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I was always wondering about the following:

If lightning strikes directly into the phone line outdoors, can the excess power coming from the phone line jump over the switching power supply of the ADSL modem into the mains in the house and damage any computers / TVs / washing machines?

Let's assume that the ADSL modem is WiFi only and that there are no cables connected to it except for the phone line and mains.

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  • \$\begingroup\$ I once had a lightning strike apparently damage the COM card in a computer, presumably after passing along the phone line and through a circa-1985 modem. But there's no plausible mechanism for lightning to enter the house on a phone line and "jump" from a WiFi modem to WiFi clients. \$\endgroup\$ – Hot Licks Jul 1 at 22:48
  • \$\begingroup\$ @HotLicks the question was about jumping from the ADSL line to the power line through the WiFi modem, and then possibly damaging other things through the power line \$\endgroup\$ – user253751 Jul 2 at 9:21
  • \$\begingroup\$ Related: I have had two multifunction printers-faxes die from a close lightning strike., I assume it came via phone line as no non phone connected equipment has even died. \$\endgroup\$ – Russell McMahon Jul 2 at 12:07
  • \$\begingroup\$ Related: A means to reduce very high energy lightning strike damage that's well documented in various places (no refs to hand) and accepted by many as valid is to add a one or two turn loop in the cable near the entry point, maybe 300mm diameter but not too critical. Having part of the loop against grounded metal is a bonus but not essential. A major lightning strike tends to break out of the cable through the insulation and take a more direct path to ground. Calculations indicate that the impedance of such a loop should be too small to make much difference. Lightning seems not to know this. \$\endgroup\$ – Russell McMahon Jul 4 at 5:02
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There are many 'entry points' to your house for electrical disturbances, once lightning has struck close by, and the most important of these is the mains. A nearby strike will lift the ground potential, and induce a voltage into any wiring.

My anecdote is that I live in a row of houses. A house 4 doors up was struck by lightning. They needed to be totally rewired and replastered. Cables had blown out of their walls. Their neighbour's wiring survived, but they lost every item of electrical equipment, including simple motor-only things like fans. Their neighbour's motor-based equipment survived, but they lost all electronics. I lost only my ADSL modem, perhaps as it was connected to mains and the phone line, but all my other electronics survived.

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    \$\begingroup\$ wha... No lightning rods, or regardless of one or how does that happen..? \$\endgroup\$ – ilkkachu Jul 2 at 7:10
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    \$\begingroup\$ It would be extremely rare in the UK for a normal 2 storey house to have a lightning rod. \$\endgroup\$ – charmer Jul 2 at 10:10
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    \$\begingroup\$ @charmer Is there also a law in the UK which forbids lightning strikes to hit such houses? \$\endgroup\$ – Dmitry Grigoryev Jul 2 at 12:03
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    \$\begingroup\$ UK: 0.46 strikes/km²/Year. Brazil:300. According to a UK vendor of lightning protection "direct hits are still the rarest form of domestic damage caused by lightning". \$\endgroup\$ – RedGrittyBrick Jul 2 at 14:10
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    \$\begingroup\$ @charmer It wasn't meant to be serious. \$\endgroup\$ – Dmitry Grigoryev Jul 6 at 7:10
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Back in my hardware days, I was a technician repairing modems for a modem manufacturer. I repaired many modems hit by lightning strikes. They all had a couple large resistors connected to the phone line that would take the brunt of it. The smell of a fried 2W resistor is quite distinctive. I could tell it was a lightning strike when I took the case cover off. At most it would char part of the circuit board. I had to grind off the charred part, glue down traces from a scrap board if needed, the solder everything back up.

I never saw a power supply get harmed. I would think other things would be more susceptible to power surges through the mains than the modem. That's how we lose most of our electronics around here. So, don't worry about it.

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  • \$\begingroup\$ I was also a computer repair tech that saw various internal modems that were "directly" hit by lightening and usually the computer itself was left undamaged as far as I could tell. I've seen not only damaged components, but traces that were vaporized, chips that literally blew up, and even holes burned completely through the cards themselves. \$\endgroup\$ – computercarguy Jul 2 at 20:18
  • \$\begingroup\$ I understand, so even if lightning hits a power line pole that also carries cable TV and phone line wires, it would eventually toast the ADSL modem but the surge through the power adapter of the ADSL model should not harm anything else in the house, am I right? \$\endgroup\$ – adamsfamily Jul 12 at 13:00
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It's unlikely. The wires of your telephone line are very thin and are close to ground at enough points to first vaporize before they can hit your washing mashine.

Let's assume that the ADSL modem is WiFi only and that there are no cables connected to it except for the phone line and mains.

If there is any vector for damage to your washing machine it's through mains, anyway.

Generally, lightning strikes into cables close to you are far rarer than people think they are (no, anecdotes don't change that). The likelihood of telephone lines being hit is smaller than that of power lines, because these typically hang higher for isolation purposes.

Multiply that with the small probability that damage to the phone system will propagate any further than the point at which the telephone line enters your house, and you'll find that you're worrying about something that is far less likely than your building collapsing onto your washing machine, computer and TV.

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  • \$\begingroup\$ I'd add that, at least in the US, the copper lines (rapidly disappearing) are protected with pairs of large gas-tube flash over devices in a box at the corner of a garage far from the entry point into the house. It's not perfect and won't hold off a direct strike. But it is pretty good at reducing some of the risks. I still have mine here despite replacing copper with fiber years ago. \$\endgroup\$ – jonk Jul 1 at 12:40
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    \$\begingroup\$ Mains power line entry into the house is probably more of a risk, I think. \$\endgroup\$ – jonk Jul 1 at 12:41
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    \$\begingroup\$ @Marcus Anecdotal data point :-). I had two multifunction phone line connected printers die about a year apart after close proximity thunder storms. On the 2nd occasion I saw the lightning through a window, close enough to make me jump, with the sound almost instantaneous. VERY impressive.The printer immediately started a death gurgle :-) - slowly rising tone from line monitor speaker. I do not know how far away the strike was physically - I wish that I'd gone looking to see. The input COULD have been via the power lines, but on both occasions only the phone line connected printer died. \$\endgroup\$ – Russell McMahon Jul 3 at 6:58
  • \$\begingroup\$ @RussellMcMahon interesting! Maybe the combination of mains+ phone connection really made it, offering a relatively badly protected current path \$\endgroup\$ – Marcus Müller Jul 3 at 7:10
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The probability of damage to electrical equipment through mains supply surges, after a nearby lightning strike, is much higher than that caused through telephone lines.

In my experience the failures have always been limited to power supply components.

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In Austria I have seen such lightning damages years ago. At that time analog modems were in use and the computer the modem was connected to was sometimes destroyed. However, only devices that were directly connected to the modem were destroyed. WiFi did not exist at that time.

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