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On October 8, 2016, at approximately 1310 EDT, while in Mode 4 for a planned refueling outage, Shearon Harris Nuclear Power Plant experienced an undervoltage (UV) condition in the switchyard for about 1.5 seconds. This triggered the UV relays for both emergency 6.9 kV buses and for several of the non-nuclear safety 6.9kV auxiliary buses, resulting in the respective supply breakers opening. At the time of the UV, the site was experiencing high winds and rain from the effects of Hurricane Matthew. Both Emergency Diesel Generators started and loaded as designed. Operations restored offsite power at 2154 EDT after verifying stable grid behavior for an extended period. Additionally, the Containment Ventilation Isolation system and the Auxiliary Feedwater system actuated and performed as designed.

The site declared an Unusual Event at 1328 EDT for loss of offsite power to emergency buses for greater than 15 minutes. At 2049 EDT, the Unusual Event was terminated.

The causes of the UV were determined to be a line fault on the Cape Fear - West End 230 kV line and equipment deficiencies associated with the Cape Fear 230 kV Substation protection relays which prevented immediate clearing of the fault.