megger handbook

5/14/2018 Megger Handbook

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MEGGER


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MEGGERTrade Mark

POCKET BOOKON INSULATIONAND CONTINUITY

TESTERS

HOW TO MAK.E INSULATION TESTSWITH NOTES ON SIMPLE CONTINUITY

TESTS

Publ ished byEVERSH ED & VIG NOLES LI M ITEDACTON LANE WORKS, CHISWICK, LONDON, WATelegram. II < Cables: Mogger, London, Telex 22583. Telephone: Chiswick 3670PUBLICATION NO. 20017 D~CEMBER 1960


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CONTENTSFOREWORDWHAT IS AN INSULATORTESTING ELECTRICAL ApPLIANCES -TESTING MOTORS AND GENERATORSTESTING HOUSE WIRING

Insulation TestsContinuity Tests -

MEGGER TESTERSFor insulation Tests onlyFor Insulation and Continuity TestsFor Continuity Tests onlyPrinciple of operationEffect of capacitanceHow to use Megger TestersHow to record results

357

11172129

32

343537

38

38

39

FOREWORD

The use of electrical apparatus and machinery in everysphere of industrial and domestic life makes their safe andcontinuous operation a matter of vital importance.

Breakdowns mean loss of output and inconvenience, andmay cause injury to individuals. The avoidance of suchbreakdowns, therefore, interests everyone using elec-tricity.

The safety of electrical installations and apparatusdepends chiefly on the condition of the insulation and thispocket book explains how this condition can be ascertainedby simple tests.

Systematic testing of insulation, which can be carriedout with the help of a Megger Insulation Tester, showswhether this insulation is healthy and helps those res-ponsible to maintain it in a satisfactory condition and soavoid interruptions or breakdowns.

The pocket book also gives simple instructions forcarrying out continuity tests to ensure that apparatus isproperly bonded to earth. The results of al l tests should berecorded in a log book. This permanent record is valuableto the maintenance engineer, for it enables him to prove thatthe plant under his charge is being maintained in a safecondition, or to indicate if repairs are necessary.


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WHAT IS AN INSULATORIn every electrical installation or piece of apparatus,

"conductors" and "insulators" are used, the conductor toprovide the path to carry the current of electricity, and theinsulator to prevent the current leaking from that path.The value of insulation is expressed in terms of itselectrical resistance, the practical unit being the megohm

(1 million ohms).The insulation resistance of different materials varies,

and the resistance of any given material will alter accordingto a number of factors the chief of which, probably, is thedegree to which it absorbs moisture. This absorption ofmoisture lowers the resistance of the material in question,while an accumulation of dirt upon the surface of thematerial, though it does not theoretically alter the insulationresistance of the material itself, does in effect do so, sinceit provides a conducting path through the dirt along thesurface of the material.The effect of moisture on insulating material is well

shown in the table in Figure 2.The safety of electrical installations and apparatus

depends on the condition of the insulation, which shouldretain the electricity in the path provided, and it is verydesirable, therefore, that the condition of this insulationshould be ascertained from time to time, to make sure thatit is not deteriorating through the accumulation of dirt,damp, or in any other way.This resistance can be measured very simply by means

of a Megger Insulation Testing Set which indicates directlyon a scale the insulation resistance of the apparatus orinstallation under test.

Figure I (opposite)The three series of Megger Insulation Testers.

Series ISeries 2 Series 35


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Time Insulation Resistance

On placing in drier .. 1000 ohms.After Ihour . . .. 1.1 megohms." 1 " . . .. 6.0 "" I! " .. .. 15.0 "" 2 " .. . . 26.0 "" 3 " .. .. 36.0 "" 4 " .. .. 40.0 "

Figure 2Tests illustrating the variation in insulation resistance,due to the presence of moisture

r ---------- -_.......~ .----------uvr i, : NLlNEl,,--,. :[ARTHED

~'-jl..iNEUTRAL o

rAULTB

TABLEACTINGASINSULATOR

- - - - - - - ~ - - - - - - - ~ - -- r !- ... ..... ~ : fSUBSTATION RETURN PATH

(ARTH THROUGHEARTHFigure 3

Diagram showing how a man may receive a shock. Theheater element being in contact with the kettle at B, theman completes the circuit LAB C DEN

6

TESTI NG ELECTRICAL APPLIANCESA great many portable appliances are connected to theelectric supply circuit by means of "adaptors" in larnp-holders, or by means of plugs and wall sockets. Electricirons, toasters, hair dryers and bowl fires are some of the

smaller apparatus used in this way.Under certain conditions a shock can be obtained bytouching the casing of a piece of apparatus in which theinsulation is defective; this will be understood fromFigure 3, where, owing to a fault, the body of the kettle isalive and a return circuit is available through the man'sbody to the earth connection at the works of the SupplyUndertaking.It should be said at once that electrical apparatus andswitches should not be placed in such positions that theycan be touched by a person standing on a wet floor unlessthe metal casings of the apparatus are adequately earthedin accordance with I.E.E. Regulation No. 401," otherwisea very severe shock may be obtained should a fault develop.Apart from the risk of shock, the insulation of theapparatus should be maintained in good condition orthe apparatus will eventually cease to function.In hospitals, institutions, or anywhere where there is alarge amount of apparatus in use, each piece should be

identified by a reference number and regular tests of theinsulation resistance should be made at least monthly andrecorded in a log-book or register. Such a log-book can,as mentioned on page 39, be obtained free of charge fromthe publishers of this book.A record kept in this manner is extremely useful as aguide and, by indicating that some weakness is developing,will enable electrical breakdowns to be avoided. In fact,the purpose of the Megger Insulation Tester is not only toshow where a fault exists, but to help to anticipate trouble,

and to find where it is necessary to apply that "stitch intime which saves nine."*I.E.E. Regulations for the Electrical Equip"..nt of Buildings (Thirteenth Edition).

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TEST 3

TEST. 2Figure 4 Tests on an electric kettle

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How to make tests on appliancesTo ensure safety, and to make certain that the apparatusis in a satisfactory working condition, the following threetests should be made with a Megger Insulation Tester (seePage 32).

Test I. To see if the electrical element has made accidentalcontact with the frame.Connect one lead of the testing set to the frame and theother lead to the two conductors (Figure 4).Turn the instrument handle at 160 r.p.m. when thereading should be several megohms.Test 2. To see if the insulation of theflexible issound.Disconnect the latter at both ends and connect one leadof the testing set to each conductor.Turn the handle at 160 r.p.m, when the reading shouldbe several megohms.Test 3. To see % / there is a complete circuit.*Replace the flexible and test between each conductorturning the handle very slowly, when the reading shouldbe zero. If a zero reading is not obtained, remove theflexible and test between the terminals of the apparatus,to see if the break is in the apparatus or inthe flexible.The actual value of the insulation resistance, as measuredin tests 1 and 2, which may be considered satisfactory variesaccording to the size of the apparatus or installation undertest. Such portable apparatus as we are considering atthe moment, if it is in good condition, will usually have aresistance of several megohms. Itwill be safe to use iftheresistance is at least one megohm, but the apparatus insuch case should be tested at short intervals to see if a faultis developing. If the test gives a figure which is muchbelow one megohm the apparatus should not be used untilthe fault has been cleared, which may be effected in manycases by cleaning the terminals, or removing and refittingthe flexible connecting wire.* This test is best tarried rmt with a Meggtr Insulation and Continuity Tester or wirha Megger Circuit Tester as both these instruments have a low range enabling theactual resistance of [he.apparatus 'Cobe measured,

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Generator No I 40 KW 110 voltsInsulation Time of TestDate Resistance. Weather condition Temperature ofMegohms. Machine

Feb. 25 15 Fine and bright Machine hot27 10 - Mondav morningMachine cold

Mar. 10 50 Cold and dry Machine hot12 30 Very cold and dry Monday morningMachine cold

Generator No 2 50 KW 110voltsFeb. 25 25 Fine and bright Machine hot

27 15 Fine and bright Monday morningMachine cold29 50 Dull and dry Hot after 12 hoursat 28 KjWs.

Mar. 1 12 Rainy After standing 12hours10 100 Cold and dry Machine hot12 50 Very cold and dry Monday morningMachine cold

Generator No.3. 135 KW. 460 voltsN h' bet dw mac. me elng erecteFeb. 24 20 - Armature only

24 25 - Field onlyMar. 5 4 - Complete machineon completion oferection8 2.5 Fine Machine hot9 30 Fine Machine cold afterstanding all day

12 75 Very cold and dry Machine cold23 20 Fine Machine hotTest after runningall day

Figure 5INSULATION TESTS ON POWER HOUSE GENERATORSwith 500 volts Constant Pressure Megger Insulation Tester10

TESTING MOTORS AND GENERATORSFaults on electrical machinery must be due to one oftwo causes. One is the absence of continuity in the con-ductor which is supposed to be carrying the current. Theother is an absence, or partial absence, of insulation.

The latter is by far the more common and the more danger-ous of the two faults. A burnt out armature, for instance.is usually due to insulation failure.It is of the utmost importance, therefore, to make regulartests of the insulation resistance of all machinery so as todetect incipient faults. The result of such tests should beentered in a log-book. Entries from a typical log book areshown in Figure 5 and it should be noted that, against eachentry, a reference is made to the state of the weather andto whether a machine has been tested when hot or cold, asthese conditions may influence the value of the insulationresistance.Effect of damp weather.

A drop in insulation resistance may often be accountedfor by damp weather. An examination, for instance, ofthe log of Generator No. 2 (Figure 5) reveals that duringthe 12 hours from February 29th to March Ist the insula-tion resistance fell from 50 to 12 megohms. It will benoted, however, that the weather conditions deterioratedduring this period. That this drop in insulation resistancewas due to the change in atmospheric conditions and not toan incipient fault was confirmed by the subsequent test onMarch 10th, when the weather was again dry and the resist-ance had risen to 100 megohms.Effect of temperature.

As regards the effect of temperature it should be notedthat the insulation resistance of motors and generators isgenerally lower when they are hot than when they are coldas the insulating varnishes used in the building of themachines have a lower resistance when hot than whencold.II


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When to make tests.It is a good plan, therefore, to make the test of a machineas soon after it has been shut down as possible, when theinsulation resistance is likely to be lowest. If, after themotor has just been shut down, the insulation resistance is

found to be satisfactory, it may be assumed that it will bebetter at any other time, provided that the machine does notstand idle for long in a humid atmosphere.Ifmachines are regularly and consistently tested, ifthesevarious relevant points are noted in the diary or log, and ifthese tests are taken whether the machine is behaving wellor not, then the work of diagnosing any trouble when itdoes arise is much simplified. Further, the likelihood ofany trouble developing to such an extent that breakdownoccurs is remote.

BONDEO CONDUiT .........

~ STARTER

EARTHINCSCREW

Figure 6Test to frame or earth on motor and switchgear

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I ns ula tio n v alu es .British Standards Specification No. 2613 (which super-sedes the former Standard Specification Nos. 168 and 169)states that where requested "Test Certificates shall recordthe insulation resistance and the temperature at which it

was measured."When a test is made on a new machine, this figureshould be obtained at the end of a run at full load when themachioe is hot and the insulation resistance is likely to beat the lowest value.It should be noted that the insulation resistance of amachine which is in good order and has been in use for asubstantial period should be higher than the original value.British Standards Specification No. 170 for Fractional

Horse-power Generators and Motors states that theinsulation resistance shall not be less than 1 megohmwhen testing at 500 volts.In general, a resistance of 1 megohm would be con-sidered satisfactory, but regular tests should be made, as acomparison of the readings obtained will give more informa-tion than the absolute value of the insulation resistance.

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Figure 7Test on motor onlyto frame or earth

Figure 8Test on field windings

and brush gear(armature excluded)

Figure 9Test on armature only

Makingroutine tests on motors and generators(a) D .C . m otor s a nd g en er ato rs

Disconnect the supply of electricity from the motorcircuit by opening the main switch and withdrawing themain fuses.Join together both terminals on the motor side of thedouble pole main switch (Figure 6) and connect these to oneterminal of the Megger Insulation Tester.Connect the other terminal of the Megger Tester toearth, using the frame of the motor or switch. * Tumthe instrument handle at 160 r.p.m. and take a reading.Ifthe insulation resistance between the terminals and theframes is found to be unsatisfactory, it must be ascertainedwhether the defect is in the starter, in the motor, or in thecables connecting the starter to the motor.To do this, disconnect the cables at the motor, andrepeat the test. If after this disconnection the defect is nolonger apparent, then it is evident that the fault does notlie in the starter and cables but in the motor.Repeat the test on the motor only.1. With the armature and field windings all connectedtogether (Figure 7) ;2. With the brushes lifted from contact with thecommutator (Figure 8) ;3. On the armature only, between the commutator andframe, the brushes being lifted (Figure 9).From these tests it can be deduced whether the defect isin the field coils and brush gear, in the armature or in both.If it is not in the armature, separate out sections of thefield windings and components and test individually untilthe defect is located.

* Thl metal frames 0/ all the electrical apparatus in the circuit should b. permanently,o",,rod togethor and earthed 10 a wat6' pipe or other good earth; This is eu ntialto . .. .. .. .. that the protectine fuse. will operate '" the ..,.", 0/ an .arlh oceurri"t onthe circuit. The efficacy of the earth-csmtinuisy conductor may b. tested with aM.gg" Insulation and Continuity Toste' (see page 29).


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Similarly,lithe defectwasoriginallytraced to the starter,separate out the various coils such as resistance coils, no-volt releasecoiland overloadcoiland test these separately.In the above description a simple rheostat starter hasbeen taken to illustrate the principles of fault location.

The exact procedure will vary with the type of starter.Thus with a contactor operated starter which, in the "off"position, disconnects all the lines to the motor, it isnecessary to make tests to earth on both the incoming andoutgoing terminals of the starter.It is sometimes found that the insulation resistance islow all round without a definite fault on any section. Thiscan usually be remedied by careful cleaning of the machine,for, when electrical machinery has been in service someorne, it is liable to become coated in placeswith metallicorother conducting dust-in particular a mixture of metallicand carbon dust, often mixed with oil, from commutators orslip rings. Such deposits form leakagepaths.

(b) A.C. mo to rs and gene ra to rsThese should be tested in a similar manner to D.C.machines with contactor operated starters, isolating thevarious circuits until the defect is located.

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TESTING HOUSE WIRINGE le ctricity Supply R egula tions 1937In the above regulations the Electricity Commissionersprescribe in effect (Section 26) that the Electricity SupplyUndertakings shall not permanently connect an installationunless they are satisfiedthat the connectionwould not causea leakageexceedingone ten-thousandth part ofthe maximumcurrent to the installation.Now this regulation pre-supposes that some form of testbe carried out, and whilst no specific tests are mentionedit is usual in practice to measure the insulation resistanceof an installation prior to connecting it up.Further, in Section 27 of these regulations it is stipulatedthat the Supply Undertakings shall not be compelled togive a supply of energy to any consumer unless they aresatisfied that all conductors and apparatus are constructed,installed and protected so as to prevent danger.The precise nature of t h e requirements are not specific-allymentioned except in sofar as this regulation is qualifiedby the statement that:" Any consumer's installationwhich complieswith theprovisions of the Institution of Electrical EngineersRegulations shall be deemed to fulfil the requirements ofthis regulation."Now, according to the I.E.E. Regulations, it is necessaryto carry out the followingtests to makesure that an installa-tion is safe:

1 . Insulation tests (Regulations501-503)*(a) Between each conductor and earth;(b ) Between the conductors themselves.2. Continuity tests (Regulations219A, 401-410).To ensure that the conduit or lead sheathing iselectrically continuous throughout and connectedto earth.3. Polarity of switches (Regulation 504).To ensure that all single pole switches arc on thelive side of the apparatus they control.*l.E.E. ReguJariowjor the Electrical ~ui fml.nt 0/ Bui/di"lls (Thirteenth Edition).

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Figure 10Testing the earth-continuityconductor of aportable electrictool

IS

Figure IIMaking aninsulation testat a switch-fuse

This book deals only with the insulation tests, and therequirements of the I.E.E. Regulations regarding insulationare set out below.The methods of making continuity tests and of testingthe polarity of switches are fully explained in a separatebooklet No. 201 entitled "Continuity and PolarityTesting," obtainable on application from Evershed &Vignoles Ltd. A few notes are also given on page 29 ofthis pocket book.

Insulation tests. I.E.E. RegulationsThese prescribe that electrical installations shall be testedon completion with a direct current pressure of not lessthan twice the working pressure, and that the insulationresistance of a completed installation shall not be less than

50 megohms divided by the number of outlets (points andswitch positions) in the circuit, although no installationneed have an insulation resistance greater than 1 megohm. . . . I " 50 megohmsi.e, mmnnum msu anon resistance =Nflo.ooutetsFor an incomplete installation, i.e. one without fittings,lamps, etc., the minimum insulation resistance should benot less than 100 megohms divided by the number ofoutlets.Where p.v.c.-insulated cables are used, the. equivalentvalues are I2l and 25 megohms divided by the number ofoutlets.

Need for periodical testingIt should be realised that the insulation resistance of thewiring in a house will not always remain the same althoughthe materials may be of the best quality, but will depend agood deal upon the amount of moisture and dirt present onthe fittings and accessories. Periodical tests of insulation

resistance on installations are therefore very desirable, sothat any deterioration due to moisture, dirt, atmosphericconditions or age may be detected and remedied.19


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ALL FUSES 'IN'

CONDUCTORSTO OTHER

BRANCH CIRCUITS

LAMPINPOSITION

SWITCH'ON'

MAIN EARTH

ALL SINGLE: POI.E SWITCHES 'ON'ALL LAMPS IN POSITION

Figure 12 Test to Earth on whole installation

2 0

fI

How to make the insulation tests(a) Insulation tests to earth

Disconnect the supply by opening the main switch andwithdrawing the main fuses,Insert all fuses at the distribution board (W, Figure 12).Insert all lamps.Close all single-pole switches.On an incomplete installation, the conductors at eachoutlet (points and switch positions) must be joined together.Join together the two contacts (L and N, Figure 12) onthe installation side on the main switch, and connect theseto one terminal of the Megger Insulation Tester.Connect the other terminal of the Megger Tester to the

conduit in which the wiring is run or, iflead-covered cableis used, to the lead sheathing. A second connection shouldalso be made to the consumer's main earth (Figure 12).This second connection is, however, unnecessary if thecontinuity and earthing of the conduit has been previouslytested, as described on page 29.Turn the handle of the Megger Insulation Tester at about160 r.p.m. and take a reading.In circuits having 2-way corridor switches, two test

readings should be taken, one switch in each pair beingchanged over before taking the second test, thus ensuringthat both inter-switch wires (P and Q, Figure 15) areincluded in the test.If the result of this test is considered satisfactory and isin accordance with the regulations previously mentioned,the installation may be pronounced as sound so far asresistance to earth is concerned,If, however, the values obtained are not sufficiently high.withdraw all fuses at the distribution fuse board and test

again. This test will only include the portion of the installa-tion between the main switch and the busbars of the fuseboard.

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If the fault does not lie in this section, proceed to thedistribution fuse board and test each branch circuit in turn(Figure 13) till the faulty circuit or circuits are discovered.These should be subjected to further tests as indicated inFigure 14 till the actual fault is located.It might be as well to suggest at this stage, that everyreading which is obtained should be written down, for it isa most annoying thing to test round a whole house andthen find that one has forgotten the readings.

ALL rUSES 'OUT'

W

LAMP INPOSITION

figure 13Test to earth on branch circuit

Connection to earth shown dotted to be used if theconductors are not enclosed in conduit or metalsheathing

z z

w

TEST A.(ONDUC TOR S F' 'H' &LAMP TO EARTH

(ONDUC TOR 'f'

wc C t:J [J

TEST BCONDUCTOR GTO EAR TH

o [] 00

c ceo

TEST C,CONDUCTOR5 C O & ' H 'TO EARTH

SWITCH'OFF'

LAMP IN

SWITCH'OFF'

SWITCH'ON'

Figure 14Test to locate fault to earth on branch circuit

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24

c:o.~5. .:

(b) In su la tio n te sts b etw ee n c on du cto rsRemove all lamps.(The main switch must be open, all fuses insertd atthe distribution board, and all single-pole switches in the

closedor "ON" position* as in the previous test.) .Connect one terminal of the Megger Insulation Testerto fuse contact L (Figure 15),and the other to the contact Nand make a test.Two readings should be taken on an installation contain-ing 2-way switches, one with both switches in one"ON" position (Figure 15) and the other with bothswitches in the alternative "ON" position; this willensure that both inter-switch wires (P and Q, Figure 15)

are included in the test.Ifthe result of the test between conductors is also satis-factory, no further insulation tests are necessary and theinstallation may be passed as sound.If, however, the results of the tests are unsatisfactory,proceed to the distribution board, withdraw all fuses andtest each branch circuit individually between conductorsuntil the faulty circuit or circuits are located.Having established the circuit upon which the fault

lies, disconnect any component part of that circuit whichit is convenient to remove. Disconnect, for instance, theflexiblefrom ceiling roses and test again to see whether theremoval of these small sections has cleared the fault.It is as well to remember that although faults dooccasionallyoccur inordinary straight runs of cable, this isnot the first place to look for trouble. It is much morefrequently the case that faults, both between conductorsand to earth, occur at switches, in ceiling roses, in lamp-

holders, in junction boxes, etc.*T" test ",hither 2",ay swilCMl art i" JM "ON" position.insert th lamp theycontrol. A M',fger Insulation Tester ."" ... e t . a botwem wntacu Land N Figure IS'fIIi1lshow zerD J / th e WJit,he~ are in zhe ~ON position,

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0 1 1 , ~Zo0::1;< I 1Wwll:I


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SWITCH

c 3 PINo: WALL! SOCKET!

TEST LEADSMAINSWITCHAND FUSES

WATER PIPEOR OTHERSUITABLE EARTH

Figure 17 Testing the earth-continuity of an installation. Goodcontact must be made between test leads and conduit under test

Figure 18 Earthing of portable apparatus. Simple method oftests if continuity of installations is sound2 8

How to make the continuity testsThe normal insulation tester is unsuitable for thesetests as the scale range is too high and the testing pressureexcessive.The instruments recommended for continuity testingare the Megger Circuit Testing Ohmmeter (page 35) andthe Megger Insulation and Continuity Tester (page 34).Continuity tests on installations must be made with thecircuit wiring dead, the procedure being as follows;-

B e fo re T e stin g1. Open the mains switch and, as a precaution, removethe main fuses.2. Connect one terminal of the testing instrument by along test lead to a rising water main (as shown inFigure 17), or to the consumer's main earth.3. This test lead should have an independent con-nection to the consumer's main earth so that theresistance of the actual earth connection is includedin the test.4. Connect the other terminal of the instrument by a

short lead to switchboxes, ceiling roses, earth-sockets of 3 pin socket-outlets, to the conduit at2 pin sockets, to the metallic frames of connectedportable apparatus, to motor frames, etc.The resistance actually measured includes that of thetesting leads, which may be measured on the instrumentand their value deducted.The net values obtained should be in accordance with th eappropriate I.E.E. Regulation No. 508(c).If the resistance is excessive the cause must be sought bymeasurements made along the line of the continuity con-ductor until the joint causing the high resistance is located.

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Figure 19 Measuring earth-continuity acrossa junction box Figure 21 Megger Insulation and Continuity Tester,Series3, Mk. III

Figure 20 MeggerCircuit Testing Ohmmeter figure 22 Megger Insulation and Continuity Tester, Series 230 31


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MEGGER TES TE RSFor insulation tests only

There are three series of instruments available forcarrying out insulation tests only. These are illustrated inFigure 1.1. Series 1 TesterThis instrument is recommended for testing high tensionequipment, transformers, mains, etc. and apparatus havinga high degree of insulation and considerable capacitance.

Ranges up to 50,000 megohms, 2,500 volts.Size 14x8x8 ins. Weight 30 lb.

2. Series 2 TesterThis is a convenient instrument for contractors andinspectors and is recommended for testing power circuits,motors, etc., operating on 500 volts, and for testing mainshaving moderate capacitance.

Ranges up to 2,000 megohms, 1,000 volts.Sizes 5t x9! X 6 1 ins. Weight 7 to 9 lb.

3. Series 3Mk. III TesterThis pocket size instrument is suitable for testing housewiring, small motors, etc., operating on voltages notexceeding 250 volts, and for testing mains of moderatecapacitance.Ranges up to 100 megohms, 500 volts.Size 5!x4x2i ins. Weight 2t lb.

32

cP

rs A t'1 1Figure 23

The word "Meg~er," which i.derived from thewords f(megohm ' an d "tester," is the exclusiveTrade Mark of Bvershed & Vignol Ltd., and isregistered in all the principal countries of theworld.

GO500 so40 "00 30 :3300 20 2

15

cOO 10a150

100

Figu re 24 .~ ,Figure 25 . ._,_"- '-,,-~~ , _ . . . . ~-.TYPICAL SCALES

OF INSULATION TESTERSFigure 23 Series I Tester (2,500 v.)Flgu r e 24 Series 2 Tester (1.000 v.)Figure 25 Series 3 Tester (500 v.)

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Fo.. insulation and continuity testsThe .instruments !or. this dual purpose are the MeggerInsulation and Continuity Testers, Series 2 or 3, illustratedon page 31. Though similar in appearance to the cor-responding insulation testers, each instrument has a seconds7ale graduated in ohms and a change-over switch enablingeither scale to be selected at will. The continuity tests aremade at low voltage.

o

4"3o1'5 MIl.IMn800kll600500~o

3'00~Oo{so

100So

( P I r QFigure 26

Series 2 (SOOv.)Figure 27

Series 3 (SOOv.)TYPICAL SCALES OF INSULATION

AND CONTINUITY TESTERS

For continuity tests onlyFor this purpose the Megger Circuit Tester illustrated onpage 30 is recommended. It consists of a direct readingohmmeter with a self-contained dry battery, the wholebeing housed in a moulded plastic case measuring 5i X4 X 2 1 ins. The ohmmeter movement being of the cross-

coil type, the readings are independent of the batteryvoltage.

o

o

5

/0

6000500040003000

20001500

Figure 28 Figure 29

TYPICAL SCALES OF MEGGERCIRCUIT TESTERS

35


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EARTH

...L.... CONTROL---:-- CIRCUIT

RESISTA,NCE

GENERATo"R

GUARD (WHEN FITTED~ DEFLECTING Cl RCUIT RES1STANCEFigure 30 Series 2 instrument arranged for insulation test

_.L_

MQ

PERMANENTM.o.GNET

o,~

-)"DEFLEC riNG

CIRCUITRESISTANCEFigure 31 Series 3 instrument arranged for insulation test

+

36Figure 32 Series 3 instrument arranged for continuity test

Principle of operationAll the Megger Insulation Testing Sets described in thishandbook consist of a hand-driven generator and a directreading ohmmeter. *The hand-driven Series 1 and 2 Testers have permanentmagnet d.c. generators (Fig. 30) whereas the Series 3Mark III Tester has an a.c. generator and a static rectifier(Fig. 31).The ohmmeter movement contains no control spring, thelatter being replaced by a control or pressure coil connectedacross the generator in series with a fixed control circuitresistance. The pointer will therefore only take up adefinite position when the generator handle is turned.The deflecting or current coil is also connected acrossthe generator and, for insulation tests, it is in series with the

resistance under test .The instrument measures the ratio of the currents inthe two coils, which will depend only on the value ofthe resistance under test, since variations in the pressuregenerated, due to varying handle speeds, affect both thecoils in the same proportion. The instrument is thereforea true ohmmeter and is calibrated in megohms andthousands of ohms.To guard against surface leakage, an instrument scaled upto 500 megohms or over is fitted with a guard terminal forconnection as shown in Figure 30. Leakage current is thusreturned directly to the generator without passing throughthe deflecting coil of the ohmmeter.In the Megger Insulation and Continuity Testers, thechange-over switch connects the deflecting coil in parallelwith the resistance under test (see Fig. 32) instead of inseries with it, and thus the scale reads in the oppositesense.+Hand-and-mcior driuen pattern Series 1 Testers , and mains operated Series 1 and2 Testers a.re also available.

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Effect of capacitanceIfa testing voltage is applied to any system of appreci-able capacitance, a current will flow into the systemuntil it is fully charged, even though the insulation isperfect. As a consequence, the indicating pointer of theMegger Insulation Tester will initially fall, subsequently

rising to the correct insulation value.If,moreover, the applied voltage varies, the generatorof the insulation tester will alternately charge and dischargethe capacitance, with the result that the pointer reading willbe unsteady.All the generators in Series 1 Testers and most of those inSeries 2 instruments have centrifugally controlled clutchesto give a steady voltage. However, these clutches are notnecessary with the Series 3 Mark III Testers, the outputvoltage being kept steady by special voltage limitingfeatures in the internal circuitry.How to use Megger Testing Sets

1. Place the instrument so that the crank may beturned conveniently. Without making any connections,turn the crank and the pointer will move promptly overthe scale and stand over the Infinity mark.2. Connect suitable leads to the instrumentterminals. Taking care that the other ends of the leads arenot in contact with anything, tum the crank again, when theneedle should once more stand over the Infinity mark;if it does not do so, there is a leak in the leads themselves.3. Connect to the circuit to be tested, according to theinstructions given in previous pages, tum the crank atabout 160 revolutions per minute, and while turning thecrank observe the position of the pointer on the scale.This shows the value of the insulation resistance undertest.4. Ifseveral successive readings show infinity resistance,touch the further ends of the test leads together whileturning the handle slowly, to make sure that the leads arenot disconnected or broken.38

How to record resultsAs explained earlier in this book the. c~mpari~on of ~nereading with another is of greater value 10 insulation t~stJ.ngthan the absolute value obtained on test. Thus, if therecord shows that for months the insulation resistance of amotor has been between 4 and 5 megohms and a test oneday gives only 1 megohm, it is then de~ira~le to m~efurther tests at short intervals to ascertain if trouble ISdeveloping. On the other hand, ifthe results of the previ-ous tests have been between 1 and 2 megohms, we need takeno special action, as this insulation resistance is in mostcases quite satisfactory.In order to assist the users of Megger Insulation Testers, thepublishers of this book have prepared a small log-book, andwill, on request, send onefree of charge to any user of Megger

Insulation Testers.

39

megger handbook

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