On a morning in early June, a manhole exploded on High Street in downtown Boston, launching its 200-pound iron cover several feet through the air and severely disrupting the morning commute.
About a month later, a transformer exploded in a manhole, this time on Beacon Hill. But despite the force of an explosion that witnesses reported could be heard several blocks away, the manhole cover stayed in place.
The difference, Eversource officials say, is that the Beacon Hill manhole was equipped with a new type of cover that took years to develop and that the company is rushing to install on the 38,000 manholes it owns and maintains in the Boston area, including the one on High Street. The new “energy release” covers have hooks that hold the heavy disks in their housings in case of an explosion and runners that allow them to lift 4 inches to release the pressure of the blast without sending the cover airborne.
In the niche world of underground electrical systems engineering, the new cover is nothing short of revolutionary.
“The beauty of it is its simplicity,” said William Ritchie, Eversource’s vice president of electrical field operations.
They’re also working, according to Eversource. In reports filed to state regulators, the company reported that manhole incidents in the Boston region have fallen, from 212 in 2018 to 62 last year.
“I would love to say that it will never happen again, but that’s a hard statement,” added Craig Hallstrom, president of Eversource’s regional operations. But the new technology, he said, is a “game changer.”
Manhole explosions and the covers they occasionally launch skyward are a problem that has long bedeviled utilities; the causes are complex and until recently solutions were elusive. But essentially, the combination of underground gas, whether leaking from supply pipes or generated by decomposing organic matter, and aging high-powered electrical equipment can be a volatile mix.
“Over the past three decades we’ve come a long way,” said George Gela, who researched manhole safety for more than two decades at the Electrical Power Research Institute lab in Lenox. “We do not understand all these issues, but at least it is possible to make some recommendations on how to detect before the event and then mitigate and prevent.”
Noreen McGuire, who leads an Eversource team working on underground lines, set out on a recent morning along Massachusetts Avenue in Dorchester to deploy one of the company’s new covers.
“Maiden voyage!” she shouted as she guided a manhole cover lifted by a remote-controlled winch into the bed of a new $200,000 specially designed Eversource truck. McGuire and a colleague placed the old cover in the back of the truck then attached the winch to a new cover, which McGuire guided into place over the manhole.
To get the two hooks in place over the opening, McGuire used her foot to angle the cover, putting one side in place, then the other.
The cover McGuire installed had vents to allow gas to escape before it builds to dangerous levels, another innovation, but not all the new covers do. A vented cover can let in more road salt that degrades underground wiring and pose a tripping hazard to those in high heels, so Eversource is careful where they are placed, according to Mark Baldwin, the company’s director of electrical field operations in Boston.
Eversource has installed about 1,400 of its new energy release covers around the city and aims to put another 5,000 in before the end of 2023, Baldwin said.
The company is focusing on manholes that are prone to explosions, those that are packed with a lot of older wiring, and are near high traffic areas such as downtown Boston. It’s replaced all manhole covers in the Back Bay and along the Boston Marathon route and has made a significant dent in the manholes in the North End, Fenway, Financial District, and along Boylston and Newbury streets, Baldwin said.
The new truck will allow the company to install 125 new covers a week, up from 40 or 50 now, Baldwin said.
But the new manhole covers are no panacea.
“It’s fairly easy to restrain the manhole,” said Glen Bertini, a Seattle-based engineer and entrepreneur. “But it doesn’t really stop the explosion.”
Indeed, two Eversource employees who were working in a manhole were hospitalized by the Beacon Hill explosion and state office buildings were shut down even though the cover was secured.
The problem is that manhole explosions that can throw heavy iron covers hundreds of feet in the air often start with a tiny spark.
The underground electrical system in Boston is old, having first been installed in the late 1880s. The system relies on a secondary network, or hundreds of redundant, lower-voltage wires that connect individual buildings to main lines.
Road salt, rats, and general wear and tear can all degrade the smaller secondary wires and the insulation that protects them, Gela said. That makes the wires vulnerable to arcing — when an electrical charge jumps between a gap in a conductor or between two conductors.
Meanwhile, gas leaking from service pipes or generated by decaying organic matter and burnt wire insulation can build up in underground conduits, providing the explosive fuel that can send a manhole cover skyward when wires arc or a transformer sparks.
Research by Bertini, who serves on an Institute of Electrical and Electronics Engineers panel currently drafting manhole safety recommendations, found that about 1 million small fires take place each week in underground electrical systems throughout North America. And, on average, about 10 of those turn into larger fires each week, and one is powerful enough to cause an explosion, Bertini said.
Ideally, utilities would use active ventilation to pump gas out of manholes, but that can be expensive, Bertini said.
At a basic level, inspections can spot issues with wiring or gas buildup before they occur.
However, the number of manhole inspections Eversource performed dropped sharply in the last four years, from 5,621 in 2018 to 3,481 in 2021, according to reports filed with the state first reported by WCVB.
Most inspections are performed while workers are doing unrelated maintenance work, and that has been curtailed during the pandemic because of reduced business activity downtown and health precautions, Hallstrom said. The company is adding all of the manholes in downtown Boston to a list of critical locations that receive more frequent and intense inspections, he said.
Experts also point to another problem. Boston’s pervasive natural gas leaks may be contributing to the city’s spate of explosions, said Nathan Phillips, a Boston University ecologist.
Phillips pointed to a March 2020 manhole explosion on Charles Street that launched a cover into the bottom of the Longfellow Bridge. He has measured significant gas leaks in the same area, he said.
“That’s a plausible causal factor that has not been aired,” he said. “This is a serious, serious oversight.”
Eversource is piloting 20 gas sensors in manholes around Boston with 100 more slated for installation by the end of the year, Hallstrom said. The sensors can detect many types of flammable gasses as well as temperature and oxygen levels, two indicators of fire, and relay that data back to Eversource in real time.
“I acknowledge that this is a very frightening situation,” Eversource’s Hallstrom said. “I do think manholes are safe. These events . . . don’t happen all that frequently, [but] we’re working really hard to prevent those from happening at all.”