Designing an electrical installation Beginner Guide Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 2 SUMMARY 1. Consumers list....5 1.1. Definition of voltage ranges..5 1.2. Installed power loads - characteristics....6 1.2.1. Induction motors......6 1.2.2. Resistive-type heating appliances and incandescent lamps(conventional or halogen)...8 2. Power balance..12 2.1. Power loading of an installation.12 2.1.1. Installed power (kW).12 2.1.2. Installed apparent power (kVA)....13 2.1.3. Estimation of actual maximum kVA demand...13 2.1.4. Example of application of factors ku and ks...16 2.1.5. Diversity factor..17 2.2. Choice of transformer rating..17 2.3. Example....18 3. Single line diagram......19 4. Study of each electrical section...21 4.1. Determination of the rated current of the protective devices.....214.2. Determination of the sections of cables.22 4.2.1. Determination of conductor size for unburied circuits..23 4.2.2. Determination of conductor size for buried circuits......27 4.2.3. Sizing the neutral conductor..31 4.2.3.1. Influence of the earthing system....31 4.2.3.2. Influence of harmonic currents...31 4.2.3.3. Protection of the neutral conductor....33 4.2.3.4. Breaking of the neutral conductor......34 4.2.3.5. Isolating of the neutral conductor...35 4.2.4. Sizing the protective earthing conductor (PE)..35 4.2.4.1. Connection..35 4.2.4.2. Type of materials....36 4.2.4.3. Conductor sizing.37 4.2.5. Calculation of Lmax. for a TN-earthed system, using the conventional method.39 4.2.6. Rules fore marine electrical cables according Bureau Veritas..40 4.3. Determination of the voltage drop.46 4.3.1. Maximum voltage drop limit.....46 Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 3 4.3.2. Calculation of voltage drop in steady load conditions..47 4.3.3. Examples.......49 4.4. Determination of the short-circuit currents......51 4.4.1. Calculation of maximum short-circuit currents in electrical ship mains according GERMANISCHER LLOYD SCC363.EXE.51 4.4.1.1. Principles of the calculation...52 4.4.1.2. Structure of the Ship Mains to be calculated..54 4.4.1.3. Asymmetric short circuit55 4.4.1.4. Remarks on input data and components.55 4.4.1.5. Simplified calculation.59 4.4.1.6. Selection of switch gear.60 4.4.4.7. The documentation.60 4.4.2. Short-circuit current calculation according BUREAU VERITAS61 4.4.2.1. Main methods.61 4.4.2.2. Theoretical considerations..62 4.4.2.3. Formulas.67 4.4.2.4. Selection of protective devices...69 4.5. Worked example of cable calculation....71 4.6. Choice of the protective devices.....74 4.6.1. The basic functions of LV switchgear.......74 4.6.2. Elementary switching devices.......72 4.6.2.1. Disconnector (or isolator)...78 4.6.2.2. Load-breaking switch.78 4.6.2.3. Bistable switch (tlrupteur)..79 4.6.2.4. Contactor80 4.6.2.5. Fuses...84 4.6.2.5. Circuit breaker87 4.6.3. Combined switchgear elements.93 4.6.3.1. Switch and fuse combinations94 4.6.3.2. Fuse - disconnector + discontactor,Fuse - switch-disconnector + discontactor.95 4.6.3.3. Circuit-breaker + contactor circuit-breaker + discontactor96 4.6.4. Selection of a circuit breaker.96 4.6.4.1. Choice of a circuit breaker..96 4.6.4.2. The selection of main and principal circuit-breakers..99 4.6.5. Protection of circuits according GL102 4.6.6. Protection of circuits according Bureau Veritas..102 4.7. Selectivity of the protections.....103 4.7.1. Cascading103 4.7.2. Discriminative tripping (selectivity)...103 5. Electrical machines........109 5.1. Induction motors....109 5.1.1. The basic functions of the motor-starters....109 5.1.2. The motor start solutions.....110 5.1.2.1. D.O.L. solutions...110 5.1.2.2. Star-delta solution.114 5.1.2.3. Star-double star solution (Dahlander connection)116 5.1.3. Variable speed drives for asynchronous motors (Altivar 38)..116 Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 4 5.1.3.1. Applications..116 5.1.3.2. Functions..117 5.1.3.3. Options.....117 5.1.3.4. Characteristics..118 5.1.3.5. Special uses..118 5.1.3.6. Connection diagrams....120 5.2. Connection diagrams for synchronous generators....126 5.3. Connection diagrams for DC- Motors.....127 5.4. Protection of motors according Bureau Veritas.....128 5.5. Protection of generators....129 5.6. Transformers.132 5.6.1. Basic principals.......132 5.6.2. Circuit symbols....133 5.6.3. Transformers types......133 5.6.4. Vectors-groups of transformers...135 5.6.5. Important equations.........138 5.6.6. Protection of transformers according Bureau Veritas.....139 6. Technical information...140 6.1. Degrees of protection provided by enclosures....140 6.2. Degrees of protection against mechanical impact......142 6.3. Minimum required degrees of protection on ships (Bureau Veritas)...143 Lexicon145 Bibliography...150 Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 5 CHAPTER1 1. Consumers list The first step for the designing of an electrical installation, whatever will be, involve the knowledge en detail of whole consumers which utilize her. In general the minimum of information required per consumer are: thetypeendthevalueofthepowersupply(alternativethree-phaseelectrical voltage, D.C. voltage etc.); the electrical power of the consumer; the rated current of the consumer; the number of consumers of the same type. In ships the principal consumers, supplied directly from the main switchboard, are:lighting equipment, power equipment,heating equipment, control & signaling, warming equipment, fire pumps, bilge pumps, radio equipment, steering gear, lateral thruster, sprinkler equipment, variable pitch propeller systems, CO2 system, auxiliary services for main engine, panels for ship and engine control. 1.1. Definition of voltage ranges IEC voltage standards and recommendations Thenominalvoltageofexisting220/380Vand240/415Vsystemsshallevolve toward the recommended value of 230/400 V. The transition period should be as short as possibleandshouldnotexceedtheyear2008.Duringthisperiod,asafirststep,the electricitysupplyauthoritiesofcountrieshaving220/380Vsystemsshouldbringthe voltage within the range 230/400 V +6 %, -10 % and those of countries having 240/415 V systems should bring the voltage within the range 230/400 V +10 %, -6 %. At the end of this transition period, the tolerance of 230/400 V 10 % should have been achieved; after Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 6 this the reduction of this range will be considered. All the above considerations apply also to the present 380/660 V value with respect to the recommended value 400/690 V. Fig.1.1. Standard voltages between 100 V and 1000 V 1.2. Installed power loads - characteristics The examination of actual values of apparent-power required by each load enables the establishment of: a declared power demand which determines the source for the supply of energy; theratingoftheHV/LVtransformer,whereapplicable(allowingforexpected increases load); levels of load current at each distribution board. 1.2.1. Induction motors Current demand The full-load current Ia supplied to the motor is given by the following formulae: 3-phase motor: cos 3000 , 1 UPIna; 1-phase motor: cos000 , 1 UPIna where Ia - current demand (in amps); Pn - nominal power (in kW of active power); U-voltagebetweenphasesfor3-phasemotorsandvoltagebetweentheterminalsfor single-phasemotors(involts).Asingle-phasemotormaybeconnectedphase-to-neutral or phase-to-phase; - per-unit efficiency, i.e. (output kW)/( input kW); cos - power factor, i.e. (kW input)/( kVA input). Subtransient current and protection setting Subtransient current peak value can be very high; typical value is about 12 to 15 times the RMS rated value Inm. Sometimes this value can reach 25 times Inm. MerlinGerincircuit-breakers,Telemecaniquecontactorsandthermalrelaysare designed to withstand motor starts with very high subtransient current (subtransient peak value can be up to 19 RMS rated value Inm). If unexpected tripping of the overcurrent protection occurs during starting, this means thestartingcurrentexceedsthenormallimits.Asaresult,somemaximumswitchgears Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 7 withstands can be reach, life time can be reduce and even some devices can be destroyed. In order to avoid such a situation, oversizing of the switchgear must be considered. Merlin Gerin and Telemecanique switchgears are designed to ensure the protection of motor starters against short circuits. According to the risk, tables show the combination of circuit breaker, contactor and thermal relay to obtain type 1 or type 2 coordination. Motor starting current Althoughhighefficiencymotorscanbefindonthemarket,inpracticetheirstarting currents are roughly the same as some of standard motors. Theuseofstart-deltastarter,staticsoftstartunitorspeeddriveconverterallowsto reduce the value of the starting current (Example: 4Ia instead of 7.5Ia). Compensation of reactive-power (kvar) supplied to induction motors Itisgenerallyadvantageousfor technicalandfinancialreasonsto reduce thecurrent supplied to induction motors. This can be achieved by using capacitors without affecting the power output of the motors. Theapplicationofthisprincipletotheoperationofinductionmotorsisgenerally referred to as power-factor improvement or power-factor correction. Theapparentpower(kVA)suppliedtoaninductionmotorcanbesignificantly reducedbytheuseofshunt-connectedcapacitors.ReductionofinputkVAmeansa corresponding reduction of input current (since the voltage remains constant). Fig. 1.2. Rated operational power and currentsDesigning an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 8 Compensationofreactive-powerisparticularlyadvisedformotorsthatoperatefor long periods at reduced power. As noted above cos = (kW input)/(kVA input) so that a kVA input reduction in kVA input will increase (i.e. improve) the value of cos. Thecurrentsuppliedtothemotor,afterpower-factorcorrection,isgivenby: ' cos / cos aI wherecos isthepowerfactorbeforecompensationandcosisthe power factor after compensation, Ia being the original current. It shouldbenoted that speed drive converter provides reactive energy compensation. Figure 1.2. below shows, in function of motor rated power, standard motor current values for several voltage supplies. 1.2.2.Resistive-typeheatingappliancesandincandescentlamps (conventional or halogen) The current demand of a heating appliance or an incandescent lamp is easily obtained from the nominal power Pn quoted by the manufacturer (i.e. cos = 1) (see Fig. 1.3.). Fig. 1.3. Current demands of resistive heating and incandescentlighting (conventional or halogen) appliances The currents are given by: 3-phase case: UPIna3 1-phase case: UPIna where U is the voltage between the terminals of the equipment. Foranincandescentlamp,theuseofhalogengasallowsamoreconcentratedlight source. The light output is increased and the lifetime of the lamp is doubled. Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 9 Note:Attheinstantofswitchingon,thecoldfilamentgivesrisetoaverybriefbut intense peak of current. Fluorescent lamps and related equipment The power Pn (watts) indicated on the tube of a fluorescent lamp does not include the power dissipated in the ballast. The current is given by: cos +UP PIn ballasta If no power-loss value is indicated for the ballast, a figure of 25% of Pn may be used. Standard tubular fluorescent lamps The power Pn (watts) indicated on the tube of a fluorescent lamp does not include the power dissipated in the ballast. The current taken by the complete circuit is given by: cos +UP PIn ballasta where U is the voltage applied to the lamp, complete with its related equipment. With (unless otherwise indicated): cos = 0.6 with no power factor (PF) correction ( capacitor); cos = 0.86 with PF correction (single or twin tubes); cos = 0.96 for electronic ballast. Fig. 1.4. Current demands and power consumptionof commonly-dimensioned fluorescent lighting tubes (at 230 V-50 Hz) Fig. 1.5. Current demands and power consumption of compactfluorescent lamps (at 230 V - 50 Hz) Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 10 If no power-loss value is indicated for the ballast, a figure of 25% of Pn may be used. Figure 1.4. gives these values for different arrangements of ballast. Compact fluorescent lamps Compact fluorescent lamps have the same characteristics of economy and long life as classicaltubes.Theyarecommonlyusedinpublicplaceswhicharepermanently illuminated (for example: corridors, hallways, bars, etc.) and can be mounted in situations otherwise illuminated by incandescent lamps (see Fig. 1.5.). Discharge lamps Thepowerinwattsindicatedonthetubeofadischargelampdoesnotincludethe powerdissipatedintheballast.Figure1.6.givesthecurrenttakenbyacompleteunit, including all associated ancillary equipment. These lamps depend on the luminous electrical discharge through a gas or vapour of a metallic compound, which is contained in a hermetically-sealed transparent envelope at a pre-determined pressure. These lamps have a long start-up time, during which the current IaisgreaterthanthenominalcurrentIn.Powerandcurrentdemandsaregivenfor differenttypesoflamp(typicalaveragevalueswhichmaydifferslightlyfromone manufacturer to another). Fig. 1.6. Current demands of discharge lamps Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 11 Note:Theselampsaresensitivetovoltagedips.Theyextinguishifthevoltagefallsto lessthan50%oftheirnominalvoltage,andwillnotre-ignitebeforecoolingfor approximately 4 minutes. Sodiumvapourlow-pressurelampshavealight-outputefficiencywhichis superior to that of all other sources. However, use of theselampsis restricted by thefactthetallow-orangecolouremittedmakescolourrecognitionpractically impossible. Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 12 CHAPTER2 2. Power balance The power balance consist in the determination, for every consumer, of the followings parameters: electrical power installed,loadfactor, timefactor end the power required by the consumer in the normal work regime. The settlement of this balance is very serious for the dimension end the correct choice of the electrical lines, of the electrical cables, of the transformers end the main generator. In general the electrical power installed must to be smaller than the electrical power of the generator. 2.1. Power loading of an installation Inordertodesignaninstallation,theactualmaximumloaddemandlikelytobe imposed on the power-supply system must be assessed. Tobasethedesignsimplyonthearithmeticsumofalltheloadsexistinginthe installation would be extravagantly uneconomical, and bad engineering practice. The aim of this chapter is to show how some factors taking into account the diversity (nonsimultaneousoperationofallappliancesofagivengroup)andutilization(e.g.an electric motor is not generally operated at its full-load capability, etc.) of all existing and projected loads can be assessed. The values given are based on experience and on records takenfromactualinstallations.Inadditiontoprovidingbasicinstallation-designdataon individual circuits, the results will provide a global value for the installation, from which therequirementsofasupplysystem(distributionnetwork,HV/LVtransformer,or generating set) can be specified. 2.1.1. Installed power (kW) The installed power is the sum of the nominal powers of all power-consuming devices in the installation. But this is not the power to be actually supplied in practice.Most electrical appliances and equipments are marked to indicate their nominal power rating (Pn). This is the case for electric motors, where the power rating refers to the output power at its driving shaft. The input power consumption will evidently be greater. Fluorescent and discharge lamps associated with stabilizing ballasts are other cases in which thenominal powerindicated on thelampisless than the power consumedby the lamp and its ballast. The power demand (kW) is necessary to choose the rated power of a generating set or battery, and where the requirements of a prime mover have to be considered. For a power Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 13 supplyfromaLVsupplynetwork,orthroughaHV/LVtransformer,thesignificant quantity is the apparent power in kVA. 2.1.2. Installed apparent power (kVA) The installed apparent power is commonly assumed to be the arithmetical sum of the kVAofindividualloads.ThemaximumestimatedkVAtobesuppliedhoweverisnot equal to the total installed kVA. The apparent-power demand of a load (which might be a single appliance) is obtained fromitsnominalpowerrating(correctedifnecessary,asnotedaboveformotors,etc.) and the application of the following coefficients: = the per-unit efficiency = output kW / input kW cos = the power factor = kW / kVA The apparent-power kVA demand of the load: Pa = Pn /( cos) From this value, the full-load current Ia taken by the load will be: Ia = (Pa103)/V for single phase-to-neutral connected load ) 3 /( ) 10 (3V P Ia a forthree-phasebalancedloadwhere:V-phase-to-neutralvoltage(volts);U-phase-to-phase voltage (volts). Itmaybenotedthat,strictlyspeaking,thetotalkVAofapparentpowerisnotthe arithmetical sum of the calculated kVA ratings of individual loads (unless all loads are at the same power factor). Itiscommonpracticehowever,tomakeasimplearithmeticalsummation,theresult ofwhichwillgiveakVAvaluethatexceedsthetruevaluebyanacceptabledesign margin. Whensomeoralloftheloadcharacteristicsarenotknown,theestimationofthe valuesofinstalledapparentpowermaybeusedtogiveaveryapproximateestimateof VA demands. 2.1.3. Estimation of actual maximum kVA demand Allindividualloadsarenotnecessarilyoperatingatfullratednominalpowernor necessarilyat thesametime.Factorskuandksallow thedeterminationofthemaximum power and apparent-power demands actually required to dimension the installation. Factor of maximum utilization (ku - load factor) Innormaloperatingconditionsthepowerconsumptionofaloadissometimesless than that indicated as its nominal power rating, a fairly common occurrence that justifies the application of an utilization factor (ku) in the estimation of realistic values. Thisfactormustbeappliedtoeachindividualload,withparticularattentionto electric motors, which are very rarely operated at full load. Inanindustrialinstallationthisfactormaybeestimatedonanaverageat0.75for motors. For incandescent-lighting loads, the factor always equals 1. Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 14 For socket-outlet circuits, thefactors depend entirely on the type of appliancesbeing supplied from the sockets concerned. Factor of simultaneity (ks - time factor) Itisamatterofcommonexperiencethatthesimultaneousoperationofallinstalled loads of a giveninstallationnever occursin practice,i.e. thereis alwayssome degree of diversityandthisfactistakenintoaccountforestimatingpurposesbytheuseofa simultaneity factor (ks). The factor ks is applied to each group of loads (e.g. being supplied from a distribution or sub-distribution board).Thedeterminationofthesefactorsistheresponsibilityofthedesigner,sinceit requiresadetailedknowledgeoftheinstallationandtheconditionsinwhichthe individualcircuitsaretobeexploited.Forthisreason,itisnotpossibletogiveprecise values for general application. Example: Factor of simultaneity for an apartment block SometypicalvaluesforthiscasearegiveninFigure2.1.,andareapplicableto domesticconsumerssuppliedat230/400V(3-phase4-wires).Inthecaseofconsumers usingelectricalheat-storageunitsforspaceheating,afactorof0.8isrecommended, regardless of the number of consumers. Fig. 2.1. Simultaneity factors in an apartment block Example (see fig. 2.2.): Fivestoreysapartmentbuildingwith25consumers,eachhaving6kVAofinstalled load. The total installed load for the building is: 36 + 24 + 30 + 36 + 24 = 150 kVA. The apparent-power supply required for the building is: 1500.46 = 69 kVA Fromfigure2.2.,itispossibletodeterminethemagnitudeofcurrentsindifferent sections of the common main feeder supplying all floors. For vertical rising mains fed at groundlevel,thecross-sectionalareaoftheconductorscanevidentlybeprogressively reduced from the lower floors towards the upper floors. These changes of conductor size are conventionally spaced by at least 3-floor intervals. In the example, the current entering the rising main at ground level is: A 1003 40010 46 . 0 1503 the current entering the third floor is: A 553 40010 63 . 0 ) 42 36 (3 + Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 15 Fig. 2.2. Application of the factor of simultaneity (kS)to an apartment block of 5 storeys Example: Factor of simultaneity for distribution boards Figure2.3.showshypotheticalvaluesofkSforadistributionboardsupplyinga number of circuitsfor which thereisnoindication of themannerin which the totalload divides between them. Ifthecircuitsaremainlyforlightingloads,itisprudenttoadoptkSvaluescloseto unity. Fig. 2.3. Factor of simultaneity for distribution boards Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 16 Example: Factor of simultaneity according to circuit function Ksfactorswhichmaybeusedforcircuitssupplyingcommonly-occurringloads,are shown in figure 2.4. Fig. 2.4. Factor of simultaneity according to circuit function 2.1.4 Example of application of factors ku and ks AnexampleintheestimationofactualmaximumkVAdemandsatalllevelsofan installation, from each load position to the point of supply (see Fig. 2.5.). Fig. 2.5. An example in estimating the maximum predicted loading of an installation (the factor values used are for demonstration purposes only) Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 17 Inthisexample,thetotalinstalledapparentpoweris126.6kVA,whichcorresponds to an actual (estimated) maximum value at the LV terminals of the HV/LV transformer of 65 kVA only. Note:inordertoselectcablesizesforthedistributioncircuitsofaninstallation,the current I (in amps) through a circuit is determined from the equation: 3103UkVAIwhere kVAis the actualmaximum 3-phase apparent-power value shown on the diagram for the circuit concerned, and U is the phase-to-phase voltage (in volts). 2.1.5. Diversity factor Thetermdiversityfactor,asdefinedinIECstandards,isidenticaltothefactorof simultaneity(ks)usedinthisguide.InsomeEnglish-speakingcountrieshowever(at the time of writing) diversity factor is the inverse of ks i.e. it is always 1. 2.2. Choice of transformer rating WhenaninstallationistobesupplieddirectlyfromaHV/LVtransformerandthe maximumapparent-powerloadingoftheinstallationhasbeendetermined,asuitable ratingforthetransformercanbedecided,takingintoaccountofthefollowing considerations (see Fig. 2.6.): the possibility of improving the power factor of the installation;anticipated extensions to the installation; installation constraints (temperature...) standard transformer ratings. Fig. 2.6. Standard apparent powers for HV/LVtransformers and related nominal currents The nominal full-load current In on the LV side of a 3-phase transformer is given by: 3103UPIan Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 18 where: Pa - kVA rating of the transformer U - phase-to-phase voltage at no-load in volts (237 V or 410 V) In is in amperes. For a single-phase transformer: VPIan310 where: V - voltage between LV terminals at no-load (in volts); In = kVA1.4 - simplified equation for 400 V (3-phase load). 2.3. Example ConsumerElectrical power Installed KW Load factor % Time factor % Power required KW anchor winch16.5000 crane3.0000 starting air compressor 1 7.580201.2 starting air compressor 2 7.5000 cargo hold ventilation stb. 3.6801002.9 engine room ventilation stb. 4.0851003.4 stearing gear 17.5601004.5 stearing gear 27.5000 hfo separator 14.0651002.6 hfo separator 24.0651002.6 hfo heater 136.05010018.0 hfo heater 224.05010012.0 seawater cooling pump 18.58010014.8 fuel pump 11.1851000.9 fuel pump 21.1000 fuel transf. pump4.080100.3 compressor aircond. plant 31.06510020.2 lighting22.0401008.8 electric stove11.750402.3 container ( 50 pcs, 10KW each) 5008070280 all others16699.5 880.5 without container194 with container474 requiredelectricalpoweratmaneuveringareawithoutcontainerand bow thruster 209.3 Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 19 CHAPTER3 3. Single line diagram Averyimportantstepinthedesigningofanelectricinstallationistherealizationof the one line diagram. Fig. 3.1. Single line diagram example The oneline diagramis the drawing of the simplified general electrical diagram. For example,for a three-phase electricalnetwork the system wires are represented through a single line which shall be marked by three little parallel lines. In the case of a three-phase systemwithfourwires,withaneutralconductor,intheonelinediagramshallappeara Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 20 singlelinemarkedbyfourlittleparallellines;sametimesthe4thlittleline,theneutral line, present on the end a little circle.Ingeneralintheonelinediagramtheconsumeraregroupatedandboundedthein blocksandsectionsdependingonthetypeandthevalueoftheelectricalsupplyandon the consumers types. Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 21 CHAPTER4 4. Study of each electrical section The study of each electrical installation must be maked methodically while respecting the following most important stages: determination of the rated current, In, of the protective devices, determination of the sections of cables, determination of the tension drop,determination of the short-circuit currents, choice of the protective devices, selectivity of the protections, verification of the protection of people. 4.1. Determination of the rated current of the protective devices The determination of the rated current, In, of each protected device is based on the maximum load current, IB. Fig. 4.1. Calculation of maximum load current IB Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 22 At the final circuits level, this current corresponds to the rated kVA of the load. In the caseofmotor-starting,orotherloadswhichtakeaninitially-highcurrent,particularly where frequent starting is concerned (e.g. lift motors, resistance-type spot welding, and so on)thecumulativethermaleffectsoftheovercurrentsmustbetakenintoaccount.Both cables and thermal type relays are affected. AtallupstreamcircuitlevelsthiscurrentcorrespondstothekVAtobesupplied, whichtakesaccountofthefactorsofsimultaneity(diversity)andutilization,ksandku respectively, as shown in figure 4.1. Thefull-loadcurrentscalculationforthemostusedloadsisdescribedinthefirst chapter of this document at the paragraph 1.2. In conclusion the rated current In of the protective devices must be equal to or greater than the maximum load current IB (fig. 4.2.). B nI I Fig. 4.2. Determination of the rated current In of protective devices 4.2. Determination of the sections of cables Thefirststepistodeterminethesizeofthephaseconductors.Inthisclausethe following cases are considered: unburied conductors, buried conductors. The tables in this clause permit the determination of the size of phase conductors for a circuit of given current magnitude. The procedure is as follows: determine an appropriate code-letter reference which takes into account: - the type of circuit (single-phase; three-phase, etc.); - the kind of installation. determinethefactorKofthecircuitconsidered,whichcoversthefollowing influences: - installation method; - circuit grouping; - ambient temperature.

IMPORTANT NOTES:ThisprocedureisacombinationofIEC60364-5-52requirementsandSchneider Electric recommendations (fig. 4.3.). Thisprocedureisalsopresentedasaprincipleforthedeterminationofthe sectionsofcables.Fordifferentmanufacturersofcablestheprinciplereststhe Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 23 samewiththespecificsvariationsdependingoftheapplicationtype,ofthe environment temperature and the necessaries protections. Forexamplefortheelectricalmarineapplicationsisutilizedonlycoppercables becauseofthesuperiorelectricpropertiesofthecopperandbecausethecross-sectionalareaofthealuminiuncablesisbiggerthanthecross-sectionalareaof coppercables.Themarinecablesarealsotobeprotectedagainstthecorrosion because the marine environment is very wet and very corrosive. Fig. 4.3. Logigram for the determination of minimum conductor size for a circuit 4.2.1. Determination of conductor size for unburied circuits Determination of the code-letter reference The size of a phase conductor is given in tables which relate: the code letter symbolizing the method of installation, and the factor of influence K. These tables distinguish unburied circuits from buried circuits. Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 24 The letter of reference (A to G) depends on the type of conductor used and its method ofinstallation. The possiblemethods ofinstallation are numerous, but the most common of them have been grouped according to four classes of similar environmental conditions, as shown below in figure 4.4. Fig. 4.4. G12 Code-letter reference, depending on type of conductor and method of installation Determination of the factor K Forcircuitswhicharenotburied,factorKcharacteristizestheconditionsof installation,andisgivenby:K=K1K2K3.Thethreecomponentfactorsdependingon differentfeatures of theinstallation. The values of thesefactors are givenin figures 4.5. to 4.7. below. Correction factor K1 (see Fig. 4.5.) Factor K1 is a measure of the influence of the method of installation. Fig. 4.5.Factor K1 according to method of circuit installation Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 25 Correction factor K2 (see Fig. 4.6.) FactorK2isameasureofthemutualinfluenceoftwocircuitsside-by-sideinclose proximity.TwocircuitsareconsideredtobeincloseproximitywhenL,thedistance between two cables, is less than double the diameter of the larger of the two cables. Fig. 4.6.Correction factor K2 for a group of conductors in a single layer When cables are installed in more than one layer a further factor, by which K2 must be multiplied, will have the following values: 2 layers: 0.80 3 layers: 0.73 4 or 5 layers: 0.70 Note:IEC60364-5-52recommendsusingacorrectionfactorwhencablesare installed in more than one layer but no values are given. Correction factor K3 (see Fig. 4.7.) FactorK3isameasureoftheinfluenceofthetemperature,accordingtothetypeof insulation. Fig.4.7. Correction factor K3for ambient temperature other than 30 0C Example A 3-phase 3-core XLPE cableislaid on a perforated cable-trayin close proximity to three other circuits, consisting of: Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 26 a 3-phase 3-core cable (circuit no. 1); three single-core cables (circuit no. 2); six single-core cables (circuit no. 3). Circuitno. 2 and no. 3 are 3-phasecircuits, thelatter comprising 2 cables per phase. Thereare,therefore,effectively53-phasecircuitstobeconsidered,asshownin figure 4.8. The ambient temperature is 40 0C. The code letter indicated in figure 4.4. is E. K1 = 1, given by figure 4.5. K2 = 0.75, given by figure 4.6. K3 = 0.91, given by figure 4.7. K = K1K2K3 = 10.750.91 = 0.68 Fig. 4.8.Example in the determination of factors K1, K2 and K3 Determination of the minimum cross-sectional area of a conductor The current Iz when divided by K gives a fictitious current Iz. Values of Iz are given in figure 4.9., together with corresponding cable sizes for different types of insulation and core material (copper or aluminium). Example The example shown in figure 4.8. for determining the value of K, will also be used to illustratethewayinwhichtheminimumcross-sectional-areaofconductorsmaybe found, by using figure 4.9. The XLPE cable to be installed will carry 23 amps per phase. Previous examples show that: the appropriate code letter is E the factor K = 0.68 Determination of the cross-sectional areas A standard value of In nearest to, but higher than 23 A is required. Two solutions are possible,onebasedonprotectionbyacircuitbreakerandthesecondonprotectionby fuses. Circuit breaker: - In = 25 A - permissible: current Iz = 25 A - fictitious current: Iz = 25 / 0.68 = 36.8 A - cross-sectional-area of conductors is found as follows: 1. In the column XLPE3 corresponding to code letter E the value of 42 A (the nearest value greater than 36.8 A) is shown to require a copper conductor c.s.a. of 4 mm2. 2. For an aluminium conductor the corresponding values are 42 A and 6 mm2. Fuses: - In = 25 A - permissible current Iz = K3In = 1.2125 = 30.3 A Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 27 - the fictitious current Iz = 30.3 / 0.68 = 40.6 A - the cross-sectional-areas,ofcopperor aluminium conductors are (in this case)foundtobethesameasthosenotedabove for a circuit-breaker protected circuit. Fig. 4.9.Case of an unburied circuit: determination of the minimum cable size derived from the code letter; conductor material; insulation material and the fictitious current Iz 4.2.2. Determination of conductor size for buried circuits Inthecaseofburiedcircuitsthedeterminationofminimumconductorsizes, necessitates the establishement of a factor K. A code letter corresponding to a method of installation is not necessary. Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 28 Determination of factor K Factor K summarizes the global influence of different conditions of installation, and is obtained by multiplying together correction factors K4, K5, K6 and K7. The values of these several factors are given in figures 4.10. to 4.13. Correction factor K4

Factor K4 is a measure of the influence of the method of installation. Fig. 4.10.Correction factor K4 related to the method of installation Correction factor K5

Factor K5 is a measure of the mutual influence of circuits placed side-by-side in close proximity. Cables are in close proximity when the distance L separating them is less than doublethediameterofthelargerofthetwocablesconcerned.Whencablesarelaidin severallayers,multiplyK5by0.8for2layers,0.73for3layers,0.7for4layersor5 layers. Fig. 4.11. Correction factor K5 for the grouping of several circuits in one layer Correction factor K6

This factor takes into account the nature and condition of the soil in which a cable (or cables) is (are) buried; notably its thermal conductivity. Fig. 4.12.Correction factor K6 for the nature of the soil Correction factor K7

This factor takes into account the influence of soil temperature if it differs from 20 0C. Example (see figure 4.15.) Asingle-phase230Vcircuitisincludedwithfourotherloadedcircuitsinaburied conduit.Thesoiltemperatureis20 0C.TheconductorsarePVCinsulatedandsupplya5 kW lighting load. The circuit is protected by a circuit breaker. K4 = 0.8, from figure 4.10. K5 = 0.6, from figure 4.11. Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 29 K6 = 1.0, from figure 4.12. K7 = 1.0, from figure 4.13. K = K4 K5 K6 K7 = 0.48 Determination of the smallest cross-sectional-area of a conductor, for buried circuits Knowing Iz and K, the corresponding cross-sectional-areas are given in figure 4.14. Fig. 4.13.Correction factor K7 for soil temperatures different than 20 0C Example This is a continuation of the previous example, for which the factors K4, K5, K6 and K7 were determined, and the factor K was found to be 0.48. Full load current: A Ib22230000 , 5 Fig. 4.15.Example for the determination of K4, K5, K6 and K7 Selection of protection: a circuit-breaker rated at 25 A would be appropriate. Maximum permanent current permitted (i.e. the circuit-breaker rating In): Iz = 25 A Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 30 Fictitious current: AKIIZZ1 . 5248 . 025' C.s.a.ofcircuitconductors:InthecolumnPVC,2conductors,acurrentof54A corresponds to a 10 mm2 copper conductor. In the case where the circuit conductors are in aluminium,thesamefictitiouscurrent(52A)wouldrequirethechoiceof16mm2 corresponding to a fictitious current value (for aluminium) of 62 A. Fig. 4.14.Case of a buried circuit: minimum c.s.a. in terms of type of conductor; type of insulation; and value of fictitious current Iz (Iz = Iz / K) Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 31 4.2.3. Sizing the neutral conductor The c.s.a. and the protection of the neutral conductor, apart from its current-carrying requirement, depend on several factors, namely: the type of earthing system, TT, TN, etc. the harmonic currents, the method of protection against indirect contact hazards according to the methods described below. The color of the neutral conductor is statutorily blue. PEN conductor, when insulated, shall be marked by one of the following methods: green-and-yellow throughout their length with, in addition, light blue markings at the terminations, or light blue throughout their length with, in addition, green-and-yellow markings at the terminations. 4.2.3.1. Influence of the type of earthing system TT, TN-S and IT schemes Single-phasecircuitsorthoseofc.s.a.16mm2(copper)25mm2(aluminium): the c.s.a. of the neutral conductor must be equal to that of the phases. Three-phase circuits of c.s.a. > 16 mm2 copper or 25 mm2 aluminium: the c.s.a. of the neutral may be chosen to be equal to that of the phase conductors, or smaller, on condition that: -thecurrentlikelytoflowthroughtheneutralinnormalconditionsislessthanthe permittedvalueIz.Theinfluenceofthe3rdandmultiplesofthe3rdharmonicmustbe given particular consideration or - the neutral conductor is protected against short-circuit. TN-C scheme Thesameconditionsapplyintheoryasthosementionedabove,butinpractice,the neutral conductor must not be open-circuited under any circumstances since it constitutes a PE as well as a neutral conductor. IT scheme Ingeneral,itisnotrecommendedtodistributetheneutralconductor,i.e.a3-phase3-wire scheme is preferred. When a 3-phase 4-wire installation is necessary, however, the conditions described above for TT and TN-S schemes are applicable. 4.2.3.2. Influence of harmonic currents Effects of order 3 and multiple of 3 harmonics Harmonics are generated by the non-linear loads of the installation (computers, ballast lighting,rectifiers,powerelectronicchoppers)andcanproducehighcurrentsinthe neutral.Inparticularorder3ormultipleof3harmonicsofthethreephaseshavea tendency to cumulate in the neutral as: fundamental currents are out-of-phase by 2/3 so that their sum is zero on the other hand, order 3 harmonics of the three phases are always positioned in thesamemannerwithrespecttotheirownfundamental,andareinphasewith each (see Fig. 4.16.). Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 32 Fig. 4.16.Order 3 harmonics are in phase and cumulate in the neutral Figure 4.17. shows the load rate of the neutral conductor in function of the percentage of harmonic 3. In practice, this maximum load rate cannot exceed 3 . Fig. 4.17.Load rate of the neutral conductor vs the percentage of harmonic 3 Reduction factors for harmonic currents in four-core and five-core cables with four cores carrying current The basic calculation of a cable concerns only cables with three loaded conductors i.e there is no current in the neutral conductor. Because of the third harmonic current, there is a current in the neutral. As a result, this neutral current creates an hot environment for the 3phaseconductorsandforthisreason,areductionfactorforphaseconductorsis necessary (see Fig. 4.18.). Reductionfactors,appliedtothecurrent-carryingcapacityofacablewiththree loadedconductors,givethecurrent-carryingcapacityofacablewithfourloaded conductors where the currentin thefourth conductor is due to harmonics. The reduction factorsalsotake theheatingeffectoftheharmoniccurrentinthephaseconductorsinto account. Where the neutral current is expected to be higher than the phase current then the cable size should be selected on the basis of the neutral current. Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 33 Wherethecablesizeselectionisbasedonaneutralcurrentwhichisnot significantlyhigherthanthephasecurrentitisnecessarytoreduce thetabulated current carrying capacity for three loaded conductors. If the neutral current is more than 135% of the phase current and the cable size is selected on the basis of the neutral current then the three phase conductors will not befullyloaded.Thereductioninheatgeneratedbythephaseconductorsoffsets the heat generated by the neutral conductor to the extent that it is not necessary to applyanyreductionfactortothecurrentcarryingcapacityforthreeloaded conductors. Fig. 4.18. Reduction factors for harmonic currents in four-core and five-core cables (according to IEC 60364) Examples Considerathree-phasecircuitwithadesignload of37Atobeinstalledusingfour-core PVC insulated cable clipped to a wall, installation method C. From figure 4.14., a 6 mm2cablewithcopperconductorshasacurrent-carryingcapacityof40Aandhenceis suitable if harmonics are not present in the circuit. If20%thirdharmonicispresent,thenareductionfactor of0.86isappliedand thedesignloadbecomes:37/0.86=43A.Forthisloada10mm2cableis necessary. If 40 % thirdharmonicis present, the cable size selectionisbased on theneutral currentwhichis:370.43=44.4Aandareductionfactorof0.86isapplied, leadingtoadesignloadof:44.4/0.86=51.6A.For thisloada10mm2cableis suitable. If 50 % thirdharmonicis present, the cable sizeis again selected on the basis of the neutral current, whichis: 370.53 = 55.5 A .In this case the ratingfactor is 1 and a 16 mm2 cable is required. 4.2.3.3. Protection of the neutral conductor (see Fig. 4.19.) Protection against overload Iftheneutralconductoriscorrectlysized(includingharmonics),nospecific protectionoftheneutralconductorisrequiredbecauseitisprotectedbythephase protection. However,inpractice,ifthec.s.a.oftheneutralconductorislowerthanthephase c.s.a, a neutral overload protection must be installed. Protection against short circuit If the c.s.a. of the neutral conductor is lower than the c.s.a. of the phase conductor, the neutral conductor must be protected against short-circuit. Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 34 Ifthec.s.a.oftheneutralconductorisequalorgreaterthanthec.s.a.ofthephase conductor,nospecificprotectionoftheneutralconductorisrequiredbecauseitis protected by the phase protection. Fig. 4.19.The various situations in which the neutral conductor may appear 4.2.3.4. Breaking of the neutral conductor (see Fig. 4.19.)

Theneedtobreakornottheneutralconductorisrelatedtotheprotectionagainst indirect contact In TN-C scheme Theneutralconductormustnotbeopen-circuitedunderanycircumstancessinceit constitutes a PE as well as a neutral conductor. Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 35 In TT, TN-S and IT schemes Intheeventofafault,thecircuitbreakerwillopenallpoles,includingtheneutral pole, i.e. the circuit breaker is omnipolar. The action can only be achieved with fuses in an indirect way, in which the operation of one or morefuses provokes amechanical trip-out of all poles of an associated series-connected load-break switch. 4.2.3.5. Isolation of the neutral conductor (see Fig. 4.19.) It is considered to be the good practice that every circuit be provided with the means for its isolation. 4.2.4. Sizing the protective earthing conductor (PE) Protective(PE)conductorsprovidethebondingconnectionbetweenallexposedand extraneousconductivepartsofaninstallation,tocreatethemainequipotentialbonding system. These conductors conduct fault current due to insulation failure (between a phase conductorandanexposedconductivepart)totheearthedneutralofthesource.P.E. conductors are connected to the main earthing terminal of the installation. PE conductors must be: insulated and coloured yellow and green (stripes), be protected against mechanical and chemical damage. 4.2.4.1. Connection

PE conductors must: not include anymeans ofbreaking the continuityof the circuit (such as a switch, removable links, etc.), connectexposedconductivepartsindividuallytothemainPEconductor,i.e.in parallel, not in series,have an individual terminal on common earthing bars in distribution boards. TT scheme ThePEconductorneednotnecessarilybeinstalledincloseproximitytothelive conductorsofthecorrespondingcircuit,sincehighvaluesofearth-faultcurrentarenot needed to operate the RCD-type of protection used in TT installations. Fig. 4.20. Direct connection of the PEN conductor to the earth terminal of an appliance Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 36 IT and TN schemes ThePEorPENconductor,aspreviouslynoted,mustbeinstalledasclosebyas possibletothecorrespondingliveconductorsofthecircuitandnoferro-magnetic materialmust beinterposed between them.A PEN conductor must alwaysbe connected directlytotheearthterminalofanappliance,withaloopedconnectionfromtheearth terminal to the neutral terminal of the appliance (see Fig. 4.20.). TN-C scheme (the neutral and PE conductor are one and the same, referred to as a PEN conductor) TheprotectivefunctionofaPENconductorhaspriority,sothatallrulesgoverning PE conductors apply strictly to PEN conductors. TN-C to TN-S transition ThePEconductorfortheinstalllationisconnectedtothePENterminalorbar(see Fig.4.21.)generallyattheoriginoftheinstallation.Downstreamofthepointof separation, no PE conductor can be connected to the neutral conductor. Fig. 4.21.The TN-C-S scheme 4.2.4.2. Types of materials Materials of the kinds mentioned below in figure 4.22. can be used for PE conductors, provided that the conditions mentioned in the last column are satisfied. Fig. 4.22.Choice of protective conductors (PE) (1) In schemes TN and IT, fault clearance is generally effected by overcurrent devices (fuses or circuit breakers)sothattheimpedanceofthefault-currentloopmustbesufficientlylowtoassurepositive protective device operation. The surest means of achieving a low loop impedance is to use a supplementary coreinthesamecableasthecircuitconductors(ortakingthesame routeasthecircuitconductors).This stratagem minimizes the inductive reactance and therefore the impedance of the loop. (2)ThePENconductorisaneutralconductorthatisalsousedasaprotectiveearthconductor.This means that a current may be flowing through it at any time (in the absence of an earth fault). For this reason an insulated conductor is recommended for PEN operation. (3)ThemanufacturerprovidesthenecessaryvaluesofRandXcomponentsoftheimpedances (phase/PE, phase/PEN) to include in the calculation of the earth-fault loop impedance. Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 37 (4)Possible,butnotrecomended,sincetheimpedanceoftheearth-faultloopcannotbeknownatthe design stage. Measurements on the completed installation are the only practical means of assuring adequate protection for persons. (5) It must allow the connection of other PE conductors. Note: these elements must carry an indivual green/yellow striped visual indication, 15 to 100 mm long (or the letters PE at less than 15 cm from each extremity). (6)Theseelementsmustbedemountableonlyifothermeanshavebeenprovidedtoensure uninterrupted continuity of protection. (7) With the agreement of the appropriate water authorities. (8) In the prefabricated pre-wired trunking and similar elements, the metallic housing may be used as a PEN conductor, in parallel with the corresponding bar, or other PE conductor in the housing. (9)Forbiddeninsomecountriesonly-universallyallowedtobeusedforsupplementaryequipotential conductors. 4.2.4.3. Conductor sizing Adiabatic method(which corresponds with that described in IEC 60724) This method, while being economical and assuring protection of the conductor against overheating,leadstosmallc.s.a.scomparedto thoseofthecorrespondingcircuitphase conductors.TheresultissometimesincompatiblewiththenecessityinITandTN schemestominimizetheimpedanceofthecircuitearth-faultloop,toensurepositive operation byinstantaneous overcurrent tripping devices. This method is used in practice, therefore, for TT installations, and for dimensioning an earthing conductor. For any size of the phase conductor: - for a period of 5 seconds or less, the relationship 2 2 2S k t I characterizes the time in seconds during which a conductor of c.s.a. S (in mm2) can be allowed to carry a current I amps,beforeitstemperaturereachesalevelwhichwoulddamagethesurrounding insulation. 2 2 2S k t I ) 1 (kt ISPEThe c.s.a. of earthing conductor between the installation earth electrode and the main earth terminal: when protected against mechanical damage: kt ISPEwithoutmechanicalprotection,butprotectedagainstcorrosionbyimpermeable cable sheath. Minimum 16 mm2 for copper or galvanized steel. withouteitheroftheaboveprotections;min.of25mm2forbarecopperand50 mm2 for bare galvanized steel. (1) When the PE conductor is separated from the circuit phase conductors, the following minimum values must be respected: - 2.5 mm2 if the PE is mechanically protected, - 4 mm2 if the PE is not mechanically protected. Simplified method ThismethodisbasedonPEconductorsizesbeingrelatedtothoseofthe correspondingcircuitphaseconductors,assumingthatthesameconductormaterialis used in each case. Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 38 ph PE phS S mm S 2162 2 216 35 16 mm S mm S mmPE ph Note:when,inaTTscheme,theinstallationearthelectrodeisbeyondthezoneof influence of the source earthing electrode, the c.s.a. of the PE conductor can be limited to 25 mm2 (for copper) or 35 mm2 (for aluminium). TheneutralcannotbeusedasaPENconductorunlessitsc.s.a.isequalto orlarger than 10 mm2 (copper) or 16 mm2 (aluminium). Moreover, a PEN conductor is not allowed in a flexible cable. Since a PEN conductor functionsalsoasaneutralconductor,itsc.s.a.cannot,inanycase,belessthanthat necessary for the neutral. This c.s.a. cannot be less than that of the phase conductors unless: the kVA rating of single-phase loads is less than 10% of the total kVA load, and Imaxlikelytopassthroughtheneutralinnormalcircumstances,islessthanthe current permitted for the cable size selected. Furthermore, protection of the neutral conductor must be assured by the protective devices provided for phase-conductor. Values of factor k to be used in the formulae Thesevaluesareidenticalinseveralnationalstandards,andthetemperaturerise ranges,togetherwithfactorkvaluesandtheuppertemperaturelimitsforthedifferent classes of insulation, correspond with those published in IEC 60724 (1984). Thedatapresentedinfigure4.23.arethosemostcommonlyneededforLV installation design. Fig. 4.23.k factor values for LV PE conductors, commonly used in nationalstandards and complying with IEC 60724 Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 39 4.2.5.CalculationofLmax.foraTN-earthedsystem,usingthe conventional method The maximum length of a circuit in a TN-earthed installation is given by the formula: aphI mS UL + ) 1 (8 . 00max where: Lmax - maximum length in metres, U0 - phase volts, 230 V for a 230/400 V system, - resistivityatnormalworkingtemperatureinohm-mm2/metre(22.510-3 for copper; 3610-3 for aluminium), Ia - trip current setting for the instantaneous operation of a circuit breaker, or Ia - thecurrentwhichassuresoperationoftheprotectivefuse concerned, in the specified time, Fig. 4.24.Calculation of L max. for a TN-earthed system, using the conventional method m = Sph / SPE Sph - cross-sectional area of the phase conductors of the circuit concerned in mm2, SPE - cross-sectional area of the protective conductor concerned in mm2. Tables Thetablesgivemaximumcircuitlengths,beyondwhichtheohmicresistanceofthe conductorswilllimitthemagnitudeoftheshort-circuitcurrenttoalevelbelowthat requiredtotripthecircuit-breaker(ortoblowthefuse)protectingthecircuit,with sufficient rapidity to ensure safety against indirect contact. Correction factor m Figure4.25.indicatesthecorrectionfactortoapplytothevaluesgiveninfigures 4.26., according to the ratio Sph/SPE, the type of circuit, and the conductor materials. The tables take into account: Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 40 the type of protection: circuit breakers or fuses, operating-current settings, cross-sectional area of phase conductors and protective conductors, type of system earthing, type of circuit breaker (i.e. B, C or D). EquivalenttablesforprotectionbyCompactandMulti9circuitbreakers(Merlin Gerin) are included in the relevant catalogues. Fig. 4.25.Correction factor to apply to the lengths givenfor TN systems (may be used for 230/400 V systems) Fig. 4.26.Maximum circuit lengths (in metres) for different sizes of copper conductor and instantaneous-tripping-current settings for general-purpose circuit breakers in 230/240 V TN system with m = 1 4.2.6. Rules for marine electrical cables according Bureau Veritas General 1.All electricalcables and wiring external to equipment shallbeatleast of aflame-retardant type, in accordance with IEC Publication 60332-1. 2. When cables are laid in bunches, cable types are to be chosen in appliance with IEC Publication 60332-3 Category A, or over means are to be provided such as not to impair their original flame-retarding properties. 3.Wherenecessaryforspecificapplicationssuchasradiofrequencyordigital communications systems, which require the use of particular types of cables, the Society may permit the use of cables with do not comply with the provisions of 1 and 2. Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 41 4. Cables which are required to havefire-resisting characteristics are to complywith the requirements stipulated in IEC Publications 60331. Choice of insulation 1.Themaximumratedoperatingtemperatureoftheinsulatingmaterialistobeat least100Chigherthanthemaximumambienttemperatureliabletooccurortobe produced in the space where the cable is installed.2. Themaximum rated conductor temperature for normal and short-circuit operation, for the type of insulating compounds normally used for shipboard cables, is not to exceed thevaluesstatedinTab1.Specialconsiderationwillbegiventootherinsulating materials. 3.PVCinsulatedcablesarenot tobeusedeitherinrefrigeratedspaces,or ondecks exposed to the weather of ships classed for unrestricted service. Table 1: Maximum rated conductor temperature Maximum rated conductor temperature (0C) Type of insulating compound Abbreviated designation Normal operation Short-circuit a)Thermoplastic: -baseduponpolyvinylchlorideorcopolymerofvinyl chloride and vinyl acetate PVC/A 60 180 b) Elastomeric or thermosetting: -baseduponethylene-propylenerubberorsimilar(EPM or EPDM) -baseduponhighmodulusorhardgradeethylene-propylene rubber - based upon cross-linked polyethylene - based upon rubber silicon -baseduponethylene-propylenerubberorsimilar(EPM or EPDM) halogen free -baseduponhighmodulusorhardgradehalogenfree ethylene propylene rubber - based upon cross-linked polyethylene halogen free - based upon rubber silicon halogen free -baseduponcross-linkedpolyolefinmaterialforhalogen free cable (1) EPR HEPR XLPE S 95 HF EPR HF HEPR HF XLPE HF S 95 HF 85 85 85 85 95 85 85 85 95 85 250 250 250 250 250 250 250 350 250 (1) Used on sheathed cable only Choice of protective covering 1.Theconductorinsulatingmaterialsaretobeenclosedinanimpervioussheathof material appropriate to the expected ambient conditions where cables areinstalledin the following locations: - on decks exposedto the weather, - in damp or wet spaces (e.g. in bathrooms), - in refrigerated spaces, - in machinery spaces and, in general, - where condensation water or harmful vapour may be present. Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 42 2. Where cables are provided with armour or metallic braid (e.g. for cables installed in hazardousareas),anoverallimpervioussheathmeanstoprotectthemetallicelements against corrosion is to be provided. 3.Animpervioussheathisnotrequiredforsingle-corecablesinstalledintubesor ducts inside accommodation spaces, in circuits with maximum system voltage 250 V.4.Inchoosingdifferenttypesofprotectivecoverings,dueconsiderationsistobe given to the mechanical action to which cable may be subjected during installation and in service. Ifthemechanicalstrengthoftheprotectivecoveringisconsideredinsufficient,the cables are to be mechanically protected (e.g. by an armour or by installation inside pipes or conduits). 5.Single-corecablesfora.c.circuitswithratedcurrentexceeding20Aaretobe either non-armoured or armoured with non-magnetic material. Cables in refrigerated spaces 1. Cables installed in refrigerated spaces are to have a watertight or impervious sheat and are to be protected against mechanical damage. If an armour is applied on the sheath, the armour is to be protected against corrosion by a further moisture-resisting covering. Cables in circuits fore fire alarm, fire detection and fire-extinguishing 1.Ingeneral,incircuitsintendedforfirealarmanddetection,emergencyfire- extinguishingservice,firetelecommunication(e.g.communicationbetweenthe navigatingbridge and themainfire control station), remote stoppingand similar control circuits for safety purposes, cables are to be of a fire-resistant type unless: - the systems are ofself-monitoring type or failing to safety, - the systems are duplicated. 2.Cablesforservicesthatarerequiredtomaintainoperationofequipmentduringa fire (e.g. cables for the general emergency alarm, the public address system when it is the onlysystemtoprovidethegeneralemergencyalarm,thefire-extinguishingmedium alarm and their power supplies) are to be of a fire-resistant type. 3.Cablesconnectingfirepumpstotheemergencyswitchboardshallbeoffire-resistant type where they pass through fire risk areas. Cables fore submerged bilge pumps 1. Cables and their connections to such pumps are to be capable of operating under a headofwaterequaltotheirdistancebelowthebulkheaddeck.Thecableistobe impervious-sheathedandarmoured,istobeinstalledincontinuouslengthsfromabove the bulkhead to the motor terminals and is to enter the air bell from the bottom. Internal wiring of switchboard and other enclosures for equipment 1.Forinstallationsinswitchboardsandotherenclosuresforequipment,single-core cables may be used without further protection (sheath). Other types of flame-retardant switchboards wiring may be accepted. Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 43 Current carrying capacity of cables 1.ThecurrentcarryingcapacityforcontinuousserviceofcablesgiveninTab2to Tab6isbasedonthemaximumpermissibleservicetemperatureoftheconductoralso indicated therein and on an ambient temperature of 450C.Table 2: Current carrying capacity, in A, in continuous service for cables based on maximum conductor operating temperature of 600C (ambient temperature 450C) Number of conductorsNominal section mm2123 or 4 1876 1.512108 2.5171412 4221915 6292520 10403428 16544638 25716050 35877461 501058974 7013511595 95165140116 120190162133 150220187154 185250213175 240290247203 300335285235 Table 3: Current carrying capacity, in A, in continuous service for cables based on maximum conductor operating temperature of 750C (ambient temperature 450C) Number of conductorsNominal section mm2123 or 4 113119 1.5171412 2.5242017 4322722 6413529 10574840 16766553 251008570 3512510688 50150128105 70190162133 95230196161 120270230189 150310264217 185350298245 240415353291 300475404333 Table 4: Current carrying capacity, in A, in continuous service for cables based on maximum conductor operating temperature of 800C (ambient temperature 450C) Number of conductorsNominal section mm2123 or 4 1151311 1.5191613 2.5262218 4353025 6453832 10635444 16847159 251109477 3514011998 50165140116 70260221182 95215183151 120300255210 150340289238 185390332273 240460391322 300530450371 Table 5: Current carrying capacity, in A, in continuous service for cables based on maximum conductor operating temperature of850C (ambient temperature 450C) Number of conductorsNominal section mm2123 or 4 1161411 1.5201714 2.5282420 4383227 6484134 10675747 16907763 2512010284 35145123102 50180153126 70225191158 95275234193 120320272224 150365310256 185415353291 240490417343 300560476392 Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 44 2. The current carrying capacity is applicable, with rough approximation, to all types of protective covering (e.g. both armoured and non-armoured cables). 3.Valuesother thenthoseshowninTab2toTab6maybeacceptedprovidedthey aredeterminedonthebasisofcalculationmethodsorexperimentalvaluesapprovedby the Society. 4.Whentheactualambienttemperatureobviouslydiffersfrom450C,thecorrection factors shown in Tab 7 may be applied to the current carrying capacity in Tab 2 to Tab 6.5. Where more than six cables are bunched together in such a way that is an absence of free air circulating around them, and the cables can be expected to be under full load simultaneously, a correction factor of 0.85 is to be applied. 7.Forsupplycablestosingleservicesforintermittentloads(e.g.cargowinchesor machinery space cranes), the current carrying capacity obtained from Tab 2 to Tab 6 may be increased by applying the correction factors given in Tab 9.Thecorrectionfactorsarecalculatedwithroughapproximationforperiodsof10 minutes, of witch 4 minutes with a constant load and 6 minutes without load. Minimum nominal cross-sectional area of conductors 1. In general theminimumallowable conductor cross-sectional areas are those given in tables above. 2.Thenominalcross-sectionalareaoftheneutralconductorinthree-phase distributionsystemsistobeequaltoatleast50%ofthecross-sectionalareasofthe phases, unless thelatter isless than or equal to 16mm2. Insuch case the cross-sectional of the neutral conductor is to be equal to that of the phase. Table 6: Current carrying capacity, in A, in continuous service for cables based on maximum conductor operating temperature of950C (ambient temperature 450C) Number of conductorsNominal section mm2123 or 4 1201714 1.5242017 2.5322722 4423629 6554739 10756453 161008570 2513511595 35165140116 50200170140 70255217179 95310264217 120360306252 150410349287 185470400329 240570485399 300660560462 Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 45 Table7: Correction factors for various ambient air temperatures Correction factors for ambient air temperature of:Maximum conductor temperature 0C 350C400C450C500C550C600C650C700C750C800C850C 601.291.151.000.82------- 751.151.081.000.910.820.710.58---- 801.131.071.000.930.850.760.650.53--- 851.121.061.000.940.870.790.710.610.50-- 951.101.051.000.950.890.840.770.710.630.550.45 Choice of cables 1. Rated voltage of any cable is to be not lower than the nominal voltage of the circuit which it is used. 2.Thenominalcross-sectionalareaofeachcableistobesufficienttosatisfythe following conditions with reference to the maximum anticipated ambient temperature: thecurrentcarryingcapacityistobenotlessthanthehighestcontinuousloadcarried by the cable, thevoltagedropinthecircuit,byfullloadonthiscircuit,isnottoexceedthe specified limits, the cross-sectional area calculated on the basis ofthe above is to be such that the temperatureincreases whichmaybe causedby overcurrents or starting transients do not damage the insulation. Table 8: Corrections factors for short-time loads - hour service1 hour service Sum of nominal cross-sectional areas of all conductors in mm2 Sum of nominal cross-sectional areas of all conductors in mm2 Cable with metallic sheath and armoured cables Cable with non-metallic sheath and non-armoured cables Cable with metallic sheath and armoured cables Cable with non-metallic sheath and non-armoured cables Correction factor up to 20up to 75up to 80up to 2301.06 21 - 4176-12581-170231-4001.10 41 - 65126-180171-250401-6001.15 66 95181-250251-430601-8001.20 96-135251-320431-600-1.25 136-180321-4006001-800-1.30 181-235401-500--1.35 236-285501-600--1.40 286-350---1.45 3. The highest continuous load carried by a cable is to be calculated on the basis of thepowerrequirementsandofdiversityfactoroftheloadsandmachinessuppliedthrough that cable.Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 46 4.Whenconductorsarecarryingthemaximumnominalservicecurrent,thevoltage drop from themain or emergencyswitchboard busbars to any pointin theinstallationis not to exceed 6% of the nominal voltage. For battery circuits with supply voltage less than 55 V, this value may be increased to 10%. Forcircuitsofnavigationlights,thevoltagedropisnottoexceed5%oftherated voltage under normal conditions. Table 9: Correction factors for intermittent service Sum of nominal cross-sectional areas of all conductors in mm2 Cable with metallic sheath and armoured cables Cable with metallic sheath and non-armoured cables Correction factor -S 51.10 -5 < S 81.15 -8 < S 161.20 S 416 < S 251.25 4 < S 725 < S 421.30 7 < S 1742 < S 721.35 17 < S 4272 < S 1401.40 42 460 V mains frequency f < 45 Hz or f > 65 Hz resistance of a generator ra > 30 % of the subtransient reactance xdsubtransient time constant of a generator Td < 10 ms internal resistance Rtraf of a transformer is larger than the internal impedance Ztraf. 2.Afterperformingthecheckroutinetheprogramstartsthecalculationofthetime dependent ac end dc current of generators, single motors and the equivalent motor. The ac and dc current are calculated for t = 0 and t = T/2. 3. Next, the equivalent generators are calculated at the points of common connection, i.e. at the main distribution as described in the following: 3.1.Calculationoftheequivalentgenerator,comprisingallactivecomponents, generators, single motors and equivalent motor. 3.2. Calculation of a number of equivalent generators in a program sequence (loop). These generators comprise all active components but neglecting one single motor in each sequence,beginningwithmotorno.1andendingwithmotorno.20,ifprovided.The number,ofthisequivalentgenerators,tobecalculatedequalsthenumberofsingle motors. 4. Finally the short circuit currents at the various fault points of the ship mains will be calculated as follows: 4.1.Calculationofthefaultcurrentsatmainbus,generatorbreakersandbreakers ofsinglemotorsisbasedmainlyontheresultsdescribedbytheabovementioneditem 2 = currents of active components. The power factor in case of a short circuit at main bus is calculated using the data of theequivalent generator, whichis comprised of all active components, as described by 3.1., see above. 4.2.Calculationofthetimedependentshortcircuitcurrentsatdistributionsand subdistributions using as active component the equivalent generator described by 3.1., see above. 4.3. Calculation of the time dependent short circuit currents at the terminals of the single motors using as active component the equivalent generators described by 3.2., see above. 5. After performing the calculation data are written into an ASCII-file and afterwards displayed on the monitor. The said file contains the following data: 5.1. All input data 5.2. The root mean square value of the ac component at the beginning of the short circuit Ik at t = 0 or t = T/2. 5.3. The peak value Ip of the current calculated at time t = T/2. 5.4. The power factor The calculation carried out by this program is based on the following assumptions in compliance with the IEC Paper: The short circuit occurs at the same time between all phases. The calculationis carried out, neglecting the time dependent characteristic of the voltageregulators,whicharepartofthegenerators.Theseregulatorsstartto increase the interval voltage and hence the currents of the generator about 100 ms afterthedropofthevoltagecausedbyashortcircuit.Thecalculationis performed, assuming that the continuous short circuit current is three times higher Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 54 than the nominal current of a generator if there are no input data (Ikd/In = 0). Ikd is the steady state current and In is the nominal current. Low ohmicshort circuitis applied to all phases. Therefore, theimpedance of the faultypoint(i.e.theconnectionfromphasetophase)isneglectedbythe calculation. System capacitances are neglected. Generatorsrunninginparallelhavethesamepowerfactor(i.e.equalproportion between active and reactive load). Harmonic distortions of the currents are neglected. 4.4.1.2. Structure of the Ship Mains to be calculated Theprogramiscapableofcalculatingshort-circuitsinashipmainsconstructedas follows: Equivalent motor - sum of small motors; G - generator; FP - fault point; M - motor; TR - transformer; Notes on the system structure: the installation of transformers is optional in any case; a max number of 5 subdistributions may be connected to each distribution board. Description of the fault points: FP.1. short circuit at main distribution (worst case), FP.2. short circuit at the generator breaker (generator side), FP.3. short circuit at distribution board, Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 55 FP.4. short circuit at subdistribution board, FP.5. short circuit at breaker of single motor (motor side), FP.6. short circuit at terminals of single motor. 4.4.1.3. Asymmetric short circuit 1. Unearthed system Alargenumberofelectricalsystemsinstalledinvesselsisoperatedwiththeneutral pointinsulatedfromtheshipshull.Insuchsystem,themostcriticalcondition concerningshort-circuitisaninsulationfaultbetweenallphasesascalculatedbythis program. 2. Earthed systems Iftheneutralpointisconnecteddirectlytotheshipshull,highercurrentsthan calculated by this program may be produced if one or two phases are in contact with the ships hull. Asymmetric failures are not calculated by this program. If a separate detailed calculationwillnotbecarriedoutitisrecommendedtomultiplythesymmetricfault currents of the generators and synchronous motors by a factor of 1.5. Incaseofanasymmetricshortcircuitasynchronousmotorssupplycurrentsonlyif two phases are connected to the ships hull because the neutral points of these motors are operatedunearthedinmostapplications.Theasymmetriccurrentsofmotorsmaybe derived from the symmetric currents using a factor of 0.866 (=square root of three divided by two). 4.4.1.4. Remarks on input data and components 1. Generators Definitions:

"dx [%] - subtransient reactance of a synchronous machine in the d-axis,

'dx [%] - transient reactance of a synchronous machine in the d-axis,

dx [%] - reactance in the d-axis, ar [%] - stator resistance, "dt [ms] - subtransient time constant, 'dt [ms] - transient time constant, dct [ms] - dc time constant. Toconvertpercentvalues(e.g. ar ofagenerator)intofigureswithdimensionand vice versa the following formula is to be used for:

aR =ar 2gU/ (100gS ),

gS - nominal power of the generator,

gU - resistance voltage of the generator,

aR - resistance of the stator winding stated in ohm,

ar - resistance of the stator winding stated in %. For the conversion of reactance the same formula is to be used in principle. Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 56 Iftheinputvaluefortheresistanceofthestatorwindingiszerotheprogramwill calculate with the following data basing on experience:

ar = 0.3 "dx gS = 1000KVA 2. Asynchronous motors Single motors All motors rated above 100 KW or exceeding the generator power by 15 % are large motorsandshouldbecalculatedindividuallyacc.totherevisedIECPublication363. Motors witch do not match above categories will be classified as small motors. Definitions: sr [%] - stator resistance, rr [%] - rotor resistance, sx [%] - stator reactance, rx [%] -rotor reactance. Ifthemotordataexceptfortheshaftpowerandthevoltagearenotavailable(i.e. inputdata=0)theprogramwillcalculatetheshortcircuitcurrentbymeansofthe following parameters taken from IEC Publication except for the efficiency and the power factor, witch are based on experience. Internal data: a) Impedance, reactance and resistances of large motors "mz = 16 % "mx = 15 % (s r mx x x + ) sr = 3.4 % rr = 2.1 % b) Impedance, reactance and resistances of small motors "mz = 20 % "mx = 18.8 % (s r mx x x + ) sr = 4.3 % rr = 2.7 % c) Time constants at 60 Hz for large and small motors "mT= 18.67 ms dcT = 11.73 ms d) Time constants at 50 Hz for large and small motors "mT= 22.4 ms dcT = 14.076 ms Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 57 e) Power factor and efficiency power factor = 0.8 efficiency = 0.9 Equivalent motor TheequivalentmotorwillbecalculatedasasmallmotordespiteoftheKWpower. ForthedefinitionofsmallandlargemotorsreferenceismadetotheIECPublication 61363-1. The fault currents of the equivalent motor will be evaluated as follows: In Iac 5- ac component of the short circuit current at t = 0n pI I 8- peak value of the short circuit current ac dcI I 2- dc component of the short circuit current at t = 0 nI -isthetheoreticalnominalcurrentoftheequivalentmotorcalculatedbymeansof main bus voltage and a power factor and an efficiency as mentioned above. Transformers The complex internal impedance of a transformer is calculated by the program, using the following input data: a) short-circuit voltage (z - component of the internal impedance) b) copper losses (r - component of the internal impedance) Inshortcircuitconditionatransformerisnormallyoperatedwithreducedvoltage, therefore losses of the iron core are not relevant. Calculation of the internal resistance and the impedance: ) 100 /(2traf traf k trafS U u Z trafZ - impedance of transformer, ku - short circuit voltage [%], trafU -nominal voltage of transformer on that side witch is opposite to the short circuit, trafS- nominal power of transformer, 2 2/traf traf cu trafS U P R trafR- internal resistance of a transformer, cuP- copper losses of a transformer. The reactance X is derived from Z and R by means of the following formula: ) (2 2traf traf trafR Z X If short circuit voltage and copper losses are not available, the following table may be used to obtain the missing data: a) 220 V transformers power [KVA]501002505001000 short-circuit voltage [%]2.03.03.03.55 copper losses [KW]1.01.52.54.07 Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 58 b) 3 kVA transformer power [KVA]500100025005000 short-circuit voltage [%]4.06.06.06.0 copper losses [KW]5.09.016.025.0 The data of tables 1 and 2 above are based on experience. Cables RandXvalues(resistancesandreactances)ofcablesarecalculatedbymeansofa program routine based on the data specified by the table below.If this internal data shall beused,theinputofthespecificr-andx-values,whichcanbefoundonseveral pages, must be zero. Cross- Section [mm x mm] Resistance R [m/m] Resistance X at50 Hz [m/m] Resistance X at 60 Hz [m/m] 3x1.513.10.1260.152 3x2.57.860.1170.140 3x44.910.1070.128 3x63.280.1000.120 3x101.9650.0980.113 3x161.230.0910.109 3x250.7860.0820.098 3x350.5600.0820.098 3x500.3930.0750.090 3x700.2800.0750.090 3x950.2060.0750.090 3x1200.1640.0720.086 Table:Resistancesandreactancesrelatedtolengthofmarinecablemadeofcopper for nominal voltages up to 1000 V. The data are based on experience.Ifthespecificr-andx-valuesareunequaltozero,theprogramwillignorethe internal data and calculate resistances and impedances of cables by means of the specific data. Under number of conductors enter the number of parallel conductors per phase. Parameters After entering the submenu parameters the following may be specified: 1. Modification of time constants - the time constants which are derived from external or internal data will remain unchanged during the calculation. 2. Modification of time constants - the time constants which are derived from external or internal data will be modified by the resistances and the reactances of passive components which are cables and transformers. The equation used for the modification of time constants are extracted unchanged from the IEC Publication 61363-1. 3. The setting modification of time constants should be standard due to fact thatthedctimeconstantsofgeneratorswillnotbederivedfromtheinputdatabut calculatedbymeansofreactancesandresistances.Ifthementionedsettingistruea possible further element of uncertainty (i.e. the dc time constant) may be eliminated from thecalculation.Theshort-circuitprogramisusingthefollowingequationfromthe calculation of the modified dc time constant of the generators acc. To the IEC Publication 61363-1: Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 59 )) ( 2 ( / ) ("acb a cab d mdcR R f X X T + +

mdcT - dc time constant of a generator , modified;

"dX - sub transientreactance of a generator;

cabX - reactance of the cable between a generator and the main distribution ; f frequency;

aR - resistance of the stator winding of a generator;

cabR - resistance of the cable between the generator and the main distribution. Theequationforthemodificationofthesubtransienttimeconstantasstatedbythe IEC Publication will change this constant, even if no cable is provided between generator and main distribution. 4. Include preload conditions of motors < false> - for the calculation of fault currents ofmotorsthepreloadconditions(i.e.motorwasrunningwithnominalloadpriorto the shortcircuit)willbeneglected.Theinternalvoltageofthemotorwillbethenominal voltage at the terminals of this machine. 5.Includepreloadconditionsofmotors-thepreloadconditionsofamotor willbetakenintoaccount.Theinternalvoltageofthismotorisnowthevoltageatthe terminalsminusthevoltagedropcausedbythecurrentandbytheinternalimpedance. The fault currents of a motor become smaller if preload conditions are not neglected. Enablebackupfileallinputdataarewrittenintoafilenamedbakonthe following occasions: before leaving the short circuit calculationbefore deleting an input page before starting a print out before starting a calculation Torecoverdata thebak-filehastoberenamedusinganappropriatecommandfrom system level. The modified file name must have the extension .dat. The status of the above mentioned parameters (true/false) is written into a file named defaultwhenleavingthesubmenuparameters.Aftereachstartontheshort calculationtheseparameterswillbesetacc.tothecontentofthedefault-file.Anew calculation will start with the setting of the previous one. Detailed ResultsInsteadofshowingthecompleteresultsafterperformingacalculationtheprogram may display detailed results enabling the operator to check the influence of parameters or to verify the results of a calculation. Toenableamoredetailedoutputenter thesubmenuparametersandspecifyunder menuitemdisplaydetailedcalculationthatpartofthecalculationwhichshallbe displayed in detail. Toproducehard-copiesofthedetailedresultsusethekeycombination+ . If a detailed calculation is selected the complete results will not be displayed. 4.4.1.5. Simplified Calculation For a simplified calculation of short-circuit currents at themain distributionsupplied by generators and motors the following equations may be used. "kgI =gnI 100 /"dx[%] Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 60 "kmI = 6 mnI"ktotI = "kgtotI + "kmtotIptotI = 2.3 "ktotI

gnI- nominal current of a single generator;

"kgI- subtransient short-circuit current of a single generator;

"kgtotI- subtransient short-circuit current of allgeneratorswhichmayoperateinparallel;

mnI- nominal current of a single motors;

"kmI- subtransient short-circuit current of a single motor;

"kmtotI - subtransientshort-circuit current of all motors, whichmaybeoperated simultaneously;

"ktotI- subtransient short-circuit current of all generators and motors;

ptotI- peak value of the short-circuit current of all motors and generators. 4.4.1.6. Selection of switch gear Beside the calculation of electrodynamics forces between busbars and heat dissipation inelectriccomponentsashort-circuitevaluationisrequiredfortheselectionofthe protective devices which are fuses and circuit breakers.For the selection of breakers the following ratings must be available: rated short-circuit making capacity; rated short-circuit breaking capacity; rated operational voltage; power factor. The rated short-circuit breaking capacity of a circuit breaker must not be less than the calculatedsymmetricac-component.Concerningthebreakingoperationdelaytimesof the switch gear may be taken into account. The rated short-circuit making capacity of a circuit-breaker must not be lees than the calculated peak value of the short-circuit current. Therated operationalvoltageofthebreakermustnotbeleesthanthevoltageat the point of installation of the breaker. If the abovementioned conditioned arenot fulfilled the circuit-breakermanufacturer may be consulted as to state whether the may be modified. 4.4.1.7. The documentation Thedocumentationofashort-circuitcalculationsubmittedforapprovalshould comprise: 1. The principle lay-out of the electrical mains which has been calculated showing all: - active components, e.g. generators and motors; - cables; - transformers; - distributions and sub distributions; - fault points with identifications markings. The electrical mains may be displayed in the form of a single line diagram. Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 61 2.Tableofallinputdatarequiredforthecalculation,e.g.listofcableswithcross section, lengths and number of parallel conductors per phase, data of motors, generators, transformers, etc. It should be clearly if data are based on assumption.3. Results of the short-circuit calculation for the worst case condition, comprising the short-circuitcurrent(peakcurrent,symmetrical,ac-currentsandpowerfactors)ofthe active components, and the short-circuit currents at all relevant fault points, e.g.: - main bus, - breaker of generators, - breaker of single motors, - distributions, - subdistribution. 4.Forpracticalreasonsthedocumentcontainingthecalculatedresultsshouldbe groupedwithregardtothefaultpointsandshouldcomprisealiststatingthemakeand typeofallinstalledbreakerswiththeirmakingcapacity,breakingcapacityandpower factors,seeexamplebelowshowingtheprocedureforasubdistributionandtwo breakers. Place of Installation: subdistribution EXAMPLE Fault currents and power factors at the place of installation, calculated values Data of breakers for 440 V operation makers data pI [kA] acI [kA] p.f.TypeMakeBreakp.f. 25160.45XYZ 63A35180.25 UVW 125A40200.3 4.4.2. Short circuit current calculation according BUREAU VERITAS 4.4.2.1. Main methods There are only 2 methods of calculation: Equivalentimpedancemethod:bythismethod,theimpedancesonnetworkare reduced to an equivalent impedance by considering the connection of impedances (series, parallel) in relation to the point of calculation. Contribution method: by thismethod, each equipmentis considered separatelyin relation to the point of calculation and the total short circuit current on the point is thesummationoftheshortcircuitcurrents ofelementsconnectedto thepoint of calculation. Anyothermethodisavariation,acombinationoraparticularcaseofthetwo methods. Real units method Bythismethod,alloperatorsaremeasureddirectlyinohm,V,A,VA.Themain problemofmethodisthattheimpedancesmustbecorrectedtothevoltagelevelofthe pointofcalculationifTXsarepresentinthenetwork;iftherearemanyvoltagelevels (many TXs) in the network, the method may be quite elaborated. Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 62 The correction formula for Z, R and X, from one voltage level to other voltage level is: 2121 2

,_

UUZ Zwhere:

1Z is the value calculated for voltage level nomU U 1 of equipment,

2Zis the corrected value from 1Uto 2U = voltage level of the point of calculation. Alternatively, the values can be calculated directly to the voltage level of the point of calculation; by this way, each Z, R and X will have one value for each point of calculation (for each voltage level). Proportional units method (pu) By this method, each equipments impedance is referred to an apparent base power Sb and to a base voltage Ub; by this way, the corrections of values from one voltage level to other voltage level are eliminated. The three (3) formulae of the method are: ] , [ ] [ ] [2V VAUSohm Z pu Zb ] , , [3A V VAUSIbbb] , , [ A pu AZIIbbkwhere: U[V]-isthenominalvoltageofequipment,regardlessofvoltagelevelofthe intended point of calculation;

bU [V] - is the voltage level of the point of calculation;

bS [VA]-isthebasepowerandhasanarbitraryvalue(e.g.1kVA,10MVA, 100MVA, 0.444VA etc), provided that is has the same value for calculation of all bI , Z, R and X in the network,

bI[Apu] - is the base current at the point of the calculation,

bZ[pu] -is equivalent or calculation Z at the point of calculation,

kI[A] - is short circuit current at the point of calculation. Byusingtheaboveformulae,thecalculationformulaeofZ,RandXarechangedin comparison to real units method. 4.4.2.2. Theoretical considerations The following steps are to be performed for calculation of short-circuit currents: Step 1: Calculation of each equipment impedance Zi. Step 2: Development of impedances diagram, based on one-line diagram system. Step 3: Calculation of short-circuit impedances Zki on the points of calculations. Parallel connection:iiiicAAA Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 63 Serial connection: ii cA A Convert from to Y:23 13 1213 121A A AA AA+ + 23 13 1223 133A A AA AA+ +

23 13 1223 122A A AA AA+ + Convert from Yto :32 12 1 12AA AA A A++ +

13 23 2 23AA AA A A++ +

23 13 1 13AA AA A A++ + Step 4: Calculation of short-circuit currents. By short-circuit is meant the contact with a very small resistance between two or more conductors being under voltage; the circuit is closed by a small resistance, the current on this circuit resulting to be of a very high value (as I = U/R). Based on the above, the following can be deduced: the short-circuit is basically a normal circuit with a small resistance as consumer; as bigger the voltage is, as higher the short-circuit current is. the calculation of the short-circuit current is simplified reduced to determining the resistance at the point of short-circuit. Therearedefinedthreeshort-circuitcurrentsaccordingtothetypeofshort-circuit, respectively (RMS values): 3P short-circuit: ( )kkkZUI33 12 3 A12 A1A2 A23 A3 A13 Designing an electrical installation (Beginner Guide) Romanian Electro Trade, Engineering & Consulting 64 2P short-circuit: ( )kkkZUI22 1P short-circuit: ( )012 Z ZUIkkk+ where: Zk - is the equivalent short-circuit impedance,Uk - is the line voltage at the point of calculation, Z0 - is the impedance of neutral conductor (=Z ); if the neutral is connected to earth by an impedance Zn, then Z0 =Z + Zn. Three-phase short-circuit This fault involves all