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.