A cast-iron manhole cover can weigh between 85 and 300 pounds (35 to 136 kg), and explosions have propelled these massive discs anywhere from 1 foot to 50 feet (0.3 to 15 m) into the air. The real problem with these explosions (aside from the risk of injury) is the loss of power in the aftermath.
In most cases, these are the events that lead to an explosion:
- Underground cables become frayed from aging, corrosive chemicals, overload or rats biting them. These cables carry on the order of 13,000 volts of electricity.
- These electrical wires heat up the paper, lead and rubber insulation.
- The insulation smolders and catches on fire, releasing gases.
- The pressure from the gas builds up inside the manhole.
- The electrical wires arc like a bolt of lightning and ignite the gases, causing a powerful explosion.
Depending on the amount of gas-pressure built up inside the manhole, the cover may flip over or be launched several feet in the air. Often, there may not be an explosion, just a lot of smoke or fire.
Some power companies are in the process of replacing solid manhole covers with slotted manhole covers. These new covers allow the gas to be released less violently, and also give an early warning to possible explosions.