THERE ARE OVER 2,000 MANHOLE FIRES/EXPLOSIONS EACH YEAR IN NYC. UNDERGROUND CABLES BECOME FRAYED FROM SNOW/ICE MELTING CHEMICALS, AGING, CORROSIVE CHEMICALS, OVERLOAD OR RATS BITING THEM

Snow/ice melting compounds in melting snow has wreaked havoc on the New York City's underground utility system, sparking nearly 600 manhole explosions, fires and other incidents since Sunday, Con Ed officials said.  The city averages about 2,100 a year.  In November, two people were hurt after an explosion in midtown.  Last month, a 71-year-old Park Slope man was injured when a manhole cover flew off and hit him in the head.  And few weeks ago, three firefighters in Brooklyn were injured when more than a dozen manholes exploded across the borough.  

Manhole fires, like those in Park Slope and others reported overnight, are often triggered by salt that leaks into the underground electrical system and corrodes wires, which then ignite, officials said.

"A lot of times after it snows and the snow melts, salt that is put down onto the sidewalk leaks into our system and can cause fires when it hits our equipment," Con Ed spokesman Alfonso Quiroz said.

"What it does is it eats away the insulation that prevents the different cables from touching each other," said FDNY Battalion Chief Thomas Schmitt. "Once that's gone, you get this effect."

The flaming wiring can then release the dangerous gas, which can leak into neighboring buildings as it builds up pressure below the manhole, causing it to explode.

A cast-iron manhole cover can weigh between 85 and 300 pounds (35 to 136 kg), and explosions have propelled these massive discs anywhere from 1 foot to 50 feet (0.3 to 15 m) into the air. The real problem with these explosions (aside from the risk of injury) is the loss of power in the aftermath.

In most cases, these are the events that lead to an explosion:

Underground cables become frayed from aging, corrosive chemicals, overload or rats biting them.  These cables carry on the order of 13,000 volts of electricity.

These electrical wires heat up the paper, lead and rubber insulation.

The insulation smolders and catches on fire, releasing gases.

The pressure from the gas builds up inside the manhole.

The electrical wires arc like a bolt of lightning and ignite the gases, causing a powerful explosion.

Depending on the amount of gas-pressure built up inside the manhole, the cover may flip over or be launched several feet in the air. Often, there may not be an explosion, just a lot of smoke or fire.

Some power companies are in the process of replacing solid manhole covers with slotted manhole covers. These new covers allow the gas to be released less violently, and also give an early warning to possible explosions.

Con Ed was still investigating what caused the Monday explosion, but Quiroz said salt was likely to blame.

"We don’t know [for sure], but considering the weather Monday that is very likely what it was," he said.

Manhole explosions typically increase during the winter, when salt applied to roads to melt ice can fray cables that lie beneath the city streets.  After the insulation is damaged, the cables then can short, causing sparking fires, smoking incidents, and blasts due to the increased gas pressure inside the vault.  Several hundred pound manhole covers can shoot in the air from that pressure built up. 

The electric utility company (Con Ed in this case) would basically have to replace sections of its underground wire infrastructure every few years to keep them from happening, a totally impossible project considering the traffic congestion issues, costs and thousands of miles of cables.  In 2014, ConEd started working on implementing a $1.3 billion electrical modernization project that will hopefully decrease the number of incidents.

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THE MANHOLE FIRE/EXPLOSION INCIDENTS ARE EXPECTED TO INCREASE DURING THE SNOW MELTING PERIOD

NEW YORK (Reuters) 

Salvatore Grillo was walking his Labrador retriever near Brooklyn's Prospect Park on a cold morning in February when he heard what sounded like a muffled explosion.

Minutes later, emergency responders were hovering over the 71-year-old as he lay unconscious on the ground. Police said an underground blast had blown a cast-iron manhole cover skyward and the heavy metallic disk came crashing down on his head.

While the seriousness of Grillo's injury is unusual, manhole "events" have become all too common in the country's largest metropolis. In the snowy first week of February, Consolidated Edison Inc, the local utility, tallied about 600 "smokers," fires and occasional explosions involving manholes, part of a seasonal surge that plagues New York every winter.

Manholes are entry points to a labyrinth of electric cables, many of them aged and decaying, that snake underneath the city streets. In winter melting snow mixed with de-icing salt can seep through, causing frayed low-voltage cables to fail. That can trigger fires, smoke and explosions that can send manhole covers flying.

"These incidents are yet another example of New York City's outdated infrastructure. It's well-known that saltwater and exposed electrical wires are a dangerous combination," said New York's Public Advocate Letitia James. "If we want a safer city, we must do more to address this issue."

Cables have an expected lifetime of about 40 years, but in Manhattan, 5 percent of low-voltage distribution cables were installed before 1930, according to a 2014 analysis. In addition, overheating and even gnawing rats can hasten the deterioration. It is the oldest electrical system in the nation.

"Everything that’s electrical has a certain limit, a certain lifetime," said William Black, a professor emeritus of mechanical engineering at the Georgia Institute of Technology.

In any single year, New York has 2,100 manhole incidents a year, or nearly six of them every day, Con Ed estimates.

The problem is so much larger in New York than elsewhere in part because the city has the country's largest underground electrical system, with its 98,000 miles (157,716 km) of cable and 264,000 manholes and service boxes.

To be sure, most manhole incidents are relatively harmless "smokers," but about 10 percent in the first week of February were more dangerous.

Mario Tama/GettyFirefighters gather at the scene of an underground blaze following an explosion near Eighth Avenue and 38th Street.

Earlier this month, a smoking manhole exploded in Brooklyn, sending a worker scrambling for safety in an incident caught on video. On the same day, a parked Mazda was torched after a manhole beneath it started spouting fire.

"Con Ed has a rather difficult situation because if you proceed to dig underneath the street you’ll see how congested it is," Black said.

The problem is attracting attention. Donovan Richards, chair of the New York City Council's Environmental Protection Committee, is considering legislation to push Con Ed to spend more on modernizing its underground infrastructure.

In 2014, Con Ed invested $1.3 billion in modernizing its electrical infrastructure, including the installation of nearly 1,600 miles (2,575 km) of underground electric cable. But that is just a fraction of the tens of thousands of miles in the system.

Manhole events "take a considerable amount of time and resources," said Con Ed spokesman Allan Drury. "But our emphasis is on public safety and we are always looking for new technologies and methods to improve our system."

For instance, the utility has been installing vented covers that allow for trapped combustible gases to dissipate more easily.

Con Ed has teamed with the Massachusetts Institute of Technology and Columbia University to develop a tool that predicts manhole incidents through statistical modeling, taking into account factors such cable age and failure history.

But Cynthia Rudin, associate professor of statistics at MIT who worked on the project, warns against focusing too much energy and money on replacing cables.

"It is not a reasonable scenario to think of replacing the whole distribution network every few years or so," Rudin said.

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3 FIREFIGHTERS INJURED IN UNDERGROUND SERVICE BOX EXPLOSION IN BROOKLYN

FRIDAY, MARCH 06, 2015

BROOKLYN, N.Y. (WABC) 

Three firefighters were injured when an underground service box exploded in Brooklyn Thursday night.

They were responding to a fire in an underground service box at 1285 Broadway in Bushwick Thursday night. Broadway remained closed in the area between Greene and Gates Friday morning.

One of the firefighters was hit by the cover to the service box. He was taken Kings County Hospital with a police escort. Two others sustained minor injuries.

None of the injuries are considered non-life threatening.

One hundred people lost power. An emergency Con Edison crew was on the scene.

Salty runoff from the street seeping into the ground electrical wiring ignited the explosion. Several manholes in the area have been smoking and the fire started beneath one.