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Why Manhole Explosions Happen in the Summer

It wasn’t a bird, a plane or Superman that blasted nearly 200 feet into the sky in Buffalo, New York, in April. It was a manhole cover.
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It wasn’t a bird, a plane or Superman that blasted nearly 200 feet into the sky in Buffalo, New York, in April. It was a manhole cover.

Triggered by an underground electrical fire that ignited some volatile gasses, the manhole cover shot into the sky as a local photojournalist filmed its brief but spectacular flight.

The unexpected blast-off was the most serious type of what are known in the utility trades as “manhole events,” an accident classification that ranges from a few puffs of smoke from a short-circuit to the dangerous and frightening explosions that hurl metal discs weighing anywhere from 70 to 300 pounds toward the heavens.

Manhole events happen almost every day somewhere in the U.S., and can be more than merely scary:

  • A bystander captured video of a manhole cover exploding 15 feet into the air in Hell’s Kitchen on Friday. Nobody was injured in the blast, but it shattered a nearby window and left more than a dozen city blocks closed at the beginning of the weekend summer commute.
  • A July manhole fire in Washington Heights in Manhattan left nearly 800 people without power for roughly nine hours in sweltering temperatures.
  • And in May, a manhole cover in Northridge, California, blew up and injured three people as they drove by.
New York is Ground Zero

A review of records and news reports by NBC News indicates that they are more likely to happen in New York City than anywhere else.

A 2014 report by Consolidated Edison Company said the Big Apple recorded 3,369 manhole events, including 32 explosions, during the year. Con Ed spokesperson Allan Drury said an explosion is defined as a manhole cover dislodging and causing injuries or property damage.

For comparison’s sake, the Los Angeles Department of Water and Power recorded only five manhole events, including three explosions, in the last two years, and ComEd in Chicago reported fewer than 10 events — none explosions — in the past year.

Drury said that while manhole events can happen at any time of year, the extra volume of electricity running through them because of heavy air conditioning use in summer typically increases the number of incidents. That explains why the utility recorded almost 500 manhole events between May and August last year.

“Cables get extremely hot because of extra volume of electricity running through them because of air condition,” he said. “During the summer we always encourage customers to use energy wisely.”

New York’s vulnerability to manhole events is attributable to the size and age of the mostly underground electrical system that keeps the city humming.

Con Ed has more than 94,000 miles of cable strung beneath New York City — the largest underground system in the world. Drury said the cable is supposed to last at least 40 years, but a 2014 MIT report analyzing the reliability of New York’s power grid found that some lines are much older than that.

“In Manhattan alone,” the report reads, “we calculated that at least 5 percent of the low-voltage distribution cables were installed prior to 1930, and many of these old cables are still functioning reliably.”

Drury said Con Ed’s biggest challenge is that numerous factors cause cables to fail, including vibrations from traffic, rats biting the wires, and most important, road salt.

“The main cause of manhole events is corrosion from salt (used to keep roads clear in snowstorms),” Drury said. “If two hypothetical pieces of cable in the same part of town have the same exposure to salt after a given storm, but one piece of cable is 40 years old and the other one is relatively new, the older one is more likely to be affected.”

A salty mystery

Salt takes such a toll on Con Ed’s cables that 68 percent of all 2014 New York City manhole events happened in January and February, when 78 percent of the city’s salt supply was used.

No one can say why, but salt seems to take more of a toll on New York’s electrical infrastructure.

ComEd, which provides electric utilities to more than 3.8 million people in Chicago and northern Illinois, recorded fewer than 10 manhole events in all of 2014, a company representative said. The utility has only 4,900 miles of underground cable — nearly 90,000 miles less than New York — but its ratio of manhole events to mile of wire is still considerably lower.

Chicago and New York use slightly different salt formulations for winter cleanups, but nothing that would account for a big difference in underground cable failure rates, according to sanitation department representatives in each city.

"NYC’s manholes might be more vulnerable to fires simply because … (they) have more cables per manhole.”

Cynthia Rudin, an associate professor of statistics at MIT who specializes in energy grid reliability and author of the MIT report, said the density of cables running through New York’s underground system may play a role in the failure rate.

“If any cable’s insulation breaks down there is a chance of a fire/explosion in that manhole,” Rudin said in an email, “which means that each of NYC’s manholes might be more vulnerable to fires simply because … (they) have more cables per manhole.”

Still, Rudin emphasized the research conducted for the MIT report did not reveal a direct cause for the large number of manhole events New York experiences.

Nearly 90 percent of the manhole incidents in the five boroughs in 2014 were caused by a breakdown in the insulation that protects the underground cables, prompting critics to urge better system upkeep and preventative measures.

Drury, the Con Ed spokesman, said Con Ed already conducts a five-year cyclical inspection program of all equipment. He referenced a May 2015 press release that stated the company is “investing $1.3 billion in new equipment installations and upgrades.”

Will vents help?

Among the new equipment are vented manhole covers, which “will help to diffuse the buildup of combustible gases associated with secondary events,” according to the 2014 report. Drury also said the company has been replacing older underground cable with corrosion-resistant, low-voltage cable, which currently makes up about 22 percent of the underground network and is less prone to sparking explosions.

A Los Angeles utility representative said similar inspection measures are common practice on the West Coast, along with infrared detection that’s used to point out failing and aging equipment.

Related: Natural Gas Overtakes Coal As Top U.S. Electricity Source

As for Chicago, the ComEd rep said vented manhole covers are already in place and thermographic cameras are used to identify underground hot spots. The company has also been assessing each of its 32,000 manholes and vaults since a modernization program started in 2012.

Despite Con Ed’s claims of progress, New York City Councilman Donovan Richards introduced a bill in June that would require the Fire Department — not the utility — to submit an annual report to the council “on manhole fires and explosions to which they responded.”

A spokesman for Richards said the proposed legislation’s goal is to collect more detailed reports of manhole events from a group — the FDNY — within the councilman’s jurisdiction. The spokesman said it is not a matter of mistrusting Con Ed’s annual reports.

“Knowing realistic numbers would allow us to better determine how serious and widespread the need is for underground infrastructure improvements,” Richards said in a statement.

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