Electrical cable failure behind Back Bay manhole incident, Eversource official says

“It was an electrical short that occurred in that manhole, and as a result, that short created fire,” Foley said in an interview Wednesday.

He said the cable is made of copper with a neoprene jacket, a flexible and heavy-duty material that is resistant to water and extreme temperatures.

Foley said crews were cutting out the damaged cable and installing a new one. He said the area is safe for pedestrians and drivers, though construction crews are still working with the city to complete repairs.

The failure generated intense heat and smoke that vented through nearby manholes connected by underground conduits, he said. There was no damage in the other holes, he said.

The Boston Fire Department evacuated several buildings Tuesday night after detecting carbon monoxide, a byproduct of electrical burning. No serious injuries were reported, and city officials later cleared the buildings for reentry, according to the fire department.

Fire officials described the incident as “explosions” involving six manholes along Newbury Street, though Foley argued the event was an electrical fault rather than a blast.

“I wouldn’t consider it an explosion, I would consider it an electrical failure,” Foley said. “By the nature of electricity flowing, there’s energy there and the energy goes from the conductor to ground and produces heat and smoke … Certainly not an explosion by the pure definition.”

He said witnesses who reported loud sounds were likely hearing electricity discharging to ground, a common occurrence in underground electrical failures.

The incident underscores the challenges of maintaining Boston’s extensive underground electrical network, much of which runs beneath streets built long before modern utilities. Unlike overhead lines, underground cables are designed to operate while submerged, but they remain vulnerable to corrosion and wear.

Foley said winter weather likely contributed to the failure, particularly after a snow storm last month dropped nearly two feet of snow in the Boston area. Snowmelt and road salt can seep into manholes, creating a corrosive environment that degrades cable connections over time, he said.

Repeated heating and cooling of cables can also cause them to expand and contract, leading to scuffing and eventual breakdown, Foley said.

Those dynamics have played a role in a series of manhole incidents in Boston over the past decade. In some cases, pressure from electrical faults has been strong enough to launch heavy cast-iron covers into the air, injuring pedestrians and damaging property.

In response, Eversource developed “energy-release” manhole covers designed to vent pressure while remaining latched in place. The covers are intended to prevent the manhole lids from becoming airborne even during serious electrical faults.

Foley said the cover at the Newbury Street “trouble hole” remained secured throughout Tuesday’s incident.

“In and around Boston, we’re about 99 percent complete” installing the energy-release covers, Foley said, adding that he was not aware of any cover being dislodged since the program began.

Eversource said it has not recorded any cover displacement at locations equipped with its newer energy-release covers since accelerating their installation in 2022.

Eversource operates roughly 45,000 manholes across the state, with about 33,000 in Eastern Massachusetts, Foley said. He said the company has completed upgrades on more than 60 percent of locations across Massachusetts, with dense urban areas like Boston and Cambridge prioritized first.

Nick Stoico can be reached at nick.stoico@globe.com.