Power Lines

Live power lines fell across a friends driveway. The were afraid drive over them. I was thinking that that would be perfectly safe, what with all the rubber insulation. And, if the live wire touched the car, I don’t think it could have electrocuted anyone inside. I did not tell them this, as I would have felt somewhat badly if I was wrong. Was I wrong, or is driving over live wires perfectly safe?

You were 100% in the wrong.

You want to stay as far away as possible for downed power lines. If they are live, there is a tremendous amount of power available. Driving over them could cause them to short to an unseen fault in the driveway causing an explosion that could easily whip a line through the window of a car. Or, puncture the gas tank, leading to a fire.

depends on what power line it could have thousands of volts. people have died when autos/trucks/tractors/earth-moving-machinery contacted power lines and the tires would melt. high voltage electricity can go through air and over rubber.

They were either the ones that go directly to the house, or just before that last transformer. What is the voltage just before they step it down to 220?

Generally, around 3,000 volts, but it could be as much as 16,000 volts.

I had no idea. Really really glad I kept my mouth shut.

What is the power in those really tall metal towers that have about 20 ceramic stand offs?

In the “if you have to ask you shouldn’t even think about even throwing rocks at them” category. :smiley:

Regional transmission lines routinely go 100-500kV. There’s a trans-Canada one that runs at 1 million volts.

They work on these lines bare-handed, by the way. In MASSIVELY insulated boom lifts. I don’t know what’s creeped me out more, watching some guy diddle with wires powered enough to turn him into a dull oily cloud, or those clips of steeplejacks going up the last 20 feet to a light fixture 1200 feet up, no safety belt.

Yeah, the rubber tire myth is prevalent and completely wrong.

Cotton is also an insulator, and you can certainly be electrocuted through your clothing. Or freeze to death in the winter. A thin bit of rubber isn’t going to offer much protection from electrocution.

The really big transmission lines can carry voltages of anywhere from 100 kV up to over a million Volts. Try taking a fluorescent light tube under a high voltage line. It’s pretty neat and terrifying how they light up without being connected to anything.

There are some up to 765kV.

They are probably somewhere between 50,000 and 100,000 volts. I’ve seen transmission voltages as low as 20,000 to 30,000 volts and I think the current record is just over a million volts at the moment.

ETA: I looked it up. I was right.

Just as a data point-
I was working in our office one afternoon, alone (the other guy had gone to lunch), when there was a blinding flash outside the window, almost simultaneously followed by the most tremendous bang, and a sizzling sound, and all the lights went out. I had a WTF! moment, and then went outside to see that the building next to us was on fire. As near as I could tell, the distribution line that fed the transformer on the pole next to our building either fatigued and broke, or the clamp that holds it to the (old, wooden) pole failed. Either way, the line fell, and when it hit the grounded casing of the transformer, shorted out and caused the bang. It also threw the line a long ways - it was well away from where it would have fallen naturally.
The sparks from the explosion set the roof of the building, and the dry grass next to it on fire. The fire was put out with only minor damage resulting.

The loudness of the explosion was really startling - it sounded like a grenade.

Where I live, they are either 110 kV, 220kV or 400 kV. In the north there are 270 kV lines.

Something worth mentioning: you don’t have to actually touch a downed HV line to be electrocuted. The electric field on the ground can be such that if you have a wide stance, you have a lethal voltage between your two feet.

Here is a picture of a logging truck that had a bit of trouble with a power line. The driver had thrown a binder cable over the top of the load. These are used to tighten up and hold the load in place. He was parked in a stupid place to be throwing cables.

http://bamaloggers.org/safety/safety-alert-powerlines/ Click on the “Hot Loading” picture to enlarge.

You can see that the hook on the end of the binder caught the power line. In the other picture you can see the result. Burned that truck right to the ground, including all the tires.

The voltage is higher with the larger stand off, so 20cm… like 30kV feeder.
1 meter insulators… 330 kV
Anyway, just because you might think its 220 volts, you don’t know for sure, and its wise to connect 220 volts or 110 volts to the car.
Block the way to keep people away. Get emergency services there immediately.

dauerbach, FWIW I think that if they had driven over them they would probably have been fine. You may have been thinking that the fact that they were surrounded by metal would keep them from electrocution. There are principles here that would make it hard to electrocute them. At least, that’s what I think.

The trouble is that “probably” isn’t good enough. beowulff et al provide examples of things that can go wrong when there’s that much power, that much voltage. With a great deal of uncertainty, I’m going to throw out there the guess that the probability of surviving driving over a live power line that has fallen on a driveway is 90%. So, a 10% chance of dying is a big problem, and I don’t have much certainty that this is correct, either.

So I also think you chose wisely to not promote them trying it.

Does this mean that power lines are not enclosed in insulating jackets? As in, given the discussion of how the magic smoke inside power lines would scoff at car and truck towers, the only jacketing on them would be to protect the cables from the elements? So therefore, it’s not just the ends of power lines that would give trouble, but their entire lengths?

You would be right about that.
Where I grew up, the lines to the house from the street had a thin anti-corrosion cover on them, but the lines at the street were bare braided copper, prettily turning blue from exposure. If they had enough insulation to block the current, they’d be too heavy and you’d need to put the poles a LOT closer together. Instead, the just put insulators on the poles.

It also made repairs easier: just weld the broken ends back together (they probably used solder). No need to try to spray on insulation, no need to string brand new wire for every break.
We had a hurricane pass by 40 miles away, and there were more than a dozen breaks in the power lines just within a quarter-mile of our house. Can’t be stringing brand new wire to fix all those, gotta splice.

Designing an insulating jacket for a 30 kV (or whatever) ACSR overhead conductor that will not breakdown when driving over it with a truck is a tall order. Even if it could be done, the disadvantages are enormous.

Compared to an uninsulated wire, the insulated wire would…

  • be much more expensive.
  • be much heavier per unit length, which would necessitate more supports.
  • be much more difficult to repair.
  • have a much lower current-carrying capacity due to a higher operating temperature for a given current.

At these voltages, the only practical solution is to not use an insulating jacket at all, and only provide ceramic insulators at the supporting points. For corrosion protection, the steel wire(s) comprising the core are usually coated with zinc. In some environments, anti-corrosion grease is also infused to “fill the gaps” between the wires that comprise the steel core. For even more protection, anti-corrosion grease is used to fill the gaps between the steel wires and the outer aluminum wires.

There are different types of “power lines”. Transmission lines run at very high voltages (tens of thousands of volts up to over a million volts), and are not insulated. These are the big guys on the tall metal towers, typically, and they shuttle power from one area to another. Transmission lines will end at a substation that will drop the voltage down to distribution level. Distribution lines will then carry the power out to the individual neighborhoods.

Distribution lines are the kind that are on telephone poles and the like. They typically operate at something like 2,000 to 10,000 volts or so, with lower voltages being common on older lines and slightly higher voltages being more common on newer ones (and sometimes you’ll have multiple voltages on different sets of lines on the same poles). Sometimes distribution lines are insulated, but often they are not. Newer lines in particular are less likely to be insulated than older lines.

Transformers convert the distribution voltage down to household voltage (240 volts) which typically feeds three or four houses. The service drop line from the transformer to the house is usually insulated.

i agree with others – you were likely wrong about it being safe, and right to have kept your mouth shut.

One other consideration – driving over the live wires could have caused one of them to touch the metal of the car (or voltage to leak to the car through the tires (most rubber tires have metal belts inside them nowadays). Now modern cars don’t have much bare metal surface inside the car, so it’s likely the occupants wouldn’t have been electrocuted. But modern cars do have a whole lot of electronic components in the engine control computer, dashboard, radio/sound system, electronic door locks, etc. Sensitive, expensive electronics systems. A few thousand volts of AC power hitting those electronics could easily cause hundreds, even thousands of dollars worth of repairs to the car.

While not nearly as bad as injury or death, still not something your friend would have appreciated.