megger insulation_testing.pdf

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A Guide To DiagnosticInsulation TestingAbove 1 kV


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Table of Cont ent s

INTRODUCTION ............................................................................................................ 2WHAT IS INSULATION ? .................................................................................................. 3

Wha t Causes Insulat ion to Deg rad e? .............................................................................. 3Electrical Stress.............................................................................................................. 3Mecha nical Stress.......................................................................................................... 3Chemica l Atta ck ............................................................................................................ 3Therm a l Stress............................................................................................................... 4Environm ent a l Cont a mina tio n .................................................................................... 4

How Ca n Pred ictive Ma inte na nce Help Me? .................................................................. 4

The Bene fit of New Techno log y ...................................................................................... 5HOW INSULATION RESISTANCE IS M EASURED ...................................................................... 6How a n Insulat ion Resista nce Tester Ope rat es ............................................................... 6Compo nen ts of Test Current ........................................................................................... 6

Capa citive Cha rg ing Current ........................................................................................ 6Absorpt ion or Pola rizat ion Current ............................................................................. 6Surfa ce Lea kag e Curren t .............................................................................................. 7Cond uction Current ...................................................................................................... 7

Conn ecting you r Insulat ion Tester .................................................................................. 8Selected Typical Conn ectio ns ........................................................................................... 9

Shielded Pow er Ca ble ................................................................................................... 9Circuit Brea ker/Bushings .............................................................................................. 9Pow er Tran sform er ..................................................................................................... 10AC Ge nera to r .............................................................................................................. 10

Insulat ion Resista nce Teste r Scale ................................................................................. 11Volt a g e Chara cteristics .................................................................................................. 12

EVALUATION AND INTERPRETATION OF RESULTS .................................................................. 13Interpretat ion of the Infinity ( ∞) Read ing .................................................................... 13

DIAGNOSTIC HIGH VOLTAGE INSULATION TESTS ................................................................ 15Spot Read ing Test ........................................................................................................... 15

Time vs. Resista nce Test ............. .............. .............. .............. ............. .............. .............. .. 17Pola rizat ion Index Test ................................................................................................... 18Step Volt a g e Test ............................................................................................................ 20Dielectric Discha rge Test ................................................................................................ 21Diffe rent Prob lems/Diffe rent Tests ............................................................................... 23

APPENDICES .............................................................................................................. 24Pot ent ial So urces of Error/Ensuring Qua lity Test Results ............................................. 24

Test Lea ds .................................................................................................................... 24Making Mea suremen ts abo ve 100 G Ω .............................................................................. 24Accuracy Sta te me nt s .................................................................................................. 24Delivery of Sta te d Volt a g e ......................................................................................... 24Interf eren ce Rejectio n ................................................................................................ 25Rules on Testing a nd Comp aring ............................................................................... 25

Testing Insulatio n Resista nce on Rot a ting Ma chinery ................................................. 26The Gua rd Termina l ....................................................................................................... 29Eff ects of Tem pera ture .................................................................................................. 31Eff ects of Humidity ........................................................................................................ 33

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NOTES I NTRODUCTION

Electrical insulation deg rade s over a period of t ime b ecause of w hich a re imposed upo n it d uring its norma l w orking life. Thebeen d esign ed to w ithsta nd th ese stresses fo r a period o f years, rega rded a s the w orking life o f t ha t insulation. This of ten runs

Abno rmal stresses can bring a bo ut a n increase in this na tural a gcan severely shorten the w orking life o f t he insulat ion. For th is rpractice to perform regular testing to identify w hethe r increa sedplace and , if possible, to identify w heth er the eff ects ma y be re

The purpo se of d iag no stic insulat ion te sting is:

To iden tify increa sed a g ing.

To identify the cause of this ag ing.

To iden tify, if possible, the mo st appro priate a ctions to correc

In its simplest fo rm diag nostic testing ta kes the fo rm of a “electrical maintenance professionals have made spot tests wha pplied to t he insula tion a nd a resistan ce is mea sured . The d iag nis l imited to “ the insulat ion is go od” or “ the insulat ion is ba d.” this diagno sis w hat do w e do a bout it? It ’s a bi t like going to tba d cough and the d octor simply te l ling you, “ You’ve got a bw ouldn’t be ha ppy to come a w ay w ith only that informat iondoctor to examine you, carry out a few tes ts, and te l l you w hycough a nd w ha t to do a bou t i t to cure the cough .

In insulation testing, a spot test on its ow n is the eq uivalent o f thyou t ha t yo u a re w ell or you a re sick. It’s minima l inf orma tion . Ttest th at is typically applied t o low -voltag e circuits w here the c

low an d eq uipment can be replaced ea sily and inexpensively. ment b eing tested is low voltag e eq uipment, these tests are typiusing a 500 or 1000 V test volta ge an d w ill be f am iliar to all ena nce personne l.

How ever, if the d octor records the results of his examination andw ith those from previous visits, then a trend m ight b e appa relead to medication being prescribed. Similarly, if insulation resare recorded a nd compa red w ith previously obta ined reading ssible to see a trend an d t o prescribe remedial actions if such are

Diagn ostic insulation t esting at voltag es abo ve 1 kV is an area iar to ma ny electrical ma intena nce person nel. The purpose of t hisfo re, is to:

• Acquaint the reader w ith ma king d iag nost ic insulat ion resist

• Provide g uidel ines for evaluat ing the results of t hese diag


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This bo oklet is ba sed on th e principles esta blished in the b oo klet “A Stit ch in Tim e… The Com plet e Guide t o Elect rical Insulat ion Test ing ” first published in1966 by the James G. Biddle Company.

W HAT IS I NSULATION ?

Every electric w ire in a f a cility, wh eth er it’s in a mo to r, ge nera to r, cable, sw itch,tran sfo rmer, or w ha tever is covered w ith some fo rm of electrical insulation.While th e w ire itself is a g oo d cond uctor (usually ma de o f copper or a luminum)of the electric current that powers electrical equipment, the insulation mustresist current a nd keep t he current in i ts path a long t he conducto r. Understa nd-ing Ohm’s Law , w hich is expressed in th e fo llow ing eq uat ion, is the key to un-

derstanding insulation testing:E = I x R

where ,

E = voltag e in volts

I = current in a mperes

R = resista nce in ohms

For a given resista nce, the higher th e voltag e, the grea ter the current. Alterna-tively, the low er the resista nce of the w ire, the mo re current t ha t flow s fo r thesame vol tag e.

No insula tion is perfect (ha s infinite resista nce), so some current do es flow a longthe insulation or through it to ground. Such a current may be insignificantlysmall for most pra ctical purposes but i t is the ba sis of insulation t esting eq uip-ment .

So w hat is “ go od” insulat ion? “ Good ” means a re la t ively high resista nce to

current f low . When used to descr ibe a n insulat ion m ater ia l, “ go od” also m eans“ the a bili ty to ma intain a high resista nce.” Mea suring resistance can t ell youhow “ good ” the insu la t ion is.

What Causes Insulation to Degrade?There a re five ba sic causes fo r insulation deg rada tion. They intera ct w ith ea chot her and cause a g radua l spiral of decline in insulation q uality.

Electr ical St ress

I n s u l a t i o n i s d e s i g n e d f o r a p a r t i c u l a r a p p l i c a t i o n . O v e r v o l t a g e s a n dundervoltag es cause a bno rmal stresses within the insulation, w hich can lead tocracking or delamina tion of the insulat ion.

M echan ical St ress

Mechanical dama ge such as hitt ing a cable w hile digg ing a trench is fa irly ob vi-ous but m echanical stresses also ma y occur from running a ma chine o ut o f b al-an ce or freque nt stops an d sta rts The resulting vibration f rom ma chine o pera-


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NOTES Therm al St ress Runn ing a piece of m a chinery in excessively ho t o r cold con d

over expansion o r cont raction of the insulation w hich migh t resfa ilures. How ever, the rma l stresses a re also incurred every timstarted or stopped . Unless the m achinery is design ed fo r intermistop an d start w ill adversely af fect th e ag ing process of the ins

En v i r o n m e n t a l Co n t a m i n a t i o n

Environment al cont am ination covers a multitude o f ag ents ranture f rom processes, to humidi ty on a mugg y da y, and even to atha t gna w their w ay into the insulat ion.

Insulation be g ins to de g rad e a s soon a s it is put in service. The igiven application will have been designed to provide good seryears under normal operat ing conditions. How ever, ab norma lhave a da mag ing effect w hich, if lef t unchecked, w i ll speed uprad a tion a nd w ill ultimat ely cau se a fa ilure in the insulation . Insuto ha ve failed if it fa ils to ad eq uat ely prevent electrical currentund esira ble pa th s. This include s current f low a cross th e ou te r oof the insulat ion (surface leakag e current), throug h the b od y o(conduction current) or fo r a va riety o f o ther reasons.

For example, pinholes or cracks can develop in the insulation ofo reign ma tte r can pen etrat e t he surface(s). These conta minantunder the ef fect of a n applied volta ge providing a low resista nfa ce lea kag e current w hich increases compared w ith dry unconfa ces. Clea ning a nd drying the insulation, ho w ever, w ill easilyt ion.

Other enem ies of insula tion ma y prod uce deteriorat ion tha t is notHow ever, once insulat ion deg rada tion ha s sta rted, the va rious iassist each ot her to increa se the rat e of decline.

How Can Predictive Maintenance Help Me?While th ere are cases w here the drop in insulation resista nce such a s wh en eq uipment is floo ded , i t usually drops gradua lly, w a rning if te sted pe riod ica lly. These reg ular checks permit pltion ing prior to service f a ilure a nd /or a shock con dition .

Without a period ic testing prog ram a ll fa ilures w ill come as a surp

inconvenient a nd q uite possibly very expensive in time and resoufore, money to rectify. For instance, take a small motor that ima terial, w hich w ill solidify if a llow ed t o stan d, a round a procexpected fa ilure of this motor w ill cost tens mayb e even hundreof do llars to rectify if dow ntime of the plan t is also calculat ed. nostic insulation te sting ha d b een included in the preventive magram i t may have been possible to plan maintenance or repl


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do w ntime. Mea suring insulation q uality on a regular ba sis is a crucial pa rt ofan y mainten an ce prog ram a s it helps predict a nd prevent electrical equipmentbreakdown.

This is particularly appropriat e now w hen w e consider tha t large parts of theelectrica l netw ork in th e USA an d Europe w ere insta lled in th e 1950s in a b urstof postwar investment. Some equipment is approaching the end of i ts designlife, w hile some ha s already e xceede d it but is sti ll operat ing sat isfa ctorily.

Since diag no stic te sting is g ene rally reserved f or mo re critica l items w e no r-mal ly, but no t a lw ays, f ind t hat diagno st ic tes ters have vol tag e out puts of 5 or10 kV, these voltag es being mo re suitab le for testing t he a ssets w hich th em-selves are usually medium voltage machines, cables, transformers, etc.

The Benefit of New TechnologyInsulat ion testers da te b ack to t he ea rly 20 th century w hen Sidney Evershed an dErnest Vigno les de veloped th eir first insulat ion te ster (w hich developed in 1903into t he MEGGER® rang e of testers).

In th e ea rly da ys, mo st instrume nt s we re ha nd -cra nked. This limited the ir a bilityto carry out tests which took an extended time to complete, and limited thevoltage stabili ty to the operator ’s abili ty to crank steadily. Later, these same

instruments were capab le of ha ving a n external mot or drive add ed w hich helpedw ith long durat ion tests but did very li t t le to improve the voltag e stab ili ty. How -ever, the rang e o f t hese instrument s ra rely exceed ed 1000 M Ω. The a na log mo ve-ment s w ere very hea vy an d a ctually served to da mp out a ny transient event s.

The a ppeara nce of electronics an d th e development of b at tery technology revo-lutionized the design of insulation t esters. Mode rn instruments are l ine o r ba t-tery-pow ered an d produce very sta ble test volta ge s under a w ide variety ofcond itions. They a re also a ble t o me a sure very sma ll current s so t ha t t heir insu-

lation resistance measuring range is extended several thousandfold into theteraohm (T Ω) rang e. Some ca n even replace the pe ncil , pape r and stopw at ch,w hich w ere forme rly used t o ma nua lly collect results, by recording d at a inmemo ry fo r lat er dow nload an d a na lysis. It is fortun at e tha t the se aston ishingenha ncements we re made since the man ufa cturers of insulat ing ma terial ha vebeen w orking ha rd, also, w ith the result that m od ern insulat ing ma terials now exhibit much hig her resista nces than tho se in t he ea rly 20 th century.

New er technology off ers enha nced performan ce so tha t estab lished procedurescan yield greater ins ights and new methods can be made avai lable . Moderninstruments d eliver sta ble volta ge over their full resistance rang e, w ith micro-processor sensitivity in the measuring circuit enabling measurements in the T Ωrang e. The combina tion of stable voltag e an d enha nced sensitivity ena bles thetester to mea sure the minuscule amoun ts of current tha t a re passed by q ualityinsulat ion in new, capita l eq uipment . According ly, soph isticate d procedu res tha trely on precise measurement have been developed and may be easily imple-

t d


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NOTES H OW I NSULATION R ESISTAN CE IS M EASURED

How an Insulation Resistance Tester OperatesThe MEGGER ® insulation t ester is a porta ble instrument tha t pread ing of insulation resistance in ohms, mego hms, gigohm s, opending on t he mod el chosen) rega rdless of t he test voltag e selinsulation, t he resistance usually read s in t he m eg ohm or higMEGG ER insulat ion t ester is essent ially a high -rang e resistan ceter) with a built-in dc g enerat or.

The instrument ’s g ene rat or, w hich ca n be ha nd -cra nked, b a tt erat ed, de velops a high d c volta ge t ha t causes several sma ll curre

over surfa ces of the insulation b eing t ested. The t o t a l currenthe o hmmeter, w hich has an a nalog indicat ing scale , d igita l re

Components of Test CurrentIf we apply a test voltage across a piece of insulation, then bresulta nt current a nd a pplying Ohm’s La w (R= E/I), w e can calta nce of the insulat ion. Unfortunat ely, more tha n one current f loto complicate matters.

Capaci t iv e Charg ing Cur ren t

We are a ll fa miliar with t he current required to cha rge th e capinsula tion be ing t ested . This current is initially larg e b ut relativdropping exponent ially to a value close to zero a s the item und erInsulat ing ma terial becomes cha rged in the same w ay a s a dieletor.

A b s o r p t i o n o r Po l a r i za t i o n Cu r r e n t

Absorption current is actua lly mad e up of up t o th ree compo nenat a d ecrea sing rate t o a value close to zero over a period of se

The f irst is caused by a ge neral drift o f f ree electrons throug hunder the effe ct of t he electric field.

The second is caused b y molecular distortion w hereb y the impo sdisto rts the neg at ive charg e o f t he electron shells circulating arot o w a r d t h e p o s i t i v evol tage.

The t hird is due to thea l i g n m e n t o f p o l a r -ized mo lecules w ithinthe electric field ap-plied. This align me ntis fairly random in an e u t r a l s t a t e , b u t

h l i fi ld


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The t hree currents are g enerally considered tog ethe r as a sing le current a nd aremainly affected b y the type and condit ion of t he bo nding ma ter ia l used in theinsulation. Altho ug h the ab sorption current approa ches zero, the process takesmuch, much long er tha n w ith capacitive current.

Orientational polarization is increased in the presence of absorbed moisturesince conta minate d ma terials are more po larized. This increases the d eg ree ofpolarization. Depo lymerization o f th e insulation a lso lead s to increased a bsorp-tion current.

Not all materials possess all three components and, indeed, material such aspolyethylene exhibits li t t le, if a ny, pola riza tion a bsorption.

Sur f ace Leakag e Curr ent The surface lea kag e current is present because t he surface o f the insulation isconta minat ed w ith moisture or salts. The current is consta nt w ith time and de-pends on the degree of ionization present, which is i tself dependent on tem-perat ure. It is of ten igno red as a separat e current, being included w ith the con-duction current below as the total leakage current.

Con du ct io n Cur ren t

Conduction current is stea dy t hroug h t he insulat ion a nd is usually representedby a very high value resistor in pa rallel w ith the capa cita nce of the insulation. Itis a compo nent o f the Lea kag e Current, w hich is the current t ha t w ould bemea sured w hen th e insulation is fully cha rged a nd f ull ab sorption ha s ta kenplace. Not e th at i t includes surface leakag e, w hich can b e reduced o r elimi-na ted by the use of t he g uard t ermina l (to b e discussed lat er).

The fo llowing g raph shows the na ture of ea ch of the components of currentw ith respect to t ime.


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NOTES The t ot al current is the sum o f t hese compone nts. (Lea kag e currone current.) It is this current t ha t ca n b e me asured directly by aor, in te rms of meg ohm s, at a particular voltag e by me an s of a t ion tester. Some instruments offer the alternatives of displayment in terms of current or a s a resistance.

Because the total current depends upon the time that the voltOhm’s La w (R = E/I) only holds, th eo retically, a t a n inf inite tiw aiting f orever bef ore ta king a read ing). It is also high ly depening f rom a ba se level of t ot al d ischarge. The f irst step in any itherefo re, to ensure tha t t he insulation is completely discharge

Please no t e:

The charging current disappears relatively rapidly as th e equipm ent test becom es charged. Larger u nit s w ith mo re capacitance w ill t ake loto be charged . This curr ent is st ored energy an d, f or saf ety reason s, mbe discharged af ter th e test . Fort unat ely, th e discharge of th is energy t aplace relat ively quickly. During t esti ng , the absorp t ion curr ent decrea relatively slow rate, depending up on t he exact nat ure of t he insulaThis sto red energ y, too , must be released at t he end o f a t est , and r equira much long er t im e to discharge th an t he capacitance charging curre

Connecting your Insulation TesterWith mo dern insulat ing m at erials there is li t t le, if an y, differencob ta ined, reg ardless of w hich w ay the t erminals are connecteolder insulation, a l i t t le known phenomenon called electroendthe low er read ing t o be obt ained w ith the posit ive terminal cg r o u n d e d s i d e o f t h e i n s u l a t i o n b e i n g t e s t e d . underground cable , thepositive terminal w ould

normal ly be connectedt o t h e o u t s i d e o f t h ecab le since this will beg r o u n d e d b y c o n t a c tw ith the soil, as show n inF i g u r e 3 . P l e a s e n o t etha t you do not connectdirectly to t he insula tionbut ra t her to the cable’s

neutral or ground.

Figure 3 : Simplistic Connect ion t o a Cable


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Selected Typical ConnectionsShie lded Pow er Cable

Connected to me asure the insulat ion resista nce betw een one conducto r an dground .

Circuit Breaker/Bushing s

Figure 4: Connection to a Shielded Pow er Cable


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NOTES Pow er Transform er

AC Genera to r

Figure 6: Connection t o a Pow er Transfo rmer


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Insulation Resistance Tester ScaleMost modern insulation testers offer displays that provide the operator with

bot h a d igi ta l read out of the result and some fo rm of ana log readout . Below isa representation of the MEGGER BM25 display.

W h e n a n i n s u l a t i o ntester is “ hoo ked up”t o t h e i t e m t o b et e s t e d , a n d a t e s t i sstarted, several thingsoccur. The t hree diff er-ent current s, capa citivecharg ing, dielectric a b-sorption, and conduc-tion/leakag e a re flow -ing. The sum o f t heset h r e e c u rr e n t s w i llcause the ins t rumentdisplay to vary w ith theread ing increa sing , ini-tially q uickly a nd thenmore slow ly w ith time.

With an a na log display, the mo vement of the po inter may provide informa tionto an experienced o perat or. Is the pointer tra veling smoot hly, or “ stuttering?”Is it rising ste a dily or intermitt ent ly dro pping b a ck? This valua ble supplemen-ta ry info rmat ion w ould be difficult or nea rly impossible to discern from theda ncing digits of a n LCD. A few examples are listed here:

As the te st volta ge increa ses an d the item und er test approaches breakdow n,corona discharge w ill cause the pointer to “ jit ter,” indicat ing to th e operat orthat the ma ximum volta ge th at t he i tem can w ithstand is being approached.This w arning ha ppens in time to terminat e the test bef ore actua l brea kdow n,an d po ssible da ma ge , occurs.

To t he experienced o perato r, the speed a t w hich t he po inter travels impartsinformat ion on the capa cita nce of the item und er test . This is a useful prop-erty in high-voltag e cable testing, an d relates to t he the oretical basis of t hemore sophisticat ed dielectric discharge test t ha t is described elsew here in th is

booklet .If t he po inter alternat ely rises and drops ba ck, i t could indicat e a rcing in th ei tem under tes t that is too smal l to cause the automatic shutdown of thetester. Such info rmat ion helps direct t he o perato r in pinpointing a problem.

Observing a pointer as i t slow s to a n a pparent ha lt (i t may still be moving, butat a “ speed” l ikened to that of a c lock hand ) can be more ag reeable to taking

Figure 8: M EGGER BM25 Display


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NOTES Digital models present no such problem, as they inform the o(within the unit’s accuracy specification) what measurement And rememb er, most will give you a value of capacitance at t he

Most MEGGER insulat ion te sters a bo ve 1 kV come w ith a n a nplay. One of the advantages of th is display is that the analometer w i ll sw ay a nd o scil la t e , indicat ing to the o perat or tha ttest has not yet reached a steady state and is sti l l under the iab sorption a nd cha rging current. This indicat ion mea ns that thetes ted longer or tha t t here is a problem. When the ana log port iobecomes steady, the instrument displays the result in an unamdirect read ing fo rm, with no multipliers or ma th t o perfo rm.

Unlike t he a nalog/digi ta l d isplay m ent ioned a bove, a n “ averagraph meter does not provide a real-time indication of insulaSome instruments offer a curved bar graph in place of a genuarc, in w hich the low end of the scale is expan ded relat ive to thba r graph ta kes read ings over t ime, performs calculat ions an d tresults. The pro blem w ith th is type o f me te r is its principal of oevent occurs when the bar graph is not taking readings, i t wilnot show n on the display. Add itiona lly, ba r graph simulat ions may not appea r to the eye the same a s the familiar pointer t ravreplicate a mechanical movement to the expected degree.

When do ing insulation testing, the mo re the operat or know s ab(during a nd a fte r the t est), the bet ter his/her decision on how problem, if o ne exists. If something is missed during a test b ecment h ad a b ar gra ph style meter, importa nt informa tion could

Voltage CharacteristicsThe o utput voltag e of an insulation te ster depends on t he resis

suring. At low resista nces, say tens of ohms, the o utput voltag ezero, ma ybe a few volts. As the resista nce loa d is increa sed sow ill increa se until it reaches the req uested volta g e. As the resifurt her, th e te st volta g e w ill slow ly increa se until a stea dy valuevalue w ill prob ab ly be sligh tly in excess of the requested nom i5104 V w he n 5000 V w a s selecte d).


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You should alw ays ensure that an insulat ion tester is provided w ith a “ loadgra ph” tha t indicat es output voltag e characteristics ag ainst loa d resista nce or,alterna tively, an integ ral voltmet er that actua lly measures the t erminal volta geduring a test a nd d isplays it cont inuously. By this mea ns you can ensure tha t a nadequate voltage is produced over the resistance range of interest.

A qua lity insulation t ester w ill have a voltag e chara cteristic tha t exhibits a sharprise in volta ge up to a level of resista nce comm ensurat e w ith go od insulation. Afa st rise time ensures a n eff ective me asurement. The volta ge characteristic show nin Figure 9 represents a g oo d cha racteristic. In t his example, the out put volta gew ill have reached 500 V at a load as low as 500 k Ω and 1000 V by 1 M Ω . Thesevalues are legislated by international standards for testing wiring in houses,

shops, etc. While this is hardly a typical use for typical diagnostic insulationtesters, i t do es provide a go od benchma rk fo r the serious ma nufa cturer. Similarfigures wo uld apply at higher voltag es. Volta ge should rise sharply up to an y-where f rom one to f ive megohms, depending on the vol tage select ion, andma intain tha t voltag e at all higher resista nces.

With low er q ua lity insula tion t esters, volta g e ram p is fa r slow er. The instru-ment s typified b y the poo r curve show n in Figure 10 do no t produce the ratedvolta g e unt il much high er resista nces have be en rea ched. Thus tests could pro -duce results tha t provide pass levels of insulation but ha ve only bee n subjectedto half the d esired t est vol tage.

E VALUATION AND I NTERPRETATION OF R ESULTS

Interpretation of the Infinity ( ∞ ) Reading

Figure 10: Poor Load Curve


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NOTES but ra ther w ant s to see a high value and “ inf inity” ( ∞) cecriterion. How ever, “ infinity” is not a m ea surement ; it is an ininsulation b eing t ested ha s a resista nce tha t exceeds the mea surof the tes ter and should a lw ays be recorded a s “ greater tha n 100ever is the highest available number on your insulation tester.ad eq uat e since the m inimum a ccepta ble value of resista nce is l ilower tha n the ma ximum reading a vailab le .

But for ma intena nce of capi ta l eq uipment , a tes ter wi th only a“ shortcha ng ing” the opera to r. For preventive/predictive ma intereadings are of no use . The operator know s tha t the t est i tem is “much more. Testers w ith extended rang e, up into terao hms (1

M Ω), afford actual measurements right from the time of instaestab lishing a long t ime line tha t g ives the ma intena nce profe s“ brea th ing room.”

Significan t cha ng es in insulat ion q uality can o ccur at high levresista nce, beyon d th e rang e of m ore limited instruments, as shlow ing g raph .

In this exam ple, a l imited ran ge tester w ould not capture this vacan clearly see tha t, a lthoug h t he last recorded insulation va lueG h f d li i i i hi i A i

Figure 11: Changes in Insulat ion Resistance at High Values


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D IAGNOSTIC H IGH V OLTAGE I NSULATION T ESTS

Diagn ostic insulat ion t ests electrically stimulate the insulation an d mea sure the

response. Dependent upo n tha t response, we can d raw some conclusions ab outthe condition of the insulation.

Diagn ostic insulation t esting covers a very w ide rang e of techniques, some ofw hich involve portable eq uipment a nd some tha t require considera ble fixedeq uipment. Here w e sha ll consider only those tests tha t ma y be performed w itha rea dily port a ble dc insula tion t ester. These a re:

• Trend ing spo t t e st s

• Tim e co n st a n t

• Po la r iza t ion Index (PI)

• Step Vol tage (SV)

• Dielectr ic Discharge (DD)

Each test g ives a d ifferent view, o r windo w, into th e condition of t he insulation;the w hole picture is only availab le wh en a ll required tests have b een completed.

Spot Reading Test

The spot reading test is the simplest o f a ll insulation t ests an d t he o ne mo stassociated w ith low er volta ge insulat ion testers; the test voltag e is applied for ashort, specific period of time (typically 60 second s as usually any capa citive charg -ing current w ill have de cayed b y this t ime) and a read ing is then ta ken. Theread ing can th en be compa red to t he minimum installation specifications. Un-less the result is cat astrophically low , it is best used w hen trended ag ainst previ-ously obt ained values.

How ever, insulation resistance is highly temperat ure depend ent, a nd thus theresults should b e corrected t o a stand ard tempe rature, usually 40 o C. While te m-perat ure effects will be covered later, a g oo d rule of thum b is tha t fo r every 10 o

C increase in t empe rat ure, th e current d oub les (resista nce ha lves). The key t oma king t he spot rea ding t est valuab le is consistent t imekeeping, eff ective recordkeeping , and trending o f results.

As no ted previously, th e increased sensitivity ava ila ble in micropro cessor-ba seddiag nostic insulation t esters allow s the opera to r to ident ify insulat ion problemsin their ea rly sta ge s rath er tha n w hen t hose problems become cata strophic. Inmany cases, the t rend is far more importa nt t han the a bsolute value.

Compare th e tw o t races in Figure 12. Appara tus “ A” show s a high insulationresista nce while Appara tus “ B” show s a low value. How ever, wh en the trend isexamined, Appara tus “ B” show s lit t le cause for concern; it ha s been a round th esame value for several years and shows every prospect of continuing in thesame vein for ma ny years to come. Conversely, the curve fo r Appara tus “ A” isdiving d rama tically an d the a ppara tus will, if not hing is do ne to prevent it , fail


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NOTES

While Apparatus “ A” ha s much higher a bsolute resista nce valutus “ B” , the t rend is qui te w orrying. Apparat us “ B” has a fa itrend, indicat ing tha t the insulat ion qua lity is proba bly accepta

Insulat ion resista nce read ings should be considered relat ively ralutely. They can va ry widely for o ne mo to r or ma chine t estedrow, yet no t mea n ba d insulat ion. As ment ioned, the importan

the t rend in read ings over a t ime period, show ing lessening resisting of coming problems. Periodic testing is, therefore, crit icma intenance of electrical equipment. The interval b etw een tests a year, once a year, etc.) depends upon the type, location, andthe eq uipment . Evaluat ing a ser ies of read ings ta ken over a numor years moves the operato r tow ard being a diagno st ician.

Period ic tests should be ma de in the sam e w ay ea ch time. Usconnections an d a pply the same t est volta ge f or the same leng

should a lso be mad e at ab out the same t emperature , or the operect the m to t he same tem perature. A record of the relative huequipment a t t he t ime of the tes t i s helpful in evaluat ing the reaas low temperatures and high humidity might sugg est condensafa ce of t he insulat ion. For this rea son it is essential to ensure thabe te sted is at a te mperat ure in excess of t he dew point, as othsation w ill form w hich w ill disto rt the reading s unless the m ea

Figure 12: Com parison of Trended Test Result s


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Condition What To Do

(a ) Fa ir t o h ig h va lues a nd • No ca use fo r co ncern

well maintained(b ) Fa ir t o h ig h va lues, b ut sho w ing a • Lo ca te and remed y t he ca use

co n st a n t t en d e ncy t o w a rd s lo w e r va l ue s a n d ch e ck t h e d o w n w a r d t re n d

(c) Low but w ell mainta ined • Condition is prob a bly all rightbut cause of low values shouldbe checked. Ma y simply be t hetype o f insula tion in use

(d ) So low as to be unsafe • Clean, dry out, o r otherw ise

raise the values before placingeq uipment in service (te st we teq uipment while drying o ut)

(e ) Fa ir or high values previously we ll • Make tests at freq uent intervalsma int a ined but sho wing sudd en unt il t he ca use o f lo w va lues islo w ering loca t ed a nd remed ied o r,

• Until the values have becomestead y a t a lower level but safefor opera tion or,

• Until values become so low tha ti t is unsafe t o keep the eq uip-ment in operation

Time vs. Resistance TestFam iliar, sta nda rdized test procedures tha t ha ve been employed f or years ben-efit from t he improved capab ilit ies of enha nced diag nostic testing. Most ba sicof these is the time-resista nce meth od . A valuab le property of insulation, b ut

one tha t must be understood , is that i t “ charges” dur ing t he course of a t esttha nks to t he m ovement of electrons a s explained previously. This movement ofelectrons constitutes a current.

Its value a s a diag nostic indicat or is ba sed o n tw o o pposing fa ctors; the currentdies aw ay as the structure reaches its final orienta tion, w hile “ leakag e” pro-mot ed b y moisture or det eriorat ion passes a compara tively larg e, consta nt cur-rent. The net result is tha t w ith “ go od ” insulation, leakag e current is relat ivelysmall and resistance rises continually as current decreases from the effects ofcharging an d dielectric ab sorption. Det eriorat ed insulation w ill pass relativelylarge amounts of leakage current a t a constant ra te for the appl ied vol tage,w hich will tend to mask the charging and ab sorpt ion effects .

Graphing the resistance reading at t ime intervals from initiation of the testyie lds a smooth r ising curve for “ go od” insulat ion, but a “ f la t” graph fo r dete-riorat ed e q uipment . The concept of t he time resista nce test is to t a ke successive

di t ifi d ti It i b d th l ti i t d f l k


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NOTES

The b enefits of the time resista nce test a re tha t i t is relat ively tem perature a nd can g ive conclusive informat ion w ithout th etests.

Polarization Index TestThe simplest implemen ta tion o f th e time resista nce test f or sorepresente d b y the popular Polarizat ion Index (PI) test , w hich reread ings follow ed b y a simple d ivision; the one-minute rea dinthe ten-minute rea ding to provide a ratio. The result is a pure nunorma lly be considered indepe nden t of tempe rature since the tthe equipment being tested is usually so great that the overalta kes place during the 10 minutes of t he t est is neg ligible.

In general, a low ratio indicates l i t t le change, hence poor inshigh ra tio indicat es the o pposite. References to t ypical PI valuethe literat ure, w hich ma kes this test very easy an d rea dily emplw e say “ in g eneral” because a s ment ioned previously there arexhibit very li t t le or no d ielectric absorption. Carrying o ut a tesrials w ould t hen produce a result very close t o 1.

Note t ha t resista nce rea dings alone are difficult to w ork w ith a

Figure 13: Time Resistance Test Graph


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teraohm rang e, and t herefore , do no t run off the g raph. The largest and new -est capi ta l eq uipment can be read ily tested to yie ld repeat ab le dat a for record-ing a nd subsequent trend eva luation. The fo llow ing chart highlights selected PIvalues and w hat t hey mean to the o perator.

Polarizat ion Index Insulat ion Condit ion

4 G o o d

Values ab ove 4 indicat e excellent eq uipment f or w hich no action is likely to b enecessary w ithin the immediate ma intena nce schedule. The ope rato r may becalled upon t o m ake crit ical judgme nts, how ever. Some high va lues of PI (ab ove5) could indicate brittle or cracked insulation; this should be fairly obvious. Asudden increase in PI grea ter tha n 20%, witho ut a ny mainten an ce having b eenperformed , should serve a s a w arning; insulation ma y hold its value fo r longperiods, but is not likely to dramatically improve all by itself.

A bene fit of t he PI test is tha t i t can provide a n indicat ion of insulat ion qua lityin ten minutes on very large pieces of equipment tha t might take a n hour ormore to ful ly charge With a spot reading t est the o perat or w ould have to w ai t

Figure 14 : Benefit o f the Polarization Test fo r Large Equipm ent


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NOTES 13.4 w as obt ained . The stato r had cooled d ow n an d no d oubcuring ph ase. Subsequent tests yielded reducing PI values until it s4.7. During routine ma intena nce, PI values do not reach these h

It is also interesting to not e tha t ma ny people have tried to use tf i lled t ransformers and cannot understand w hy a known go od t rth em results close to 1. The a nsw er is simple. PI testing is not a ppfilled t ransfo rmers. The con cept d epen ds on t he relat ively rigsolid insulating ma terials, w here a bsorption energy is req uiredthe electronic structure of comparatively fixed molecules agavoltage field. Because this process can go to a theoretical sta(at “ infinite t ime,” w hich ob viously can not be a chieved in th

but can be reasona bly approxima ted ), the result is a stea dy dimrent a s molecules rea ch their “ final” alignmen t. Because the PI this phenome non, i t canno t be successfully applied t o fluid mapassage of test current throug h a n o il-filled sample creat es convtha t cont inually sw irl the oil , resulting in a cha otic lack of structthe ba sic premise upo n w hich t he PI test rests.

Step Voltage TestSince g oo d insulat ion is resistive, an increa se in te st volta ge wcrea se in current w ith a result tha t t he resistance rema ins consttion f rom t his could signify def ective insulat ion. At low er test vV or 1000 V, i t is q uite possible t ha t t hese def ects migh t b e unothe volta ge rises w e reach a point w here ionizat ion can t ake plaor cavities, resulting in an increase in current, a nd therefo re a rinsulation resistance. Not e th at i t is not necessary to reach t hefor the insulation for these defects to become apparent, sincelooking f or ioniza tion in the defe ct.

The Step Volta g e test f ollow s exactly this principle a nd ca n be

fully at voltag es rea ching 2500 V an d upw ards. The Step Voltaemployed a s an un dervoltag e or overvolta ge t est. How ever, it bered tha t a n overvol tag e tes t can lead to a catas t rophic fa i lurebreaks dow n because high volta ge tes t se ts have a lot of powundervoltag e test carried o ut b y an insulat ion tester has relat ivavailable and it is therefore far less likely to result in a destruct

A recognized standard procedure is to increase voltage in fivone-minute increments a nd record t he f inal insulation resistanc

Any ma rked or unusual resista nce reduction is an indicat ion of ness. Mode rn electronics allow s these rea dings to be ca ptured a

Follow ing a re some possible results from a Step Volta ge test o500 to 2500 volts and w hat they mean to the o perator :

No app recia ble d ifferen ce in va lues - Insulation is in reliab le co


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The g raphs in Figure 15 are taken from a mot or tha t w as da mp a nd d irty (low ertrace) an d a fte r cleaning a nd d rying (upper trace).

In g eneral, if a deviation of 25% in resistance me asurements is ob served over

the rang e o f successive volta ge s, i t is an indicat ion of the presence of moistureor othe r cont am ination. Localized physical dama ge ma y be further revea led bybreakdow n or a rcing. A “ stut ter ing” or “ jit tery” pointer movement can a nt ici -pat e this condition as the breakdo w n volta ge is nea red. It ma y be desirable toterminate the t est a t such point b efore insulat ion b reakdow n fur ther deter io-ra tes the condi t ion o f t he tes t i tem.

Like the PI test , the Step Volta ge test is a repeat ab le, self-evaluat ing te st tha t,because o f i ts short durat ion, is free o f e xtran eous influences like t emperat ure

effect .Dielectric Discharge TestThe Dielectric Discha rge t est (DD) is a rela tively new test m eth od tha t w a s de -veloped by EdF, France’s nat ional po w er utili ty, a nd ba sed on years of resea rch.While the o ther meth od s ment ioned mea sure the currents flow ing during thecharging process, the DD te st measures the current t ha t flow s during dischargeof the test sample. As such, it is not a pure insulation resistance test b ut ra theran a djunct to trad itiona l insulation tests.

Th e c h a r g e t h a t i sstored during an insu-lation test is automati-cally discharged at thee n d o f t h e t e s t w h e nthe insulat ion tes ter ’s

Figure 15: Step Volt age Step Graph


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NOTES The rat e of discharge d epend s only on t he discha rge resistors aof stored charg e from t he insulation. How ever, the capa citive charapidly until the voltage across the insulation has reduced to

tha t t ime, the effe ct of leaka ge currents will be ne gligible. So of dielectric a bsorpt ion is left . This is kno w n a s dielectric rea bsmirror ima ge of the dielectric ab sorption.

Figure 17: Reabsorpt ion Current s

The cap a citive current q uickly de cays from a high va lue w ith atime constan t (a f ew second s). The a bsorption (or reab sorptiocharge) current always starts at a high level but has a much lstant (up to ma ny minutes). It is caused by th e d ipoles ran do miment within the insulation and the electron shell returning to shape . This ha s th e eff ect of a current f low ing if th e discha rconnected, o r a voltag e reappea ring on t he sample if it is left oidly removing the effe cts of leakag e a nd ca pacitive currents alli ty of in terpret ing the d egree of polar izat ion o f t he insulat ion amoisture and other polarization effects.

The t est i tem is first cha rged fo r anyw here from 10 to 30 minuta g e unt il full ab sorption ha s ta ken place. (The MEGGER insulaaut oma te t his test charg e the test sample fo r 30 minutes.) At t hta nce is fully charg ed a nd t he d ielectric a bsorpt ion is essent iallyleakag e current continues to flow. At this point th e test voltag ethe insulation is discharged throug h t he instrument ’s interna l


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Current flow ing a ft er 1 minute (nA)Test Volt a g e (V) x Cap a cita nce ( µF)

The mea surement is tempe rature depe nda nt, so it is important to t est at a ref-erence temperature or to record the temperature.

Insulation in high voltage equipment often consists of layers, each having itsow n capa cita nce and associated leakag e resistance. When insulat ion is built upin this w ay, the a im is to ma ke each layer such tha t t he volta ge stress is sharedeq ually betw een la yers. When t he insulat or is discharged , ea ch layer ’s cha rgew ill decrea se equa lly until there is no volta ge remaining.

When a layer is faulty bet w een g oo d layers, its leakag e resista nce w ill decrea se

w hile capa cita nce is likely to rema in the same . A stand ard insulation t est w ill bedet ermined by the go od layers, and not l ikely to reveal this condition. But dur-ing dielectric discharge, t he time consta nt o f t he fa ulty layer w ill misma tch theot hers to yield a h igher DD value. A low DD value indicates tha t rea bsorptioncurrent is de caying q uickly, an d t he t ime consta nt o f ea ch la yer is simila r. A highvalue indicat es that reab sorption exhibits long relaxation t imes, w hich ma y pointto a problem.

Typica l con dition s from pra ctica l resea rch, primarily carried out on g ene rat orsby EdF, arrived a t t he f igures of merit in t he f ollow ing t ab le. This technique w asdeveloped for high vol tag e g enerators but ha s applicat ion on any mult ilayeredinsulation.

DD Value (in mA V -1F-1) Insulation Condition> 7 Ba d

4 - 7 Po o r2 - 4 Quest io na b le

< 2 OK

Different Problems/Different TestsAs w e ha ve just seen, th e Dielectric Discha rge Test can b e used t o ide nt ify prob-lems in a single layer of multilayer insulation. Other test methods might notpoint to problems on this specific type of insulating structure. Similarly, thePolarization Index test is particularly valuable in revealing moisture ingress, oilsoaks, a nd similar pe rvasive cont am ination. These invading conta minant s pro-vide convenient paths for electrical leakage, which damages the surroundinginsulation a nd event ually burns through a s a “ short.” This type of problem isrevealed a t a lmost a ny tes t voltag e a nd w ill appear a s a character ist ical ly “ f la t”PI. Moisture and contaminants will also bring down the readings, but this re-q uires a previous value for comparison; th e PI test ha s the a dvan ta ge of m akingan internal compa rison.

How ever, othe r prob lems ma y seem t o “ pass” a PI or simple Spot Rea ding t est byi ldi g h igh i t l t g i lt g S h bl i l d l li d


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NOTES A PPENDICES

Potential Sources of Error/Ensuring Quality Test Results

The fo llow ing section ident ifies several area s of p ot ent ial errte sting a bo ve 1 kV. These fa ctors may b e of less impo rta nce in 1increased vol tages and sensi t iv i t ies make them cr i t ical fortesting.

Test Leads

Bew are of instrument s w ith low q uality leads wh ose volta ge rthe t est volta ge s employed. It is extremely importa nt t ha t th e orents dur ing a measurement are those that are developed by

under te st . If th e lead s them selves prod uce lea kag e, you ma y beinsulation resista nce rather tha n the item unde r test .

All lea ds supplied w ith MEGG ER insulat ion te sters a re high q uahave been tes ted to w ithsta nd vol tages w ell above the highest terat ed b y the pa rticular instrument. Even then , it is importa nt leakag e by prevent ing the lead s f rom conta ct ing ea ch other, particularly w at er.

M a k i n g M e a su r e m e n t s a b o v e 1 0 0 G Ω

Mea surement s up to 100 G Ω can be ma de w ithout a ny speciasuming tha t t he lead s are reasona bly clean a nd d ry. The g uard later) can be used t o remove t he eff ects of surface leakag e if neprecautions are required when measuring resistances above 1leakage current can spoi l the qual i ty of the readings taken. fol lowing:

Test lea ds should not be a llow ed t o to uch each othe r or asince this w ill induce leaka g e pa th s.

Sharp points at the test lead connections should be avoidedencourage corona discharge.

Inst rument tes t jacks should be d eep so tha t unw anted leakacur between the terminals.

Accuracy Sta t ement s

Pay close att ention t o a n insulation te ster ’s accuracy sta teme ntmere plus/minus percent a g e fo r digita l units. The sta te men t muplus/minus a n umb er of dig its, as no d igita l display can fix itssig nificant dig it, or l.s.d.) to a sing le numb er. Accuracies specifieread ing” indicat e the same error at a ll points on the scale.

Ana log stat ement s l isted as “ percent o f scale” or “ full scale dcan b e d eceptive. Because the accuracy interval is based on the fit translates into an increasing percentage error as the readingl i h i l I h d h b f i


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D e l i v er y o f St a t e d Vo l t a g e

Volta ge reg ulation is indicat ed fo r an insulat ion tester with a loa d g raph in theinstruction ma nual show ing the output volta ge ag ainst resistance loa d. The loadcurve ensures th a t, a t t ypica l insulatio n resistan ce values, th e insulat ion te ster isdelivering full rate d te st volta ge t o th e test i tem. While this ma y appea r to beob vious, it is not ne cessarily the case unless so sta ted by th e ma nufa cturer of agiven tester. A poorly-regulat ed t ester may loa d d ow n und er a high-resista nceload so tha t the insulation of the t est item m ay a ctually be experiencing only afract ion o f the ra ted t est vol tage, w hich the t ransformer can output o nly underma ximum conditions. Such instrument at ion is not l ikely to come provided w itha loa d curve.

It w as t his condition t ha t inspectors from specifying a ge ncies, like UL ®, discov-ered among “ tes ters” that w ere “ jury-r igg ed” from on-hand t ransformers andot her compo nent s at job sites to perfo rm high pot ential tests. The inad eq uaciesof such systems lea d t o th e highly specific lang uag e perta ining to o utput volt-ag e tha t no w common ly appe ars in the sta nda rds litera ture. MEGGER insula-t ion tes ters confo rm by del ivering a nd m aintaining the ra t ed t est vol tag e o ncea minimum loa d comm ensurat e w ith typical insulation va lues (ge nerally 1 to 10M Ω, depe nding on mod el and voltag e selection) is applied. Test voltag e is typi-cally a f ew volts above ra ted, b ut should no t d rop below it , ma inta ining the

integrity of the test and the repeatabili ty when performing scheduled preven-tive maintenance. If exceptionally precise reporting data is mandated, somemodels display the actual tes t voltag e in ad di t ion t o t he selected voltag e a ndthis informat ion is included a mong the d at a provided a t the conclusion.

In te r f e rence Reject io n

Interference is the electrical noise prod uced a t a variety o f f requen cies, wh ichcan a ppea r in t he sample b eing t ested. It is usually induced currents or volta ge sfrom ad jacent eq uipment a nd is very comm on in substations, particularly high

voltag e substat ions w here po w er freq uencies predominat e. This electrical no isesuperimposes an ac signal on the dc test current and can cause considerablevar ia t ions in readings and may prevent the operator g et t ing a reading a t a ll if i tis be yond th e cap a bilities of your instrument . As an e xample, 4 mA of 50/60 Hznoise is fairly typical of the electrical noise that can be encountered in largesubsta tio ns (400+ kV).

Be aw are of the capa bi lity of the insulat ion tester being used to cancel out theeffects of this ac noise effectively, resulting in the abili ty to make measure-men ts in increa sing ly mo re difficult cond itions.Not a ll no ise is limited to pow erfrequen cies, how ever. To a ccommo da te o ther freq uencies some top of the ra ngeinstruments incorporate further software fil ters that can eliminate the effectsof this noise. It is importan t th at the instrument you use is ma tched t o t he levelof interference anticipated.

Rules on Test in g and Com par ing


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NOTES It is tempting to try to back up tests with additional readings.some ad justment t o t he test i tem o r setup, or someone else ma yaccepting t he result a nd w ish to verify it . But a n insulation t e

multimeter! High -voltag e testing beha ves very much like thecert aint y Principle (you cannot know both the speed and postron) applied t o insulat ion. Tha t is to say, the act o f me asuring being m ea sured, so tha t subseq uent rea dings are not b eing ta kthe same i tem.

As has been d escribed , the act o f a pplying a n insulation t est polat ing ma teria l. This eff ectively chan g es its electrical conf iguratric properties. Because insulating material is, by design, a pocan ta ke considerable t ime for “ relaxat ion,” or the re turn to rantion, to occur. Immed iately upon terminat ion of a test , the item precisely the same piece of eq uipment tha t i t wa s bef ore the te sfo llow -up test w ill be aff ected, sometimes considerab ly, by the th e first t est. Which mea sureme nt is correct? They b o t h are! expected to give a correct measurement of the condition of ththe time o f t est . Furthermore, industry-stand ard discharge procsufficient f or the institution o f a repeat test . Such procedures asonnel saf ety, not q ualification of the t est item . Residua l cha rgehours, or even days, that may be below human percept ion yetsensitive met er. Eq uipment should b e left groun ded fo r several hably until the next day, before additional testing is done. Anfa ctors, especially tempera ture, must not be o verloo ked.

This do es not mea n t ha t o n-the-spot retesting should n ever be prelative informa tion, i t ma y be q uite valuable. But i t must be tive. Do not expect the rea dings to agree .

Tw o different o perat ors ma y also not o bserve the same deg re

respect to procedure. Temperat ure is one fa ctor. If t he eq uipmenperhaps to check performan ce, then rete sted , the second test iscompa rable to the first . Time of test is also rea dily overloo kedma y rigidly time the t est wh ile ano ther merely w aits for staread ing. This can result in mea surement s being ta ken at differentime-resista nce curve (as ha s been illustrat ed under t he “ Spot-and ag ain the tw o results wi ll not b e comparable .

If this seems like excessive a tte ntion to det ail , consider the stanOrgan izations like UL ® a nd ASTM ® do not write procedures th“ hook up a met er and ta ke a reading.” Rat her, they specifyincluding setup, procedure, and characteristics of the test instresults can be considered in conformance. Standard maintenade serve no less diligen ce.

Testing Insulation Resistance of Rotating Machinery


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Individua ls/orga nizations tha t ma nufa cture rot at ing ma chines.

Individua ls/orga nizations tha t a re responsible fo r the a ccepta nce of new ro-tating machines.

Individua ls/orga nizations tha t te st and ma intain rota ting m achines.

Individua ls/orga nizations th at opera te rota ting ma chines.

Meg ge r recomm ends tha t an yone involved in testing and /or mainta ining rot at -ing ma chinery review this sta nda rd in det ail . We w ill provide some of the high-lights.

IEEE Std 43-2000 recom men ds a procedu re f or m ea suring insulat ion resista nceof arma ture and field w indings in rota ting ma chines rat ed 1 hp, 750 W or grea teran d applies to synchronous ma chines, induction m achines, dc ma chines an d syn-chrono us cond ensers. It d oes not apply to fractiona l horsepow er machines. Italso recommends the insulation test voltag e (ba sed o n w inding rating) and mini-mum a ccepta ble values of insulat ion resista nce for ac and dc rota ting ma chinew indings.

The f ollow ing chart provides guidelines for the dc voltag e to be a pplied d uringan insulation resistance test. Note th at voltag es up to 10 kV are recommend edfo r windings rated a t g reate r than 12 kV.

*Winding Rated Insulation Resistance TestVoltage (V) Direct Voltage (V)

12,000 5000-10,000

* Rated line-to- line volt age for th ree-phase ac machines, line-to-g roun d voltage for single-phase machines, and rat ed direct voltage for dc machines or field w inding s.

The stand ard recommen ds that ea ch phase be isolat ed a nd t ested separat ely (iffea sible) as this approach a llow s compa risons to be ma de b etw een pha ses. Thetwo phases not being tes ted should be grounded to the same ground as thestato r core or rotor b od y. When a ll pha ses are te sted simultan eously, only theinsulation to ground is tested. Insulation resistance measurements should bema de w ith a ll external eq uipment (cab les, capa cito rs, surge arresters, etc.) dis-connected a nd g rounded as these items may influence the resistance reading . Acommon ground should be used to prevent stray losses in the ground circuittha t could effect th e test results.

The sta nda rd calls out b oth the insulation resista nce test a nd t he po lariza tion


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NOTES The recommend ed m inimum va lues for PI are ba sed o n th e the rinsulating m at erials and apply to all insulat ing ma terials rega rtio n pe r IEC 60085-01: 1984. The PI te st is no t a pplicab le to no n

w indings. Be aw are t ha t a very high PI (grea ter tha n 8) for vashellac mica-folium, or a sphaltic sta to r w indings may indicatetion ha s been therma lly ag ed a nd ma y be a t risk of fa ilure. Phcan b e used to confirm if th e insulation is dry and britt le.

Thermal Class Rating Minimum PI Value

Cla ss A 1.5

Cla ss B 2.0

Cla ss F 2.0

Cla ss H 2.0

The recommen ded minimum insulat ion resista nce af ter o ne minube d etermined f rom the f ollow ing chart. The minimum resistancof a three-phase armature winding tested w ith the ot her two gbe a pproxima tely tw ice tha t of the ent ire w inding. If ea ch pharately (with guard circuits being used on the phases not unde

served minimum resista nce should b e th ree times the ent ire w inMinimum InsulationResistance (M Ω) Test Specimen

kV* + 1 Fo r mo st w ind ing s ma d e b efo re afield w indings, an d ot hers not d es

100 Fo r mo st d c a rm a t ure a nd a c wa ft er ab out 1970 (fo rm-w oun d coi

5 Fo r mo st ma chines w it h ra nd o m-wcoils a nd f orm-w oun d coils rat ed b

* kV is the rated machine terminal to t erminal voltage in rms kV.The rat ing of the m achine determines w hether th e mot or w indingthe minimum value fo r either the insulation resistance t est or achieve the minimum fo r both t ests.

Machine Rating Evaluation Criteria

10, 000 kVA o r le ss Sh o u ld h a v e EITHER a v a lu e o f t h evalue of the insulation resista nce above the minimum recommended

Ab o ve 10, 000 kVA Sh o u ld h a ve BO TH a v a lu e o f t h evalue of the insulation resistance above the minimum recommended


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The Guard TerminalSome insulation t esters ha ve tw o t ermina ls, ot hers have three. As these a re dctesters, tw o o f t he te rmina ls are t he + an d -. The th ird (if present) is a gua rd. Itdoes not have to be used a nd m an y operat ors use insulation te sters satisfa cto-rily w ithout ever employing t he gu ard. How ever, it a ffo rds the o perato r an ex-tra function f or dia gn osis of e q uipment prob lems. The g uard is a shunt circuittha t diverts surface leakag e current around the mea surement f unction. If pa ral-lel leakag e pa ths exist , a g uard conne ction w ill elimina te t hose from th e mea -surement , and g ive a more precise reading o f the leakag e bet w een the remain-ing elements.

Figure 18 : Use of th e Guard Terminal on a Power Cable

As an example, dirt a nd mo isture on a transfo rmer bushing w ill promo te sur-fa ce leakag e betw een the + and – connect ions , thereby bringing do w n the read-ing and possibly giving a false impression that the bushing is defective. Con-nect ing t he gua rd to a ba re wire w rapped around t he bushing w il l in terceptthis current and yield a measurement based predominant ly upon leakaget h rough de fects in the ceramic.

It is most important no t to confuse the guard w ith a ground. Connect ing the

guard and re turn lead to the same element of the tes t i tem only shunts thecurrent that is supposed to be measured, and thereby short-circuits the mea-surement function. When selecting a tester, consider:

The g oa ls of testing (ba sic insta lla tion checks do n’t g enera lly require a g ua rd).

The electrical compo sition o f t he items to be tested (mot ors and tran sfo rmerscan b e tested for leakage b etw een w indings with ground leakag e eliminated)


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NOTES Testers w ith gua rds generally cost a bit mo re tha n tw o-terminama ny applications, a tw o-terminal mode l wo n’t be imparting tof informat ion that can be accrued by insulat ion t est ing.

Something that is often forgotten is the difference in the capgua rd circuit . The gua rding capa bility of the insulation tester isporta nt w hen mea suring lea ky insulation tha n the usually quot eaccuracy figure, w hich ma y be 5%. Consider th e fo llow ing examcase w here the surfa ce lea kag e pa th is 200 times less tha n th e rinsulation.

Figure 19: Guard Terminal Diagram

Here w e show an insulato r of value 100 M Ω tha t w e w ish to mand conta minated a nd so i t has a surface leakag e pat h of 500our tes t volta ge f rom the posit ive a nd neg at ive terminals w ithocircuit , 20 times as much current w ill flow throug h t he surfacpared w ith the current f low ing through t he insulat ion w e w ishw e w ill rea d a resista nce of only 497 k Ω .

If w e “ gua rd” the sample, here show n as being g uarded such leakage resistance equally on either side of the guard connecable to e l iminate the effect of the surface leakage to a cer tamuch w e eliminat e the eff ect of the surface leakag e is ba sed ocuitry of t he insulation te ster used. Depend ing on the instrume


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Effects of TemperatureTemperat ure variations can ha ve a significan t effect on insulation resista nceread ings. Resistance drops ma rkedly w ith an increase in temperat ure for th esame piece of apparatus. Each type of insulating material has a different de-g ree of resistance chang e w ith tempera ture. Temperat ure correction fa ctor tab leshave been developed for various types of electrical apparatus and can be ac-quired from the manufacturer. Failing that, i t is recommended that you de-velop your ow n correction f acto r tab les by recording t w o resista nce values forthe same piece of eq uipment a t tw o different temperatures. A graph ma y thenbe plot ted of resistance (on a loga rithmic scale) ag ainst tempera ture (on a l in-ea r scale). The g raph is a straight l ine a nd ma y be extrapolat ed t o a ny tempera -ture so tha t correction fa ctors ma y be rea d d irectly.

In lieu of d eta iled da ta , the “ rule-of -thumb ” is tha t fo r every 10 o C increase intem perature, ha lve t he resistance; or f or every 10 o C decrea se in tem perature,do ub le the resista nce. For exam ple, a 100 G Ω resista nce a t 20 o C becom es 25 G Ωat 40 o C.

Why is tempe rature correction importa nt? Consider the follow ing example ofa mo to r tested a t various times of t he year at d iffering t emperat ures (all withina 15 o ba nd). The te mperat ure ad justments w ere ma de using th e rule-of-thumbcorrection.

Insulation Temperature Temp. AdjustedDate Resistance (M Ω) oF Insulation Resistance (M Ω)

01-Jan-90 15,000 68 14,990

01-Jun-90 9,000 80 14,276

01-Jan-91 14,500 68 14,490

01-Jun-91 8,500 82 14,56201-Jan-92 14,300 68 14,290

01-Jun-92 8,700 81 14,341

01-Jan-93 14,500 68 14,490

01-Jun-93 8,900 81 14,671

01-Jan-94 14,200 69 14,748

01-Jun-94 8,900 80 14,117

01-Jan-95 13,600 68 13,59101-Jun-95 8,900 78 13,071

01-Jan-96 13,500 66 12,491

01-Jun-96 7,500 80 11,896

01-Jan-97 11,300 68 11,292


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NOTES If the same da ta is corrected for temperature and plot t ed, the gprovide a valuab le picture of the de terioration of t he insulatio

Temperat ure correction is particularly importan t w hen testing wag es and at higher levels of sensitivity.

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

Figure 20: Insulation Resistance Graph Not Corrected fo r Temperature

4,000

6,000

8,000

10,000

12,000

14,000

16,000

Insulation Resistance Graph - XYZ Motor(not corrected for temperature)

Insulation Resistance Graph - XYZ Motor(corrected for temperature)


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Effects of HumidityHumidity (moisture conte nt) has a n eff ect upon insulation resistance, but i t can-not be q uant i f ied as neat ly as can temperature effect because different types of

insulation w ill absorb moisture to varying deg rees, as w ill varying a ge s and con-ditions of th e same t ype. The be st t ha t can b e said is tha t humidity is a fa ctortha t should not b e overloo ked w hen evaluat ing test results. Unlike tempe ra-ture, humidity’s effe ct is not a consta nt g radient a nd a s long a s the t emperat ureremains above the dew point, humidity will not appreciably affect insulationreadings.

Increasing hum idity in th e surround ing (amb ient) air can a ffe ct insulat ion resis-ta nce to varying d eg rees. If eq uipment opera tes reg ularly abo ve the dew -point

tem perature (the t emperat ure at w hich the mo isture vapor in air cond enses asa liquid), the t est rea ding w ill not be af fected much by the humidity. Even if th eeq uipment to be t ested is idle, the sam e is true — so long as i ts tempe rature iskept above the dew point (and the insulat ion surfaces are f ree of contami-na nts, such as certa in lints and a cids or salts, w hich ha ve the property o f a bsorb-ing moisture).

In electrical equipment , w e’re concerned primarily with t he cond itions on theexposed surfa ces w here mo isture conde nses an d af fects the o verall resista nceof the insulat ion. Studies show , how ever, tha t de w w ill form in the cracks an dcrevices of insulat ion be fo re it is visibly evide nt o n th e surfa ce. Dew -point m ea -surement s w ill provide a clue a s to w hethe r such invisible cond itions might ex-ist, altering the test results.

Humidity effects req uire grea ter at tent ion as test voltag es increa se because thehigher vol tag es can promote ionizat ion much more readi ly tha n a t low volt -ag es. As a result , humidity tha t do esn’t prod uce a not icea ble effect a t 1 kV ma yprod uce perplexing ly low rea ding s at 5 kV. This is not necessa rily a pro blem.The diffe rence in response at tw o d ifferent voltag es can be used to det ect mois-

ture and tes ts carr ied o ut g uarded a nd ung uarded can b e used to detect surfacemoisture or interna l moisture.

Ingress ProtectionSome w here in the f ine print of most te st eq uipment product b ulletins is an IPrating , a numb er that gives the o perat or vita l info rmat ion. In fa ct, the IP ratinglets the opera to r know w heth er a piece of t est eq uipment is suited fo r his/herapplication and test environment.

“ IP” sta nds for “ ingress protect ion.” That is the d egree to w hich the inst ru-ment can w ithstan d invasion b y foreign ma tt er. The IP ra ting system h a s bee nesta blished by t he IEC ( Inter nat ion al Elect rot echni cal Com m ission ), in the ir Sta n-da rd 529, and is used a s a g uide to help the operato r protect the l ife of theinstrument . It a lso can help the o perat or ma ke a more informe d purchase deci-sion by ensuring tha t the piece of te st eq uipment is designed to w ork in theenvironm ent (s) th a t h e/she fa ces


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NOTES The first digit ref ers to pa rticulate ingress, reflecting the deg reobjects can penetra te t he enclosure . A level of “ 5” indicat es “ duw ell as protected f rom invasion w ith a w ire do w n to 1.0 mm. T

higher categ ory: “ dust t ight .” The second digit refers to moist“ 4” m ea ns resista nce to “ splashing w at er, any direction.” The 5 through 8 indicat e “ je t t ing w at er” a nd “ temporary” o r “ consion.

So w ha t? Well, suppose an instrument und er considera tion w aIP43. Wha t w ould tha t tell the ope rato r abo ut i ts usabili ty? Coug hly utilized in a q uarry or cement plan t? Hardly! The parindicat es “ ob jects eq ual or grea ter tha n 1 mm.” Tha t’s a b ouldeto pa rticles typica lly prod uced b y industrial processes. Flying duunit out of commission.

Suppose the unit is rated at IP42. A moisture rat ing o f 2 indicatter. Therefore, i t w ould n ot be resista nt to flying spray. Acquirinfo r an en vironment tha t exceeds its IP capa bilit ies likely meansto r will need a not her very soon. Wha t a bo ut a rating o f IP40? Aof 0 mea ns that t he unit is not prot ected a ga inst a ny liquid ing

The fo llow ing charts provide a g uide to va rious IP ratings and to the operator :

Protection Against Access to Hazardous Parts (First Digit)

Number Description

0 No n-pro tect ed

1 Pro t ect ed ag ainst access w it h b a ck o f ha nd (

2 Pro tect ed ag a inst access w it h jo int ed fing er

3 Pro t ect ed a ga inst a ccess w it h a to o l (2.5 mm

4, 5, 6 Pro tect ed ag a inst access w it h a w ire (1.0 mm

Protection Against Ingress of Solid Foreign Objects (First Digit)

Number Description

0 No n-pro t ected

1 Ob ject s eq ua l o r g rea t er t ha n 50 mm2 Ob ject s eq ua l o r g rea t er tha n 12.5 mm

3 Ob ject s eq ua l o r g rea t er tha n 2.5 mm

4 Ob ject s eq ua l o r g rea t er t ha n 1 mm

5 D t t t d


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Protection against Ingress of Liquids (Second Digit)

Number Description

0 No n-pro t ected1 Wa t er d ripping vert ica lly

2 Wa t er d ripping , enclo sure t ilt ed up to 15 °

3 Spra ying w a ter, up t o 60 ° ang le from vertical

4 Spla shing w a t er, a ny d irect io n

5 Jet t ing w a t er, a ny d irect io n

6 Po w erf ul je t t ing w a t er, a ny d irect io n

7 Tempo ra ry immersio n in w a ter

8 Co nt inuo us immersio n in w a ter

High Potential TestingThere is no t ruly singular def inition o f t he “ high po tent ial” test . It is comm onlyused, but its definition is situa tiona l, in the “ eye of the b eholder” it migh t besaid. Basically, a high potential test is an electrical stress test conducted at avoltag e tw o or more t imes ra ted vol tag e and sometimes known a s a Withstandor P roo f Test.

Since the test is conducted a t a volta ge considera bly high er than t he rated volt-ag e of the eq uipment being tes ted, it i s known as an o vervolta ge tes t unlikethe high volta ge insulat ion test, wh ich is ge nerally applied at a voltag e below the ra t ed vol tag e of the eq uipment . The a ct of o vervoltag e tes t ing creates ab-normal stresses in the test sample and can contribute to the acceleration ofag ing in insulat ion. Indee d, some sta nda rds require the volta ge t o be increa sed

until the te st sam ple breaks do w n.I f an overvo l t age t e s t i s to be app l i ed , i t i s normal p rac t i ce to app ly anundervoltag e PI test bef oreha nd t o pre-q ualify the insulat ion.

High po tent ial tests may be carried o ut w ith ac or dc voltag es, as appropriate.Sam ples w ith considerable capa cita nce w ill appea r as a short circuit to a n a c testrequiring a t est set w ith very larg e pow er capa bilit ies to overcome t he capa ci-tive charging currents. In situa tions such as th is, i t is quite no rmal to apply a dctes t w i th the eq uivalent pea k.

Current (nA) Readings vs. Resistance (M Ω ) ReadingsInsulat ion testers measure current an d t hen convert i t into a resista nce reading .Why do w e do this? Well, predomina ntly, it’s trad ition. Go od insulation pro-duces a hig h reading w hile poo r insulat ion produces a low read ing. Also, go odinsulation is predom inantly resistive. If w e do uble the t est volta ge , w e do uble


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NOTES Burn CapabilityFull-function insulat ion testers abo ve 1 kV of ten include a “ burfeature that may never be used; yet i t does have a viable fu

narrow range o f a pplicat ion.

Insulat ion testers w ill genera te high volta ge s into sign ificant rever, if a breakdo w n o ccurs w ithin t he insulat ion, the resista ncrent increa ses, and t he volta ge drops. If left to i ts ow n de vices the b reakdow n arc to extinguish, the resista nce to increa se, andincrease w hich in turn ca uses breakdo w n a nd so on. This continnot a llow the mea surement of resistance and indeed could openlarge b urn tracks. Rat her tha n cause further da ma ge , most in

w il l shut do w n.However, i f you want to f ind the locat ion of the breakdown tremely inconvenient. For t his reason some instrument s offer alectab le “ burn” mode; the a utoma tic shutdow n is overr idden aarc is mainta ined. It must be understoo d, how ever, tha t t he incircuit limita tion is still in effe ct. The t ester w ill not pro vide a “ dfunction enables the operator to localize or identify the fault spark or wisp of smoke o r perhaps by use of an ionization d etew indings can be identified, covered w ith insulat ing varnish, anreturned to service. In cable maintenance, a high potential tehigher currents than insulation te sters is used to “ brea k dowta nce faul t , convert ing i t to an “ open” tha t is much more ea siarc reflection techniques.

Drying Out Electrical EquipmentElectricity an d w at er do no t fo rm a ha ppy partnership an d so isary to “ dry out” insulation. This ma y be do ne to remove surfperhaps to drive moisture from the internals of the insulationpieces of eq uipment ha ve in-built h ea ter coils w hich can be uspose. However, several other methods are also available for dequipment .

The m ost satisfactory solution to the problem involves placing an oven w ith suitab le temperat ure cont rol and proper a ir circulinfrared lamps ma y be used w hen t his is not possible, or a suitabbe built a round the ma chine, using stea m coils or electric resistfo r a source of h ea t. Openings must b e provided fo r the free circ

ot herw ise t he e xpulsion o f moisture w ould simply result in a nmidity inside t he d rying cham ber. Blow ers ma y be used t o increament .

Vacuum d rying ha s been a lso eff ectively used t o expedite th e rment to service, but this method requires extra precaut ions an dunderta ken by experienced personnel


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This w ill prevent no t o nly the rapid th ermal det eriorat ion of the insulation b utda mag e from the high vapor pressures that w ould be obta ined if s team w ereproduced.

If drying is required, records help d etermine w hen the insulation is moisturefree. As an e xample of the importa nce of pa st read ings, consider a mot or tha t’sbeen flood ed. After a clea nup, a spot rea ding w ith the MEGGER tester show s 15M Ω. If pa st records show ed t he insulation resistance to run from 10 to 20 M Ω,the mot or w ould be in good shape. If , on the o ther hand, past records show edthe normal resistance values to run from 100 to 150 M Ω , the operator wouldknow tha t mo isture w as sti ll present in the mo tor w indings.

During drying operat ions, w hen insulat ion resista nce values are used a s an indi-

cato r of the suita bili ty of w indings for service or for application o f te st poten-tial , the drying must be continued for a sufficient t ime to make sure that thevalues are reliable. Often the resista nce curve w ill ta ke one or mo re sharp dipsbe fo re leveling o ff or cont inuing t o increase in a po sitive direction. This is dueto moisture w orking out of the w inding s. When t he ma chine is completely driedout , further w ork is req uired t o remo ve any rema ining dust. This ma y be d onethroug h t he use o f d ry compressed air at pressure not exceeding 40 psi.

Follow ing is the t ypical drying -out curve for a dc mot or arma ture, w hich show show insulation resista nce chang es. During th e first pa rt of the run, the resis-ta nce decreases because of the h igher tempe rature. Then it rises at a consta nttem perature a s drying proceeds. Finally, i t r ises to a high va lue, as room tem-perature (20 ° C) is reached.


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NOTES There is a significa nt cavea t w hen te sting w et insulation w ith an iw et eq uipment is suscept ible to voltag e breakdow n. If w indinga lot o f mo isture even low voltag es can puncture insulat ion. Th

erator should be very careful before applying high voltages. MEGGER insulation te sters allow the t est volta ge t o be set a t low of 25 volts to a high of 5000 volts in 25-volt increme nt s.

Test Item DischargePerhaps a t school you w ere taug ht to discharge a capa citor anthe capacitor with the terminals shorted together. Did you evsince you ha ve discharged the capacitor a nd ma ybe had checkedno volta ge a cross the t erminals, you neede d to short the te rmin

The rea son is the dielectric ab sorption current. If the termina ls a rethe e nergy stored b y dielectric absorption w ill slow ly relea se withmigrat ing to one t erminal and po sitive cha rge to the po sitive tperiod of t ime this charge can build up to a dangerous levelor iginal test vol tage, a nd w ith a considerable amo unt of enerThis en erg y ca n kill.

At the end of an insulation test the test sample closely resemcapacitor; there remains a considerable amount of energy st

insula tion dielectric.There is an importa nt “ rule of th umb” on charging a nd discha rgtest. This rule sugg ests that the opera to r discharge the item undtimes as long as i t w as tested . If t he o perato r performs a 10-mishe should a llow the un it to d ischarge f or 50 minutes.

A go od q uality instrument w ill aut oma tically discharge t he tesas a test is completed or interrupted. Some lower quality instseparate discharge selection knob, o r sw itch, w hich a dd s a step

step is forgo t ten, the t est i tem can be d ead ly for the next persoit .

MEGGER insulation t esters also det ect th e volta ge across the tethe d ischarge pha se an d w ill show this voltag e until i t has falleAt th is point, the item is safe t o ha ndle.

How ever, all we ha ve discharged at this point is the stored capaexplained at the sta rt of t his bo oklet, any capa cita nce is charge d rat the start of a test . Similarly, the capacitive charge is disch

quickly at the end of a test . But the dielectric absorption currlonger to g o in and also t akes much longer to come out .

Thus w hile t he sample is immed iately saf e to ha ndle, if the teshorted t hey w ill gradua lly acquire charge and become da ngerSo, unless the eq uipment is go ing b ack into service, ensure tha


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Charging Time for Large EquipmentOne quest ion w e are of ten a sked is, “ How long w ill it ta ke to charge a pa r t icu-lar piece of eq uipment?” The an swer is, “ We don ’t know ! ”

Why not? Well, the answ er depends on the actua l configurat ion of the pa rticu-lar piece o f e q uipment concerned . For exa mple, t he MEGG ER S1-5010 specifiesa charging ra te o f “ less than 5 seconds per microf ara d w ith 2 mA short circuitcurrent ” an d “ 2.5 secon ds per microfa rad w ith 5 mA short circuit current.” Thus,i f you know the capacita nce of the t est sample you can w ork out t he chargingt ime; i t doesn’t mat ter i f i t i s a motor, a cable or jus t a s lab of insulat ingmaterial.

Motor Driven Insulation TestersAnother question we a re frequent ly asked is “ What happened t o the o ld wo odenbo x mot or driven insulat ion testers?” Some peo ple seemed to th ink tha t theyset the stand ard f or insulat ion testing and still do .

These mo tor dr iven w ood en b oxes, w i th an external mot or w ere producedbet w een 1910 and 1972 an d used t he o rigina l Evershed pa tent ed “ Cross CoilsOhmmet er.” This w as a large hea vy movemen t tha t, as the nam e sugg ests, hadtw o coils set a t a n a ng le to ea ch oth er. This w as th e f irst “ True Ohmmet er.” Theconstruct ion of t he movement ha d b enef i ts and draw ba cks.

The ma in benef it w as, because of the w eight of the mo vement , it ha d consider-ab le inertia a nd w as, therefo re, quite insensitive to interference or transientevents. This resulted in a very smoo th mot ion. Unfortuna tely, the sheer w eightof the mo vement mad e i t fa ir ly del icat e a nd so the inst ruments needed to beha ndled w ith care. Furthermore, the instruments neede d to be leveled b efo reuse and w as, therefo re, supplied w ith a spirit level on t he scale a nd a djusta blefe et . The mo vemen ts w ere also fa irly insensitive w ith ma ximum resista nce ca-pab ilit ies tha t could be mea sured in high Meg ohm s or low Gigo hms.

Alternat ive pow er sources we re developed. The o ld gene rator w as big and hea vyas anyone w ho ha s t ried t o ha nd crank one of these old inst ruments w il l a t tes t ;you cer ta inly wouldn’t want to do a PI tes t whi le hand cranking, but i f youlacked a ma ins supply there w as no a lternative.

Technology a dvan ces mea nt th at “ electronic movemen ts” could be used w hichw ere more rugged and more accurat e . New low -voltag e generat ors w ere de-veloped w hich made hand cranking much ea sier and then ult imately bat t erytechnolog y enab led pure bat tery pow er to b e employed. This resulted in the

long t erm, very sta ble pow er supplies that w e see toda y.The use of electronics ha s resulted in lig ht er, more rug g ed , more a ccurat e in-strument s tha t respond m ore q uickly. They can provide mo re info rmat ion, w hichresults in us seeing transient events tha t w ere previously tot ally hidd en b y therelat ive instab ili ty of t he pow er supply an d the inertia of t he movemen t.


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NOTES I NSULATION T ESTERS A VAILABLE FROM M EGGER ®

10 kV Insulation Resistance Tester

The MG 10-01 extend s the e nd user’s testing capab ility by o fferin10 kV mea suring ran g e. The MG 10-01 ha s bee n designe d fo r anyofa ctures, uses or ma intains rotat ing m achinery.

IEEE Std . 43-2000 (“ IEEE Recom me nd ed Pra ctice f or Testing Insulaof Rot at ing Ma chinery” ) recomm end s using up to 10 kV w hen mlat ion resistan ce test on w inding s rat ed a t g rea ter th a n 12 kVallow s

megger insulation_testing.pdf

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