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.
Trang 1AS ITCHINTIME
W W W M
E G G
E
R
C O M
“A Stitch i Time ”
US $9.95 GBP £5.95
The Cmpete
Gie t Eectrica
Trang 2 A I T C H I N T IM E A S ITC IN TIME
“A Stitch I Time”
The Cmpete Gie t
Eectrica Isati T estig
Copright 2006
W W M
E G
E
C
OM
WHAT IS “Good” InSulATIon?
Ever eectric wire i our pat – whether it’s i a motor, geerator, cabe, switch, 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
To 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 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
Figre 1–Cmparis water w (a) with eectric crret (b).
WHAT IS “GOOd” InSUlATIOn? 3
WHAT MAkES InSUlATIOn GO BAd? 4
HOW InSUlATIOn RESISTAnCE IS MEASUREd 5
HOW TO InTERPRET RESISTAnCE REAdInGS 6
FACTORS AFFECTInG InSUlATIOn RESISTAnCE REAdInGS 8
TyPES OF InSUlATIOn RESISTAnCE TESTS 10
TEST VOlTAGE VS EqUIPMEnT RATInG 16
AC TESTInG VS dC 17
USE OF dC dIElECTRIC TEST SET 18
TESTS dURInG dRyInG OUT OF EqUIPMEnT 18
EFFECT OF TEMPERATURE On InSUlATIOn RESISTAnCE 21 EFFECTS OF HUMIdITy 23
PREPARATIOn OF APPARATUS TO TEST 2
SAFETy PRECAUTIOnS 2
COnnECTIOnS FOR TESTInG InSUlATIOn RESISTAnCE OF ElECTRICAl EqUIPMEnT 27
AddITIOnAl nOTES ABOUT USInG A MEGGER InSUlATIOn TESTER 33
InTERPRETATIOn-MInIMUM VAlUES 36
MInIMUM VAlUES FOR InSUlATIOn RESISTAnCE 38
TESTS USInG MUlTI-VOlTAGE MEGGER InSUlATIOn TESTERS 42
STEP-VOlTAGE METHOd 48
USE OF A GUARd TERMInAl 50
BUSHInGS, POTHEAdS And InSUlATORS 54
OUTdOOR OIl CIRCUIT BREAkERS 57
SETTInG UP A MAInTEnAnCE PROGRAM 60
HOW OFTEn SHOUld yOU TEST? 60 MEGGER 5 And 10 kV InSUlATIOn TESTERS 62 MEGGER 1 kV InSUlATIOn TESTERS 64
Trang 3 A I T C H I N T IM E A S ITC IN TIME
WHAT IS “Good” InSulATIon?
Ever eectric wire i our pat – whether it’s i a motor, geerator, cabe, switch, 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
To 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 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
Figre 1–Cmparis water w (a) with eectric crret (b).
WHAT IS “GOOd” InSUlATIOn? 3
WHAT MAkES InSUlATIOn GO BAd? 4
HOW InSUlATIOn RESISTAnCE IS MEASUREd 5
HOW TO InTERPRET RESISTAnCE REAdInGS 6
FACTORS AFFECTInG InSUlATIOn RESISTAnCE REAdInGS 8
TyPES OF InSUlATIOn RESISTAnCE TESTS 10
TEST VOlTAGE VS EqUIPMEnT RATInG 16
AC TESTInG VS dC 17
USE OF dC dIElECTRIC TEST SET 18
TESTS dURInG dRyInG OUT OF EqUIPMEnT 18
EFFECT OF TEMPERATURE On InSUlATIOn RESISTAnCE 21 EFFECTS OF HUMIdITy 23
PREPARATIOn OF APPARATUS TO TEST 2
SAFETy PRECAUTIOnS 2
COnnECTIOnS FOR TESTInG InSUlATIOn RESISTAnCE OF ElECTRICAl EqUIPMEnT 27
AddITIOnAl nOTES ABOUT USInG A MEGGER InSUlATIOn TESTER 33
InTERPRETATIOn-MInIMUM VAlUES 36
MInIMUM VAlUES FOR InSUlATIOn RESISTAnCE 38
TESTS USInG MUlTI-VOlTAGE MEGGER InSUlATIOn TESTERS 42
STEP-VOlTAGE METHOd 48
USE OF A GUARd TERMInAl 50
BUSHInGS, POTHEAdS And InSUlATORS 54
OUTdOOR OIl CIRCUIT BREAkERS 57
SETTInG UP A MAInTEnAnCE PROGRAM 60
HOW OFTEn SHOUld yOU TEST? 60 MEGGER 5 And 10 kV InSUlATIOn TESTERS 62 MEGGER 1 kV InSUlATIOn TESTERS 64
Oce starte, the ieret eemies te to ai each other, permittig excessive curret through the isuatio
Sometimes the rop i isuatio resistace is sue, as whe euipmet
is ooe Usua, however, it rops 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, 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
o-estructive; that is, it oes ot cause eterioratio o the isuatio
Figre 2–Typica Megger test istrmet h-p t measre isati resistace.
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:
E = I x R where, 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, “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 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 wire,
cabe, motors, a so o have cotiua improve their isuatios or
services i iustr nevertheess, eve toa, 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 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
Trang 4 A I T C H I N T IM E A S ITC IN TIME
Oce starte, the ieret eemies te to ai each other, permittigexcessive curret through the isuatio
Sometimes the rop i isuatio resistace is sue, as whe euipmet
is ooe Usua, however, it rops graua, givig pet o warig,i chece perioica Such checs permit pae recoitioig beoreservice aiure I there are o checs, a motor with poor isuatio, orexampe, ma ot o be agerous to touch whe votage is appie,but aso be subject to bur out What was goo isuatio has become apartia couctor
HoW InSulATIon RESISTAnCE IS MEASuREdyou 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 theisuatio (resistace ecreases i temperature or moisture) With a itterecor-eepig a commo sese, however, ou ca get a goo picture othe 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 gooisuatio, 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 speciacostructio with both curret a votage cois, eabig true ohms to berea irect, iepeet o the actua votage appie This metho is o-
estructive; that is, it oes ot cause eterioratio o the isuatio
Figre 2–Typica Megger test istrmet h-p t measre isati resistace.
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:
E = I x Rwhere, E = votage i vots
I = curret i amperes
R = resistace i ohmsnote, 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, “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 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 wire,
cabe, motors, a so o have cotiua improve their isuatios or
services i iustr nevertheess, eve toa, 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 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
Whether ou test moth, twice a ear, or oce a ear epes upo thetpe, ocatio, a importace o the euipmet For exampe, a smapump 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 oureuipmet
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 orthe same egth o time Aso ou shou mae tests at about the sametemperature, or correct them to the same temperature A recor o thereative humiit ear the euipmet at the time o the test is aso hepui evauatig the reaig a tre later sectios cover temperaturecorrectio a humiit eects
I summar, here are some geera observatios about how ou ca iterpretperioic isuatio resistace tests, a what ou shou o with the resut:
(a) Fair t high aes n case r ccer
a we maitaie
(b) Fair t high aes, lcate a remey the case a
cstat teecytwars wer aes
(c) lw bt we maitaie Citi is prbaby a right, bt
case w aes sh bechece
() S w as t be sae Cea, ry t, r therwise raise the
aes bere pacig eipmet iserice (Test wet eipmet whie
ryig t.)(e) Fair r high aes, Mae tests at reet iteras tipreisy we the case w aes is catemaitaie bt shwig a remeie; r ti the aes
ee bt sae r perati; r ti
aes becme s w that it is saet eep the eipmet i perati
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, 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 4–Typi ca recr isati resistace a mi 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.
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
Trang 5 A I T C H I N T IM E A S ITC IN TIME
Whether ou test moth, twice a ear, or oce a ear epes upo thetpe, ocatio, a importace o the euipmet For exampe, a smapump 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 oureuipmet
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 orthe same egth o time Aso ou shou mae tests at about the sametemperature, or correct them to the same temperature A recor o thereative humiit ear the euipmet at the time o the test is aso hepui evauatig the reaig a tre later sectios cover temperaturecorrectio a humiit eects
I summar, here are some geera observatios about how ou ca iterpretperioic isuatio resistace tests, a what ou shou o with the resut:
(a) Fair t high aes n case r ccer
a we maitaie
(b) Fair t high aes, lcate a remey the case a
cstat teecytwars wer aes
(c) lw bt we maitaie Citi is prbaby a right, bt
case w aes sh bechece
() S w as t be sae Cea, ry t, r therwise raise the
aes bere pacig eipmet iserice (Test wet eipmet whie
ryig t.)(e) Fair r high aes, Mae tests at reet iteras tipreisy we the case w aes is catemaitaie bt shwig a remeie; r ti the aes
ee bt sae r perati; r ti
aes becme s w that it is saet eep the eipmet i perati
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, 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 4–Typi ca recr isati resistace a mi 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.
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 5–Cres shwig cmpets crret measre rig dC testig isati.
note aso i Fig 5 that the chargig curret isappears reative rapi
as the euipmet uer test becomes charge larger uits with morecapacitace wi tae oger to be charge This curret aso is the storeeerg iitia ischarge ater our test, b short-circuitig a grouigthe isuatio.AlWAYS TAkE THIS SAFETY MEASuRE
you ca see urther rom Fig 5 that the absorptio curret ecreases at areative sow rate, epeig upo the exact ature o the isuatio Thisstore 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 astea vaue that is costat or the appie votage, as show i Fig 5 Aicrease o eaage curret with time is a warig o troube, as iscusse ithe tests escribe i the oowig sectio
FACToRS AFFECTInG InSulATIon 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, 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 ca 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
Trang 6 A I T C H I N T IM E A S ITC IN TIME
Figre 5–Cres shwig cmpets crret measre rig dC testig isati.
note aso i Fig 5 that the chargig curret isappears reative rapi
as the euipmet uer test becomes charge larger uits with morecapacitace wi tae oger to be charge This curret aso is the storeeerg iitia ischarge ater our test, b short-circuitig a grouigthe isuatio.AlWAYS TAkE THIS SAFETY MEASuRE
you ca see urther rom Fig 5 that the absorptio curret ecreases at areative sow rate, epeig upo the exact ature o the isuatio Thisstore 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 astea vaue that is costat or the appie votage, as show i Fig 5 Aicrease o eaage curret with time is a warig o troube, as iscusse ithe tests escribe i the oowig sectio
FACToRS AFFECTInG InSulATIon 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, 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 ca 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
I the apparatus ou are testig has ver sma capacitace, such as ashort ru o house wirig, the spot reaig test is a that is ecessar.However, most euipmet is capacitive a so our ver frst spot reaigo 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, maiteaceproessioas 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 1,000volts of operating voltage, with a minimum value of one megohm
For exampe, a motor rate at 2,400 vots shou have a miimum isuatioresistace o 2.4 megohms I practice, megohm reaigs orma arecosierab 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 basiso jugig the actua isuatio coitio A persistet owwar tre
is usua air warig o troube ahea, eve though the reaigs ma behigher tha the suggeste miimum sae vaues Eua true, as og as
our perioic reaigs are cosistet, the ma be o, eve though owertha the recommee miimum vaues The curves o Fig 7 show tpicabehavior o isuatio resistace uer varig pat operatig coitios.The curves were potte rom spot reaigs tae with a Megger istrumetover a perio o moths
With a bacgrou ow o how time aects the meaig o istrumet
reaigs, et’s cosier three commo test methos: (1) short-time or spot
reaig; (2) time-resistace; a (3) step or muti-votage tests
TYPES oF InSulATIon 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, 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 6–Typica cre isati resistace (i meghms) with time r the “shrt time” r
“spt-reaig” test meth.
Trang 70 ASTITCHIN TIME A S ITC IN TIME
I the apparatus ou are testig has ver sma capacitace, such as ashort ru o house wirig, the spot reaig test is a that is ecessar.However, most euipmet is capacitive a so our ver frst spot reaigo 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, maiteaceproessioas 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 1,000volts of operating voltage, with a minimum value of one megohm
For exampe, a motor rate at 2,400 vots shou have a miimum isuatioresistace o 2.4 megohms I practice, megohm reaigs orma arecosierab 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 basiso jugig the actua isuatio coitio A persistet owwar tre
is usua air warig o troube ahea, eve though the reaigs ma behigher tha the suggeste miimum sae vaues Eua true, as og as
our perioic reaigs are cosistet, the ma be o, eve though owertha the recommee miimum vaues The curves o Fig 7 show tpicabehavior o isuatio resistace uer varig pat operatig coitios.The curves were potte rom spot reaigs tae with a Megger istrumetover a perio o moths
With a bacgrou ow o how time aects the meaig o istrumet
reaigs, et’s cosier three commo test methos: (1) short-time or spot
reaig; (2) time-resistace; a (3) step or muti-votage tests
TYPES oF InSulATIon 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, 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 6–Typica cre isati resistace (i meghms) with time r the “shrt time” r
“spt-reaig” test meth.
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 othe absorptio eect o goo isuatio compare to that o moist orcotamiate isuatio yousimp tae successive reaigs at specifc timesa ote the iereces i reaigs (see curves, Fig 8) Tests b this methoare sometimes reerre to as absorptio tests
note that goo isuatio shows a cotiua icrease i resistace (esscurret – 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 isuatioshows this charge eect over a time perio much oger tha the timereuire to charge the capacitace o the isuatio
I the isuatio cotais much moisture or cotamiats, the absorptioeect is mase b a high eaage curret which stas at a air costatvaue, 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.
The time-resistace test is o vaue aso because it is iepeet o
euipmet size The icrease i resistace or cea a r isuatio occursi the same maer whether a motor is arge or sma you ca, thereore,compare severa motors a estabish staars or ew oes, regaress otheir horsepower ratigs
Figre 7–Typica behair isati resistace er a peri mths er aryig peratig
citis, (cres ptte rm spt reaigs with a Megger istrmet).
Trang 8 ASTITCHIN TIME A S ITC 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 othe absorptio eect o goo isuatio compare to that o moist orcotamiate isuatio yousimp tae successive reaigs at specifc timesa ote the iereces i reaigs (see curves, Fig 8) Tests b this methoare sometimes reerre to as absorptio tests
note that goo isuatio shows a cotiua icrease i resistace (esscurret – 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 isuatioshows this charge eect over a time perio much oger tha the timereuire to charge the capacitace o the isuatio
I the isuatio cotais much moisture or cotamiats, the absorptioeect is mase b a high eaage curret which stas at a air costatvaue, 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.
The time-resistace test is o vaue aso because it is iepeet o
euipmet size The icrease i resistace or cea a r isuatio occursi the same maer whether a motor is arge or sma you ca, thereore,compare severa motors a estabish staars or ew oes, regaress otheir horsepower ratigs
Figre 7–Typica behair isati resistace er a peri mths er aryig peratig
citis, (cres ptte rm spt reaigs with a Megger istrmet).
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 isuseu i recorig iormatio about isuatio I the ratio is a 10-miutereaig ivie b a 1-miute reaig, the vaue is cae the poarizatioiex
With ha-crae Megger istrumets, it’s a ot easier or ou to ru thetest 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, to get the poarizatioiex Tabe I gives vaues o the ratios a correspoig reativecoitios o the isuatio that the iicate
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
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
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
InSUlATIOnCOndITIOn 60/30-SECOndRATIO (POlARIZATIO10/1-MInUTE RATIOn IndEX)
dagerousquestioabeGoo
Exceet
—1.0 to 1.251.4 to 1.6Above 1.6**
less tha 11.0 to 2***
2 to 4Above 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 wiigwhich wi ai uer shoc coitios or urig starts Forprevetive maiteace, the motor wiig shou beceae,treate, a rie to restore wiig exibiit
***These resuts wou be satisactor or euipmet withver ow capacitace such as short rus o house wirig
TABLE I—Condition of Insulation Indicated by
Dielectric Absorption Ratios*
Trang 9 ASTITCHIN TIME A S ITC IN TIME
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 isuseu i recorig iormatio about isuatio I the ratio is a 10-miutereaig ivie b a 1-miute reaig, the vaue is cae the poarizatioiex
With ha-crae Megger istrumets, it’s a ot easier or ou to ru thetest 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, to get the poarizatioiex Tabe I gives vaues o the ratios a correspoig reativecoitios o the isuatio that the iicate
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
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
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
InSUlATIOnCOndITIOn 60/30-SECOndRATIO (POlARIZATIO10/1-MInUTE RATIOn IndEX)
dagerousquestioabeGoo
Exceet
—1.0 to 1.251.4 to 1.6Above 1.6**
less tha 11.0 to 2***
2 to 4Above 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 wiigwhich wi ai uer shoc coitios or urig starts Forprevetive maiteace, the motor wiig shou beceae,treate, a rie to restore wiig exibiit
***These resuts wou be satisactor or euipmet withver ow capacitace such as short rus o house wirig
TABLE I—Condition of Insulation Indicated by
Dielectric Absorption Ratios*
AC TESTInG vS dC
Up to ow, we’ve tae about testig with dC votage, but ou wi hearo AC testig a ee to ow the ierece Remember that we spoeo the is o curret prouce i isuatio b dC? (The iitia surge ochargig curret, the rop with time to absorptio curret, a the, atermore 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
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 somespecife 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, icotrast to the dC test which is basica o-estructive
I a AC test votage has bee use a ou wat to use dC tests asa aterative, ou wi ee to icrease the maximum dC test votagesomewhat 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 eitherpasses or oes’t pass the test With the dC test, ou get a more uaitativepicture; ou ca meter the eaage curret as ou icrease the votage aobtai specifc vaues o isuatio resistace
As the size o our euipmet icreases, there are aso mare ecoomicavatages i dC over AC testig As the test votage icreases, both thecost 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 chargigcurret which becomes a remais ver high i the arger machies Asexpaie previous, i dC testig, this curret rops rapi ater the iitiachargig perio
TEST volTAGE vS EquIPMEnT RATInG
Commo use dC test votages or routie maiteace are as oows:
Eipmet AC Ratig dC Test vtage
up to 100 vots 100 a 250 vots
440 to 550 vots 500 a 1,000 vots2,400 vots 1,000 to 2,500 vots
or higher4,160 vots a above 1,000 to 5,000 vots,
or higherTest 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 namepate 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
Trang 10 ASTITCHIN TIME A S ITC IN TIME
AC TESTInG vS dC
Up to ow, we’ve tae about testig with dC votage, but ou wi hearo AC testig a ee to ow the ierece Remember that we spoeo the is o curret prouce i isuatio b dC? (The iitia surge ochargig curret, the rop with time to absorptio curret, a the, atermore 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
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 somespecife 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, icotrast to the dC test which is basica o-estructive
I a AC test votage has bee use a ou wat to use dC tests asa aterative, ou wi ee to icrease the maximum dC test votagesomewhat 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 eitherpasses or oes’t pass the test With the dC test, ou get a more uaitativepicture; ou ca meter the eaage curret as ou icrease the votage aobtai specifc vaues o isuatio resistace
As the size o our euipmet icreases, there are aso mare ecoomicavatages i dC over AC testig As the test votage icreases, both thecost 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 chargigcurret which becomes a remais ver high i the arger machies Asexpaie previous, i dC testig, this curret rops rapi ater the iitiachargig perio
TEST volTAGE vS EquIPMEnT RATInG
Commo use dC test votages or routie maiteace are as oows:
Eipmet AC Ratig dC Test vtage
up to 100 vots 100 a 250 vots
440 to 550 vots 500 a 1,000 vots2,400 vots 1,000 to 2,500 vots
or higher4,160 vots a above 1,000 to 5,000 vots,
or higherTest 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 namepate 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
There are various was to r out eectrica euipmet, epeig upoits size a portabiit you ca use a bast o hot air, a ove, circuatioo curret through couctors, or a combiatio o techiues loca patcoitios a aciities, together with iormatio rom the euipmetmauacturers, ca serve as a guie to the best metho or our particuareuipmet
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 oprevious tests o the apparatus Whe rig out is reuire, such recorsare aso hepu to etermie whe the isuatio is ree o moisture.noTE: Wet eipmet is ssceptibe t tage breaw Therere, ysh se a w-tage Megger tester (100 r 250 vdC), at easti the eary stages a ryig-t r I a w-tage istrmet
is t reaiy aaiabe, sw craig a 500-t tester may besbstitte
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 mitrigthe ryig prcess I a ihm measremet is btaie, isatihas bee thrghy satrate, bt may be recaimabe Aterateytest a ry, watchig the reaigs rise ti they reach the Meghmrage, 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-hpmotor that’s bee ooe Ater a cea-up, a spot reaig with the Meggertester shows 1.5 megohms Oha, ou’ probab sa this is o What’smore, 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 toru 10 or 20 megohms, the ou wou ow that water was sti i themotor wiigs
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, both i uait a 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,
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 resistace b the
same test methos as outie or the Megger istrumet; that is, the
short-time, 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
Trang 11 ASTITCHIN TIME A S ITC IN TIME
There are various was to r out eectrica euipmet, epeig upoits size a portabiit you ca use a bast o hot air, a ove, circuatioo curret through couctors, or a combiatio o techiues loca patcoitios a aciities, together with iormatio rom the euipmetmauacturers, ca serve as a guie to the best metho or our particuareuipmet
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 oprevious tests o the apparatus Whe rig out is reuire, such recorsare aso hepu to etermie whe the isuatio is ree o moisture.noTE: Wet eipmet is ssceptibe t tage breaw Therere, ysh se a w-tage Megger tester (100 r 250 vdC), at easti the eary stages a ryig-t r I a w-tage istrmet
is t reaiy aaiabe, sw craig a 500-t tester may besbstitte
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 mitrigthe ryig prcess I a ihm measremet is btaie, isatihas bee thrghy satrate, bt may be recaimabe Aterateytest a ry, watchig the reaigs rise ti they reach the Meghmrage, 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-hpmotor that’s bee ooe Ater a cea-up, a spot reaig with the Meggertester shows 1.5 megohms Oha, ou’ probab sa this is o What’smore, 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 toru 10 or 20 megohms, the ou wou ow that water was sti i themotor wiigs
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, both i uait a 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,
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 resistace b the
same test methos as outie or the Megger istrumet; that is, the
short-time, 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
EFFECT oF TEMPERATuRE on InSulATIon RESISTAnCEThe resistace o isuatig materias ecreases mare with a icrease itemperature As we’ve see, however, tests b the time-resistace a step-votage methos are reative iepeet o temperature eects, givigreative vaues
I ou wat to mae reiabe comparisos betwee reaigs, ou shoucorrect the reaigs to a base temperature, such as 20°C, or tae a ourreaigs 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 / 2megohm
at 40°C
Each tpe o isuatig materia wi have a ieret egree o resistacechage with temperature Factors have bee eveope, however, tosimpi the correctio o resistace vaues Tabe II gives such actors orrotatig euipmet, trasormers a cabe you mutip the reaigs ouget b the actor correspoig to the temperature (which ou ee tomeasure)
For exampe, assume ou have a motor with Cass A isuatio a ou get areaig 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) aobtai the actor 4.80 your correcte vaue o resistace is the:
(reaig (correctio (correcte
Cass A 68°F or 20°C)isuatio
at 104°F)
Figre 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, 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
time-resistace 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)
Trang 120 ASTITCHIN TIME A S ITC IN TIME
EFFECT oF TEMPERATuRE on InSulATIon RESISTAnCEThe resistace o isuatig materias ecreases mare with a icrease itemperature As we’ve see, however, tests b the time-resistace a step-votage methos are reative iepeet o temperature eects, givigreative vaues
I ou wat to mae reiabe comparisos betwee reaigs, ou shoucorrect the reaigs to a base temperature, such as 20°C, or tae a ourreaigs 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 / 2megohm
at 40°C
Each tpe o isuatig materia wi have a ieret egree o resistacechage with temperature Factors have bee eveope, however, tosimpi the correctio o resistace vaues Tabe II gives such actors orrotatig euipmet, trasormers a cabe you mutip the reaigs ouget b the actor correspoig to the temperature (which ou ee tomeasure)
For exampe, assume ou have a motor with Cass A isuatio a ou get areaig 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) aobtai the actor 4.80 your correcte vaue o resistace is the:
(reaig (correctio (correcte
Cass A 68°F or 20°C)isuatio
at 104°F)
Figre 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, 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
time-resistace 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)
nomograph o temperature correctio vaues or Megger reaigs(correcte to 25°C) For rotatig machier with Cass B isuatio
EFFECTS oF HuMIdITY
We have spoe at various poits i this maua about the presece omoisture i isuatio a its ver mare eect upo resistace vaues youmight expect, thereore, that icreasig humiit (moisture cotet) i thesurrouig (ambiet) air cou aect isuatio resistace A it ca tovarig egrees
I our euipmet operates reguar above the ew-poit temperature (thetemperature at which the moisture vapor i air coeses as a iui), thetest reaig orma wi ot be aecte much b the humiit Eve i theeuipmet to be teste is ie, the same is true – so og as its temperature is
ept above the ew poit
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*
T MP ROTATInGEqUIP. CABl S
°
C F ° C l A S
A C l A S B O I
l F I l d T R A S O R M E R S
- C O E n T U R A l C O E G
R S P R F O R M A C E
- n T U R A l H E A T R E I
S . n T U R A l H E A T R E I
S . &
P R F O R
M . G
R S
- O Z O E R E I
S . n T U R A l G
R S
- V A R n I S H E d C A M B I C I M P R E G A T d
P A P R
0.25 0.36 0.50 0.74 1.00 1.40 1.98 2.80 3.95 5.60 7.85 11.20 15.85 22.40 31.75 44.70
0.25 0.40 0.61 1.00 1.47 2.27 3.52 5.45 8.45 13.10 20.00
0.12 0.23 0.46 1.00 1.83 3.67 7.32 14.60 29.20 54.00 116.00
0.47 0.60 0.76 1.00 1.24 1.58 2.00 2.55 3.26 4.15 5.29 6.72 8.58
0.42 0.56 0.73 1.00 1.28 1.68 2.24 2.93 3.85 5.08 6.72 8.83 11.62 15.40 20.30 26.60
0.22 0.37 0.58 1.00 1.53 2.48 4.03 6.53 10.70 17.10 27.85 45.00 73.00 118.00 193.00 313.00
0.14 0.26 0.49 1.00 1.75 3.29 6.20 11.65 25.00 41.40 78.00
0.10 0.20 0.43 1.00 1.94 4.08 8.62 18.20 38.50 81.00 170.00 345.00 775.00
0.28 0.43 0.64 1.00 1.43 2.17 3.20 4.77 7.15 10.70 16.00 24.00 36.00
*Corrected to 20 °C for rotating equipment and transformers; 15.6°C for cable.
Trang 13 ASTITCHIN TIME A S ITC IN TIME
nomograph o temperature correctio vaues or Megger reaigs(correcte to 25°C) For rotatig machier with Cass B isuatio
EFFECTS oF HuMIdITY
We have spoe at various poits i this maua about the presece omoisture i isuatio a its ver mare eect upo resistace vaues youmight expect, thereore, that icreasig humiit (moisture cotet) i thesurrouig (ambiet) air cou aect isuatio resistace A it ca tovarig egrees
I our euipmet operates reguar above the ew-poit temperature (thetemperature at which the moisture vapor i air coeses as a iui), thetest reaig orma wi ot be aecte much b the humiit Eve i theeuipmet to be teste is ie, the same is true – so og as its temperature is
ept above the ew poit
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*
T MP ROTATInGEqUIP. CABl S
°
C F ° C l A S
A C l A S B O I
l F I l d T R A S O R M E R S
- C O E n T U R A l C O E G
R S P R F O R M A C E
- n T U R A l H E A T R E I
S . n T U R A l H E A T R E I
S . &
P R F O R
M . G
R S
- O Z O E R E I
S . n T U R A l G
R S
- V A R n I S H E d C A M B I C I M P R E G A T d
P A P R
0.25 0.36 0.50 0.74 1.00 1.40 1.98 2.80 3.95 5.60 7.85 11.20 15.85 22.40 31.75 44.70
0.25 0.40 0.61 1.00 1.47 2.27 3.52 5.45 8.45 13.10 20.00
0.12 0.23 0.46 1.00 1.83 3.67 7.32 14.60 29.20 54.00 116.00
0.47 0.60 0.76 1.00 1.24 1.58 2.00 2.55 3.26 4.15 5.29 6.72 8.58
0.42 0.56 0.73 1.00 1.28 1.68 2.24 2.93 3.85 5.08 6.72 8.83 11.62 15.40 20.30 26.60
0.22 0.37 0.58 1.00 1.53 2.48 4.03 6.53 10.70 17.10 27.85 45.00 73.00 118.00 193.00 313.00
0.14 0.26 0.49 1.00 1.75 3.29 6.20 11.65 25.00 41.40 78.00
0.10 0.20 0.43 1.00 1.94 4.08 8.62 18.20 38.50 81.00 170.00 345.00 775.00
0.28 0.43 0.64 1.00 1.43 2.17 3.20 4.77 7.15 10.70 16.00 24.00 36.00
*Corrected to 20° C for rotating equipment and transformers; 15.6° C for cable.
Megger istrumet i use, i which case othig wou be gaie bseparatig the compoets, because the isuatio resistace o each partwou be sti higher
For a iitia test, it ma be ecessar to separate the compoet parts, evethough abor a expese are ivove, a test each oe separate Asomae a test o a the compoets coecte together With this iormatioo recor, it ma ot be ecessar to separate the compoets o uturetests uess uaccoutab ow reaigs are observe
3 discharge Capacitace
It is ver importat that capacitace be ischarge, both beore a atera isuatio resistace test It shou be ischarge or a perio about ourtimes as og as test votage was appie i a previous test
Megger istrumets are reuet euippe with ischarge circuits or thispurpose I a ischarge uctio is ot provie, a ischarge stic shou beuse leave high capacitive apparatus (i.e., capacitors, arge wiigs, etc.)short circuite uti rea to re-eergize
4 Crret leaage at SwitchesWhe apparatus is shut ow or the isuatio resistace test, mae surethat the reaigs are ot aecte b eaage over or through switches or
use bocs, etc Such eaage ma mas the true isuatio resistace o theapparatus uer test See Use o a Gar Termia, page 50
Or, what ma be more serious, curret rom a eergize ie ma ea itothe apparatus a cause icosistet reaigs, particuar i the ive ie is
dC However, such eaage usua ca be etecte b watchig the poitero the Megger istrumet at the momet the test eas are coecte tothe apparatus a beore the istrumet is operate Beore maig theseobservatios, be sure that a capacitace is ischarge b short circuitig orgrouig the apparatus
CAUTION:Never connect a Megger insulation tester to energizedlines or equipment Never use the tester or any of its leads oraccessories for any purpose not described in this book
The aforementioned statement assumes that the insulation surfaces
are free of contaminants, such as certain lints and acids or salts,
which have the property of absorbing moisture (they’re called
“hygroscopic” or “deliquescent” materials by chemists) Their
presence could unpredictably affect your readings; they should be
removed before tests are made
I 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, 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 SeeSaety 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
Trang 14 ASTITCHIN TIME A S ITC IN TIME
Megger istrumet i use, i which case othig wou be gaie bseparatig the compoets, because the isuatio resistace o each partwou be sti higher
For a iitia test, it ma be ecessar to separate the compoet parts, evethough abor a expese are ivove, a test each oe separate Asomae a test o a the compoets coecte together With this iormatioo recor, it ma ot be ecessar to separate the compoets o uturetests uess uaccoutab ow reaigs are observe
3 discharge Capacitace
It is ver importat that capacitace be ischarge, both beore a atera isuatio resistace test It shou be ischarge or a perio about ourtimes as og as test votage was appie i a previous test
Megger istrumets are reuet euippe with ischarge circuits or thispurpose I a ischarge uctio is ot provie, a ischarge stic shou beuse leave high capacitive apparatus (i.e., capacitors, arge wiigs, etc.)short circuite uti rea to re-eergize
4 Crret leaage at SwitchesWhe apparatus is shut ow or the isuatio resistace test, mae surethat the reaigs are ot aecte b eaage over or through switches or
use bocs, etc Such eaage ma mas the true isuatio resistace o theapparatus uer test See Use o a Gar Termia, page 50
Or, what ma be more serious, curret rom a eergize ie ma ea itothe apparatus a cause icosistet reaigs, particuar i the ive ie is
dC However, such eaage usua ca be etecte b watchig the poitero the Megger istrumet at the momet the test eas are coecte tothe apparatus a beore the istrumet is operate Beore maig theseobservatios, be sure that a capacitace is ischarge b short circuitig orgrouig the apparatus
CAUTION:Never connect a Megger insulation tester to energizedlines or equipment Never use the tester or any of its leads oraccessories for any purpose not described in this book
The aforementioned statement assumes that the insulation surfaces
are free of contaminants, such as certain lints and acids or salts,
which have the property of absorbing moisture (they’re called
“hygroscopic” or “deliquescent” materials by chemists) Their
presence could unpredictably affect your readings; they should be
removed before tests are made
I 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, 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 SeeSaety 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
Slight sparking may be encountered:
(1) Whe attachig the test eas to euipmet i which the capacitacehas ot bee compete ischarge
(2) durig a test, arcig through or over aut isuatio
(3) Foowig a test whe capacitace is ischarge
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 TESTInG InSulATIon RESISTAnCE
oF ElECTRICAl EquIPMEnTThe oowig iagrams show how to coect a Megger isuatio tester tovarious tpes o eectrica euipmet The iagrams aso show i pricipehow euipmet must be iscoecte rom other circuits beore theistrumet is coecte
These iustratios are tpica a wi serve as guies or testig isuatioresistace 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 measreswhateer resistace is cecte betwee its termias This mayice series r parae eaage paths thrgh isati r erits srace
SAFETY PRECAuTIonS
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
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 oPreparati 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 There is, however, a hazar whe testig euipmet
ocate i iammabe or exposive atmospheres
Trang 15 ASTITCHIN TIME A S ITC IN TIME
Slight sparking may be encountered:
(1) Whe attachig the test eas to euipmet i which the capacitacehas ot bee compete ischarge
(2) durig a test, arcig through or over aut isuatio
(3) Foowig a test whe capacitace is ischarge
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 TESTInG InSulATIon RESISTAnCE
oF ElECTRICAl EquIPMEnTThe oowig iagrams show how to coect a Megger isuatio tester tovarious tpes o eectrica euipmet The iagrams aso show i pricipehow euipmet must be iscoecte rom other circuits beore theistrumet is coecte
These iustratios are tpica a wi serve as guies or testig isuatioresistace 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 measreswhateer resistace is cecte betwee its termias This mayice series r parae eaage paths thrgh isati r erits srace
SAFETY PRECAuTIonS
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
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 oPreparati 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 There is, however, a hazar whe testig euipmet
ocate i iammabe or exposive atmospheres
Coectios or testig the isuatio resistace o a 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
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
Trang 16 ASTITCHIN TIME A S ITC IN TIME
Coectios or testig the isuatio resistace o a 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
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
I6 Pwer CabesCoectios or testig the isuatio resistace o a power cabe Whetestig cabe, it is usua best to iscoect at both es i orer to testthe cabe b itse, a to avoi error ue to eaage across or throughswitchboars or paeboars See aso use Gar Termia, page 50
Figre 17
Figre 18
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
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
5 Ctr, Sigaig & Cmmicati Cabes
Figre 16
Coectios or testig isuatio resistace o oe wire i a
muti-couctor cabe agaist a other wires a sheath coecte together
Trang 170 ASTITCHIN TIME A S ITC IN TIME
I6 Pwer CabesCoectios or testig the isuatio resistace o a power cabe Whetestig cabe, it is usua best to iscoect at both es i orer to testthe cabe b itse, a to avoi error ue to eaage across or throughswitchboars or paeboars See aso use Gar Termia, page 50
Figre 17
Figre 18
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
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
5 Ctr, Sigaig & Cmmicati Cabes
Figre 16
Coectios or testig isuatio resistace o oe wire i a
muti-couctor cabe agaist a other wires a sheath coecte together
AddITIonAl noTES ABouT uSInG A MEGGER InSulATIon TESTERTestig leas
Ierior or eective testig eas wi cause erroeous a miseaig resutso isuatio resistace tests Tae care i this respect
uisate leas
To avoi error ue to the isuatio o eas, pace the Megger istrumetcose to the ugroue termia or couctor o the apparatus uer testa coect a short piece o ight bare wire irect rom the lie termiao the istrumet to the apparatus I the Guar termia is use, it ma betreate simiar no 18 or 20 gauge, soi wire wi sufce Support the eao b its coectios to the istrumet a the apparatus
With this metho o coectig rom the lie termia, the uait o theisuatio, i a, o the Earth or Grou ea becomes uimportat.Isate leas
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, sige-couctor no 14 strae wire is recommee.The outer jacet shou be smooth, with o outer brai lugs shou beftte or attachig to the istrumet termias, a stout sprig cips arerecommee or coectig to the apparatus or circuit uer test Acoveiet egth o ea ma be use Joits are to be avoie
Ater coectig the eas to the istrumet, a just beore coectigthem to the apparatus, mae sure there is o ea rom ea to ea do thisb operatig the istrumet, which shou rea Ifit do ot correct sight
ea eaage b attemptig to reset the Ifit Ajuster o a high-rageistrumet The touch the test es o the eas together to mae surethe are ot iscoecte or broe
Curret testig with the high-rage (50,000 megohms) Megger isuatiotesters 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 shiecoecte to Guar, prevets a eaages over its termiatios or throughthe ea isuatig materia, rom beig measure
7 Pwer Trasrmers
Figre 19
Coectios or testig isuatio resistace o a trasormer high votage
wiig a bushigs, a the high tesio iscoect switch, i parae,
with reerece to the ow votage wiig a grou note that the ow
votage wiig is groue or this test
8 AC Geeratrs
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