BY TOM DALTON
---- — SALEM — The only silver lining in this week’s downtown power outage is that nobody was hurt when an underground explosion blew several manhole covers off their placements.
The four manhole covers that dislodged near the corner of Central and Front streets weighed about 120 pounds each, according to National Grid. They can weigh more than 200 pounds, officials said.
One of the cast-iron covers that came loose near Front and Central streets struck a car and did minor damage.
National Grid said the four manhole covers “dislodged,” meaning they moved only a slight distance. A manhole explosion several years ago near Essex and Crombie streets reportedly sent the covers several feet into the air.
Both National Grid and the city are still investigating the cause of the power outage and apparent transformer failure Tuesday afternoon.
But what about the manhole covers? What can make something so heavy fly up into the air — if only a few inches?
“When we have a manhole dislodge like this, it’s generally some form of combustible gas in the manhole,” said David Gendall of National Grid. “What we have seen in the past is that when a cable burns, it gives off vapors and, in the right circumstances, they can accumulate. And those vapors, with a source of ignition, can ignite.”
Sometimes, the source is a natural-gas leak. In this case, a preliminary investigation by National Grid has ruled that out. The source of the gas is believed to be the burning cables, officials said.
Speaking in general, city electrician John Giardi said transformers can overload and cause electrical cables to start breaking down. When the insulation around the cable gets overheated and starts to burn, it releases gases, which seep into the manhole and build up over time.
“The jacket on the cable starts giving off gases from the meltdown,” he said.
Any kind of spark can trigger an explosion, he said.
Mayor Kim Driscoll spoke to National Grid yesterday about the possibility of replacing these manhole covers, and perhaps others in pedestrian areas, with safer covers that vent the gas without dislodging. Those discussions will continue.
National Grid said yesterday that customers at The Essex condominiums who had been switched to generator power after the explosion were brought back online yesterday morning. The remaining customers relying on generator power were scheduled to resume normal service at some point last night.
Crews still hadn’t located the cable that overloaded as of yesterday afternoon, a spokesman said.
Neil H. Dempsey contributed to this report.
Tom Dalton can be reached at firstname.lastname@example.org.