Southern Calirornia Edison Company

MEMBERAIEE DURING recent years, there hasbeen aconsiderable increase intheunderstanding oflightning behavior, andinthe performance oflightning protective devices. Thispaperwasprepared atthe suggestion oftheAIEEProtective Devices Committee, inorder that acomparisonofthepresent lightning protection practices ofutilities invarious sections of theUnited States will beavailable. In theareaserved bytheSouthern California EdisonCompany, lightning stormsare veryinfrequent andthemethodsof lightning protection maybequite different fromthose required byutilities situated wherethere isagreat deal oflightning. Inthecompany's service area, there is anextreme variation intopography which includes mountainranges,sheltered valleys, widedesert areas, andcoastal plains. Theisokeraunic level varies from 0to15,butrelative freedom fromlightningdoesnotsimplify theproblem of protection. Ifthere werenolightning, arresters would notberequired anywhere andifthere wereagreat deal oflightning, arresters wouldbeusedeverywhere, but sincethelightning frequency on the systemvaries widely, a careful analysis mustbemadeofeachcasetodetermine ifarresters should beinstalled. Because ofthelack ofexactinformation on thevariation ofisokeraunic level within thearea, a survey wasmadein 1953anda local isokeraunic mapprepared.Thecompanyhasalso evaluated theprotection afforded apparatus when several lines areconnected toastation. Entirely satisfactory protection isnow obtained witha minimumnumberof arrester installations. Itappears thaton this system, notmorethan30percentof thepowertransformer bankswill need