Ref er enced Publ i cat i ons. . . . . . . . . . . . . . . . . . . . . . 13 Def i ni t i ons. . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Par t 2 - Requi r ement s

4 Gener al . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74a Faci l i t y Gr oundi ng Syst em Funct i ons. . . . . . . . . . . . . . 74a( 1) Syst em Gr oundi ng . . . . . . . . . . . . . . . . . . . . . 74a( 2 ) Equi pment Gr oundi ng. . . . . . . . . . . . . . . . . . . . 74a( 3) I sol at ed Gr oundi ng . . . . . . . . . . . . . . . . . . . . 74a( 4) Li ght ni ng Pr ot ect i on Gr oundi ng . . . . . . . . . . . . . . 74a( 5) St at i c Gr oundi ng . . . . . . . . . . . . . . . . . . . . . 74a( 6) Si gnal Ref er enc e Gr i d Gr oundi ng. . . . . . . . . . . . . . 7

4b Si ngl e Syst em. . . . . . . . . . . . . . . . . . . . . . . . . 84b( 1 ) Mi ni mum Requi r e ment s . . . . . . . . . . . . . . . . . . . 84b( 2) Bondi ng t o Ot her Sys t e ms . . . . . . . . . . . . . . . . . 84c Compl i ance . . . . . . . . . . . . . . . . . . . . . . . . . . 84d Gr oundi ng and Ov er al l Sy st em Per f or manc e . . . . . . . . . . . 8

5 Syst em Gr oundi ng . . . . . . . . . . . . . . . . . . . . . . . . . 95a Gener al . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95b Neut r al and Gr ound Connec t i ons . . . . . . . . . . . . . . . . 95c Separ at el y Der i ved Syst ems . . . . . . . . . . . . . . . . . . 95c( 1 ) Dr y Type Tr ansf or mer s. . . . . . . . . . . . . . . . . . . 95c ( 2 ) St andby or Emer genc y Gener at or s . . . . . . . . . . . . . . 105d Gr oundi ng El ec t r ode Sy st em Res i s t anc e. . . . . . . . . . . . . 10

5d( 1 ) Low Resi st ance Myt h. . . . . . . . . . . . . . . . . . . . 105d( 2) Basewi de Appl i cat i ons. . . . . . . . . . . . . . . . . . . 105d( 3) Commer c i al and I ndus t r i al Appl i c at i ons . . . . . . . . . . 115d( 4) Communi c at i onel ec t r oni c s Appl i c at i ons . . . . . . . . . . 115d( S) Haz ar dous Ex pl os i v es Appl i c at i ons . . . . . . . . . . . . . 115d( 6) Ot her Appl i cat i ons . . . . . . . . . . . . . . . . . . . . 115d( 7 ) Obt ai ni ng Bet t er Gr o unds . . . . . . . . . . . . . . . . . 115e Gr ound Faul t Pr ot ect i on. . . . . . . . . . . . . . . . . . . . 12

6 Equi pment Gr oundi ng. . . . . . . . . . . . . . . . . . . . . . . . 126a Subs t i t ut es f or Equi pment Gr oundi ng Conduc t or s . . . . . . . . 126b Separ at e Gr ounds . . . . . . . . . . . . . . . . . . . . . . . 126c Equi pment Gr oundi ng Conduc t or s . . . . . . . . . . . . . . . . 1 3

7 Li ght ni ng Pr ot ect i on . . . . . . . . . . . . . . . . . . . . . . . 137a Nonconvent i onal Syst ems. . . . . . . . . . . . . . . . . . . . 13

i



7b Changes t o AFM 88- 9, Chapt er 3 . . . . . . . . . . . . . . . . 1 37b( 1) Ai r Ter mi nal Types . . . . . . . . . . . . . . . . . . . . 137b( 2 ) Hazar d ous Locat i ons. . . . . . . . . . . . . . . . . . . . 137b( 3) Ter mi nal s. . . . . . . . . . . . . . . . . . . . . . . . . 137b( 4) St ai nl ess St eel Gr ound Rods. . . . . . . . . . . . . . . . 137b( 5) Hi gh Compr es si on Connec t i ons . . . . . . . . . . . . . . . 13

7c Bondi ng. . . . . . . . . . . . . . . . . . . . . . . . . . . . 137d Subsyst em Resi st ance . . . . . . . . . . . . . . . . . . . . . 147e Ar r est or s. . . . . . . . . . . . . . . . . . . . . . . . . . . 147f Spaci ng f or Li ght ni ng Rods . . . . . . . . . . . . . . . . . . 14

8 Gr oundi ng and Bondi ng f or St at i c Pr ot ec t i on. . . . . . . . . . . . 148a Syst em I nt egr at i on . . . . . . . . . . . . . . . . . . . . . . 148b St at i c Gr o und Resi st ance . . . . . . . . . . . . . . . . . . . 158c Except i on. . . . . . . . . . . . . . . . . . . . . . . . . . . 158d Conduct or Si ze . . . . . . . . . . . . . . . . . . . . . . . . 15

9 El ec t r oni c Dat a Pr ocess i ng9. a Saf et y . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

9a( 1) Pr emi ses Wi r i ng. . . . . . . . . . . . . . . . . . . . . . 159a( 2 ) Noi se and Dat a Er r or s. . . . . . . . . . . . . . . . . . . 169b Desi gn and Const r uct i on. . . . . . . . . . . . . . . . . . . . 169c Zer o Si gnal Ref er ence Gr i d . . . . . . . . . . . . . . . . . . 169c( 1) Rai sed Fl oor s . . . . . . . . . . . . . . . . . . . . . . . 169c( 2) Saf et y Connect i on. . . . . . . . . . . . . . . . . . . . . 169c( 3) Li ght ni ng Pr ot ect i on . . . . . . . . . . . . . . . . . . . 179d I sol at ed Gr ound Equi pment . . . . . . . . . . . . . . . . . . . 179d( 1) Recept acl es. . . . . . . . . . . . . . . . . . . . . . . . 179d( 2 ) Har dwi r ed Equi pment . . . . . . . . . . . . . . . . . . . . 179d( 3) I sol at i on Tr ansf or mer s . . . . . . . . . . . . . . . . . . 17

Li s t of Fi gur es

Fi gure 1 ( page 2) Typi cal syst em ground i ng schemat i c

Fi gur e 2 ( page 3) Typi cal equi pment gr oundi ng schemat i c

Fi gure 3 ( page 4) Typi cal i sol at ed gr oundi ng schemat i c

Fi gur e 4 ( page 4) Li ght ni ng pr ot ec t i on f or var i ous r oof t ypes

Fi gur e 5 ( page 5) St at i c gr oundi ng of char ged equi pment or bodi es

Fi gure 6 ( page 6) Typi cal schemat i c of an equi pot ent i al p l ane wi t h opt i onal

gr ound st r ap i n ADP syst em gr ound conduct or s

i i



Par t 1 - Gener al

1. I nt r oduc t i on. Thi s doc ument s pec i f i es and c l ar i f i es bas i c el ec t r i c algr oundi ng, s t at i c and l i ght ni ng pr ot ec t i on c r i t er i a f or t he mos t common Ai rFor c e appl i c at i ons . Ref er t o t he l i s t of publ i c at i ons bel ow f or addi t i onali nf or mat i on, def i ni t i ons and f or appl i c at i ons not c ov er ed.

2. Ref er enced Publ i cat i ons.

a. AFR 88- 4 , Chapt er 11, Cr i t er i a f or Dat a Pr ocess i ng Fac i l i t y Des i gnand Const r uct i on.

b. AFR 88- 15, Cr i t er i a and St andar ds f or Ai r For ce Const r uc t i on ( Dr af tVer s i on, Jan 86) .

c . AFM 88- 9, Chapt er 3, El ec t r i cal Des i gn, Li ght ni ng and St at i cEl ec t r i c i t y Pr ot ec t i on.

d. AFR 91- 38, Mai nt enance of El ect r i ca l Gr oundi ng Syst ems.

e. AFP 91- 43, Mai nt enance Respons i bi l i t i es f or Ai r For ce Gr oundi ngSys t ems.

f . AFR 127- 100, Expl osi ves Saf et y St andar ds.

g. DOD 6055. 9- STD, Ammuni t i on and Expl osi ves Saf et y St andar ds.

h. Eng i neer i ng Techn i cal Let t er ( ETL) 86- 17, Power Condi t i oni ng andCont i nuat i on I nt er f aci ng Equi pment ( PCCI E) .

i . Feder al I nf o rmat i on Pr ocess i ng St andar ds ( FI PS) Publ i cat i on 94,Gui del i ne on El ec t r i cal Power f or ADP I ns t al l at i ons .

j . I ns t i t ut e of El ec t r i c al a nd El ec t r oni c Engi neer s ( I EEE) St andar d141, Recommended Pr ac t i ce f or El ec t r i cal Power Di s t r i but i on f or I ndus t r i alPl ant s .

k. I EEE St andar d 142, Recommended Pr act i ce f or Gr oundi ng of I ndust r i aland Commer ci al Power Sys t ems ( Gr een Book) .

l . I EEE St andar d 446, Recommended Pr act i ce f or Emer gency and St andbyPower ( Or ange Book) .

m. Mi l i t ar y St andar d ( MI L- STD) 188- 124A, Gr oundi ng, Bondi ng andShi el di ng f or Common Long Haul / Tact i cal Communi cat i on Syst ems.

n. Mi l i t ar y Handbook ( MI L- HDBK) 419A, Gr oundi ng, Bondi ng, andShi el di ng f or El ec t r oni c Equi pment and Fac i l i t i es .

1



o. Nat i onal Fi r e Pr ot ec t i on Assoc i at i on ( NFPA) 70, Nat i onal El ec t r i calCode.

p. Recommended Pr ac t i ce on St at i c El ec t r i c i t y .

q. NFPA 78, L i ght n i ng Pr ot ect i on Code.

r . USAF Handbook f or t he Desi gn and Const r uct i on of HEMP/ TEMPESTShi el ded Fac i l i t i es .

3. Def i ni t i ons.

a. Sys t em Gr oundi ng. The i nt ent i onal connec t i on of a c i r cui tconduc t or , such as t he neut r al of a gener at or , t r ans f or mer or bui l di ngs er v i c e, t o ear t h ei t her di r ec t l y ( s ol i d gr oundi ng) or t hr ough a r es i s t anc eor r eact ance. See Fi gur e 1.

2


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b. Equi pment Gr oundi ng. The connect i on t o ear t h of a l l noncur r entcar r y i ng met al par t s of an el ec t r i cal wi r i ng sys t em. See Fi gur e 2.

c . I sol at ed Gr oundi ng. An i nsul at ed ground i ng conduct or whi ch r unswi t h t he c i r cui t conduc t or s ( i nc l udi ng t he " gr een wi r e" or equi pment

gr oundi ng conduct or ) and per mi t t ed t o pass t hr ough one or mor e panel boar dswi t hout connect i on t o t he panel boar d gr oundi ng t er mi nal and connect ed at t heequi pment gr oundi ng conduct or t e rmi nal of t he appl i cabl e der i ved syst em orser v i ce ent r ance t r ansf or mer . Al t hough not r ecommended except wher e r equi r edby t he NEC, t h i s t ype of gr oundi ng i s somet i mes used t o r educeel ec t r omagnet i c i n t er f er ence or " noi se" on t he gr ound c i r cui t . THI S I S NOT A" SEPARATE" OR " DEDI CATED" GROUNDI NG WHI CH I S UNSAFE AND NOT PERMI TTED BY THENEC. See Fi gur e 3.

3



d. Li ght ni ng Pr ot ec t i on Gr oundi ng. Li ght ni ng pr ot ec t i on i s r equi r edi n f ac i l i t i es t o pr ov i de a nondes t r uc t i v e pat h t o gr ound f or l i ght ni ngcur r ent : t o pr event danger ous s i de f l ashes bet ween met al l i c obj ec t s i n, on ornear a f ac i l i t y ; and t o l i mi t damagi ng ov er v ol t ages i nduc ed on t he el ec t r i c alsys t em. Addi t i onal pr ot ec t i on agai ns t over vol t ages and sur ges i nvol ves t he

di s si pat i on t o ear t h of l i ght ni ng i nduc ed ov er v ol t ages t hr ough ar r es t or t y pedev i ces i ns t al l ed at var i ous l ocat i ons wi t hi n t he sys t em. See Fi gur e 4 f ort ypi cal ai r t er mi nal i nst al l at i ons t o pr ot ect bui l di ngs agai nst di r ec tl i ght ni ng s t r ok es .



e. St a t i c Gr oundi ng. Conduct i ve pat hs bet ween t he- obj ec t s ( bondi ng)and t o ear t h ( g r o undi ng) whi c h wi l l equal i z e any pot ent i al di f f er enc esbet ween t hem and d i ss i pat e any e l ec t r i cal charge. See Fi gure 5.

f . Si gnal Ref er ence Gr i d Gr oundi ng. Met al gr i d t o cont r ol s t at i cchar ge and pr ov i de an equi pot ent i al conduct i ng pl ane t o whi ch hi gh f r equencys i gnal c i r c ui t s ar e r e f er enc ed, t her eby mi ni mi z i ng i nt er f er enc e and noi s e.Thi s gener al ar r angement gr eat l y r educes t he noi se cur r ent s whi ch wi l l f l owi n gr ounded si gnal c i r cui t s as wel l as i n t he saf et y equi pment gr oundconduct ors t o each power ed sens i t i ve el ec t r on i c equi pment . I t compl i es wi t hcode r equi r ement s and has none of t he hazar ds o f t he " i sol a t ed ear t hel ec t r ode gr ound" connect i on. See Fi gur e 6.

5



Opt i onal Gr ound St r ap i n ADP Syst em Gr ound Conduct or s]

6



Par t 2 - Requi r ement s

4. Gener al Requi r ement s . Pr oper el ec t r i cal g r oundi ng i s essent i al f orper sonne l and equi pment saf et y , as wel l as f o r sat i s f ac t or y equi pmentoper at i on. I mpr oper met hods and t echni ques can cr eat e ext r eme saf et y andf i r e haz ar ds , r es ul t i n per s onnel i nj ur y or deat h, and/ or c os t l y damage t o

t he f aci l i t y and i t s cont ent s.

a. Fac i l i t y Gr oundi ng Sys t em Func t i ons . A saf e and ef f ec t i vegr oundi ng s ys t em f or a f ac i l i t y mus t per f or m a number of f unc t i ons t oi nc l ude:

( 1) Sys t em Gr oundi ng. Sys t em gr oundi ng i s r equi r ed f or al lf ac i l i t i es . I t p r o vi des a s t abl e s ys t em v ol t age r ef er enc e f or equi pmentoper at i on; l i mi t s over vol t ages caused by l i ght ni ng, swi t chi ng sur ges , gr oundf aul t s and ot her condi t i ons ; and enabl es pr oper oper at i on of c i r cui tpr ot ec t i ve dev i ces by pr ov i di ng a l ow i mpedance pat h f or f aul t cur r ent .

( 2) Equ i pment Gr oundi ng. Equi pment g roundi ng i s r equ i r ed f or a l l

f ac i l i t i es . I t el i mi nat es t he v ol t age s hoc k haz ar d t o per s onnel c aus ed byuni nt ent i onal c ont ac t of an ener gi z ed c i r c ui t c onduc t or wi t h i t s met al f r ameor enc l osur e, and al so pr ov i des a nondes t r uc t i ve cur r ent car r y i ng pat h f orf aul t c ur r ent unt i l i t c an be i nt er r upt ed by a pr ot ec t i v e dev i c e.

( 3) I sol at ed Gr oundi ng El ec t r odes . Connect i ons t o " separ a t e" or" dedi cat ed" gr oundi ng el ec t r odes whi ch ar e not bonded t o t he f ac i l i t ygr oundi ng el ect r ode sys t em ar e pr ohi bi t ed by t he NEC.

( 4) Li ght ni ng Pr ot ec t i on Gr oundi ng. Di r ec t st r ok e pr ot ec t i oni nv ol v es i ns t al l i ng one or mor e ai r t er mi nal s ( l i ght ni ng r ods , mas t s orel ev at ed/ c at enar y wi r es ) des i gned t o i nt er c ept l i ght ni ng st r ok es , andconnect i ng t hem t o ear t h by down conduct ors and/ or t he met a l bui l di ng f r ame

or s ki n. Ref er enc e Fi gur e 4. Pr ot ec t i on of ex pl os i v es f ac i l i t i es r equi r e t hepr ot ect i on cont ai ned i n AFR 127- 100 and AFR 91- 43.

( 5) St at i c Gr oundi ng. St at i c pr ot ect i on i s r equi r ed i n f aci l i t i esas descr i bed i n subsequent par agraphs, t ypi cal l y wher e f l ammabl e or expl osi vemat er i al s ar e handl ed or pr oc es sed. I t pr ev ent s st at i c el ec t r i c al c har ges onobj ect s such as equi pment , mat er i al s and oper at i ng per sonne l f r omaccumul at i ng t o a pot ent i al whi ch can di schar ge f r om t hem t o gr ound or t oanot her c har ged obj ec t of di f f er ent pot ent i al , c r e at i ng a f i r e or e xpl os i on.

( 6) Si gnal Ref er ence Gr i d Gr oundi ng. Si gnal r ef er ence gr i ds ,whi ch ar e di scussed i n mor e det ai l l at er , shoul d gener al l y be pr ov i ded i ndat a pr ocessi ng/ comput er r oom i ns t al l at i ons and

7



ot her ar eas wher e sens i t i ve el ec t r oni c equi pment ar e i ns t al l ed andi nt er connect ed by h i gh f r equency s i gnal cabl es. These gr i ds must be gr oundedf or saf et y , j us t as al l ot her conduc t i ng member s near ener gi zed el ec t r i calconduct or s must be gr ounded. At t he same t i me, t hey must mi ni mi ze t headver se ef f ect s of any cur r ent s whi ch may be pr esent on t he gr oundi ng syst em.

b. Si ngl e Syst em Requi r ement . The equi pment or subsyst ems used t oper f or m t hese f unc t i ons mus t be i nt egr at ed i nt o a s i ngl e gr oundi ng syst em f ort he f ac i l i t y. SO- CALLED " SEPARATE" OR " DEDI CATED" GROUNDS ARE UNSAFE FORBOTH PERSONNEL AND EQUI PMENT, AND THEI R I NTENDED PURPOSE ( TYPI CALLYACCEPTABLE EQUI PMENT OPERATI ON) CAN BE ACHI EVED BY THE SAFE AND EFFECTI VEMETHODS DESCRI BED BELOW.

( 1) As a mi ni mum, al l gr oundi ng el ec t r odes , gr ound t e rmi na l s andl oop c onduc t or / c ount er poi s e f or a f ac i l i t y i nc l udi ng t hos e us ed f or l i ght ni ngand st at i c pr ot ec t i on, syst em and equi pment g round i ng, t el ephone and ant ennasyst ems, as wel l as t he met al f r ames of t he bui l d i ng and under gr ound met alwat er pi pi ng, shal l be bonded t oget her i n accor dance wi t h Nat i onal El ec t r i calCode ( NEC) Ar t 250-81 t o f o rm t he gr oundi ng e l ec t r ode sys t em f or t he

f ac i l i t y . NOTE THAT UNDERGROUND METAL GAS PI PI NG MUST NOT BE USED AS AGROUNDI NG ELECTRODE.

( 2) Bondi ng bet ween ot her gr oundi ng medi a i n t he f ac i l i t y wi l l bepr ov i ded as spec i f i ed by ot her par agraphs of t hi s ETL and as per mi t t ed i n t her ef er enced publ i cat i ons . Thi s i s necessar y t o mi ni mi ze t he pot ent i aldi f f er enc es bet ween nonc ur r ent c ar r y i ng met al obj ec t s wi t hi n t he f ac i l i t ywhi ch can r esul t f r om l i ght ni ng, gr ound f aul t and ot her condi t i ons . SUCHDI FFERENCES CREATE SHOCK HAZARDS AND CAN ALSO RESULT I N DESTRUCTI VE SI DEFLASHES AND I MPROPER OPERATI ON OF AND DAMAGE TO EQUI PMENT.

c . Compl i ance wi t h Ref er enced Publ i cat i ons . Al l g r oundi ng, s t at i c andl i ght ni ng pr ot ec t i on f eat ur es f or a f ac i l i t y shal l c ompl y wi t h t he appl i c abl e

pr ov i s i ons of t he r e f er ences l i s t ed i n Par agr aph 2 of t hi s ETL except ass pec i f i c al l y not ed.

d. Gr oundi ng and Over al l El ec t r i cal Sys t em Per f ormance. A saf e andef f ec t i v e gr oundi ng sy st em wi l l pr ec l ude many el ec t r i c al s ys t em per f or manc epr ob l ems, but i t i s no t a panacea. I mpr oper or i nadequat e des i gn ori ns t al l at i on of many ot her a spec t s of t he f ac i l i t y ' s el ec t r i c s ys t em c anr esul t i n unaccept abl e equi pment opera t i on and/ or hazar dous cond i t i ons.Fai l ur e t o pr ov i de or i mpr oper i ns t al l at i on or appl i c at i on of s ur gear r est or s , power condi t i oni ng equi pment , and dedi cat ed f eeder s f or t echni call oads ar e j ust some exampl es of def i c i enci es whi ch can cause pr obl ems of t enat t r i but ed t o " i nadequat e gr oundi ng" . The des i gn, i nv es t i gat i on

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and ev al uat i on of a f ac i l i t y ' s el ec t r i c al s ys t em mus t , t her ef or e, c onc ent r at eon many f eat ures of t he el ect r i cal sys t em, no t j us t t he gr oundi ng sys t em.

5. Syst em Gr oundi ng Requi r ement s. The f o l l owi ng par agr aphs cont ai n gener alsys t em gr oundi ng r equi r ement s and c l ar i f i cat i ons of some spec i f i cr equi r ement s whi ch ar e commonl y mi sunder st ood and vi ol at ed.

a. Gener al . El ec t r i c al s y st ems f or al l f ac i l i t i es mus t be gr ounded i naccor dance wi t h t he r equi r ement s of t he NEC and I EEE St andar d 142 ( Gr eenBook) . Sol i d gr oundi ng i s t he pre f er r ed met hod f o r l ow vol t age ( 1000V orl ess) syst ems. Resi st ance gr oundi ng may be consi der ed f or medi um vol t agesys t ems and f or i ns t al l at i ons wi t h mul t i pl e gener at or s , wher e i t may benecessar y t o l i mi t f au l t cur r ent and pr event equi pment damage.

b. Neut r al and Gr ound Connect i ons. NEC Sect i on 250- 23 r equi r es t hatt he gr ounded c i r cui t conduct or such as t he neut r al of a 480/ 277V or 208/ 120Vel ec t r i cal syst em mus t be connec t ed t o t he gr oundi ng el ec t r ode syst em at t het r ansf or mer suppl y i ng t he ser v i ce and at t he ser v i ce ent r ance on t he supp l ys i de of t he ser v i ce di sconnec t i ng means . I t al so r equi r es t hat no ot her

gr oundi ng connect i on be made on t he l oad s i de of t he ser v i ce d i sconnect i ngmeans except i n t he case of a separ a t el y der i ved sys t em. Thi s i nc l udesavoi d i ng connect i ons bet ween t he neut r a l and any gr oundi ng e l ect r odes,equi pment gr oundi ng conduct or s or noncur r ent car r y i ng met al par t s of t hewi r i ng syst em. Fai l ur e t o mai nt ai n t hi s separ at i on bet ween neut r al and" gr ound" t hr oughout t he f ac i l i t y can r es ul t i n unwant ed neut r al r et ur ncur r ent f l owi ng on t he gr oundi ng sys t em. THI S I S A SHOCK AND FI RE HAZARD ANDCAN ALSO LEAD TO I NADVERTENT OPERATI ON OF GROUND FAULT CI RCUI T I NTERRUPTI ONEQUI PMENT, AND I MPROPER OPERATI ON OR DAMAGE TO OTHER EQUI PMENT.

c . Separ at e l y Der i ved Sys t ems. A separa t el y der i ved syst em i s def i nedby t he NEC Sect i on 250- 5 as a pr emi ses wi r i ng syst em whose power i s der i vedf r om a gener at or , t r ans f or mer , or conver t er wi ndi ngs and has no di r ec t

connec t i on, i nc l udi ng a sol i dl y connec t ed gr ounded ci r cui t conduc t or( neut r al ) , t o suppl y conduc t or s i n anot her sys t em. The neut r al of asepara t e l y der i ved syst em must be connect ed t o a near by gr oundi ng el ect r ode,pr ef er abl y s t r uc t ur al met al ( Sec t i on 250- 26) . Addi t i onal l y , a bondi ng j umperi s r equi r ed bet ween t he gr ounded ci r cui t conduc t or ( neut r al ) and t heequi pment gr oundi ng conduct or s ( met a l enc l osur es , condu i t and " green wi r e" )of t he separ a t el y der i ved sys t em.

( 1) Dr y t y pe t r ans f or mer s ( i s ol at i on and non- i s ol at i on t y pes ) ar ecommon sour ces of separa t el y der i ved syst ems i n a f ac i l i t y . They ar egener al l y connect ed i n a del t a- wye conf i gur at i on. To compl y wi t h t he NEC andt o mi ni mi ze t he i mpedance t o gr ound, t he neut r al o f t he t r ans f or mer secondar ymus t be connec t ed t o ei t her t he near es t avai l abl e, ef f ec t i vel y gr ounded

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st r uct ur al met al member of t he st r uct ur e or gr ounded wat er p i pe. Onl y whennei t her i s avai l abl e can any ot her gr oundi ng el ec t r ode ( g r ound r od,count er poi se, et c ) be used. Add i t i onal l y , NEC Sect i on 250- 81 r equi r es t heabove gr oundi ng el ec t r ode( s ) t o be connec t ed t o t he f ac i l i t y gr oundi ngel ect r ode syst em. THE COMMON AND WRONG PRACTI CE OF I SOLATI NG GROUND SYSTEMSTO REDUCE ELECTRI CAL NOI SE ON COMPUTER SYSTEMS VI OLATES THESE ARTI CLES. Thi s

wi l l be di scussed i n mor e det ai l i n subsequent par agr aphs on s i gnal r ef er encegr i d gr oundi ng.

( 2) St andby or emer gency gener at or s ar e al so common sour ces ofsepara t e l y der i ved sys t ems. However , a gener at or connect ed t o a f ac i l i t yt hr ough a t r ans f er s wi t c h i s not a s epar at el y der i v ed s ys t em i f t he neut r alconduct or r emai ns connect ed t o t he nor mal commer ci al power sour ce neut r alaf t er t r ans f er ( i . e. , t he neut r al i s not s wi t c hed al ong wi t h t he phas ec onduc t or s ) . I n t hi s cas e, t he r equi r ed c onnec t i on of t he neut r al t o t hef ac i l i t y ' s gr oundi ng el ec t r ode syst em f or bot h t he commer c i al power sour ceand t he gener at or i s made onl y on t he suppl y si de of t he commer ci al powerser v i ce di sconnect i ng means. Pr ov i d i ng an addi t i onal connect i on bet ween t hegenera t or neut r al and a gr oundi ng el ec t r ode at t he genera t or wou l d be a

gr oundi ng connect i on on t he l oad s i de of t he ser v i ce d i sconnect i ng means anda v i ol at i on o f t he NEC. Ref er t o I EEE St andard 446 f or add i t i onali nf or mat i on and r equi r ement s on gr oundi ng emer gency and st andby gener at or s.

d. Gr oundi ng El ec t r ode Sys t em Res i s t ance. The r es i s t ance of t hegr oundi ng e l ect r ode syst em must be l ow enough t o ensur e gr ound f aul t cur r entcan be r et ur ned t o i t s sour ce wi t hout c r eat i ng unaccept abl e over vol t ages ont he nonc ur r ent c ar r y i ng par t s of t he wi r i ng s ys t em. Addi t i onal l y , t he f aul tc ur r ent mus t be of s uf f i c i ent magni t ude ( par t i c ul ar l y at l oc at i ons r e mot ef r om t he sour ce) t o oper at e over cur r ent dev i ces .

( 1) Low Res i st anc e " Myt h" . LOW RESI STANCE GROUNDI NG OF ELECTRODESYSTEMS I S DESI RABLE BUT NOT ESSENTI AL FOR A SAFE AND EFFECTI VE GROUNDI NG

SYSTEM. I n most f ac i l i t i es , a groundi ng syst em wi t h a gr oundi ng el ec t r odesyst em of r el at i vel y hi gh r esi st ance can st i l l per f or m pr oper l y i f pot ent i aldi f f er ences bet ween t he var i ous noncur r ent car r y i ng met al obj ec t s i n t hef aci l i t y ar e mi ni mi zed usi ng pr oper bondi ng t echni ques. LI KEWI SE, A SYSTEMWI TH A GROUNDI NG ELECTRODE SYSTEM OF VERY LOW RESI STANCE MAY STI LL BE VERYUNSAFE AND I NEFFECTI VE I F OTHER ASPECTS OF THE GROUNDI NG SYSTEM AREI MPROPERLY DESI GNED OR I NSTALLED.

( 2) Basewi de App l i cat i ons . For s t andar d commer c i al and i ndus t r i alsyst ems, I EEE St andar d 142 def i nes an ef f ect i vel y gr ounded syst em ( one whi chi s adequat el y f r ee f r om t r ans i ent over vol t ages ) as one whi ch i s gr oundedt hr ough a l ow enough i mpedance such t hat f or al l syst em condi t i ons , t he r at i o

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of zer o sequence r eact ance t o posi t i ve sequence r eact ance ( X0/ X1) i spos i t i ve and l ess t han 3, and t he r at i o of zer o sequence r es i s t ance t opos i t i ve sequence r eac t ance ( R0/ X1) i s posi t i ve and l ess t han 1. Thi sens ur es t hat l i ne- t o- gr ound t r ans i ent v ol t ages wi l l be l i mi t ed t o 1. 5 t i mest he s ys t em ( per uni t ) v ol t age. Not e t hat p r o per appl i c at i on of t hi s c r i t er i ar equi r es an anal ys i s whi ch cons i der s t he t ot al sys t em r eac t ance i nc l udi ng

t hat of any gener at or s , t r ans f or mer s and r ot at i ng machi nes . I t i s ,t her ef or e, more appr opr i at e f or basewi de syst ems and l a rge i ndus t r i alcompl exes, r at her t han f or i ndi v i dual f ac i l i t i es. Si mi l ar c r i t er i a ar e al sopr ov i ded i n t he st andard f o r r es i s t ance grounded sys t ems.

( 3) Commer c i al and I ndus t r i al Fac i l i t i es. For most commer c i al andi ndus t r i al f ac i l i t i es , t he r equi r ed r es i s t anc e f or t he gr oundi ng el ec t r odesys t em i s der i ved f r om t he NEC Sec t i on 250- 84 whi ch s t at es t hat a si ngl egr oundi ng el ect r ode cons i s t i ng o f a r od, pi pe , or pl at e whi ch does not have ar esi s t ance t o gr ound of 25 ohms or l ess shal l be augment ed by one addi t i onalel ec t r ode. Thi s 25 ohm v al ue i s of t en i nt er pr et ed as t he r equi r ement f or t hegr oundi ng el ec t r ode syst em and r epr esent s a t ar get whi ch i s eas i l y achi evabl eat most f aci l i t i es ( dependi ng on soi l condi t i ons) , par t i cul ar l y i f ot her

r equi r ement s f or bondi ng t oget her of al l avai l abl e gr oundi ng el ec t r odes( Sect i on 250- 81) ar e met .

( 4) Communi cat i onel ec t r oni cs Appl i cat i ons . Gr oundi ng el ec t r odesyst ems f or Communi cat i ons- El ec t r oni cs ( C- E) f ac i l i t i es cover ed by MI L- STD188- 124 and MI L- HDBK- 419 must have a maxi mum r esi st ance of 10 ohms. However ,t hi s shoul d not be i nt er pr et ed as t he r equi r ement f or al l f ac i l i t i escont ai ni ng el ec t r oni c dat a pr ocessi ng and ot her el ec t r oni c equi pment . Des i gnof t hese f aci l i t i es must compl y wi t h AFR 88- 4, Chapt er 11, and FI PS Pub 94 asdi scussed i n subsequent par agr aphs.

( 5) Hazar dous Expl osi ves Appl i cat i ons. AFR 127- 100 cont ai nsgr oundi ng r equi r ement s f or hazar dous expl os i ves f ac i l i t i es . To ensur e

adequat e l i ght ni ng pr ot ec t i on, t he t ar get max i mum gr oundi ng el ec t r ode sys t emr es i s t ance i s 10 ohms. The r egu l at i on spec i f i es anal yses and pr ocedures f orpr ov i di ng an opt i mum syst em r es i s t ance when 10 ohms or l ess i s not achi evabl edue t o s oi l c ondi t i ons .

( 6) Ot her Appl i cat i ons. AFR 91- 43 cont ai ns a t abl e summar i z i ngr es i s t anc e r equi r ement s f or gr oundi ng el ec t r ode sy st ems at ot her f ac i l i t i es( el ect r i cal s ubst at i ons, ai r cr af t par ki ng apr ons, et c) .

( 7) Obt ai ni ng Bet t er Gr ounds . I f t he gr oundi ng el ec t r ode sys t emr es i s t ance i s not sat i s f ac t or y and an adequat e l evel of saf et y or equi pmentper f or mance i s not achi evab l e due t o poor soi l condi t i ons ( r ocky , sandy orshal l ow soi l s ) af t er a l l NEC r equi r ement s have been met , an anal ys i s

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( i nc l udi ng soi l r es i s t i v i t y measur ement s ) o f t he var i ous met hods f orobt ai ni ng l ower r es i s t ance such as deeper r od penet r at i on, par al l el r ods ,l oop conduct or / count er poi se, soi l r epl acement and concr et e encapsul at i on mustbe per f or med. THE MOST PRACTI CAL AND COST EFFECTI VE METHODS WHI CH WI LLENSURE AN ADEQUATE LEVEL OF SAFETY AND ACCEPTABLE EQUI PMENT PERFORMANCE MUSTBE I MPLEMENTED.

e. Gr ound Faul t Pr ot ect i on. NEC Sect i on 230- 95 r equi r es gr ound f aul tpr ot ec t i on of equi pment f or sol i dl y gr ounded wye el ec t r i cal ser v i ces of mor et han 150V t o gr ound, but not exceedi ng 600V phase t o phase f or each ser vi cedi sc onnec t i ng means r at ed 1000A or mor e. THI S I S NECESSARY TO PREVENTEQUI PMENT DAMAGE FROM HI GH I MPEDANCE ( ARCI NG) GROUND FAULTS WHI CH MAY HAVESUFFI CI ENT ENERGY TO CAUSE EQUI PMENT HEATI NG AND BURN DOWN, BUT NOT ENOUGHCURRENT FLOW TO ACTI VATE PROTECTI VE DEVI CES. However , t o pr event shut downof t he ent i r e f ac i l i t y due t o gr ound f aul t s on f eeder s , gr ound f aul tpr ot ec t i on wi t h " zone i sol at i on" f eat ur es mus t be pr ov i ded on al l f eederc i r c ui t s t o c oor di nat e t he pr ot ec t i on and l i mi t t he por t i on of t he s ys t emaf f ect ed. An except i on t o t hi s i s NEC Sect i on 215-10: Feeder gr ound f aul tpr ot ect i on ( GFP) i s not r equi r ed wher e GFP of equi pment i s pr ovi ded on t he

suppl y s i de of t he f eeder .

6. Equi pment Gr oundi ng Requi r ement s. As pr evi ousl y st at ed, equi pmentgr oundi ng i s necessar y t o el i mi nat e t he vol t age shock hazar d t o per sonnelcaused by uni nt ent i onal cont ac t of an ener gi zed ci r cui t ( phase) conduc t orwi t h i t s met al f r ame or enc l osur e, and al so pr ov i des a nondes t r uc t i ve cur r entc ar r y i ng pat h f or f aul t c ur r ent unt i l i t c an be i nt er r upt ed by a pr ot ec t i v edevi ce. The NEC r equi r ement s i n Ar t 250 must be met wi t h t he f o l l owi ngex cept i ons and cl ar i f i c at i ons .

a. No Subst i t ut es f or Equi pment Gr oundi ng Conduct or s. The noncur r entcar r y i ng met al par t s of a l l equi pment must be connect ed t o an equi pmentgr oundi ng conduc t or syst em whi ch pr ov i des an el ec t r i cal l y cont i nuous pat h f or

f aul t c ur r ent t o r et ur n di r ec t l y t o t he s our c e of t he s ys t em s uppl y i ng t heequi pment ( bui l di ng ser v i ce or separ at el y der i ved syst em, whi chever i sappl i c abl e) . The ear t h, whi c h i s a r el at i v el y poor c onduc t or , i s not anaccept abl e equi pment gr oundi ng conduct or . Ther ef or e, connect i ons t o near bypar t s of t he gr oundi ng el ec t r ode sys t em such as s t r uc t ur al s t eel orcount er poi se/ gr ound l oop conduct or whi ch may be desi r ab l e f or l i ght ni ngpr ot ect i on or ot her pur poses ar e per mi t t ed, BUT ARE NOT A SUBSTI TUTE FOR ANEQUI PMENT GROUNDI NG CONDUCTOR TO THE SYSTEM SOURCE.

b. No Separ at e Gr ounds. Connect i ons t o " separ at e" or " dedi cat ed"gr oundi ng el ec t r odes whi ch ar e not bonded t o t he f ac i l i t y gr oundi ng el ec t r odesyst em are pr ohi bi t ed by t he NEC. Thi s i s necessar y t o pr event dangerouspot ent i al di f f er enc es f r om dev el opi ng dur i ng l i ght ni ng and f aul t c ondi t i ons

bet ween equi pment " gr ounded" i n t hi s manner and any near by met al obj ect s

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connec t ed t o ot her g r oundi ng el ec t r odes . Addi t i onal i nf or mat i on on t hi sr es t r i c t i on and on t he pr oper appl i c at i on and i ns t al l at i on of " i s ol at ed"gr ounds i s cover ed i n t he par agr aphs on e l ec t r oni c dat a pr ocess i ng and s i gnalr ef er ence gr i ds .

c. Equi pment Gr oundi ng Conduct or s. A separ at e, appr opr i at e l y s i zed,

" gr een ( o r bar e) wi r e" equi pment gr oundi ng conduc t or i s r equi r ed f or al lf eeder a nd br anc h c i r c ui t s i n al l f ac i l i t i es . Met al l i c c ondui t may not beused t o sat i s f y t he r equi r ement . Thi s par agr aph super seded AFR 88- 15, par a16- 8 on " gr een wi r e" r equi r ement s.

7. Li ght ni ng Pr ot ec t i on. Li ght ni ng pr ot ec t i on s ys t ems pr ov i de f or t he s af edi s si pat i on of l i ght ni ng s t r ok es and s ur ges i nt o t he ear t h and l i mi t t het r ans i ent ov er v ol t ages i nduc ed on t he el ec t r i c al s ys t em t o ac cept abl e l ev el s .The r equi r ement s of AFM 88- 9, Chapt er 3, and NFPA 78 appl y f or st andar df ac i l i t i es wi t h t he f ol l owi ng c l ar i f i c at i ons and ex cept i ons . AFR 127- 100s pec i f i es addi t i onal r equi r ement s f or h az ar dous ex pl os i v es f ac i l i t i es .

a. Nonconvent i onal Sys t ems. Onl y l i gh t ni ng pr ot ec t i on sys t ems

r ecogni zed by t he NFPA and Under wr i t er s Labor at or i es may be used.Nonconvent i onal syst ems such as d i ss i pat i on ar r ays and t hose usi ngr adi oac t i v e l i ght ni ng r o ds ar e not ac cept abl e.

b. Manual Changes and Addi t i ons. The f o l l owi ng addi t i ons and changest o AFM 88- 9, Chapt er 3 appl y:

( 1) Ai r Ter mi nal Ty pes . I n addi t i on t o t he t r adi t i onal i nt egr alsyst em of ai r t er mi nal s ( commonl y known as l i ght ni ng r ods) , syst ems us i ngover head wi r es ( cat enar i es) , mast s or f ar aday cage i n accor dance wi t h NFPA 78ar e accept abl e. Mast s and over head wi r e syst ems have r educed mai nt enancer equi r ement s and af f or d a hi gh l ev el of l i ght ni ng pr ot ec t i on. Ex pl os i v ef ac i l i t i es wi t h per i met er s over 300 f eet must use a mast or over head wi r e

syst em per AFR 91- 43.

( 2) Hazar dous Locat i ons . Li gh t ni ng pr o t ec t i on sys t ems f orhazar dous expl osi ves f aci l i t i es must compl y wi t h AFR 127- 100.

( 3) Ter mi nal s . I nt egr al t y pe ai r t er mi nal s mus t hav e poi nt ed t i ps ;capped t er mi nal s ar e no l onger per mi t t ed. Ter mi nal s must be copper( pr ef er r ed) or a l umi num i n accor dance wi t h NFPA 78. Use a t er mi nal 18 i nchesl ong.

( 4) St ai nl ess s t eel gr ound r ods ar e subj ec t t o cor r os i onunder gr ound and ar e no l onger per mi t t ed.

( 5) Hi gh Compr essi on Connect i ons. I n addi t i on t o spot wel d andexot her mi c wel d t ype connect i ons, h i gh compr essi on connect i ons ar e a l soaccept abl e f or gr oundi ng and bondi ng.

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c. Bondi ng. Adequat e bondi ng of met al obj ect s and bodi es i n, on andnear a f ac i l i t y t o t he l i ght ni ng pr ot ec t i on s ys t em i s one of t he mos tc r i t i c al as pec t s of e f f ec t i v e l i ght ni ng pr ot ec t i on. Thi s ens ur es pot ent i alequa l i zat i on and f r eedom f r om danger ous s i def l ashes. Chapt er 3 of NFPA 78c ont ai ns det ai l s on mi ni mum bondi ng r e qui r ement s f or or di nar y f ac i l i t i es .

d. Gr ound Ter mi nal Subsyst em Resi st ance. NFPA 78 does not speci f y amax i mum r es i s t ance f or t he gr ound t er mi nal subsys t em ( connec t i on of t hel i ght ni ng pr ot ec t i on s ys t em t o ear t h) f or o r d i nar y s t r uc t ur es . Howev er , i tdoes r equi r e appl i cat i on of spec i f i c t echni ques such as l onger and mul t i pl egr ound r ods and l oop conduct or / count er poi se t o r educe t he r es i s t ance i n h i ghr esi st i v i t y soi l condi t i ons ( s andy, gr avel l y or " shal l ow" s oi l ) . I t al sor equ i r es bond i ng of t he gr ound t er mi nal subsys t em t o ot her gr oundi ng medi a,c ons i s t ent wi t h t he NEC r equi r ement f or t he f ac i l i t y ' s gr oundi ng el ec t r odesys t em.

e. Appl i c at i on of Sur ge/ Li ght ni ng Ar r es t or s . Tr ans i ent ov er v ol t agesand s ur ges on a f ac i l i t y ' s el ec t r i c al s ys t em c an be pr oduc ed by l i ght ni ng,swi t chi ng, gr ound f aul t s and ot her condi t i ons . Li mi t i ng t hese sur ges t o an

ac cept abl e l ev el r equi r es pr oper appl i c at i on of s ur ge ar r es t or s at al l l ev el sof t he el ec t r i c al s ys t em, i nc l udi ng bot h t he i nt er i or and ex t er i or di s t r i bu-t i on sys t em. Ar r es t er s i ns t al l ed wi t hi n a f ac i l i t y may be over whel med anddes t r oyed by sur ges i nduced on i ncomi ng di s t r i but i on l i nes i f t hat por t i on oft he syst em has not been adequat el y pr ot ect ed. As a mi ni mum, appr opr i at ec l ass sur ge ar r es t or s must be pr ov i ded at t he power t r ans f ormer suppl y i ng t hef aci l i t y and i n t he f aci l i t y ' s mai n di st r i but i on panel or s wi t chgear .Addi t i onal sur ge pr ot ect i on and power condi t i oni ng may be prov i ded by t heuser f or hi s i ndi v i dua l equ i pment i t ems i n accor dance wi t h ETL 86- 17.

f . Spaci ng f r om Li ght ni ng Rods. NEC Sect i on 250-46 r equi r es a t l eas ta 6- f oot c l ear ance bet ween l i ght n i ng r od conduct or s and met al r aceways,enc l osur es , f r ames and ot her noncur r ent car r y i ng met al par t s of el ec t r i cal

equi pment . Ot her wi se, t hi s equi pment must be bonded t o t he l i gh t ni ng r odconduct or s.

8. Gr oundi ng and Bondi ng f or St at i c Pr ot ec t i on. St at i c pr ot ec t i onr equi r ement s i n AFM 88- 9, Chapt er 3 and NFPA 77 appl y wi t h t he except i ons andc l ar i f i cat i ons bel ow. AFR 127- 100 spec i f i es addi t i onal r equi r ement s f orhaz ar dous ex pl os i v es f ac i l i t i es .

a. Syst em I nt egr at i on. Bondi ng and gr oundi ng ar e common t echni quesf or s t at i c pr ot ec t i on. Bondi ng or e l ec t r i c al l y c onnec t i ng t wo or mor econduc t i ve obj ec t s equal i zes t he pot ent i al bet ween t he obj ec t s . Gr oundi ngmi ni mi zes t he pot ent i a l di f f er ences bet ween t he ob j ec t s and t he gr ound bypr ov i di ng a connec t i on bet ween t hem and t he ear t h t o dr ai n of f s t at i c char ges

bef or e t hey r each a spar k i ng pot ent i al . Al t hough s t at i c gr ounds ar e not par tof t he f ac i l i t y ' s el ec t r i c al power s ys t em, t hey mus t be pr oper l y i nt egr at ed

14



i nt o t he ov er al l f ac i l i t y gr oundi ng s ys t em and not appl i ed i n i s ol at i on.

b. St at i c Gr ound Res i s t ance. Al l g r ounds used f or s t at i c pr ot ec t i oni n Ai r For c e f ac i l i t i es i nc l udi ng t hos e f or ai r c r af t and f uel t ank s mus t hav ea maxi mum r es i s t ance o f 10, 000 ohms. Thi s i s i n cont r ast t o AFM 88- 9 ,Chapt er 3, whi ch r ecommends mai nt ai ni ng an aver age r ange of bet ween 25, 000 t o

100, 000 ohms ( r esi st ances as hi gh as a megohm can of t en pr ovi de an adequat el eak age pat h f or s t at i c el ec t r i c i t y ) t o l i mi t f aul t c ur r ent and av oi d s hoc khazar d t o per sonne l . Any danger of el ec t r i cal shock hazar d caused by t he10, 000 ohm val ue can be el i mi nat ed by pr oper bondi ng t o ot her gr oundi ng medi a( equ i pment gr oundi ng conduct or s , l i ght ni ng pr ot ec t i on sys t em component s ,gr oundi ng el ect r ode sys t em) .

c . " Except i on" . A spec i f i c except i on t o t he bondi ng r equi r ement oft he pr ev i ous par agraph i nvol ves s t at i c bus bars i n hazar dous expl os i vesf ac i l i t i es per AFR 127-100. These s t at i c bus bar s must be used onl y f ors t at i c gr ound i ng and must not be connect ed t o t el ephone gr ounds, el ec t r i calcondui t or ot her near by gr oundi ng medi a. However , t he gr oundi ng e l ect r odesf or t hese bar s must be bonded t o t he f ac i l i t y ' s gr oundi ng e l ec t r ode sys t em.

d. Conduc t or Si ze. Bondi ng conduc t or s used f or st at i c pr ot ec t i onshoul d be #6 mi ni mum ( bar e or i nsul at ed) sol el y f or mechani cal s t r engt h si nces t at i c el ec t r i cal cur r ent s ar e ver y smal l ( mi c r oamps) . However , anygr oundi ng conduct or adequat e f or power c i r cui t s ( s i zed i n accor dance wi t h t heNEC) or l i ght ni ng pr ot ec t i on i s gener al l y adequat e f or pr ot ec t i on f r om s t at i cel ect r i c i t y.

9. El ect r oni c Dat a Pr ocessi ng Equi pment . St andar d gr oundi ng t echni queswhi ch ar e adequat e f or equi pment oper at i ng at 60 HZ may be t ot a l l y i nadequat ef or el ec t r oni c dat a pr ocess i ng equi pment whi ch may oper at e at c l ock ands i gnal f r equenc i es wel l above 10 MHZ. The under l y i ng r eason f or t hi s i s t hei nherent l y hi gh i mpedance o f s t andar d conduct or s / wi r es at t hese f r equenc i es .

I n l engt hs gr eat er t han 1/ 20 of a wavel engt h ( about 4 f eet at 10 MHZ) , t heyar e ver y i nef f ec t i ve at equal i z i ng pot ent i al and can l ead t o r esonancecond i t i ons and el ect r omagnet i c and capac i t i ve coup l i ng of noi se andi nt er f er enc e ont o s i gnal c i r c ui t s . Spec i al t ec hni ques ar e t her ef or e r equi r edt o pr ov i de a saf e and ef f ec t i ve gr oundi ng and s i gnal r ef er enc i ng sys t em f ort hi s equ i pment .

a. Saf et y Fi r s t . Accept abl e per f or mance of el ec t r oni c equi pment i snot an excuse t o v i ol a t e appl i cabl e saf et y r equi r ement s i n gover ni ng codessuch as t he NEC and NFPA 78. The 1990 NEC cont ai ns r evi si ons andcl ar i f i cat i ons t o speci f i cal l y pr ohi bi t t hi s.

( 1) NEC Ar t 100 cont a i ns a r ev i sed def i ni t i on of " pr emi ses

wi r i ng" . Such wi r i ng ( whi ch i s subj ec t t o NEC r equi r ement s ) i nc l udes al li nt er i or and ex t er i or wi r i ng and i nt er connec t ed equi pment on t he l oad si de of

15



t he ser v i ce and t he out put of any separ at el y der i ved sys t em ( such as at r ans f or mer , gener at or , c omput er power di s t r i but i on uni t , or uni nt er r upt i bl epower suppl y) .

( 2) NEC Ar t 250- 21 now s t at es t hat cur r ent s whi ch i nt r oduce no i seor dat a er r or s i n el ec t r oni c equi pment ar e not cons i der ed t o be obj ec t i onabl e

cur r ent s t hat al l ow modi f i cat i on of NEC groundi ng requi r ement s .

b. Desi gn and Const r uct i on Requi r ement s. AFM 88- 4, Chapt er 11cont ai ns des i gn and cons t r uc t i on r equi r ement s f or el ec t r oni c dat a pr ocess i ngf ac i l i t i es . By r ef er enc e, i t i nc or por at es t he gui del i nes of FI PS Pub 94whi ch addr esses saf e and ef f ec t i ve ground i ng and s i gnal r ef er enc i ngt echni ques . Communi cat i onsel ec t r oni c f ac i l i t i es and f aci l i t i es wher e TEMPESTand HEMP ar e consi der at i ons must be desi gned i n accor dance wi t h MI L- HDBK419A, MI L- STD 188- 124A, and t he USAF Handbook f or t he Desi gn and Const r uct i onof HEMP and TEMPEST Shi el ded Faci l i t i es, as appr opr i at e.

c. Zer o Si gnal Ref er ence Gr i d ( ZSRG) . As pr evi ousl y s t at ed, ZSRGs ar epr ov i ded i n comput er r ooms and o t her f ac i l i t i es wher e e l ec t r oni c equi pment

i t ems ar e i nt er connect ed by hi gh f r equency dat a and s i gnal cabl es. They ar eused t o cont r ol s t at i c char ge and pr ov i de an equi pot ent i al conduc t i ng pl anet o whi ch hi gh f r equency s i gnal c i r cui t s ar e r ef er enced, t her eby mi ni mi z i ngi nt er f er ence and noi se.

( 1) Rai sed Fl oor s. The i deal ZSRG woul d be a cont i nuous sheet ofmat er i al wi t h good sur f ace conduc t i v i t y ( copper , al umi num, z i nc pl at ed s t eel )t o whi ch al l dat a/ s i gnal cabl e shi el ds and equi pment encl osur es coul d bebonded ( r ef er enced) by ver y shor t connect i ons . However , t h i s i s notpr ac t i cal f r om a c ost and i nst al l at i on st andpoi nt . Al t er nat i ves t o t hi swhi ch exhi bi t accept ab l y l ow i mpedance at hi gh f r equenc i es ar e gr i ds ofc opper wi r e or s t r aps whi c h ar e i ns t al l ed under t he r ai s ed f l oor t y pi c al l yf ound i n el ec t r oni c da t a pr ocess i ng r ooms. One di sadvant age of t hese opt i ons

i s t he i nc r e as e i n l engt h of t he connec t i ons f r om t he equi pment t o t he gr i ddue t o t he he i ght of t he r a i sed f l oor . Thi s di sadvant age can be over come andcos t s r educed by usi ng t he r ai sed f l oor i t sel f as t he ZSRG. These f l oor swi l l have accept abl y l ow i mpedance i f t hey empl oy car ef ul l y i ns t al l edbol t ed- down st r i nger s an t hey ar e sui t abl y pl at ed wi t h t i n or z i nc t o per mi tl ow r es i s t ance pr essur e connect i ons .

( 2) Saf et y Connect i on. ZSRGs must be gr ounded f or saf et y, j ust asal l ot her conduct i ng member s near ener gi zed el ec t r i cal conduct or s must begr ounded. The conduct or whi ch gr ounds t he gr i d at some conveni ent poi nt t ot he equi pment gr ound bus i n a br anc h c i r c ui t di s t r i but i on ci r c ui t br eak erpanel shoul d be a gr een wi r e encl osed i n condui t and of a conduct or s i zeappr opr i at e ( per t he NEC) t o t he l ar gest phase conduct or ampaci t y br ought

i nt o t he comput er r oom.

16



( 3) Li ght ni ng Pr ot ec t i on. To pr ov i de adequat e pr ot ec t i on f r oml i ght ni ng i nduc ed ov er v ol t ages , any met al obj ec t s ( s t r uc t ur al s t eel ,el ec t r i cal and communi cat i on condui t s , wat er pi pes , ai r condi t i oni ng duc t s ,et c) passi ng t hr ough or near t he ZSRG must be bonded t o i t . Al so, I FBUI LDI NG STRUCTURAL STEEL I S NOT AVAI LABLE, MULTI PLE CONNECTI ONS SHOULD BEMADE, i f pr ac t i cal , be t ween t he ZSRG and t he f ac i l i t y ' s gr oundi ng el ect r ode

sys t em.

d. I sol at ed Gr ound ( I G) Equi pment . Cer t ai n i sol at ed gr oundi ngt echni ques ar e per mi t t ed by t he NEC. However , t hey shoul d not be conf usedwi t h "separ at e" or " dedi cat ed" grounds wh i ch ar e unsaf e and not per mi t t ed .Addi t i onal l y , s i nc e t hey ar e of ques t i onabl e val ue f or hi gh f r equenc ygr oundi ng and may , dur i ng l i ght ni ng c ondi t i ons , ex hi bi t pot ent i al di f f er enc esbet ween t he " i sol at ed" equi pment gr ound and ot her par t s ( such as met alcondui t and enc l osur es ) of t he f ac i l i t y ' s equi pment gr oundi ng conduc t orsyst em, t hey shoul d be used onl y when r equi r ed by gover ni ng codes ( hospi t a lappl i cat i ons) or when sat i s f ac t ory equi pment oper a t i on cannot be achi eved byany ot her appr oved met hod.

( 1) I G Recept ac l es . I n I G r ecept ac l es , t he gr oundi ng pi n i si nsul at ed f r om t he r ecept acl e body and mount i ng yoke. Per NEC Ar t 250- 74,t h i s gr oundi ng p i n must not be connect ed t o any separ at e or dedi cat ed gr ound,but must - beconnect ed ( by an i nsul at ed conduct or r un wi t h t he c i r cui tconduc t or s ) t o t he equi pment gr oundi ng conduc t or t er mi nal of t he el ec t r i cals our c e s uppl y i ng t he equi pment ( bui l di ng ser v i c e or f i r s t s epar at el y der i v eds ys t em, whi c hev er i s appl i c abl e) . Addi t i onal l y , t he body of t he r ec ept ac l e( and met al out l et box and condui t ) must be connect ed t o t he " gr een wi r e"f ac i l i t y equi pment gr oundi ng conduc t or .

( 2) Har dwi r ed Equi pment . The 1990 NEC ( Ar t 250- 75) per mi t s t hei nst al l at i on of an i ns ul at i ng spacer o r f i t t i ng t o i nt er r upt t he el ec t r i c alcont i nui t y of a met al l i c r aceway sys t em at t he poi nt of connec t i on t o a met al

enc l osur e i f t he met al condui t i s gr ounded at i t s suppl y end and an equi pmentgr oundi ng conduct or i s r un t hr ough t he condui t i nt o t he met al encl osur e andi s connect ed t o t he equi pment gr ound i ng t er mi nal o f t he enc l osur e. As i n t hecase of I G r ecept acl es, t h i s equi pment gr oundi ng conduct or must not beconnect ed t o a separ at e or dedi cat ed gr ound, but t o t he equi pment gr oundi ngt er mi nal of t he sour ce supp l y i ng t he equi pment .

( 3) I sol at i on Tr ans f or mer s . I sol at i on t r ans f or mer mus t compl ywi t h t he NEC r equi r ement s f or separ at el y der i ved syst ems and must not be usedt o est ab l i sh separ at e or dedi cat ed grounds f o r equ i pment . As di scussed i npar agr aph 5c above, t he neut r a l , encl osur e ( and equi pment gr oundi ng conduct orof t he t r ans f or mer ' s sour ce) , shi el d and equi pment gr oundi ng conduct or sys t emof t he i sol at i on t r ans f or mer mus t be connec t ed t o t he near es t ef f ec t i vel y

17



gr ounded st r uct ur a l s t eel o r met al wat er pi pe t o mi ni mi ze t he i mpedance t ogr ound. Any ot her t ype of gr oundi ng el ec t r ode must not be used i f ei t her i savai l abl e. SEPARATE OR DEDI CATED GROUNDI NG ELECTRODES ARE UNSAFE ANDPROHI BI TED BY NEC Ar t 250- 81.

18



I ndex of Oct 90

ENGI NEERI NG TECHNI CAL LETTERS ( ETL)

SECTI ON A - CURRENT ETLs

ETL Number Ti t l e Dat e I ssued

82- 2 Ener gy Ef f i c i ent Equi pment 10 Nov 8283- 1 Desi gn of Cont r ol Syst ems f or HVAC 16 Feb 83

Change No. 1 t o ETL 83- 1, U. S. Ai r 22 Jul 87For ce St andar di zed Heat i ng, Vent i l at i ng& Ai r Condi t i oni ng ( HVAC) Cont r ol Syst ems

83- 3 I nt er i or Wi r i ng Syst ems, AFM 88- 15 2 Mar 83Par a & 7- 3

83- 4 EMCS Dat a Tr ansmi ss i on Medi a ( DTM) 3 Apr 83Consi dera t i ons

83- 7 Pl umbi ng, AFM 88- 8, Chapt er 4 30 Aug 8383- 8 Use of Ai r - t o- Ai r Uni t ar y Heat Pumps 15 Sep 83

83- 9 I nsul at i on 14 Nov 8384- 2 Comput er Ener gy Anal ysi s 27 Mar 84

Change 1 Ref : HQ USAF/ LEEEU Msg031600Z MAY 84 1 Jun 84

84- 7 MCP Ener gy Conser vat i on I nvest ment 13 Jun 84Pr ogr am ( ECI P)

84- 10 Ai r For ce Bui l d i ng Const r uct i on and 1 Aug 84t he Use of Ter mi t i c i des

86- 2 Ener gy Management and Cont r ol Sys t ems 5 Feb 86( EMCS)

86- 4 Pai nt s and Pr ot ect i ve Coat i ngs 12 May 8686- 5 Fuel s Use Cr i t er i a f or Ai r For ce 22 May 86

Cons t r uc t i on

86- 8 Aqueous Fi l m For mi ng Foam Wast e 4 Jun 86Di schar ge Ret ent i on and Di sposal

86- 9 Lodgi ng Faci l i t y Desi gn Gui de 4 Jun 8686- 10 Ant i t er r or i sm Pl anni ng and 13 Jun 86

Desi gn Gui dance86- 14 Sol ar Appl i cat i ons 15 Oct 8686- 16 Di r ec t Di gi t al Cont r o l Heat i ng 9 Dec 86

Vent i l a t i on and Ai r Condi t i oni ng Sys t ems87- 1 Lead Ban Reqmt s of Dr i nki ng Wat er 15 Jan 8787- 2 Vol at i l e Or gani c Compounds 4 Mar 8787- 4 Ener gy Budget Fi gur es ( EBFS) f or 13 Mar 87

Fac i l i t i es i n t he Mi l i t ar y Cons t r uc t i on Pr ogr am87- 5 Ut i l i t y Met er s i n New and Renovat ed 13 Jul 87

Faci l i t i es87- 9 Pr ewi r i ng 21 Oct 87

At ch 3 ( 1 of 3)



I ndex of Oct 90ENGI NEERI NG TECHNI CAL LETTERS ( ETL)

SECTI ON A - CURRENT ETLs

ETL Number Ti t l e Dat e I ssued

88- 2 Phot ovol t ai c Appl i cat i ons 21 Jan 8888- 3 Des i gn St andar ds f or Cr i t i cal Fac i l i t i es 15 Jun 8888- 4 Rel i abi l i t y & Mai nt ai nabi l i t y ( R&M) 24 Jun 88

Desi gn Checkl i s t88- 5 Cat hodi c Pr ot ect i on 2 Aug 8888- 6 Heat Di st r i but i on Syst ems Out s i de of 1 Aug 88

Bui l di ngs88- 8 Chl or of l uor ocar bon ( CFC) L i mi t at i on i n 4 Oct 88

Heat i ng, Vent i l at i ng and Ai r - Condi t i oni ng( HVAC) Sys t ems

88- 9 Radon Reduct i on i n New Faci l i t y 7 Oct 88Cons t r uc t i on

88- 10 Pr ewi r ed Wor kst at i ons Gui de Speci f i cat i on 29 Dec 8889- 2 St andar d Gui del i nes f or Submi ssi on of 23 May 89

Faci l i t y Oper at i ng and Mai nt enance Manual s89- 3 Fac i l i t y Fi r e Pr ot ec t i on Cr i t er i a f or 9 J un 89

El ec t r oni c Equi pment I ns t al l at i ons89- 4 Sys t ems Fur n i t ur e Gui de Spec i f i cat i on 6 Jul 8989- 5 Ai r For ce I nt er i or Des i gn Pol i cy not yet89- 6 Power Condi t i oni ng and Cont i nuat i on

I nt er f aci ng Equi pment ( PCCI E) i n t heMi l i t ar y Const r uc t i on Pr ogram ( MCP) 7 Sep 89

89- 7 Desi gn of Ai r For ce Cour t r ooms 29 Sep 8990- 1 Bui l t - Up Roof ( BUR) Repai r / Repl acement

Gui de Spec i f i cat i on 23 Jan 90

90- 2 Gener al Pol i cy f o r Pr ewi r ed Works t a t i onsand Syst ems Fur ni t ur e 26 Jan 90

90- 3 TEMPEST Pr ot ec t i on f o r Fac i l i t i esChange 1 Ref : HQ USAF/ LEEDE Lt r

dat ed 20 Apr i l 90, Same Subj ect 20 Apr 9090- 4 1990 Ener gy Pr i ces and Di scount Fact or s

f or Li f e- Cyc l e Cost Anal ys i s 24 May 9090- 5 Fuel and Lube Oi l Bul k St or age Capaci t y

f or Emer gency Gener at or s 26 Jul 9090- 6 El ec t r i cal Syst em Gr oundi ng, St at i c and

Li ght ni ng Pr ot ec t i on 3 Oc t 90

At ch 3 ( 2 of 3)3 Oct 90



SECTI ON B - OBSOLETE ETLs Oct 90

No. Dat e St at us

82- 1 10 Nov 82 Super seded by ETL 83- 10, 86- 1, 87- 4

82- 3 10 Nov 82 Super seded by ETL 83- 5, 84- 282- 4 10 Nov 82 Super seded by ETL 84- 782- 5 10 Nov 82 Super seded by ETL 84- 1, 86- 13, 86- 1482- 6 30 Dec 82 Cancel l ed82- 7 30 Nov 82 Cancel l ed83- 2 16 Feb 83 Super seded by ETL 84- 383- 6 24 May 83 Cancel l ed

84- 3 21 Mar 84 Cancel l ed84- 4 10 Apr 84 Super seded by ETL 86- 7, 86- 159 87- 584- 5 7 May 84 Super seded by ETL 84- 89 86- 11, 86- 18, 88- 684- 6 Not I ssued Cancel l ed/ Not Used84- 9 5 Jul 84 Super seded by ETL 88- 7

86- 3 21 Feb 86 Super seded by ETL 86- 486- 6 3 Jun 86 Super seded by ETL 86- 11, 86- 18, 88- 686- 7 3 Jun 86 Super seded by ETL 86- 1586- 12 3 Jul 86 Super seded by ETL 90- 286- 13 18 Aug 86 Super seded by ETL 86- 1486- 15 13 Nov 86 Super seded by ETL 87- 586- 17 17 Dec 86 Super seded by ETL 89- 686- 18 18 Dec 86 Super seded by ETL 88- 687- 3 12 Mar 87 Super seded by ETL 87- 6, ETL 88- 587- 6 21 Aug 87 Super seded by ETL- 88- 587- 7 14 Oct 87 Super seded by ETL 89- 1Chg 1 30 Dec 87 Super seded by ETL 90- 188- 1 5 Jan 88 Super seded by ETL 89- 2

88- 7 24 Aug 88 Super seded by ETL 90- 3 ( TEMPEST Por t i on)89- 1 6 Feb 89 Super seded by ETL 90- 4

At ch 3 ( 3 of 3)

electrical system grounding- static grounding and lightning protection...

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