Overhead powerlines carry high-voltage electricity from power plants to businesses and homes across the U.S. But did you know these wires are typically bare?
When we say the wires are bare, the high-voltage power lines don’t have insulation, jacketing, or sheathing. Rather than being coated with a protective layer, they use the air around them as a protective element.
Sometimes, a coating may be applied to the wire, as in the case of transformer riser wire, but it’s not technically insulation. Instead, the coating helps reduce faults caused by weather, vibrations, or crossed leads.
To the average person, leaving so much electrical power unchecked sounds incredibly dangerous, but there are several reasons why the lines aren’t insulated.
If you apply too much stress to a guitar string when you tighten it, it might snap. The same thing applies to distribution lines.
High-voltage transmission lines are subject to many conditions, including ice, wind gusts, and constantly changing temperatures. Even thin layers of insulation add weight to a wire, and the insulation needed to protect these wires would be very thick.
As more weight is added to the wire, stress and weight will cause it to sag. More weight also means more instability and less conductivity. This is because the wire needs to dissipate heat – if the wire’s insulation is too thick, it could be difficult for heat to leave the wire.
On the other hand, uninsulated conductors dissipate heat quickly, allowing the wire to stay cooler and maintain higher conductivity.
Do you know how much it would cost to insulate hundreds of thousands of miles of power line wire?
If you guessed “a helluva lot,” you’re right.
Cost is the primary reason utilities don’t go out of their way to insulate powerline wire. Adding a thick layer of insulation to the long-distance lines would drastically increase the cost, so lines are often left bare as a cost-saving measure.
Think about the last time you could touch a high-voltage power line. Unless you were up there performing line maintenance, the answer is probably never.
High-voltage overhead lines are kept high enough off the ground to keep them from becoming dangerous to people or animals (except for birds, squirrels, and other critters capable of reaching them), using the air surrounding the conductors as insulation. Air is typically a great insulator, but high pressure can cause it to conduct electricity in rare circumstances.
Being way off the ground and on top of steel transmission towers also helps because the chances of a passerby getting close enough to create an arc or otherwise electrocute themselves are slim. Birds, however, are a different story.
If you’ve ever driven by transmission towers and seen rows of crows sitting on the wires, you might wonder how they haven’t been electrocuted by the high-voltage electricity under their feet.
It all lies in electrical grounding. Although birds perch on the wires, they aren’t grounded. Electricity is always looking for the path of least resistance to ground. Luckily, a bird’s body isn’t as good of a conductor as copper or aluminum – nor does it shorten the path. As a result, the bird is unharmed while the electricity stays on the wire.
Everything changes if the bird becomes grounded at some point and touches a wire. If that happens, or if a large-winged bird accidentally touches two wires, it will be electrocuted.
Lastly, the lines don’t need insulation because they’re spaced out far enough on the transmission tower to avoid arcing. In other cases, spacers are used to keep a safe distance between conductors. Power is also sent through the lines in phases to prevent arcing and other issues from happening.
It’s no secret that people aren’t always excited about living near high-voltage power lines, and they aren’t the most appealing structures to look at. However, moving the lines underground does more than make the landscape a little prettier – it can reduce the risk of outages due to unforeseen weather events and other mishaps.
In portions of the United States, more transmission lines are moving underground. Unlike overhead transmission lines, underground power lines must be insulated to prevent accidental shocks while digging or performing routine maintenance.
Products like Underground Feeder (UF) cables must be direct burial rated and buried at least two feet (24 inches) underground to avoid accidentally digging into it.
Other materials like tracer wire may be buried alongside the lines for locating later. To meet American Public Works Association (APWA) guidelines, the tracer wire should be red, denoting an electrical line.