Here’s Why Manholes Explode Each Winter in New York City


Besides train delays, canceled flights, and countless piles of lovely slush, the city’s wintry conditions are also causing manhole covers to spontaneously pop out of the ground and, in some cases, do serious damage to pedestrians. Within the last 48 hours, there have been more than 200 cases of manholes smoking, catching fire, or even exploding, according to Con Ed.

Two incidences were recently reported in Park Slope, Brooklyn. The first explosion, at 11:20 a.m on February 2, sent a 70-pound manhole cover flying into the head of 71-year-old Sal Grillo, who was out with his dog. Grillo was rushed to the Lutheran Medical Center; fire department officials say he’s lucky to be alive. Another victim of the incident was Marge Contorno, 93, who was cut by flying shards of glass after the violent blast shattered her window.

The second explosion occurred at 4 a.m on February 3. No one was hurt, but six nearby buildings were evacuated owing to high levels of carbon monoxide in the area.

Con Ed spokesman Phillip O’Brien says the flying manhole covers often appear at this time of year, when heavy snowfall is followed by the spreading of salt on the roads. This salt-and-snow combo subsequently escapes underground into the electrical system, he explains, where some of the cables already sport nicks and tears from overhead traffic vibrations, natural wear and tear, and gnawing rats. The briny mixture seeps into those cuts and erodes the insulation on underground electrical wires, causing them to spark, smoke up, and, in extreme cases, explode. O’Brien says it’s not necessarily related to NYC’s aging infrastructure. Some cables are old and some are new, and either can be affected by corrosion due to salt and snow.

In fact, manhole explosions were worse in 2014 because of dreadful snowstorms, which consequently meant more salt-spreading. One preventive measure Con Ed has used for a long time: installing vented manhole covers, which have more openings, thus allowing smoke and built-up gas to escape more easily. The approach, however, only mitigates the effect; the salt still trickles down to the cables through the numerous holes.

Manhole explosions are common in other parts of the U.S. — and, indeed, the Northern Hemisphere — in winter. No breakthroughs have been made, at least from Con Ed, in determining when a manhole is likely to “pop.”

“This is a common occurrence each winter, especially in the periods of weather like we’re experiencing,” reiterates fire department spokesman Frank Dwyer. He urges New Yorkers to call 911 immediately if they hear crackling sounds outside, or if carbon detectors go off in homes or in other structures surrounding a troubling manhole.

The clip below is from a manhole explosion in 2014:

In 2009, three research scientists from Columbia University’s Center for Computational Learning Systems collaborated with Con Ed engineers to create a manhole profiling tool that ranked the structures in order to help predict when serious events like smoke, fire or an explosion occur in a manhole. In a later publication of the paper in 2011, titled, “21st-Century Data Miners Meet 19th-Century Electrical Cables” the researchers said, “New York City has the world’s oldest grid, and we computed that over five percent of Manhattan’s low-voltage underground cables were installed before 1930.” Interestingly, the article also revealed that NYC alone has more than 94,000 miles of underground cable, enough to wrap around the Earth three and a half times. “There is simply too much cable to replace or individually monitor–we’re not even close,” it reads.

Four years later, after Hurricane Sandy’s onslaught on the city, Con Ed drafted storm-hardening measures, which involved fortifying electric systems from future storms. According to the initiative, in order to protect their underground tunnels against flooding, Con Ed installed “reinforced concrete tunnel entrances” designed to prevent or greatly reduce water intrusion. Pumping equipment and back-up generators were also set up to remove water that might get in. Con Ed spokesman Sidney Alvarez says the utility company has made significant strides in ensuring public safety. In the past, a manhole fire would result in mass power outages, but now “when we have a manhole fire, there are different feeders that control it,” Alvarez says. As part of the storm-hardening initiative, Con Ed installed “smart” switches that reduce the number of homes and businesses that lose power when there is a fault in the system. The smart switches automatically disconnect affected parts of the electric grid so that unaffected areas continue to have uninterrupted power while repairs are being made.

Manhole Profiling Tool To Help Determine Serious Events

And New Yorkers, as usual, panic on Twitterverse: