在双母线带旁路母线接线中且母联断路器兼作旁路断路器时，正常运行情况下母联断路器起到（  ）的作用。 

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69483' and id < 4500442 and id<>'4500442' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69483' and id > 4500442 and id<>'4500442' order by id asc limit 5