The basic function of switchgear is tocontrol supply of electric power andto protect the equipment in the eventof abnormal conditions. Uninterruptedsupply of electric power is the need oftoday's industry and is catered to bythe switchgear and controlgear in anyplant.To meet the expectations of userindustry, switchgear has to be reliableand safe. It should also be able towithstand adequate number ofoperations - in healthy conditions aswell as in abnormal/unhealthysituations. Manufacturers as well ascustomers, both concur on theseaspects.However, reliability, adequacy aresubjective parameters. Unless theyare quantified, the expectations ofusers and the capabilities of theproducts cannot be matched.Thus, defining reliability, safety andadequacy in terms of quantifiableparameters is essential from theJanuary-March 1996user's as well as the manufacturer'spoint of view.This activity is jointly done by a teamcomprising representatives of userindustry, manufacturers and leadingtechnical institutes. The quantifiableparameters are laid down in the formof standards.Development in technology, innova-tions, availability of better materials,superior manufacturing processesand techniques result in improvementof products and their characteristic.At the same time, field experiencegives a clue for further development.It is realised that the field situationsare different from the laboratoryconditions and that product perform-ance differs in actual working condi-tions. Expansions and increasingloads demand higher source ratings,which demand protective deviceswith higher withstand capabilities.All these demand continuousupgradation of products.To meet the demands of industry,International Standards (formulatedby IEC) for Low Voltage Switchgearand Controlgear were revised in1988. Earlier, these standards werehaving different numbers and someof the terms were not clearly definedor quantified. Now, standards for alllow voltage switchgear andcontrolgear are grouped togetherunder a new specification - IEC-947.Bureau of Indian Standards adoptedthe new text in 1993 and a newstandard - IS 13947 was publishedin 1993.In this issue of L&T Current Trends,we bring you an update on majorrevisions carried out in the standardson low voltage switchgear andcontrolgear. Highlights of generalrules are covered in this issue. In thenext issue, product standards will becovered.Issued by : Switchgear Contracts Division LARSEN & TOUBRO LIMITED Powai Works, Bombay 400 072.Prospect / RetrospectContinuousUpgradationBEST WISHESFOR AHAPPYNEWYEARIn a major revision, InternationalElectrotechnical Commission (IEC)introduced a new specification, IEC947, on low voltage switchgear andcontrolgear in 1988. Bureau of IndianStandards also decided to adopt thetext of IEC 947 and consequently, in1993, introduced a new dual numberspecification IS 13947/IEC pub 947on switchgear and controlgearproducts. However, keeping in viewapplicability of this specification inIndian conditions, National Annexhas been added to IEC text to covertypical requirements e.g. use ofaluminum conductors, ambientconditions, preferred voltages, etc.This specification is divided into twodocuments.(1) General rules (Part 1) :All those parts of various productsstandards which can be consideredas general have been consolidated inthis first part. These requirements areapplicable to all the products viz.contactors, starters, air circuit break-ers, moulded case circuit breakers,switches and fuse combination units.(2) Product standard :Specific testing and performancerequirements of the products arecovered by relevant product stand-ards. These are :Part 2 : Circuit breakersPart 3 :Switches, disconnectors, switchdisconnectorsand fuse combination unitsPart 4 : Contactors and motorstarters(Section 1 : Electromechanicalcontactors and starters)Part 5 : Control circuit devices andswitching elementsPart 6 : Multiple function devicesPart 7 : Ancillary equipmentsIn pages to follow, we shall discusssignificant revisions that have beenintroduced in IS 13947.PART 1: GENERAL RULES :This standard states those generalrules and requirements which arecommon to low voltage switchgearproducts e.g. definitions; character-istics; normal service, mounting andtransport conditions; informationsupplied with the equipment; con-structional and performance require-ments and verification of theserequirements.Since a product standard is read inconjunction with general rules, it maynot require and hence may omit ageneral rule or it may add to it but willnot deviate from it.Following are the major revisionsintroduced in part 1 :(I) DEFINITIONS :Disconnector:Definition of disconnector has beenenlarged. A disconnector has nowbeen defined as a mechanicaldevice which in the open positioncomplies with the requirements ofisolating function.This definition differs from thepresent one (IEV 441-14-05) whichrefers to only isolating distance. Asper the new definition, a disconnectorneeds to meet performance require-ments in addition to minimum isolat-ing distance.(II) RATINGS AND LIMITINGVALUES :(i)Conventional thermal currents :Following two new limiting cur-rents have been introduced todefine thermal characteristics ofthe product.(a) Conventional free air thermalcurrent (lth)(b) Conventional enclosed thermalcurrent (lthe)These currents are defined as themaximum values of currents to beused for temperature rise tests onunenclosed equipment (lth) or onequipment in smallest enclosurespecified by the manufacturer (lthe).These currents are not the ratingsand need not be marked on theequipment.(ii) Rated impulse withstand voltage(Uimp) :This is a new concept for low voltageswitchgear products.So far attention was not given to thenature and effect of overvoltagesoriginating from causes like loadfluctuations, switching operations,resonances, faults and lighteningdischarges on low voltage (LV)systems.These overvoltages affect commonLV installations and equipment suchas motors, switchgears, appliancesetc. Various utilities and manufactur-ers have come across failures of LVequipment attributable to theseovervoltages. Increased applicationsof miniaturized electronics in indus-trial products underscore need toF e a t u r eIntroduction to IS 13947 :A new Indian Standard onSwitchgear and Controlgear Products- H.T. Mistry, Senior Executive-Design, Switchgear Design and Product Developmentconsider overvoltage phenomena inLV system.In view of above, in order to evaluateadequacy of design of low voltageswitchgear products to withstandovervoltages, concept of ratedimpulse withstand voltage has beenintroduced.Preferred values of rated impulsewithstand voltage are specified innew IS. These are reproduced intable 1.Overvoltage category for the equip-ment is decided by its location in theinstallation. (Origin of installation,distribution, load etc.).Preferred value of impulse withstandvoltage reduces as the equipment islocated further away from the sourceof energy.To be suitable for applicationfor a given location/overvoltagecategory, rated impulse withstandvoltage of the equipment should beequal to or above the preferredvalue for that location/overvoltagecategory.(III) NORMAL SERVICECONDITIONS :Pollution degrees :In order to define environmentalconditions more precisely and toevaluate clearance and creepagedistances following four degrees ofpollution are established.Pollution degree 1:No pollution or only dry, non-conduc-tive pollution occurs.Pollution degree 2:Normally non-conductive pollutionocccurs. Occasionally, however, atemporary conductivity caused bycondensation may be expected.Pollution degree 3:Conductive pollution occurs or dry,non-conductive pollution occurswhich becomes conductive due tocondensation.Pollution degree 4:The pollution generates persistentconductivity caused, for instance, byconductive dust or rain or snow.Unless otherwise specified equip-ment for industrial application isgenerally suitable for use in pollutiondegree 3 environment.(IV) CONSTRUCTIONAL REQUI-REMENTS :Constructional requirements asregards insulating materials, termi-nals, clearance and creepagedistances have now been made moredetailed.(a) Materials :The suitability of insulating materialsis verified with respect to its resist-ance to abnormal heat and firethrough elaborate tests. These testsare either conducted on the equip-ment (glow wire test) or on specimenof the material (hot wire ignition andarc ignition tests).(b) Terminals :Constructional requirements e.g.strength, size etc. for terminalssuitable for unprepared copperconductors are evaluated throughfollowing tests.(i)Tests for verification of mechani-cal strength(ii)Flexion test (Test for damage toconductors and their accidentalloosening)(iii) Pull out test(iv) Test for insertability of conduc-torsRequirements for terminals suitablefor aluminum conductors are underconsideration.(c) Clearance and creepage dis-tances : Clearance:By definition, clearance is the dis-tance between two conductive partsalong a string stretched the shortestway between these conductive parts.The requirement of clearance in air isdetermined by transient overvoltagesand micro environment.Minimum clearance values havebeen specified in new IS on the basisof-rated impulse withstand voltage and-pollution degreeStipulated minimum clearance valuesfor various rated impulse withstandvoltages are given in table II.Clearance is verified through a typeTABLE IPREFERRED VALUES OF IMPULSE WITHSTAND VOLTAGEPREFERRED VALUES OF IMPULSE WITHSTANDVOLTAGE i n kV (1.2/50 S)OVER VOLTAGE CATEGORYMAXIMUM VALUE OFRATED OPERATIONALVAOLTAGE TO EARTHVIVORIGIN OFINSTALLATIONLEVELIIIDISTRIBUTIONCIRCUITLEVELIILOAD LEVELISPECIALLYPROTECTEDLEVEL50 1.5 0.8 0.5 0.33100 2.5 1.5 0.8 0.5150 4 2.5 1.5 0.8300 6 4 2.5 1.5600 8 6 4 2.51000 12 8 6 4TABLE IIMINIMUM CLEARANCE IN AIRMINIMUM CLEARANCE, in mmPOLLUTION DEGREERATED IMPULSEWITHSTANDVOLTAGEUimp, kV (1.2/50 S) 1 2 3 40.33 0.01 0.2 0.8 1.60.5 0.04 0.2 0.8 1.60.8 0.1 0.2 0.8 1.61.5 0.5 0.5 0.8 1.62.5 1.5 1.5 1.5 1.64 3 3 3 36 5.5 5.5 5.5 5.58 8 8 8 812 14 14 14 14test for verification of dielectricproperties.Creepage distance : Creepagedistance is defined as the shortestdistance along the surface of aninsulating material between twoconductive parts.Recommendations for creepagedistances are based on a consider-able amount of empirical data.To account for deterioration ofproperties of insulating materialsminimum creepage distances speci-fied consider following fundamentalfactors-Rated insulation voltage orworking voltage-Environmental conditions orpollution degree-Insulating materials usedInsulating materials can be roughlycharacterized according to thedamage they suffer due to flow andinterruption of leakage current oncontaminated surfaces. Though nodirect relationship exists, it has beenfound by tests and experience thatinsulating materials with highercomparative tracking index (CTI)have higher relative performance.Hence for the purpose of specifyingminimum creepage distance insulat-ing materials have been classifiedinto four material groups as given intable III.Minimum creepage distances for lowvoltage switchgear products aregiven in table IV.Creepage distance is verified byactual measurement as a part of thetype test for verification of dielectricproperties.(v) VERIFICATION OF PERFORM-ANCE REQUIREMENTS :General test conditions e.g. mountingarrangements, test circuits, permissi-ble variations on test parameters etc.for all nature of tests are given in thispart.In national annex aluminum conduc-tors to be used for various tests areincluded. The specified cross sec-tions are corresponding to coppersections specified in IEC text. Theseare compared in table v.TABLE IIIMATERIAL GROUPSMATERIAL GROUP I 600