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“A Stitch i Time...”
The Cmpete Gie t Eectrica Isati Isati Testig Testig
W W W .M E G G E R . C O M A STITCH IN TIME
“A Stitch I Time” The Cmpete Gie t Eectrica Isati Testig Copright 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? ........................................................................3 WHAT MAkES InSUlATIOn GO BAd? ................................................................4 HOW InSUlATIOn RESISTAnCE IS MEASUREd .................................................. .................................................. 5 HOW TO InTERPRET RESISTAnCE REAdInGS .................................................... .................................................... 6 FACTORS AFFECTInG InSUlATIOn RESISTAnCE REAdInGS .............................. .............................. 8 TyPES OF InSUlATIOn RESISTAnCE TESTS.......................................................10 TEST VOlTAGE VS. EqUIPMEnT RATInG..........................................................16 AC TESTInG VS. dC ........................................................................................... ............................................................................................. 17 USE OF dC dIElECTRIC TEST SET ...................................................................... ...................................................................... 18 TESTS dURInG dRyInG OU T OF EqUIPMEnT ..................................................18 EFFECT OF TEMPERATURE On InSUlATIOn RESISTAnCE................................2 1 EFFECTS OF HU MIdITy ..................................................................................... ....................................................................................... 23 PREPARATIOn PREPARA TIOn OF APPARATUS TO TEST ........................................................... ........................................................... 2 4 SAFETy PRECAUTIOnS ...................................................................................... ...................................................................................... 2 6 COnnECTIOnS FOR TESTInG InSUlATIOn RESISTAnCE OF ElECTRICAl EqUIPMEnT .................................................................................. .................................................................................. 27
WHAT IS “Good” InSulATIon? Ever eectric wire i our pat – whether it’s i a motor motor,, geerator, cabe, switch, trasormer, trasormer, etc. – is careu covere with some orm o eectrica isuatio. The wire itse is usua copper or aumium, which is ow to be a goo couctor o the eectric curret that powers our euipmet. The isuatio must be just the opposite rom a couctor: it shou resist curret a eep the curret i its path aog the couctor. couctor. To uersta uersta isuatio testig ou rea o’t ee to go ito the mathematics o eectricit, but oe simpe euatio – Ohm’s aw – ca be ver hepu i appreciatig ma aspects. Eve i ou’ve bee expose to this aw beore, it ma be a goo iea to review it i the ight o isuatio testig. The purpose o isuatio arou a couctor is much ie that o a pipe carrig water, a Ohm’s aw o eectricit ca be more easi uerstoo b a compariso with water ow. I Fig. 1 we show this compariso. Pressure Pressure o water rom a pump causes ow aog the pipe (Fig. 1a). I the pipe were to sprig a ea, ou’ waste water a ose some water pressure. With eectricit, votage is ie the pump pressure, causig eectricit to ow aog the copper wire (Fig. 1b). As i a water pipe, there is some resistace to ow, but it is much ess aog the wire tha it is through the isuatio.
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
Figre 1–Cmparis water w (a) with eectric crret (b).
OUTdOOR OIl CIRCUIT BREAkERS...................................................................57 BREAkERS ...................................................................57 SETTInG UP A MAInTEnAnCE PROGRAM ....................................................... ....................................................... 60 HOW OFTEn SHOUld yOU TEST? ....................................................................6 0 MEGGER 5 And 10 kV InSUlATIOn TESTERS ..................................................6 2 MEGGER 1 kV InSUlATIOn TESTERS................................................................6 TESTERS................................................................6 4
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? ........................................................................3 WHAT MAkES InSUlATIOn GO BAd? ................................................................4 HOW InSUlATIOn RESISTAnCE IS MEASUREd .................................................. .................................................. 5 HOW TO InTERPRET RESISTAnCE REAdInGS .................................................... .................................................... 6 FACTORS AFFECTInG InSUlATIOn RESISTAnCE REAdInGS .............................. .............................. 8 TyPES OF InSUlATIOn RESISTAnCE TESTS.......................................................10 TEST VOlTAGE VS. EqUIPMEnT RATInG..........................................................16 AC TESTInG VS. dC ........................................................................................... ............................................................................................. 17 USE OF dC dIElECTRIC TEST SET ...................................................................... ...................................................................... 18 TESTS dURInG dRyInG OU T OF EqUIPMEnT ..................................................18 EFFECT OF TEMPERATURE On InSUlATIOn RESISTAnCE................................2 1 EFFECTS OF HU MIdITy ..................................................................................... ....................................................................................... 23 PREPARATIOn PREPARA TIOn OF APPARATUS TO TEST ........................................................... ........................................................... 2 4 SAFETy PRECAUTIOnS ...................................................................................... ...................................................................................... 2 6 COnnECTIOnS FOR TESTInG InSUlATIOn RESISTAnCE OF ElECTRICAl EqUIPMEnT .................................................................................. .................................................................................. 27
WHAT IS “Good” InSulATIon? Ever eectric wire i our pat – whether it’s i a motor motor,, geerator, cabe, switch, trasormer, trasormer, etc. – is careu covere with some orm o eectrica isuatio. The wire itse is usua copper or aumium, which is ow to be a goo couctor o the eectric curret that powers our euipmet. The isuatio must be just the opposite rom a couctor: it shou resist curret a eep the curret i its path aog the couctor. couctor. To uersta uersta isuatio testig ou rea o’t ee to go ito the mathematics o eectricit, but oe simpe euatio – Ohm’s aw – ca be ver hepu i appreciatig ma aspects. Eve i ou’ve bee expose to this aw beore, it ma be a goo iea to review it i the ight o isuatio testig. The purpose o isuatio arou a couctor is much ie that o a pipe carrig water, a Ohm’s aw o eectricit ca be more easi uerstoo b a compariso with water ow. I Fig. 1 we show this compariso. Pressure Pressure o water rom a pump causes ow aog the pipe (Fig. 1a). I the pipe were to sprig a ea, ou’ waste water a ose some water pressure. With eectricit, votage is ie the pump pressure, causig eectricit to ow aog the copper wire (Fig. 1b). As i a water pipe, there is some resistace to ow, but it is much ess aog the wire tha it is through the isuatio.
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
Figre 1–Cmparis water w (a) with eectric crret (b).
OUTdOOR OIl CIRCUIT BREAkERS...................................................................57 BREAkERS ...................................................................57 SETTInG UP A MAInTEnAnCE PROGRAM ....................................................... ....................................................... 60 HOW OFTEn SHOUld yOU TEST? ....................................................................6 0 MEGGER 5 And 10 kV InSUlATIOn TESTERS ..................................................6 2 MEGGER 1 kV InSUlATIOn TESTERS................................................................6 TESTERS................................................................6 4
A STITCH IN TIME
A S S TI TI TC TC H H IN IN T T IM IM E
Commo sese tes us that the more votage we have, the more curret there’ be. Aso, the ower the resistace o the wire, the more curret or the same votage. Actua, this is Ohm’s aw, which is expresse this wa i euatio orm:
where,
E=IxR E = votage i vots I = curret i amperes R = resistace i ohms
note, however, that o isuatio is perect (that is, has ifite resistace) so some eectricit oes ow aog the isuatio or through it to grou. Such a curret ma o be a miioth o a ampere (oe microampere) but it is the basis o isuatio testig euipmet. note aso that a higher votage tes to cause more curret through the isuatio. This sma amout o curret wou ot, o course, harm goo isuatio but wou be a probem i the isuatio has eteriorate. now, to sum up our aswer to the uestio “what is ‘goo’ isuatio?” We have see that, essetia essetia,, “goo” meas a reative high resistace to curret. Use to escribe a isuatio materia, “goo” wou aso mea “the abiit to eep a high resistace.” So, a suitabe wa o measurig resistace ca te ou how “goo” the isuatio is. Aso, i ou tae measuremets at reguar perios, ou ca chec tres towar its eterioratio (more o this ater).
WHAT WHA T MAkES InSulATIon Go BAd? Whe our pat eectrica sstem a euipmet are ew, the eectrica isuatio shou be i top otch shape. Furthermore, mauacturers o o wire, cabe, motors, a so o have cotiua improve their isuatios or services i iustr iustr.. nevertheess, eve toa, isuatio isuatio is subject to ma eects which ca cause it to ai – mechaica amage, vibratio, excessive heat or co, irt, oi, corrosive vapors, moisture rom processes, or or just the humiit o a mugg a. I various egrees, these eemies o isuatio are at wor as time goes o – combie with the eectrica stresses that exist. As pi hoes or cracs eveop, moisture a oreig matter peetrate the suraces o the isuatio, proviig a ow resistace path or eaage curret.
A S S TI TI TC TC H H IN IN T T IM IM E
Oce starte, the ieret eemies te to ai each other other,, permittig excessive curret through the isuatio. Sometimes the rop i isuatio resistace is sue, as whe euipmet is ooe. Usua, however, however, it rops graua graua,, givig pet o warig, i chece perioica. Such checs permit pae recoitioig beore service aiure. I there are o checs, a motor with poor isuatio, or exampe, ma ot o be agerous to touch whe votage is appie, but aso be subject to bur out. What was goo isuatio has become a partia couctor.
HoW InSulATIon RESISTAnCE IS MEASuREd you have see that goo isuatio has high resistace; poor isuatio, reative ow resistace. The actua resistace vaues ca be higher or ower, epeig upo such actors as the temperature or moisture cotet o the isuatio (resistace ecreases i temperature or moisture). With a itte recor-eepig a commo sese, however, however, ou ca get a goo picture o the isuatio coitio rom vaues that are o reative. The Megger isuatio tester is a sma, portabe istrumet that gives ou a irect reaig o isuatio resistace i ohms or megohms. For goo isuatio, the resistace usua reas i the megohm rage. The Megger isuatio tester is essetia a high-rage resistace meter (ohmmeter) with a buit-i irect-curret geerator. This meter is o specia costructio with both curret a votage cois, eabig true ohms to be rea irect, iepeet o the actua votage appie. This metho is oestructive; that is, it oes ot cause eterioratio o the isuatio.
Figre 2–Typica Megger test istrmet h-p t measre isati resistace.
A STITCH IN TIME
Commo sese tes us that the more votage we have, the more curret there’ be. Aso, the ower the resistace o the wire, the more curret or the same votage. Actua, this is Ohm’s aw, which is expresse this wa i euatio orm:
where,
E=IxR E = votage i vots I = curret i amperes R = resistace i ohms
note, however, that o isuatio is perect (that is, has ifite resistace) so some eectricit oes ow aog the isuatio or through it to grou. Such a curret ma o be a miioth o a ampere (oe microampere) but it is the basis o isuatio testig euipmet. note aso that a higher votage tes to cause more curret through the isuatio. This sma amout o curret wou ot, o course, harm goo isuatio but wou be a probem i the isuatio has eteriorate. now, to sum up our aswer to the uestio “what is ‘goo’ isuatio?” We have see that, essetia essetia,, “goo” meas a reative high resistace to curret. Use to escribe a isuatio materia, “goo” wou aso mea “the abiit to eep a high resistace.” So, a suitabe wa o measurig resistace ca te ou how “goo” the isuatio is. Aso, i ou tae measuremets at reguar perios, ou ca chec tres towar its eterioratio (more o this ater).
WHAT WHA T MAkES InSulATIon Go BAd? Whe our pat eectrica sstem a euipmet are ew, the eectrica isuatio shou be i top otch shape. Furthermore, mauacturers o o wire, cabe, motors, a so o have cotiua improve their isuatios or services i iustr iustr.. nevertheess, eve toa, isuatio isuatio is subject to ma eects which ca cause it to ai – mechaica amage, vibratio, excessive heat or co, irt, oi, corrosive vapors, moisture rom processes, or or just the humiit o a mugg a. I various egrees, these eemies o isuatio are at wor as time goes o – combie with the eectrica stresses that exist. As pi hoes or cracs eveop, moisture a oreig matter peetrate the suraces o the isuatio, proviig a ow resistace path or eaage curret.
Sometimes the rop i isuatio resistace is sue, as whe euipmet is ooe. Usua, however, however, it rops graua graua,, givig pet o warig, i chece perioica. Such checs permit pae recoitioig beore service aiure. I there are o checs, a motor with poor isuatio, or exampe, ma ot o be agerous to touch whe votage is appie, but aso be subject to bur out. What was goo isuatio has become a partia couctor.
HoW InSulATIon RESISTAnCE IS MEASuREd you have see that goo isuatio has high resistace; poor isuatio, reative ow resistace. The actua resistace vaues ca be higher or ower, epeig upo such actors as the temperature or moisture cotet o the isuatio (resistace ecreases i temperature or moisture). With a itte recor-eepig a commo sese, however, however, ou ca get a goo picture o the isuatio coitio rom vaues that are o reative. The Megger isuatio tester is a sma, portabe istrumet that gives ou a irect reaig o isuatio resistace i ohms or megohms. For goo isuatio, the resistace usua reas i the megohm rage. The Megger isuatio tester is essetia a high-rage resistace meter (ohmmeter) with a buit-i irect-curret geerator. This meter is o specia costructio with both curret a votage cois, eabig true ohms to be rea irect, iepeet o the actua votage appie. This metho is oestructive; that is, it oes ot cause eterioratio o the isuatio.
Figre 2–Typica Megger test istrmet h-p t measre isati resistace.
A STITCH IN TIME
A S S TI TI TC TC H H IN IN T T IM IM E
The geerator ca be ha-crae or ie-operate to eveop a high dC votage which causes a sma curret through a over suraces o the isuatio beig teste (Fig. 2). This curret (usua at a appie votage o 500 vots or more) is measure b the ohmmeter, ohmmeter, which has a iicatig scae. Fig. 3 shows a tpica scae, which reas icreasig resistace vaues rom et up to ifit, or a resistace too high to be measure.
Figre 3–Typica scae the Megger isati tester.
HoW To InTERPRET RESISTAnCE REAdInGS As previous metioe, isuatio resistace reaigs shou be cosiere reative. The ca be uite ieret or oe motor or machie teste three as i a row, et ot mea ba isuatio. What rea matters is the tre i reaigs over a time perio, showig esseig resistace a warig o comig probems. Perioic testig is, thereore, our best approach to prevetive maiteace o eectrica euipmet, usig recor cars as show i Fig. 4.
Figre 4–Typica 4–Typica recr isati resistace a mi mtr. mtr. Cre A shws test aes as measre; Cre B shws same aes crrecte t 20°C (see page 22), giig a efite wwar tre twar a sae citi. Reerse sie car (at right) is se t recr the test ata.
Oce starte, the ieret eemies te to ai each other other,, permittig excessive curret through the isuatio.
A S S TI TI TC TC H H IN IN T T IM IM E
Whether ou test moth, twice a ear, or oce a ear epes upo the tpe, ocatio, a importace o the euipmet. For exampe, a sma pump motor or a short cotro cabe ma be vita to a process i our pat. Experiece is the best teacher i settig up the scheue perios or our euipmet. you shou mae these perioic tests i the same wa each time. That is, with the same test coectios a with the same test votage appie or the same egth o time. Aso ou shou mae tests at about the same temperature, or correct them to the same temperature. A recor o the reative humiit ear the euipmet at the time o the test is aso hepu i evauatig the reaig a tre. later sectios cover temperature correctio a humiit eects. I summar, here are some geera observatios about how ou ca iterpret iterpret perioic isuatio resistace tests, a what ou shou o with the resut: Citi
What T d
(a) Fair t high aes a we maitaie.
n case r ccer.
(b) Fair t high aes, bt shwig a cstat teecy twars wer aes.
lcate a remey the case a chec the wwar tre.
(c) lw bt we maitaie.
Citi is prbaby a right, bt case w aes sh be chece.
() S w as t be sae.
Cea, ry t, r therwise raise the aes bere pacig eipmet i serice. (Test wet eipmet whie ryig t.)
(e) Fair r high aes, preisy we maitaie bt shwig se werig.
Mae tests at reet iteras ti the case w aes is cate a remeie; r ti the aes hae becme steay at a wer ee bt sae r perati; perati; r ti aes becme s w that it is sae t eep the eipmet i perati.
A STITCH IN TIME
The geerator ca be ha-crae or ie-operate to eveop a high dC votage which causes a sma curret through a over suraces o the isuatio beig teste (Fig. 2). This curret (usua at a appie votage o 500 vots or more) is measure b the ohmmeter, ohmmeter, which has a iicatig scae. Fig. 3 shows a tpica scae, which reas icreasig resistace vaues rom et up to ifit, or a resistace too high to be measure.
Figre 3–Typica scae the Megger isati tester.
HoW To InTERPRET RESISTAnCE REAdInGS As previous metioe, isuatio resistace reaigs shou be cosiere reative. The ca be uite ieret or oe motor or machie teste three as i a row, et ot mea ba isuatio. What rea matters is the tre i reaigs over a time perio, showig esseig resistace a warig o comig probems. Perioic testig is, thereore, our best approach to prevetive maiteace o eectrica euipmet, usig recor cars as show i Fig. 4.
Figre 4–Typica 4–Typica recr isati resistace a mi mtr. mtr. Cre A shws test aes as measre; Cre B shws same aes crrecte t 20°C (see page 22), giig a efite wwar tre twar a sae citi. Reerse sie car (at right) is se t recr the test ata.
A S S TI TI TC TC H H IN IN T T IM IM E
Whether ou test moth, twice a ear, or oce a ear epes upo the tpe, ocatio, a importace o the euipmet. For exampe, a sma pump motor or a short cotro cabe ma be vita to a process i our pat. Experiece is the best teacher i settig up the scheue perios or our euipmet. you shou mae these perioic tests i the same wa each time. That is, with the same test coectios a with the same test votage appie or the same egth o time. Aso ou shou mae tests at about the same temperature, or correct them to the same temperature. A recor o the reative humiit ear the euipmet at the time o the test is aso hepu i evauatig the reaig a tre. later sectios cover temperature correctio a humiit eects. I summar, here are some geera observatios about how ou ca iterpret iterpret perioic isuatio resistace tests, a what ou shou o with the resut: Citi
What T d
(a) Fair t high aes a we maitaie.
n case r ccer.
(b) Fair t high aes, bt shwig a cstat teecy twars wer aes.
lcate a remey the case a chec the wwar tre.
(c) lw bt we maitaie.
Citi is prbaby a right, bt case w aes sh be chece.
() S w as t be sae.
Cea, ry t, r therwise raise the aes bere pacig eipmet i serice. (Test wet eipmet whie ryig t.)
(e) Fair r high aes, preisy we maitaie bt shwig se werig.
Mae tests at reet iteras ti the case w aes is cate a remeie; r ti the aes hae becme steay at a wer ee bt sae r perati; perati; r ti aes becme s w that it is sae t eep the eipmet i perati.
A STITCH IN TIME
FACToRS AFFECTInG InSulATIon RESISTAnCE RESISTAnCE REAdInGS Remember that the measure resistace (o the isuatio) wi be etermie b the votage appie a the resutat curret (R = E/I). There are a umber o thigs that aect curret, icuig temperature o the isuatio a humiit humiit,, as metioe i the previous sectio. Right ow, et’s just cosier the ature o curret through isuatio a the eect o how og votage is appie. Curret through a aog isuatio is mae up part o a reative stea curret i eaage paths over the isuatio surace. Eectricit aso ows through the voume o the isuatio. Actua, as show i Fig. 5, our tota curret comprises three compoets: 1. Capacitace Chargig Crret Curret that starts out high a rops ater the isuatio has bee charge to u votage (much ie water ow i a gare hose whe ou frst tur o the spigot). 2. Absrpti Crret Aso a iitia high curret which the rops (or reasos iscusse uer the sectio Time-Resistace Metho). 3. Ccti r leaage Crret A sma essetia stea curret both through a over the isuatio.
As show i Fig. 5, the tota curret is the sum o the three compoets a it is this curret that ca be measure irect b a microammeter, or i terms o megohms at a particuar votage b meas o a Megger istrumet (ohmmeter). Because the tota curret epes upo the time that the votage is appie, ou c a see ow wh Ohm’s law R = E/I o hos, theoretica, at a ifite time (that is, ou’ have to wait orever beore taig a reaig). I practice, as ou wi see i the test methos escribe beow,ou rea a vaue that is the apparet resistace – a useu vaue to iagose troubes, which is what ou wat to o.
A S S TI TI TC TC H H IN IN T T IM IM E
Figre 5–Cres shwig cmpets crret measre rig dC testig isati.
note aso i Fig. 5 that the chargig curret isappears reative reative rapi as the euipmet uer test becomes charge. larger uits with more capacitace wi tae oger to be charge. This curret aso is the store eerg iitia ischarge ater our test, b short-circuitig a grouig the isuatio. AlWAYS TAkE THIS SAFETY MEASuRE. you ca see urther rom Fig. 5 that the absorptio curret ecreases at a reative sow rate, epeig upo the exact ature o the isuatio. This store eerg, eerg, too, must be reease at the e o a test, a reuires a oger time tha the capacitace chargig curret – about our times as og as the votage was appie. With goo isuatio, the couctio or eaage curret shou bui up to a stea vaue that is costat or the appie votage, as show i Fig. 5. A icrease o eaage curret with time is a warig o troube, as iscusse i the tests escribe i the oowig sectio.
A STITCH IN TIME
FACToRS AFFECTInG InSulATIon RESISTAnCE RESISTAnCE REAdInGS Remember that the measure resistace (o the isuatio) wi be etermie b the votage appie a the resutat curret (R = E/I). There are a umber o thigs that aect curret, icuig temperature o the isuatio a humiit humiit,, as metioe i the previous sectio. Right ow, et’s just cosier the ature o curret through isuatio a the eect o how og votage is appie. Curret through a aog isuatio is mae up part o a reative stea curret i eaage paths over the isuatio surace. Eectricit aso ows through the voume o the isuatio. Actua, as show i Fig. 5, our tota curret comprises three compoets: 1. Capacitace Chargig Crret Curret that starts out high a rops ater the isuatio has bee charge to u votage (much ie water ow i a gare hose whe ou frst tur o the spigot). 2. Absrpti Crret Aso a iitia high curret which the rops (or reasos iscusse uer the sectio Time-Resistace Metho). 3. Ccti r leaage Crret A sma essetia stea curret both through a over the isuatio.
As show i Fig. 5, the tota curret is the sum o the three compoets a it is this curret that ca be measure irect b a microammeter, or i terms o megohms at a particuar votage b meas o a Megger istrumet (ohmmeter). Because the tota curret epes upo the time that the votage is appie, ou c a see ow wh Ohm’s law R = E/I o hos, theoretica, at a ifite time (that is, ou’ have to wait orever beore taig a reaig). I practice, as ou wi see i the test methos escribe beow,ou rea a vaue that is the apparet resistace – a useu vaue to iagose troubes, which is what ou wat to o.
A S S TI TI TC TC H H IN IN T T IM IM E
With a bacgrou ow o how time aects the meaig o istrumet reaigs, et’s cosier three c ommo test methos: (1) (1) short-time or spot reaig; (2) time-resistace time-resistace;; a (3) step or muti-votage muti-votage tests.
TYPES oF InSulATIon RESISTAnCE RESISTAnCE TESTS Shrt-Time r Spt-Reaig Test
I this metho, ou simp coect the Megger istrumet across the isuatio to be teste a operate it or a short, short, specifc time perio (60 secos is usua recommee). As show schematica i Fig. 6, ou’ve simp pice a poit o a curve o icreasig resistace vaues; uite ote the vaue wou be ess or 30 secos, more or 60 secos. Bear i mi aso that temperature a humiit, as we as coitio o our isuatio aect our reaig.
Figre 5–Cres shwig cmpets crret measre rig dC testig isati.
note aso i Fig. 5 that the chargig curret isappears reative reative rapi as the euipmet uer test becomes charge. larger uits with more capacitace wi tae oger to be charge. This curret aso is the store eerg iitia ischarge ater our test, b short-circuitig a grouig the isuatio. AlWAYS TAkE THIS SAFETY MEASuRE. you ca see urther rom Fig. 5 that the absorptio curret ecreases at a reative sow rate, epeig upo the exact ature o the isuatio. This store eerg, eerg, too, must be reease at the e o a test, a reuires a oger time tha the capacitace chargig curret – about our times as og as the votage was appie. With goo isuatio, the couctio or eaage curret shou bui up to a stea vaue that is costat or the appie votage, as show i Fig. 5. A icrease o eaage curret with time is a warig o troube, as iscusse i the tests escribe i the oowig sectio.
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I the apparatus ou are testig has ver sma capacitace, such as a short ru o house wirig, the spot reaig test is a that is ecessar. However,, most euipmet is capacitive a so our ver frst spot reaig However o euipmet i our pat, with o prior tests, ca be o a rough guie as to how goo or ba the isuatio is. For ma ears, maiteace proessioas have use the oe-megohm rue to estabish the aowabe ower imit or isuatio resistace. The rue 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 exampe, a motor rate at 2,400 vots shou have a miimum isuatio resistace o 2.4 megohms. I practice, megohm reaigs orma are cosierab above this miimum vaue i ew euipmet or whe isuatio is i goo coitio. B taig reaigs perioica a recorig them, ou have a better basis o jugig the actua isuatio coitio. A persistet owwar tre is usua air warig o troube ahea, eve though the reaigs ma be higher tha the suggeste miimum sae vaues. Eua true, as og as our perioic reaigs are cosistet, the ma be o, eve though ower tha the recommee miimum vaues. The curves o Fig. 7 show tpica behavior o isuatio resistace uer varig pat operatig coitios. The curves were potte rom spot reaigs tae with a M egger istrumet over a perio o moths.
Figre 6–Typica cre isati resistace (i meghms) with time r the “shrt time” r “spt-reaig” test meth.
0
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With a bacgrou ow o how time aects the meaig o istrumet reaigs, et’s cosier three c ommo test methos: (1) (1) short-time or spot reaig; (2) time-resistace time-resistace;; a (3) step or muti-votage muti-votage tests.
TYPES oF InSulATIon RESISTAnCE RESISTAnCE TESTS Shrt-Time r Spt-Reaig Test
I this metho, ou simp coect the Megger istrumet across the isuatio to be teste a operate it or a short, short, specifc time perio (60 secos is usua recommee). As show schematica i Fig. 6, ou’ve simp pice a poit o a curve o icreasig resistace vaues; uite ote the vaue wou be ess or 30 secos, more or 60 secos. Bear i mi aso that temperature a humiit, as we as coitio o our isuatio aect our reaig.
I the apparatus ou are testig has ver sma capacitace, such as a short ru o house wirig, the spot reaig test is a that is ecessar. However,, most euipmet is capacitive a so our ver frst spot reaig However o euipmet i our pat, with o prior tests, ca be o a rough guie as to how goo or ba the isuatio is. For ma ears, maiteace proessioas have use the oe-megohm rue to estabish the aowabe ower imit or isuatio resistace. The rue 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 exampe, a motor rate at 2,400 vots shou have a miimum isuatio resistace o 2.4 megohms. I practice, megohm reaigs orma are cosierab above this miimum vaue i ew euipmet or whe isuatio is i goo coitio. B taig reaigs perioica a recorig them, ou have a better basis o jugig the actua isuatio coitio. A persistet owwar tre is usua air warig o troube ahea, eve though the reaigs ma be higher tha the suggeste miimum sae vaues. Eua true, as og as our perioic reaigs are cosistet, the ma be o, eve though ower tha the recommee miimum vaues. The curves o Fig. 7 show tpica behavior o isuatio resistace uer varig pat operatig coitios. The curves were potte rom spot reaigs tae with a M egger istrumet over a perio o moths.
Figre 6–Typica cre isati resistace (i meghms) with time r the “shrt time” r “spt-reaig” test meth.
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Time-Resistace Meth
This metho is air iepeet o temperature a ote ca give ou cocusive iormatio without recors o past tests. It is base o the absorptio eect o goo isuatio compare to that o moist or cotamiate isuatio. you you simp tae successive reaigs at specifc times a ote the iereces i reaigs (see curves, Fig. 8). Tests b this metho are sometimes reerre to as absorptio tests. note that goo isuatio shows a cotiua icrease i resistace (ess curret – see curve A) over a perio o time (i the orer o 5 to 10 miutes). This is cause b the absorptio curret we spoe o earier; goo isuatio shows this charge eect over a time perio much oger tha the time reuire to charge the capacitace o the isuatio. I the isuatio cotais much moisture or cotamiats, the absorptio absorptio eect is mase b a high eaage curret which stas at a air costat vaue, eepig the resistace reaig ow (remember: R = E/I).
Figre 8–Typica cres shwig ieectric absrpti eect i a “time-resistace” test, mae capacitie eipmet sch as a arge mtr wiig. Figre 7–Typica behair isati resistace er a peri mths er aryig peratig citis, (cres ptte rm spt reaigs with a Megger istrmet).
ASTITCH AST ITCHIN INTIME TIME
The time-resistace time-resistace test is o vaue aso because it is iepeet o euipmet size. The icrease i resistace or cea a r isuatio occurs i the same maer whether a motor is arge or sma. you ca, thereore, compare severa motors a estabish staars or ew oes, regaress o their horsepower ratigs. A STITCH IN TIME
Time-Resistace Meth
This metho is air iepeet o temperature a ote ca give ou cocusive iormatio without recors o past tests. It is base o the absorptio eect o goo isuatio compare to that o moist or cotamiate isuatio. you you simp tae successive reaigs at specifc times a ote the iereces i reaigs (see curves, Fig. 8). Tests b this metho are sometimes reerre to as absorptio tests. note that goo isuatio shows a cotiua icrease i resistace (ess curret – see curve A) over a perio o time (i the orer o 5 to 10 miutes). This is cause b the absorptio curret we spoe o earier; goo isuatio shows this charge eect over a time perio much oger tha the time reuire to charge the capacitace o the isuatio. I the isuatio cotais much moisture or cotamiats, the absorptio absorptio eect is mase b a high eaage curret which stas at a air costat vaue, eepig the resistace reaig ow (remember: R = E/I).
Figre 8–Typica cres shwig ieectric absrpti eect i a “time-resistace” test, mae capacitie eipmet sch as a arge mtr wiig. Figre 7–Typica behair isati resistace er a peri mths er aryig peratig citis, (cres ptte rm spt reaigs with a Megger istrmet).
The time-resistace time-resistace test is o vaue aso because it is iepeet o euipmet size. The icrease i resistace or cea a r isuatio occurs i the same maer whether a motor is arge or sma. you ca, thereore, compare severa motors a estabish staars or ew oes, regaress o their horsepower ratigs. A STITCH IN TIME
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Fig. 9 shows how a 60-seco test wou appear or goo a perhaps ba isuatio. Whe the isuatio is i goo shape, the 60-seco reaig is higher tha the 30-seco reaig.
dieectric Absrpti Rati
The ratio o two time-resistace reaigs (such as a 60-seco reaig ivie b a 30-seco reaig) is cae a ieectric absorptio ratio. It is useu i recorig iormatio about isuatio. I the ratio is a 10-miute 10-miute reaig ivie b a 1-miute reaig, the vaue is cae the poarizatio iex. With ha-crae Megger istrumets, it’s a ot easier or ou to ru the test or o 60 secos, taig our frst reaig at 30 secos. I ou have a ie-operate Megger istrumet, ou’ get best resuts b ruig the test 10 miutes, taig reaigs at 1- a at 10-miutes, 10-miutes, to get the poarizatio iex. Tabe I gives vaues o the ratios a correspoig correspoig reative coitios o the isuatio that the iicate.
TABLE I—Condition of Insulation Indicated by Dielectric Absorption Ratios* InSUlATIOn COndITIOn
60/30-SECOnd RATIO
10/1-MInUTE RATIO (POlARIZATIOn IndEX) (POlARIZATIOn
Figre 9–Typica car pt a time-resistace r be-reaig test.
A urther avatage o this oube-reaig test, as it is sometimes cae, is that it gives ou a cearer picture, eve whe a spot reaig sas the isuatio oos fe. For exampe, et’s sa the spot reaig o a schroous motor was 10 megohms. now, et’s assume that the oube-reaig chec shows that the isuatio resistace hos stea at 10 megohms whie ou ho votage up to 60 secos. This meas there ma be irt or moisture i the wiigs that bears watchig. O the other ha, i the poiter shows a graua icrease betwee the 30-seco a the 60-seco checs, the ou’re reasoab sure the wiigs are i goo shape. Time-resistace tests o arge rotatig eectrica machier – especia with Time-resistace high operatig votage – reuire high isuatio resistace rages a a ver costat test votage. A heav-ut Megger test set, ie-operate, serves this ee. Simiar, such a istrumet is better aapte or arge cabes, bushigs, trasormers a switchgear switchgear..
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dagerous
—
less tha 1
questioabe
1.0 to 1.25
1.0 to 2***
Goo
1.4 to 1.6
2 to 4
Exceet
Above 1.6**
Above 4**
*These vaues must be cosiere tetative a reative— subject to experiece with the time-resistace metho over a perio o time. **I some cases, with motors, vaues approximate 20% higher tha show here iicate a r britte wiig which wi ai uer shoc coitios or urig starts. For prevetive maiteace, the motor wiig shou be ceae,treate, a rie to restore wiig exibiit. ***These resuts wou be satisactor or euipmet with ver ow capacitace such as short rus o house wirig.
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Fig. 9 shows how a 60-seco test wou appear or goo a perhaps ba isuatio. Whe the isuatio is i goo shape, the 60-seco reaig is higher tha the 30-seco reaig.
dieectric Absrpti Rati
The ratio o two time-resistace reaigs (such as a 60-seco reaig ivie b a 30-seco reaig) is cae a ieectric absorptio ratio. It is useu i recorig iormatio about isuatio. I the ratio is a 10-miute 10-miute reaig ivie b a 1-miute reaig, the vaue is cae the poarizatio iex. With ha-crae Megger istrumets, it’s a ot easier or ou to ru the test or o 60 secos, taig our frst reaig at 30 secos. I ou have a ie-operate Megger istrumet, ou’ get best resuts b ruig the test 10 miutes, taig reaigs at 1- a at 10-miutes, 10-miutes, to get the poarizatio iex. Tabe I gives vaues o the ratios a correspoig correspoig reative coitios o the isuatio that the iicate.
TABLE I—Condition of Insulation Indicated by Dielectric Absorption Ratios* InSUlATIOn COndITIOn
60/30-SECOnd RATIO
10/1-MInUTE RATIO (POlARIZATIOn IndEX) (POlARIZATIOn
Figre 9–Typica car pt a time-resistace r be-reaig test.
A urther avatage o this oube-reaig test, as it is sometimes cae, is that it gives ou a cearer picture, eve whe a spot reaig sas the isuatio oos fe. For exampe, et’s sa the spot reaig o a schroous motor was 10 megohms. now, et’s assume that the oube-reaig chec shows that the isuatio resistace hos stea at 10 megohms whie ou ho votage up to 60 secos. This meas there ma be irt or moisture i the wiigs that bears watchig. O the other ha, i the poiter shows a graua icrease betwee the 30-seco a the 60-seco checs, the ou’re reasoab sure the wiigs are i goo shape. Time-resistace tests o arge rotatig eectrica machier – especia with Time-resistace high operatig votage – reuire high isuatio resistace rages a a ver costat test votage. A heav-ut Megger test set, ie-operate, serves this ee. Simiar, such a istrumet is better aapte or arge cabes, bushigs, trasormers a switchgear switchgear..
dagerous
—
less tha 1
questioabe
1.0 to 1.25
1.0 to 2***
Goo
1.4 to 1.6
2 to 4
Exceet
Above 1.6**
Above 4**
*These vaues must be cosiere tetative a reative— subject to experiece with the time-resistace metho over a perio o time. **I some cases, with motors, vaues approximate 20% higher tha show here iicate a r britte wiig which wi ai uer shoc coitios or urig starts. For prevetive maiteace, the motor wiig shou be ceae,treate, a rie to restore wiig exibiit. ***These resuts wou be satisactor or euipmet with ver ow capacitace such as short rus o house wirig.
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TEST volTAGE vS. EquIPMEnT RATInG
AC TESTInG vS. dC
Commo use dC test votages or routie maiteace are as oows:
Up to ow, we’ve tae about testig with dC votage, but ou wi hear o AC testig a ee to ow the ierece. Remember that we spoe o the is o curret prouce i isuatio b dC? (The iitia surge o chargig curret, the rop with time to absorptio curret, a the, ater ater more time, the stea couctio curret.) We saw that i isuatio testig, the couctio or eaage curret is the oe that gives us the iormatio we ee.
Eipmet AC Ratig
dC Test vtage
up to 100 vots
100 a 250 vots
440 to 550 vots
500 a 1,000 vots
2,400 vots
1,000 to 2,500 vots or higher
4,160 vots a above
1,000 to 5,000 vots, or higher
Test votages use or proo testig o euipmet are cosierab higher tha those use or routie maiteace. Athough there are o pubishe iustr staars or dC maximum proo test votages to be use with rotatig euipmet, the scheue give beow is customari use. For specifc recommeatios o our euipmet, ou shou cosut the mauacturer o the euipmet. Pr Test vtages r Rtatig Eipmet:
Factor AC Test = 2 x n amepate Ratig + 1000 vots dC Proo Test o Istaatio = 0.8 x Factor AC Test x 1.6 dC Proo Test Ater Service = 0.6 x Factor AC Test x 1.6 Exampe:
Motor with 2,400 VAC amepate ratig– Factor AC Test = 2(2,400) +1,000 = 5,800 VAC Max. dC Test o Istaatio = 0.8(5,800)1.6 = 7,424 VdC Max. dC Test Ater Service = 0.6(5,800)1.6 = 5,568 VdC
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I cotrast, testig with AC gives a chargig curret that is extreme arge compare to the other is; eaage curret is reative mior. AC reuet is use or high-potetia testig; votage is icrease to some specife poit to see i the isuatio ca sta that particuar votage. It is a GO/nO-GO tpe o test a ca cause eterioratio o the isuatio, i cotrast to the dC test which is basica o-estructive. I a AC test votage has bee use a ou wat to use dC tests as a aterative, ou wi ee to icrease the maximum dC test votage somewhat to obtai euivaet resuts. I some cases, AC testig ma be more suitabe or proo testig o euipmet (that is, seeig that the euipmet meets prescribe staars). you ru the votage up to the seecte vaue a the euipmet either passes or oes’t pass the test. With With the dC test, ou get a more uaitative picture; ou ca meter the the eaage curret as ou icrease the votage a obtai specifc vaues o isuatio resistace. As the size o our euipmet icreases, there are aso mare ecoomic avatages i dC over AC testig. As the test votage icreases, both the cost a weight o AC euipmet go up much aster tha with comparabe dC test euipmet. This is because the AC test set must supp the chargig curret which becomes a remais ver high i the arger machies. As expaie previous, i dC testig, this curret rops rapi ater the iitia chargig perio.
A STITCH IN TIME
TEST volTAGE vS. EquIPMEnT RATInG
AC TESTInG vS. dC
Commo use dC test votages or routie maiteace are as oows:
Up to ow, we’ve tae about testig with dC votage, but ou wi hear o AC testig a ee to ow the ierece. Remember that we spoe o the is o curret prouce i isuatio b dC? (The iitia surge o chargig curret, the rop with time to absorptio curret, a the, ater ater more time, the stea couctio curret.) We saw that i isuatio testig, the couctio or eaage curret is the oe that gives us the iormatio we ee.
Eipmet AC Ratig
dC Test vtage
up to 100 vots
100 a 250 vots
440 to 550 vots
500 a 1,000 vots
2,400 vots
1,000 to 2,500 vots or higher
4,160 vots a above
1,000 to 5,000 vots, or higher
Test votages use or proo testig o euipmet are cosierab higher tha those use or routie maiteace. Athough there are o pubishe iustr staars or dC maximum proo test votages to be use with rotatig euipmet, the scheue give beow is customari use. For specifc recommeatios o our euipmet, ou shou cosut the mauacturer o the euipmet. Pr Test vtages r Rtatig Eipmet:
Factor AC Test = 2 x n amepate Ratig + 1000 vots dC Proo Test o Istaatio = 0.8 x Factor AC Test x 1.6 dC Proo Test Ater Service = 0.6 x Factor AC Test x 1.6 Exampe:
Motor with 2,400 VAC amepate ratig– Factor AC Test = 2(2,400) +1,000 = 5,800 VAC Max. dC Test o Istaatio = 0.8(5,800)1.6 = 7,424 VdC Max. dC Test Ater Service = 0.6(5,800)1.6 = 5,568 VdC
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I summar, dC test sets are empoe amost excusive or high-votage maiteace a fe testig or the oowig reasos: 1. lower cost 2. lighter weight 3. Smaer size 4. no-estructive 5. Better iormatio, iormatio, both i uait a uatit uatit
uSE oF dC dIElECTRIC TEST SET The Megger istrumet, reaig irect i ohms a megohms o isuatio resistace, is our best bet or routie i-pat maiteace. However,, some pats, particuar with higher votage ratigs i euipmet, However use aother Megger prouct – the ieectric test set. So, ou shou be aware o this istrumet a its use i isuatio resistace measuremets. The ieectric test set ca be use to etermie isuatio isuatio resistace b the same test methos as outie or the Megger istrumet; that is, the shorttime, time-resistace a step-votage tests. It is esige or other uses, too, but or isuatio testig it provies: (1) a ajustabe output votage a (2) a moitorig o the resutat curret i micro-amperes. The Megger dC dieectric Test Sets are avaiabe with votage outputs ragig rom 5 V up to 160 V. The curves o Fig. 5 are potte as curret versus time, as are curves or isuatio measuremets o tpica euipmet give ear the e o this maua. Megger suppies graph paper which maes it eas to pot megohms o isuatio resistace rom our votage a curret reaigs.
TESTS duRInG dRYInG ouT oF EquIPMEnT Wet eectrica euipmet is a commo hazar ace b a maiteace egieers. I the euipmet is wet rom resh water, ou go right ahea rig it out. However, i ou’ve got sat water, ou must frst wash awa the sat with resh water. Otherwise, ou’ eave ver corrosive eposits o sat o meta a isuatig suraces as we as i crevices o the isuatio. With moisture, such eposits orm a ver goo couctor o eectricit. Aso, ou shou remove oi or grease rom the isuatio, usig a suitabe sovet.
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I cotrast, testig with AC gives a chargig curret that is extreme arge compare to the other is; eaage curret is reative mior. AC reuet is use or high-potetia testig; votage is icrease to some specife poit to see i the isuatio ca sta that particuar votage. It is a GO/nO-GO tpe o test a ca cause eterioratio o the isuatio, i cotrast to the dC test which is basica o-estructive. I a AC test votage has bee use a ou wat to use dC tests as a aterative, ou wi ee to icrease the maximum dC test votage somewhat to obtai euivaet resuts. I some cases, AC testig ma be more suitabe or proo testig o euipmet (that is, seeig that the euipmet meets prescribe staars). you ru the votage up to the seecte vaue a the euipmet either passes or oes’t pass the test. With With the dC test, ou get a more uaitative picture; ou ca meter the the eaage curret as ou icrease the votage a obtai specifc vaues o isuatio resistace. As the size o our euipmet icreases, there are aso mare ecoomic avatages i dC over AC testig. As the test votage icreases, both the cost a weight o AC euipmet go up much aster tha with comparabe dC test euipmet. This is because the AC test set must supp the chargig curret which becomes a remais ver high i the arger machies. As expaie previous, i dC testig, this curret rops rapi ater the iitia chargig perio.
A STITCH IN TIME
There are various was to r out eectrica euipmet, epeig upo its size a portabiit portabiit.. you ca use a bast o hot air, a ove, circuatio o curret through couctors, or a combiatio o techiues. loca pat coitios a aciities, together with iormatio rom the euipmet mauacturers, ca serve as a guie to the best metho or our particuar euipmet. I some cases, or with certai euipmet, rig out ma ot be ecessar. you ca chec this b isuatio resistace tests, i ou have recors o previous tests o the apparatus. Whe rig out is reuire, such recors are aso hepu to etermie whe the isuatio is ree o moisture. noTE: Wet eipmet is ssceptibe t tage breaw. Therere, y sh se a w-tage Megger tester (100 r 250 vdC), at east i the eary stages a ryig-t r. I a w-tage istrmet is t reaiy aaiabe, sw craig a 500-t tester may be sbstitte. May testers hae a aitia test rage measrig i ihms (W). This measremet is typicay mae at y a ew ts, a is the iea iitia measremet t be mae e eipmet. This rage measres bew the Meghm rage, a ca, therere, prie a acta measremet t se as a bechmar i mitrig the ryig prcess. I a ihm measremet is btaie, isati has bee thrghy satrate, bt may be recaimabe. Ateratey test a ry, watchig the reaigs rise ti they reach the Meghm rage, at which time higher tage tests ca be saey empye.
As a exampe o how importat past reaigs are, et’s oo at a 100-hp motor that’s that’s bee ooe. Ater a cea-up, a spot reaig with the Megger tester shows 1.5 megohms. Oha, ou’ probab probab sa this is o. What’s more, i past recors showe the isuatio resistace to ru betwee 1 a 2 megohms, ou’ be sure. O the other ha, i past recors showe the orma resistace vaues to ru 10 or 20 megohms, the ou wou ow that water was sti i the motor wiigs.
A STITCH IN TIME
I summar, dC test sets are empoe amost excusive or high-votage maiteace a fe testig or the oowig reasos: 1. lower cost 2. lighter weight 3. Smaer size 4. no-estructive 5. Better iormatio, iormatio, both i uait a uatit uatit
uSE oF dC dIElECTRIC TEST SET The Megger istrumet, reaig irect i ohms a megohms o isuatio resistace, is our best bet or routie i-pat maiteace. However,, some pats, particuar with higher votage ratigs i euipmet, However use aother Megger prouct – the ieectric test set. So, ou shou be aware o this istrumet a its use i isuatio resistace measuremets. The ieectric test set ca be use to etermie isuatio isuatio resistace b the same test methos as outie or the Megger istrumet; that is, the shorttime, time-resistace a step-votage tests. It is esige or other uses, too, but or isuatio testig it provies: (1) a ajustabe output votage a (2) a moitorig o the resutat curret i micro-amperes. The Megger dC dieectric Test Sets are avaiabe with votage outputs ragig rom 5 V up to 160 V. The curves o Fig. 5 are potte as curret versus time, as are curves or isuatio measuremets o tpica euipmet give ear the e o this maua. Megger suppies graph paper which maes it eas to pot megohms o isuatio resistace rom our votage a curret reaigs.
TESTS duRInG dRYInG ouT oF EquIPMEnT Wet eectrica euipmet is a commo hazar ace b a maiteace egieers. I the euipmet is wet rom resh water, ou go right ahea rig it out. However, i ou’ve got sat water, ou must frst wash awa the sat with resh water. Otherwise, ou’ eave ver corrosive eposits o sat o meta a isuatig suraces as we as i crevices o the isuatio. With moisture, such eposits orm a ver goo couctor o eectricit. Aso, ou shou remove oi or grease rom the isuatio, usig a suitabe sovet.
There are various was to r out eectrica euipmet, epeig upo its size a portabiit portabiit.. you ca use a bast o hot air, a ove, circuatio o curret through couctors, or a combiatio o techiues. loca pat coitios a aciities, together with iormatio rom the euipmet mauacturers, ca serve as a guie to the best metho or our particuar euipmet. I some cases, or with certai euipmet, rig out ma ot be ecessar. you ca chec this b isuatio resistace tests, i ou have recors o previous tests o the apparatus. Whe rig out is reuire, such recors are aso hepu to etermie whe the isuatio is ree o moisture. noTE: Wet eipmet is ssceptibe t tage breaw. Therere, y sh se a w-tage Megger tester (100 r 250 vdC), at east i the eary stages a ryig-t r. I a w-tage istrmet is t reaiy aaiabe, sw craig a 500-t tester may be sbstitte. May testers hae a aitia test rage measrig i ihms (W). This measremet is typicay mae at y a ew ts, a is the iea iitia measremet t be mae e eipmet. This rage measres bew the Meghm rage, a ca, therere, prie a acta measremet t se as a bechmar i mitrig the ryig prcess. I a ihm measremet is btaie, isati has bee thrghy satrate, bt may be recaimabe. Ateratey test a ry, watchig the reaigs rise ti they reach the Meghm rage, at which time higher tage tests ca be saey empye.
As a exampe o how importat past reaigs are, et’s oo at a 100-hp motor that’s that’s bee ooe. Ater a cea-up, a spot reaig with the Megger tester shows 1.5 megohms. Oha, ou’ probab probab sa this is o. What’s more, i past recors showe the isuatio resistace to ru betwee 1 a 2 megohms, ou’ be sure. O the other ha, i past recors showe the orma resistace vaues to ru 10 or 20 megohms, the ou wou ow that water was sti i the motor wiigs.
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EFFECT oF TEMPERATuRE on InSulATIon RESISTAnCE The resistace o isuatig materias ecreases mare with a icrease i temperature. As we’ve see, however however,, tests b the time-resistace a stepvotage methos are reative iepeet o temperature eects, givig reative vaues. I ou wat to mae reiabe comparisos betwee reaigs, ou shou correct the reaigs to a base temperature, such as 20°C, or tae a our reaigs at approximate the same temperature (usua ot ifcut to o). We wi cover some geera guies to temperature correctio. Oe thumb rue is: For every 10°C increase in temperature, halve the resistance; or, for every 10°C decrease, double the resistance. For exampe, a two-megohm resistace at 20°C reuces to 1 / 2 megohm at 40°C. Each tpe o isuatig materia wi have a ieret egree o resistace chage with temperature. Factors have bee eveope, however, to simpi the correctio o resistace vaues. Tabe II gives such actors or rotatig euipmet, trasormers a cabe. you mutip the reaigs ou get b the actor correspoig to the temperature (which ou ee to measure). Figre 10-Typica 10-Typica ryig cre where e-mite reaigs isati resistace are tae eery r hrs.
The tpica rig-out curve or a dC motor armature (Fig. 10) shows how isuatio resistace chages. durig the frst part o the ru, the resistace actua ecreases because o the higher temperature. The it rises at a costat temperature as rig procees. Fia, Fia, it rises to a high vaue, as room temperature (20°C) is reache. note that i ou couct isuatio resistace tests urig rig, a ou have reaigs o previous tests o the r euipmet, ou’ ow whe ou’ve reache the sae vaue or the uit. you ma preer to use a timeresistace test, tae perioica (sa, oce a shit), usig the ieectric absorptio ratio or poarizatio iex to oow r-out progress (o ee to correct or temperature). 0
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For exampe, assume ou have a motor with Cass A isuatio a ou get a reaig o 2.0 megohms at a temperature (i the wiigs) o 104°F (40°C). From Tabe II ou rea across at 104°F to the ext coum (or Cass A) a obtai the actor 4.80. your correcte vaue o resistace is the: 2.0 megohms x 4.80 = (reaig (correctio ((correcte correcte at 104°F) actor or Cass A isuatio at 104°F)
9.6 megohms reaig or 68°F or 20°C)
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EFFECT oF TEMPERATuRE on InSulATIon RESISTAnCE The resistace o isuatig materias ecreases mare with a icrease i temperature. As we’ve see, however however,, tests b the time-resistace a stepvotage methos are reative iepeet o temperature eects, givig reative vaues. I ou wat to mae reiabe comparisos betwee reaigs, ou shou correct the reaigs to a base temperature, such as 20°C, or tae a our reaigs at approximate the same temperature (usua ot ifcut to o). We wi cover some geera guies to temperature correctio. Oe thumb rue is: For every 10°C increase in temperature, halve the resistance; or, for every 10°C decrease, double the resistance. For exampe, a two-megohm resistace at 20°C reuces to 1 / 2 megohm at 40°C. Each tpe o isuatig materia wi have a ieret egree o resistace chage with temperature. Factors have bee eveope, however, to simpi the correctio o resistace vaues. Tabe II gives such actors or rotatig euipmet, trasormers a cabe. you mutip the reaigs ou get b the actor correspoig to the temperature (which ou ee to measure). Figre 10-Typica 10-Typica ryig cre where e-mite reaigs isati resistace are tae eery r hrs.
The tpica rig-out curve or a dC motor armature (Fig. 10) shows how isuatio resistace chages. durig the frst part o the ru, the resistace actua ecreases because o the higher temperature. The it rises at a costat temperature as rig procees. Fia, Fia, it rises to a high vaue, as room temperature (20°C) is reache. note that i ou couct isuatio resistace tests urig rig, a ou have reaigs o previous tests o the r euipmet, ou’ ow whe ou’ve reache the sae vaue or the uit. you ma preer to use a timeresistace test, tae perioica (sa, oce a shit), usig the ieectric absorptio ratio or poarizatio iex to oow r-out progress (o ee to correct or temperature). 0
For exampe, assume ou have a motor with Cass A isuatio a ou get a reaig o 2.0 megohms at a temperature (i the wiigs) o 104°F (40°C). From Tabe II ou rea across at 104°F to the ext coum (or Cass A) a obtai the actor 4.80. your correcte vaue o resistace is the: 2.0 megohms x 4.80 = (reaig (correctio ((correcte correcte at 104°F) actor or Cass A isuatio at 104°F)
9.6 megohms reaig or 68°F or 20°C)
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note that the resistace is amost fve times greater at 68°F (20°C), as compare to the reaig tae at 104°F. The reerece temperature or cabe is give as 60°F (15.6°C), but the importat poit is to be cosistet a correct to the same base. TABlE II-Temperatre Crrecti Factrs* TEMP.
ROTATInG EqUIP.
S R E M R l d E O A l F l S R I E U F - n d T l I A A R O O T C n
CABlES S E . S - T & C . . R S I R n T T S G S S G A I . E I l S l S M R l M A E A E A R R E R R R R R O O U E F U T U T F n S d T A T A T - R O R R E A E A E A O E Z C G P n H n H P O n
d E T d E C A I n H S G I R B E R n R E R M P P A A M A V C I P
A S S A C F l C
B S S A l C
0 5 10 15.6
32 41 50 60
0.21 0.31 0.45 0.71
0.40 0.50 0.63 0.81
0.25 0.36 0.50 0.74
0.25 0.40 0.61 1.00
0.12 0.23 0.46 1.00
0.47 0.60 0.76 1.00
0.42 0.56 0.73 1.00
0.22 0.37 0.58 1.00
0.14 0.26 0.49 1.00
0.10 0.20 0.43 1.00
0.28 0.43 0.64 1.00
20 25 30 35
68 77 86 95
1.00 1.48 2.20 3.24
1.00 1.25 1.58 2.00
1.00 1.40 1.98 2.80
1.47 1.83 2.27 3.67 3.52 7.32 5.45 14.60
1.24 1.58 2.00 2.55
1.28 1.68 2.24 2.93
1.53 1.75 2.48 3.29 4.03 6.20 6.53 11.65
1.94 4.08 8.62 18.20
1.43 2.17 3.20 4.77
2.50 3.95 8.45 29.20 3.26 3.15 5.60 13.10 54.00 4.15 3.98 7.85 20.00 116.00 5.29 5.00 11.20 6.72
3.85 5.08 6.72 8.83
°
°
40 45 50 55
104 4.80 113 7.10 122 10.45 131 15.50
60 65 70 75
140 149 158 167
22.80 6.30 15.85 34.00 7.90 22.40 50.00 10.00 31.75 74.00 12.60 44.70
10.70 25.00 38.50 7.15 17.10 41.40 81.00 10.70 27.85 78.00 170.00 16.00 45.00 345.00 24.00
8.58 11.62 73.00 15.40 118.00 20.30 193.00 26.60 313.00
775.00 36.00
nomograph o temperature correctio vaues or Megger reaigs (correcte to 25°C). For rotatig machier with Cass B isuatio.
*Corrected to 20° 20 ° C for rotating equipment and transformers; 15.6 15.6°° C for cable.
EFFECTS oF HuMIdITY We have spoe at various poits i this maua about the presece o moisture i isuatio a its ver mare eect upo resistace vaues. you might expect, thereore, that icreasig humiit (moisture cotet) i the surrouig (ambiet) air cou aect isuatio resistace. A it ca to varig egrees. I our euipmet operates reguar above the ew-poit temperature (the temperature at which the moisture vapor i air coeses as a iui), the test reaig orma wi ot be aecte much b the humiit humiit.. Eve i the euipmet to be teste is ie, the same is true – so og as its temperature is ept above the ew poit.
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note that the resistace is amost fve times greater at 68°F (20°C), as compare to the reaig tae at 104°F. The reerece temperature or cabe is give as 60°F (15.6°C), but the importat poit is to be cosistet a correct to the same base. TABlE II-Temperatre Crrecti Factrs* TEMP.
ROTATInG EqUIP.
S R E M R l d E O A l F l S R I E U F - n d T l I A A R O O T C n
CABlES S E . S - T & C . . R S n T T G I R G S S A I I . S E l S l S M R l M A E A E A R R E R R R R R O n E O U T U T U F F S d - R T A T A T O R R E A E A E A O E Z C G P n H n H P O n
d E T d E C A I n H S R I B G E R n R E R M P P A A M A V C I P
A S S A C F l C
B S S A l C
0 5 10 15.6
32 41 50 60
0.21 0.31 0.45 0.71
0.40 0.50 0.63 0.81
0.25 0.36 0.50 0.74
0.25 0.40 0.61 1.00
0.12 0.23 0.46 1.00
0.47 0.60 0.76 1.00
0.42 0.56 0.73 1.00
0.22 0.37 0.58 1.00
0.14 0.26 0.49 1.00
0.10 0.20 0.43 1.00
0.28 0.43 0.64 1.00
20 25 30 35
68 77 86 95
1.00 1.48 2.20 3.24
1.00 1.25 1.58 2.00
1.00 1.40 1.98 2.80
1.47 1.83 2.27 3.67 3.52 7.32 5.45 14.60
1.24 1.58 2.00 2.55
1.28 1.68 2.24 2.93
1.53 1.75 2.48 3.29 4.03 6.20 6.53 11.65
1.94 4.08 8.62 18.20
1.43 2.17 3.20 4.77
2.50 3.95 8.45 29.20 3.26 3.15 5.60 13.10 54.00 4.15 3.98 7.85 20.00 116.00 5.29 5.00 11.20 6.72
3.85 5.08 6.72 8.83
°
°
40 45 50 55
104 4.80 113 7.10 122 10.45 131 15.50
60 65 70 75
140 149 158 167
22.80 6.30 15.85 34.00 7.90 22.40 50.00 10.00 31.75 74.00 12.60 44.70
10.70 25.00 38.50 7.15 17.10 41.40 81.00 10.70 27.85 78.00 170.00 16.00 45.00 345.00 24.00
8.58 11.62 73.00 15.40 118.00 20.30 193.00 26.60 313.00
775.00 36.00
nomograph o temperature correctio vaues or Megger reaigs (correcte to 25°C). For rotatig machier with Cass B isuatio.
*Corrected to 20° 20 ° C for rotating equipment and transformers; 15.6 15.6°° C for cable.
EFFECTS oF HuMIdITY We have spoe at various poits i this maua about the presece o moisture i isuatio a its ver mare eect upo resistace vaues. you might expect, thereore, that icreasig humiit (moisture cotet) i the surrouig (ambiet) air cou aect isuatio resistace. A it ca to varig egrees. I our euipmet operates reguar above the ew-poit temperature (the temperature at which the moisture vapor i air coeses as a iui), the test reaig orma wi ot be aecte much b the humiit humiit.. Eve i the euipmet to be teste is ie, the same is true – so og as its temperature is ept above the ew poit.
The aforementioned statement assumes that the insulation s urfaces are free of contaminants, such as certain lints and acids or salts, which have the property of absorbing moisture (they’re called “hygroscopic” or “deliquescent” materials by chemists). Their presence could unpredictably affect your readings; they should be removed before tests are made.
I eectrica euipmet we’re cocere primari with the coitios o the expose suraces where moisture coeses a aects the overa resistace o the isuatio. Stuies show, however, however, that ew wi orm i the cracs a crevices o isuatio beore it is visib eviet o the surace. dew-poit measuremets wi give ou a cue as to whether such ivisibe coitios might exist, aterig the test resuts. As a part o our maiteace recors, thereore, it’s a goo iea to mae ote at east o whether the surrouig air was r or humi whe the test was mae. Aso, whether the temperature was above or beow the ambiet. Whe ou test vita euipmet, recor the ambiet wet a r bub temperatures, rom which ew-poit a percet reative or absoute humiit ca be obtaie.
PREPARATIon oF APPARATuS To TEST 1. Tae ot Serice
Shut ow the apparatus. Ope switches. de-eergize. discoect rom other euipmet a circuits, icuig eutra a protective (worme’s temporar) grou coectios. See Saety Precatis, page 26. 2. Mae Sre Jst What is Ice i the Test
Ispect the istaatio ver careu to etermie just what euipmet is coecte a wi be icue i the test, especia i it is ifcut or expesive to iscoect associate apparatus a circuits. Pa particuar attetio to couctors that ea awa rom the istaatio. This is ver importat, because the more euipmet that is icue i a test, the ower the reaig wi be, a the true isuatio resistace o the apparatus i uestio ma be mase b that o the associate euipmet. It is awas possibe, o course, that the isuatio resistace o the compete istaatio (without iscoectig everthig) wi be satisactori high, especia or a spot chec. Or, it ma be higher tha the rage o the
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Megger istrumet i use, i which case othig wou be gaie b separatig the compoets, because the isuatio resistace o each part wou be sti higher. For a iitia test, it ma ma be ecessar to separate the compoet parts, eve though abor abor a expese are ivove, a test each oe separate separate.. Aso mae a test o a the compoets coecte together. together. With this iormatio o recor, it ma ot be ecessar to separate the c ompoets o uture tests uess uaccoutab ow reaigs are observe. 3. discharge Capacitace
It is ver importat that capacitace be ischarge, both beore a ater a isuatio resistace test. It shou be ischarge or a perio about our times as og as test votage was appie i a previous test. Megger istrumets are reuet euippe with ischarge c ircuits or this purpose. I a ischarge uctio is ot provie, a ischarge stic shou be use. leave high capacitive apparatus (i.e., capacitors, arge wiigs, etc.) short circuite uti rea to re-eergize. 4. Crret leaage at Switches
Whe apparatus is shut ow or the isuatio resistace test, mae sure that the reaigs are ot aecte b eaage over or through switches switches or use bocs, etc. Such eaage ma mas the true isuatio resistace o the apparatus uer test. See Use o a Gar Termia, page 50. Or, what ma be more serious, curret rom a eergize ie ma ea ito the apparatus a cause icosistet reaigs, particuar i the ive ie is dC. However, such eaage usua ca be etecte b watchig the poiter o the Megger istrumet at the momet the test eas are c oecte to the apparatus a beore the istrumet is operate. Beore maig these observatios, be sure that a capacitace is ischarge b short circuitig or grouig the apparatus. CAUTION: Never connect a Megger insulation tester to energized lines or equipment. Never use the tester or any of its leads or accessories for any purpose not described in this book.
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The aforementioned statement assumes that the insulation s urfaces are free of contaminants, such as certain lints and acids or salts, which have the property of absorbing moisture (they’re called “hygroscopic” or “deliquescent” materials by chemists). Their presence could unpredictably affect your readings; they should be removed before tests are made.
I eectrica euipmet we’re cocere primari with the coitios o the expose suraces where moisture coeses a aects the overa resistace o the isuatio. Stuies show, however, however, that ew wi orm i the cracs a crevices o isuatio beore it is visib eviet o the surace. dew-poit measuremets wi give ou a cue as to whether such ivisibe coitios might exist, aterig the test resuts. As a part o our maiteace recors, thereore, it’s a goo iea to mae ote at east o whether the surrouig air was r or humi whe the test was mae. Aso, whether the temperature was above or beow the ambiet. Whe ou test vita euipmet, recor the ambiet wet a r bub temperatures, rom which ew-poit a percet reative or absoute humiit ca be obtaie.
PREPARATIon oF APPARATuS To TEST 1. Tae ot Serice
Shut ow the apparatus. Ope switches. de-eergize. discoect rom other euipmet a circuits, icuig eutra a protective (worme’s temporar) grou coectios. See Saety Precatis, page 26. 2. Mae Sre Jst What is Ice i the Test
Ispect the istaatio ver careu to etermie just what euipmet is coecte a wi be icue i the test, especia i it is ifcut or expesive to iscoect associate apparatus a circuits. Pa particuar attetio to couctors that ea awa rom the istaatio. This is ver importat, because the more euipmet that is icue i a test, the ower the reaig wi be, a the true isuatio resistace o the apparatus i uestio ma be mase b that o the associate euipmet. It is awas possibe, o course, that the isuatio resistace o the compete istaatio (without iscoectig everthig) wi be satisactori high, especia or a spot chec. Or, it ma be higher tha the rage o the
Megger istrumet i use, i which case othig wou be gaie b separatig the compoets, because the isuatio resistace o each part wou be sti higher. For a iitia test, it ma ma be ecessar to separate the compoet parts, eve though abor abor a expese are ivove, a test each oe separate separate.. Aso mae a test o a the compoets coecte together. together. With this iormatio o recor, it ma ot be ecessar to separate the c ompoets o uture tests uess uaccoutab ow reaigs are observe. 3. discharge Capacitace
It is ver importat that capacitace be ischarge, both beore a ater a isuatio resistace test. It shou be ischarge or a perio about our times as og as test votage was appie i a previous test. Megger istrumets are reuet euippe with ischarge c ircuits or this purpose. I a ischarge uctio is ot provie, a ischarge stic shou be use. leave high capacitive apparatus (i.e., capacitors, arge wiigs, etc.) short circuite uti rea to re-eergize. 4. Crret leaage at Switches
Whe apparatus is shut ow or the isuatio resistace test, mae sure that the reaigs are ot aecte b eaage over or through switches switches or use bocs, etc. Such eaage ma mas the true isuatio resistace o the apparatus uer test. See Use o a Gar Termia, page 50. Or, what ma be more serious, curret rom a eergize ie ma ea ito the apparatus a cause icosistet reaigs, particuar i the ive ie is dC. However, such eaage usua ca be etecte b watchig the poiter o the Megger istrumet at the momet the test eas are c oecte to the apparatus a beore the istrumet is operate. Beore maig these observatios, be sure that a capacitace is ischarge b short circuitig or grouig the apparatus. CAUTION: Never connect a Megger insulation tester to energized lines or equipment. Never use the tester or any of its leads or accessories for any purpose not described in this book.
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SAFETY PRECAuTIonS
Slight sparking may be encountered:
Observe a rues or saet whe taig euipmet out o service. Boc out iscoect switches. Test or oreig or iuce votages. App worme’s grous.
(1) Whe attachig the test eas to euipmet i which the capacitace has ot bee compete ischarge
Remember that whe worig arou high votage euipmet there is awas a possibiit o votages beig iuce i apparatus uer test or ies to which it is coecte, because o proximit to eergize high votage euipmet. Thereore, rather tha removig a worme’s grou i orer to mae a test, it is more avisabe to iscoect the apparatus, such as a trasormer or circuit breaer, rom the expose bus or ie, eavig the atter groue. Use rubber goves whe coectig the test eas to the apparatus a whie operatig the Megger istrumet.
Apparatus Under Test Must Not Be Live! See page 24 o Preparati Apparats r Test. I eutra or other grou coectios have to be iscoecte, mae sure the are ot carrig curret at the time, a that whe iscoecte o other euipmet wi ac ecessar protectio. Pa particuar attetio to couctors that ea awa rom the circuit beig teste a mae sure the have bee proper iscoecte rom a source o votage. Shc Hazar rm Test vtage
Observe the votage ratig o the Megger istrumet a regar it with appropriate cautio. large eectrica euipmet a cabes usua have sufciet capacitace to store up a agerous amout o eerg rom the test curret. Mae sure this capacitace is ischarge ater the test a beore haig the test eas. See aso discharge Capacitace, page 25. Expsi a Fire Hazar
So ar as is ow, there is o fre hazar i the orma use o a Megger isuatio tester. tester. There is, however, however, a hazar whe testig euipmet ocate i iammabe or exposive atmospheres.
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(2) durig a test, arcig through or over aut isuatio isuatio (3) Foowig a test whe capacitace is ischarge
CAUTION: Do not use the instrument in an explosive atmosphere. Sggestis:
For (1) a (3): Arrage permaet istae grouig aciities a test eas to a poit where istrumet coectios ca be mae i a sae atmosphere. For (2): Use ow-votage testig istrumets, or a series resistace. For (3): do ot iscoect the test eas or at east 30 to 60 secos oowig a test, aowig time or capacitace ischarge.
ConnECTIonS FoR T ESTInG InSulATIon RESISTAnCE oF ElECTRICAl EquIPMEnT The oowig iagrams show how to coect a Megger isuatio tester tester to various tpes o eectrica euipmet. The iagrams aso show i pricipe how euipmet must be iscoecte rom other circuits beore the istrumet is coecte. These iustratios are tpica a wi serve as guies or testig isuatio resistace o practica a tpes o apparatus a couctors. Beore proceeig with tests, rea the artice o Preparati Apparats r Test, page 24.
REMEMBER! The Megger isati resistace tester measres REMEMBER!The whateer resistace is cecte betwee its termias. This may ice series r parae eaage paths thrgh isati r er its srace.
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SAFETY PRECAuTIonS
Slight sparking may be encountered:
Observe a rues or saet whe taig euipmet out o service. Boc out iscoect switches. Test or oreig or iuce votages. App worme’s grous.
(1) Whe attachig the test eas to euipmet i which the capacitace has ot bee compete ischarge
Remember that whe worig arou high votage euipmet there is awas a possibiit o votages beig iuce i apparatus uer test or ies to which it is coecte, because o proximit to eergize high votage euipmet. Thereore, rather tha removig a worme’s grou i orer to mae a test, it is more avisabe to iscoect the apparatus, such as a trasormer or circuit breaer, rom the expose bus or ie, eavig the atter groue. Use rubber goves whe coectig the test eas to the apparatus a whie operatig the Megger istrumet.
Apparatus Under Test Must Not Be Live! See page 24 o Preparati Apparats r Test. I eutra or other grou coectios have to be iscoecte, mae sure the are ot carrig curret at the time, a that whe iscoecte o other euipmet wi ac ecessar protectio. Pa particuar attetio to couctors that ea awa rom the circuit beig teste a mae sure the have bee proper iscoecte rom a source o votage. Shc Hazar rm Test vtage
Observe the votage ratig o the Megger istrumet a regar it with appropriate cautio. large eectrica euipmet a cabes usua have sufciet capacitace to store up a agerous amout o eerg rom the test curret. Mae sure this capacitace is ischarge ater the test a beore haig the test eas. See aso discharge Capacitace, page 25. Expsi a Fire Hazar
So ar as is ow, there is o fre hazar i the orma use o a Megger isuatio tester. tester. There is, however, however, a hazar whe testig euipmet ocate i iammabe or exposive atmospheres.
(2) durig a test, arcig through or over aut isuatio isuatio (3) Foowig a test whe capacitace is ischarge
CAUTION: Do not use the instrument in an explosive atmosphere. Sggestis:
For (1) a (3): Arrage permaet istae grouig aciities a test eas to a poit where istrumet coectios ca be mae i a sae atmosphere. For (2): Use ow-votage testig istrumets, or a series resistace. For (3): do ot iscoect the test eas or at east 30 to 60 secos oowig a test, aowig time or capacitace ischarge.
ConnECTIonS FoR T ESTInG InSulATIon RESISTAnCE oF ElECTRICAl EquIPMEnT The oowig iagrams show how to coect a Megger isuatio tester tester to various tpes o eectrica euipmet. The iagrams aso show i pricipe how euipmet must be iscoecte rom other circuits beore the istrumet is coecte. These iustratios are tpica a wi serve as guies or testig isuatio resistace o practica a tpes o apparatus a couctors. Beore proceeig with tests, rea the artice o Preparati Apparats r Test, page 24.
REMEMBER! The Megger isati resistace tester measres REMEMBER!The whateer resistace is cecte betwee its termias. This may ice series r parae eaage paths thrgh isati r er its srace.
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1. AC Mtrs & Startig Eipmet
Figre 11
Coectios or testig the isuatio resistace o a motor motor,, startig euipmet a coectig ies, i parae. note that the starter switch is i the “o” positio or the test. It is awas preerabe to iscoect the compoet parts a test them separate i orer to etermie where weaesses exist.
3. Wirig Istaati
Figre 13
Coectios or testig to grou each circuit separate, worig rom the istributio pae.
2. dC Geeratrs & Mtrs
Figre 12
With the brushes raise as iicate, the brush riggig a fe cois ca be teste separate rom the armature. liewise the armature ca be teste b itse. With the brushes owere, the test wi be that o brush riggig, fe cois a armature combie.
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Figre 14
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1. AC Mtrs & Startig Eipmet
Figre 11
Coectios or testig the isuatio resistace o a motor motor,, startig euipmet a coectig ies, i parae. note that the starter switch is i the “o” positio or the test. It is awas preerabe to iscoect the compoet parts a test them separate i orer to etermie where weaesses exist.
3. Wirig Istaati
Figre 13
Coectios or testig to grou each circuit separate, worig rom the istributio pae.
2. dC Geeratrs & Mtrs
Figre 12
With the brushes raise as iicate, the brush riggig a fe cois ca be teste separate rom the armature. liewise the armature ca be teste b itse. With the brushes owere, the test wi be that o brush riggig, fe cois a armature combie.
Figre 14
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Coectios at the mai power boar, rom which poit the etire sstem ca be teste to grou at oe time, proviig a switches i the istributio pae are cose. 4. Appiaces, Meters, Istrmets & Misceaes Eectrica Apparats
I6. Pwer Cabes
Coectios or testig the isuatio resistace o a power cabe. Whe testig cabe, it is usua best to iscoect at both es i orer to test the cabe b itse, a to avoi error ue to eaage across or through switchboars or paeboars. See aso use Gar Termia, page 50.
Figre 15
Coectios or testig a appiace. The test is mae betwee the couctor (the heatig uit, motor, etc.) a expose meta parts. The apparatus must be iscoecte rom a source o power a pace o some isuatig materia.
Figre 17
5. Ctr, Sigaig & Cmmicati Cabes
Figre 16
Coectios or testig isuatio resistace o oe wire i a muticouctor cabe agaist a other wires a sheath coecte together. together.
0
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Figre 18
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Coectios at the mai power boar, rom which poit the etire sstem ca be teste to grou at oe time, proviig a switches i the istributio pae are cose. 4. Appiaces, Meters, Istrmets & Misceaes Eectrica Apparats
I6. Pwer Cabes
Coectios or testig the isuatio resistace o a power cabe. Whe testig cabe, it is usua best to iscoect at both es i orer to test the cabe b itse, a to avoi error ue to eaage across or through switchboars or paeboars. See aso use Gar Termia, page 50.
Figre 15
Coectios or testig a appiace. The test is mae betwee the couctor (the heatig uit, motor, etc.) a expose meta parts. The apparatus must be iscoecte rom a source o power a pace o some isuatig materia.
Figre 17
5. Ctr, Sigaig & Cmmicati Cabes
Figre 16
Coectios or testig isuatio resistace o oe wire i a muticouctor cabe agaist a other wires a sheath coecte together. together.
0
Figre 18
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7. Pwer Trasrmers
AddITIonAl noTES ABouT uSInG A ME GGER InSulATIon TESTER Testig leas
Ierior or eective testig eas wi cause erroeous a miseaig resuts o isuatio resistace tests. Tae care i this respect. uisate leas
To avoi error ue to the isuatio o eas, pace the Megger istrumet cose to the ugroue termia or couctor o the apparatus uer test a coect a short piece o ight bare wire irect rom the lie termia o the istrumet to the apparatus. I the Guar termia is use, it ma be treate simiar. simiar. no. 18 or 20 gauge, soi wire wi sufce. Support the ea o b its coectios to the istrumet a the apparatus. With this metho o coectig rom the lie termia, the uait o the isuatio, i a, o the Earth or Grou ea becomes uimportat. Figre 19
Isate leas
Coectios or testig isuatio resistace o a trasormer high votage wiig a bushigs, a the high tesio iscoect switch, i parae, with reerece reerece to the ow votage wiig a grou. note that the ow votage wiig is groue or this test. 8. AC Geeratrs
Where epeece is pace o the isuatio o eas, the must be urabe a o the best uait isuatig materia. Oi resistat, sthetic, rubber-isuate, rubber-isuat e, sige-couctor no. 14 strae wire is recommee. The outer jacet shou be smooth, with o outer brai. lugs shou be ftte or attachig to the istrumet termias, a stout sprig cips are recommee or coectig to the apparatus or circuit uer test. A coveiet egth o ea ma be use. Joits are to be avoie. Ater coectig the eas to the istrumet, a just beore coectig them to the apparatus, mae sure there is o ea rom ea to ea. do this b operatig the istrumet, which shou rea Ifit. do ot correct sight ea eaage b attemptig to reset the Ifit Ajuster o a high-rage istrumet. The touch the test es o the eas together to mae sure the are ot iscoecte or broe.
Figre 20
With this coectio, the isuatio resistace wi be that o the geerator stator wiig a coectig cabe combie. To test either the stator wiig or the cabe itse, the cabe must be iscoecte at the machie.
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Curret testig with the high-rage (50,000 megohms) megohms) Megger isuatio testers reuires that the lie test ea be maitaie at a high vaue so that it wi ot eter ito the measuremet. The shiee test ea,with the shie coecte to Guar, prevets prevets a eaages over its termiatios or through the ea isuatig materia, rom beig measure.
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7. Pwer Trasrmers
AddITIonAl noTES ABouT uSInG A ME GGER InSulATIon TESTER Testig leas
Ierior or eective testig eas wi cause erroeous a miseaig resuts o isuatio resistace tests. Tae care i this respect. uisate leas
To avoi error ue to the isuatio o eas, pace the Megger istrumet cose to the ugroue termia or couctor o the apparatus uer test a coect a short piece o ight bare wire irect rom the lie termia o the istrumet to the apparatus. I the Guar termia is use, it ma be treate simiar. simiar. no. 18 or 20 gauge, soi wire wi sufce. Support the ea o b its coectios to the istrumet a the apparatus. With this metho o coectig rom the lie termia, the uait o the isuatio, i a, o the Earth or Grou ea becomes uimportat. Figre 19
Isate leas
Coectios or testig isuatio resistace o a trasormer high votage wiig a bushigs, a the high tesio iscoect switch, i parae, with reerece reerece to the ow votage wiig a grou. note that the ow votage wiig is groue or this test. 8. AC Geeratrs
Where epeece is pace o the isuatio o eas, the must be urabe a o the best uait isuatig materia. Oi resistat, sthetic, rubber-isuate, rubber-isuat e, sige-couctor no. 14 strae wire is recommee. The outer jacet shou be smooth, with o outer brai. lugs shou be ftte or attachig to the istrumet termias, a stout sprig cips are recommee or coectig to the apparatus or circuit uer test. A coveiet egth o ea ma be use. Joits are to be avoie. Ater coectig the eas to the istrumet, a just beore coectig them to the apparatus, mae sure there is o ea rom ea to ea. do this b operatig the istrumet, which shou rea Ifit. do ot correct sight ea eaage b attemptig to reset the Ifit Ajuster o a high-rage istrumet. The touch the test es o the eas together to mae sure the are ot iscoecte or broe.
Figre 20
With this coectio, the isuatio resistace wi be that o the geerator stator wiig a coectig cabe combie. To test either the stator wiig or the cabe itse, the cabe must be iscoecte at the machie.
Curret testig with the high-rage (50,000 megohms) megohms) Megger isuatio testers reuires that the lie test ea be maitaie at a high vaue so that it wi ot eter ito the measuremet. The shiee test ea,with the shie coecte to Guar, prevets prevets a eaages over its termiatios or through the ea isuatig materia, rom beig measure.
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Istrctis r use
Shrt-Time Reaigs
The utagge e o the shiee ea is to be coecte to the lie a Guar termias o the Megger istrumet – the e termia to lie a the sie (shie) termia to Guar. The cip o the lie ea is coecte to the apparatus uer test i the reguar wa. The outboar Guar termia ma be coecte to that part o the apparatus uer test which the user wishes to guar. The couctor empoe i maig this coectio must be isuate or the votage ratig o the Megger istrumet use.
For short-time reaigs o isuatio resistace, operate the istrumet or a efite egth o time, either 30 secos or 1 miute, a rea at the e o that time. Cotiue c raig steai at sip spee uti the reaig has bee tae. Mae uture tests with the same egth o operatig time.
Eect Capacitace
Capacitace i apparatus uer test must be charge up to the rate dC votage o the Megger isuatio tester tester,, a maitaie maitaie or 30 to 60 secos beore a fa reaig is tae. Mae sure that capacitace is ischarge, b short circuitig a grouig the apparatus beore coectig the test eas. See discharge Capacitace, page 25. noTE: Capacitace cases the piter t s wig twars zer whie the istrmet is beig brght p t spee, a t swig scae bey ifity whe the geeratr is swig w. This is simpy the charge wig it a t the capacitace a thrgh the eectig ci the hmmeter.
Capacitace eects are most oticeabe i arge geerators, i power a commuicatio cabe more tha a ew hure eet i egth, a i capacitors. I geera, these eects are sma with capacitace o ess tha 0.01 F. It becomes more oticeabe as capacitace a/or the sesitivit o the istrumet icreases. The heav-ut series o Megger isuatio testers ca be use o arge capacitors with goo resuts, particuar whe operate rom the power ie rather tha the ha cra.
Time-Resistace Meth
Whe usig a ha-crae istrumet, operate cotiuous or 1 miute. Tae a reaig at the e o the frst 30 secos a aother reaig at the e o the miute. Whe usig a motor-rive or rectifer-operate istrumet, istrumet, the time itervas are usua 1 miute a 10 miutes rom the time the testig votage is appie. Or, time-resistace curves ma be tae over a perio o 10 to 30 miutes or oger. vtage Scaes
Some isuatio testers ma be suppie with a votage scae to chec or the absece o votage beore isuatio testig. As expaie i the Saety Precatis sectio, however, isuatio testers shou ever be coecte to eergize ies or euipmet whe operate i a o the Isuatio Test or Resistace Test moes.
operatig Time
A ver importat cosieratio i maig isuatio resistace tests is the time reuire or the reaig o isuatio resistace resistace to reach a maximum. The time reuire to charge the geometric capacitace is ver short – usua o more tha a ew secos – a that which causes urther ea i reachig u charge is a ieectric absorptio eect. It ma be a matter o miutes or eve hours or this eectrifcatio time to be compete, a or the poiter to reach a absoute maximum.
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Istrctis r use
Shrt-Time Reaigs
The utagge e o the shiee ea is to be coecte to the lie a Guar termias o the Megger istrumet – the e termia to lie a the sie (shie) termia to Guar. The cip o the lie ea is coecte to the apparatus uer test i the reguar wa. The outboar Guar termia ma be coecte to that part o the apparatus uer test which the user wishes to guar. The couctor empoe i maig this coectio must be isuate or the votage ratig o the Megger istrumet use.
For short-time reaigs o isuatio resistace, operate the istrumet or a efite egth o time, either 30 secos or 1 miute, a rea at the e o that time. Cotiue c raig steai at sip spee uti the reaig has bee tae. Mae uture tests with the same egth o operatig time.
Eect Capacitace
Capacitace i apparatus uer test must be charge up to the rate dC votage o the Megger isuatio tester tester,, a maitaie maitaie or 30 to 60 secos beore a fa reaig is tae. Mae sure that capacitace is ischarge, b short circuitig a grouig the apparatus beore coectig the test eas. See discharge Capacitace, page 25. noTE: Capacitace cases the piter t s wig twars zer whie the istrmet is beig brght p t spee, a t swig scae bey ifity whe the geeratr is swig w. This is simpy the charge wig it a t the capacitace a thrgh the eectig ci the hmmeter.
Capacitace eects are most oticeabe i arge geerators, i power a commuicatio cabe more tha a ew hure eet i egth, a i capacitors. I geera, these eects are sma with capacitace o ess tha 0.01 F. It becomes more oticeabe as capacitace a/or the sesitivit o the istrumet icreases. The heav-ut series o Megger isuatio testers ca be use o arge capacitors with goo resuts, particuar whe operate rom the power ie rather tha the ha cra.
Time-Resistace Meth
Whe usig a ha-crae istrumet, operate cotiuous or 1 miute. Tae a reaig at the e o the frst 30 secos a aother reaig at the e o the miute. Whe usig a motor-rive or rectifer-operate istrumet, istrumet, the time itervas are usua 1 miute a 10 miutes rom the time the testig votage is appie. Or, time-resistace curves ma be tae over a perio o 10 to 30 miutes or oger. vtage Scaes
Some isuatio testers ma be suppie with a votage scae to chec or the absece o votage beore isuatio testig. As expaie i the Saety Precatis sectio, however, isuatio testers shou ever be coecte to eergize ies or euipmet whe operate i a o the Isuatio Test or Resistace Test moes.
operatig Time
A ver importat cosieratio i maig isuatio resistace tests is the time reuire or the reaig o isuatio resistace resistace to reach a maximum. The time reuire to charge the geometric capacitace is ver short – usua o more tha a ew secos – a that which causes urther ea i reachig u charge is a ieectric absorptio eect. It ma be a matter o miutes or eve hours or this eectrifcatio time to be compete, a or the poiter to reach a absoute maximum.
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InTERPRETATIon-MInIMuM vAluES Isuatio resistace o eectrica euipmet is aecte b ma variabes such as the euipmet esig; the tpe o isuatig materia use, icuig biers a impregatig compous; the thicess o the isuatio a its area; ceaiess, moisture a temperature. For isuatio resistace reaigs to be a cocusive measure o the coitio o the euipmet beig teste, these variabes must be tae ito cosieratio. Ater the euipmet has bee put ito service, actors such as the esig o the euipmet, the i o isuatig materia use, a its thicess a area cease to be variabes, aowig miimum isuatio resistace vaues to be estabishe withi reasoabe toeraces. The variabes that must be cosiere ater the euipmet ha bee put ito service, a at the time that the isuatio resistace measuremets are beig mae, are ceaiess, moisture, temperature a mechaica amage (such as ractures).
Thereore, it is strog recommee that recors o perioic tests be ept, because persistet owwar tres tres i isuatio resistace usua give air warig o impeig troube, eve though the actua vaues ma be higher tha the suggeste miimum sae vaues. Coverse, aowaces must be mae or euipmet i service showig Coverse, perioic test vaues ower tha the suggeste miimum sae vaues, so og as the vaues remai stabe or cosistet. I such cases, ater ue cosieratio has bee give to temperature a humiit coitios at the time o test, there ma be o ee or cocer. This coitio ma be cause b uiorm istribute eaages o a harmess ature, a ma ot be the resut o a agerous ocaize weaess. Here agai, recors o isuatio resistace tests over a perio o time revea chages which ma justi ivestigatio. The tre o the curve ma be more sigifcat tha the umerica vaues themseves.
G Hseeepig
The oe-Meghm Re
The most importat reuiremets i the reiabe operatio o eectrica euipmet are ceaiess, a the eimiatio o moisture peetratio ito the isuatio. This ma be cosiere as goo houseeepig, a is essetia i the maiteace o a tpes o eectrica euipmet. The ver act that isuatio resistace is aecte b moisture a irt, with ue aowaces or temperature, maes the Megger isuatio tester the vauabe too that it is i eectrica maiteace. It is at oce a gauge o ceaiess a goo houseeepig as we as a etector o eterioratio a impeig troube.
For ma ears oe megohm has bee wie use as a air aowabe ower imit or isuatio resistace o oriar iustria eectrica euipmet rate up to 1000 vots, a is sti recommee or those who ma ot be too amiiar with isuatio resistace testig practices, or who ma ot wish to approach the probem rom a more techica poit o view.
What Reaigs t Expect – Periic Tests
Severa criteria or miimum vaues o isuatio resistace have bee eveope a are summarize here. The shou serve as a guie or euipmet i service. However, perioic tests o euipmet i service wi usua revea reaigs cosierab higher tha the suggeste miimum sae vaues.
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For euipmet rate above 1000 vots the oe megohm rue is usua state as a miimum o oe megohm per thousa vots. Athough this rue is somewhat arbitrar, a ma be criticize as acig a egieerig ouatio, it has stoo the test o a goo ma ears o practica experiece. It gives some assurace assurace that euipmet is ot too wet or irt a has save ma a uecessar breaow. More recet stuies o the probem, however, have resute i ormuas or miimum vaues o isuatio resistace that are base o the i o isuatig materia use a the eectrica a phsica imesios o the tpes o euipmet uer cosieratio.
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InTERPRETATIon-MInIMuM vAluES Isuatio resistace o eectrica euipmet is aecte b ma variabes such as the euipmet esig; the tpe o isuatig materia use, icuig biers a impregatig compous; the thicess o the isuatio a its area; ceaiess, moisture a temperature. For isuatio resistace reaigs to be a cocusive measure o the coitio o the euipmet beig teste, these variabes must be tae ito cosieratio. Ater the euipmet has bee put ito service, actors such as the esig o the euipmet, the i o isuatig materia use, a its thicess a area cease to be variabes, aowig miimum isuatio resistace vaues to be estabishe withi reasoabe toeraces. The variabes that must be cosiere ater the euipmet ha bee put ito service, a at the time that the isuatio resistace measuremets are beig mae, are ceaiess, moisture, temperature a mechaica amage (such as ractures).
Thereore, it is strog recommee that recors o perioic tests be ept, because persistet owwar tres tres i isuatio resistace usua give air warig o impeig troube, eve though the actua vaues ma be higher tha the suggeste miimum sae vaues. Coverse, aowaces must be mae or euipmet i service showig Coverse, perioic test vaues ower tha the suggeste miimum sae vaues, so og as the vaues remai stabe or cosistet. I such cases, ater ue cosieratio has bee give to temperature a humiit coitios at the time o test, there ma be o ee or cocer. This coitio ma be cause b uiorm istribute eaages o a harmess ature, a ma ot be the resut o a agerous ocaize weaess. Here agai, recors o isuatio resistace tests over a perio o time revea chages which ma justi ivestigatio. The tre o the curve ma be more sigifcat tha the umerica vaues themseves.
G Hseeepig
The oe-Meghm Re
The most importat reuiremets i the reiabe operatio o eectrica euipmet are ceaiess, a the eimiatio o moisture peetratio ito the isuatio. This ma be cosiere as goo houseeepig, a is essetia i the maiteace o a tpes o eectrica euipmet. The ver act that isuatio resistace is aecte b moisture a irt, with ue aowaces or temperature, maes the Megger isuatio tester the vauabe too that it is i eectrica maiteace. It is at oce a gauge o ceaiess a goo houseeepig as we as a etector o eterioratio a impeig troube.
For ma ears oe megohm has bee wie use as a air aowabe ower imit or isuatio resistace o oriar iustria eectrica euipmet rate up to 1000 vots, a is sti recommee or those who ma ot be too amiiar with isuatio resistace testig practices, or who ma ot wish to approach the probem rom a more techica poit o view.
What Reaigs t Expect – Periic Tests
Severa criteria or miimum vaues o isuatio resistace have bee eveope a are summarize here. The shou serve as a guie or euipmet i service. However, perioic tests o euipmet i service wi usua revea reaigs cosierab higher tha the suggeste miimum sae vaues.
More recet stuies o the probem, however, have resute i ormuas or miimum vaues o isuatio resistace that are base o the i o isuatig materia use a the eectrica a phsica imesios o the tpes o euipmet uer cosieratio.
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MInIMuM vAluES FoR InSulA InSulATIon TIon RESISTAnCE Rtatig Machiery
The IEEE guie, “Recommee Practices or Testig Isuatio Isuatio Resistace Resistace o Rotatig Machier”, eas with the probem o maig a iterpretig isuatio resistace measuremets or rotatig machier. It reviews the actors which aect or chage isuatio resistace characteristics, outies a recommes uiorm methos or maig tests, a presets ormuas or the cacuatio o approximate miimum isuatio resistace vaues or various tpes o AC a dC rotatig machier. machier. The guie states: “The recommee miimum isuatio resistace R m or ateratig-curre ateratig-currett a irect-curret machie armature wiigs a or fe wiigs o ateratig-curret a irect-curret machies ca be etermie b: Rm = V + 1 where: Rm
=
V
=
recommee miimum isuatio resistace i megohms megohms at 40°C o the etire machie wiig rate machie termia to termia potetia, i iovots
I appicatios where the machie is vita, it has bee cosiere goo practice to iitiate recoitioig shou the isuatio resistace, havig bee we above the miimum vaue give b E 2, rop appreciab to ear that eve.” It is recmmee that thse wh perate a maitai rtatig machiery btai cpies the IEEE pbicati, “Recmmee Practices r Testig Isati Resistace Rtatig Machiery”, which ca be btaie by writig the IEEE at 345 East 47th St., new Yr, nY, nY, 10017. Bshigs
I the case o outoor oi circuit breaer bushigs, experiece has show that a bushig, with its assembe associate isuatig members, shou, or reiabe operatio, have a isuatio resistace vaue above 10,000 megohms at 20°C. This assumes that the oi withi the ta is i goo coitio, that the breaer is separate rom its extera coectios to other euipmet, a that the porceai weather shie is guare. This meas that each compoet such as the strippe bushig itse, cross-member,, it ro, ower arcig shie, etc., shou have a isuatio cross-member resistace i excess o that vaue.
For euipmet rate above 1000 vots the oe megohm rue is usua state as a miimum o oe megohm per thousa vots. Athough this rue is somewhat arbitrar, a ma be criticize as acig a egieerig ouatio, it has stoo the test o a goo ma ears o practica experiece. It gives some assurace assurace that euipmet is ot too wet or irt a has save ma a uecessar breaow.
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A compoets which are superfcia cea a r a have vaues ess tha 10,000 megohms are usua eteriorate itera, b the presece o moisture or carboize paths, to such a extet that the are ot reiabe or goo service uess recoitioe. This is particuar so whe operatig uer surge coitios such as urig ightig isturbaces. I the case o the strippe bushig itse, the ower stem a upper weather shie must be either perect cea or guare beore it is coeme as ureiabe because o a isuatio resistace vaue ess tha 10,000 megohms. What has bee sai or strippe oi circuit breaer bushigs aso appies to bushigs or other euipmet, such as trasormers. Sice bushigs a other associate members have ver high isuatio resistace vaues orma,, a Megger isuatio tester havig a rage o at east 10,000 orma megohms is ecessar to test such euipmet. Megger istrumets havig rages up to 50,000 megohms wi permit observatio o eterioratig tres i bushigs beore the reach the uestioabe vaue o 10,000 megohms. Cabe a Cctrs
Cabe a couctor istaatios preset a wie variatio o coitios rom the poit o view o the resistace o the isuatio. These coitios resut rom the ma is o isuatig materias use, the votage ratig or isuatio thicess, a the egth o the circuit ivove i the measuremet. Furthermore, such circuits usua exte over great istaces, a ma be subjecte to wie variatios i temperature, which wi have a eect o the isuatio resistace vaues obtaie. The termias o cabes a couctors wi aso have a eect o the test vaues uess the are cea a r, or guare. The Isuate Cabe Egieers Associatio (ICEA) gives miimum vaues o isuatio resistace i its specifcatios or various tpes o cabes a couctors. These miimum vaues are or ew, sige-couctor wire a cabe ater beig subjecte to a AC high votage test a base o a dC test potetia o 500 vots appie or oe miute at a temperature o 60°F.
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MInIMuM vAluES FoR InSulA InSulATIon TIon RESISTAnCE Rtatig Machiery
The IEEE guie, “Recommee Practices or Testig Isuatio Isuatio Resistace Resistace o Rotatig Machier”, eas with the probem o maig a iterpretig isuatio resistace measuremets or rotatig machier. It reviews the actors which aect or chage isuatio resistace characteristics, outies a recommes uiorm methos or maig tests, a presets ormuas or the cacuatio o approximate miimum isuatio resistace vaues or various tpes o AC a dC rotatig machier. machier. The guie states: “The recommee miimum isuatio resistace R m or ateratig-curre ateratig-currett a irect-curret machie armature wiigs a or fe wiigs o ateratig-curret a irect-curret machies ca be etermie b: Rm = V + 1 where: Rm
=
V
=
recommee miimum isuatio resistace i megohms megohms at 40°C o the etire machie wiig rate machie termia to termia potetia, i iovots
I appicatios where the machie is vita, it has bee cosiere goo practice to iitiate recoitioig shou the isuatio resistace, havig bee we above the miimum vaue give b E 2, rop appreciab to ear that eve.” It is recmmee that thse wh perate a maitai rtatig machiery btai cpies the IEEE pbicati, “Recmmee Practices r Testig Isati Resistace Rtatig Machiery”, which ca be btaie by writig the IEEE at 345 East 47th St., new Yr, nY, nY, 10017. Bshigs
I the case o outoor oi circuit breaer bushigs, experiece has show that a bushig, with its assembe associate isuatig members, shou, or reiabe operatio, have a isuatio resistace vaue above 10,000 megohms at 20°C. This assumes that the oi withi the ta is i goo coitio, that the breaer is separate rom its extera coectios to other euipmet, a that the porceai weather shie is guare. This meas that each compoet such as the strippe bushig itse, cross-member,, it ro, ower arcig shie, etc., shou have a isuatio cross-member resistace i excess o that vaue.
A compoets which are superfcia cea a r a have vaues ess tha 10,000 megohms are usua eteriorate itera, b the presece o moisture or carboize paths, to such a extet that the are ot reiabe or goo service uess recoitioe. This is particuar so whe operatig uer surge coitios such as urig ightig isturbaces. I the case o the strippe bushig itse, the ower stem a upper weather shie must be either perect cea or guare beore it is coeme as ureiabe because o a isuatio resistace vaue ess tha 10,000 megohms. What has bee sai or strippe oi circuit breaer bushigs aso appies to bushigs or other euipmet, such as trasormers. Sice bushigs a other associate members have ver high isuatio resistace vaues orma,, a Megger isuatio tester havig a rage o at east 10,000 orma megohms is ecessar to test such euipmet. Megger istrumets havig rages up to 50,000 megohms wi permit observatio o eterioratig tres i bushigs beore the reach the uestioabe vaue o 10,000 megohms. Cabe a Cctrs
Cabe a couctor istaatios preset a wie variatio o coitios rom the poit o view o the resistace o the isuatio. These coitios resut rom the ma is o isuatig materias use, the votage ratig or isuatio thicess, a the egth o the circuit ivove i the measuremet. Furthermore, such circuits usua exte over great istaces, a ma be subjecte to wie variatios i temperature, which wi have a eect o the isuatio resistace vaues obtaie. The termias o cabes a couctors wi aso have a eect o the test vaues uess the are cea a r, or guare. The Isuate Cabe Egieers Associatio (ICEA) gives miimum vaues o isuatio resistace i its specifcatios or various tpes o cabes a couctors. These miimum vaues are or ew, sige-couctor wire a cabe ater beig subjecte to a AC high votage test a base o a dC test potetia o 500 vots appie or oe miute at a temperature o 60°F.
A STITCH IN TIME
ASTITCH AST ITCHIN INTIME TIME
These staar miimum vaues (or sige-couctor cabe) are base o the oowig ormua:
The isuatio resistace o oe couctor o a muticouctor cabe to a others a sheath is:
R = k og 10 d /
R = k og 10 d /
where:
where:
R
=
megohms per 1000 eet o cabe
k
=
costat or isuatig materia
d
=
outsie iameter o couctor isuatio
=
iameter o couctor
d
= iameter over isuatio isuatio o euivaet sige-couctor sige-couctor cabe = + 2c + 2b
=
iameter o couctor (or sector cabes euas iameter rou couctor o same cross-sectio)
c
=
thicess o couctor isuatio
b
=
thicess o jacet isuatio
o
Miimm vaes k at 60° F.
(a imesis mst be expresse i same its)
Isuatio Tpe Impregate Paper ................................................................... ................................................................... 2,640 Varishe Cambric .................................................................... .................................................................... 2,460 Thermopastic-Poethee ........................................ above 50,000 Composite Poethee ......................................................... ......................................................... 30,000 Thermopastic-Povi: Povi Chorie 60°C ........................................................ 500 Povi Chorie 75°C ..................................................... 2,000 Grae
natura Rubber
Sthetic Rubber
Trasrmers
Acceptabe isuatio resistace vaues or r a compou-fe trasormers shou be comparabe to those or Cass A rotatig machier, athough o staar miimum vaues are avaiabe. Oi-fe trasormers or votage reguators preset a specia probem i that the coitio o the oi has a mare iuece o the isuatio resistace o the wiigs. I the absece o more reiabe ata the oowig ormua is suggeste:
Coe...............................................................................................950
R = CE √VA
Perormace ....................................... 10,560............................2,000 Heat Resistat..................................... 10,560............................2,000
R
=
miimum 1-miute 500-vot dC isuatio resistace i megohms rom wiig to grou, with other wiig or wiigs guare, or rom wiig to wiig with core guare
C
=
a costat or 20°C measuremets
E
=
votage ratig o wiig uer test
VA =
rate capacit o wiig uer test
Ozoe Resistat...................... 10,000 (But) ............................2,000 kerite ..........................................................................................4,000 See pages 44 a 45 or Tabes o log 10 d /
For tests o wiig to grou with the other wiig or wiigs groue, the vaues wi be much ess tha that give b the ormua. R i this ormua is base o r, aci-ree, suge-ree suge-ree oi, a bushigs a termia boars that are i goo coitio. 0
ASTITCH AST ITCHIN INTIME TIME
A STITCH IN TIME
These staar miimum vaues (or sige-couctor cabe) are base o the oowig ormua:
The isuatio resistace o oe couctor o a muticouctor cabe to a others a sheath is:
R = k og 10 d /
R = k og 10 d /
where:
where:
R
=
megohms per 1000 eet o cabe
k
=
costat or isuatig materia
d
=
outsie iameter o couctor isuatio
=
iameter o couctor
d
= iameter over isuatio isuatio o euivaet sige-couctor sige-couctor cabe = + 2c + 2b
=
iameter o couctor (or sector cabes euas iameter rou couctor o same cross-sectio)
c
=
thicess o couctor isuatio
b
=
thicess o jacet isuatio
o
Miimm vaes k at 60° F.
(a imesis mst be expresse i same its)
Isuatio Tpe Impregate Paper ................................................................... ................................................................... 2,640 Varishe Cambric .................................................................... .................................................................... 2,460 Thermopastic-Poethee ........................................ above 50,000 Composite Poethee ......................................................... ......................................................... 30,000 Thermopastic-Povi: Povi Chorie 60°C ........................................................ 500 Povi Chorie 75°C ..................................................... 2,000 Grae
natura Rubber
Sthetic Rubber
Trasrmers
Acceptabe isuatio resistace vaues or r a compou-fe trasormers shou be comparabe to those or Cass A rotatig machier, athough o staar miimum vaues are avaiabe. Oi-fe trasormers or votage reguators preset a specia probem i that the coitio o the oi has a mare iuece o the isuatio resistace o the wiigs. I the absece o more reiabe ata the oowig ormua is suggeste:
Coe...............................................................................................950
R = CE √VA
Perormace ....................................... 10,560............................2,000 Heat Resistat..................................... 10,560............................2,000
R
=
miimum 1-miute 500-vot dC isuatio resistace i megohms rom wiig to grou, with other wiig or wiigs guare, or rom wiig to wiig with core guare
C
=
a costat or 20°C measuremets
E
=
votage ratig o wiig uer test
VA =
rate capacit o wiig uer test
Ozoe Resistat...................... 10,000 (But) ............................2,000 kerite ..........................................................................................4,000 See pages 44 a 45 or Tabes o log 10 d /
For tests o wiig to grou with the other wiig or wiigs groue, the vaues wi be much ess tha that give b the ormua. R i this ormua is base o r, aci-ree, suge-ree suge-ree oi, a bushigs a termia boars that are i goo coitio. 0
A STITCH IN TIME
ASTITCH AST ITCHIN INTIME TIME
vaes C at 20°C
60-Hertz Tae oi-fe tpe Utae oi-fe tpe dr or compou-fe tpe
1.5 30.0 30.0
25-Hertz 1.0 20.0 20.0
This ormua is itee or sige-phase trasormers. trasormers. I the trasormer uer test is o the three-phase tpe, a the three iiviua wiigs are beig teste as oe, the: E
=
VA =
votage ratig o oe o the sige-phase wiigs (phase to phase or eta coecte uits a phase to eutra or star coecte uits) rate capacit o the compete three-phase wiig uer test
o the ohmmeter ca be expaie b reerrig to Fig. 21. I this fgure, the chage which ma occur i eaage curret ater the absorptio curret has isappeare is show potte i terms o isuatio resistace agaist three ieret votages. note that there there is o chage i resistace show i the fgure betwee 500 a 1000 vots, iicatig o chage i the isuatio as a resut o appig these two votages. This is a assumptio, but is a coitio which is ot ucommo i practice. I the isuatio cotiues to be stabe at 2500 vots, there wi be o chage i the isuatio resistace vaue obtaie, which is show b the otte extesio o the horizota ie i the fgure. Whe o-iear coitios appear at a higher votage, the the votage resistace curve reveas this ver ver cear b a ower resistace vaue, iicate b the owwar curve i the fgure. The fgure, thereore, reveas the simpicit o etermiig the chage i isuatio stabiit b usig three fxe votages which are easi reproucibe whe maig three-votage tests o a routie basis.
TESTS uSInG MulTI-volTAGE MulTI-volTAGE MEGGER InSulATIon TESTE RS Maiteace practice tres iicate the vaue o testig isuatio with dC votages at eves somewhat higher tha the pea vaue o the rate AC votage o the euipmet beig teste. Such dC tests have i some cases bee show to revea o-estructive icipiet weaesses i isuatio isuatio which cou ot otherwise be ou, except possib b partia ischarge etectio at o-estructive AC test votage eves. The techiue ivoves the appicatio o two or more dC votages, a critica observig a reuctio o isuatio resistace at the higher votage. A mare or uusua reuctio i isuatio resistace or a prescribe icrease i appie votage is a iicatio o icipiet weaess. It is importat to metio that the merits o this techiue arise rom more recet ivestigatios which iicate that rather high dC votage ca be use to etect weaesses without amagig the isuatio. The maximum vaue o votage which shou be use wi epe arge o the ceaiess a ress o the isuatio to be teste. I maig tests o isuatio at such dC votages, the ohmmeter metho has at east two avatages. First, prescribe fxe votages are switche ito use, a oe istrumet measuremet mae with the irect reaig ohmmeter.. This is a simpe a reproucibe metho compare to oe i ohmmeter which ma choices o votage are avaiabe. Aother importat avatage
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Figre 21
We wish to emphasize that the curve i Fig. 21 iicates the resistace chage ue to eaage curret o, a ot the absorptio curret which ma appear or a perio o time with each chage i votage. It ma be ecessar to wait a appreciabe amout o time ater each votage chage or the absorptio curret to isappear beore taig a reaig.
A STITCH IN TIME
vaes C at 20°C
60-Hertz Tae oi-fe tpe Utae oi-fe tpe dr or compou-fe tpe
1.5 30.0 30.0
25-Hertz 1.0 20.0 20.0
This ormua is itee or sige-phase trasormers. trasormers. I the trasormer uer test is o the three-phase tpe, a the three iiviua wiigs are beig teste as oe, the: E
=
VA =
votage ratig o oe o the sige-phase wiigs (phase to phase or eta coecte uits a phase to eutra or star coecte uits) rate capacit o the compete three-phase wiig uer test
o the ohmmeter ca be expaie b reerrig to Fig. 21. I this fgure, the chage which ma occur i eaage curret ater the absorptio curret has isappeare is show potte i terms o isuatio resistace agaist three ieret votages. note that there there is o chage i resistace show i the fgure betwee 500 a 1000 vots, iicatig o chage i the isuatio as a resut o appig these two votages. This is a assumptio, but is a coitio which is ot ucommo i practice. I the isuatio cotiues to be stabe at 2500 vots, there wi be o chage i the isuatio resistace vaue obtaie, which is show b the otte extesio o the horizota ie i the fgure. Whe o-iear coitios appear at a higher votage, the the votage resistace curve reveas this ver ver cear b a ower resistace vaue, iicate b the owwar curve i the fgure. The fgure, thereore, reveas the simpicit o etermiig the chage i isuatio stabiit b usig three fxe votages which are easi reproucibe whe maig three-votage tests o a routie basis.
TESTS uSInG MulTI-volTAGE MulTI-volTAGE MEGGER InSulATIon TESTE RS Maiteace practice tres iicate the vaue o testig isuatio with dC votages at eves somewhat higher tha the pea vaue o the rate AC votage o the euipmet beig teste. Such dC tests have i some cases bee show to revea o-estructive icipiet weaesses i isuatio isuatio which cou ot otherwise be ou, except possib b partia ischarge etectio at o-estructive AC test votage eves. The techiue ivoves the appicatio o two or more dC votages, a critica observig a reuctio o isuatio resistace at the higher votage. A mare or uusua reuctio i isuatio resistace or a prescribe icrease i appie votage is a iicatio o icipiet weaess. It is importat to metio that the merits o this techiue arise rom more recet ivestigatios which iicate that rather high dC votage ca be use to etect weaesses without amagig the isuatio. The maximum vaue o votage which shou be use wi epe arge o the ceaiess a ress o the isuatio to be teste. I maig tests o isuatio at such dC votages, the ohmmeter metho has at east two avatages. First, prescribe fxe votages are switche ito use, a oe istrumet measuremet mae with the irect reaig ohmmeter.. This is a simpe a reproucibe metho compare to oe i ohmmeter which ma choices o votage are avaiabe. Aother importat avatage
ASTITCH AST ITCHIN INTIME TIME
Figre 21
We wish to emphasize that the curve i Fig. 21 iicates the resistace chage ue to eaage curret o, a ot the absorptio curret which ma appear or a perio o time with each chage i votage. It ma be ecessar to wait a appreciabe amout o time ater each votage chage or the absorptio curret to isappear beore taig a reaig.
A STITCH IN TIME
To better uersta the techiue o maig isuatio resistace tests at two or more votages, the oowig steps are suggeste, usig a iustria or tractio motor casse i the 300- to 1000-vot rage as a exampe: 1. Mae a oe-miute Megger istrumet test at 500 vots to serve as a basis o compariso or subseuet steps. 2. Ater a careu ceaig operatio mae a seco 500-vot test to etermie the eectiveess o the ceaig. 3. I the oe-miute isuatio resistace vaue is suborma, or i the 60 seco/30 seco isuatio resistace ratio is o greater tha oe at this poit the a rig operatio ma be esirabe beore usig a higher test votage. However, maig aother test at 1000 vots a comparig these reaigs with those rom the 500-vot test wi hep i etermiig the ee or rig. I the 1000-vot test vaue is appreciab ess tha that at 500 vots, the a rig operatio shou be perorme. O the other ha, i the 1000-vot a 500-vot test vaues are approximate the same, it is reasoabe to assume that the ecisio to perorm a rig operatio ca be eerre uti ater the ext step.
presece o moisture. I the isuatio resistace is frst teste o the shorttime reaig basis (frst at oe votage eve a the at a higher potetia), a ower vaue o isuatio resistace atthe higher dC test votage usua iicates the presece o moisture. The appie votages shou preerab be i the ratio o 1 to 5. Experiece has iicate that a chage o 25% i the isuatio resistace vaue, with a 1 to 5 ratio i test votages, is usua ue to the presece o a excessive amout o moisture. This metho is ot base base o a ieectric absorptio pheomea, but it oes reate to the Evershe Eect. As with time-resistace measuremets, the muti-votage metho o testig isuatio resistace has icrease vaue whe mae o a perioic or scheue basis.
4. Mae a Megger istrumet test at 2500 vots. I there is o appreciabe ierece i the 500- a 2500-vot test vaues, goo eviece exists that the motor i uestio is i reiabe coitio as ar as its isuatio is cocere. O the other ha, i there is a appreciabe ierece i the two, there is goo eviece that more thorough recoitioig is cae or.. I the isuatio ais uer the 2500-vot test, ater oowig steps 1, or 2 a 3, we beieve there is a ieihoo that the motor i uestio wou ai i service eve though a attempt were mae to recoitio it o the basis o ow-votage tests o.
The muti-votage metho ca aso be hepu i etermiig the presece o excessive moisture i the isuatio o euipmet rate at votages euivaet to or greater tha the highest votage avaiabe rom the mutivotage Megger istrumet beig use. I other wors, eve though the highest Megger istrumet votage avaiabe oes ot stress the isuatio beo its ratig, a two-votage test ca, evertheess, ote revea the
ASTITCH AST ITCHIN INTIME TIME
Figre 22-Time Resistace Test Frms
A STITCH IN TIME
To better uersta the techiue o maig isuatio resistace tests at two or more votages, the oowig steps are suggeste, usig a iustria or tractio motor casse i the 300- to 1000-vot rage as a exampe: 1. Mae a oe-miute Megger istrumet test at 500 vots to serve as a basis o compariso or subseuet steps. 2. Ater a careu ceaig operatio mae a seco 500-vot test to etermie the eectiveess o the ceaig. 3. I the oe-miute isuatio resistace vaue is suborma, or i the 60 seco/30 seco isuatio resistace ratio is o greater tha oe at this poit the a rig operatio ma be esirabe beore usig a higher test votage. However, maig aother test at 1000 vots a comparig these reaigs with those rom the 500-vot test wi hep i etermiig the ee or rig. I the 1000-vot test vaue is appreciab ess tha that at 500 vots, the a rig operatio shou be perorme. O the other ha, i the 1000-vot a 500-vot test vaues are approximate the same, it is reasoabe to assume that the ecisio to perorm a rig operatio ca be eerre uti ater the ext step.
presece o moisture. I the isuatio resistace is frst teste o the shorttime reaig basis (frst at oe votage eve a the at a higher potetia), a ower vaue o isuatio resistace atthe higher dC test votage usua iicates the presece o moisture. The appie votages shou preerab be i the ratio o 1 to 5. Experiece has iicate that a chage o 25% i the isuatio resistace vaue, with a 1 to 5 ratio i test votages, is usua ue to the presece o a excessive amout o moisture. This metho is ot base base o a ieectric absorptio pheomea, but it oes reate to the Evershe Eect. As with time-resistace measuremets, the muti-votage metho o testig isuatio resistace has icrease vaue whe mae o a perioic or scheue basis.
4. Mae a Megger istrumet test at 2500 vots. I there is o appreciabe ierece i the 500- a 2500-vot test vaues, goo eviece exists that the motor i uestio is i reiabe coitio as ar as its isuatio is cocere. O the other ha, i there is a appreciabe ierece i the two, there is goo eviece that more thorough recoitioig is cae or.. I the isuatio ais uer the 2500-vot test, ater oowig steps 1, or 2 a 3, we beieve there is a ieihoo that the motor i uestio wou ai i service eve though a attempt were mae to recoitio it o the basis o ow-votage tests o.
The muti-votage metho ca aso be hepu i etermiig the presece o excessive moisture i the isuatio o euipmet rate at votages euivaet to or greater tha the highest votage avaiabe rom the mutivotage Megger istrumet beig use. I other wors, eve though the highest Megger istrumet votage avaiabe oes ot stress the isuatio beo its ratig, a two-votage test ca, evertheess, ote revea the
ASTITCH AST ITCHIN INTIME TIME
STEP-volTAGE METHod I this metho, ou ee a muti-votage Megger istrumet to app two or more votages i steps, ie 500 vots a the 1000 vots. you oo or a reuctio o isuatio resistace at the higher votage. I the resistace is ower, it’s it’s a sig o a isuatio weaess that shows up o at the higher votage. Fig. 23 shows a exampe i which, istea o progressive icreasig the votage, ou frst test at a ow votage (such as 500 vots) a the, ater ischargig the sampe, ou test agai at a higher votage (such as 2500 vots). A ierece i the two tests i terms o megohms wi show sigs o weaess at the higher votage – a warig to ivestigate urther. urther. As the coitios withi the sampe eteriorate, the higher-votage pot, as show i Fig. 23, wi reuce i megohms compare to the ower-votage oe, a its upwar sope wi be ess.
Figre 22-Time Resistace Test Frms
A STITCH IN TIME
now, whe the votage is icrease i steps to prouce eectrica stresses which approach or excee those ecoutere i service, the oca wea spots iuece the overa isuatio resistace more a more. The resistace o such oca auts geera ecreases rapi as the eectrica stress i them icreases beo a certai imit. The pot o cosecutive Megger istrumet reaigs cear shows the sharp rop whe this occurs (see Fig. 24).
Figre 23-Typica cres with the “step-tage” test.
The theor behi the step-votage techiue is a bit compex, but we’ tr to eep it simpe. Moisture a irt i isuatio are usua reveae b tests at votages ar beow those expecte i service. H owever owever,, eects o agig or mechaica amage i air cea a r isuatio ma ot be reveae at such ow stress.
ASTITCH AST ITCHIN INTIME TIME
Figre 24-Test cres by the step-tage meth, cmparig rests with g a ba isati. Cre 1 (wer pt) shws efite rp i resistace with icreasig tage, iicatig a prbem. Cre 2 (pper pt) shws citis i the same mtr wiig ater ceaig, baig a impregatig perati.
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STEP-volTAGE METHod I this metho, ou ee a muti-votage Megger istrumet to app two or more votages i steps, ie 500 vots a the 1000 vots. you oo or a reuctio o isuatio resistace at the higher votage. I the resistace is ower, it’s it’s a sig o a isuatio weaess that shows up o at the higher votage. Fig. 23 shows a exampe i which, istea o progressive icreasig the votage, ou frst test at a ow votage (such as 500 vots) a the, ater ischargig the sampe, ou test agai at a higher votage (such as 2500 vots). A ierece i the two tests i terms o megohms wi show sigs o weaess at the higher votage – a warig to ivestigate urther. urther. As the coitios withi the sampe eteriorate, the higher-votage pot, as show i Fig. 23, wi reuce i megohms compare to the ower-votage oe, a its upwar sope wi be ess.
now, whe the votage is icrease i steps to prouce eectrica stresses which approach or excee those ecoutere i service, the oca wea spots iuece the overa isuatio resistace more a more. The resistace o such oca auts geera ecreases rapi as the eectrica stress i them icreases beo a certai imit. The pot o cosecutive Megger istrumet reaigs cear shows the sharp rop whe this occurs (see Fig. 24).
Figre 23-Typica cres with the “step-tage” test.
The theor behi the step-votage techiue is a bit compex, but we’ tr to eep it simpe. Moisture a irt i isuatio are usua reveae b tests at votages ar beow those expecte i service. H owever owever,, eects o agig or mechaica amage i air cea a r isuatio ma ot be reveae at such ow stress.
Figre 24-Test cres by the step-tage meth, cmparig rests with g a ba isati. Cre 1 (wer pt) shws efite rp i resistace with icreasig tage, iicatig a prbem. Cre 2 (pper pt) shws citis i the same mtr wiig ater ceaig, baig a impregatig perati.
A STITCH IN TIME
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you o ee to eep the test votage costat betwee steps or about 60 secos. This short perio wi ot aect the tre i resistace chage. The time perio, however, shou awas be the same or a give piece o euipmet. A the absorptio curret ma ot have isappeare i this time, but our resistace measuremets wi be mae o the same basis a thereore wi be meaigu. your resuts are iepeet o the isuatio materia a its temperature because ou’re ooig at the chage i resistace – ot the absoute resistace vaues.
The isuatio o a eectrica apparatus has two couctig or eaage paths – oe through the isuatig materia a the other over its suraces. B proviig a thir test termia i the path o the surace eaage, eaage, it is separate ito two parts, ormig a three-termia etwor as show i Fig. 25a. I practice, this thir termia ma be provie as show i Figures 26 to 38.
As is true with the spot-reaig a time-resistace measuremets, the step-votage metho is more vauabe to ou whe repeate o a perioic, scheue basis. The step-votage metho is particuar useu i etermiig the presece o excessive moisture or other cotamiats i the isuatio o euipmet that is rate at votages euivaet to or greater tha the highest votage avaiabe i our muti-votage Megger istrumet. I other wors, eve though our highest votage oes ot stress the isuatio beo its ratig, a two-votage test ca, evertheess, ote revea the presece o such cotamiats.
Figre 25a
For exampe, assume that ou frst test isuatio resistace o a short-time reaig basis – at a votage o 500 V dC, a the at a higher potetia o, sa, 2500 VdC. Eve though the atter votage ma be omia i reatio to the votage ratig o our euipmet, a ower vaue o isuatio resistace at the higher test votage usua iicates the presece o c otamiate ractures or other eaage paths through the isuatio to grou. Figre 25b
The appie votages shou preerab be i the ratio o 1 to 5 or greater (500 a 2500, or exampe). Resuts to ate show that a chage o 25% i the isuatio resistace vaue, with a 1 to 5 ratio i test votages, is usua ue to the presece o a excessive amout o moisture or other cotamiat.
uSE oF A GuARd TERMInAl A Megger isuatio testers havig rages o 1000 megohms a higher are euippe with Guar termias. The purpose o this termia is to provie aciities or maig a three-termia etwor measuremet, so that the resistace o oe o two possibe paths ca be etermie irect. irect. It has the urther or secoar purpose o proviig a source o dC votage o goo reguatio a o imite curret capacit. 0
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Figre 26-Shwig hw t se the Gar termia t eimiate the eects srace eaage acrss expse isati at e e a cabe. See as Figres 28, 30 a 31.
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you o ee to eep the test votage costat betwee steps or about 60 secos. This short perio wi ot aect the tre i resistace chage. The time perio, however, shou awas be the same or a give piece o euipmet. A the absorptio curret ma ot have isappeare i this time, but our resistace measuremets wi be mae o the same basis a thereore wi be meaigu. your resuts are iepeet o the isuatio materia a its temperature because ou’re ooig at the chage i resistace – ot the absoute resistace vaues.
The isuatio o a eectrica apparatus has two couctig or eaage paths – oe through the isuatig materia a the other over its suraces. B proviig a thir test termia i the path o the surace eaage, eaage, it is separate ito two parts, ormig a three-termia etwor as show i Fig. 25a. I practice, this thir termia ma be provie as show i Figures 26 to 38.
As is true with the spot-reaig a time-resistace measuremets, the step-votage metho is more vauabe to ou whe repeate o a perioic, scheue basis. The step-votage metho is particuar useu i etermiig the presece o excessive moisture or other cotamiats i the isuatio o euipmet that is rate at votages euivaet to or greater tha the highest votage avaiabe i our muti-votage Megger istrumet. I other wors, eve though our highest votage oes ot stress the isuatio beo its ratig, a two-votage test ca, evertheess, ote revea the presece o such cotamiats.
Figre 25a
For exampe, assume that ou frst test isuatio resistace o a short-time reaig basis – at a votage o 500 V dC, a the at a higher potetia o, sa, 2500 VdC. Eve though the atter votage ma be omia i reatio to the votage ratig o our euipmet, a ower vaue o isuatio resistace at the higher test votage usua iicates the presece o c otamiate ractures or other eaage paths through the isuatio to grou. Figre 25b
The appie votages shou preerab be i the ratio o 1 to 5 or greater (500 a 2500, or exampe). Resuts to ate show that a chage o 25% i the isuatio resistace vaue, with a 1 to 5 ratio i test votages, is usua ue to the presece o a excessive amout o moisture or other cotamiat.
uSE oF A GuARd TERMInAl A Megger isuatio testers havig rages o 1000 megohms a higher are euippe with Guar termias. The purpose o this termia is to provie aciities or maig a three-termia etwor measuremet, so that the resistace o oe o two possibe paths ca be etermie irect. irect. It has the urther or secoar purpose o proviig a source o dC votage o goo reguatio a o imite curret capacit. 0
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Figre 26-Shwig hw t se the Gar termia t eimiate the eects srace eaage acrss expse isati at e e a cabe. See as Figres 28, 30 a 31.
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There are aso cases, such as ou i two wiig trasormers or muticouctor cabes, where a three-termia etwor is orme as show i Fig. 25b. Figures 30 a 33 a others show practica appicatios o this orm o three-termia etwor. I maig a three-termia test ivovig o oe measuremet, the lie termia o the Megger istrumet is coecte to Termia 1, Fig. 25a, the Guar termia to Termia 3, a the Earth termia to Termia 2. This wi give the true vaue o r 12, provie r 23 a r13 are ot too ow i vaue. The eg r23, which is coecte across the Megger istrumet geerator, geerator, shou be about 1 megohm or higher to prevet excessive oa o the geerator, a maitai satisactor geerator votage. I usig the Guar termia, particuar i the case o motor-rive or rectifer-operate Megger istrumets,aso mae certai that there is o chace o a arc-over betwee the guare termia o the sampe a grou. Such a arc-over ma cause uesirabe arcig at the commutator o the istrumet geerator.
Figre 28-Shwig se the Gar cecti t eimiate the eect eaage t gr, as i Figre 26, a as the eect eaage t ajacet cctrs. nte that the Gar wire is wrappe ar the expse isati a as is cecte t the ajacet cctrs. d t cse this iagram with Figre 26, where the Gar wire ges y t the expse isati, a the ajacet cctrs are gre.
The eg r 13, which shuts the Megger eectig coi, shou be at east 100 megohms or a measurig accurac o approximate 1%. The 1% accurac fgure is base o the R' baast resistor beig 1 megohm, which is tpica. For more precise etermiatios o accurac, obtai the exact vaue o R' b writig to Megger a givig the seria umber o the istrumet i use.
Figre 29-T eimiate the eect srace eaage i measrig the tre resistace a isatig member, sch as a it r i a circit breaer.
Figre 27-Shwig hw t se the Gar cecti t eimiate the eects srace eaage acrss expse isati at bth es a cabe whe a spare cctr i the cabe is aaiabe r cmpetig the Gar cecti.
ASTITCH AST ITCHIN INTIME TIME
A STITCH IN TIME
There are aso cases, such as ou i two wiig trasormers or muticouctor cabes, where a three-termia etwor is orme as show i Fig. 25b. Figures 30 a 33 a others show practica appicatios o this orm o three-termia etwor. I maig a three-termia test ivovig o oe measuremet, the lie termia o the Megger istrumet is coecte to Termia 1, Fig. 25a, the Guar termia to Termia 3, a the Earth termia to Termia 2. This wi give the true vaue o r 12, provie r 23 a r13 are ot too ow i vaue. The eg r23, which is coecte across the Megger istrumet geerator, geerator, shou be about 1 megohm or higher to prevet excessive oa o the geerator, a maitai satisactor geerator votage. I usig the Guar termia, particuar i the case o motor-rive or rectifer-operate Megger istrumets,aso mae certai that there is o chace o a arc-over betwee the guare termia o the sampe a grou. Such a arc-over ma cause uesirabe arcig at the commutator o the istrumet geerator.
Figre 28-Shwig se the Gar cecti t eimiate the eect eaage t gr, as i Figre 26, a as the eect eaage t ajacet cctrs. nte that the Gar wire is wrappe ar the expse isati a as is cecte t the ajacet cctrs. d t cse this iagram with Figre 26, where the Gar wire ges y t the expse isati, a the ajacet cctrs are gre.
The eg r 13, which shuts the Megger eectig coi, shou be at east 100 megohms or a measurig accurac o approximate 1%. The 1% accurac fgure is base o the R' baast resistor beig 1 megohm, which is tpica. For more precise etermiatios o accurac, obtai the exact vaue o R' b writig to Megger a givig the seria umber o the istrumet i use.
Figre 29-T eimiate the eect srace eaage i measrig the tre resistace a isatig member, sch as a it r i a circit breaer.
Figre 27-Shwig hw t se the Gar cecti t eimiate the eects srace eaage acrss expse isati at bth es a cabe whe a spare cctr i the cabe is aaiabe r cmpetig the Gar cecti.
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The highest accurac is esire i cases as show i Fig. 25a, or where the true resistace o each eg is wate as i the case o Fig. 25b, three measuremets are reuire a the oowig euatios 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 reaigs i megohms measure across the termias o the etwor which are coecte to the lie a Earth termias o the Megger istrumet with Termias 3, 1 a 2 respective coecte to the istrumet Guar termia. R' is the vaue o the baast resistace i megohms o the istrumet i use. I maig these three measuremets, o o coect the lie termia o the istrumet to the groue termia o the etwor, as a eaage over the istrumet case betwee the Earth termia a grou wi shut the resistace beig measure.
Figre 31-Cectis r testig isati resistace betwee e wire a a ther wires cecte, witht beig aecte by eaage t gr.
BuSHInGS, PoTHEAdS And InSulAToR InSulAToRS S
Figre 32-Shwig se sprig car as a Gar cecti t eimiate the eects srace eaage. The eice er test mst be iscecte rm a ther eipmet.
Figre 30-Cectis r testig isati resistace betwee e wire a gr, witht beig aecte by eaage t ther wires. nte se Gar cecti.
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A STITCH IN TIME
The highest accurac is esire i cases as show i Fig. 25a, or where the true resistace o each eg is wate as i the case o Fig. 25b, three measuremets are reuire a the oowig euatios 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 reaigs i megohms measure across the termias o the etwor which are coecte to the lie a Earth termias o the Megger istrumet with Termias 3, 1 a 2 respective coecte to the istrumet Guar termia. R' is the vaue o the baast resistace i megohms o the istrumet i use. I maig these three measuremets, o o coect the lie termia o the istrumet to the groue termia o the etwor, as a eaage over the istrumet case betwee the Earth termia a grou wi shut the resistace beig measure.
Figre 31-Cectis r testig isati resistace betwee e wire a a ther wires cecte, witht beig aecte by eaage t gr.
BuSHInGS, PoTHEAdS And InSulAToR InSulAToRS S
Figre 32-Shwig se sprig car as a Gar cecti t eimiate the eects srace eaage. The eice er test mst be iscecte rm a ther eipmet.
Figre 30-Cectis r testig isati resistace betwee e wire a gr, witht beig aecte by eaage t ther wires. nte se Gar cecti.
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ouTdooR oIl CIRCuIT BREAkERS The our iustratios (Fig. 35 through 38) show the usua methos o testig bushigs a associate parts o a outoor oi circuit breaer, a the accompaig tabe iicates the test proceure b steps. I the test vaues are beow 10,000 megohms i a o the our steps, the ta shou be owere or raie so that the exc essive osses ca be isoate b urther tests a ivestigatios. I the test vaues are beow 50,000 megohms i test #1, the tre o the coitio o the particuar bushig ivove shou be watche b maig more reuet tests.
Test Breaer Bushig Bushig Bushig Positio Eergize Guare Groue
Figre 33-Cectis r testig isati resistace a trasrmer high tage wiig a bshigs, a the high tesi iscect switch i parae with reerece t gr bt witht beig aecte by eaage betwee the high a w tage wiigs thrgh se the Gar cecti.
Part Measure
1
ope
1 (2 to guar)
1
……
2
ope
1
1
2
3
ope
1&2
1&2
……
Bushig 1 i parae with cross member Bushig 1 & 2 i parae
4
cose
1&2
1&2
……
Bushig 1 & 2 i parae with it ro
Bushig 1
Figre 34-Cectis r testig isati resistace betwee high a w tage wiigs witht beig aecte by eaage t gr.
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ouTdooR oIl CIRCuIT BREAkERS The our iustratios (Fig. 35 through 38) show the usua methos o testig bushigs a associate parts o a outoor oi circuit breaer, a the accompaig tabe iicates the test proceure b steps. I the test vaues are beow 10,000 megohms i a o the our steps, the ta shou be owere or raie so that the exc essive osses ca be isoate b urther tests a ivestigatios. I the test vaues are beow 50,000 megohms i test #1, the tre o the coitio o the particuar bushig ivove shou be watche b maig more reuet tests.
Test Breaer Bushig Bushig Bushig Positio Eergize Guare Groue
Figre 33-Cectis r testig isati resistace a trasrmer high tage wiig a bshigs, a the high tesi iscect switch i parae with reerece t gr bt witht beig aecte by eaage betwee the high a w tage wiigs thrgh se the Gar cecti.
Part Measure
1
ope
1 (2 to guar)
1
……
2
ope
1
1
2
3
ope
1&2
1&2
……
Bushig 1 i parae with cross member Bushig 1 & 2 i parae
4
cose
1&2
1&2
……
Bushig 1 & 2 i parae with it ro
Bushig 1
Figre 34-Cectis r testig isati resistace betwee high a w tage wiigs witht beig aecte by eaage t gr.
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Figre 35-Step 1
Figre 37-Step 3
Figre 36-Step 2
Figre 38-Step 4
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Figre 35-Step 1
Figre 37-Step 3
Figre 36-Step 2
Figre 38-Step 4
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SETTInG uP A MAInTEnAnCE PRoGRAM
HoW oFTEn SHould You TEST?
To start isuatio testig testig there is oe geera rue: Put frst thigs frst. That is, review a the eectrica euipmet a cassi it as to reative importace. For exampe, i that AC motor aie i epartmet A, how wou it aect overa pat prouctio? your prouctio peope ca certai hep i this a shou be vita itereste i the iea.
That epes upo the size a compexit o our pat. Eve ietica uits ca ier i the reuire chec perios; experiece is our best guie. I geera, however, worig apparatus – motors, geerators etc. – are more ie to eveop isuatio weaesses, as compare to wirig, isuators a the ie. A test scheue or worig euipmet shou be estabishe, varig rom ever 6 to 12 moths, epeig o the size o euipmet a severit o the surrouig atmospheric coitios. For wirig a the ie, tests oce a ear are geera sufciet, uess the pat coitios are uusua severe.
I time permits at the start, test ever piece o eectrica euipmet a mae out a recor car. Possib at frst ou wi have to combie severa uits but it wi pa i the og ru to have test recors or each iiviua uit. The, i isuatio weaesses o show up, our job o tracig ow the oeig part wi be easier easier.. Show i Fig. 40 is oe orm o test recor car, which is avaiabe rom Megger.. Foowig is the tpe o iormatio, which becomes more vauabe Megger to ou as tests are repeate at itervas: 1. name a ocatio o the euipmet 2. dates a vaues o test resuts (recor actua reaig at the time o test) 3. Rage, votage, a seria umber o the Megger istrumet use 4. Temperature Temperature o the apparatus (aso, particuar or arge uits, wet a r bub temperatures – or humiit a ew-poit etermiatios) 5. Isuatio resistace measuremet correcte or temperature 6. A pot o cosecutive reaigs to show tre a eabe ou to aticipate aiures
Figre 40
0
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A STITCH IN TIME
SETTInG uP A MAInTEnAnCE PRoGRAM
HoW oFTEn SHould You TEST?
To start isuatio testig testig there is oe geera rue: Put frst thigs frst. That is, review a the eectrica euipmet a cassi it as to reative importace. For exampe, i that AC motor aie i epartmet A, how wou it aect overa pat prouctio? your prouctio peope ca certai hep i this a shou be vita itereste i the iea.
That epes upo the size a compexit o our pat. Eve ietica uits ca ier i the reuire chec perios; experiece is our best guie. I geera, however, worig apparatus – motors, geerators etc. – are more ie to eveop isuatio weaesses, as compare to wirig, isuators a the ie. A test scheue or worig euipmet shou be estabishe, varig rom ever 6 to 12 moths, epeig o the size o euipmet a severit o the surrouig atmospheric coitios. For wirig a the ie, tests oce a ear are geera sufciet, uess the pat coitios are uusua severe.
I time permits at the start, test ever piece o eectrica euipmet a mae out a recor car. Possib at frst ou wi have to combie severa uits but it wi pa i the og ru to have test recors or each iiviua uit. The, i isuatio weaesses o show up, our job o tracig ow the oeig part wi be easier easier.. Show i Fig. 40 is oe orm o test recor car, which is avaiabe rom Megger.. Foowig is the tpe o iormatio, which becomes more vauabe Megger to ou as tests are repeate at itervas: 1. name a ocatio o the euipmet 2. dates a vaues o test resuts (recor actua reaig at the time o test) 3. Rage, votage, a seria umber o the Megger istrumet use 4. Temperature Temperature o the apparatus (aso, particuar or arge uits, wet a r bub temperatures – or humiit a ew-poit etermiatios) 5. Isuatio resistace measuremet correcte or temperature 6. A pot o cosecutive reaigs to show tre a eabe ou to aticipate aiures
Figre 40
0
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MEGGER 5 v & 10 v InSulATIon TESTERS Where critica, high-capita euipmet is ivove, the itrouctio o ew a improve isuatig materias is re-writig the boo o isuatio testig. Euipmet with operatig votages above 1 V reuires commesurate higher test votages. Moer materias, whe ew or ear i their ie cces, ca have isuatio vaues ito rages that were previous umeasure. your o isuatio tester ma ot be u aeuate to meet the emas o a rigorous a thorough program o prevetive/ preictive maiteace o moer euipmet. To be u i coormace with the most moer testig reuiremets, Megger oers a ami o the highest uait isuatio testers at votages above 1 V. At the core o high-votage testig...5 V...the MIT510 a MIT520 aor the highest eve o uait testig aog with prime saet, coveiece a portabiit. MIT510 oers seectabe test votages at 250, 500, 1000, 2500, a 5000 V, thereb maig the tester suitabe or appicatios otherwise perorme with a staar hahe moe, i aitio to the more emaig high-votage appicatios. The MIT520 icreases the uctioait b permittig test votage to be set i a 10 V icremet rom 50 V to 1 V, the i 25 V steps a the wa to 5 V. The testers measure to 15 Tera-o Tera-ohms, hms, thereb maig them u suitabe or critica istaatio tests a estabishmet o reiabe base ata or ew high-capita euipmet. Test resuts are ispae o Megger’s patete eectroic igita/aaog arc, thereb aorig the precisio a assurace o a igita reaig combie with the te-tae poiter trave orma reserve or mechaica movemets.
The ew 5 V isuatio resistace testers rom Megger are esige specifca to assist ou with the testig a maiteace o high votage eectrica euipmet. Mais or batter powere n digita/Aaog bacit ispa n Seectabe test votage rom 250 to 5000V n Automatic IR test n
Measures to 15 T W n Compete with caibratio certifcate n
Megger MIT510
The ew 10 V isuatio resistace testers rom Megger are aso esige specifca to assist ou with the testig a maiteace o high votage eectrica euipmet. A cases are rugge a eas to carr, beig mae o tough popropee a achievig a igress protectio ratig o IP65. Mais or batter powere n digita/aaog bacit ispa
Megger MIT1020
n
Variabe test votage 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
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MEGGER 5 v & 10 v InSulATIon TESTERS Where critica, high-capita euipmet is ivove, the itrouctio o ew a improve isuatig materias is re-writig the boo o isuatio testig. Euipmet with operatig votages above 1 V reuires commesurate higher test votages. Moer materias, whe ew or ear i their ie cces, ca have isuatio vaues ito rages that were previous umeasure. your o isuatio tester ma ot be u aeuate to meet the emas o a rigorous a thorough program o prevetive/ preictive maiteace o moer euipmet. To be u i coormace with the most moer testig reuiremets, Megger oers a ami o the highest uait isuatio testers at votages above 1 V. At the core o high-votage testig...5 V...the MIT510 a MIT520 aor the highest eve o uait testig aog with prime saet, coveiece a portabiit. MIT510 oers seectabe test votages at 250, 500, 1000, 2500, a 5000 V, thereb maig the tester suitabe or appicatios otherwise perorme with a staar hahe moe, i aitio to the more emaig high-votage appicatios. The MIT520 icreases the uctioait b permittig test votage to be set i a 10 V icremet rom 50 V to 1 V, the i 25 V steps a the wa to 5 V. The testers measure to 15 Tera-o Tera-ohms, hms, thereb maig them u suitabe or critica istaatio tests a estabishmet o reiabe base ata or ew high-capita euipmet. Test resuts are ispae o Megger’s patete eectroic igita/aaog arc, thereb aorig the precisio a assurace o a igita reaig combie with the te-tae poiter trave orma reserve or mechaica movemets.
The ew 5 V isuatio resistace testers rom Megger are esige specifca to assist ou with the testig a maiteace o high votage eectrica euipmet. Mais or batter powere digita/Aaog bacit ispa n Seectabe test votage rom 250 to 5000V n Automatic IR test n n
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Measures to 15 T W Compete with caibratio certifcate
Megger MIT510
The ew 10 V isuatio resistace testers rom Megger are aso esige specifca to assist ou with the testig a maiteace o high votage eectrica euipmet. A cases are rugge a eas to carr, beig mae o tough popropee a achievig a igress protectio ratig o IP65. Mais or batter powere n digita/aaog bacit ispa
Megger MIT1020
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Variabe test votage 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
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MEGGER MIT400 SERIES A ew ie o high perormace testers rate to CAT IV 600 V provies greater saet whe testig at higher votage eves. The MIT400 Series oer test votages rom 10 to 1000V, a icues a specia user-seectabe 10 V to 100 V i 1 V icremets tester or ow votage isuatio testig reuiremets. The uits icue a exceptioa isuatio resistace measuremet rage that extes rom 20 Gigohms to 200 Gigohms, with the optio to ispa the isuatio test votage or eaage curret o the secoar ispa. Megger MIT410
Time isuatio testig or PI a dAR testig is avaiabe o certai moes. A 200 mA cotiuit measuremet is avaiabe with a 0.01-ohm resoutio or ast accurate cabe resistace measuremet. This ca be set to 20 mA where coormit to eectrica testig reguatios is ot reuire, exteig batter ie.
For those who preer the traitioa mechaica aaog tpe o istrumet, the MIT310A oers everthig i the MIT310, but with a movig coi ispa. The aaog ispa is ehace b bac ecas o a white bacgrou, to give high cotrast a improve visibiit. The ami is compete b the u-eature MIT320 a MIT330. The ormer as a iohm rage or measuremets betwee the extremes o cotiuit a high votage. With this eature, measuremets ca be mae cotiuous rom 0.01 W to 999 M W. A bacit ispa is urther ehace b bacit seectios, to mae both the setup a resuts visibe i poor ight. The MIT320 aso oers auibe aarms o both the c otiuit a megohm rages, ajustabe across the etire rage. Fia, the MIT330 oers everthig escribe i the MIT320, pus storage o up to 1000 resuts, owoaig through a USB port via icue sotware, a a ispa iicatio o remaiig memor.
MEGGER MIT200 SERIES nee o basic 1 V isuatio a cotiuit testig, with ecoom a paramout cosieratio? no ee to sacrifce! Megger oers the o top uait ie o iexpesive testers o the maret...the MIT200 Series. Though esige with ecoom i mi, these testers oer o oss o the uait a reiabiit that the ame Megger impies. Furthermore, there has bee o sacrifce o saet, which is a serious probem with iexpesive testers that have cut cost at the ris o arc ash a operator error.
MEGGER MIT300 SERIES The MIT300 Series is esige aog Megger’s amiiar pricipe o asceig eatures with o oss o basic uait. Five moes comprise the ami, each buit o a basic esig esurig the uait a reiabiit that are Megger’s traemars. MIT300 is the simpest moe, oerig core uctios o highest uait but o aitioa eatures that ma prove reuat i basic appicatios. For appicatios where a 1 V test must be avoie, the MIT300 oers a 250 a 500 V test o. Combie with cotiuit a votage warig, pus a the coveiece a saet eatures that istiguish the ami, this is the moe o choice or the simpest appicatios. I basic uctio is sti the goa but a 1 V test reuire, the MIT310 is the perect ft. With the possibiit o worig at higher votages, sae operatio has bee urther ehace b the icusio o a eaut votmeter.
Megger MIT230
Four moes are avaiabe, ierig o i the test votages oere: MIT200 is a 500 V moe or staar appicatios; MIT210 is a 1 V moe where higher test votage is i orer; MIT220 oers two votages, 250 a 500, or more sesitive appicatios where a 1 V test is to be avoie; or u-spectrum appicatios, MIT230 oers a three votages, 250, 500 a 1 V.
Megger MIT300 Series
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For those who preer the traitioa mechaica aaog tpe o istrumet, the MIT310A oers everthig i the MIT310, but with a movig coi ispa. The aaog ispa is ehace b bac ecas o a white bacgrou, to give high cotrast a improve visibiit.
MEGGER MIT400 SERIES A ew ie o high perormace testers rate to CAT IV 600 V provies greater saet whe testig at higher votage eves.
The ami is compete b the u-eature MIT320 a MIT330. The ormer as a iohm rage or measuremets betwee the extremes o cotiuit a high votage. With this eature, measuremets ca be mae cotiuous rom 0.01 W to 999 M W. A bacit ispa is urther ehace b bacit seectios, to mae both the setup a resuts visibe i poor ight. The MIT320 aso oers auibe aarms o both the c otiuit a megohm rages, ajustabe across the etire rage.
The MIT400 Series oer test votages rom 10 to 1000V, a icues a specia user-seectabe 10 V to 100 V i 1 V icremets tester or ow votage isuatio testig reuiremets. The uits icue a exceptioa isuatio resistace measuremet rage that extes rom 20 Gigohms to 200 Gigohms, with the optio to ispa the isuatio test votage or eaage curret o the secoar ispa.
Fia, the MIT330 oers everthig escribe i the MIT320, pus storage o up to 1000 resuts, owoaig through a USB port via icue sotware, a a ispa iicatio o remaiig memor.
Megger MIT410
Time isuatio testig or PI a dAR testig is avaiabe o certai moes. A 200 mA cotiuit measuremet is avaiabe with a 0.01-ohm resoutio or ast accurate cabe resistace measuremet. This ca be set to 20 mA where coormit to eectrica testig reguatios is ot reuire, exteig batter ie.
MEGGER MIT200 SERIES nee o basic 1 V isuatio a cotiuit testig, with ecoom a paramout cosieratio? no ee to sacrifce! Megger oers the o top uait ie o iexpesive testers o the maret...the MIT200 Series. Though esige with ecoom i mi, these testers oer o oss o the uait a reiabiit that the ame Megger impies. Furthermore, there has bee o sacrifce o saet, which is a serious probem with iexpesive testers that have cut cost at the ris o arc ash a operator error.
MEGGER MIT300 SERIES The MIT300 Series is esige aog Megger’s amiiar pricipe o asceig eatures with o oss o basic uait. Five moes comprise the ami, each buit o a basic esig esurig the uait a reiabiit that are Megger’s traemars. MIT300 is the simpest moe, oerig core uctios o highest uait but o aitioa eatures that ma prove reuat i basic appicatios. For appicatios where a 1 V test must be avoie, the MIT300 oers a 250 a 500 V test o. Combie with cotiuit a votage warig, pus a the coveiece a saet eatures that istiguish the ami, this is the moe o choice or the simpest appicatios.
Megger MIT230
I basic uctio is sti the goa but a 1 V test reuire, the MIT310 is the perect ft. With the possibiit o worig at higher votages, sae operatio has bee urther ehace b the icusio o a eaut votmeter.
Four moes are avaiabe, ierig o i the test votages oere: MIT200 is a 500 V moe or staar appicatios; MIT210 is a 1 V moe where higher test votage is i orer; MIT220 oers two votages, 250 a 500, or more sesitive appicatios where a 1 V test is to be avoie; or u-spectrum appicatios, MIT230 oers a three votages, 250, 500 a 1 V.
Megger MIT300 Series
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Megger makes more than just insulation resistance testers Megger also makes high quality instruments for the following electrical testing applications: n
Earth/Ground Testing
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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
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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 4271BronzeWay Dallas,TX75237 The word “Megger” is a registered trademark.
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