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“A Stitch i Time...”
The Cmpete Gie t Eectrica Isati Testig
W W W .B i d d l e
M e g g e r
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.C O M
“A Stitch I Time” The Cmpete Gie t Eectrica Isati Testig Cpright 2006
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ConTEnTS
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WHAT IS “Good” InSUlATIon? ........................................................................ 3 WHAT MAkeS InSUlATIon Go BAd? ................................................................ 4 HoW InSUlATIon ReSISTAnCe IS MeASURed .................................................. 5 HoW To InTeRPReT ReSISTAnCe ReAdInGS .................................................... 6 FACToRS AFFeCTInG InSUlATIon ReSISTAnCe ReAdInGS .............................. 8 TyPeS oF InSUlATIon ReSISTAnCe TeSTS....................................................... 10 TeST VolTAGe VS. eqUIPMenT RATInG.......................................................... 16 AC TeSTInG VS. dC ............................................................................................ 17 USe oF dC dIeleCTRIC TeST SeT ...................................................................... 18 TeSTS dURInG dRyInG oUT oF eqUIPMenT.................................................. 18 eFFeCT oF TeMPeRATURe on InSUlATIon ReSISTAnCe................................ 21 eFFeCTS oF HUMIdITy ...................................................................................... 23 PRePARATIon oF APPARATUS To TeST ........................................................... 24 SAFeTy PReCAUTIonS ...................................................................................... 26 ConneCTIonS FoR TeSTInG InSUlATIon ReSISTAnCe oF eleCTRICAl eqUIPMenT .................................................................................. 27 AddITIonAl noTeS ABoUT USInG A MeGGeR InSUlATIon TeSTeR ........... 33 InTeRPReTATIon-MInIMUM VAlUeS .............................................................. 36 MInIMUM VAlUeS FoR InSUlATIon ReSISTAnCe .......................................... 38 TeSTS USInG MUlTI-VolTAGe MeGGeR InSUlATIon TeSTeRS...................... 42 STeP-VolTAGe MeTHod .................................................................................. 48 USe oF A GUARd TeRMInAl ............................................................................ 50 BUSHInGS, PoTHeAdS And InSUlAToRS ....................................................... 54 oUTdooR oIl CIRCUIT BReAkeRS................................................................... 57 SeTTInG UP A MAInTenAnCe PRoGRAM ....................................................... 60 HoW oFTen SHoUld yoU TeST? .................................................................... 60 MeGGeR 5 And 10 kV InSUlATIon TeSTeRS.................................................. 62
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WHAT IS “Good” InSulATIon? evr ctric wir i ur pat – whthr it’s i a mtr, gratr, cab, switch, trasrmr, tc. – is caru cvr with sm rm ctrica isuati. Th wir its is usua cppr r aumium, which is w t b a g cuctr th ctric currt that pwrs ur uipmt. Th isuati must b just th ppsit rm a cuctr: it shu rsist currt a p th currt i its path ag th cuctr. T ursta isuati tstig u ra ’t t g it th mathmatics ctricit, but simp uati – ohm’s aw – ca b vr hpu i apprciatig ma aspcts. ev i u’v b xps t this aw br, it ma b a g ia t rviw it i th ight isuati tstig. Th purps isuati aru a cuctr is much i that a pip carrig watr, a ohm’s aw ctricit ca b mr asi urst b a cmparis with watr w. I Fig. 1 w shw this cmparis. Prssur watr rm a pump causs w ag th pip (Fig. 1a). I th pip wr t sprig a a, u’ wast watr a s sm watr prssur. With ctricit, vtag is i th pump prssur, causig ctricit t w ag th cppr wir (Fig. 1b). As i a watr pip, thr is sm rsistac t w, but it is much ss ag th wir tha it is thrugh th isuati.
Figre 1–Cmparis water w (a) with eectric crret (b).
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Cmm ss ts us that th mr vtag w hav, th mr currt thr’ b. As, th wr th rsistac th wir, th mr currt r th sam vtag. Actua, this is ohm’s aw, which is xprss this wa i uati rm:
whr,
e=IxR e = vtag i vts I = currt i amprs
R = rsistac i hms nt, hwvr, that isuati is prct (that is, has ifit rsistac) s sm ctricit s w ag th isuati r thrugh it t gru. Such a currt ma b a miith a ampr ( micrampr) but it is th basis isuati tstig uipmt. nt as that a highr vtag ts t caus mr currt thrugh th isuati. This sma amut currt wu t, curs, harm g isuati but wu b a prbm i th isuati has trirat. nw, t sum up ur aswr t th usti “what is ‘g’ isuati?” W hav s that, sstia, “g” mas a rativ high rsistac t currt. Us t scrib a isuati matria, “g” wu as ma “th abiit t p a high rsistac.” S, a suitab wa masurig rsistac ca t u hw “g” th isuati is. As, i u ta masurmts at rguar pris, u ca chc trs twar its trirati (mr this atr).
WHAT MAkES InSulATIon Go BAd? Wh ur pat ctrica sstm a uipmt ar w, th ctrica isuati shu b i tp tch shap. Furthrmr, mauacturrs wir, cab, mtrs, a s hav ctiua imprv thir isuatis r srvics i iustr. nvrthss, v ta, isuati is subjct t ma cts which ca caus it t ai – mchaica amag, vibrati, xcssiv hat r c, irt, i, crrsiv vaprs, mistur rm prcsss, r just th humiit a mugg a. I varius grs, ths mis isuati ar at wr as tim gs – cmbi with th ctrica strsss that xist. As pi hs r cracs vp, mistur a rig mattr ptrat th suracs th isuati, prviig a w rsistac path r aag currt.
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oc start, th irt mis t t ai ach thr, prmittig xcssiv currt thrugh th isuati. Smtims th rp i isuati rsistac is su, as wh uipmt is . Usua, hwvr, it rps graua, givig pt warig, i chc priica. Such chcs prmit pa rcitiig br srvic aiur. I thr ar chcs, a mtr with pr isuati, r xamp, ma t b agrus t tuch wh vtag is appi, but as b subjct t bur ut. What was g isuati has bcm a partia cuctr.
HoW InSulATIon RESISTAnCE IS MEASuREd yu hav s that g isuati has high rsistac; pr isuati, rativ w rsistac. Th actua rsistac vaus ca b highr r wr, pig up such actrs as th tmpratur r mistur ctt th isuati (rsistac crass i tmpratur r mistur). With a itt rcr-pig a cmm ss, hwvr, u ca gt a g pictur th isuati citi rm vaus that ar rativ. Th Mggr isuati tstr is a sma, prtab istrumt that givs u a irct raig isuati rsistac i hms r mghms. Fr g isuati, th rsistac usua ras i th mghm rag. Th Mggr isuati tstr is sstia a high-rag rsistac mtr (hmmtr) with a buit-i irct-currt gratr. This mtr is spcia cstructi with bth currt a vtag cis, abig tru hms t b ra irct, ipt th actua vtag appi. This mth is structiv; that is, it s t caus trirati th isuati.
Figre 2–Typica Megger test istrmet h-p t measre isati resistace.
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Th gratr ca b ha-cra r i-prat t vp a high dC vtag which causs a sma currt thrugh a vr suracs th isuati big tst (Fig. 2). This currt (usua at a appi vtag 500 vts r mr) is masur b th hmmtr, which has a iicatig sca. Fig. 3 shws a tpica sca, which ras icrasig rsistac vaus rm t up t ifit, r a rsistac t high t b masur.
Figre 3–Typica scae the Megger isati tester.
HoW To InTERPRET RESISTAnCE REAdInGS As prvius mti, isuati rsistac raigs shu b csir rativ. Th ca b uit irt r mtr r machi tst thr as i a rw, t t ma ba isuati. What ra mattrs is th tr i raigs vr a tim pri, shwig ssig rsistac a warig cmig prbms. Priic tstig is, thrr, ur bst apprach t prvtiv maitac ctrica uipmt, usig rcr cars as shw i Fig. 4.
Figre 4–Typica recr isati resistace a mi mtr. Crve A shws test vaes as measre; Crve B shws same vaes crrecte t 20°C (see page 22), givig a efite wwar tre twar a sae citi. Reverse sie car (at right) is se t recr the test ata.
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Whthr u tst mth, twic a ar, r c a ar ps up th tp, cati, a imprtac th uipmt. Fr xamp, a sma pump mtr r a shrt ctr cab ma b vita t a prcss i ur pat. expric is th bst tachr i sttig up th schu pris r ur uipmt. yu shu ma ths priic tsts i th sam wa ach tim. That is, with th sam tst cctis a with th sam tst vtag appi r th sam gth tim. As u shu ma tsts at abut th sam tmpratur, r crrct thm t th sam tmpratur. A rcr th rativ humiit ar th uipmt at th tim th tst is as hpu i vauatig th raig a tr. latr sctis cvr tmpratur crrcti a humiit cts. I summar, hr ar sm gra bsrvatis abut hw u ca itrprt priic isuati rsistac tsts, a what u shu with th rsut: Citi
What T d
(a) Fair t high vaes a we maitaie.
n case r ccer.
(b) Fair t high vaes, bt shwig a cstat teecy twars wer vaes.
lcate a remey the case a chec the wwar tre.
(c) lw bt we maitaie.
Citi is prbaby a right, bt case w vaes sh be chece.
() S w as t be sae.
Cea, ry t, r therwise raise the vaes bere pacig eipmet i service. (Test wet eipmet whie ryig t.)
(e) Fair r high vaes, previsy we maitaie bt shwig se werig.
Mae tests at reet itervas ti the case w vaes is cate a remeie; r ti the vaes have becme steay at a wer eve bt sae r perati; r ti vaes becme s w that it is sae t eep the eipmet i perati.
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FACToRS AFFECTInG InSulATIon RESISTAnCE REAdInGS Rmmbr that th masur rsistac ( th isuati) wi b trmi b th vtag appi a th rsutat currt (R = e/I). Thr ar a umbr thigs that act currt, icuig tmpratur th isuati a humiit, as mti i th prvius scti. Right w, t’s just csir th atur currt thrugh isuati a th ct hw g vtag is appi. Currt thrugh a ag isuati is ma up part a rativ sta currt i aag paths vr th isuati surac. ectricit as ws thrugh th vum th isuati. Actua, as shw i Fig. 5, ur tta currt cmpriss thr cmpts: 1. Capacitace Chargig Crret Currt that starts ut high a rps atr th isuati has b charg t u vtag (much i watr w i a gar hs wh u frst tur th spigt). 2. Absrpti Crret As a iitia high currt which th rps (r rass iscuss ur th scti Tim-Rsistac Mth). 3. Ccti r leaage Crret A sma sstia sta currt bth thrugh a vr th isuati.
As shw i Fig. 5, th tta currt is th sum th thr cmpts a it is this currt that ca b masur irct b a micrammtr, r i trms mghms at a particuar vtag b mas a Mggr istrumt (hmmtr). Bcaus th tta currt ps up th tim that th vtag is appi, u ca s w wh ohm’s law R = e/I hs, thrtica, at a ifit tim (that is, u’ hav t wait rvr br taig a raig). I practic, as u wi s i th tst mths scrib bw, u ra a vau that is th appart rsistac – a usu vau t iags trubs, which is what u wat t .
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Figre 5–Crves shwig cmpets crret measre rig dC t estig isati.
nt as i Fig. 5 that th chargig currt isappars rativ rapi as th uipmt ur tst bcms charg. largr uits with mr capacitac wi ta gr t b charg. This currt as is th str rg iitia ischarg atr ur tst, b shrt-circuitig a gruig th isuati. AlWAYS TAkE THIS SAFETY MEASuRE. yu ca s urthr rm Fig. 5 that th absrpti currt crass at a rativ sw rat, pig up th xact atur th isuati. This str rg, t, must b ras at th a tst, a ruirs a gr tim tha th capacitac chargig currt – abut ur tims as g as th vtag was appi. With g isuati, th cucti r aag currt shu bui up t a sta vau that is cstat r th appi vtag, as shw i Fig. 5. A icras aag currt with tim is a warig trub, as iscuss i th tsts scrib i th wig scti.
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With a bacgru w hw tim acts th maig istrumt raigs, t’s csir thr cmm tst mths: (1) shrt-tim r spt raig; (2) tim-rsistac; a (3) stp r muti-vtag tsts.
TYPES oF InSulATIon RESISTAnCE TESTS Shrt-Time r Spt-Reaig Test
I this mth, u simp cct th Mggr istrumt acrss th isuati t b tst a prat it r a shrt, spcifc tim pri (60 scs is usua rcmm). As shw schmatica i Fig. 6, u’v simp pic a pit a curv icrasig rsistac vaus; uit t th vau wu b ss r 30 scs, mr r 60 scs. Bar i mi as that tmpratur a humiit, as w as citi ur isuati act ur raig.
Figre 6–Typica crve isati resistace (i meghms) with time r the “shrt time” r “spt-reaig” test meth.
0
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I th apparatus u ar tstig has vr sma capacitac, such as a shrt ru hus wirig, th spt raig tst is a that is cssar. Hwvr, mst uipmt is capacitiv a s ur vr frst spt raig uipmt i ur pat, with prir tsts, ca b a rugh gui as t hw g r ba th isuati is. Fr ma ars, maitac prssias hav us th -mghm ru t stabish th awab wr imit r isuati rsistac. Th ru ma b stat: Insulation resistance should be approximately one megohm for each 1,000 volts of operating voltage, with a minimum value of one megohm.
Fr xamp, a mtr rat at 2,400 vts shu hav a miimum isuati rsistac 2.4 mghms. I practic, mghm raigs rma ar csirab abv this miimum vau i w uipmt r wh isuati is i g citi. B taig raigs priica a rcrig thm, u hav a bttr basis jugig th actua isuati citi. A prsistt wwar tr is usua air warig trub aha, v thugh th raigs ma b highr tha th suggst miimum sa vaus. eua tru, as g as ur priic raigs ar csistt, th ma b , v thugh wr tha th rcmm miimum vaus. Th curvs Fig. 7 shw tpica bhavir isuati rsistac ur varig pat pratig citis. Th curvs wr ptt rm spt raigs ta with a Mggr istrumt vr a pri mths.
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Figre 7–Typica behavir isati resistace ver a peri mths er varyig peratig citis, (crves ptte rm spt reaigs with a Megger istrmet).
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Time-Resistace Meth
This mth is air ipt tmpratur a t ca giv u ccusiv irmati withut rcrs past tsts. It is bas th absrpti ct g isuati cmpar t that mist r ctamiat isuati. yu simp ta succssiv raigs at spcifc tims a t th ircs i raigs (s curvs, Fig. 8). Tsts b this mth ar smtims rrr t as absrpti tsts. nt that g isuati shws a ctiua icras i rsistac (ss currt – s curv A) vr a pri tim (i th rr 5 t 10 miuts). This is caus b th absrpti currt w sp arir; g isuati shws this charg ct vr a tim pri much gr tha th tim ruir t charg th capacitac th isuati. I th isuati ctais much mistur r ctamiats, th absrpti ct is mas b a high aag currt which stas at a air cstat vau, pig th rsistac raig w (rmmbr: R = e/I).
Figre 8–Typica crves shwig ieectric absrpti eect i a “time-resistace” test, mae capacitive eipmet sch as a arge mtr wiig.
Th tim-rsistac tst is vau as bcaus it is ipt uipmt siz. Th icras i rsistac r ca a r isuati ccurs i th sam mar whthr a mtr is arg r sma. yu ca, thrr, cmpar svra mtrs a stabish staars r w s, rgarss thir hrspwr ratigs.
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Fig. 9 shws hw a 60-sc tst wu appar r g a prhaps ba isuati. Wh th isuati is i g shap, th 60-sc raig is highr tha th 30-sc raig.
Figre 9–Typica car pt a t ime-resistace r be-reaig test.
A urthr avatag this ub-raig tst, as it is smtims ca, is that it givs u a carr pictur, v wh a spt raig sas th isuati s f. Fr xamp, t’s sa th spt raig a schrus mtr was 10 mghms. nw, t’s assum that th ub-raig chc shws that th isuati rsistac hs sta at 10 mghms whi u h vtag up t 60 scs. This mas thr ma b irt r mistur i th wiigs that bars watchig. o th thr ha, i th pitr shws a graua icras btw th 30-sc a th 60-sc chcs, th u’r rasab sur th wiigs ar i g shap. Tim-rsistac tsts arg rtatig ctrica machir – spcia with high pratig vtagA–hav-ut ruir high isuati rsistac rags a a vr cstat tst vtag. Mggr tst st, i-prat, srvs this . Simiar, such a istrumt is bttr aapt r arg cabs, bushigs, trasrmrs a switchgar.
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dieectric Absrpti Rati
Th rati tw tim-rsistac raigs (such as a 60-sc raig ivi b a 30-sc raig) is ca a ictric absrpti rati. It is usu i rcrig irmati abut isuati. I th rati is a 10-miut raig ivi b a 1-miut raig, th vau is ca th parizati ix. With ha-cra Mggr istrumts, it’s a t asir r u t ru th tst r 60 scs, taig ur frst raig at 30 scs. I u hav a i-prat Mggr istrumt, u’ gt bst rsuts b ruig th tst 10 miuts, taig raigs at 1- a at 10-miuts, t gt th parizati ix. Tab I givs vaus th ratis a crrspig rativ citis th isuati that th iicat.
TABLE I—Condition of Insulation Indicated by Dielectric Absorption Ratios* InSUlATIon CondITIon
60/30-SeCond RATIo
10/1-MInUTe RATIo (PolARIZATIon IndeX)
dagrus
—
lss tha 1
qustiab
1.0 t 1.25
1.0 t 2***
G
1.4 t 1.6
2 t 4
exct
Abv 1.6**
Abv 4**
*Ths vaus must b csir ttativ a rativ— subjct t xpric with th tim-rsistac mth vr a pri tim. **I sm cass, with mtrs, vaus apprximat 20% highr tha shw hr iicat a r britt wiig which wi ai ur shc citis r urig starts. Fr prvtiv maitac, th mtr wiig shu b ca,trat, a ri t rstr wiig xibiit. ***Ths rsuts wu b satisactr r uipmt with vr w capacitac such as shrt rus hus wirig.
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TEST VolTAGE VS. EquIPMEnT RATInG Cmm us dC tst vtags r ruti maitac ar as ws: Eipmet AC Ratig
dC Test Vtage
up t 100 vts
100 a 250 vts
440 t 550 vts
500 a 1,000 vts
2,400 vts
1,000 t 2,500 vts r highr
4,160 vts a abv
1,000 t 5,000 vts, r highr
Tst vtags us r pr tstig uipmt ar csirab highr tha ths us r ruti maitac. Athugh thr ar pubish iustr staars r dC maximum pr tst vtags t b us with rtatig uipmt, th schu giv bw is custmari us. Fr spcifc rcmmatis ur uipmt, u shu csut th mauacturr th uipmt. Pr Test Vtages r Rtatig Eipmet:
Factr AC Tst = 2 x nampat Ratig + 1000 vts dC Pr Tst Istaati = 0.8 x Factr AC Tst x 1.6 dC Pr Tst Atr Srvic = 0.6 x Factr AC Tst x 1.6 Exampe:
Mtr with 2,400 VAC ampat ratig– Factr AC Tst = 2(2,400) +1,000 = 5,800 VAC Max. dC Tst Istaati = 0.8(5,800)1.6 = 7,424 VdC Max. dC Tst Atr Srvic = 0.6(5,800)1.6 = 5,568 VdC
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AC TESTInG VS. dC Up t w, w’v ta abut tstig with dC vtag, but u wi har AC tstig a t w th irc. Rmmbr that w sp th is currt pruc i isuati b dC? (Th iitia surg chargig currt, th rp with tim t absrpti currt, a th, atr mr tim, th sta cucti currt.) W saw that i isuati tstig, th cucti r aag currt is th that givs us th irmati w . I ctrast, tstig with AC givs a chargig currt that is xtrm arg cmpar t th thr is; aag currt is rativ mir. AC rut is us r high-pttia tstig; vtag is icras t sm spcif pit t s i th isuati ca sta that particuar vtag. It is a Go/no-Go tp tst a ca caus trirati th isuati, i ctrast t th dC tst which is basica -structiv. I a AC tst vtag has b us a u wat t us dC tsts as a atrativ, u wi t icras th maximum dC tst vtag smwhat t btai uivat rsuts. I sm cass, AC tstig ma b mr suitab r pr tstig uipmt (that is, sig that th uipmt mts prscrib staars). yu ru th vtag up t th sct vau a th uipmt ithr passs r s’t pass th tst. With th dC tst, u gt a mr uaitativ pictur; u ca mtr th aag currt as u icras th vtag a btai spcifc vaus isuati rsistac. As th siz ur uipmt icrass, thr ar as mar cmic avatags i dC vr AC tstig. As th tst vtag icrass, bth th cst a wight AC uipmt g up much astr tha with cmparab dC tst uipmt. This is bcaus th AC tst st must supp th chargig currt which bcms a rmais vr high i th argr machis. As xpai prvius, i dC tstig, this currt rps rapi atr th iitia chargig pri.
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I summar, dC tst sts ar mp amst xcusiv r high-vtag maitac a f tstig r th wig rass: 1. 2. 3. 4. 5.
lwr cst lightr wight Smar siz n-structiv Bttr irmati, bth i uait a uatit
uSE oF dC dIElECTRIC TEST SET
Th Mggr istrumt , raig irct i hms a mghms isuati rsistac, is ur bst bt r ruti i-pat maitac. Hwvr, sm pats, particuar with highr vtag ratigs i uipmt, us athr Mggr pruct – th ictric tst st. S, u shu b awar this istrumt a its us i isuati rsistac masurmts. Th ictric tst st ca b us t trmi isuati rsistac b th sam tst mths as uti r th Mggr istrumt; that is, th shrttim, tim-rsistac a stp-vtag tsts. It is sig r thr uss, t, but r isuati tstig it prvis: (1) a ajustab utput vtag a (2) a mitrig th rsutat currt i micr-amprs. Th Mggr dC dictric t 160 V.Tst Sts ar avaiab with vtag utputs ragig rm 5 V up Th curvs Fig. 5 ar ptt as currt vrsus tim, as ar curvs r isuati masurmts tpica uipmt giv ar th this maua. Mggr suppis graph papr which mas it as t pt mghms isuati rsistac rm ur vtag a currt raigs.
TESTS duRInG dRYInG ouT oF EquIPMEnT Wt ctrica uipmt is a cmm hazar ac b a maitac girs. I th uipmt is wt rm rsh watr, u g right aha rig it ut. Hwvr, i u’v gt sat watr, u must frst wash awa th sat with rsh watr. othrwis, u’ av vr crrsiv psits sat mta a isuatig suracs as w as i crvics th isuati. With mistur, such psits rm a vr g cuctr ctricit. As, u shu rmv i r gras rm th isuati, usig a suitab svt.
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Thr ar varius was t r ut ctrica uipmt, pig up its siz a prtabiit. yu ca us a bast ht air, a v, circuati currt thrugh cuctrs, r a cmbiati tchius. lca pat citis a aciitis, tgthr with irmati rm th uipmt mauacturrs, ca srv as a gui t th bst mth r ur particuar uipmt. I sm cass, r with crtai uipmt, rig ut ma t b cssar. yu ca chc this b isuati rsistac tsts, i u hav rcrs prvius tsts th apparatus. Wh rig ut is ruir, such rcrs ar as hpu t trmi wh th isuati is r mistur. noTE: Wet eipmet is ssceptibe t vtage breaw. Therere, y sh se a w-vtage Megger tester (100 r 250 VdC), at east i the eary stages a ryig-t r. I a w-vtage istrmet is t reaiy avaiabe, sw craig a 500-vt tester may be sbstitte. May testers have a aitia test rage measrig i ihms (W). This measremet is typicay mae at y a ew vts, a is the iea iitia measremet t be mae e eipmet. This rage measres bew the Meghm rage, a ca, therere, prvie a acta measremet t se as a bechmar i mitrig the ryig prcess. I a ihm measremet is btaie, isati has bee thrghy satrate, bt may be recaimabe. Ateratey test a ry, watchig the reaigs rise ti they reach the Meghm rage, at which time higher vtage tests ca be saey empye.
As a xamp hw imprtat past raigs ar, t’s at a 100-hp mtr that’s b . Atr a ca-up, a spt raig with th Mggr tstr shws 1.5 mghms. oha, u’ prbab sa this is . What’s mr, i past rcrs shw th isuati rsistac t ru btw 1 a 2 mghms, u’ b sur. o th thr ha, i past rcrs shw th rma rsistac vaus t ru 10 r 20 mghms, th u wu w that watr was sti i th mtr wiigs.
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Figre 10-Typica ryig crve where e-mite reaigs isati resistace are tae every r hrs.
Th tpica rig-ut curv r a dC mtr armatur (Fig. 10) shws hw isuati rsistac chags. durig th frst part th ru, th rsistac actua crass bcaus th highr tmpratur. Th it riss at a cstat tmpratur as rig prcs. Fia, it riss t a high vau, as rm tmpratur (20°C) is rach. nt that i u cuct isuati rsistac tsts urig rig, a u hav raigs prvius tsts th r uipmt, u’ w wh u’v rach th sa vau r th uit. yu ma prr t us a timrsistac tst, ta priica (sa, c a shit), usig th ictric absrpti rati r parizati ix t w r-ut prgrss ( t crrct r tmpratur). 0
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EFFECT oF TEMPERATuRE on InSulATIon RESISTAnCE Th rsistac isuatig matrias crass mar with a icras i tmpratur. As w’v s, hwvr, tsts b th tim-rsistac a stpvtag mths ar rativ ipt tmpratur cts, givig rativ vaus. I u wat t ma riab cmpariss btw raigs, u shu crrct th raigs t a bas tmpratur, such as 20°C, r ta a ur raigs at apprximat th sam tmpratur (usua t ifcut t ). W wi cvr sm gra guis t tmpratur crrcti. o thumb ru is: For every 10°C i ncrease in temperature, halve the resistance; or, for every 10°C decrease, double the resistance. Fr xamp, a tw-mghm rsistac at 20°C rucs t 1/2 mghm at 40°C. each tp isuatig matria wi hav a irt gr rsistac chag with tmpratur. Factrs hav b vp, hwvr, t simpi th crrcti rsistac vaus. Tab II givs such actrs r rtatig uipmt, trasrmrs a cab. yu mutip th raigs u gt b th actr crrspig t th tmpratur (which u t masur). Fr xamp, assum u hav a mtr with Cass A isuati a u gt a raig 2.0 mghms at a tmpratur (i th wiigs) 104°F (40°C). Frm Tab II u ra acrss at 104°F t th xt cum (r Cass A) a btai th actr 4.80. yur crrct vau rsistac is th: 2.0 mghms (raig at 104°F)
x
4.80 = (crrcti actr r Cass A isuati at 104°F)
9.6 mghms (crrct raig r 68°F r 20°C)
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nt that th rsistac is amst fv tims gratr at 68°F (20°C), as cmpar t th raig ta at 104°F. Th rrc tmpratur r cab is giv as 60°F (15.6°C), but th imprtat pit is t b csistt a crrct t th sam bas. TABlE II-Temperatre Crrecti Factrs* RoTATInG eqUIP.
TeMP.
C °
F °
A S S A l C
B S S A l C
S R e M dR l e o F Il S F - n Il A R oT
CABleS
l A R e U dT oA C n
e C n A e d -S oR C G
l MA R R oU F T R A e P n
. T S I S e R T A e H
l A R U T A n
& . T S I S e R T A e H
S R G . R M o F R e P
. T IS S e
S R G l R e A R nU oT Z A on
d e C H S I I R nB R M AA V C
G n e R R e P P A M I P
0 5 10 15.6
32 41 50 60
0.21 0.31 0.45 0.71
0.40 0.50 0.63 0.81
0.25 0.36 0.50 0.74
0.25 0.40 0.61 1.00
0.12 0.23 0.46 1.00
0.47 0.60 0.76 1.00
0.42 0.56 0.73 1.00
0.22 0.37 0.58 1.00
0.14 0.26 0.49 1.00
0.10 0.20 0.43 1.00
0.28 0.43 0.64 1.00
20 25 30 35
68 77 86 95
1.00 1.48 2.20 3.24
1.00 1.25 1.58 2.00
1.00 1.40 1.98 2.80
1.47 1.83 2.27 3.67 3.52 7.32 5.45 14.60
1.24 1.58 2.00 2.55
1.28 1.68 2.24 2.93
1.53 1.75 2.48 3.29 4.03 6.20 6.53 11.65
1.94 4.08 8.62 18.20
1.43 2.17 3.20 4.77
2.50 3.95 8.45 29.20 3.26 3.15 5.60 13.10 54.00 4.15 3.98 7.85 20.00 116.00 5.29 5.00 11.20 6.72
3.85 5.08 6.72 8.83
40 45 50 55
104 4.80 113 7.10 122 10.45 131 15.50
60 65 70 75
140 149 158 167
22.80 6.30 15.85 34.00 7.90 22.40 50.00 10.00 31.75 74.00 12.60 44.70
10.70 25.00 38.50 7.15 17.10 41.40 81.00 10.70 27.85 78.00 170.00 16.00 45.00 345.00 24.00
8.58 11.62 73.00 15.40 118.00 20.30 193.00 26.60 313.00
775.00 36.00
*Corrected to 20° C for rotating equipment and transformers; 15.6° C for cable.
d e T A
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nmgraph tmpratur crrcti vaus r Mggr raigs (crrct t 25°C). Fr rtatig machir with Cass B isuati.
EFFECTS oF HuMIdITY W hav sp at varius pits i this maua abut th prsc mistur i isuati a its vr mar ct up rsistac vaus. yu might xpct, thrr, that icrasig humiit (mistur ctt) i th surruig (ambit) air cu act isuati rsistac. A it ca t varig grs. I ur uipmt prats rguar abv th w-pit tmpratur (th tmpratur at which th mistur vapr i air css as a iui), th tst raig rma wi t b act much b th humiit. ev i th uipmt t b tst is i, th sam is tru – s g as its tmpratur is pt abv th w pit.
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The aforementioned statement assumes that the insulation surfaces are free of contaminants, such as certain lints and acids or salts, which have the property of absorbing moisture (they’re called “hygroscopic” or “deliquescent” materials by chemists). Their presence could unpredictably affect your readings; they should be removed before tests are made.
I ctrica uipmt w’r ccr primari with th citis th xps suracs whr mistur css a acts th vra rsistac th isuati. Stuis shw, hwvr, that w wi rm i th cracs a crvics isuati br it is visib vit th surac. dw-pit masurmts wi giv u a cu as t whthr such ivisib citis might xist, atrig th tst rsuts. As a part ur maitac rcrs, thrr, it’s a g ia t ma t at ast whthr th surruig air was r r humi wh th tst was ma. As, whthr th tmpratur was abv r bw th ambit. Wh u tst vita uipmt, rcr th ambit wt a r bub tmpraturs, rm which w-pit a prct rativ r absut humiit ca b btai.
PREPARATIon oF APPARATuS To TEST 1. Tae ot Service
Shut w th apparatus. op switchs. d-rgiz. discct rm thr uipmt a circuits, icuig utra a prtctiv (wrm’s tmprar) gru cctis. S Saety Precatis, pag 26. 2. Mae Sre Jst What is Ice i the Test
Ispct th istaati vr caru t trmi just what uipmt is cct a wi b icu i th tst, spcia i it is ifcut r xpsiv t iscct assciat apparatus a circuits. Pa particuar attti t cuctrs that a awa rm th istaati. This is vr imprtat, bcaus th mr uipmt that is icu i a tst, th wr th raig wi a th tru isuati rsistac th apparatus i usti ma bb, mas b that th assciat uipmt. It is awas pssib, curs, that th isuati rsistac th cmpt istaati (withut iscctig vrthig) wi b satisactri high, spcia r a spt chc. or, it ma b highr tha th rag th
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Mggr istrumt i us, i which cas thig wu b gai b sparatig th cmpts, bcaus th isuati rsistac ach part wu b sti highr. Fr a iitia tst, it ma b cssar t sparat th cmpt parts, v thugh abr a xps ar ivv, a tst ach sparat. As ma a tst a th cmpts cct tgthr. With this irmati rcr, it ma t b cssar t sparat th cmpts utur tsts uss uaccutab w raigs ar bsrv. 3. discharge Capacitace
It is vr imprtat that capacitac b ischarg, bth br a atr a isuati rsistac tst. It shu b ischarg r a pri abut ur tims as g as tst vtag was appi i a prvius tst. Mggr istrumts ar rut uipp with ischarg circuits r this purps. I a ischarg ucti is t prvi, a ischarg stic shu b us. lav high capacitiv apparatus (i.., capacitrs, arg wiigs, tc.) shrt circuit uti ra t r-rgiz. 4. Crret leaage at Switches
Wh apparatus is shut w r th isuati rsistac tst, ma sur that th raigs ar t act b aag vr r thrugh switchs r us bcs, tc. Such aag ma mas th tru isuati rsistac th apparatus ur tst. S Us a Gar Termia, pag 50. or, what ma b mr srius, currt rm a rgiz i ma a it th apparatus a caus icsistt raigs, particuar i th iv i is dC. Hwvr, such aag usua ca b tct b watchig th pitr th Mggr istrumt at th mmt th tst as ar cct t th apparatus a br th istrumt is prat. Br maig ths bsrvatis, b sur that a capacitac is ischarg b shrt circuitig r gruig th apparatus. CAUTION: Never connect a use Megger insulation tester toleads energized lines or equipment. Never the tester or any of its or accessories for any purpose not described in this book.
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SAFETY PRECAuTIonS obsrv a rus r sat wh taig uipmt ut srvic. Bc ut iscct switchs. Tst r rig r iuc vtags. App wrm’s grus. Rmmbr that wh wrig aru high vtag uipmt thr is awas a pssibiit vtags big iuc i apparatus ur tst r is t which it is cct, bcaus prximit t rgiz high vtag uipmt. Thrr, rathr tha rmvig a wrm’s gru i rr t ma a tst, it is mr avisab t iscct th apparatus, such as a trasrmr r circuit brar, rm th xps bus r i, avig th attr gru. Us rubbr gvs wh cctig th tst as t th apparatus a whi pratig th Mggr istrumt.
Apparatus Under Test Must Not Be Live! S pag 24 Preparati Apparats r Test. I utra r thr gru cctis hav t b iscct, ma sur th ar t carrig currt at th tim, a that wh iscct thr uipmt wi ac cssar prtcti. Pa particuar attti t cuctrs that a awa rm th circuit big tst a ma sur th hav b prpr iscct rm a surc vtag. Shc Hazar rm Test Vtage
obsrv th vtag ratig th Mggr istrumt a rgar it with apprpriat cauti. larg ctrica uipmt a cabs usua hav sufcit capacitac t str up a agrus amut rg rm th tst currt. Ma sur this capacitac is ischarg atr th tst a br haig th tst as. S as discharge Capacitace, pag 25. Expsi a Fire Hazar
S ar as is w, thr is fr hazar i th rma us a Mggr isuati tstr. Thr is, hwvr, a hazar wh tstig uipmt cat i iammab r xpsiv atmsphrs.
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Slight sparking may be encountered:
(1) Wh attachig th tst as t uipmt i which th capacitac has t b cmpt ischarg (2) durig a tst, arcig thrugh r vr aut isuati (3) Fwig a tst wh capacitac is ischarg
CAUTION: Do not use the instrument in an explosive atmosphere.
Sggestis:
Fr (1) a (3): Arrag prmat ista gruig aciitis a tst as t a pit whr istrumt cctis ca b ma i a sa atmsphr. Fr (2): Us w-vtag tstig istrumts, r a sris rsistac. Fr (3): d t iscct th tst as r at ast 30 t 60 scs wig a tst, awig tim r capacitac ischarg.
ConnECTIonS FoR TESTInG InSulATIon RESISTAnCE oF ElECTRICAl EquIPMEnT Th wig iagrams shw hw t cct a Mggr isuati tstr t varius tps ctrica uipmt. Th iagrams as shw i pricip hw uipmt must b iscct rm thr circuits br th istrumt is cct. Ths iustratis ar tpica a wi srv as guis r tstig isuati rsistac practica a tps apparatus a cuctrs. Br prcig with tsts, ra th artic Preparati Apparats r Test, pag 24.
REMEMBER! The Megger isati resistace tester measres whatever resistace is cecte betwee its termias. This may ice series r parae eaage paths thrgh isati r ver its srace.
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1. AC Mtrs & Startig Eipmet
Figre 11
Cctis r tstig th isuati rsistac a mtr, startig uipmt a cctig is, i para. nt that th startr switch is i th “” psiti r th tst. It is awas prrab t iscct th cmpt parts a tst thm sparat i rr t trmi whr wasss xist. 2. dC Geeratrs & Mtrs
Figre 12
With th brushs rais as iicat, th brush riggig a f cis ca b tst sparat rm th armatur. liwis th armatur ca b tst b its. With th brushs wr, th tst wi b that brush riggig, f cis a armatur cmbi.
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3. Wirig Istaati
Figre 13
Cctis r tstig t gru ach circuit sparat, wrig rm th istributi pa.
Figre 14
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Cctis at th mai pwr bar, rm which pit th tir sstm ca b tst t gru at tim, prviig a switchs i th istributi pa ar cs. 4. Appiaces, Meters, Istrmets & Misceaes Eectrica Apparats
Figre 15
Cctis r tstig a appiac. Th tst is ma btw th cuctr (th hatig uit, mtr, tc.) a xps mta parts. Th apparatus must b iscct rm a surc pwr a pac sm isuatig matria. 5. Ctr, Sigaig & Cmmicati Cabes
Figre 16
Cctis r tstig isuati rsistac wir i a muticuctr cab agaist a thr wirs a shath cct tgthr.
0
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I6. Pwer Cabes
Cctis r tstig th isuati rsistac a pwr cab. Wh tstig cab, it is usua bst t iscct at bth s i rr t tst th cab b its, a t avi rrr u t aag acrss r thrugh switchbars r pabars. S as use Gar Termia, pag 50.
Figre 17
Figre 18
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7. Pwer Trasrmers
Figre 19
Cctis r tstig isuati rsistac a trasrmr high vtag wiig a bushigs, a th high tsi iscct switch, i para, with rrc t th w vtag wiig a gru. nt that th w vtag wiig is gru r this tst. 8. AC Geeratrs
Figre 20
With this ccti, th isuati rsistac wi b that th gratr statr wiig a cctig cab cmbi. T tst ithr th statr wiig r th cab its, th cab must b iscct at th machi.
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AddITIonAl noTES ABouT uSInG A MEGGER InSulATIon TESTER Testig leas
Irir r ctiv tstig as wi caus rrus a misaig rsuts isuati rsistac tsts. Ta car i this rspct. uisate leas
T avi rrr u t th isuati as, pac th Mggr istrumt cs t th ugru trmia r cuctr th apparatus ur tst a cct a shrt pic ight bar wir irct rm th li trmia th istrumt t th apparatus. I th Guar trmia is us, it ma b trat simiar. n. 18 r 20 gaug, si wir wi sufc. Supprt th a b its cctis t th istrumt a th apparatus. With this mth cctig rm th li trmia, th uait th isuati, i a, th earth r Gru a bcms uimprtat. Isate leas
Whr pc is pac th isuati as, th must b urab a th bst uait isuatig matria. oi rsistat, sthtic, rubbr-isuat, sig-cuctr n. 14 stra wir is rcmm. Th utr jact shu b smth, with utr brai. lugs shu b ftt r attachig t th istrumt trmias, a stut sprig cips ar rcmm r cctig t th apparatus r circuit ur tst. A cvit gth a ma b us. Jits ar t b avi. Atr cctig th as t th istrumt, a just br cctig thm t th apparatus, ma sur thr is a rm a t a. d this b pratig th istrumt, which shu ra Ifit. d t crrct sight a aag b attmptig t rst th Ifit Ajustr a high-rag istrumt. Th tuch th tst s th as tgthr t ma sur th ar t iscct r br. Currt tstig with th high-rag (50,000 mghms) Mggr isuati tstrs ruirs tst a b maitai at aa, highwith vau sshi that it wi t tr that it th th li masurmt. Th shi tst th cct t Guar, prvts a aags vr its trmiatis r thrugh th a isuatig matria, rm big masur.
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Istrctis r use
Th utagg th shi a is t b cct t th li a Guar trmias th Mggr istrumt – th trmia t li a th si (shi) trmia t Guar. Th cip th li a is cct t th apparatus ur tst i th rguar wa. Th utbar Guar trmia ma b cct t that part th apparatus ur tst which th usr wishs t guar. Th cuctr mp i maig this ccti must b isuat r th vtag ratig th Mggr istrumt us. Eect Capacitace Capacitac i apparatus ur tst must b charg up t th rat dC vtag th Mggr isuati tstr, a maitai r 30 t 60 scs br a fa raig is ta. Ma sur that capacitac is ischarg, b shrt circuitig a gruig th apparatus br cctig th tst as. S discharge Capacitace, pag 25. noTE: Capacitace cases the piter t swig twars zer whie the istrmet is beig brght p t spee, a t swig scae bey ifity whe the geeratr is swig w. This is simpy the charge wig it a t the capacitace a thrgh the eectig ci the hmmeter.
Capacitac cts ar mst ticab i arg gratrs, i pwr a cmmuicati cab mr tha a w hur t i gth, a i capacitrs. I gra, ths cts ar sma with capacitac ss tha 0.01 F. It bcms mr ticab as capacitac a/r th ssitivit th istrumt icrass. Th hav-ut sris Mggr isuati tstrs ca b us arg capacitrs with g rsuts, particuar wh prat rm th pwr i rathr tha th ha cra. operatig Time
A vr imprtat csirati i maig isuati rsistac tsts is th tim ruir r th raig isuati rsistac t rach a maximum. Th tim ruir t charg th gmtric capacitac is vr shrt – usua mr tha a w scs – a that which causs urthr a i rachig u charg is a ictric absrpti ct. It ma b a mattr miuts r v hurs r this ctrifcati tim t b cmpt, a r th pitr t rach a absut maximum.
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Shrt-Time Reaigs
Fr shrt-tim raigs isuati rsistac, prat th istrumt r a fit gth tim, ithr 30 scs r 1 miut, a ra at th that tim. Ctiu craig stai at sip sp uti th raig has b ta. Ma utur tsts with th sam gth pratig tim. Time-Resistace Meth
Wh usig a ha-cra istrumt, prat ctiuus r 1 miut. Ta a raig at th th frst 30 scs a athr raig at th th miut. Wh usig a mtr-riv r rctifr-prat istrumt, th tim itrvas ar usua 1 miut a 10 miuts rm th tim th tstig vtag is appi. or, tim-rsistac curvs ma b ta vr a pri 10 t 30 miuts r gr. Vtage Scaes
Sm isuati tstrs ma b suppi with a vtag sca t chc r th absc vtag br isuati tstig. As xpai i th Saety Precatis scti, hwvr, isuati tstrs shu vr b cct t rgiz is r uipmt wh prat i a th Isuati Tst r Rsistac Tst ms.
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InTERPRETATIon-MInIMuM VAluES Isuati rsistac ctrica uipmt is act b ma variabs such as th uipmt sig; th tp isuatig matria us, icuig birs a imprgatig cmpus; th thicss th isuati a its ara; caiss, mistur a tmpratur. Fr isuati rsistac raigs t b a ccusiv masur th citi th uipmt big tst, ths variabs must b ta it csirati. Atr th uipmt has b put it srvic, actrs such as th sig th uipmt, th i isuatig matria us, a its thicss a ara cas t b variabs, awig miimum isuati rsistac vaus t b stabish withi rasab tracs. Th variabs that must b csir atr th uipmt ha b put it srvic, a at th tim that th isuati rsistac masurmts ar big ma, ar caiss, mistur, tmpratur a mchaica amag (such as racturs). G Hseeepig
Th mst imprtat ruirmts i th riab prati ctrica uipmt ar caiss, a th imiati mistur ptrati it th isuati. This ma b csir as g huspig, a is sstia i th maitac a tps ctrica uipmt. Th vr act that isuati rsistac is act b mistur a irt, with u awacs r tmpratur, mas th Mggr isuati tstr th vauab t that it is i ctrica maitac. It is at c a gaug caiss a g huspig as w as a tctr trirati a impig trub. What Reaigs t Expect – Periic Tests
Svra critria r miimum vaus isuati rsistac hav b vp a ar summariz hr. Th shu srv as a gui r uipmt i srvic. Hwvr, priic tsts uipmt i srvic wi usua rva raigs csirab highr tha th suggst miimum sa vaus.
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Thrr, it is strg rcmm that rcrs priic tsts b pt, bcaus prsistt wwar trs i isuati rsistac usua giv air warig impig trub, v thugh th actua vaus ma b highr tha th suggst miimum sa vaus. Cvrs, awacs must b ma r uipmt i srvic shwig priic tst vaus wr tha th suggst miimum sa vaus, s g as th vaus rmai stab r csistt. I such cass, atr u csirati has b giv t tmpratur a humiit citis at th tim tst, thr ma b r ccr. This citi ma b caus b uirm istribut aags a harmss atur, a ma t b th rsut a agrus caiz wass. Hr agai, rcrs isuati rsistac tsts vr a pri tim rva chags which ma justi ivstigati. Th tr th curv ma b mr sigifcat tha th umrica vaus thmsvs. The oe-Meghm Re
Fr ma ars mghm has b wi us as a air awab wr imit r isuati rsistac riar iustria ctrica uipmt rat up t 1000 vts, a is sti rcmm r ths wh ma t b t amiiar with isuati rsistac tstig practics, r wh ma t wish t apprach th prbm rm a mr tchica pit viw. Fr uipmt rat abv 1000 vts th mghm ru is usua stat as a miimum mghm pr thusa vts. Athugh this ru is smwhat arbitrar, a ma b criticiz as acig a girig uati, it has st th tst a g ma ars practica xpric. It givs sm assurac that uipmt is t t wt r irt a has sav ma a ucssar braw. Mr rct stuis th prbm, hwvr, hav rsut i rmuas r miimum vaus isuati rsistac that ar bas th i isuatig matria us a th ctrica a phsica imsis th tps uipmt ur csirati.
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MInIMuM VAluES FoR InSulATIon RESISTAnCE Rtatig Machiery
Th Ieee gui, “Rcmm Practics r Tstig Isuati Rsistac Rtatig Machir”, as with th prbm maig a itrprtig isuati rsistac masurmts r rtatig machir. It rviws th actrs which act r chag isuati rsistac charactristics, utis a rcmms uirm mths r maig tsts, a prsts rmuas r th cacuati apprximat miimum isuati rsistac vaus r varius tps AC a dC rtatig machir. Th gui stats: “Th rcmm miimum isuati rsistac Rm r atratig-currt a irct-currt machi armatur wiigs a r f wiigs atratig-currt a irct-currt machis ca b trmi b: Rm = V + 1 whr: Rm
=
rcmm miimum isuati rsistac i mghms at 40°C th tir machi wiig
V
=
rat machi trmia t trmia pttia, i ivts
I appicatis whr th machi is vita, it has b csir g practic t iitiat rcitiig shu th isuati rsistac, havig b w abv th miimum vau giv b e 2, rp apprciab t ar that v.” It is recmmee that thse wh perate a maitai rtatig machiery btai cpies the IEEE pbicati, “Recmmee Practices r Testig Isati Resistace Rtatig Machiery”, which ca be btaie by writig the IEEE at 345 East 47th St., new Yr, nY, 10017. Bshigs
I th cas utr i circuit brar bushigs, xpric has shw that a bushig, with its assmb assciat isuatig mmbrs, shu, r riab prati, hav a isuati rsistac vau abv 10,000 mghms at 20°C. This assums that th i withi th ta is i g citi, that th brar is sparat rm its xtra cctis t thr uipmt, a that th prcai wathr shi is guar. This mas that ach cmpt such as th stripp bushig its, crss-mmbr, it r, wr arcig shi, tc., shu hav a isuati rsistac i xcss that vau.
ASTITCHINTIME
A cmpts which ar suprfcia ca a r a hav vaus ss tha 10,000 mghms ar usua trirat itra, b th prsc mistur r carbiz paths, t such a xtt that th ar t riab r g srvic uss rciti. This is particuar s wh pratig ur surg citis such as urig ightig isturbacs. I th cas th stripp bushig its, th wr stm a uppr wathr shi must b ithr prct ca r guar br it is cm as uriab bcaus a isuati rsistac vau ss tha 10,000 mghms. What has b sai r stripp i circuit brar bushigs as appis t bushigs r thr uipmt, such as trasrmrs. Sic bushigs a thr assciat mmbrs hav vr high isuati rsistac vaus rma, a Mggr isuati tstr havig a rag at ast 10,000 mghms is cssar t tst such uipmt. Mggr istrumts havig rags up t 50,000 mghms wi prmit bsrvati triratig trs i bushigs br th rach th ustiab vau 10,000 mghms. Cabe a Cctrs
Cab a cuctr istaatis prst a wi variati citis rm th pit viw th rsistac th isuati. Ths citis rsut rm th ma is isuatig matrias us, th vtag ratig r isuati thicss, a th gth th circuit ivv i th masurmt. Furthrmr, such circuits usua xt vr grat istacs, a ma b subjct t wi variatis i tmpratur, which wi hav a ct th isuati rsistac vaus btai. Th trmias cabs a cuctrs wi as hav a ct th tst vaus uss th ar ca a r, r guar. Th Isuat Cab egirs Assciati (ICeA) givs miimum vaus isuati rsistac i its spcifcatis r varius tps cabs a cuctrs. Ths miimum vaus ar r w, sig-cuctr wir a cab atr big subjct t a AC high vtag tst a bas a dC tst pttia 500 vts appi r miut at a tmpratur 60°F.
ASTITCHINTIME
Ths staar miimum vaus (r sig-cuctr cab) ar bas th wig rmua: R = k g10 d/ whr: R
=
mghms pr 1000 t cab
k
=
cstat r isuatig matria
d
=
utsi iamtr cuctr isuati
=
iamtr cuctr
Miimm Vaes k at 60° F.
Isuati Tp Imprgat Papr ................................................................... 2,640 Varish Cambric .................................................................... 2,460 Thrmpastic-Pth ........................................ abv 50,000 Cmpsit Pth ......................................................... 30,000 Thrmpastic-Pvi: Pvi Chri 60°C ........................................................ 500 Pvi Chri 75°C ..................................................... 2,000 Gra
natura Rubbr
Sthtic Rubbr
C ...............................................................................................950 Prrmac ....................................... 10,560............................2,000 Hat Rsistat..................................... 10,560............................2,000 oz Rsistat...................... 10,000 (But) ............................2,000 krit ..........................................................................................4,000 S pags 44 a 45 r Tabs lg10 d/
0
ASTITCHINTIME
Th isuati rsistac cuctr a muticuctr cab t a thrs a shath is: R = k g10 d/ whr: d
= iamtr vr isuati uivat sig-cuctr cab = + 2c + 2b
=
iamtr cuctr (r sctr cabs uas iamtr ru cuctr sam crss-scti)
c
=
thicss cuctr isuati
b
=
thicss jact isuati
(a imesis mst be expresse i same its)
Trasrmers
Accptab isuati rsistac vaus r r a cmpu-f trasrmrs shu b cmparab t ths r Cass A rtatig machir, athugh staar miimum vaus ar avaiab. oi-f trasrmrs r vtag rguatrs prst a spcia prbm i that th citi th i has a mar iuc th isuati rsistac th wiigs. I th absc mr riab ata th wig rmua is suggst: R=
Ce √VA
R
=
miimum 1-miut 500-vt dC isuati rsistac i mghms rm wiig t gru, with thr wiig r wiigs guar, r rm wiig t wiig with cr guar
C
=
a cstat r 20°C masurmts
e
=
vtag ratig wiig ur tst
VA =
rat capacit wiig ur tst
Fr tsts wiig t gru with th thr wiig r wiigs gru, th vaus wi b much ss tha that giv b th rmua. R i this rmua is bas r, aci-r, sug-r i, a bushigs a trmia bars that ar i g citi.
ASTITCHINTIME
Vaes C at 20°C
60-Hrtz Ta i-f tp Uta i-f tp dr r cmpu-f tp
1.5 30.0 30.0
25-Hrtz 1.0 20.0 20.0
This rmua is it r sig-phas trasrmrs. I th trasrmr ur tst is th thr-phas tp, a th thr iiviua wiigs ar big tst as , th: e
=
VA =
vtag ratig th sig-phas wiigs (phas t phas r ta cct uits a phas t utra r star cct uits) rat capacit th cmpt thr-phas wiig ur tst
TESTS uSInG MulTI-VolTAGE MEGGER InSulATIon TESTERS Maitac practic trs iicat th vau tstig isuati with dC vtags at vs smwhat highr tha th pa vau th rat AC vtag th uipmt big tst. Such dC tsts hav i sm cass b shw t rva -structiv icipit wasss i isuati which cu t thrwis b u, xcpt pssib b partia ischarg tcti at -structiv AC tst vtag vs. Th tchiu ivvs th appicati tw r mr dC vtags, a critica bsrvig a ructi isuati rsistac at th highr vtag. A mar r uusua ructi i isuati rsistac r a prscrib icras i appi vtag is a iicati icipit wass. It is imprtat t mti that th mrits this tchiu aris rm mr rct ivstigatis which iicat that rathr high dC vtag ca b us t tct wasss withut amagig th isuati. Th maximum vau vtag which shu b us wi p arg th caiss a rss th isuati t b tst. I maig tsts isuati at such dC vtags, th hmmtr mth has at ast tw avatags. First, prscrib fx vtags ar switch it us, a istrumt masurmt ma with th irct raig hmmtr. This is a simp a rprucib mth cmpar t i which ma chics vtag ar avaiab. Athr imprtat avatag
ASTITCHINTIME
th hmmtr ca b xpai b rrrig t Fig. 21. I this fgur, th chag which ma ccur i aag currt atr th absrpti currt has isappar is shw ptt i trms isuati rsistac agaist thr irt vtags. nt that thr is chag i rsistac shw i th fgur btw 500 a 1000 vts, iicatig chag i th isuati as a rsut appig ths tw vtags. This is a assumpti, but is a citi which is t ucmm i practic. I th isuati ctius t b stab at 2500 vts, thr wi b chag i th isuati rsistac vau btai, which is shw b th tt xtsi th hrizta i i th fgur. Wh -iar citis appar at a highr vtag, th vtag rsistac curv rvas this vr car b a wr rsistac vau, iicat b th wwar curv i th fgur. Th fgur, thrr, rvas th simpicit trmiig th chag i isuati stabiit b usig thr fx vtags which ar asi rprucib wh maig thr-vtag tsts a ruti basis.
Figre 21
W wish t mphasiz that th curv i Fig. 21 iicats th rsistac chag u t aag currt , a t th absrpti currt which ma appar r a pri tim with ach chag i vtag. It ma b cssar t wait a apprciab amut tim atr ach vtag chag r th absrpti currt t isappar br taig a raig.
ASTITCHINTIME
D
d 0 1
G o l F o S E u l A V
S E H C N I – S S E N K C I H T N O I T A L U S N I
4 3 2 .
1 2 9 .
9 1 2 .
4 6 3 5 9 9 0 2 4 2 .8 .8 .7 .6 .5
5 0 9 8 5 7 9 6 2 9 6 3 4 . 4 . 4 . 3 . 3 . 3 .
1 6 2 6 9 1 8 6 4 2 3 . 2 . 2 . 2 . 2 .
5 4 7 3 3 1 0 8 7 6 2 . 2 . 1 . 1 . 1 .
7 4 6 0 7 5 5 4 4 2 1 . 1 . 1 . 1 . 1 .
4 6 0 3 3 1 1 0 9 1 . 1 . 1 . 0 .
3 0 2 .
6 9 8 1 6 6 7 9 2 0 8 . 7 . 6 . 6 . 5 .
3 0 9 9 7 0 6 4 0 7 4 2 4 . 4 . 4 . 3 . 3 . 3 .
5 1 8 2 6 9 7 4 3 1 2 . 2 . 2 . 2 . 2 .
3 3 6 3 3 0 9 7 6 5 2 . 1 . 1 . 1 . 1 .
8 4 7 1 9 4 4 3 3 1 1 . 1 . 1 . 1 . 1 .
5 4 0 3 6 7 1 0 9 8 .1 .1 .0 .0
8 8 1 .
6 3 .8
1 5 .7
0 7 .6
4 9 .5
3 8 .4
0 5 4 .
8 1 4 .
8 8 3 .
9 5 3 .
8 2 3 .
2 0 3 .
8 7 2 .
5 5 2 .
3 3 2 .
8 1 2 .
3 0 2 .
0 9 1 .
1 8 1 .
5 6 1 .
2 5 1 .
3 4 1 .
9 3 1 .
5 3 1 .
8 2 1 .
2 2 1 .
1 1 1 .
2 0 1 .
4 5 9 0 .
8 9 8 0 .
2 1 8 0 .
2 7 1 .
4 0 .8
0 2 .7
1 4 .6
6 6 .5
3 5 .4
6 2 .4
5 9 .3
6 6 .3
8 3 .3
9 0 .3
4 8 .2
1 6 2 .
8 3 2 .
8 1 2 .
4 0 2 .
9 8 1 .
7 7 1 .
8 6 1 .
3 5 1 .
1 4 1 .
3 3 1 .
9 2 1 .
5 2 1 .
0 2 1 .
3 1 1 .
3 0 .1
6 4 9 .0
9 9 8 .0
0 3 8 .0
0 5 7 .0
6 5 1 .
0 6 9 7 1 7 8 0 3 3 .7 .6 .6 .5 4 .
1 1 3 6 8 4 0 7 4 1 8 6 4 . 3 . 3 . .3 2 . 2 .
2 1 2 9 5 4 2 0 8 7 2 . 2 . 2 . .1 1 .
4 5 1 0 2 6 5 4 3 2 1 . 1 . 1 . .1 1 .
3 8 5 8 4 4 1 1 0 0 9 1 . 1 . 1 . .1 0 .
0 7 8 0 .
0 1 8 0 .
1 6 7 0 .
7 8 6 0 .
1 4 1 .
2 1 5 5 3 3 5 7 0 0 .7 .6 .5 .5 .4
3 5 8 3 7 4 7 4 1 9 6 4 .3 .3 .3 .2 .2 .2
3 4 7 3 0 2 0 8 7 6 .2 .2 .1 .1 .1
0 2 9 9 1 5 4 2 1 1 .1 .1 .1 .1 .1
4 8 8 5 9 4 0 0 9 9 .1 .1 .0 .0
6 5 8 0 .
9 8 7 .0
4 3 7 .0
1 9 6 .0
3 2 6 .0
5 2 1 .
1 9 6 .
1 1 6 .
8 3 .5
0 7 .4
3 7 .3
6 4 .3
8 1 .3
3 9 .2
9 6 .2
4 4 .2
3 2 .2
4 0 .2
5 8 .1
8 6 .1
7 5 .1
5 4 .1
6 3 .1
8 2 .1
6 1 .1
7 0 .1
0 0 .1
1 7 9 0 .
3 4 9 0 .
5 9 8 0 .
1 5 8 0 .
0 7 7 0 .
8 0 7 0 .
8 5 6 0 .
9 1 6 0 .
8 5 5 0 .
9 0 1 .
5 4 6 .
8 6 5 .
8 9 4 .
2 3 4 .
0 4 3 .
4 1 3 .
9 8 2 .
5 6 2 .
3 4 2 .
0 2 2 .
1 0 2 .
3 8 .1
6 6 .1
1 5 .1
0 4 .1
0 3 .1
1 2 .1
4 1 .1
3 0 .1
2 5 9 .0
8 8 8 .0
1 6 8 .0
6 3 8 0 .
3 9 7 0 .
5 5 7 0 .
1 8 6 0 .
5 2 6 0 .
1 8 5 0 .
6 4 5 0 .
2 9 4 0 .
4 9 . 0
4 0 3 2 5 9 2 5 9 0 .5 .5 .4 .3 .3
1 7 6 5 5 7 8 5 3 1 9 7 .2 .2 .2 .2 .1 .1
1 6 2 3 3 6 4 3 2 1 .1 .1 .1 .1 .1
5 1 3 6 9 0 5 0 9 9 8 .1 .0 .0 .0
9 6 7 .0
9 4 7 0 .
7 2 7 0 .
7 8 6 0 .
6 5 6 0 .
0 9 5 0 .
1 4 5 0 .
2 0 5 0 .
2 7 4 0 .
5 2 4 0 .
8 7 0 .
7 7 4 7 7 3 6 0 4 6 5 . 4 . 4 . 3 . 2 .
5 4 4 6 8 2 4 2 0 8 6 5 2 . 2 . 2 . 1 . 1 . 1 .
3 8 5 3 4 6 3 2 1 0 9 1 . 1 . 1 . 1 . 0 .
7 9 8 0 .
5 4 8 0 .
3 6 7 0 .
6 8 6 0 .
5 6 6 0 .
4 4 6 0 .
0 8 5 0 .
1 5 5 0 .
0 0 5 0 .
6 5 4 .0
3 2 4 .0
7 9 3 .0
7 5 3 .0
3 6 0 .
0 7 4 .
5 0 4 .
8 4 3 .
6 9 2 .
6 0 2 .
6 2 1 .
4 1 1 .
3 2 9 0 .
7 4 0 .
2 9 3 .
4 3 3 .
3 8 2 .
9 3 2 .
0 / 1
0 / 0 / 0 / 2 3 4
. . G . M . C W . r Ao
.l o S
2 3 8 .
8 4 7 .
9 6 6 .
0 5 5 .
5 2 2 .
0 8 4 .
7 8 1 .
8 4 4 .
1 7 1 .
6 1 4 .
5 5 1 .
2 8 3 .
9 3 1 .
4 5 3 .
7 2 3 .
1 0 3 .
2 0 1 .
6 7 2 .
9 5 2 .
0 5 2 .
7 2 .2
6 1 .2
8 9 .1
4 5 8 0 .
7 8 7 0 .
1 3 7 .0
8 8 6 .0
0 2 6 .0
0 0 ,0 0 5 2
0 0 ,0 0 0 3
0 0 0 , 0 5 3
0 0 0 , 0 0 4
0 0 0 , 0 0 5
3 8 .1
0 0 7 0 .
2 7 .1
7 6 .1
3 6 .1
5 5 .1
8 4 .1
4 3 .1
5 2 .1
7 1 .1
8 0 .1
3 9 9 0 .
. r t S
4 2 0 1 1 1 8 6
ASTITCHINTIME
5 1 5 .
5 4 3 2 1
0 0 0 , 0 0 6
0 0 0 , 0 0 7
0 0 0 , 0 5 7
0 0 0 , 0 0 8
0 0 0 , 0 0 9
0 0 0 , 0 0 0 , 1
0 0 0 , 0 5 2 , 1
0 0 0 , 0 0 5 , 1
0 0 0 , 0 5 7 , 1
0 0 0 , 0 0 0 , 2
0 0 0 , 0 0 5 , 2
. e g a p t x e n n d e in t n c s e a V
D
d 0 1
G o l F o S E u l A V
S E H C N I – S S E N K C I H T N O I T A L U S N I
8 3 4 .
4 0 9 6 .8 .7
0 8 0 2 1 2 7 4 0 6 .7 .6 .6 6 . .5
5 0 7 5 2 2 9 5 2 0 .5 .4 .4 .4 .4
9 9 3 7 7 7 5 4 1 9 .3 .3 .3 .3 .2
1 3 7 5 5 8 7 6 5 4 2 . 2 . 2 . .2 2 .
5 0 7 7 0 2 1 9 8 7 2 . 2 . 1 . .1 1 .
2 2 4 .
0 8 8 .
6 4 7 .
7 0 7 .
6 6 6 .
8 2 6 .
0 9 5 .
9 4 5 .
4 1 5 .
0 8 4 .
7 4 4 .
5 1 4 .
2 9 3 .
9 6 3 .
0 5 3 .
4 3 3 .
9 0 3 .
9 8 2 .
3 7 .2
6 6 .2
0 6 .2
9 4 .2
9 3 .2
9 1 .2
4 0 .2
1 9 .1
1 8 1 .
5 6 1 .
7 0 4 .
6 6 8 .
4 3 7 .
4 9 6 .
3 5 .6
5 1 .6
8 7 .5
8 3 .5
2 0 .5
9 6 .4
6 3 .4
5 0 .4
3 8 .3
0 6 .3
1 4 .3
6 2 .3
1 0 .3
1 8 .2
6 6 2 .
9 5 2 .
2 5 2 .
2 4 2 .
2 3 2 .
2 1 2 .
7 9 1 .
5 8 .1
6 7 .1
0 6 .1
1 9 3 .
1 0 5 2 8 . 7 .
0 0 3 5 5 8 4 0 6 2 6 . 6 . 6 . 5 . 5 .
1 8 5 5 3 9 5 2 9 7 4 . 4 . 4 . 3 . 3 .
1 2 7 2 3 5 3 1 9 7 3 . 3 . 3 . .2 2 .
8 1 5 4 5 5 5 4 3 2 .2 .2 .2 .2 .2
6 0 0 0 6 0 9 8 7 5 .2 1 . 1 . 1 . 1 .
5 7 3 .
2 6 6 2 3 0 .9 .8 7 .
7 5 9 3 3 6 2 8 5 1 6 . 6 . 5 . .5 5 .
9 6 4 4 3 7 4 1 8 6 4 . 4 . 4 . .3 3 .
1 3 8 4 5 4 2 0 8 6 3 . 3 . 3 . .2 2 .
0 3 7 7 8 5 4 3 2 1 2 . 2 . 2 . 2 . 2 .
9 5 4 4 0 9 8 7 6 5 1 . 1 . 1 . 1 . 1 .
9 5 3 .
6 0 9 .
1 2 8 .
9 9 .6
2 5 .6
3 1 .6
6 7 .5
0 4 .5
1 0 .5
6 6 .4
5 3 .4
3 0 .4
3 7 .3
2 5 .3
1 3 3 .
3 1 3 .
8 9 2 .
5 7 2 .
6 5 2 .
2 4 2 .
6 3 2 .
0 3 2 .
9 1 2 .
1 1 2 .
2 9 1 .
9 7 1 .
8 6 1 .
9 5 1 .
4 4 1 .
4 4 3 .
9 8 8 .
4 0 8 .
6 7 6 .
7 3 6 .
8 9 5 .
2 6 5 .
6 2 5 .
7 8 4 .
4 5 4 .
2 2 4 .
2 9 3 .
2 6 3 .
2 4 3 .
1 2 .3
3 0 .3
9 8 .2
6 6 .2
8 4 .2
4 3 .2
8 2 .2
2 2 .2
2 1 2 .
3 0 2 .
6 8 1 .
2 7 1 .
2 6 1 .
3 5 1 .
9 3 1 .
8 2 3 .
1 7 .8
7 8 .7
0 6 .6
2 2 .6
3 8 .5
7 4 .5
2 1 .5
4 7 .4
1 4 .4
0 1 .4
0 8 .3
1 5 .3
1 3 .3
0 1 3 .
3 9 2 .
9 7 2 .
7 5 2 .
9 3 2 .
5 2 2 .
0 2 2 .
4 1 .2
4 0 .2
6 9 .1
9 7 .1
6 6 .1
5 5 .1
5 4 .1
4 3 .1
3 1 3 .
3 0 3 5 7 4 8 . 7 . 6 .
6 8 2 7 1 0 6 3 9 6 6 . 5 . 5 . 4 . 4 .
8 7 7 9 0 2 9 6 3 2 .4 3 . 3 . .3 3 .
0 3 9 8 0 0 8 6 4 3 .3 .2 .2 .2 .2
7 1 6 6 9 1 1 0 9 8 .2 .2 2 . .1 1 .
2 9 9 1 8 7 5 4 4 2 1 . 1 . 1 . .1 1 .
7 9 2 .
4 3 8 .
1 5 7 .
6 2 6 .
9 8 5 .
1 5 5 .
6 1 5 .
2 8 4 .
5 4 4 .
4 1 4 .
4 8 3 .
5 5 3 .
7 2 3 .
9 0 3 .
9 8 2 .
2 7 2 .
9 5 2 .
8 3 2 .
1 2 2 .
9 0 2 .
3 0 2 .
8 9 1 .
9 8 1 .
1 8 1 .
5 6 1 .
3 5 1 .
3 4 1 .
5 3 1 .
2 2 1 .
1 8 2 .
4 1 8
1 3 .7
8 0 .6
2 7 .5
5 3 .5
0 0 .5
6 6 .4
1 3 .4
9 9 3 .
0 7 3 .
2 4 3 .
5 1 3 .
6 9 2 .
8 7 2 .
2 6 2 .
9 4 2 .
8 2 2 .
2 1 2 .
9 9 1 .
4 9 1 .
9 8 1 .
0 8 1 .
3 7 1 .
7 5 1 .
6 4 1 .
6 3 1 .
8 2 1 .
7 1 1 .
6 6 2 .
3 9 7 .
2 1 7 .
0 9 5 .
4 5 5 .
7 1 5 .
3 8 4 .
0 5 4 .
5 1 4 .
5 8 .3
6 5 .3
9 2 .3
2 0 .3
4 8 .2
6 6 .2
0 5 .2
6 3 .2
8 1 .2
3 0 2 .
1 9 1 .
5 8 1 .
0 8 1 .
2 7 1 .
5 6 1 .
0 5 1 .
9 3 1 .
0 3 1 .
2 2 1 .
1 1 1 .
0 5 2 .
5 6 1 1 0 4 5 7 9 7 .9 .8 .7 .6 .5
. . G . M . C W . r Ao
l. o S
5 0 6 3 9 3 0 6 3 9 5 . 5 . 4 . .4 3 .
9 2 5 9 2 6 4 1 8 7 3 . 3 . 3 . .2 2 .
4 9 7 8 3 5 3 2 0 9 2 . 2 . 2 . .2 .1
1 6 2 4 7 8 7 7 6 5 .1 .1 .1 .1 .1
2 2 3 6 5 4 3 2 1 0 .1 .1 .1 .1 .1
0 0 ,0 0 0 3
0 0 0 , 0 0 7
0 0 0 , 0 5 2 , 1
. r t S
4 2 0 8 6 1 1 1
5 4 3 2 1
0 / 0 / 0 / 0 / 1 2 3 4
0 0 ,0 0 5 2
0 0 0 , 0 5 3
0 0 0 , 0 0 4
0 0 ,0 0 0 5
0 0 ,0 0 0 6
0 0 0 , 0 5 7
0 0 0 , 0 0 8
0 0 0 , 0 0 9
0 0 0 , 0 0 0 , 1
0 0 0 , 0 0 5 , 1
0 0 0 , 0 5 7 , 1
ASTITCHINTIME
0 0 0 , 0 0 0 , 2
0 0 0 , 0 0 5 , 2
T bttr ursta th tchiu maig isuati rsistac tsts at tw r mr vtags, th wig stps ar suggst, usig a iustria r tracti mtr cass i th 300- t 1000-vt rag as a xamp: 1. Ma a -miut Mggr istrumt tst at 500 vts t srv as a basis cmparis r subsut stps. 2. Atr a caru caig prati ma a sc 500-vt tst t trmi th ctivss th caig. 3. I th -miut isuati rsistac vau is subrma, r i th 60 sc/30 sc isuati rsistac rati is gratr tha at this pit th a rig prati ma b sirab br usig a highr tst vtag. Hwvr, maig athr tst at 1000 vts a cmparig ths raigs with ths rm th 500-vt tst wi hp i trmiig th r rig. I th 1000-vt tst vau is apprciab ss tha that at 500 vts, th a rig prati shu b prrm. o th thr ha, i th 1000-vt a 500-vt tst vaus ar apprximat th sam, it is rasab t assum that th cisi t prrm a rig prati ca b rr uti atr th xt stp. 4. Ma a Mggr istrumt tst at 2500 vts. I thr is apprciab irc i th 500- a 2500-vt tst vaus, g vic xists that
th mtr i usti is i riab citi as ar as its isuati is ccr. o th thr ha, i thr is a apprciab irc i th tw, thr is g vic that mr thrugh rcitiig is ca r. I th isuati ais ur th 2500-vt tst, atr wig stps 1, 2 a 3, w biv thr is a iih that th mtr i usti wu ai i srvic v thugh a attmpt wr ma t rciti it th basis w-vtag tsts . Th muti-vtag mth ca as b hpu i trmiig th prsc xcssiv mistur i th isuati uipmt rat at vtags uivat t r gratr tha th highst vtag avaiab rm th mutivtag Mggr istrumt big us. I thr wrs, v thugh th highst Mggr istrumt vtag avaiab s t strss th isuati b its ratig, a tw-vtag tst ca, vrthss, t rva th
ASTITCHINTIME
prsc mistur. I th isuati rsistac is frst tst th shrttim raig basis (frst at vtag v a th at a highr pttia), a wr vau isuati rsistac at th highr dC tst vtag usua iicats th prsc mistur. Th appi vtags shu prrab b i th rati 1 t 5. expric has iicat that a chag 25% i th isuati rsistac vau, with a 1 t 5 rati i tst vtags, is usua u t th prsc a xcssiv amut mistur. This mth is t bas a ictric absrpti phma, but it s rat t th evrsh ect. As with tim-rsistac masurmts, th muti-vtag mth tstig isuati rsistac has icras vau wh ma a priic r schu basis.
Figre 22-Time Resistace Test Frms
ASTITCHINTIME
STEP-VolTAGE METHod I this mth, u a muti-vtag Mggr istrumt t app tw r mr vtags i stps, i 500 vts a th 1000 vts. yu r a ructi isuati rsistac at th highr vtag. I th rsistac is wr, it’s a sig a isuati wass that shws up at th highr vtag. Fig. 23 shws a xamp i which, ista prgrssiv icrasig th vtag, u frst tst at a w vtag (such as 500 vts) a th, atr ischargig th samp, u tst agai at a highr vtag (such as 2500 vts). A irc i th tw tsts i trms mghms wi shw sigs wass at th highr vtag – a warig t ivstigat urthr. As th citis withi th samp trirat, th highr-vtag pt, as shw i Fig. 23, wi ruc i mghms cmpar t th wr-vtag , a its upwar sp wi b ss.
Figre 23-Typica crves with the “step-vtage” test.
Th thr bhi th stp-vtag tchiu is a bit cmpx, but w’ tr t p it simp. Mistur a irt i isuati ar usua rva b tsts at vtags ar bw ths xpct i srvic. Hwvr, cts agig r mchaica amag i air ca a r isuati ma t b rva at such w strss.
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nw, wh th vtag is icras i stps t pruc ctrica strsss which apprach r xc ths cutr i srvic, th ca wa spts iuc th vra isuati rsistac mr a mr. Th rsistac such ca auts gra crass rapi as th ctrica strss i thm icrass b a crtai imit. Th pt cscutiv Mggr istrumt raigs car shws th sharp rp wh this ccurs (s Fig. 24).
Figre 24-Test crves by the step-vtage meth, cmparig rests with g a ba isati. Crve 1 (wer pt) shws efite rp i resistace with icreasig vtage, iicatig a prbem. Crve 2 (pper pt) shws citis i the same mtr wiig ater ceaig, baig a impregatig perati.
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yu t p th tst vtag cstat btw stps r abut 60 scs. This shrt pri wi t act th tr i rsistac chag. Th tim pri, hwvr, shu awas b th sam r a giv pic uipmt. A th absrpti currt ma t hav isappar i this tim, but ur rsistac masurmts wi b ma th sam basis a thrr wi b maigu. yur rsuts ar ipt th isuati matria a its tmpratur bcaus u’r ig at th chag i rsistac – t th absut rsistac vaus. As is tru with th spt-raig a tim-rsistac masurmts, th stp-vtag mth is mr vauab t u wh rpat a priic, schu basis. Th stp-vtag mth is particuar usu i trmiig th prsc xcssiv mistur r thr ctamiats i th isuati uipmt that is rat at vtags uivat t r gratr tha th highst vtag avaiab i ur muti-vtag Mggr istrumt. I thr wrs, v thugh ur highst vtag s t strss th isuati b its ratig, a tw-vtag tst ca, vrthss, t rva th prsc such ctamiats. Fr xamp, assum that u frst tst isuati rsistac a shrt-tim raig basis – at a vtag 500 VdC, a th at a highr pttia , sa, 2500 VdC. ev thugh th attr vtag ma b mia i rati t th vtag ratig ur uipmt, a wr vau isuati rsistac at th highr tst vtag usua iicats th prsc ctamiat racturs r thr aag paths thrugh th isuati t gru. Th appi vtags shu prrab b i th rati 1 t 5 r gratr (500 a 2500, r xamp). Rsuts t at shw that a chag 25% i th isuati rsistac vau, with a 1 t 5 rati i tst vtags, is usua u t th prsc a xcssiv amut mistur r thr ctamiat.
uSE oF A GuARd TERMInAl A Mggr isuati tstrs havig rags 1000 mghms a highr ar uipp with Guar trmias. Th purps this trmia is t prvi aciitis r maig a thr-trmia twr masurmt, s that th rsistac tw pssib paths ca b trmi irct. It has th urthr r scar purps prviig a surc dC vtag g rguati a imit currt capacit. 0
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Th isuati a ctrica apparatus has tw cuctig r aag paths – thrugh th isuatig matria a th thr vr its suracs. B prviig a thir tst trmia i th path th surac aag, it is sparat it tw parts, rmig a thr-trmia twr as shw i Fig. 25a. I practic, this thir trmia ma b prvi as shw i Figurs 26 t 38.
Figre 25a
Figre 25b
Figre 26-Shwig hw t se the Gar termia t eimiate the eects srace eaage acrss expse isati at e e a cabe. See as Figres 28, 30 a 31.
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Thr ar as cass, such as u i tw wiig trasrmrs r muticuctr cabs, whr a thr-trmia twr is rm as shw i Fig. 25b. Figurs 30 a 33 a thrs shw practica appicatis this rm thr-trmia twr. I maig a thr-trmia tst ivvig masurmt, th li trmia th Mggr istrumt is cct t Trmia 1, Fig. 25a, th Guar trmia t Trmia 3, a th earth trmia t Trmia 2. This wi giv th tru vau r 12, prvi r23 a r13 ar t t w i vau. Th g r , which is cct acrss th Mggr istrumt gratr, shu 23 b abut 1 mghm r highr t prvt xcssiv a th gratr, a maitai satisactr gratr vtag. I usig th Guar trmia, particuar i th cas mtr-riv r rctifr-prat Mggr istrumts, as ma crtai that thr is chac a arc-vr btw th guar trmia th samp a gru. Such a arc-vr ma caus usirab arcig at th cmmutatr th istrumt gratr. Th g r13, which shuts th Mggr ctig ci, shu b at ast 100 mghms r a masurig accurac apprximat 1%. Th 1% accurac fgur is bas th R' baast rsistr big 1 mghm, which is tpica. Fr mr prcis trmiatis accurac, btai th xact vau R' b writig t Mggr a givig th sria umbr th istrumt i us.
Figre 27-Shwig hw t se the Gar cecti t eimiate the eects srace eaage acrss expse isati at bth es a cabe whe a spare cctr i the cabe is avaiabe r cmpetig the Gar cecti.
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Figre 28-Shwig se the Gar cecti t eimiate the eect eaage t gr, as i Figre 26, a as the eect eaage t ajacet cctrs. nte that the Gar wire is wrappe ar the expse isati a as is cecte t the ajacet cctrs. d t cse this iagram with Figre 26, where the Gar wire ges y t the expse isati, a the ajacet cctrs are gre.
Figre 29-T eimiate the eect srace eaage i measrig the tre resistace a isatig member, sch as a it r i a circit breaer.
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Th highst accurac is sir i cass as shw i Fig. 25a, r whr th tru rsistac ach g is wat as i th cas Fig. 25b, thr masurmts ar ruir a th wig uatis ar us: r12
=
R12 R13 – (R')2 R13 + R'
r23
=
R12 R23 – (R')2 R12 + R'
r13
=
R12 R13 – (R')2 R12 + R'
whr R12, R23 a R13 ar th actua raigs i mghms masur acrss th trmias th twr which ar cct t th li a earth trmias th Mggr istrumt with Trmias 3, 1 a 2 rspctiv cct t th istrumt Guar trmia. R' is th vau th baast rsistac i mghms th istrumt i us. I maig ths thr masurmts, cct th li trmia th istrumt t th gru trmia th twr, as a aag vr th istrumt cas btw th earth trmia a gru wi shut th rsistac big masur.
BuSHInGS, PoTHEAdS And InSulAToRS
Figre 30-Cectis r testig isati resistace betwee e wire a gr, witht beig aecte by eaage t ther wires. nte se Gar cecti.
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Figre 31-Cectis r testig isati resistace betwee e wire a a ther wires cecte, witht beig aecte by eaage t gr.
Figre 32-Shwig sprig car a Gar cecti eimiate the eects srace eaage. The evicese er test mst beas iscecte rm a t ther eipmet.
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Figre 33-Cectis r testig isati resistace a trasrmer high vtage wiig a bshigs, a the high tesi iscect switch i parae with reerece t gr bt witht beig aecte by eaage betwee the high a w vtage wiigs thrgh se the Gar cecti.
Figre 34-Cectis r testig isati resistace betwee high a w vtage wiigs witht beig aecte by eaage t gr.
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ouTdooR oIl CIRCuIT BREAkERS Th ur iustratis (Fig. 35 thrugh 38) shw th usua mths tstig bushigs a assciat parts a utr i circuit brar, a th accmpaig tab iicats th tst prcur b stps. I th tst vaus ar bw 10,000 mghms i a th ur stps, th ta shu b wr r rai s that th xcssiv sss ca b isat b urthr tsts a ivstigatis. I th tst vaus ar bw 50,000 mghms i tst #1, th tr th citi th particuar bushig ivv shu b watch b maig mr rut tsts. Tst Brar Bushig Bushig Bushig Psiti ergiz Guar Gru
Part Masur
1
p
1 (2 t guar)
1
……
2
p
1
1
2
3
p
1&2
1&2
……
Bushig 1 i para with crss mmbr Bushig 1 & 2 i para
4
cs
1&2
1&2
……
Bushig 1 & 2 i para with it r
Bushig 1
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Figre 35-Step 1
Figre 36-Step 2
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Figre 37-Step 3
Figre 38-Step 4
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SETTInG uP A MAInTEnAnCE PRoGRAM T start isuati tstig thr is gra ru: Put frst thigs frst. That is, rviw a th ctrica uipmt a cassi it as t rativ imprtac. Fr xamp, i that AC mtr ai i partmt A, hw wu it act vra pat pructi? yur pructi pp ca crtai hp i this a shu b vita itrst i th ia. I tim prmits at th start, tst vr pic ctrica uipmt a ma ut a rcr car. Pssib at frst u wi hav t cmbi svra uits but it wi pa i th g ru t hav tst rcrs r ach iiviua uit. Th, i isuati wasss shw up, ur jb tracig w th ig part wi b asir. Shw i Fig. 40 is rm tst rcr car, which is avaiab rm Mggr. Fwig is th tp irmati, which bcms mr vauab t u as tsts ar rpat at itrvas: 1. nam a cati th uipmt 2. dats a vaus tst rsuts (rcr actua raig at th tim tst) 3. Rag, vtag, a sria umbr th Mggr istrumt us 4. Tmpratur th apparatus (as, particuar r arg uits, wt a r bub tmpraturs – r humiit a w-pit trmiatis) 5. Isuati rsistac masurmt crrct r tmpratur 6. A pt cscutiv raigs t shw tr a ab u t aticipat aiurs
Figre 40
0
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HoW oFTEn SHould You TEST? That ps up th siz a cmpxit ur pat. ev itica uits ca ir i th ruir chc pris; xpric is ur bst gui. I gra, hwvr, wrig apparatus – mtrs, gratrs tc. – ar mr i t vp isuati wasss, as cmpar t wirig, isuatrs a th i. A tst schu r wrig uipmt shu b stabish, varig rm vr 6 t 12 mths, pig th siz uipmt a svrit th surruig atmsphric citis. Fr wirig a th i, tsts c a ar ar gra sufcit, uss th pat citis ar uusua svr.
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MEGGER 5 V & 10 V InSulATIon TESTERS Whr critica, high-capita uipmt is ivv, th itructi w a imprv isuatig matrias is r-writig th b isuati tstig. euipmt with pratig vtags abv 1 V ruirs cmmsurat highr tst vtags. Mr matrias, wh w r ar i thir i ccs, ca hav isuati vaus it rags that wr prvius umasur. yur isuati tstr ma t b u auat t mt th mas a rigrus a thrugh prgram prvtiv/ prictiv maitac mr uipmt. T b u i crmac with th mst mr tstig ruirmts, Mggr rs a ami th highst uait isuati tstrs at vtags abv 1 V. At th cr high-vtag tstig...5 V...th MIT510 a MIT520 ar th highst v uait tstig ag with prim sat, cvic a prtabiit. MIT510 rs sctab tst vtags at 250, 500, 1000, 2500, a 5000 V, thrb maig th tstr suitab r appicatis thrwis prrm with a staar hah m, i aiti t th mr maig high-vtag appicatis. Th MIT520 icrass th uctiait b prmittig tst vtag t b st i a 10 V icrmt rm 50 V t 1 V, th i 25 V stps a th wa t 5 V. Th tstrs masur t 15 Tra-hms, thrb maig thm u suitab r critica istaati tsts a stabishmt riab bas ata r w high-capita uipmt. Tst rsuts ar ispa Mggr’s patt ctric igita/aag arc, thrb arig th prcisi a assurac a igita raig cmbi with th t-ta pitr trav rma rsrv r mchaica mvmts.
ASTITCHINTIME
Th w 5 V isuati rsistac tstrs rm Mggr ar sig spcifca t assist u with th tstig a maitac high vtag ctrica uipmt. n
Mais r battr pwr
digita/Aag bacit ispa nSctab tst vtag rm 250 t 5000V nAutmatic IR tst n
Masurs t 15 TW nCmpt with caibrati crtifcat n
Megger MIT510
Th w 10 V isuati rsistac tstrs rm Mggr ar as sig spcifca t assist u with th tstig a maitac high vtag ctrica uipmt. A cass ar rugg a as t carr, big ma tugh pprp a achivig a igrss prtcti ratig IP65. Mais r battr pwr ndigita/aag bacit ispa nVariab tst vtag rm 50 V t 10 V nAutmatic IR, PI, dAR, SV a dd tsts nMasurs t 15 T W (5 V) a 35 TW (10 V)
Megger MIT1020
n
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MEGGER MIT300 SERIES Th MIT300 Sris is sig ag Mggr’s amiiar pricip ascig aturs with ss basic uait. Fiv ms cmpris th ami, ach buit a basic sig surig th uait a riabiit that ar Mggr’s tramars. MIT300 is th simpst m, rig cr uctis highst uait but aitia aturs that ma prv ruat i basic appicatis. Fr appicatis whr a 1 V tst must b avi, th MIT300 rs a 250 a 500 V tst . Cmbi with ctiuit a vtag warig, pus a th cvic a sat aturs that istiguish th ami, this is th m chic r th simpst appicatis. I basic ucti is sti th ga but a 1 V tst ruir, th MIT310 is th prct ft. With th pssibiit wrig at highr vtags, sa prati has b urthr hac b th icusi a aut vtmtr. Fr ths wh prr th traitia mchaica Megger MIT Series aag tp istrumt, th MIT310A rs vrthig i th MIT310, but with a mvig ci ispa. Th aag ispa is hac b bac cas a whit bacgru, t giv high ctrast a imprv visibiit. Th ami is cmpt b th u-atur MIT320 & MIT330. Th rmr as a ihm rag r masurmts btw th xtrms ctiuit a high vtag. With this atur, masurmts ca b ma ctiuus rm 0.01 W t 999 MW. A bacit ispa is urthr hac b bacit sctis, t ma bth th stup a rsuts visib i pr ight. Th MIT320 as rs auib aarms bth th ctiuit a mghm rags, ajustab acrss th tir rag. Fia, th MIT330 rs vrthig scrib i th MIT320, pus strag up t 1000 rsuts, waig thrugh a USB prt via icu stwar, a a ispa iicati rmaiig mmr.
ASTITCHINTIME
MEGGER MIT200 SERIES n basic 1 V isuati a ctiuit tstig, with cm a paramut csirati? n t sacrifc! Mggr rs th tp uait i ixpsiv tstrs th mart... th MIT200 Sris. Thugh sig with cm i mi, ths tstrs r ss th uait a riabiit that th am Mggr impis. Furthrmr, thr has b sacrifc sat, which is a srius prbm with ixpsiv tstrs that hav cut cst at th ris arc ash a pratr rrr.
Megger MIT230
Fur ms ar avaiab, irig i th tst vtags r: MIT200 is a 500 V m r staar appicatis; MIT210 is a 1 V m whr highr tst vtag is i rr; MIT220 rs tw vtags, 250 a 500, r mr ssitiv appicatis whr a 1 V tst is t b avi; r u-spctrum appicatis, MIT230 rs a thr vtags, 250, 500 a 1 V.
ASTITCHINTIME
Megger maes mre tha jst isati resistace testers Mggr as mas high uait istrumts r th wig ctrica tstig appicatis: n
earth/Gru Tstig
n n n n n
n
Battr Tstig
n
Ra Tstig oi Tstig Circuit Brar Tstig Pwr quait Aasis lw Rsistac Tstig
n n n n
Watthur Mtr Tstig Trasrmr Tstig Cab Faut Tstig Pwr Factr Tstig Hi Pt Tstig
Mggr mauacturs ctrica tst a maitac istrumts r ctric pwr, prcss mauacturig, buiig wirig, girig srvics a cmmuicatis.
Visit ur wbsit r ca assistac at www.BiddleMggr.cm.
MeterCenter - 2046 W. Peninsula Circle, Chandler, AZ 85248 (800) 230-6008 ASTITCHINTIME Th wr “Mggr” is a rgistr tramar.
M e G -4 8 6 /M Il /4 M /R v e /2 /2 0 0 6