▁of ▁service ▁for ▁planned ▁maintenance . ▁After ▁the ▁power ▁loss , ▁ED G ▁ 1 - A ▁failed ▁to ▁start ▁due ▁to ▁a ▁protect ive ▁safety ▁trip . ▁The ▁resulting ▁loss ▁of ▁elect rical ▁power ▁in ▁the ▁plant ' s ▁" v ital ▁circ uits " ▁shut ▁down ▁the ▁resid ual ▁heat ▁removal ▁( R HR ) ▁p ump ▁that ▁was ▁cool ing ▁the ▁core ▁of ▁Unit   1 ▁( which ▁was ▁near ing ▁the ▁end ▁of ▁a ▁ref uel ing ▁out age ) ▁and ▁prevent ed ▁the ▁backup ▁R HR ▁from ▁activ ating . ▁Even ▁though ▁Unit   1 ▁was ▁off line ▁at ▁the ▁time , ▁resid ual ▁heat ▁from ▁the ▁natural ▁decay ▁of ▁the ▁radio active ▁fuel ▁must ▁be ▁removed ▁to ▁prevent ▁a ▁dangerous ▁rise ▁in ▁core ▁temperature . ▁While ▁the ▁non - s af ety ▁power ▁was ▁not ▁interrupted , ▁there ▁was ▁no ▁physical ▁connection ▁between ▁the ▁vital ▁and ▁non - v ital ▁elect rical ▁trains , ▁prevent ing ▁the ▁vital ▁trains ▁from ▁receiving ▁power ▁from ▁the ▁una ffect ed ▁path ▁through ▁the ▁U AT s . ▁ ▁At ▁ 9 : 4 0   a . m ., ▁the ▁plant ▁operators ▁declared ▁a ▁site ▁area ▁emer gency ▁( SA E ) ▁per ▁existing ▁procedures ▁which ▁called ▁for ▁an ▁S AE ▁whenever ▁" v ital " ▁power ▁is ▁lost ▁for ▁more ▁than ▁ 1 5   min utes . ▁At ▁ 9 : 5 6   a . m ., ▁after ▁trying ▁multiple ▁times ▁to ▁start ▁the ▁ 1 -