Investigators have determined the cause of an airfield lighting outage at Chicago's O'Hare International Airport that forced the closure of several runways.
Chicago Department of Aviation spokesman Gregg Cunningham says an electrical equipment failure occurred during a switchover from one power source to another. The switch then had to be made manually.
He said Thursday night's 10-minute outage on four runways was a "very unusual" occurrence and that the traveling public was never at risk.
Two of the affected runways were in use at the time and had to be closed, causing some delays of flights headed to one of the nation's busiest airports.
The Federal Aviation Administration's separate approach lighting leading to the runways was not affected.
Cunningham said O'Hare's multiple power systems are meant to prevent outages.