Megger-book-the-complete-guide-to-electric

US $9.95 GBP £5.95 “A Stitch i Time ” The Cmpete Gie t Eectrica Isati Isati  Testig Testig W W W M E   G  G E  R   C   O M A STITCH IN TIME  “A Stitch I Time” The Cmpete Gie t Eectrica Isati Testig Copright 2006 W W W M E   G  G E  R   C   O M  A STITCH IN TIME A S S TI TI TC TC H H IN IN T T IM IM E ConTEnTS PAGE WHAT IS “GOOd” In SUlATIOn? WHAT MAkES InSUlATIOn GO BAd? HOW InSUlATIOn RESISTAnCE IS MEASUREd HOW TO InTERPRET RESISTAnCE REAdInGS FACTORS AFFECTInG InSUlATIOn RESISTAnCE REAdInGS 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 OU T OF EqUIPMEnT 18 EFFECT OF TEMPERATURE On InSUlATIOn RESISTAnCE EFFECTS OF HU MIdITy 23 PREPARATIOn PREPARA TIOn OF APPARATUS TO TEST SAFETy PRECAUTIOnS COnnECTIOnS FOR TESTInG InSUlATIOn RESISTAnCE OF ElECTRICAl EqUIPMEnT 27  WHAT IS “Good” InSulATIon? Ever eectric wire i our pat – whether it’s i a motor motor,, geerator, cabe, switch, trasormer, trasormer, etc – is careu covere with some orm o eectrica isuatio The wire itse is usua copper or aumium, which is ow to be a goo couctor o the eectric curret that powers our euipmet The isuatio must be just the opposite rom a couctor: it shou resist curret a eep the curret i its path aog the couctor couctor To uersta uersta isuatio testig ou rea o’t ee to go ito the mathematics o eectricit, but oe simpe euatio – Ohm’s aw – ca be ver hepu i appreciatig ma aspects Eve i ou’ve bee expose to this aw beore, it ma be a goo iea to review it i the ight o isuatio testig The purpose o isuatio arou a couctor is much ie that o a pipe carrig water, a Ohm’s aw o eectricit ca be more easi uerstoo b a compariso with water ow I Fig we show this compariso Pressure Pressure o water rom a pump causes ow aog the pipe (Fig 1a) I the pipe were to sprig a ea, ou’ waste water a ose some water pressure With eectricit, votage is ie the pump pressure, causig eectricit to ow aog the copper wire (Fig 1b) As i a water pipe, there is some resistace to ow, but it is much ess aog the wire tha it is through the isuatio AddITIOnAl nOTES ABOUT USInG A MEGGER InSUlATIOn TESTER 33 InTERPRETATIOn-MInIMUM InTERPRETA TIOn-MInIMUM VAlUES 36 MInIMUM VAlUES FOR InSUlATIOn RESISTAnCE 38 TESTS USInG MUlTI-VOlT MUlTI-VOlTAGE AGE MEGGER InSUlATIOn TESTERS 42 STEP-VOlTAGE STEP-VOlT AGE METHOd 48 USE OF A GUARd TERMInAl 50 BUSHInGS, POTHEAdS And InSUlATORS 54 Figre 1–Cmparis  water w (a) with eectric crret (b) OUTdOOR OIl CIRCUIT BREAkERS 57 BREAkERS 57 SETTInG UP A MAInTEnAnCE PROGRAM 60 HOW OFTEn SHOUld yOU TEST? MEGGER And 10 kV InSUlATIOn TESTERS MEGGER kV InSUlATIOn TESTERS TESTERS  A S S TI TI TC TC H H IN IN T T IM IM E A STITCH IN TIME  ConTEnTS PAGE WHAT IS “GOOd” In SUlATIOn? WHAT MAkES InSUlATIOn GO BAd? HOW InSUlATIOn RESISTAnCE IS MEASUREd HOW TO InTERPRET RESISTAnCE REAdInGS FACTORS AFFECTInG InSUlATIOn RESISTAnCE REAdInGS 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 OU T OF EqUIPMEnT 18 EFFECT OF TEMPERATURE On InSUlATIOn RESISTAnCE EFFECTS OF HU MIdITy 23 PREPARATIOn PREPARA TIOn OF APPARATUS TO TEST SAFETy PRECAUTIOnS COnnECTIOnS FOR TESTInG InSUlATIOn RESISTAnCE OF ElECTRICAl EqUIPMEnT 27 WHAT IS “Good” InSulATIon? Ever eectric wire i our pat – whether it’s i a motor motor,, geerator, cabe, switch, trasormer, trasormer, etc – is careu covere with some orm o eectrica isuatio The wire itse is usua copper or aumium, which is ow to be a goo couctor o the eectric curret that powers our euipmet The isuatio must be just the opposite rom a couctor: it shou resist curret a eep the curret i its path aog the couctor couctor To uersta uersta isuatio testig ou rea o’t ee to go ito the mathematics o eectricit, but oe simpe euatio – Ohm’s aw – ca be ver hepu i appreciatig ma aspects Eve i ou’ve bee expose to this aw beore, it ma be a goo iea to review it i the ight o isuatio testig The purpose o isuatio arou a couctor is much ie that o a pipe carrig water, a Ohm’s aw o eectricit ca be more easi uerstoo b a compariso with water ow I Fig we show this compariso Pressure Pressure o water rom a pump causes ow aog the pipe (Fig 1a) I the pipe were to sprig a ea, ou’ waste water a ose some water pressure With eectricit, votage is ie the pump pressure, causig eectricit to ow aog the copper wire (Fig 1b) As i a water pipe, there is some resistace to ow, but it is much ess aog the wire tha it is through the isuatio AddITIOnAl nOTES ABOUT USInG A MEGGER InSUlATIOn TESTER 33 InTERPRETATIOn-MInIMUM InTERPRETA TIOn-MInIMUM VAlUES 36 MInIMUM VAlUES FOR InSUlATIOn RESISTAnCE 38 TESTS USInG MUlTI-VOlT MUlTI-VOlTAGE AGE MEGGER InSUlATIOn TESTERS 42 STEP-VOlTAGE STEP-VOlT AGE METHOd 48 USE OF A GUARd TERMInAl 50 BUSHInGS, POTHEAdS And InSUlATORS 54 Figre 1–Cmparis  water w (a) with eectric crret (b) OUTdOOR OIl CIRCUIT BREAkERS 57 BREAkERS 57 SETTInG UP A MAInTEnAnCE PROGRAM 60 HOW OFTEn SHOUld yOU TEST? MEGGER And 10 kV InSUlATIOn TESTERS MEGGER kV InSUlATIOn TESTERS TESTERS  A STITCH IN TIME A S S TI TI TC TC H H IN IN T T IM IM E Commo sese tes us that the more votage we have, the more curret there’ be Aso, the ower the resistace o the wire, the more curret or the same votage Actua, this is Ohm’s aw, which is expresse this wa i euatio orm: where, E=IxR E = votage i vots I = curret i amperes R = resistace i ohms note, however, that o isuatio is perect (that is, has ifite resistace) so some eectricit oes ow aog the isuatio or through it to grou Such a curret ma o be a miioth o a ampere (oe microampere) but it is the basis o isuatio testig euipmet note aso that a higher votage tes to cause more curret through the isuatio This sma amout o curret wou ot, o course, harm goo isuatio but wou be a probem i the isuatio has eteriorate now, to sum up our aswer to the uestio “what is ‘goo’ isuatio?” We have see that, essetia essetia,, “goo” meas a reative high resistace to curret Use to escribe a isuatio materia, “goo” wou aso mea “the abiit to eep a high resistace.” So, a suitabe wa o measurig resistace ca te ou how “goo” the isuatio is Aso, i ou tae measuremets at reguar perios, ou ca chec tres towar its eterioratio (more o this ater) WHAT WHA T MAkES InSulATIon Go BAd? Whe our pat eectrica sstem a euipmet are ew, the eectrica isuatio shou be i top otch shape Furthermore, mauacturers o o wire, cabe, motors, a so o have cotiua improve their isuatios or services i iustr iustr nevertheess, eve toa, isuatio isuatio is subject to ma eects which ca cause it to ai – mechaica amage, vibratio, excessive heat or co, irt, oi, corrosive vapors, moisture rom processes, or or just the humiit o a mugg a I various egrees, these eemies o isuatio are at wor as time goes o – combie with the eectrica stresses that exist As pi hoes or cracs eveop, moisture a oreig matter peetrate the suraces o the isuatio, proviig a ow resistace path or eaage curret  A S S TI TI TC TC H H IN IN T T IM IM E  Oce starte, the ieret eemies te to ai each other other,, permittig excessive curret through the isuatio Sometimes the rop i isuatio resistace is sue, as whe euipmet is ooe Usua, however, however, it rops graua graua,, givig pet o warig, i chece perioica Such checs permit pae recoitioig beore service aiure I there are o checs, a motor with poor isuatio, or exampe, ma ot o be agerous to touch whe votage is appie, but aso be subject to bur out What was goo isuatio has become a partia couctor HoW InSulATIon RESISTAnCE IS MEASuREd you have see that goo isuatio has high resistace; poor isuatio, reative ow resistace The actua resistace vaues ca be higher or ower, epeig upo such actors as the temperature or moisture cotet o the isuatio (resistace ecreases i temperature or moisture) With a itte recor-eepig a commo sese, however, however, ou ca get a goo picture o the isuatio coitio rom vaues that are o reative The Megger isuatio tester is a sma, portabe istrumet that gives ou a irect reaig o isuatio resistace i ohms or megohms For goo isuatio, the resistace usua reas i the megohm rage The Megger isuatio tester is essetia a high-rage resistace meter (ohmmeter) with a buit-i irect-curret geerator This meter is o specia costructio with both curret a votage cois, eabig true ohms to be rea irect, iepeet o the actua votage appie This metho is oestructive; that is, it oes ot cause eterioratio o the isuatio Figre 2–Typica Megger test istrmet h-p t measre isati resistace A STITCH IN TIME  Commo sese tes us that the more votage we have, the more curret there’ be Aso, the ower the resistace o the wire, the more curret or the same votage Actua, this is Ohm’s aw, which is expresse this wa i euatio orm: where, E=IxR E = votage i vots I = curret i amperes R = resistace i ohms note, however, that o isuatio is perect (that is, has ifite resistace) so some eectricit oes ow aog the isuatio or through it to grou Such a curret ma o be a miioth o a ampere (oe microampere) but it is the basis o isuatio testig euipmet note aso that a higher votage tes to cause more curret through the isuatio This sma amout o curret wou ot, o course, harm goo isuatio but wou be a probem i the isuatio has eteriorate now, to sum up our aswer to the uestio “what is ‘goo’ isuatio?” We have see that, essetia essetia,, “goo” meas a reative high resistace to curret Use to escribe a isuatio materia, “goo” wou aso mea “the abiit to eep a high resistace.” So, a suitabe wa o measurig resistace ca te ou how “goo” the isuatio is Aso, i ou tae measuremets at reguar perios, ou ca chec tres towar its eterioratio (more o this ater) WHAT WHA T MAkES InSulATIon Go BAd? Whe our pat eectrica sstem a euipmet are ew, the eectrica isuatio shou be i top otch shape Furthermore, mauacturers o o wire, cabe, motors, a so o have cotiua improve their isuatios or services i iustr iustr nevertheess, eve toa, isuatio isuatio is subject to ma eects which ca cause it to ai – mechaica amage, vibratio, excessive heat or co, irt, oi, corrosive vapors, moisture rom processes, or or just the humiit o a mugg a I various egrees, these eemies o isuatio are at wor as time goes o – combie with the eectrica stresses that exist As pi hoes or cracs eveop, moisture a oreig matter peetrate the suraces o the isuatio, proviig a ow resistace path or eaage curret  Sometimes the rop i isuatio resistace is sue, as whe euipmet is ooe Usua, however, however, it rops graua graua,, givig pet o warig, i chece perioica Such checs permit pae recoitioig beore service aiure I there are o checs, a motor with poor isuatio, or exampe, ma ot o be agerous to touch whe votage is appie, but aso be subject to bur out What was goo isuatio has become a partia couctor HoW InSulATIon RESISTAnCE IS MEASuREd you have see that goo isuatio has high resistace; poor isuatio, reative ow resistace The actua resistace vaues ca be higher or ower, epeig upo such actors as the temperature or moisture cotet o the isuatio (resistace ecreases i temperature or moisture) With a itte recor-eepig a commo sese, however, however, ou ca get a goo picture o the isuatio coitio rom vaues that are o reative The Megger isuatio tester is a sma, portabe istrumet that gives ou a irect reaig o isuatio resistace i ohms or megohms For goo isuatio, the resistace usua reas i the megohm rage The Megger isuatio tester is essetia a high-rage resistace meter (ohmmeter) with a buit-i irect-curret geerator This meter is o specia costructio with both curret a votage cois, eabig true ohms to be rea irect, iepeet o the actua votage appie This metho is oestructive; that is, it oes ot cause eterioratio o the isuatio Figre 2–Typica Megger test istrmet h-p t measre isati resistace A STITCH IN TIME A S S TI TI TC TC H H IN IN T T IM IM E The geerator ca be ha-crae or ie-operate to eveop a high dC votage which causes a sma curret through a over suraces o the isuatio beig teste (Fig 2) This curret (usua at a appie votage o 500 vots or more) is measure b the ohmmeter, ohmmeter, which has a iicatig scae Fig shows a tpica scae, which reas icreasig resistace vaues rom et up to ifit, or a resistace too high to be measure Figre 3–Typica scae  the Megger isati tester HoW To InTERPRET RESISTAnCE REAdInGS As previous metioe, isuatio resistace reaigs shou be cosiere reative The ca be uite ieret or oe motor or machie teste three as i a row, et ot mea ba isuatio What rea matters is the tre i reaigs over a time perio, showig esseig resistace a warig o comig probems Perioic testig is, thereore, our best approach to prevetive maiteace o eectrica euipmet, usig recor cars as show i Fig Figre 4–Typica 4–Typica recr  isati resistace  a mi mtr mtr Cre A shws test aes as measre; Cre B shws same aes crrecte t 20°C (see page 22), giig a efite wwar tre twar a sae citi Reerse sie  car (at right) is se t recr the test ata  Oce starte, the ieret eemies te to ai each other other,, permittig excessive curret through the isuatio A S S TI TI TC TC H H IN IN T T IM IM E  Whether ou test moth, twice a ear, or oce a ear epes upo the tpe, ocatio, a importace o the euipmet For exampe, a sma pump motor or a short cotro cabe ma be vita to a process i our pat Experiece is the best teacher i settig up the scheue perios or our euipmet you shou mae these perioic tests i the same wa each time That is, with the same test coectios a with the same test votage appie or the same egth o time Aso ou shou mae tests at about the same temperature, or correct them to the same temperature A recor o the reative humiit ear the euipmet at the time o the test is aso hepu i evauatig the reaig a tre later sectios cover temperature correctio a humiit eects I summar, here are some geera observatios about how ou ca iterpret iterpret perioic isuatio resistace tests, a what ou shou o with the resut: Citi What T d (a) Fair t high aes a we maitaie n case r ccer (b) Fair t high aes, bt shwig a cstat teecy twars wer aes lcate a remey the case a chec the wwar tre (c) lw bt we maitaie Citi is prbaby a right, bt case  w aes sh be chece () S w as t be sae Cea, ry t, r therwise raise the aes bere pacig eipmet i serice (Test wet eipmet whie ryig t.) (e) Fair r high aes, preisy we maitaie bt shwig se werig Mae tests at reet iteras ti the case  w aes is cate a remeie; r ti the aes hae becme steay at a wer ee bt sae r perati; perati; r ti aes becme s w that it is sae t eep the eipmet i perati A STITCH IN TIME  The geerator ca be ha-crae or ie-operate to eveop a high dC votage which causes a sma curret through a over suraces o the isuatio beig teste (Fig 2) This curret (usua at a appie votage o 500 vots or more) is measure b the ohmmeter, ohmmeter, which has a iicatig scae Fig shows a tpica scae, which reas icreasig resistace vaues rom et up to ifit, or a resistace too high to be measure Figre 3–Typica scae  the Megger isati tester HoW To InTERPRET RESISTAnCE REAdInGS As previous metioe, isuatio resistace reaigs shou be cosiere reative The ca be uite ieret or oe motor or machie teste three as i a row, et ot mea ba isuatio What rea matters is the tre i reaigs over a time perio, showig esseig resistace a warig o comig probems Perioic testig is, thereore, our best approach to prevetive maiteace o eectrica euipmet, usig recor cars as show i Fig Figre 4–Typica 4–Typica recr  isati resistace  a mi mtr mtr Cre A shws test aes as measre; Cre B shws same aes crrecte t 20°C (see page 22), giig a efite wwar tre twar a sae citi Reerse sie  car (at right) is se t recr the test ata  A S S TI TI TC TC H H IN IN T T IM IM E Whether ou test moth, twice a ear, or oce a ear epes upo the tpe, ocatio, a importace o the euipmet For exampe, a sma pump motor or a short cotro cabe ma be vita to a process i our pat Experiece is the best teacher i settig up the scheue perios or our euipmet you shou mae these perioic tests i the same wa each time That is, with the same test coectios a with the same test votage appie or the same egth o time Aso ou shou mae tests at about the same temperature, or correct them to the same temperature A recor o the reative humiit ear the euipmet at the time o the test is aso hepu i evauatig the reaig a tre later sectios cover temperature correctio a humiit eects I summar, here are some geera observatios about how ou ca iterpret iterpret perioic isuatio resistace tests, a what ou shou o with the resut: Citi What T d (a) Fair t high aes a we maitaie n case r ccer (b) Fair t high aes, bt shwig a cstat teecy twars wer aes lcate a remey the case a chec the wwar tre (c) lw bt we maitaie Citi is prbaby a right, bt case  w aes sh be chece () S w as t be sae Cea, ry t, r therwise raise the aes bere pacig eipmet i serice (Test wet eipmet whie ryig t.) (e) Fair r high aes, preisy we maitaie bt shwig se werig Mae tests at reet iteras ti the case  w aes is cate a remeie; r ti the aes hae becme steay at a wer ee bt sae r perati; perati; r ti aes becme s w that it is sae t eep the eipmet i perati A STITCH IN TIME  FACToRS AFFECTInG InSulATIon RESISTAnCE RESISTAnCE REAdInGS Remember that the measure resistace (o the isuatio) wi be etermie b the votage appie a the resutat curret (R = E/I) There are a umber o thigs that aect curret, icuig temperature o the isuatio a humiit humiit,, as metioe i the previous sectio Right ow, et’s just cosier the ature o curret through isuatio a the eect o how og votage is appie Curret through a aog isuatio is mae up part o a reative stea curret i eaage paths over the isuatio surace Eectricit aso ows through the voume o the isuatio Actua, as show i Fig 5, our tota curret comprises three compoets: Capacitace Chargig Crret Curret that starts out high a rops ater the isuatio has bee charge to u votage (much ie water ow i a gare hose whe ou frst tur o the spigot) Absrpti Crret Aso a iitia high curret which the rops (or reasos iscusse uer the sectio Time-Resistace Metho) Ccti r leaage Crret A sma essetia stea curret both through a over the isuatio As show i Fig 5, the tota curret is the sum o the three compoets a it is this curret that ca be measure irect b a microammeter, or i terms o megohms at a particuar votage b meas o a Megger istrumet (ohmmeter) Because the tota curret epes upo the time that the votage is appie, ou c a see ow wh Ohm’s law R = E/I o hos, theoretica, at a ifite time (that is, ou’ have to wait orever beore taig a reaig) I practice, as ou wi see i the test methos escribe beow,ou rea a vaue that is the apparet resistace – a useu vaue to iagose troubes, which is what ou wat to o  A S S TI TI TC TC H H IN IN T T IM IM E Figre 5–Cres shwig cmpets  crret measre rig dC testig  isati note aso i Fig that the chargig curret isappears reative reative rapi as the euipmet uer test becomes charge larger uits with more capacitace wi tae oger to be charge This curret aso is the store eerg iitia ischarge ater our test, b short-circuitig a grouig the isuatio AlWAYS TAkE THIS SAFETY MEASuRE you ca see urther rom Fig that the absorptio curret ecreases at a reative sow rate, epeig upo the exact ature o the isuatio This store eerg, eerg, too, must be reease at the e o a test, a reuires a oger time tha the capacitace chargig curret – about our times as og as the votage was appie With goo isuatio, the couctio or eaage curret shou bui up to a stea vaue that is costat or the appie votage, as show i Fig A icrease o eaage curret with time is a warig o troube, as iscusse i the tests escribe i the oowig sectio A STITCH IN TIME  FACToRS AFFECTInG InSulATIon RESISTAnCE RESISTAnCE REAdInGS Remember that the measure resistace (o the isuatio) wi be etermie b the votage appie a the resutat curret (R = E/I) There are a umber o thigs that aect curret, icuig temperature o the isuatio a humiit humiit,, as metioe i the previous sectio Right ow, et’s just cosier the ature o curret through isuatio a the eect o how og votage is appie Curret through a aog isuatio is mae up part o a reative stea curret i eaage paths over the isuatio surace Eectricit aso ows through the voume o the isuatio Actua, as show i Fig 5, our tota curret comprises three compoets: Capacitace Chargig Crret Curret that starts out high a rops ater the isuatio has bee charge to u votage (much ie water ow i a gare hose whe ou frst tur o the spigot) Absrpti Crret Aso a iitia high curret which the rops (or reasos iscusse uer the sectio Time-Resistace Metho) Ccti r leaage Crret A sma essetia stea curret both through a over the isuatio As show i Fig 5, the tota curret is the sum o the three compoets a it is this curret that ca be measure irect b a microammeter, or i terms o megohms at a particuar votage b meas o a Megger istrumet (ohmmeter) Because the tota curret epes upo the time that the votage is appie, ou c a see ow wh Ohm’s law R = E/I o hos, theoretica, at a ifite time (that is, ou’ have to wait orever beore taig a reaig) I practice, as ou wi see i the test methos escribe beow,ou rea a vaue that is the apparet resistace – a useu vaue to iagose troubes, which is what ou wat to o  A S S TI TI TC TC H H IN IN T T IM IM E With a bacgrou ow o how time aects the meaig o istrumet reaigs, et’s cosier three c ommo test methos: (1) (1) short-time or spot reaig; (2) time-resistace time-resistace;; a (3) step or muti-votage muti-votage tests TYPES oF InSulATIon RESISTAnCE RESISTAnCE TESTS Shrt-Time r Spt-Reaig Test I this metho, ou simp coect the Megger istrumet across the isuatio to be teste a operate it or a short, short, specifc time perio (60 secos is usua recommee) As show schematica i Fig 6, ou’ve simp pice a poit o a curve o icreasig resistace vaues; uite ote the vaue wou be ess or 30 secos, more or 60 secos Bear i mi aso that temperature a humiit, as we as coitio o our isuatio aect our reaig Figre 5–Cres shwig cmpets  crret measre rig dC testig  isati note aso i Fig that the chargig curret isappears reative reative rapi as the euipmet uer test becomes charge larger uits with more capacitace wi tae oger to be charge This curret aso is the store eerg iitia ischarge ater our test, b short-circuitig a grouig the isuatio AlWAYS TAkE THIS SAFETY MEASuRE you ca see urther rom Fig that the absorptio curret ecreases at a reative sow rate, epeig upo the exact ature o the isuatio This store eerg, eerg, too, must be reease at the e o a test, a reuires a oger time tha the capacitace chargig curret – about our times as og as the votage was appie With goo isuatio, the couctio or eaage curret shou bui up to a stea vaue that is costat or the appie votage, as show i Fig A icrease o eaage curret with time is a warig o troube, as iscusse i the tests escribe i the oowig sectio A STITCH IN TIME  I the apparatus ou are testig has ver sma capacitace, such as a short ru o house wirig, the spot reaig test is a that is ecessar However,, most euipmet is capacitive a so our ver frst spot reaig However o euipmet i our pat, with o prior tests, ca be o a rough guie as to how goo or ba the isuatio is For ma ears, maiteace proessioas have use the oe-megohm rue to estabish the aowabe ower imit or isuatio resistace The rue ma be state: Insulation resistance should be approximately one megohm for each 1,000 volts of operating voltage, with a minimum value of one megohm For exampe, a motor rate at 2,400 vots shou have a miimum isuatio resistace o 2.4 megohms I practice, megohm reaigs orma are cosierab above this miimum vaue i ew euipmet or whe isuatio is i goo coitio B taig reaigs perioica a recorig them, ou have a better basis o jugig the actua isuatio coitio A persistet owwar tre is usua air warig o troube ahea, eve though the reaigs ma be higher tha the suggeste miimum sae vaues Eua true, as og as our perioic reaigs are cosistet, the ma be o, eve though ower tha the recommee miimum vaues The curves o Fig show tpica behavior o isuatio resistace uer varig pat operatig coitios The curves were potte rom spot reaigs tae with a M egger istrumet over a perio o moths Figre 6–Typica cre  isati resistace (i meghms) with time r the “shrt time” r “spt-reaig” test meth 0 ASTITCH AST ITCHIN INTIME TIME A STITCH IN TIME  With a bacgrou ow o how time aects the meaig o istrumet reaigs, et’s cosier three c ommo test methos: (1) (1) short-time or spot reaig; (2) time-resistace time-resistace;; a (3) step or muti-votage muti-votage tests TYPES oF InSulATIon RESISTAnCE RESISTAnCE TESTS Shrt-Time r Spt-Reaig Test I this metho, ou simp coect the Megger istrumet across the isuatio to be teste a operate it or a short, short, specifc time perio (60 secos is usua recommee) As show schematica i Fig 6, ou’ve simp pice a poit o a curve o icreasig resistace vaues; uite ote the vaue wou be ess or 30 secos, more or 60 secos Bear i mi aso that temperature a humiit, as we as coitio o our isuatio aect our reaig I the apparatus ou are testig has ver sma capacitace, such as a short ru o house wirig, the spot reaig test is a that is ecessar However,, most euipmet is capacitive a so our ver frst spot reaig However o euipmet i our pat, with o prior tests, ca be o a rough guie as to how goo or ba the isuatio is For ma ears, maiteace proessioas have use the oe-megohm rue to estabish the aowabe ower imit or isuatio resistace The rue ma be state: Insulation resistance should be approximately one megohm for each 1,000 volts of operating voltage, with a minimum value of one megohm For exampe, a motor rate at 2,400 vots shou have a miimum isuatio resistace o 2.4 megohms I practice, megohm reaigs orma are cosierab above this miimum vaue i ew euipmet or whe isuatio is i goo coitio B taig reaigs perioica a recorig them, ou have a better basis o jugig the actua isuatio coitio A persistet owwar tre is usua air warig o troube ahea, eve though the reaigs ma be higher tha the suggeste miimum sae vaues Eua true, as og as our perioic reaigs are cosistet, the ma be o, eve though ower tha the recommee miimum vaues The curves o Fig show tpica behavior o isuatio resistace uer varig pat operatig coitios The curves were potte rom spot reaigs tae with a M egger istrumet over a perio o moths Figre 6–Typica cre  isati resistace (i meghms) with time r the “shrt time” r “spt-reaig” test meth 0 A STITCH IN TIME ASTITCH AST ITCHIN INTIME TIME  Time-Resistace Meth This metho is air iepeet o temperature a ote ca give ou cocusive iormatio without recors o past tests It is base o the absorptio eect o goo isuatio compare to that o moist or cotamiate isuatio you you simp tae successive reaigs at specifc times a ote the iereces i reaigs (see curves, Fig 8) Tests b this metho are sometimes reerre to as absorptio tests note that goo isuatio shows a cotiua icrease i resistace (ess curret – see curve A) over a perio o time (i the orer o to 10 miutes) This is cause b the absorptio curret we spoe o earier; goo isuatio shows this charge eect over a time perio much oger tha the time reuire to charge the capacitace o the isuatio I the isuatio cotais much moisture or cotamiats, the absorptio absorptio eect is mase b a high eaage curret which stas at a air costat vaue, eepig the resistace reaig ow (remember: R = E/I) Figre 8–Typica cres shwig ieectric absrpti eect i a “time-resistace” test, mae  capacitie eipmet sch as a arge mtr wiig Figre 7–Typica behair  isati resistace er a peri  mths er aryig peratig citis, (cres ptte rm spt reaigs with a Megger istrmet)  ASTITCH AST ITCHIN INTIME TIME The time-resistace time-resistace test is o vaue aso because it is iepeet o euipmet size The icrease i resistace or cea a r isuatio occurs i the same maer whether a motor is arge or sma you ca, thereore, compare severa motors a estabish staars or ew oes, regaress o their horsepower ratigs A STITCH IN TIME  Time-Resistace Meth This metho is air iepeet o temperature a ote ca give ou cocusive iormatio without recors o past tests It is base o the absorptio eect o goo isuatio compare to that o moist or cotamiate isuatio you you simp tae successive reaigs at specifc times a ote the iereces i reaigs (see curves, Fig 8) Tests b this metho are sometimes reerre to as absorptio tests note that goo isuatio shows a cotiua icrease i resistace (ess curret – see curve A) over a perio o time (i the orer o to 10 miutes) This is cause b the absorptio curret we spoe o earier; goo isuatio shows this charge eect over a time perio much oger tha the time reuire to charge the capacitace o the isuatio I the isuatio cotais much moisture or cotamiats, the absorptio absorptio eect is mase b a high eaage curret which stas at a air costat vaue, eepig the resistace reaig ow (remember: R = E/I) Figre 8–Typica cres shwig ieectric absrpti eect i a “time-resistace” test, mae  capacitie eipmet sch as a arge mtr wiig Figre 7–Typica behair  isati resistace er a peri  mths er aryig peratig citis, (cres ptte rm spt reaigs with a Megger istrmet)  The time-resistace time-resistace test is o vaue aso because it is iepeet o euipmet size The icrease i resistace or cea a r isuatio occurs i the same maer whether a motor is arge or sma you ca, thereore, compare severa motors a estabish staars or ew oes, regaress o their horsepower ratigs A STITCH IN TIME ASTITCH AST ITCHIN INTIME TIME Fig shows how a 60-seco test wou appear or goo a perhaps ba isuatio Whe the isuatio is i goo shape, the 60-seco reaig is higher tha the 30-seco reaig  dieectric Absrpti Rati The ratio o two time-resistace reaigs (such as a 60-seco reaig ivie b a 30-seco reaig) is cae a ieectric absorptio ratio It is useu i recorig iormatio about isuatio I the ratio is a 10-miute 10-miute reaig ivie b a 1-miute reaig, the vaue is cae the poarizatio iex With ha-crae Megger istrumets, it’s a ot easier or ou to ru the test or o 60 secos, taig our frst reaig at 30 secos I ou have a ie-operate Megger istrumet, ou’ get best resuts b ruig the test 10 miutes, taig reaigs at 1- a at 10-miutes, 10-miutes, to get the poarizatio iex Tabe I gives vaues o the ratios a correspoig correspoig reative coitios o the isuatio that the iicate TABLE I—Condition of Insulation Indicated by Dielectric Absorption Ratios* InSUlATIOn COndITIOn 60/30-SECOnd RATIO 10/1-MInUTE RATIO (POlARIZATIOn IndEX) (POlARIZATIOn Figre 9–Typica car pt  a time-resistace r be-reaig test A urther avatage o this oube-reaig test, as it is sometimes cae, is that it gives ou a cearer picture, eve whe a spot reaig sas the isuatio oos fe For exampe, et’s sa the spot reaig o a schroous motor was 10 megohms now, et’s assume that the oube-reaig chec shows that the isuatio resistace hos stea at 10 megohms whie ou ho votage up to 60 secos This meas there ma be irt or moisture i the wiigs that bears watchig O the other ha, i the poiter shows a graua icrease betwee the 30-seco a the 60-seco checs, the ou’re reasoab sure the wiigs are i goo shape Time-resistace tests o arge rotatig eectrica machier – especia with Time-resistace high operatig votage – reuire high isuatio resistace rages a a ver costat test votage A heav-ut Megger test set, ie-operate, serves this ee Simiar, such a istrumet is better aapte or arge cabes, bushigs, trasormers a switchgear switchgear  ASTITCH AST ITCHIN INTIME TIME dagerous — less tha questioabe 1.0 to 1.25 1.0 to 2*** Goo 1.4 to 1.6 to Exceet Above 1.6** Above 4** *These vaues must be cosiere tetative a reative— subject to experiece with the time-resistace metho over a perio o time **I some cases, with motors, vaues approximate 20% higher tha show here iicate a r britte wiig which wi ai uer shoc coitios or urig starts For prevetive maiteace, the motor wiig shou be ceae,treate, a rie to restore wiig exibiit ***These resuts wou be satisactor or euipmet with ver ow capacitace such as short rus o house wirig A STITCH IN TIME  Fig shows how a 60-seco test wou appear or goo a perhaps ba isuatio Whe the isuatio is i goo shape, the 60-seco reaig is higher tha the 30-seco reaig dieectric Absrpti Rati The ratio o two time-resistace reaigs (such as a 60-seco reaig ivie b a 30-seco reaig) is cae a ieectric absorptio ratio It is useu i recorig iormatio about isuatio I the ratio is a 10-miute 10-miute reaig ivie b a 1-miute reaig, the vaue is cae the poarizatio iex With ha-crae Megger istrumets, it’s a ot easier or ou to ru the test or o 60 secos, taig our frst reaig at 30 secos I ou have a ie-operate Megger istrumet, ou’ get best resuts b ruig the test 10 miutes, taig reaigs at 1- a at 10-miutes, 10-miutes, to get the poarizatio iex Tabe I gives vaues o the ratios a correspoig correspoig reative coitios o the isuatio that the iicate TABLE I—Condition of Insulation Indicated by Dielectric Absorption Ratios* InSUlATIOn COndITIOn 60/30-SECOnd RATIO 10/1-MInUTE RATIO (POlARIZATIOn IndEX) (POlARIZATIOn Figre 9–Typica car pt  a time-resistace r be-reaig test A urther avatage o this oube-reaig test, as it is sometimes cae, is that it gives ou a cearer picture, eve whe a spot reaig sas the isuatio oos fe For exampe, et’s sa the spot reaig o a schroous motor was 10 megohms now, et’s assume that the oube-reaig chec shows that the isuatio resistace hos stea at 10 megohms whie ou ho votage up to 60 secos This meas there ma be irt or moisture i the wiigs that bears watchig O the other ha, i the poiter shows a graua icrease betwee the 30-seco a the 60-seco checs, the ou’re reasoab sure the wiigs are i goo shape Time-resistace tests o arge rotatig eectrica machier – especia with Time-resistace high operatig votage – reuire high isuatio resistace rages a a ver costat test votage A heav-ut Megger test set, ie-operate, serves this ee Simiar, such a istrumet is better aapte or arge cabes, bushigs, trasormers a switchgear switchgear  dagerous — less tha questioabe 1.0 to 1.25 1.0 to 2*** Goo 1.4 to 1.6 to Exceet Above 1.6** Above 4** *These vaues must be cosiere tetative a reative— subject to experiece with the time-resistace metho over a perio o time **I some cases, with motors, vaues approximate 20% higher tha show here iicate a r britte wiig which wi ai uer shoc coitios or urig starts For prevetive maiteace, the motor wiig shou be ceae,treate, a rie to restore wiig exibiit ***These resuts wou be satisactor or euipmet with ver ow capacitace such as short rus o house wirig A STITCH IN TIME ASTITCH AST ITCHIN INTIME TIME  TEST volTAGE vS EquIPMEnT RATInG AC TESTInG vS dC Commo use dC test votages or routie maiteace are as oows: Up to ow, we’ve tae about testig with dC votage, but ou wi hear o AC testig a ee to ow the ierece Remember that we spoe o the is o curret prouce i isuatio b dC? (The iitia surge o chargig curret, the rop with time to absorptio curret, a the, ater ater more time, the stea couctio curret.) We saw that i isuatio testig, the couctio or eaage curret is the oe that gives us the iormatio we ee Eipmet AC Ratig dC Test vtage up to 100 vots 100 a 250 vots 440 to 550 vots 500 a 1,000 vots 2,400 vots 1,000 to 2,500 vots or higher 4,160 vots a above 1,000 to 5,000 vots, or higher Test votages use or proo testig o euipmet are cosierab higher tha those use or routie maiteace Athough there are o pubishe iustr staars or dC maximum proo test votages to be use with rotatig euipmet, the scheue give beow is customari use For specifc recommeatios o our euipmet, ou shou cosut the mauacturer o the euipmet Pr Test vtages r Rtatig Eipmet: Factor AC Test = x n amepate Ratig + 1000 vots dC Proo Test o Istaatio = 0.8 x Factor AC Test x 1.6 dC Proo Test Ater Service = 0.6 x Factor AC Test x 1.6 Exampe: Motor with 2,400 VAC amepate ratig– Factor AC Test = 2(2,400) +1,000 = 5,800 VAC Max dC Test o Istaatio = 0.8(5,800)1.6 = 7,424 VdC Max dC Test Ater Service = 0.6(5,800)1.6 = 5,568 VdC  ASTITCH AST ITCHIN INTIME TIME I cotrast, testig with AC gives a chargig curret that is extreme arge compare to the other is; eaage curret is reative mior AC reuet is use or high-potetia testig; votage is icrease to some specife poit to see i the isuatio ca sta that particuar votage It is a GO/nO-GO tpe o test a ca cause eterioratio o the isuatio, i cotrast to the dC test which is basica o-estructive I a AC test votage has bee use a ou wat to use dC tests as a aterative, ou wi ee to icrease the maximum dC test votage somewhat to obtai euivaet resuts I some cases, AC testig ma be more suitabe or proo testig o euipmet (that is, seeig that the euipmet meets prescribe staars) you ru the votage up to the seecte vaue a the euipmet either passes or oes’t pass the test With With the dC test, ou get a more uaitative picture; ou ca meter the the eaage curret as ou icrease the votage a obtai specifc vaues o isuatio resistace As the size o our euipmet icreases, there are aso mare ecoomic avatages i dC over AC testig As the test votage icreases, both the cost a weight o AC euipmet go up much aster tha with comparabe dC test euipmet This is because the AC test set must supp the chargig curret which becomes a remais ver high i the arger machies As expaie previous, i dC testig, this curret rops rapi ater the iitia chargig perio A STITCH IN TIME  TEST volTAGE vS EquIPMEnT RATInG AC TESTInG vS dC Commo use dC test votages or routie maiteace are as oows: Up to ow, we’ve tae about testig with dC votage, but ou wi hear o AC testig a ee to ow the ierece Remember that we spoe o the is o curret prouce i isuatio b dC? (The iitia surge o chargig curret, the rop with time to absorptio curret, a the, ater ater more time, the stea couctio curret.) We saw that i isuatio testig, the couctio or eaage curret is the oe that gives us the iormatio we ee Eipmet AC Ratig dC Test vtage up to 100 vots 100 a 250 vots 440 to 550 vots 500 a 1,000 vots 2,400 vots 1,000 to 2,500 vots or higher 4,160 vots a above 1,000 to 5,000 vots, or higher Test votages use or proo testig o euipmet are cosierab higher tha those use or routie maiteace Athough there are o pubishe iustr staars or dC maximum proo test votages to be use with rotatig euipmet, the scheue give beow is customari use For specifc recommeatios o our euipmet, ou shou cosut the mauacturer o the euipmet Pr Test vtages r Rtatig Eipmet: Factor AC Test = x n amepate Ratig + 1000 vots dC Proo Test o Istaatio = 0.8 x Factor AC Test x 1.6 dC Proo Test Ater Service = 0.6 x Factor AC Test x 1.6 Exampe: Motor with 2,400 VAC amepate ratig– Factor AC Test = 2(2,400) +1,000 = 5,800 VAC Max dC Test o Istaatio = 0.8(5,800)1.6 = 7,424 VdC Max dC Test Ater Service = 0.6(5,800)1.6 = 5,568 VdC  ASTITCH AST ITCHIN INTIME TIME I summar, dC test sets are empoe amost excusive or high-votage maiteace a fe testig or the oowig reasos: lower cost lighter weight Smaer size no-estructive Better iormatio, iormatio, both i uait a uatit uatit uSE oF dC dIElECTRIC TEST SET The Megger istrumet, reaig irect i ohms a megohms o isuatio resistace, is our best bet or routie i-pat maiteace However,, some pats, particuar with higher votage ratigs i euipmet, However use aother Megger prouct – the ieectric test set So, ou shou be aware o this istrumet a its use i isuatio resistace measuremets The ieectric test set ca be use to etermie isuatio isuatio resistace b the same test methos as outie or the Megger istrumet; that is, the shorttime, time-resistace a step-votage tests It is esige or other uses, too, but or isuatio testig it provies: (1) a ajustabe output votage a (2) a moitorig o the resutat curret i micro-amperes The Megger dC dieectric Test Sets are avaiabe with votage outputs ragig rom V up to 160 V The curves o Fig are potte as curret versus time, as are curves or isuatio measuremets o tpica euipmet give ear the e o this maua Megger suppies graph paper which maes it eas to pot megohms o isuatio resistace rom our votage a curret reaigs TESTS duRInG dRYInG ouT oF EquIPMEnT Wet eectrica euipmet is a commo hazar ace b a maiteace egieers I the euipmet is wet rom resh water, ou go right ahea rig it out However, i ou’ve got sat water, ou must frst wash awa the sat with resh water Otherwise, ou’ eave ver corrosive eposits o sat o meta a isuatig suraces as we as i crevices o the isuatio With moisture, such eposits orm a ver goo couctor o eectricit Aso, ou shou remove oi or grease rom the isuatio, usig a suitabe sovet  ASTITCH AST ITCHIN INTIME TIME I cotrast, testig with AC gives a chargig curret that is extreme arge compare to the other is; eaage curret is reative mior AC reuet is use or high-potetia testig; votage is icrease to some specife poit to see i the isuatio ca sta that particuar votage It is a GO/nO-GO tpe o test a ca cause eterioratio o the isuatio, i cotrast to the dC test which is basica o-estructive I a AC test votage has bee use a ou wat to use dC tests as a aterative, ou wi ee to icrease the maximum dC test votage somewhat to obtai euivaet resuts I some cases, AC testig ma be more suitabe or proo testig o euipmet (that is, seeig that the euipmet meets prescribe staars) you ru the votage up to the seecte vaue a the euipmet either passes or oes’t pass the test With With the dC test, ou get a more uaitative picture; ou ca meter the the eaage curret as ou icrease the votage a obtai specifc vaues o isuatio resistace As the size o our euipmet icreases, there are aso mare ecoomic avatages i dC over AC testig As the test votage icreases, both the cost a weight o AC euipmet go up much aster tha with comparabe dC test euipmet This is because the AC test set must supp the chargig curret which becomes a remais ver high i the arger machies As expaie previous, i dC testig, this curret rops rapi ater the iitia chargig perio A STITCH IN TIME  There are various was to r out eectrica euipmet, epeig upo its size a portabiit portabiit you ca use a bast o hot air, a ove, circuatio o curret through couctors, or a combiatio o techiues loca pat coitios a aciities, together with iormatio rom the euipmet mauacturers, ca serve as a guie to the best metho or our particuar euipmet I some cases, or with certai euipmet, rig out ma ot be ecessar you ca chec this b isuatio resistace tests, i ou have recors o previous tests o the apparatus Whe rig out is reuire, such recors are aso hepu to etermie whe the isuatio is ree o moisture noTE: Wet eipmet is ssceptibe t tage breaw Therere, y sh se a w-tage Megger tester (100 r 250 vdC), at east i the eary stages  a ryig-t r I a w-tage istrmet is t reaiy aaiabe, sw craig  a 500-t tester may be sbstitte May testers hae a aitia test rage measrig i ihms (W) This measremet is typicay mae at y a ew ts, a is the iea iitia measremet t be mae  e eipmet This rage measres bew the Meghm rage, a ca, therere, prie a acta measremet t se as a bechmar i mitrig the ryig prcess I a ihm measremet is btaie, isati has bee thrghy satrate, bt may be recaimabe Ateratey test a ry, watchig the reaigs rise ti they reach the Meghm rage, at which time higher tage tests ca be saey empye As a exampe o how importat past reaigs are, et’s oo at a 100-hp motor that’s that’s bee ooe Ater a cea-up, a spot reaig with the Megger tester shows 1.5 megohms Oha, ou’ probab probab sa this is o What’s more, i past recors showe the isuatio resistace to ru betwee a megohms, ou’ be sure O the other ha, i past recors showe the orma resistace vaues to ru 10 or 20 megohms, the ou wou ow that water was sti i the motor wiigs A STITCH IN TIME  MInIMuM vAluES FoR InSulA InSulATIon TIon RESISTAnCE Rtatig Machiery The IEEE guie, “Recommee Practices or Testig Isuatio Isuatio Resistace Resistace o Rotatig Machier”, eas with the probem o maig a iterpretig isuatio resistace measuremets or rotatig machier It reviews the actors which aect or chage isuatio resistace characteristics, outies a recommes uiorm methos or maig tests, a presets ormuas or the cacuatio o approximate miimum isuatio resistace vaues or various tpes o AC a dC rotatig machier machier The guie states: “The recommee miimum isuatio resistace R m or ateratig-curre ateratig-currett a irect-curret machie armature wiigs a or fe wiigs o ateratig-curret a irect-curret machies ca be etermie b: Rm = V + where: Rm = V = recommee miimum isuatio resistace i megohms megohms at 40°C o the etire machie wiig rate machie termia to termia potetia, i iovots I appicatios where the machie is vita, it has bee cosiere goo practice to iitiate recoitioig shou the isuatio resistace, havig bee we above the miimum vaue give b E 2, rop appreciab to ear that eve.” It is recmmee that thse wh perate a maitai rtatig machiery btai cpies  the IEEE pbicati, “Recmmee Practices r Testig Isati Resistace  Rtatig Machiery”, which ca be btaie by writig the IEEE at 345 East 47th St., new Yr, nY, nY, 10017 Bshigs I the case o outoor oi circuit breaer bushigs, experiece has show that a bushig, with its assembe associate isuatig members, shou, or reiabe operatio, have a isuatio resistace vaue above 10,000 megohms at 20°C This assumes that the oi withi the ta is i goo coitio, that the breaer is separate rom its extera coectios to other euipmet, a that the porceai weather shie is guare This meas that each compoet such as the strippe bushig itse, cross-member,, it ro, ower arcig shie, etc., shou have a isuatio cross-member resistace i excess o that vaue  A compoets which are superfcia cea a r a have vaues ess tha 10,000 megohms are usua eteriorate itera, b the presece o moisture or carboize paths, to such a extet that the are ot reiabe or goo service uess recoitioe This is particuar so whe operatig uer surge coitios such as urig ightig isturbaces I the case o the strippe bushig itse, the ower stem a upper weather shie must be either perect cea or guare beore it is coeme as ureiabe because o a isuatio resistace vaue ess tha 10,000 megohms What has bee sai or strippe oi circuit breaer bushigs aso appies to bushigs or other euipmet, such as trasormers Sice bushigs a other associate members have ver high isuatio resistace vaues orma,, a Megger isuatio tester havig a rage o at east 10,000 orma megohms is ecessar to test such euipmet Megger istrumets havig rages up to 50,000 megohms wi permit observatio o eterioratig tres i bushigs beore the reach the uestioabe vaue o 10,000 megohms Cabe a Cctrs Cabe a couctor istaatios preset a wie variatio o coitios rom the poit o view o the resistace o the isuatio These coitios resut rom the ma is o isuatig materias use, the votage ratig or isuatio thicess, a the egth o the circuit ivove i the measuremet Furthermore, such circuits usua exte over great istaces, a ma be subjecte to wie variatios i temperature, which wi have a eect o the isuatio resistace vaues obtaie The termias o cabes a couctors wi aso have a eect o the test vaues uess the are cea a r, or guare The Isuate Cabe Egieers Associatio (ICEA) gives miimum vaues o isuatio resistace i its specifcatios or various tpes o cabes a couctors These miimum vaues are or ew, sige-couctor wire a cabe ater beig subjecte to a AC high votage test a base o a dC test potetia o 500 vots appie or oe miute at a temperature o 60°F A STITCH IN TIME ASTITCH AST ITCHIN INTIME TIME These staar miimum vaues (or sige-couctor cabe) are base o the oowig ormua:  The isuatio resistace o oe couctor o a muticouctor cabe to a others a sheath is: R = k og 10 d /  R = k og 10 d /  where: where: R = megohms per 1000 eet o cabe k = costat or isuatig materia d = outsie iameter o couctor isuatio  = iameter o couctor d = iameter over isuatio isuatio o euivaet sige-couctor sige-couctor cabe =  + 2c + 2b  = iameter o couctor (or sector cabes  euas iameter rou couctor o same cross-sectio) c = thicess o couctor isuatio b = thicess o jacet isuatio o Miimm vaes  k at 60° F (a imesis mst be expresse i same its) Isuatio Tpe Impregate Paper 2,640 Varishe Cambric 2,460 Thermopastic-Poethee above 50,000 Composite Poethee 30,000 Thermopastic-Povi: Povi Chorie 60°C 500 Povi Chorie 75°C 2,000 Grae natura Rubber Sthetic Rubber Trasrmers Acceptabe isuatio resistace vaues or r a compou-fe trasormers shou be comparabe to those or Cass A rotatig machier, athough o staar miimum vaues are avaiabe Oi-fe trasormers or votage reguators preset a specia probem i that the coitio o the oi has a mare iuece o the isuatio resistace o the wiigs I the absece o more reiabe ata the oowig ormua is suggeste: Coe .950 R = CE √VA Perormace 10,560 2,000 Heat Resistat 10,560 2,000 R = miimum 1-miute 500-vot dC isuatio resistace i megohms rom wiig to grou, with other wiig or wiigs guare, or rom wiig to wiig with core guare C = a costat or 20°C measuremets E = votage ratig o wiig uer test VA = rate capacit o wiig uer test Ozoe Resistat 10,000 (But) 2,000 kerite 4,000 See pages 44 a 45 or Tabes o log 10 d /  For tests o wiig to grou with the other wiig or wiigs groue, the vaues wi be much ess tha that give b the ormua R i this ormua is base o r, aci-ree, suge-ree suge-ree oi, a bushigs a termia boars that are i goo coitio 0 ASTITCH AST ITCHIN INTIME TIME A STITCH IN TIME  These staar miimum vaues (or sige-couctor cabe) are base o the oowig ormua: The isuatio resistace o oe couctor o a muticouctor cabe to a others a sheath is: R = k og 10 d /  R = k og 10 d /  where: where: R = megohms per 1000 eet o cabe k = costat or isuatig materia d = outsie iameter o couctor isuatio  = iameter o couctor d = iameter over isuatio isuatio o euivaet sige-couctor sige-couctor cabe =  + 2c + 2b  = iameter o couctor (or sector cabes  euas iameter rou couctor o same cross-sectio) c = thicess o couctor isuatio b = thicess o jacet isuatio o Miimm vaes  k at 60° F (a imesis mst be expresse i same its) Isuatio Tpe Impregate Paper 2,640 Varishe Cambric 2,460 Thermopastic-Poethee above 50,000 Composite Poethee 30,000 Thermopastic-Povi: Povi Chorie 60°C 500 Povi Chorie 75°C 2,000 Grae natura Rubber Sthetic Rubber Trasrmers Acceptabe isuatio resistace vaues or r a compou-fe trasormers shou be comparabe to those or Cass A rotatig machier, athough o staar miimum vaues are avaiabe Oi-fe trasormers or votage reguators preset a specia probem i that the coitio o the oi has a mare iuece o the isuatio resistace o the wiigs I the absece o more reiabe ata the oowig ormua is suggeste: Coe .950 R = CE √VA Perormace 10,560 2,000 Heat Resistat 10,560 2,000 R = miimum 1-miute 500-vot dC isuatio resistace i megohms rom wiig to grou, with other wiig or wiigs guare, or rom wiig to wiig with core guare C = a costat or 20°C measuremets E = votage ratig o wiig uer test VA = rate capacit o wiig uer test Ozoe Resistat 10,000 (But) 2,000 kerite 4,000 See pages 44 a 45 or Tabes o log 10 d /  For tests o wiig to grou with the other wiig or wiigs groue, the vaues wi be much ess tha that give b the ormua R i this ormua is base o r, aci-ree, suge-ree suge-ree oi, a bushigs a termia boars that are i goo coitio 0 A STITCH IN TIME ASTITCH AST ITCHIN INTIME TIME vaes  C at 20°C 60-Hertz Tae oi-fe tpe Utae oi-fe tpe dr or compou-fe tpe 1.5 30.0 30.0 25-Hertz 1.0 20.0 20.0 This ormua is itee or sige-phase trasormers trasormers I the trasormer uer test is o the three-phase tpe, a the three iiviua wiigs are beig teste as oe, the: E = VA = votage ratig o oe o the sige-phase wiigs (phase to phase or eta coecte uits a phase to eutra or star coecte uits) rate capacit o the compete three-phase wiig uer test  o the ohmmeter ca be expaie b reerrig to Fig 21 I this fgure, the chage which ma occur i eaage curret ater the absorptio curret has isappeare is show potte i terms o isuatio resistace agaist three ieret votages note that there there is o chage i resistace show i the fgure betwee 500 a 1000 vots, iicatig o chage i the isuatio as a resut o appig these two votages This is a assumptio, but is a coitio which is ot ucommo i practice I the isuatio cotiues to be stabe at 2500 vots, there wi be o chage i the isuatio resistace vaue obtaie, which is show b the otte extesio o the horizota ie i the fgure Whe o-iear coitios appear at a higher votage, the the votage resistace curve reveas this ver ver cear b a ower resistace vaue, iicate b the owwar curve i the fgure The fgure, thereore, reveas the simpicit o etermiig the chage i isuatio stabiit b usig three fxe votages which are easi reproucibe whe maig three-votage tests o a routie basis TESTS uSInG MulTI-volTAGE MulTI-volTAGE MEGGER InSulATIon TESTE RS Maiteace practice tres iicate the vaue o testig isuatio with dC votages at eves somewhat higher tha the pea vaue o the rate AC votage o the euipmet beig teste Such dC tests have i some cases bee show to revea o-estructive icipiet weaesses i isuatio isuatio which cou ot otherwise be ou, except possib b partia ischarge etectio at o-estructive AC test votage eves The techiue ivoves the appicatio o two or more dC votages, a critica observig a reuctio o isuatio resistace at the higher votage A mare or uusua reuctio i isuatio resistace or a prescribe icrease i appie votage is a iicatio o icipiet weaess It is importat to metio that the merits o this techiue arise rom more recet ivestigatios which iicate that rather high dC votage ca be use to etect weaesses without amagig the isuatio The maximum vaue o votage which shou be use wi epe arge o the ceaiess a ress o the isuatio to be teste I maig tests o isuatio at such dC votages, the ohmmeter metho has at east two avatages First, prescribe fxe votages are switche ito use, a oe istrumet measuremet mae with the irect reaig ohmmeter This is a simpe a reproucibe metho compare to oe i ohmmeter which ma choices o votage are avaiabe Aother importat avatage  ASTITCH AST ITCHIN INTIME TIME Figre 21 We wish to emphasize that the curve i Fig 21 iicates the resistace chage ue to eaage curret o, a ot the absorptio curret which ma appear or a perio o time with each chage i votage It ma be ecessar to wait a appreciabe amout o time ater each votage chage or the absorptio curret to isappear beore taig a reaig A STITCH IN TIME  vaes  C at 20°C 60-Hertz Tae oi-fe tpe Utae oi-fe tpe dr or compou-fe tpe 1.5 30.0 30.0 25-Hertz 1.0 20.0 20.0 This ormua is itee or sige-phase trasormers trasormers I the trasormer uer test is o the three-phase tpe, a the three iiviua wiigs are beig teste as oe, the: E = VA = votage ratig o oe o the sige-phase wiigs (phase to phase or eta coecte uits a phase to eutra or star coecte uits) rate capacit o the compete three-phase wiig uer test o the ohmmeter ca be expaie b reerrig to Fig 21 I this fgure, the chage which ma occur i eaage curret ater the absorptio curret has isappeare is show potte i terms o isuatio resistace agaist three ieret votages note that there there is o chage i resistace show i the fgure betwee 500 a 1000 vots, iicatig o chage i the isuatio as a resut o appig these two votages This is a assumptio, but is a coitio which is ot ucommo i practice I the isuatio cotiues to be stabe at 2500 vots, there wi be o chage i the isuatio resistace vaue obtaie, which is show b the otte extesio o the horizota ie i the fgure Whe o-iear coitios appear at a higher votage, the the votage resistace curve reveas this ver ver cear b a ower resistace vaue, iicate b the owwar curve i the fgure The fgure, thereore, reveas the simpicit o etermiig the chage i isuatio stabiit b usig three fxe votages which are easi reproucibe whe maig three-votage tests o a routie basis TESTS uSInG MulTI-volTAGE MulTI-volTAGE MEGGER InSulATIon TESTE RS Maiteace practice tres iicate the vaue o testig isuatio with dC votages at eves somewhat higher tha the pea vaue o the rate AC votage o the euipmet beig teste Such dC tests have i some cases bee show to revea o-estructive icipiet weaesses i isuatio isuatio which cou ot otherwise be ou, except possib b partia ischarge etectio at o-estructive AC test votage eves The techiue ivoves the appicatio o two or more dC votages, a critica observig a reuctio o isuatio resistace at the higher votage A mare or uusua reuctio i isuatio resistace or a prescribe icrease i appie votage is a iicatio o icipiet weaess It is importat to metio that the merits o this techiue arise rom more recet ivestigatios which iicate that rather high dC votage ca be use to etect weaesses without amagig the isuatio The maximum vaue o votage which shou be use wi epe arge o the ceaiess a ress o the isuatio to be teste I maig tests o isuatio at such dC votages, the ohmmeter metho has at east two avatages First, prescribe fxe votages are switche ito use, a oe istrumet measuremet mae with the irect reaig ohmmeter This is a simpe a reproucibe metho compare to oe i ohmmeter which ma choices o votage are avaiabe Aother importat avatage  ASTITCH AST ITCHIN INTIME TIME Figre 21 We wish to emphasize that the curve i Fig 21 iicates the resistace chage ue to eaage curret o, a ot the absorptio curret which ma appear or a perio o time with each chage i votage It ma be ecessar to wait a appreciabe amout o time ater each votage chage or the absorptio curret to isappear beore taig a reaig A STITCH IN TIME  To better uersta the techiue o maig isuatio resistace tests at two or more votages, the oowig steps are suggeste, usig a iustria or tractio motor casse i the 300- to 1000-vot rage as a exampe: Mae a oe-miute Megger istrumet test at 500 vots to serve as a basis o compariso or subseuet steps Ater a careu ceaig operatio mae a seco 500-vot test to etermie the eectiveess o the ceaig I the oe-miute isuatio resistace vaue is suborma, or i the 60 seco/30 seco isuatio resistace ratio is o greater tha oe at this poit the a rig operatio ma be esirabe beore usig a higher test votage However, maig aother test at 1000 vots a comparig these reaigs with those rom the 500-vot test wi hep i etermiig the ee or rig I the 1000-vot test vaue is appreciab ess tha that at 500 vots, the a rig operatio shou be perorme O the other ha, i the 1000-vot a 500-vot test vaues are approximate the same, it is reasoabe to assume that the ecisio to perorm a rig operatio ca be eerre uti ater the ext step presece o moisture I the isuatio resistace is frst teste o the shorttime reaig basis (frst at oe votage eve a the at a higher potetia), a ower vaue o isuatio resistace atthe higher dC test votage usua iicates the presece o moisture The appie votages shou preerab be i the ratio o to Experiece has iicate that a chage o 25% i the isuatio resistace vaue, with a to ratio i test votages, is usua ue to the presece o a excessive amout o moisture This metho is ot base base o a ieectric absorptio pheomea, but it oes reate to the Evershe Eect As with time-resistace measuremets, the muti-votage metho o testig isuatio resistace has icrease vaue whe mae o a perioic or scheue basis Mae a Megger istrumet test at 2500 vots I there is o appreciabe ierece i the 500- a 2500-vot test vaues, goo eviece exists that the motor i uestio is i reiabe coitio as ar as its isuatio is cocere O the other ha, i there is a appreciabe ierece i the two, there is goo eviece that more thorough recoitioig is cae or I the isuatio ais uer the 2500-vot test, ater oowig steps 1, or a 3, we beieve there is a ieihoo that the motor i uestio wou ai i service eve though a attempt were mae to recoitio it o the basis o ow-votage tests o The muti-votage metho ca aso be hepu i etermiig the presece o excessive moisture i the isuatio o euipmet rate at votages euivaet to or greater tha the highest votage avaiabe rom the mutivotage Megger istrumet beig use I other wors, eve though the highest Megger istrumet votage avaiabe oes ot stress the isuatio beo its ratig, a two-votage test ca, evertheess, ote revea the  ASTITCH AST ITCHIN INTIME TIME Figre 22-Time Resistace Test Frms A STITCH IN TIME  To better uersta the techiue o maig isuatio resistace tests at two or more votages, the oowig steps are suggeste, usig a iustria or tractio motor casse i the 300- to 1000-vot rage as a exampe: Mae a oe-miute Megger istrumet test at 500 vots to serve as a basis o compariso or subseuet steps Ater a careu ceaig operatio mae a seco 500-vot test to etermie the eectiveess o the ceaig I the oe-miute isuatio resistace vaue is suborma, or i the 60 seco/30 seco isuatio resistace ratio is o greater tha oe at this poit the a rig operatio ma be esirabe beore usig a higher test votage However, maig aother test at 1000 vots a comparig these reaigs with those rom the 500-vot test wi hep i etermiig the ee or rig I the 1000-vot test vaue is appreciab ess tha that at 500 vots, the a rig operatio shou be perorme O the other ha, i the 1000-vot a 500-vot test vaues are approximate the same, it is reasoabe to assume that the ecisio to perorm a rig operatio ca be eerre uti ater the ext step presece o moisture I the isuatio resistace is frst teste o the shorttime reaig basis (frst at oe votage eve a the at a higher potetia), a ower vaue o isuatio resistace atthe higher dC test votage usua iicates the presece o moisture The appie votages shou preerab be i the ratio o to Experiece has iicate that a chage o 25% i the isuatio resistace vaue, with a to ratio i test votages, is usua ue to the presece o a excessive amout o moisture This metho is ot base base o a ieectric absorptio pheomea, but it oes reate to the Evershe Eect As with time-resistace measuremets, the muti-votage metho o testig isuatio resistace has icrease vaue whe mae o a perioic or scheue basis Mae a Megger istrumet test at 2500 vots I there is o appreciabe ierece i the 500- a 2500-vot test vaues, goo eviece exists that the motor i uestio is i reiabe coitio as ar as its isuatio is cocere O the other ha, i there is a appreciabe ierece i the two, there is goo eviece that more thorough recoitioig is cae or I the isuatio ais uer the 2500-vot test, ater oowig steps 1, or a 3, we beieve there is a ieihoo that the motor i uestio wou ai i service eve though a attempt were mae to recoitio it o the basis o ow-votage tests o The muti-votage metho ca aso be hepu i etermiig the presece o excessive moisture i the isuatio o euipmet rate at votages euivaet to or greater tha the highest votage avaiabe rom the mutivotage Megger istrumet beig use I other wors, eve though the highest Megger istrumet votage avaiabe oes ot stress the isuatio beo its ratig, a two-votage test ca, evertheess, ote revea the  ASTITCH AST ITCHIN INTIME TIME STEP-volTAGE METHod I this metho, ou ee a muti-votage Megger istrumet to app two or more votages i steps, ie 500 vots a the 1000 vots you oo or a reuctio o isuatio resistace at the higher votage I the resistace is ower, it’s it’s a sig o a isuatio weaess that shows up o at the higher votage Fig 23 shows a exampe i which, istea o progressive icreasig the votage, ou frst test at a ow votage (such as 500 vots) a the, ater ischargig the sampe, ou test agai at a higher votage (such as 2500 vots) A ierece i the two tests i terms o megohms wi show sigs o weaess at the higher votage – a warig to ivestigate urther urther As the coitios withi the sampe eteriorate, the higher-votage pot, as show i Fig 23, wi reuce i megohms compare to the ower-votage oe, a its upwar sope wi be ess Figre 22-Time Resistace Test Frms A STITCH IN TIME  now, whe the votage is icrease i steps to prouce eectrica stresses which approach or excee those ecoutere i service, the oca wea spots iuece the overa isuatio resistace more a more The resistace o such oca auts geera ecreases rapi as the eectrica stress i them icreases beo a certai imit The pot o cosecutive Megger istrumet reaigs cear shows the sharp rop whe this occurs (see Fig 24) Figre 23-Typica cres with the “step-tage” test The theor behi the step-votage techiue is a bit compex, but we’ tr to eep it simpe Moisture a irt i isuatio are usua reveae b tests at votages ar beow those expecte i service H owever owever,, eects o agig or mechaica amage i air cea a r isuatio ma ot be reveae at such ow stress  ASTITCH AST ITCHIN INTIME TIME Figre 24-Test cres by the step-tage meth, cmparig rests with g a ba isati Cre (wer pt) shws efite rp i resistace with icreasig tage, iicatig a prbem Cre (pper pt) shws citis  i the same mtr wiig ater ceaig, baig a impregatig perati A STITCH IN TIME  STEP-volTAGE METHod I this metho, ou ee a muti-votage Megger istrumet to app two or more votages i steps, ie 500 vots a the 1000 vots you oo or a reuctio o isuatio resistace at the higher votage I the resistace is ower, it’s it’s a sig o a isuatio weaess that shows up o at the higher votage Fig 23 shows a exampe i which, istea o progressive icreasig the votage, ou frst test at a ow votage (such as 500 vots) a the, ater ischargig the sampe, ou test agai at a higher votage (such as 2500 vots) A ierece i the two tests i terms o megohms wi show sigs o weaess at the higher votage – a warig to ivestigate urther urther As the coitios withi the sampe eteriorate, the higher-votage pot, as show i Fig 23, wi reuce i megohms compare to the ower-votage oe, a its upwar sope wi be ess now, whe the votage is icrease i steps to prouce eectrica stresses which approach or excee those ecoutere i service, the oca wea spots iuece the overa isuatio resistace more a more The resistace o such oca auts geera ecreases rapi as the eectrica stress i them icreases beo a certai imit The pot o cosecutive Megger istrumet reaigs cear shows the sharp rop whe this occurs (see Fig 24) Figre 23-Typica cres with the “step-tage” test The theor behi the step-votage techiue is a bit compex, but we’ tr to eep it simpe Moisture a irt i isuatio are usua reveae b tests at votages ar beow those expecte i service H owever owever,, eects o agig or mechaica amage i air cea a r isuatio ma ot be reveae at such ow stress  Figre 24-Test cres by the step-tage meth, cmparig rests with g a ba isati Cre (wer pt) shws efite rp i resistace with icreasig tage, iicatig a prbem Cre (pper pt) shws citis  i the same mtr wiig ater ceaig, baig a impregatig perati A STITCH IN TIME ASTITCH AST ITCHIN INTIME TIME you o ee to eep the test votage costat betwee steps or about 60 secos This short perio wi ot aect the tre i resistace chage The time perio, however, shou awas be the same or a give piece o euipmet A the absorptio curret ma ot have isappeare i this time, but our resistace measuremets wi be mae o the same basis a thereore wi be meaigu your resuts are iepeet o the isuatio materia a its temperature because ou’re ooig at the chage i resistace – ot the absoute resistace vaues  The isuatio o a eectrica apparatus has two couctig or eaage paths – oe through the isuatig materia a the other over its suraces B proviig a thir test termia i the path o the surace eaage, eaage, it is separate ito two parts, ormig a three-termia etwor as show i Fig 25a I practice, this thir termia ma be provie as show i Figures 26 to 38 As is true with the spot-reaig a time-resistace measuremets, the step-votage metho is more vauabe to ou whe repeate o a perioic, scheue basis The step-votage metho is particuar useu i etermiig the presece o excessive moisture or other cotamiats i the isuatio o euipmet that is rate at votages euivaet to or greater tha the highest votage avaiabe i our muti-votage Megger istrumet I other wors, eve though our highest votage oes ot stress the isuatio beo its ratig, a two-votage test ca, evertheess, ote revea the presece o such cotamiats Figre 25a For exampe, assume that ou frst test isuatio resistace o a short-time reaig basis – at a votage o 500 V dC, a the at a higher potetia o, sa, 2500 VdC Eve though the atter votage ma be omia i reatio to the votage ratig o our euipmet, a ower vaue o isuatio resistace at the higher test votage usua iicates the presece o c otamiate ractures or other eaage paths through the isuatio to grou Figre 25b The appie votages shou preerab be i the ratio o to or greater (500 a 2500, or exampe) Resuts to ate show that a chage o 25% i the isuatio resistace vaue, with a to ratio i test votages, is usua ue to the presece o a excessive amout o moisture or other cotamiat uSE oF A GuARd TERMInAl A Megger isuatio testers havig rages o 1000 megohms a higher are euippe with Guar termias The purpose o this termia is to provie aciities or maig a three-termia etwor measuremet, so that the resistace o oe o two possibe paths ca be etermie irect irect It has the urther or secoar purpose o proviig a source o dC votage o goo reguatio a o imite curret capacit 0 ASTITCH AST ITCHIN INTIME TIME Figre 26-Shwig hw t se the Gar termia t eimiate the eects  srace eaage acrss expse isati at e e  a cabe See as Figres 28, 30 a 31 A STITCH IN TIME  you o ee to eep the test votage costat betwee steps or about 60 secos This short perio wi ot aect the tre i resistace chage The time perio, however, shou awas be the same or a give piece o euipmet A the absorptio curret ma ot have isappeare i this time, but our resistace measuremets wi be mae o the same basis a thereore wi be meaigu your resuts are iepeet o the isuatio materia a its temperature because ou’re ooig at the chage i resistace – ot the absoute resistace vaues The isuatio o a eectrica apparatus has two couctig or eaage paths – oe through the isuatig materia a the other over its suraces B proviig a thir test termia i the path o the surace eaage, eaage, it is separate ito two parts, ormig a three-termia etwor as show i Fig 25a I practice, this thir termia ma be provie as show i Figures 26 to 38 As is true with the spot-reaig a time-resistace measuremets, the step-votage metho is more vauabe to ou whe repeate o a perioic, scheue basis The step-votage metho is particuar useu i etermiig the presece o excessive moisture or other cotamiats i the isuatio o euipmet that is rate at votages euivaet to or greater tha the highest votage avaiabe i our muti-votage Megger istrumet I other wors, eve though our highest votage oes ot stress the isuatio beo its ratig, a two-votage test ca, evertheess, ote revea the presece o such cotamiats Figre 25a For exampe, assume that ou frst test isuatio resistace o a short-time reaig basis – at a votage o 500 V dC, a the at a higher potetia o, sa, 2500 VdC Eve though the atter votage ma be omia i reatio to the votage ratig o our euipmet, a ower vaue o isuatio resistace at the higher test votage usua iicates the presece o c otamiate ractures or other eaage paths through the isuatio to grou Figre 25b The appie votages shou preerab be i the ratio o to or greater (500 a 2500, or exampe) Resuts to ate show that a chage o 25% i the isuatio resistace vaue, with a to ratio i test votages, is usua ue to the presece o a excessive amout o moisture or other cotamiat uSE oF A GuARd TERMInAl A Megger isuatio testers havig rages o 1000 megohms a higher are euippe with Guar termias The purpose o this termia is to provie aciities or maig a three-termia etwor measuremet, so that the resistace o oe o two possibe paths ca be etermie irect irect It has the urther or secoar purpose o proviig a source o dC votage o goo reguatio a o imite curret capacit 0 ASTITCH AST ITCHIN INTIME TIME Figre 26-Shwig hw t se the Gar termia t eimiate the eects  srace eaage acrss expse isati at e e  a cabe See as Figres 28, 30 a 31 A STITCH IN TIME  There are aso cases, such as ou i two wiig trasormers or muticouctor cabes, where a three-termia etwor is orme as show i Fig 25b Figures 30 a 33 a others show practica appicatios o this orm o three-termia etwor I maig a three-termia test ivovig o oe measuremet, the lie termia o the Megger istrumet is coecte to Termia 1, Fig 25a, the Guar termia to Termia 3, a the Earth termia to Termia This wi give the true vaue o r 12, provie r 23 a r13 are ot too ow i vaue The eg r23, which is coecte across the Megger istrumet geerator, geerator, shou be about megohm or higher to prevet excessive oa o the geerator, a maitai satisactor geerator votage I usig the Guar termia, particuar i the case o motor-rive or rectifer-operate Megger istrumets,aso mae certai that there is o chace o a arc-over betwee the guare termia o the sampe a grou Such a arc-over ma cause uesirabe arcig at the commutator o the istrumet geerator Figre 28-Shwig se  the Gar cecti t eimiate the eect  eaage t gr, as i Figre 26, a as the eect  eaage t ajacet cctrs nte that the Gar wire is wrappe ar the expse isati a as is cecte t the ajacet cctrs d t cse this iagram with Figre 26, where the Gar wire ges y t the expse isati, a the ajacet cctrs are gre The eg r 13, which shuts the Megger eectig coi, shou be at east 100 megohms or a measurig accurac o approximate 1% The 1% accurac fgure is base o the R' baast resistor beig megohm, which is tpica For more precise etermiatios o accurac, obtai the exact vaue o R' b writig to Megger a givig the seria umber o the istrumet i use Figre 29-T eimiate the eect  srace eaage i measrig the tre resistace  a isatig member, sch as a it r i a circit breaer Figre 27-Shwig hw t se the Gar cecti t eimiate the eects  srace eaage acrss expse isati at bth es  a cabe whe a spare cctr i the cabe is aaiabe r cmpetig the Gar cecti  ASTITCH AST ITCHIN INTIME TIME A STITCH IN TIME  There are aso cases, such as ou i two wiig trasormers or muticouctor cabes, where a three-termia etwor is orme as show i Fig 25b Figures 30 a 33 a others show practica appicatios o this orm o three-termia etwor I maig a three-termia test ivovig o oe measuremet, the lie termia o the Megger istrumet is coecte to Termia 1, Fig 25a, the Guar termia to Termia 3, a the Earth termia to Termia This wi give the true vaue o r 12, provie r 23 a r13 are ot too ow i vaue The eg r23, which is coecte across the Megger istrumet geerator, geerator, shou be about megohm or higher to prevet excessive oa o the geerator, a maitai satisactor geerator votage I usig the Guar termia, particuar i the case o motor-rive or rectifer-operate Megger istrumets,aso mae certai that there is o chace o a arc-over betwee the guare termia o the sampe a grou Such a arc-over ma cause uesirabe arcig at the commutator o the istrumet geerator Figre 28-Shwig se  the Gar cecti t eimiate the eect  eaage t gr, as i Figre 26, a as the eect  eaage t ajacet cctrs nte that the Gar wire is wrappe ar the expse isati a as is cecte t the ajacet cctrs d t cse this iagram with Figre 26, where the Gar wire ges y t the expse isati, a the ajacet cctrs are gre The eg r 13, which shuts the Megger eectig coi, shou be at east 100 megohms or a measurig accurac o approximate 1% The 1% accurac fgure is base o the R' baast resistor beig megohm, which is tpica For more precise etermiatios o accurac, obtai the exact vaue o R' b writig to Megger a givig the seria umber o the istrumet i use Figre 29-T eimiate the eect  srace eaage i measrig the tre resistace  a isatig member, sch as a it r i a circit breaer Figre 27-Shwig hw t se the Gar cecti t eimiate the eects  srace eaage acrss expse isati at bth es  a cabe whe a spare cctr i the cabe is aaiabe r cmpetig the Gar cecti  A STITCH IN TIME ASTITCH AST ITCHIN INTIME TIME  The highest accurac is esire i cases as show i Fig 25a, or where the true resistace o each eg is wate as i the case o Fig 25b, three measuremets are reuire a the oowig euatios are use: r12 = R12 R13 – (R')2 R13 + R' r23 = R12 R23 – (R')2 R12 + R' r13 = R12 R13 – (R')2 R12 + R' where R12, R23 a R13 are the actua reaigs i megohms measure across the termias o the etwor which are coecte to the lie a Earth termias o the Megger istrumet with Termias 3, a respective coecte to the istrumet Guar termia R' is the vaue o the baast resistace i megohms o the istrumet i use I maig these three measuremets, o o coect the lie termia o the istrumet to the groue termia o the etwor, as a eaage over the istrumet case betwee the Earth termia a grou wi shut the resistace beig measure Figre 31-Cectis r testig isati resistace betwee e wire a a ther wires cecte, witht beig aecte by eaage t gr BuSHInGS, PoTHEAdS And InSulAToR InSulAToRS S Figre 32-Shwig se  sprig car as a Gar cecti t eimiate the eects  srace eaage The eice er test mst be iscecte rm a ther eipmet Figre 30-Cectis r testig isati resistace betwee e wire a gr, witht beig aecte by eaage t ther wires nte se  Gar cecti  ASTITCH AST ITCHIN INTIME TIME A STITCH IN TIME  The highest accurac is esire i cases as show i Fig 25a, or where the true resistace o each eg is wate as i the case o Fig 25b, three measuremets are reuire a the oowig euatios are use: r12 = R12 R13 – (R')2 R13 + R' r23 = R12 R23 – (R')2 R12 + R' r13 = R12 R13 – (R')2 R12 + R' where R12, R23 a R13 are the actua reaigs i megohms measure across the termias o the etwor which are coecte to the lie a Earth termias o the Megger istrumet with Termias 3, a respective coecte to the istrumet Guar termia R' is the vaue o the baast resistace i megohms o the istrumet i use I maig these three measuremets, o o coect the lie termia o the istrumet to the groue termia o the etwor, as a eaage over the istrumet case betwee the Earth termia a grou wi shut the resistace beig measure Figre 31-Cectis r testig isati resistace betwee e wire a a ther wires cecte, witht beig aecte by eaage t gr BuSHInGS, PoTHEAdS And InSulAToR InSulAToRS S Figre 32-Shwig se  sprig car as a Gar cecti t eimiate the eects  srace eaage The eice er test mst be iscecte rm a ther eipmet Figre 30-Cectis r testig isati resistace betwee e wire a gr, witht beig aecte by eaage t ther wires nte se  Gar cecti  A STITCH IN TIME ASTITCH AST ITCHIN INTIME TIME  ouTdooR oIl CIRCuIT BREAkERS The our iustratios (Fig 35 through 38) show the usua methos o testig bushigs a associate parts o a outoor oi circuit breaer, a the accompaig tabe iicates the test proceure b steps I the test vaues are beow 10,000 megohms i a o the our steps, the ta shou be owere or raie so that the exc essive osses ca be isoate b urther tests a ivestigatios I the test vaues are beow 50,000 megohms i test #1, the tre o the coitio o the particuar bushig ivove shou be watche b maig more reuet tests Test Breaer Bushig Bushig Bushig Positio Eergize Guare Groue Figre 33-Cectis r testig isati resistace  a trasrmer high tage wiig a bshigs, a the high tesi iscect switch i parae with reerece t gr bt witht beig aecte by eaage betwee the high a w tage wiigs thrgh se  the Gar cecti Part Measure ope (2 to guar) …… ope 1 ope 1&2 1&2 …… Bushig i parae with cross member Bushig & i parae cose 1&2 1&2 …… Bushig & i parae with it ro Bushig Figre 34-Cectis r testig isati resistace betwee high a w tage wiigs witht beig aecte by eaage t gr  ASTITCH AST ITCHIN INTIME TIME A STITCH IN TIME  ouTdooR oIl CIRCuIT BREAkERS The our iustratios (Fig 35 through 38) show the usua methos o testig bushigs a associate parts o a outoor oi circuit breaer, a the accompaig tabe iicates the test proceure b steps I the test vaues are beow 10,000 megohms i a o the our steps, the ta shou be owere or raie so that the exc essive osses ca be isoate b urther tests a ivestigatios I the test vaues are beow 50,000 megohms i test #1, the tre o the coitio o the particuar bushig ivove shou be watche b maig more reuet tests Test Breaer Bushig Bushig Bushig Positio Eergize Guare Groue Figre 33-Cectis r testig isati resistace  a trasrmer high tage wiig a bshigs, a the high tesi iscect switch i parae with reerece t gr bt witht beig aecte by eaage betwee the high a w tage wiigs thrgh se  the Gar cecti Part Measure ope (2 to guar) …… ope 1 ope 1&2 1&2 …… Bushig i parae with cross member Bushig & i parae cose 1&2 1&2 …… Bushig & i parae with it ro Bushig Figre 34-Cectis r testig isati resistace betwee high a w tage wiigs witht beig aecte by eaage t gr  ASTITCH AST ITCHIN INTIME TIME Figre 35-Step Figre 37-Step Figre 36-Step Figre 38-Step  ASTITCH AST ITCHIN INTIME TIME A STITCH IN TIME  A STITCH IN TIME  Figre 35-Step Figre 37-Step Figre 36-Step Figre 38-Step  A STITCH IN TIME ASTITCH AST ITCHIN INTIME TIME  SETTInG uP A MAInTEnAnCE PRoGRAM HoW oFTEn SHould You TEST? To start isuatio testig testig there is oe geera rue: Put frst thigs frst That is, review a the eectrica euipmet a cassi it as to reative importace For exampe, i that AC motor aie i epartmet A, how wou it aect overa pat prouctio? your prouctio peope ca certai hep i this a shou be vita itereste i the iea That epes upo the size a compexit o  our pat Eve ietica uits ca ier i the reuire chec perios; experiece is our best guie I geera, however, worig apparatus – motors, geerators etc – are more ie to eveop isuatio weaesses, as compare to wirig, isuators a the ie A test scheue or worig euipmet shou be estabishe, varig rom ever to 12 moths, epeig o the size o euipmet a severit o the surrouig atmospheric coitios For wirig a the ie, tests oce a ear are geera sufciet, uess the pat coitios are uusua severe I time permits at the start, test ever piece o eectrica euipmet a mae out a recor car Possib at frst ou wi have to combie severa uits but it wi pa i the og ru to have test recors or each iiviua uit The, i isuatio weaesses o show up, our job o tracig ow the oeig part wi be easier easier Show i Fig 40 is oe orm o test recor car, which is avaiabe rom Megger Foowig is the tpe o iormatio, which becomes more vauabe Megger to ou as tests are repeate at itervas: name a ocatio o the euipmet dates a vaues o test resuts (recor actua reaig at the time o test) Rage, votage, a seria umber o the Megger istrumet use Temperature Temperature o the apparatus (aso, particuar or arge uits, wet a r bub temperatures – or humiit a ew-poit etermiatios) Isuatio resistace measuremet correcte or temperature A pot o cosecutive reaigs to show tre a eabe ou to aticipate aiures Figre 40 0 ASTITCH AST ITCHIN INTIME TIME A STITCH IN TIME  SETTInG uP A MAInTEnAnCE PRoGRAM HoW oFTEn SHould You TEST? To start isuatio testig testig there is oe geera rue: Put frst thigs frst That is, review a the eectrica euipmet a cassi it as to reative importace For exampe, i that AC motor aie i epartmet A, how wou it aect overa pat prouctio? your prouctio peope ca certai hep i this a shou be vita itereste i the iea That epes upo the size a compexit o  our pat Eve ietica uits ca ier i the reuire chec perios; experiece is our best guie I geera, however, worig apparatus – motors, geerators etc – are more ie to eveop isuatio weaesses, as compare to wirig, isuators a the ie A test scheue or worig euipmet shou be estabishe, varig rom ever to 12 moths, epeig o the size o euipmet a severit o the surrouig atmospheric coitios For wirig a the ie, tests oce a ear are geera sufciet, uess the pat coitios are uusua severe I time permits at the start, test ever piece o eectrica euipmet a mae out a recor car Possib at frst ou wi have to combie severa uits but it wi pa i the og ru to have test recors or each iiviua uit The, i isuatio weaesses o show up, our job o tracig ow the oeig part wi be easier easier Show i Fig 40 is oe orm o test recor car, which is avaiabe rom Megger Foowig is the tpe o iormatio, which becomes more vauabe Megger to ou as tests are repeate at itervas: name a ocatio o the euipmet dates a vaues o test resuts (recor actua reaig at the time o test) Rage, votage, a seria umber o the Megger istrumet use Temperature Temperature o the apparatus (aso, particuar or arge uits, wet a r bub temperatures – or humiit a ew-poit etermiatios) Isuatio resistace measuremet correcte or temperature A pot o cosecutive reaigs to show tre a eabe ou to aticipate aiures Figre 40 0 A STITCH IN TIME ASTITCH AST ITCHIN INTIME TIME MEGGER v & 10 v InSulATIon TESTERS Where critica, high-capita euipmet is ivove, the itrouctio o ew a improve isuatig materias is re-writig the boo o isuatio testig Euipmet with operatig votages above V reuires commesurate higher test votages Moer materias, whe ew or ear i their ie cces, ca have isuatio vaues ito rages that were previous umeasure your o isuatio tester ma ot be u aeuate to meet the emas o a rigorous a thorough program o prevetive/  preictive maiteace o moer euipmet To be u i coormace with the most moer testig reuiremets, Megger oers a ami o the highest uait isuatio testers at votages above V At the core o high-votage testig V the MIT510 a MIT520 aor the highest eve o uait testig aog with prime saet, coveiece a portabiit MIT510 oers seectabe test votages at 250, 500, 1000, 2500, a 5000 V, thereb maig the tester suitabe or appicatios otherwise perorme with a staar hahe moe, i aitio to the more emaig high-votage appicatios The MIT520 icreases the uctioait b permittig test votage to be set i a 10 V icremet rom 50 V to V, the i 25 V steps a the wa to V The testers measure to 15 Tera-o Tera-ohms, hms, thereb maig them u suitabe or critica istaatio tests a estabishmet o reiabe base ata or ew high-capita euipmet Test resuts are ispae o Megger’s patete eectroic igita/aaog arc, thereb aorig the precisio a assurace o a igita reaig combie with the te-tae poiter trave orma reserve or mechaica movemets  The ew V isuatio resistace testers rom Megger are esige specifca to assist ou with the testig a maiteace o high votage eectrica euipmet Mais or batter powere n digita/Aaog bacit ispa n Seectabe test votage rom 250 to 5000V n Automatic IR test n Measures to 15 T W n Compete with caibratio certifcate n Megger MIT510 The ew 10 V isuatio resistace testers rom Megger are aso esige specifca to assist ou with the testig a maiteace o high votage eectrica euipmet A cases are rugge a eas to carr, beig mae o tough popropee a achievig a igress protectio ratig o IP65 Mais or batter powere n digita/aaog bacit ispa Megger MIT1020 n Variabe test votage rom 50 V to 10 V Automatic IR, PI, dAR, SV a dd tests n Measures to 15 T W (5 V) a 35 T W (10 V) n n  ASTITCH AST ITCHIN INTIME TIME A STITCH IN TIME  MEGGER v & 10 v InSulATIon TESTERS Where critica, high-capita euipmet is ivove, the itrouctio o ew a improve isuatig materias is re-writig the boo o isuatio testig Euipmet with operatig votages above V reuires commesurate higher test votages Moer materias, whe ew or ear i their ie cces, ca have isuatio vaues ito rages that were previous umeasure your o isuatio tester ma ot be u aeuate to meet the emas o a rigorous a thorough program o prevetive/  preictive maiteace o moer euipmet To be u i coormace with the most moer testig reuiremets, Megger oers a ami o the highest uait isuatio testers at votages above V At the core o high-votage testig V the MIT510 a MIT520 aor the highest eve o uait testig aog with prime saet, coveiece a portabiit MIT510 oers seectabe test votages at 250, 500, 1000, 2500, a 5000 V, thereb maig the tester suitabe or appicatios otherwise perorme with a staar hahe moe, i aitio to the more emaig high-votage appicatios The MIT520 icreases the uctioait b permittig test votage to be set i a 10 V icremet rom 50 V to V, the i 25 V steps a the wa to V The testers measure to 15 Tera-o Tera-ohms, hms, thereb maig them u suitabe or critica istaatio tests a estabishmet o reiabe base ata or ew high-capita euipmet Test resuts are ispae o Megger’s patete eectroic igita/aaog arc, thereb aorig the precisio a assurace o a igita reaig combie with the te-tae poiter trave orma reserve or mechaica movemets The ew V isuatio resistace testers rom Megger are esige specifca to assist ou with the testig a maiteace o high votage eectrica euipmet Mais or batter powere digita/Aaog bacit ispa n Seectabe test votage rom 250 to 5000V n Automatic IR test n n n n Measures to 15 T W Compete with caibratio certifcate Megger MIT510 The ew 10 V isuatio resistace testers rom Megger are aso esige specifca to assist ou with the testig a maiteace o high votage eectrica euipmet A cases are rugge a eas to carr, beig mae o tough popropee a achievig a igress protectio ratig o IP65 Mais or batter powere n digita/aaog bacit ispa Megger MIT1020 n Variabe test votage rom 50 V to 10 V Automatic IR, PI, dAR, SV a dd tests n Measures to 15 T W (5 V) a 35 T W (10 V) n n  A STITCH IN TIME ASTITCH AST ITCHIN INTIME TIME MEGGER MIT400 SERIES A ew ie o high perormace testers rate to CAT IV 600 V provies greater saet whe testig at higher votage eves The MIT400 Series oer test votages rom 10 to 1000V, a icues a specia user-seectabe 10 V to 100 V i V icremets tester or ow votage isuatio testig reuiremets The uits icue a exceptioa isuatio resistace measuremet rage that extes rom 20 Gigohms to 200 Gigohms, with the optio to ispa the isuatio test votage or eaage curret o the secoar ispa Megger MIT410 Time isuatio testig or PI a dAR testig is avaiabe o certai moes A 200 mA cotiuit measuremet is avaiabe with a 0.01-ohm resoutio or ast accurate cabe resistace measuremet This ca be set to 20 mA where coormit to eectrica testig reguatios is ot reuire, exteig batter ie For those who preer the traitioa mechaica aaog tpe o istrumet, the MIT310A oers everthig i the MIT310, but with a movig coi ispa The aaog ispa is ehace b bac ecas o a white bacgrou, to give high cotrast a improve visibiit The ami is compete b the u-eature MIT320 a MIT330 The ormer as a iohm rage or measuremets betwee the extremes o cotiuit a high votage With this eature, measuremets ca be mae cotiuous rom 0.01 W to 999 M W A bacit ispa is urther ehace b bacit seectios, to mae both the setup a resuts visibe i poor ight The MIT320 aso oers auibe aarms o both the c otiuit a megohm rages, ajustabe across the etire rage Fia, the MIT330 oers everthig escribe i the MIT320, pus storage o up to 1000 resuts, owoaig through a USB port via icue sotware, a a ispa iicatio o remaiig memor MEGGER MIT200 SERIES nee o basic V isuatio a cotiuit testig, with ecoom a paramout cosieratio? no ee to sacrifce! Megger oers the o top uait ie o iexpesive testers o the maret the MIT200 Series Though esige with ecoom i mi, these testers oer o oss o the uait a reiabiit that the ame Megger impies Furthermore, there has bee o sacrifce o saet, which is a serious probem with iexpesive testers that have cut cost at the ris o arc ash a operator error MEGGER MIT300 SERIES The MIT300 Series is esige aog Megger’s amiiar pricipe o asceig eatures with o oss o basic uait Five moes comprise the ami, each buit o a basic esig esurig the uait a reiabiit that are Megger’s traemars MIT300 is the simpest moe, oerig core uctios o highest uait but o aitioa eatures that ma prove reuat i basic appicatios For appicatios where a V test must be avoie, the MIT300 oers a 250 a 500 V test o Combie with cotiuit a votage warig, pus a the coveiece a saet eatures that istiguish the ami, this is the moe o choice or the simpest appicatios I basic uctio is sti the goa but a V test reuire, the MIT310 is the perect ft With the possibiit o worig at higher votages, sae operatio has bee urther ehace b the icusio o a eaut votmeter  Megger MIT230 Four moes are avaiabe, ierig o i the test votages oere: MIT200 is a 500 V moe or staar appicatios; MIT210 is a V moe where higher test votage is i orer; MIT220 oers two votages, 250 a 500, or more sesitive appicatios where a V test is to be avoie; or u-spectrum appicatios, MIT230 oers a three votages, 250, 500 a V Megger MIT300 Series  ASTITCH AST ITCHIN INTIME TIME A STITCH IN TIME  For those who preer the traitioa mechaica aaog tpe o istrumet, the MIT310A oers everthig i the MIT310, but with a movig coi ispa The aaog ispa is ehace b bac ecas o a white bacgrou, to give high cotrast a improve visibiit MEGGER MIT400 SERIES A ew ie o high perormace testers rate to CAT IV 600 V provies greater saet whe testig at higher votage eves The ami is compete b the u-eature MIT320 a MIT330 The ormer as a iohm rage or measuremets betwee the extremes o cotiuit a high votage With this eature, measuremets ca be mae cotiuous rom 0.01 W to 999 M W A bacit ispa is urther ehace b bacit seectios, to mae both the setup a resuts visibe i poor ight The MIT320 aso oers auibe aarms o both the c otiuit a megohm rages, ajustabe across the etire rage The MIT400 Series oer test votages rom 10 to 1000V, a icues a specia user-seectabe 10 V to 100 V i V icremets tester or ow votage isuatio testig reuiremets The uits icue a exceptioa isuatio resistace measuremet rage that extes rom 20 Gigohms to 200 Gigohms, with the optio to ispa the isuatio test votage or eaage curret o the secoar ispa Fia, the MIT330 oers everthig escribe i the MIT320, pus storage o up to 1000 resuts, owoaig through a USB port via icue sotware, a a ispa iicatio o remaiig memor Megger MIT410 Time isuatio testig or PI a dAR testig is avaiabe o certai moes A 200 mA cotiuit measuremet is avaiabe with a 0.01-ohm resoutio or ast accurate cabe resistace measuremet This ca be set to 20 mA where coormit to eectrica testig reguatios is ot reuire, exteig batter ie MEGGER MIT200 SERIES nee o basic V isuatio a cotiuit testig, with ecoom a paramout cosieratio? no ee to sacrifce! Megger oers the o top uait ie o iexpesive testers o the maret the MIT200 Series Though esige with ecoom i mi, these testers oer o oss o the uait a reiabiit that the ame Megger impies Furthermore, there has bee o sacrifce o saet, which is a serious probem with iexpesive testers that have cut cost at the ris o arc ash a operator error MEGGER MIT300 SERIES The MIT300 Series is esige aog Megger’s amiiar pricipe o asceig eatures with o oss o basic uait Five moes comprise the ami, each buit o a basic esig esurig the uait a reiabiit that are Megger’s traemars MIT300 is the simpest moe, oerig core uctios o highest uait but o aitioa eatures that ma prove reuat i basic appicatios For appicatios where a V test must be avoie, the MIT300 oers a 250 a 500 V test o Combie with cotiuit a votage warig, pus a the coveiece a saet eatures that istiguish the ami, this is the moe o choice or the simpest appicatios Megger MIT230 I basic uctio is sti the goa but a V test reuire, the MIT310 is the perect ft With the possibiit o worig at higher votages, sae operatio has bee urther ehace b the icusio o a eaut votmeter Four moes are avaiabe, ierig o i the test votages oere: MIT200 is a 500 V moe or staar appicatios; MIT210 is a V moe where higher test votage is i orer; MIT220 oers two votages, 250 a 500, or more sesitive appicatios where a V test is to be avoie; or u-spectrum appicatios, MIT230 oers a three votages, 250, 500 a V Megger MIT300 Series  A STITCH IN TIME ASTITCH AST ITCHIN INTIME TIME Megger makes more than just insulation resistance testers Megger also makes high quality instruments for the following electrical testing applications: n Earth/Ground Testing n Battery Testing  Megger makes more than just insulation resistance testers Megger also makes high quality instruments for the following electrical testing applications: n n n n n n Earth/Ground Testing Relay Testing Oil Testing Circuit Breaker Testing Power Quality Analysis Low Resistance Testing n n n n n n Battery Testing Watthour Meter Testing Transforme ransformerr Testing Cable Fault Testing Power Factor Testing Hi Pot Testing Megger manufactures electrical test and maintenance instruments for electric power, process manufacturing, building wiring, engineering services and communicatio communications ns Visit our website for local assistance worldwide at www.megger.com Megger 4271BronzeWay Dallas,TX75237 The word “Megger” is a registered trademark 66 6 6 A AST STIT ITCH CHI IN NTI TIME ME M E   G -4   8   6   /   M I   L   /   4  M  /    R   e v  F   /    9   /   2   0   0   6