How dangerous is a downed power line if you're inside a house?

Factual Questions
1

I’ve always heard about how safe one is from lightning (as long as you don’t touch plumbing and aren’t right by an open door or window) because of the “Faraday cage” effect.

Does this hold also for downed power lines? Last night, during a severe thunderstorm, my family had the scary experience of being inside maybe thirty feet from such a line, which popped and crackled and flared brightly through our curtains (not to mention smelling godawful). I was afraid for us to do anything but kind of stay in the center of the room and not touch anything electrical until it stopped (which seemed like forever but was probably only twenty or thirty minutes–yes, we called 911). Were we actually perfectly safe as long as we stayed inside (and no fire started–fortunately it was raining very hard)?

To give you an idea, this is what the lawn looks like the next day.

What if the lines had landed right on our wall or roof instead of in the grass? Or on a tree next to our windows?

2

… and you thought drinking the water at Grateful Dead concerts was dangerous …

Otherwise I have no idea, 30 minutes is a long time for this level of current and not trip a breaker or melt a fuseable link somewhere. If it was draped over your house or anything more flammable that wet grass, then you’d likely start a fire.

Was the wire insulated or just bare?

3

The Faraday cage effect is vastly overstated in most amateur writings. There are legitimate ways in which lightning or downed power lines will tend to mostly flow through the exterior of a car or building. But for something like lightning, just 0.001% of the bolt rattling around inside the house or car is far more than enough to incinerate you. You’re vastly safer inside a car or building than outside. But that still doesn’t mean you’re safe.

Had that line fallen on your house it would almost certainly have set the roof on fire. Likewise had it landed on a tree.

4

If it was the line that attaches to your house then it probably was at most 250 volts which means that you’d have to pretty much touch the line or what it was touching to get shocked.

It it was the distribution line (running from power pole to power pole) then it’s voltage is in the thousands (a lot of distribution is at 12,000 volts). If your house was in direct contact with a distribution line then I would expect it to get damaged and possibly burn starting at the point of contact. I think it would take some extraordinary circumstances for an occupant of the house to get shocked. Not impossible to be shocked but highly improbable.

If your house is in contact with a power line then the best course of action is to notify authorities and stay inside close to an exit as far away from the point of contact as possible. Exit only if the dwelling catches fire and poses a breathing or burning risk.

5

I’ve had tree limbs fall on household power lines, and instead of breaking the line the weatherhead was ripped off the building. So I had a “hot” power line on the ground and the house itself had electricity everyplace it should have been. Even after two weeks there was no damage to the grass it was lying on (the utility company gave priority to houses without power).

6

I never saw it close up, but I’ve got to assume it was a regular, insulated wire (which was running down the alley, BTW, so presumably a distribution line). However, I can see how the photos I linked to might suggest otherwise.

BubbaDog, your advice sounds very sensible.

7

Only “service drop” power lines (those running from the pole to the house) are insulated. All other power lines are bare.

8

Some distribution lines are insulated. It’s pretty rare and mostly on older systems.

For those who don’t know what the different power lines are, transmission lines are used to carry power from one area to another. These tend to run at scary high voltages (tens of thousands to hundreds of thousands of volts) and are not insulated. Transmission lines carry the power from the generators to the substations. From the substations, distribution lines carry the power out to the individual areas where it is used. Distribution voltages are anywhere from about 3,000 volts to maybe 12,000 or so, with older systems tending to be lower in voltage and newer systems tending to be higher. In some areas the distribution voltages can even be as high as 25,000 to 30,000 volts.

Transmission lines are always above ground. Distribution lines are usually above ground in older systems and sometimes underground in newer systems. Transformers are used to drop the voltage from the distribution line down to what your house uses (typically 240 volts off of a split phase transformer) and one transformer will typically feed 3 or 4 houses. If you have overhead distribution lines, the transformers will be the big can-like things you see hanging on the poles. If you have underground lines, the transformers are in metal boxes on the ground.

The drop cable from the transformer to your house will usually be insulated. This runs at 240 volts.

Obviously, the higher the voltage, the bigger the risk. What happens when the power line hits the ground is that there will often be a voltage gradient from the point of impact, spreading out through the ground. So the point where the wire touches the ground might be at say 12,000 volts, and a point 3 feet away might be at 6,000 volts, and a point 8 feet away might be at close to 0 volts. So even if you are standing on the ground and not touching the line, there could be a voltage potential difference of a couple of thousand volts between your feet, which is more than enough to cause you a very severe shock.

Another thing is that power lines, kinda like the wires in your house, have circuit breakers of sorts. If you draw too much current from the line, it trips out and shuts off. But there is one very important difference between the breakers in your house and the protective devices on power lines. If you blow a breaker, you just walk down to your basement and flip the switch and you’re done. To do the same thing on a power line might require a lineman to drive several miles to get to where he can flip the switch. So power lines have what are called automatic reclosers. This is a device that automatically flips the switch back on. Most power line faults are caused by things like trees blowing into power lines, so they are very temporary faults. Reclosers are programmable, but they are usually set to try a couple of times fairly quickly, and then once after a short pause, then once after a longer pause, and then they give up. So when your power goes off, you’ll often see your lights flicker a couple of times within the first couple of seconds, then once after maybe 10 or 15 seconds, then once after about 2 minutes, and after that, nothing until the power comes back on. That’s the recloser trying to restart the line. After the last attempt fails, then a lineman has to go out and find the problem with the line, and then he has to manually reset the breaker.

So if a power line falls, it might be dead as you walk up to it, but then as you stand there, maybe you hit the 2 minute mark that the recloser is programmed for, and the recloser energizes the line and you get shocked (and maybe killed).

The long and short of it is that staying away from downed power lines is smart.

Staying inside and staying away from anything electrical was a good thing to do.

9

Good to know!

I’m genuinely surprised to learn that the distribution lines are not insulated. Huh.

So from what you are describing, isn’t it odd that this line lay there sparking and burning for so long?

10

It’s a little odd but it’s not unheard of.

What usually happens is that you’ll get enough fault current flowing that it will trip the line out. Sometimes though, the fault current (the current flowing where it shouldn’t go, like through the grass and into the ground) will not be quite high enough to trip the line. Then you can get things burning, sometimes for a fairly long time.

Here’s a video of a power line shorting into at tree that takes a good 5 minutes or so before it causes the line to trip. Note the recloser starting the line back up after it trips out, near the end of the video. After it trips out the second time it stays tripped.

11

Whoa, so that tree just gradually grew to where it was a hazard on a windy day? This is a good example of why I don’t understand not insulating the wires.

12

Much cheaper to go whack the bad trees every so often than insulating ALL the lines.

Always follow the money.

13

I see what you mean, but this puts me to mind of an older gent I was talking to a few years back who described how when he was young and people were wiring their homes with electricity for the first time, they would just “string up bailing wire” (I don’t know what that is, but the implication was that it was a fire hazard, that the current codes are much more stringent in terms of safety standards). I don’t think they used fuses either. So that was presumably cheaper, but just too hazardous. How is this different? Wouldn’t the deal that burned up the grass outside my house have been less hazardous had the wire been insulated? We have so many old trees in this town (it’s a “Tree City, U.S.A.”) and violent storms (being in the middle of Tornado Alley), it doesn’t make sense to me.

14

Voltage is irrelevant. It’s amperage that kills.

Static electricity shocks can easily be in the thousands of volts.

That guy was probably talking about knob and tube wiring, which is downright scary stuff.

15

Voltage is NOT irrelevant.
You can’t have high amperage though a large resistance without high voltage.

16

Once the wire is broken, the broken end isn’t insulated. So insulation will have no benefit for a downed line.

Insulation might have benefit for lines being leaned on by trees. But it’s much cheaper to cut ack the trees when they get too big versus insulate every foot of every line. It’s also the case that insulation needs to be thicker for higher voltage lines. Transmission & distribution lines at tens of thousands of volts might need several inches of insulation, plus a few more inches for a safety factor.

Insulation also interferes with line cooling. High voltage lines get warm from the energy going through and need to dissipate that into the air or else they get hot enough to soften.

17

So that’s uninsulated except where it goes through the knobs/tubes? :eek:

18

You might want to get your facts straight.

https://www.physics.ohio-state.edu/~p616/safety/fatal_current.html

A difference of an entire order of magnitude means that voltage, is, in fact, irrelevant.

And this company sells items that are explicitly marketed as “Low Voltage, High Current”–something that you are claiming is impossible.

http://www.atecorp.com/category/low-voltage-high-current.aspx

19

Posting this shows a complete lack of understanding.
Voltage is also known as EMF - ElectroMotive Force. It is the Force that moves the electrons, which are what makes Current. You can’t have high current (through high resistance, like a human body) with out high voltage.

You example of a static shock is also incorrect. If that voltage was applied from a source that allowed it to be maintained continuously (like, a high-voltage power supply) it would be potentially deadly. The reason static shocks aren’t fatal is the total charge involved is tiny.

Which brings up the real point - electrocution is much more a function of POWER than Current.

20

Yes. And if your house was built with knob and tube, there’s no requirement to replace it. So there are a lot of homes out there that still have knob and tube wiring. My friend has knob and tube in part of his house.

Personally, the stuff gives me the willies. If I had it in my house I’d get rid of it ASAP.