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Chapter One: ShiPs' electricalsystems - safetY and maintenanceShiF' Ehctricd SystemElectrical DisgramsElectrical SefelyElcctric ShocklnsulalbD ResistsnceTestingInsulalion

'festing

Continuily TestinS,MultimctersDbdc TestsCurrenl ClampmetersLive-Line TestersGencral Eleclrical Maintenance

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I ntroduclion

l his chaptcr Prcscnls an overview ol a shiP's clcctr ic l sl stcnr and dcseribcs various types of

circuit rl iagrams uscd i| l clectric t work. l lasic clectrical :alely precautions and electrical

testing fl lcthods itrc outl incd toScther wilh a dcscripttott of gcncral clcctrical muinl€nance.

Ships' clcctricNl system

Auxil iary scrvices on btlarrl ship range frrttn cngitrc rtxrnl putltps and lans' dcck winches and

windlasscs tr.r gcncral l ighting, catcrirrg and air conditionrng. Elcctrical power is used todrive

the nrajority ,rf thcse auxil iary scrviccs. ' l

hc clectricl lpo$cr svslclt l ()t l bturd ship is designed

to providc a sccurc supply t0 tl l Irads with i ldcquittc htl i l l in pr()tccti()n f()t lhe equipment and

opcrarinS, pcrsttntrcl. ' fhc

gcncral schctne of a ship s ch;ctrieal Powcr system is conlmon t(t

near ly a l l sh ips .

)ao IG€IIERATOR

l{o 2OENERAIOR

€1,€RGEirYG€NERAIOR

'l hc gcncrators (l iotnctintes callcrl i thctnators) producc the clcetrieal prwcr' lt is collcclcd al

the main switchboard and thcn tl istrihutcd t(! thc various auxil iary *.-rviccs conlpri l inE lhe

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clectrical load. An emergency generator and €mcrgency swirchboard mainrain supplies in thecvent of a main power failure.compare thisgcneral layout with the system on yourship. Notc the grear similariries and alsonote the differences - all ships' systems differ in some res1rcct.The generators may bc driven by a diesel engine. by a steam or gas turbine, or by the mainpropulsion engine. The rype of prime mover is derermined by the design of rhe ship and bycconomic factors.The power rating ofth€ generators is determined by the power demand of the electrical load.Large passengcr ships have three or four large generators rated at 2MW or more to supply theertcnsivc hotel services on board. A cargo ship may have two main generators lypically ratedfrom 350 to l000kW which are sufficient to supply the engine room auxiliaries while at sca andthc winches or cranes for handling cargo while in port. The limiied load required during ancmergency requires that the emergency generators may be rated from about l0kW for a smallcoaster to about 300kW or more for a cargo liner. The shipbuilder must estimaie the numberand power rating ofthe required generators by assessing the pnwerdemand of the load for allsituations whether at sea or in port.Electrical power on board ship is commonly generated ar 440V,60Hz (somerimes 380V, 50Hz). These values have been adopted because they are standard shore practice in theAmericas and in Europe. Ships with a very large electrical power demand may be designed tooperate at 3.3kV and even 6.6kV, or higher.The British Standard and IEC definirion of LOW VOLTAGE is between 50V ac and 10fi)Vac (the IEC give this definirion ro harmonize British and European standards).Lighting and other domestic supplies usually operate ar I | 5V or 220V, single-phase. Trans-formers are used to reduce rhe /0r(,)V geoeraaed voltage to this safer volrage lev€I. Whereponable equipment is to be used in dangerous, hot and damp locations, it is advisable toop€rate at 55V or even 24V supplied again by a srep-d()wn rransformer. Occasionally,transformers are also used to step up volrages. e.g. supplying a large 3.3kV bow thrustelmotor from a 4zl0V switchboard supply.Batteries for various services operate at I2V or 24V but somerimcs higher voltages are used.

Elcctricrl dlrgrrms

There are variOus typ€s of diagram whieh attcnrpt t(| sh()w how an clcctrical circuit operates.Symbols are used to represent items of equipment. Thc shipbuiltter provides a complete sct ofships' electrical diagrams. lt is imgrrtant that you \iudy rhese dragrams to bc abl,j to read andunderstand them compctently, and to use them as an aid in kxating electricat faults.A BLOCK DIAGRAM shows in simplified form the main inrer-relationships of the elemenrsin a sysl€m, and how the system works or may bc operai€d. Such diagrams are oft€n used todepict control systems and oiher complcx relarionships.

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Diagrams like this state the tuncti()n ol cach bkrck but usually givc n. inlornrution of thccomfxlnents in each bklck nor how thc bkxks arc intcrconnccted

A SYSTEM DIAGRAM shows thc murn lcltures of a systcm and irs b()unds. withoutnecessarily showing causc- lo-effcct. lts matn use is to i l lustratC lhc ways ()f operating the

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sFtcm. Detril is omined in order to make the diagram as clear as possible, and so easilyundcrstood.

A CIRCUIT DIACRAM shows, lN FULL, the functioning of a circuit. All csscntial p.tt!rnd conncctiom are dcpicted by means of symbols arranged to show the opcration es clcrly.r pcriblc bur without rcgard lo the physical layout of lhe various ilcms, thcir p$lt orconncc'tions. o (

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Thc clectrical connections of rhe above m(Nor starteJ are clearly shown in the simPlestpoesible way. A most important F)int is that no attempl is made to show thc movinS conlaclsof e rclay or conlactor alongside the coil that operatcs therh (whcrc they aac actually

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physically located). l '. teadnumb€r oi leil€r.For cxarnPle: contactor coil

the coil and its related contacls are identified by a common

'cc' drives two auxiliary contacts identified as 'cc l' and 'cr2'.

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Each wire connection is identified by a number which may be found on a numbcred sleeve on

cach wirc at its termination on a component in ihe actual starter'

Allhough there are international agreements as to ihe symbol to be uscd 1o r€present

elecrricil components you must be prepared to meel various different symbols representing

thc samc oomponent. For example: a coil can be represenled as

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The use of a circuil diagram is to enable the reader to understand the operation of the circuit,

to follow each scquence in the operation from lhe moment of initiatinB the operation (e.g. by

prescing a'stan' bution) to the final act (e.g. starting of the motor)' lf the equipment fails to

operatJ correctly, lhe reader can follow the sequence of operalions until he comes to the

opcntion that has failed. He can then examine all the components involved in that faulty

opcration (and only thos€ components need be examined) and so locate the fauhy compo-

nint. He has no need to examine other components that are known to function corteclly and

have no influence on the fauli; his work is simplified. A circuit diagram is an essential tool in

trouble shooting.A WIRING DIAGRAM shows lhe detailed wiring and conneciions between componcnts or

itcms of equipment, and in some cas€s the routeingofthcsc connections- A wiring diagmm of

an ilem ofequipmenl shows the components in the approximate Posilion th€y occuPy in rhc

actual equipment. The compon€nt may be shown complete (e.9. a contaclor coil togethcr

with all Ge conracts it drives) or may bc simply represented by a blcrck with th€ necessarry

terminals clearly marked. Different thicknesses of line can be used to diffclcntiale belween

power and control circuii conneclions. 'I'he wiring diagram below is of the same starter shown

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A wiring diagram may be of a fairly simple circuit, but it is quite difficult to use it to work outthc sequential opcration of the circuit. The purpose of a wiring diagram is to imtruct the wiringmrn how to construct and connect the equipment. lt is of little use in trouble shooting apart fromidcntifying the cxact position of suspect components and tcrminals.

QUESTION

What are you to do if difficulties arise in locating a fault on an item of equipment and only awiring diagram is available?

ANSWER

It may well save time and trouble to conv€rt the wiring diagram into a much simpler and morcuseful circuit diagram. When converting a wiring diagram into a circuit diagram certain basicrules and conventions should be followed.l. Every sequence should be drawn from left to right and from top to bottom (where

prssible).2. Each stage should be in order of occurrence from left to right.3. All contacts and components which are in series should be drawn in a straight line

(where possible) with the component they conlrol.4. All contacts and components which are in parallel should be drawn side by side and at

the same level to emphasise their parallel function.5. All major components op€rating at bus-bar voltage should be drawn at lhe same level

(or aligned horizontally) to help identify the rer;uired components quickly.6. All contacts should be shown 'open'or 'closed' as in their NORMAL or unenergiscd

condition.

There are other conventions but lhese cover the main points of g<xrd systematic diagrams.Block, system, circuit and wiring diagrams are the basrc types in gcneral use for electricalwork. Other lypes of diagram arc some timcs uscd to give intirrmalron for which thc basictypes ar€ unsuitatrle (e.9. a pictorial vicw of a c()nlponcnt).You should study the ghip's electrical diagrams so as to gain an underslanding of them whcncarrying out mainle nance or fault finrling. Ships' diagrams should be regarded as an csscntialtgol when carrying out work on electrical equipment.

Eloctricd sefety

Before attempting any elecrrical work, there are mme basic safety precautions you must b€arin mind. The possible dangers arising from the misuse of electrical equipment are well known.Electric shcck and fire can caus€ loss of life and damage to equipment.Regulations exist to control the conslruction, installation, oJrration and mainlenance ofelectrical equipment so that dangcr is eliminated as far as possible. Minimum acceptablestandards of safety are issued by various hodicsjncluding national govcrnrnents, inte rnation-al governmental conventions (e.g. SOLAS), national and intcrnational standards associa-tions (e.g. BSS and IEC), learned societies (e.g. IEE), classification socictics (e.g. Lloyds),etc. Where danger arises it is usually duc to accidcnt, neglect or somc other contravention ofthe regulations.Ships'staff must operate equipment in a safe nrann€r and nraintain it in a safe condition at alltimes. Failure to do so will causc dangcr with p,ossible disastrous consequenccs. Ships' staffshould keep in mind an essential list of DO's and DONT's when working with electricalequipment.DO get to know the ship's electricalsystcm and equipmcnt. Study ships'diagramsto pinpointthe location of switches and protection devices supplying distribution boards and essentialitems of eqdipment. Write down this information in a note book. Note the normal indicationson switchboard instruments so that abnormal operation can be quickly detected.DO operate equipment according to manufacturcrs' recommendations.DO maintain equipment according to manufacturers' recommendations or shipowners'

ts3Hffff:tll""iililil;. covers and doors are securety firted anrt ihar all botrs an<t firingsare fitted and tight.DO inform the Officcr of thc Watch beforc shutting"down equipmcnt for maintenance.

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DO switch ttff and lock off supplies. rcmove fuses. and display warning noticcs bcforeremoving covers of equipment for maintenance.DO confirm that circuits arc DEAD (by using a voltage tcstcr) bcforc touching conrJuctorsand terminals.DON'T touch live conductors under any pretext.DON'T touch rotating parts.DON'T leave live conductors or rotating parts exposed.DON'T overload equipment.DON'T neglect or abuse equipment.You should think 'safety' at all times and so develop a safety conscious attitude. This may wellsave your life and the lives of others. Most accidents occur due to a momentary loss ofconcentration or attempts to short-circuit standard safety procedures. DO NOT let thishappen to YOU.

Ehctric shock

Nearly everyone has experienced an electric shock at some time. At best ir is an unpleasantexperience, at worst it is fatal. Anyone who has access to live electrical equipment must befully aware of first aid and safety procedures related to electric shock as described in relevantsafety acts. copies of these safery procedures should bc displayed on board ship.Electric shock is due to the flow of current through your body. This is ofien from hand to handor from hand to foot. A shock current as low as l5mA ac or dc may be fatal. Obviously the sizeof shock current is related to the applicd voltage and your body resistance. Unfirrtunarely,your body resistance goes down as the applied voltage goes up. This means that rhe shockcurrenl is further increased at high voltages. The size of your body resistance also depends onother factors such as your state of health. the degree of contact with live wires and thcperspiration or dampness on y()ur skin. l 'ypical dry full contact hody rcsistancc is about5UnQ at 25V falling to abour 2(XXIQ ar 2-5(,V.

QUESTTON

What would the equivhlent shock currcnr levels be ar 25V and 250V?

ANSWNR

5mA and l25mA.

Voltages of about filV and bekrw are regarded as reasonably safe for portablc hantl tools.This is why special step-down isolating transfornrcrs are used with portablc rrxlls an{handlamps. 'Ihese transft)rmers supply the tool or lamp at I l0V ac but because rhc seconclarywinding is centre-tapped to earth, the maximum shock vol tage to carth is 55V.Electric shock is often accompanied by falling. which may causc adtlirional physical injuryand require first aid action, lf the shock victim is unconscious. resuscitation must take pririrityover first aid methods. Check the resuscitation techniqucs displaycd on rhe clccrric sho,ckposters displayed on board.

Insulatiron resistance

All electrical equipment has insulation. 'l"hc purpose of the insularion is to kcep electriccurrents in thc conductors and to prevent contact with live wires. 'Ihc

elcctrical resistance ofinsulation must be very high (MC2) to prevent currcnt 'lcaking' awry from conducttlrs.Insulalion resistance is the resistancc of thc insulalirln measured (u) bctwecn conductor anclearth or (b) between conductor and other c(,nductor(s). The insuhtiorr resistancc includesthe resistance of the insu.lation matcrial and also the rcsistancc of any surfucc dcposits of dirt.oil, moisture, etc.

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Surfrcc depxits cnn rcducc the inculation resi$tsncc. Thc flow of leakugc curre ntlr throughruch surfilce dc;xrsits is called 'TRACKING', Equipment mu$t be nruintaincd in o clcrncondition to prsvcnt trucking und to muintein a high value of insulation resistancc (usually atlcast IMO),Inrulatkrn millcrills are non-mctallic and have vcry few of the generally good physicrlpropcrtics essocialed with mctnls. Insulation is adversely affected by many factors such sshumidity. lemperelure. clcctrical and mechsnical stresl, vibrrtion, chemicals, oil, dirt end, ofcoursct old uge.Traditional insulstion malerisls include cotton. silk, papcr. ctc, Thcy msy bc cithcr dry ortrcelcd with euitable varnishcs or rcsins to cxclude moisturc and othcr harmful substrnccr.Othcr materials include mica. glass fibre. etc., and morc modarn materirls such m PVC rndothcr photict and compounds. An cxtensivcly uscd insulant not nolmElly considercd rr eninrulation matcrial is air.The majority of insulation materials in common usc cannot withstand tcmpcraturcs much inexccss of ltf0oC, All elcctrical equipmcnt heats up when,carrling lbarl current with thccon$equcnt rise in tempcralure, This temperature rise is above that of the ambient €ooling airlemperuture, All electrical equipment is constructed and reted to work satisfactorily in amaximum amhient uir temperature of 45'C (Lloyds), Under these conditions the expcctcdtcmperature rise will nol exceed the permitted temperature limit set for the insulationmrterial, lt is therefore the insulation matcrial that dictates the maximum permitted opclst.ing temperature of thc electrical equipment.For thir purposc insulation is elassificd according to t[c maximum tcmpcrstu]c ot which it irrefe io opcrate, Various clasEcs of ineulation are listed'in British Stbndards but normrlly onlyclattcr A, E and B are uscd for marinc elcctrical cquipment, Thc mexlmum tcmpclsturcrlbwcd for cach of thesc clarrer is:

CLASS AEB

55'C CLASS F 105.C7rc H l$.c&fC C l3{fC +

Thesc are stearly surfaee temperatures measured with equipment stopped and no flow ofcooling rir. 'llot spot'temperatures of 105"C (Clasr A) and l30o(i (Class B) arCgencrrllyscceptcd ar normal at the centre of coils and windings of machrnes with thesc rurfrcctempcreturcs, A machin€ operating continuously with these hot sErt temp€ratures wouldhrve an expected life of l5 to 2() years before the insulation failed completely. Howevcr, thclife would be halved firr every llPC above these allowed hrlt spol temperaturcs.

Tcsting

This *ctirln kxrks al thc various elcclrical teslinB operalirlnr you may nced ttl carry out. andat lhe instrumenls vou will necd.

'l 'hc main lests are tirr:

- lnsulation Rcsistancc- Circuit Continuilv- Com;xrncnt Resistancc- Vrrltagc- Currenl

Inrulrtlon te$lnt

A mclsurenr*n, Jrt lhc insulation resistunce givcs rlnc of thc besl guidcs to rhe stalc of hcalrhrll clcctrical cquipmcnt.

'[ 'hc rcsistance should be mcasurcd hctwccn insulated conductors

und curth. and hetwcc'n conduclors.An insulatiolt lcstcr is a high rclding rcsistilncc nrelcr using a high tcsr vttltagr,. - usuully 500Vdc.

'I 'hc lssl r'(tl l lgc is produccd cithcr by un intcrnul hand-drivcn gl.ncrut()r or by o bnttery

and clcclronic rollugc changcr. A tcst vtiltagc trf 5(X)V dc is suitable tirr tcsting ships'equipment rated rl l{(lV . Tcst voltagcs of l(Ix)V und S{XIf V arc usctl for high voltagc systcmson board ship.

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Therc are rcvernl mnnufacturers of insulation testers available. thc MEGGE,R INSULA-TION TESTER hcing one of the most common.

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Short thc probcr together, switch to 'M(l' and turn the handle or prcs3 thc button orrockerswitch. Thc pointcr should indicate '0Q' resistance. Some inrtrumcnts hrve r bettcrychcck rwitch pooition also. The cquipment to be tcrtcd muit be disconncctcd and lockcd ofirmrding to rtandard rafety procedures.A Mcggcr type lR terter can be uscd to chcck whcthcr the circuit to bc tcsted ir alive. Switchthc inrtrument io 'MO' and conncct the probcs to pairs of equipment tcrminalr. DO NOTPrc$ thc button or lurn thc handle. lf the grinter deflects the circuit is'livc'. lf thc circuit it'dead', it ic then safe to press the button (turn handle), Confirm that a rclirblc crrthconnoction ir obteincd by connecting lhe probcs lo two rcparatc carth F)ints on thccquipmcnt frame while testing for continuity.Meacure and log the phase-to'pha*,e insulation rcsistance valucr. Thrce readingt should trmcarured:U - V . V - W . W - U ,

Measure and log the phase-to-earth insulatitln resistance value's. Three readings should bcmeasured:u - E . v - E . w - E .

Insulation resistance decrcases wilh increase of temJrcrature .1

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QUESTION

Why slrould thc measurcmcnt of the insulation resistrncc of r machinc ideally bc madc whilc thcmrchinc is hot?

ANSWER

lnsulation bccomes more'leaky' (its IR valuc falls) at high temperatures. So testing while hotshows the realistic lR value at, or near, working temperature. lnsulation resistance can varyconsiderably with changing atmosphericconditions. A single readinggives littlc information.Flowever, the regular recording of insulation resistance readings may show a downward lrendwhich indicates impending trouble which can be remedied by preventive maintenancc.

Example of an IR log:

Confhuity tcclin3

An insulation tester normally also incorporales a continuity test facilily. This is a lowresistance insirumcnt for mcasurinS, the continuity (or otherwisc) of conductors. lt can bcused to measure thc low resistancc of cables. motor windings. transformer windings, canhi[tstraps. etc. The pro,cedure for usc is much as for that of thc insulation lester.PROVE the correct tlperation of the instrumcnt.ISOLATE and lock off the equipment io be testedPROVE the equipment to be dead.Switch the instrumenr ro'Q'or'continuiiy'. Connect thc probes to the circuit. OPcrarc lhcswitch/handle and check the indication on the 'Q' scale. Lng all readings.

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In the case of three-phase motors and transformers. etc. the comparison betwecn readings isusually more inrportant than the ahsolute valdc of the rcadings. All rcadirrgs should trcidentical - if onc reading is significantly smaller than the others this could indicate thepossibility of short-circuited turns in the winding being measured. Conversely, a high con-tinuity resistance value indicates a high resistance fault or an open-circuit (e.g. a looseconnection).Some models of insulation/cohtinuity testers provide means to measure 'ac voltage' andresistance in the 'kQ' range.

Multimcterc

Routine etectrical test work involves measuring amps, volts and ohms. This is most conve-niently done using a multimeter with all the necessary functions and ranges. 'fhe

instrumentmay be the traditional analogue type (pointer and scale) or the more modern digital type witha numerical display.Range selection can be by rotary selector switches or a range of push buttons depending uponthe model.In all instrument models an internal battery is fitted for use when measuring resistance.Before measuring the resistance of a comp,onent it is essential rhat the componenr circuit isswitched off, locked off. and any capacitors discharged. The instrument is likely to bedamaged oiherwise.The instrument should be proved for correcl operation before use. The manufacturets'instructions should be carefully followed for this but a general procedure is as follows:Use the CORRECT probe leads and insert into the correct sockets on the meter. lf themultimeter is an ANALOCUE type: Ensure the pointer indicates zero - adjust if necessary.Set selector switches to '51' and connect probe tips together. Pointer should deflecl to indicate0Q - Adjust trimmingcontrols. Check each resistance range in this way. Set selectorswitch to'acV'(highest range). Connect piobes to a suitable known live supply (wirh CARE) such asthe electrical workshop test panel. Pointcr should indicate correct voltage.

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Instrument battery failure is checkecl with the instrument sel to read'Q' with the probe tipsconnected together. lf the pointer fails to reach '0Q' after adjustment of the resistance rangeadjust knob, the battery must be replaced. The instrument should be switched OFF when notin use to conserve battery life.

I f the mblt imeter is a DIGITAL rypc:Switch on and connect probe tips together. Sct selector switches to 'dcV' (highest range).Display should indicate zero (Obcf ). Rep.ot for all 'dcV' selector swirch prsitions anrl notemovement of the decimal point. Scparate probe tips. Sct sclcctor switches to'Q' (highestrange). Display should indicate'OL'(over-range) or '1i l ) ' (depcnds upon model). Connectprobe tips together - display should indicate zero (OOO). Repeat for all 'Q' selector switchpositions and noteimovemenl of the decimal ptlint. Set selector switches to 'acV' (highestrangc). Connect protrcs to a suitable known live suppty. Display should inrJicatc c()rreclvoltage. Test the dc voltage range also and note thc polarity indication on the nt!.ter.

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In:rrunrcnr hatrcrv fli lure is indicatcd hy thc numcric display. The tlisplay may include'BT'()r the 6ecirntl point may blink. or some other display mcans may be used. 'I 'he instrumenl

shguld be switchc{ 'OFF' when not in usc ttl conserve baltery life.

These simple 'proving tcsts' should be performed every time before using the instrument for.real'. lt is obvigusly rlungerous to touch conductorb believing them to be 'dead' having

checkcd thcm with a faulty instrument.

To MEASURE RESTSTANCE the procedure to be a<lopted should be:

PROVE the correct operation of the instrument.ISOLATE and lock off the equipment to be tested.PROVE the equipment to be dead'SWTTCH the insrrument to the appropriate resistance range, connect the probes to'the

equipment and note the resistance value.Disconnect the probes and switch the instrument to OFF.

To MEASURE VOLTAGE:PROVE the crrrrect instrument operation.SWI'ICH the instrumenr ro the HIGHEST voltage range (either acV or dcV as appropriate).

CONNECT the probes to the terminals being tested. Takc great care not to touch the probe tipl

and remember that the equipment being tested b LIVE.NOTE rhe voltage reading" If a LOWER voltage range would give a more accurate reading'

adjust the selector switches accordingly.No harm will be causerl to the instrumenl by operating the selector range swilches while still

connected ro a live supply. BUT CREAT CARE must be taken nol to switch into either thc

current or resistance mo{e. This woukJ almost certainly operate the instrumeni overlord

device and may cause severe damage to the instrument and danger to yourself. Take your

time t6 op€rate the selector switches rturing the operation and THINK about whal ydu are

doing.Disconnect the probes and switch the instrument to'OFF''

To MEASURE CURRENT:Most test inslruments can only measure up to a fcw amps (usually l0A or less)'

'I 'he cufrcnt

measuring facility is inrenderl only for light-current componenls, and in particular, fol

electroniC circuits. The insrrument will almost certainly be damaged if it is used to fieasurc

the current of motors and other power circuits.The basic currenr range can be exlended by using external shunts (dc) and currenl transfor'

mers (ac). '[hcse acccssrtries are gene rally purchased separately from the inslrument manu'

facturers.The procedure trt be used lo measure. currenl in a light'current circuit is as follows:

PROVE thc corrcct instrument operation.SWll'Cll rhc insrrumcnr rtr thc lllGtlEsTcurrent range (either acA ordcA as appropriate).

Turn ()Fl- thc powcr ttt the circuil to be tested and discharge all capacitors.

OPEN tlre circuit in which current is trl be measured - removing a fusc-link often gives a

convcnicnt poinl ti lr currcnt measurcment.

Securely conncct the prrlbes in series wirh the kratl in which current is to be measured. Turn

ON. rhe power to the iircuir being tested, R.l,.l lhc currcnt value oli the meter display. Turn

OFF rhi power to rhe circuit being testerJ and discharge all capacitgrs. Disconnect lhc tesl

probes and swirch the instrument to OFF. Reconn.ect the circuit thut was being testcd.

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Diodc tcsts

Diodes can be tested using a digital type instrumenr using the following pr<redure:PROVE the correct instrument operation.SWITCH the instrument to diode test. --ffIn this test the instrument drives a small dc current (a few mA) through the diode while it alsoacts as a voltmeter to measure the volt-drop across it. If lhe dbde is stil l in circuit. turn off thepower to the circuit, discharge all capacitors and remove fuses.CONNECT the probes across the diode.READ the forward volt drop across the diode. This should be between 500 and 9(Xf mV for ahealthy silicon diode. REVERSE the probe connections and the display should indicate'over-range'.

lf the display indicates over-range in both directions the diode is 'open-circuit' faulted.If the display indicates less than I V in both directions, the diode may be short-circuit faulted.The associated diode circuitry may be giving false readings and the diode must be discon-nected from the circuit then retested.

Currcnt clrmpmcters

Power currents can be measured simply by means of a clampmeter. The instrument 'tongs'

are clipped round a single insulated conductor - the circuit is not interrupted. The current isread off either from a digital display or an analogue display.

Many modern clampmeters are virtually multimeters with the addition of facilities to measurevoltage and resistance as well as measuring currents up to lffnA.CARE must be taken when measuring the current in UNINSULATED conductors.

QUF^STION

What would aclampmeter inrJicate if ctippeO around,a.3-core cablc which is known io bccarrying l00A ac to a motor?

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ANSWER

Zero. This is because the clampmeter monitors the magnetic flux around the cable which isproduced by the current. ln a balanced 3-core (or 2-core for that matter) cablc, the net flux iszero - hence no indication. This is why the clampm0ter is only connected arcund oneconductor.

Live-llm teslcrs

When equipment is to be inspected for maintenance it is important that supplies be switchcdOFF and locked OFF. The equipment must then b€ PROVED to be dead ro eliminate thcdanger of electric shock. A live-line (or voltage) tester is a simplc device to check whcther ornot a voltage exists at lerminals.Live-line testers are of various types. Some light up (e.g. screwdriver type with a neonindicator). some make a noise, others operate mechanical indicators (flags) to indicate thcapproximate value of voltage. It is important that voltage ibsters themselves be PROVED tooperale correctly before use. This can be conveniently carried out at lhe e lectrical workshoptest panel.Home-made test lamps should not be used as they can be dangerous.

Gcncrd dccfricrl mrfoilenemc

All equipment is subject to wear and tear. €ventually reaching rhe end of its useful life when itmust be replaced. As equipme nt nears the end of its lifc its condition can delerioratc to suchan extent as to be a danger to p€rsonnel and other plant. The purpose of maintename,therefore, is to extend the useful life by repair and/or replacement of defective parts and lomaintain it in a safe and serviceable condition.The marine environment is particularly arduous for electrical equipment due to the damp.salt'laden atmosphere. extremes of temperature and constant vibration. Shipboard equip-ment is in particular need of correct maintenance. The continuous operation of equipment onboard ship demands high operating efficiency and optimum economy in ortler to keep downcosts io maintain financial competitiveness.Nearly all equipment NEEDS maintenance. An efficient maintenance engineer must tct toknow his plant. He.must be able to check shipc'drawings and diagrams and relate rhiin toactual equipment. Equipment must be kept under continuous observation so that normelheahhy operating conditions become known. and abnormal operation becomes quicklyapparent. Faults can then be pin-pointed and corrected before a breakdown occurs. Maintc-nance can be classified as: Breakdown maintenance. Planned maintenancc and Conditionmonitoring.Breakdown maintenance (corrective maintenance) is that in which equipment is left un-touched until a breakdown occurs. At this time the equipme nt is re paired or replaccd and anyother specified maintenance prrxedure carried out.Planned maintenance (preventive maintenance) is that in which equipment is regulartyinspected and maintained according to a laid rlown rimetable and se t of procedurer speiifyin!the actual work to be done at particular times in order to prevent faiture of equipment.Condition monitoring (anothe r form of preventive maintenance) is that in which equipmentis regularly monitorcd and tested. Whcn monitoring indicates that breakdown is immincnt,the equipment is repaired or replaced and any other specified maintenance proccdurescarried out. Regular insulation testing and vibration testing arb two forms oi conditionmonitor ing.There are several disadvantages in breakdown maintenance:l. A serious breakdown of equipment may cause sufficient down time to put rhe shipoul of

commission until it is repaired.2. If seve ral.breakdowns occur simultaneously the available manpower on board ship may

not be able to cope adequately, resulting in delays.3. Some ilems of equipment may need thc specialist services of the manufacturer to carry

out repairs which may cause furrher delays.

Planned mainienancc is carried out at fixed regular intervals whether rhe equipment needs itor not and the aim is to prevent breakdown. This type of maintenance has the followingadvantages:l. Fewcr brcakdowns and reduced tlown time prlxlutes highdr levels.of operating cffi-

ciency.

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2. Maintenancc is carricd out lt timcs favourablc to the rrpcration of thc plant,3. More effective latxrur util isulion bccause maintenance is carricd oui at time s lavourahle

to ships' staff.4. Replacement equipment can be ordered in advance.5. Equipment is maintaincd in a safe condition with reduced possiblc ditngcrs.6. Where specialist manufacturers' scrviccs are required these can be obtained al pre-

planned and convenient times.7. Replacement of short life components at scheduled times.Condition monitoring is also carried out at fixed rcgular intervals. The aim is to forestallbreakdown by predicting probable failure from the TREND shown by the monitoring results.The advantage of this type of maintenance is that equipment is not subjected to unnecessarymaintenance.Equipment is regularly condition-monitored according to a monitoring schedule. Measure-ments are taken of insulation resistance. temperature and vibration (of motoqs). Contactsand other parts subject to deterioration are inspected. All findings are recortled in anhistorical record file. No maintenance is carried out until fin<fings indicate that it is ahsolutelynecessary. The equipment is then either replaced. repaired or subjected lrl a major overhaulas specified on a job card.A ricords system is required. The recorcled measurements of insulalion resistance may showa falling trend indicating a progressive degradation of insulation. The equipment should beinspected and repaired before the insulation resistance falls to a dangerously klw value.The recorded measurements of the vibration of a motor may follow a rising trend indicatingprogressive bearing deterioration. Bearings should be replaced before a final bearing failure(rccurs.. lmmediate repair or maintenance is probably not necessary but should be put in handat the earliest convenient mom€nl.

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