Every time someone explains voltage, it is inevitable that they explain why a squirrel can run across a power line with immunity, because the squirrel potential is the same as the power line. This implies that overhead lines are not insulated (I’m not sure why not). When power lines come down during storms, why don’t the damn things spark like crazy and cause all kinds of fires? Or do they?
High-voltage lines do spark like crazy, and can cause fires. But residential lines aren’t high enough voltage to do that.
And there are all kinds of fuses and breakers and the like in the power grid. I know this because one of them blew up outside my window a few years back.
They do spark like crazy when they fall: Downed power line burns hole in road
The top of the utility pole normally carries two phase current @ 15,000 volts (YMMV). A few years ago, a garbage truck drove down a street with it’s fangs up. Took down a pole. About 15 feet from the brake on the sidewalk is a foot long area that eroded away about an inch of concrete with an arc from the falling lines. It’s since been filled in with a patch of concrete.
During the recent Mississippi River flood, the power company was out in force patrolling the river banks near their huge power line crossing. Apparently the water was so high that a boat would be close enough to allow arcing between the power line and boat.
I’ve also seen the evidence on a track hoe boom which got too close to a high-transmission line. Zap!
Presumably said squirrel would have to jump cleanly onto the power line, rather than stepping onto it from the pole, otherwise, fried squirrel?
“If you see a downed power line, assume it is energized and call 911.”
That’s what the message on our local power company’s phone line says any time I call to get information about an outage.
I have never felt ballsy enough to question it.
If he didn’t jump, then he would have to climb up the length of an insulated mounting point for the wire. If the insulator was short enough, I suppose he could bridge the gap between the wire and the pole and electrocute himself, but apparently this is pretty rare.
As noted, sometimes they do spark like crazy and cause fires, but only if there’s a malfunction in the equipment that’s supposed to automatically open the circuit when a high-current short or arc-fault like that is detected. Under normal circumstances, when the wire shorts to ground you would expect a brief flash of high current before an interruptor elsewhere (likely back at the nearest substation) opens the circuit and stops the flow of current. If arc-fault detection equipment isn’t working - or if the short-circuit current isn’t high enough to trigger the interrupter - then the downed wire will keep dumping current/power/heat into the ground where it lies.
Just to show that they are indeed uninsulated and powerful, here’s a tree branch shorting two lines. The interrupter doesn’t recognize the problem until the branch vaporizes and develops a full-on arc (near end of video).
In the event that a squirrel spans two wires of differing potential, yes, they can get zapped. (warning: video of dead, already-zapped squirrel hanging from wire)
Crispy Critter (Worksafe, but may be unsettling for PETA members.)
Also answers the question, “How do squirrels toast their nuts?”
If they were insulated they would be more expensive and heavier; the only reason I can come up with to insulate them would be to slightly increase safety if they came down. Even if they were insulated, when they came down you would still have a little bit of bare wire where they snapped, so they would still be dangerous.
Some years back, a car driven by a local teen ran into a utility pole (kid was blinded by the sun and probably speeding), and the line came down. I don’t know if the line was sparking or not, but the driver got out and attempted to lift the line off the hood of the car ( I assume he was in shock and not thinking.). Instant electrocution. His dad, the local fire chief, was first on the scene. He grabbed the line (I am not making this up.), I guess to pull it away from the kid and was also electrocuted. An EMT ran up, panicked (STILL not making this up) and grabbed the line. Father and son both died, EMT survived. Whatever safety mechanisms there may be on power lines/utility poles, they’re still mighty dangerous when they’re down.
Recently the power company installed some insulated lines in some areas were trees frequently took out the power lines. I think it’s mainly there to prevent a failure from a temporary touching of the 2 wires caused by swinging after a tree hit, though if the wires end up touching for long enough the insulation will break down and cause a short and cut the power.
If the power lines that go down my street aren’t insulated, doesn’t this mean that if I throw a chain up there landing across two of them that it will cause a big short?
Yes, provided it spans the correct pair of cables. There may be multiple cables operating at the same potential (to carry more current), and shorting those two cables wouldn’t do anything; you’d need to short a “hot” cable to a ground cable.
There are different types of power lines.
“Transmission lines” are used to transmit voltages from one area to another (say from the power plant to the nearby city). They are typically very high in voltage (tens of thousands of volts up to about half a million volts) and are not insulated. These are typically the big towers that you see. Transmission lines go to a substation, and from there the power is distributed to where it needs to go for use. Transmission lines are pretty high up, but people do on occasion climb the towers and try to win themselves a Darwin award. Transmission voltages will arc pretty far, so you don’t have to touch the line to get shocked. All you need to do is get close enough for the voltage to arc to you. Transmission lines are usually spaced far enough apart that throwing a chain over two lines would be difficult, although you do often see pairs of wires at the same voltage. Throwing a chain over a pair of wires at the same voltage wouldn’t cause any harm.
“Distribution lines” are the smaller lines that run around neighborhoods, distributing power to everyone’s homes. These are lower in voltage, typically somewhere between 3,000 and 10,000 volts. Some are insulated and some aren’t. If they aren’t insulated, throwing a chain across them would certainly cause a fault.
You will also have a power line from the transformer to your home. The transformer converts the distribution voltage to your home’s voltage. A typical transformer will serve three or four homes, and the lines from there to your home are typically 240 volts. These lines are usually insulated.
When power lines fall, they do tend to arc, but as was already mentioned, there are protective devices that work like the breakers in your house to shut the power off when there is a fault. Unlike your home’s breakers though, the power company uses what are called automatic reclosers. Most power system faults are temporary, like a tree blowing into a line or two lines blowing together in the wind. Reclosers are like automatically resetting breakers, so that the power company doesn’t have to send out a lineman to reset the line for every fault. Reclosers are programmable, but they are typically programmed to try a couple of times fairly quickly (within a couple of seconds) then once more after a couple of minutes, and if there is still a fault then, they give up and a lineman has to come out and figure out what the problem is.
If a power line goes down, you may touch it and think that it is safe, and then the recloser tries to turn the line back on and you get shocked with 10,000 volts (which will tend to ruin your day). It’s also possible (especially if the line is insulated) that it doesn’t draw enough fault current to trip, and still has 10,000 volts on it.
Don’t go anywhere near downed power lines, and if one happens to fall on your car, just stay put inside of it and wait for someone from the power company to come along and shut off the line so that they can safely get you out.
Oh, and to fully answer the OP’s question, the “why not” is because it would cost too much to insulate all of those wires. They only do it when necessary.
Power lines aren’t insulated? I always thought they were.
No. When you see them up close, they look just like twisted steel cable. Mainly because that’s all they are.
Aluminum. Steel is too heavy and electrical conductivity is too low.