A commenter recently pointed out I neglected to mention manhole explosions in my previous post on the manhole covers of Washington, D.C. While I came across reports of these explosions in my research, I dismissed them as isolated incidents. It appears I was wrong.

As explained by the website Howstuffworks, manhole cover explosions are caused when the eletrical cables ignite gasses underground. They can launch a 85-to-300-pound electrical manhole cover like the one to the left as far as 50 feet, and explosions have injured or killed construction workers and pedestrians, shattered windows, and overturned cars. Between 2000 and 2003, there were over 100 manhole explosions or fires in the District of Columbia. It’s unclear how many have happened since, although several incidents have been reported by the media. In May 2005, an explosion near the World Bank closed a four-block radius for hours, causing commuter headaches. Earlier this year, an early-morning manhole explosion and fire near the Mall caused major traffic backups after the incident knocked out stoplights at dozens of intersections, causing DDOT to put to use some of the 200 small generators it purchased for use in emergencies to keep traffic lights operating.

The most notable explosion in recent years was a massive March 2000 incident in Georgetown that sent six manholes flying and shattered storefront windows. Although a $40 million project was completed to upgrade the underground utilities, the Georgetown student newspaper The Hoya reported another explosion in 2003 — exactly three years later. After the spectacular Georgetown explosion, PEPCO blamed a Washington Gas crew’s probe for the damage that resulted in the explosion, and announced they would install vented manhole covers (like the one seen to the right) to “reduce threats to public safety from displaced manhole covers.”

However, the Georgetown incident was far from isolated: between February and July, 2000, over 20 separate manhole events were reported. In response, the DC Office of the People’s Counsel asked the DC Public Service Commission to initiate a formal report to examine the phenomenon. The investigation resulted in a major 174-page report (PDF) completed by Stone & Webster Consultants and released in December 2001. In it, the authors concluded that “it is our professional opinion that overloading is a primary factor in cable and splice failures, which may ultimately lead to manhole smoking, fires and explosions.” The report recommended improved maintenance and record keeping by PEPCO, as well as technical changes to minimize the possibility of overloaded cables underground.

Although they found the integrity of the electrical network acceptable or good in most places, the report cited examples of manholes crowded with cables, flooded with water, containing leaking transformers, or containing hot cables with faulty connections, particularly in Georgetown and Adams Morgan. Here are a couple of the more troubling images from the report — we can only hope they have been remedied in the years since.

As the table to the right shows, manhole fires and explosions are by no means unique to Washington. In fact, compared with many other large electrical utilities in the country, in 2000 PEPCO’s number of manhole “incidents” per 1,000 covers was .7, far below ConEdison’s 4 or Florida Power and Light’s rate of 14. A quick Google News search found recent explosions in Reno, Nevada, Hartford, Connecticut, and Boston, Massachusetts. It seems the occasional underground explosion is the inevitable result of running high-voltage electrical wires and equipment underground, underscoring the importance of routine inspection of these facilities.

> DC Public Service Commission Formal Case Number 991, 12/7/01, “Assessment of the Underground Distribution System of the Potomac Electric Power Company” (PDF, 4 mb)
> DC Office of the People’s Counsel: “Investigation into Manhole Explosions,” “Overview of Manhole Explosions 2000-2003”
> How Stuff Works: How Exploding Manholes Work