A new Indian Standard on Switchgear and …

1 Prospect/RetrospectIS/IEC specifica -tionIn the previous three issues of L&TCurrent Trends, we have publishedan article on IS/IEC specification forLow Voltage Switchgear andControlgear (IS 13947) in three readers have written to us togive a condensed summary of thesethree this issue, we are publishing thesummary alongwith important tableswhich were included in main our efforts are to make thissummary as exhaustive as possible,so that it can be preserved by ourreaders as a handy reference, if anyclarification is required, we suggestthat our readers refer to the originalIndian December 1996 In a major revision, InternationalElectrotechnical Commission (IEC)

2 Introduced a new specification , IEC947, on low voltage Switchgear andcontrolgear in 1988. Bureau ofIndian standards also decided toadopt the text of IEC 947 and conse-quently, in1993, introduced a newdual number specification IS 13947/IEC pub 947 on Switchgear andcontrolgear products. NationalAnnex has been added to IEC text inIS to cover typical requirements of aluminium conductors, ambi-ent conditions, preferred This specification is divided into ) General rules (Part 1) :All those parts of various prooductstandards which can be consideredas general have been consolidated inthis first ) Product standardSpecific testing and performancerequirements of the products arecovered by relevant product stand-ards.

3 These are :Part 2 : Circuit breakersPart 3 : Switches, disconnectors,switch disconnectors and fusecombination unitsPart 4 : Contactors and motorstarters (section 1 : Electromechan-cal contactors and starters)Part 5 : Control circuit devices andswitching elementsPart 6 : Multiple function devicesIssued by : Switchgear Contracts Division LARSEN & TOUBRO LIMITED Powai Works, Mumbai 400 to IS 13947 :A new Indian Standard onSwitchgear and Controlgear Products- Summary- Mistry, Senior Executive-Design, Switchgear Design and Product DevelopmentPart 7 : Ancillary equipmentsPART 1 : GENERAL RULES :This Standard states those generalrules and requirements which arecommon to low voltage switchgearproducts definitions; characteris-tics; normal service, mounting andtransport conditions; information tobe supplied with the equipment;constructional and performancerequirements and verification ofthese are the major revisionsintroduced in part 1 :I) DEFINITIONS :Disconnector:Definition of disconnector has beenenlarged.

4 A disconnector has nowbeen defined as a mechanical devicewhich in the open position complieswith the requirements of isolatingfunction. As per the new definition, adisconnector needs to meet perform-ance requirements in addition tominimum isolating ) RATINGS AND LIMITINGVALUES :i) Conventional thermal currentsFollowing two new limiting currentshave been introduced to definethermal characteristics of the ) Conventional free air thermalcurrent (Ith)b) Conventional enclosed thermalcurrent (Ithe)These currents are defined as themaximum values of currents to beused for temperature rise tests onunenclosed equipment (Ith) or onequipment in smallest enclosurespecified by the manufacturer (Ithe).

5 These currents are not the ratingsand need not be marked on ) Rated impulse withstandvoltage (Uimp) :Overvoltages originate from causeslike load fluctuations, switchingoperations, resonances, faults andlightening discharges on low voltage(LV) overvoltages affect commonLV installations and equipment. Inview of this, in order to evaluateadequacy of design of low voltageswitchgear products to withstandovervoltages, concept of ratedimpulse withstand voltage has values of rated impulsewithstand voltage are specified innew IS. Theses are reproduced intable category for the equip-ment is decided by its location in theinstallation.

6 (Origin of installation,distribution, load etc.)Preferred value of impulse withstandvoltage reduces as the equipment islocated further away from the sourceof ) NORMAL SERVICECONDITIONS :Pollution degrees :In order to define environmentalconditions more precisely and toevaluate clearance and creepagedistances following four pollutiondegrees are degree 1 :No pollution or only dry, non-conduc-tive pollution degree 2 :Normally non-conductive pollutionoccurs. Occasionally, however, atemporary conductivity caused bycondensation may be degree 3 :Conductive pollution occurs or dry,non-conductive pollution occurswhich becomes conductive due degree 4 :The pollution generates persistentconductivity caused, for instance, byconductive dust or rain of otherwise specified equip-ment or industrial application isgenerally suitable for use inpollution degree 3 ) CONSTRUCTIONALREQUIREMENTS :a) Materials.

7 The suitability of insulating material isverified with respect to its resistanceto abnormal heat and fire throughelaborate tests. These tests areeither conducted on the equipment(glow wire test) or on specimen of thematerial (hot wire ignition and arcignition tests).b) Terminals :Constructional requirements , size etc. for terminalssuitable for unprepared copperconductors are evaluated throughfollowing ) Tests for verification of mechani-cal strengthii) Flexion test (test for damage toconductors and their accidentalloosening)iii) Pull out testiv) Test for insertability of conduc-torsc) Clearance and creepagedistances :Clearance :Minimum clearance values havebeen specified in new IS on the basisof rated impulse withstand voltageand pollution degree.

8 Clearance isverified through a type a test forverification of dielectric account for deterioration ofproperties of insulating materialsminimum creepage distance arespecified based on following factors- Rated insulation 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 relatively better for the purpose of specifyingminimum creepage distance,insulating materials have beenclassified into four material groups interms of CTI, as given in table creepage distances for lowvoltage Switchgear products aregiven in table 2 : CIRCUIT BREAKERSThis Standard replaces IS 2516(Parts 1&2/Sec.)

9 1)-1985I) CHARACTERISTICS :a) Utilization category :In the new IS, utilization categorieshave been specified for circuitbreakers also. These have beendefined with reference to whether a circuit breaker isintended for selectivity by means ofan intentional time delay under shortcircuit breakers not specificallyintended for selectivity can beassigned utilization category A case circuit breakers need not have shorttime withstand rating (Icw).Circuit breakers specifically meant forselectivity are assigned utilizationcategory B. Air circuit breakers withintentional time delay for tripping inshort circuit condition can be as-signed utilization category B.

10 Thesebreakers will also be assigned shorttime withstand rating (Icw).b) Short circuit ratings :Following two new short circuitratings have been ) Rated ultimate short circuitbreaking capacity (Icu) :Ultimate short circuit breakingcapacity is assigned by the the term indicates, continuedservice from the breaker is notexpected after the breaker hascleared a short circuit fault of rating is similar to P1 categoryrating defined in IS ) Service short circuit breakingcapacity (Ics) :Service short circuit breaking capac-ity is also assigned by the manufac-turer and can be expressed aspercentage of ultimate short circuitbreaking capacity (Icu).