SALEM — The series of powerful manhole explosions that rocked the downtown Saturday night was not caused by a natural gas leak but most likely by a buildup of gas vapors from burning underground high-voltage cables, according to a memo National Grid sent to city officials.
"... We believe we had a secondary system failure that began to smolder, and we believe the burning of the cable insulation caused a buildup of gas in the underground manholes," according to a company e-mail obtained by The Salem News. "When the gas ignited in the manhole, the explosion dislodged four manhole covers."
In an earlier interview, a company spokeswoman had suggested burning insulation as a possible explanation and described that kind of gas as "volatile vapors" to distinguish it from the natural gas that flows through pipes to utility customers.
However, National Grid said there was a natural gas leak Monday on Crombie Street, the scene of the explosions, but the company insisted that the leak was found nearly two days after the initial incident and was unrelated to the blast.
Natural gas, the company said, played no role in the explosions around 10:45 p.m. Saturday that launched four manhole covers into the air and caused property damage at the corner of Crombie and Essex streets.
"We are still investigating the incident, but it is unrelated to natural gas," National Grid spokeswoman Amy Zorich said last night.
Late yesterday, National Grid laid out the sequence of events leading up to the explosions, which took place during a busy Saturday night with clusters of onlookers standing on street corners watching smoke rise from manholes on Essex Street just moments before the blast.
No injuries were reported. More than 1,100 customers lost power, which was restored by midnight Sunday, the company said.
A gas repairman from National Grid was in the city earlier Saturday night after a water main break at Washington and Harbor streets damaged a gas main.
After the explosion, that repairman went over to Crombie and Essex streets and took meter readings that showed no traces of natural gas, the company said. It is apparently those readings that have convinced the company that the blast was not triggered by a natural gas leak.
On Monday, National Grid finished repairs to the water main and also responded to a report of a natural gas leak at 7 Crombie St., a condominium building that was damaged by the explosions. In fact, two of the exploding manholes were right in front of that building. That gas leak was repaired that same day, the company said.
The company fixed gas leaks on Summer and Broad streets Tuesday and a leak on Highland Avenue yesterday.
"We want to stress that we are looking at these as separate events, that our gas system is safe and that we have fixed all leaks in the area," Zorich said.
The company has made repeated checks of the Essex and Crombie street area, in vehicles and on foot, and has found no traces of gas, the spokeswoman said.
National Grid supplies both electricity and natural gas to downtown Salem.
The company plans to meet with Mayor Kim Driscoll and give her a full report of the incident in the next few days.
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