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A friend of mine was telling me about a storm that knocked down a power line over at their place, and it got me to thinking. Why are downed power lines dangerous? I don't see any good reason for it.

It seems to me that if they can put a GFI on a hair dryer that will kill power within a fraction of a second if it falls into the sink or tub and shorts out, they ought to be able to do the same thing to a power line. Is there any reason why the basic concept can't work at that scale? Or if not, why aren't they standard equipment on power lines?

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I find myself questioning the degree to which this is a physics question. I mean, we can compute the potential difference between a person's feet as they approach (indeed, I got a question on my comps that called for exactly this), but that doesn't seem to be the focus here. –  dmckee Aug 21 '11 at 17:26
    
@dmckee: The main question is, is there any physical reason why they can't scale it up to power lines or is it just a question of cost, etc? –  Mason Wheeler Aug 21 '11 at 20:57
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Actually, just to argue your point further, if it remained fully energized, then the utility would be loosing significant power through it and if was shorted to ground it would decrease the voltage in the local area on the grid significantly. Stability couldn't be maintained either. So they obviously cut the power to the line in some sense. It's not about the bulk power bandwidth, it absolutely must be about the residual energy stored in the system. –  AlanSE Aug 22 '11 at 1:46
    
FYI - Air can only sustain 30kV/inch before it arcs. Come any closer and ZAP. –  ja72 Aug 22 '11 at 13:28
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IMO power outages are always better than live downed power lines. Power can. be restored later if nasserary after the downed power line is cut. –  Yuhong Bao May 31 '12 at 7:13

3 Answers 3

Even if you could cheaply and efficenctly engineer a GFI to work on 250,000V high current lines, and you could shut down the grid to fit them how often do downed lines hurt people compared to lawn mowers going over extention leads?

Even then I suspect that if you could disconnect the other end of the line there would be enough charge stored in the cable between you and the GFI to make you very unhappy

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But when a line goes down and lands on a chain-link fence, or in a puddle of water or something like that, it stays dangerous for a long time, until someone manages to kill power to the line. That's the scenario I find ridiculous. My friend's whole neighborhood is unsafe right now because the line fell on a fence, and all the fences connect to each other, and now they're all electrified. They've got firemen going around telling everyone to stay inside, etc. A GFI would solve that. –  Mason Wheeler Aug 21 '11 at 4:10
    
Also, even if it didn't land on something particularly conductive, if it just fell on the ground it would still ground. The idea is that by the time anyone comes along who could be hurt by it, the GFI would have killed the current and the charge would have grounded out. Only real way for someone to get hurt would be if the line fell on them or something like that. –  Mason Wheeler Aug 21 '11 at 4:15
    
@Mason when a line breaks you can kill the power when you detect that you have a break. The problem is that the nearest place you can disconnect the power may not be just the broken section. It's better to leave a broken live line fenced off than turn off power to a dozen city blocks containing a hospital or fire station –  Martin Beckett Aug 21 '11 at 4:18
    
Maybe, but isn't that why we have a power grid? Isn't that specifically why we have a grid (implying redundancy built in to the system) and not a simple "power circuit"? So that you can cut off small sections when needed without having to take large chunks of it down? –  Mason Wheeler Aug 21 '11 at 4:35
    
A grid isn't the same as a net, you ultimatley only have one cable to a customer. If a larger powerline goes down the last point you can switch it may also connect to other lines - the problem is that high power remote switches are big, expensive and create reliability problems themselves, so you have them on a large scale and then manually operated pole/transformer switches on a smaller scale that you have to send a crew to –  Martin Beckett Aug 21 '11 at 4:53

Downed power lines are dangerous because they may still be live.

A downed power line (or any other ground fault) can be deadly even if you do not touch the line itself, due to the phenomenon of step-potential and earth-potential rise. Check this Wikipedia reference for more info on step-potential: http://en.wikipedia.org/wiki/Earth_potential_rise#Step.2C_touch.2C_and_mesh_Voltage

To answer your second question: Power grids do use electrical protection systems to stop the flow of current in event of a fault. However, for an LV line (110Vac in America, ~240Vac most other places) you may find the nearest upstream protection device is just a fuse. For MV lines (1000Vac+) the protection is likely to be a device called an auto-recloser or sectionaliser, which are more sophisticated than fuses but still cannot be relied on to make a downed line safe.

An example of why a downed line might remain live is if distributed generation (e.g. solar panels or a local wind turbine) have been connected to the grid downstream from the nearest protection device. The distributed generation could potentially continue to feed the downed line even after the protection system has operated. (If the DG has been well designed and implemented it should have its own protection system to prevent this, but it remains a possibility.)

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The entire network can still receive lightning from storms, however distant, and deliver them in your body. If you think you can turn off the storm then it will not be dangerous.

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Recently (in my country) some distant storm killed a professional that was repairing a power line while the electrical power was off in the substations. Ahh, it can happen in your country. I fail to see why my answer is not considered a correct one. –  Helder Velez Nov 28 '11 at 12:14

protected by Qmechanic Apr 22 at 9:21

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