Pretty weird lookin’!
Photo: Sophie Rosenbaum (AP)

You’ve surely heard about Thursday’s incident at a Con Edison substation in New York City that temporarily knocked out power at LaGuardia airport and set the night skies glowing with an eerie aquamarine. We wondered: What made it so BLUE?

Con Ed initially tweeted a laughably understated comment to describe the enormous explosion seen by thousands, if not hundreds of thousands of people: “There was a brief electrical fire at our substation in Astoria which involved some electrical transformers and caused a transmission dip in the area.”

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But this morning, Con Ed said that it was not a fire, nor was it a transformer explosion, according to WABC-TV: “The electrical fault on the 138,000-volt equipment caused a sustained electrical arc flash that was visible across a wide area. The affected equipment was isolated to a single section within the substation.”

Arc flashes occur when electrical current passes through a normally non-conductive medium, like air, on its way between two electrodes. This ionizes the medium and creates an “electrical arc” between the electrodes, characterized by a high current, high temperature, and the visible flash—the result of ionization in the medium. This concept is employed in arc welding, and even in lamps during the late 19th century. Despite other reports, it’s not the same as lightning, which is a spark or discharge. Arcs are typically sustained over a longer period of time.

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The blue color was the result of excited air molecules. If you remember back to high school, different excited gases release different colors. Past transformer explosions have also resulted in bluish glows.

“It’s a corona,” Peter Sauer, professor of electrical and computer engineering at the University of Illinois, told Gizmodo. “It happens whenever there’s an arc. There’s a bluish light that comes off of corona discharge on high-voltage transmission lines, especially when it’s humid. It’s the ionization of the air.”

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Basically, it’s a short circuit on the relatively high-voltage line, he said. It could have been caused by failed insulation, debris, or something else; we won’t know for sure until Con Ed investigates.

Arc flashes are especially bright and hot, and companies employ various methods to stop these arcs forming between electrodes. Clearly, one of those methods failed yesterday. So, though eerie, the blue flash wasn’t aliens and wasn’t even that strange, necessarily. It was just some faulty electrical equipment and a lot of ionized air.