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It isn't like lightning strikes the wires. And surely the whole apparatus is designed to be water-proof. So what is it that causes power outages during thunderstorms?

Nick Alexeev
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Bob
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    Why do you think lightning does *not* strike the wires? – Connor Wolf Jun 25 '12 at 21:49
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    Actually, lightning DOES strike transmission lines. However most transmission lines are equipped with a protective earth wire that runs above the three phase conductors. This protects the transmission line in the same way as a lightning rod - by being the highest conductive object that's connected the ground (and hence the most attractive target for lightning.) – Li-aung Yip Sep 09 '13 at 05:10

3 Answers3

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Considering the number of lightning strikes on or near power lines, power going out as a result of one is actually quite rare. Overhead power lines do have cables that run above them mostly for the purpose of catching lightning strikes. These cable are then periodically grounded at poles.

Lightning is a massive current surge, and sometimes stuff happens. It's not economical to protect against every last lightning scenario, so the systems are designed to keep on going with most lightning strikes. However, just the right strike at just the right place at the right time can still cause excessive current someplace, which trips a breaker, which shuts down power for a while. Breakers on intermediate distribution lines usually reset themselves after a few seconds a few times in a row. This is why you are more likely to see power blip out for a few seconds after a lightning strike than a real outage a crew has to come out and fix, which will take 10s of minutes to hours.

Actual outages due to lightning are actually more likely due to lightning making a tree branch fall accross a intermediate feeder line, which then shorts the line and causes a breaker to trip. The big cross country high voltage transmission lines are quite well protected against lightning, and you very rarely hear of a outage caused by a strike to them. Usually lightning outages are geographically small since it is the intermediate feeders (a few kV to a few 10s of kV) that are effected by tree branches and the like.

Olin Lathrop
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  • Excellent point on the tree branches. I wrote my answer thinking the whole time there was something else more simple that I was missing. – TheFuzzyGiggler Jun 25 '12 at 23:35
  • @TheFuzzyGiggler: Yeap, sadly the elements such as strong wind can break something like several dozen trees along the power line and some of them will just break the wires. That happens a lot in rural areas on 6-10 kilovolt lines - one storm can take several villages (hundred houses each) out. – sharptooth Jun 26 '12 at 08:00
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    My parents got an excessive voltage hit when a high voltage feeder line fall onto a lower voltage feeder line. The inside of the fuse box was black; the plastic portion of one lamp's plug was sent a few inches from the wall while the metal prongs stayed in the socket. The bulb was cracked near the base, and the lamp socket and bulb base ended together up with about a 1/16"x3/8" slot parallel to the threads. Nasty stuff. – supercat Jun 09 '14 at 16:09
  • @supercat A long ago account here told on a telephone leaping off a wall when an 11kV lines was dropped onto phone lines. | Friends had worms crawl out of the ground and COLD water boil in the pipes when 230VAC house feed polarity was inverted :-). Pole and main house fuses did not blow. So maybe only a few 10's of kW. – Russell McMahon Oct 13 '14 at 06:45
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Actually it's not terribly uncommon for lightning to strike the lines, but they have lightning arrestor protection that shunts it to ground.

What you're most likely wondering about is induced current. The power lines act like an antenna and lightning strikes cause electromagnetic disturbance across a wide range of frequencies.

Just like a radio station transmitter induces a current in your radios recieving antenna, the same thing happens to power lines during a lightning strike.

The induced current can be VERY large momentarily and sometimes it's enough to blow the transformer or breaker on the lines.

That's the simple answer anyways, there is a lot of physics behind it if you're really intereted.

TheFuzzyGiggler
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The other answers cover the main points, however there are a few other minor factors:

A lightning strike not hitting the power lines can still raise the local ground potential, possibly moving it far enough away from the "unaffected" power lines that various systems trip out.

The EMF/radio waves can induce currents on plenty of things, a lightning strike is a massive transient that will get picked up by anything with a wire in it - including power lines and phone lines (which look a lot like massive antennas in RF terms, and massive antennas are massively sensitive).

Additionally, first-hand experience of the telephone network shows that any large network of wires strung around the countryside (over or underground) is very much going to act as a lightning magnet as it's more conductive than most of the rest of the countryside hence offers a really attractive pathway to any stray electrons, plus it usually leads somewhere a long way away with a more attractive potential to head towards.

Also, someone somewhere will always have connected part of your network to a nice big tall thing, be it telephone pole, pylon, lamp-post, cellphone tower, etc. etc.

John U
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