A major power outage occurred in the southeastern city of Ulsan and county of Ulju Wednesday afternoon, causing damage and inconveniencing residents in the region.

According to authorities, the sudden blackout began at around 3:37 p.m. in Ulsan's Nam District as well as part of Ulju County, affecting over 155-thousands households as authorities fielded nearly 30 complaints of people trapped in elevators.

Police were dispatched to roads in the area after more than 120 complaints about issues deriving from a lack of traffic lights, while patients at hospitals were forced to return home as computers and medical equipment were down.

On Thursday, the Korea Electric Power Corporation(KEPCO) apologized to Ulsan and Ulju residents for the inconvenience, pledging a thorough investigation into the cause and the establishment of preventive measures.

KEPCO assumes the outage was caused by breakage of internal insulation of a switchgear during replacement work.