IEC_81-239e-DC.pdf-dir81formatpdftype-DCfile239e.pdf

81/239/DCFor IEC use only2003-12-19INTERNATIONAL ELECTROTECHNICAL COMMISSIONTECHNICAL COMMITTEE No.81:LIGHTNING PROTECTIONIEC 62305-1,Ed.1:Protection against lightning Part 1:General principlesAccording to the decision taken at last TC 81 meeting(see 81/236/RM item 6),National Committees areasked for opinion on the following editorial proposal:1.to delete in IEC 62305-3 the Annex B(informative):Lightning current flowing through externalconductive parts and installations entering the structure(see doc.81/xxx/CDV)2.to delete in IEC 62305-4 the Annex E(informative):Surges due to lightning at different installationpoint(see doc.81/238/CDV)3.to add to IEC 62305-1 a new Annex E(informative)incorporating the Annex B of IEC 62305-3 andthe Annex E of IEC 62305-4.The proposed new Annex E of IEC 62305-1 is attached.Comments should be submitted using the IEC Electronic voting/commenting system by the NationalCommittees(See AC/15/2001).Comments to be returned by 2004-02-27.2 81/239/DCAnnex E(informative)Surges due to lightning at different installation pointsFor dimensioning of conductors,SPD and apparatus the threat due to surges at the particularinstallation point of these components shall be determined.Surges can arise from(partial)lightningcurrents(typical current waveform 10/350),from induction effects into installation loops and asremaining threat downstream of SPD(typical current waveform 8/20).The threat due to thesesurges must be lower than the withstand of the used components defined by adequate tests.ForSPD such tests are given in IEC 60364-1:Class I test defines an impulse test current Iimp defined via peak value,charge and specificenergy,which is comparable with the 10/350 surge;Class II test defines an impulse test current In 8/20;Class III test uses a combination wave test,defining the open circuit voltage Uoc 1,2/50 andthe short circuit current Isc 8/20 of an 2 combination wave generator.E.1 Surges due to flashes to the structure(Source of damage S1)E.1.1 Surges flowing through external conductive parts and lines connected to the structureWhen conducted to earth,the lightning current is divided between the earth termination system,theexternal conductive parts and the lines,directly or via SPD connected to them.If If=ke I is the part of the lightning current relevant to each external conductive part or line,then ke dependson:-their number,-their conventional earthing impedance for underground parts,or their earth resistance,whereoverhead parts connect to underground,for overhead parts,-the conventional earthing impedance of the earth termination system.)ZZ*nn(*ZZZke21211+=for underground installation)ZZ*nn(*ZZZke12122+=for overhead installationwhereZis the conventional earthing impedance of the earth termination system;Z1is the conventional earthing impedance of the external parts or lines(Table B.1)runningunderground;Z2is the earth resistance of the earthing arrangement which connect to ground the overhead line.Ifthe earth resistance of the earthing point is not known,the value of Z1 of table B.1 can be usedwith the resistivity relevant to the place of earthing point.3 81/239/DCNOTE This value is supposed in the above formula to be the same for each earthing point.If it is not the case,morecomplex equations need to be used.n1is the overall number of external parts or lines running underground;n2is the overall number of external parts or lines running overhead;Iis the lightning current relevant to the considered LPS type(see 4.2 and Table 2)Assuming as first approximation,that one half of the lightning current flows in the earth terminationsystem and that Z2=Z1,the value of ke may be evaluated by:ke=0,5/(n1+n2)for an external conductive part or lineIf entering lines(e.g.electrical and telecommunication lines)are unscreened or not routed in metalconduit,each of the n conductors of the line carries an equal part of the lightning currentke=ke/n for each of the n conductors of an unshielded line.For shielded entering lines bonded at the entrance values of current ke are given by:ke=ke Rs/(n Rs+Rc)for each of the n conductors of a shielded service.withRsohmic resistance for unit length of shield;Rcohmic resistance for unit length of inner conductor.Table B.1 Conventional earthing impedance values Z and Z1 according to the resistivity of thesoilConventional earthing impedance related to the type of LPSZ mZ1 IIIIII IV1002005001 0002 0003 00081316222835461010101046101515154610204060NOTE Values reported in Table B.1 are referred to conventional earthing impedance of a buried conductor underimpulse condition(10/350 s).4 81/239/DCE.1.2 Factors influencing the sharing of the lightning current in the power linesFor detailed calculations it should be mentioned,that several factors can influence the amplitudeand the waveshape of such surges:the cable length can influence current sharing and time characteristics due to L/R-ratio different impedances of neutral and phase conductors can influence current sharing among line