A Little Bit More About Transformer Fires
Since we've had two major electrical fires in Washington this week, it seemed like a good opportunity to dive a bit more into the nature of these nasty fires. Turns out these fires are more common than you'd think, and they can be extremely dangerous.
It's looking like both last night's fire near the White House, and Thursday's fire near the Washington Hilton began in underground vaults containing power transformers. Here's where I get out my old physics textbooks, fire up the Google and talk about electrical fires.
Basic stuff, what is a transformer? Simply put a transformer transfers electric current from one circuit to another. You've seen a transformer before, it's that gray drum on a power pole that resembles a trash can. The transformers on power poles, or in the city in underground vaults are used to transfer current from higher voltage distribution lines to lower voltage that can be used by buildings and equipment.
So we've got a underground box that's job is to take high voltage power from the grid and step it down to a useful voltage for customers. Sounds simple enough, right? Well, it is all just basic physics to get the job done, but we're dealing with high voltage and high current and as we've seen it can get messy.
A transformer fire generally begins when the current begins arcing. Simply put this means the insulation in the transformer has failed and live current is now taking a shortcut somewhere else, usually to the ground. This is where things get incredible. In a power distribution transformer, an arc can contain a huge amount of current, sometimes tens of thousands of amps. The temperature of an arc can reach 30,000 degrees Fahrenheit, far hotter than the surface of the sun. These arcs may not immediately trip a breaker in the power distribution system, and the extreme heat causes all sorts of nasty problems. The thick black smoke seen pouring from a transformer fire is caused by vaporized copper combining with oxygen in the air to form thick soot. Molten copper can also be thrown into the air. There have been instances of bystanders being seriously injured by the flash explosions caused by transformers, you can read one story from Pittsburgh.
The arc often melts the internal parts of the transformers (generally a series of coils) and many transformers are insulated using mineral oil, which can be vaporized in a fire and turned into an explosive gas. Also, carbon monoxide can be released during the event, which can be especially dangerous and explosive in confined spaces.
We've seen speculation that standing water could have led to the recent transformer fires here in Washington. It's likely a combination of factors, aging transformers, high demand, and the presence of standing water. Transformers are everywhere, and are generally extremely reliable pieces of infrastructure. When problems arise, though, the situation can get dangerous very quickly. In response to Thursday's fire in Dupont, firefighters used a dry fire suppression chemical called Purple-K to fight the blaze. These fires are not easy for the fire department to extinguish, and the process requires a good deal of cooperation between the utility workers and firefighters.
Also of note, transformer fires are also responsible for many of the manhole explosions we have become accustomed to. Unfortunately it appears as the power infrastructure ages, while demand increases, these incidents may become even more common.
The lesson? If you see smoke coming from a manhole or sidewalk grate, get away from it and call the fire department. If you're a nerd like me and want to read more about transformer fires and see some impressive videos and photos, you can check out this site.