SALEM — National Grid has agreed to a city request that it replace a handful of manhole covers downtown after an underground explosion this month blew four of them out of place.
There were no injuries during the Jan. 7 blast, but one vehicle was damaged when it was struck by one of the 120-pound covers. The explosion occurred apparently after an overloaded cable caused a transformer to fail at Front and Lafayette streets.
Sixteen covers near the intersection of Front, Central and Lafayette streets will be replaced with National Grid’s current standard model for new manholes, said company spokesman Jake Navarro.
“All manhole covers have some sort of holes in them,” said Navarro. “The new manhole covers are just going to have more and larger holes.”
Covers have holes in them for two reasons: first, because they allow gases to escape, thus minimizing the chance of accumulation and explosion, and second, because workers use them to test the air below ground before heading down to work, Navarro said.
Mayor Kim Driscoll had asked National Grid to look into making the covers more secure. One possible solution was a type of cover with latches that would allow it to rise only so far during an explosion, but this was apparently dropped in favor of the current model.
In addition to offering better ventilation, Navarro said the new covers have a wider diameter, will be easier for workers to remove and will require new frames, thus necessitating some street work.
He also said the company’s decision to replace the covers shouldn’t be taken to mean it expects future explosions in the area.
“We’re confident that the cause of the issues earlier in January has been resolved,” he said in an email. “Although you can never take the risk of underground disturbances down to zero, they occur very rarely, and there is no reason to expect issues in this particular location going forward.”
Last month’s explosion echoed previous incidents in Salem. In August, two blown fuses triggered a fire that caused several covers to pop off near Bridge and Wall streets, and in July 2010, another fire caused manhole explosions at Crombie and Essex streets. There were no injuries in either of these blasts, though the earlier incident did cause property damage.
As of last week, 10 of the manhole covers had been replaced. Work was expected to be finished this week.
Neil H. Dempsey can be reached at firstname.lastname@example.org.