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Ask Sam: How Do Power Lines Grow Through Trees?

Flickr Creative Commons | Matthew Prosser


 Every other Friday on Morning Edition NHPR’s Sam Evans-Brown tracks down answers to questions about the environment and outdoors for our listeners in a segment we call “Ask Sam."


Zach from Maryland asks: "I was riding my bike recently along the Anacostia River, and I noticed big pieces of wood through which the power lines were growing. How does this happen? How do power lines grow through pieces of trees - that are then presumably cut off so you just have these pieces of trees hanging there?

 So the shortest answer here is that the wire is insulated!


There are places where electric lines are totally bare, which means nothing can touch them. That’s usually true in more rural areas where running a lot of insulated wire would be really expensive, and big transmission lines tend to be bare as well. 


If a branch so much as rested on a bare line, it could spark or catch fire.


But in more densely populated places (like where Zach lives, outside Washington D.C.) the lines probably have some sort of shielding. Which means wood can touch them, even slowly grow around them, without causing a short. 


But underneath that insulation, live wires can still be dangerous. Greg Bruton, a senior electrical engineer at National Grid and board member of the Boston chapter of the American Association of Blacks in Energy, (who would like you to know about their student energy contest this month)says that when people are called in to deal with tree issues, they tend to keep their distance. 


“You send a guy up the line who typically cuts down trees for a living, he’s not really keen about taking a chainsaw to something that could potentially kill him."


So often, the safest and easiest thing to do is just leave a bit of wood dangling there. 


If we want to get technical, it's also possible that Zach may not have been looking at a power line. Utility poles carry all sorts of different lines. Back in ye olden days, Greg Bruton says, crews would sometimes mix up which was which.


"You’d have people receiving a phone call and instantly getting electrocuted because they ran power to the phone line.”


Eek! Thankfully, that all has been standardized. 


Up at the top is what’s calledthe primary: usually three wires, held up by an arm at the very top of the pole. The expression "three-phase-power" (for the energy geeks out there) refers to all three of those together.


Below the primary, is the secondary. That's the wire that carries electricity to our homes and businesses after a transformer (it's the grey cylindrical box you see at the top)lowers the voltage. 


Everything else below the secondary isn’t actually electricity: it’s our phone, TV or internet.


Those don't carry the same risks as the powerlines higher up - so it's possible Zach saw a chunk of wood that had grown over one of those lines instead.

Sam Evans-Brown, is host ofNHPR’s Outside/In which you can subscribe to where-ever you get your podcasts. If you’d like to submit a question you can record it as a voice memo on your smartphone and send it to, OR call the hotline, 1-844-GO-OTTER, OR submit it here.


Sam Evans-Brown has been working for New Hampshire Public Radio since 2010, when he began as a freelancer. He shifted gears in 2016 and began producing Outside/In, a podcast and radio show about “the natural world and how we use it.” His work has won him several awards, including two regional Edward R. Murrow awards, one national Murrow, and the Overseas Press Club of America's award for best environmental reporting in any medium. He studied Politics and Spanish at Bates College, and before reporting was variously employed as a Spanish teacher, farmer, bicycle mechanic, ski coach, research assistant, a wilderness trip leader and a technical supporter.
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