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I have a strange phenomenon in our mountain cabin, on the Italian alps: lightning strike on the nearby (not so near: < 2 miles) causes audible / visible sparks on:

  • electric / fire wood water heater pipes (very frequently)
  • LED spotlights (rarely)

Both have the bodies grounded.

Details:

  1. In the dark I also see LEDs lighting up (just a weak flash)
  2. The nearest lighting I saw was > 1/2 miles from cabin so I cannot say how the phenomenon is affected by distance from strikes
  3. Electric system and ground are new and tested. I already asked to our electrician but he says that our ground is good, and he don't know what can cause the phenomenon (this is why I write here :)
  4. It seems that sparks occours only with metal bodies physically attached to a wall that has the ground on the other side (the cabin is on a slope; the ground level on the back is about 4 meters higher than on the front. Wall is made of stones and concrete, about 30" width)
  5. The cabin is one of the last one of the electric branch on which we are connected
  6. Water heater has a metal exhaust pipe (about 10 feets long) that run inside the masonry flue

For my knowledge strikes are too far to have a direct role, so I tend to thinking about spikes on the power line (the last mile is underground); but a spark means a LOT of potential difference - and not destructive ones + no failure on the electronic devices I have plugged in 365 days/year means very little current, so I tend to exclude the power line.

Yes, I'm confused :)

Thank you very much

Ps: I found a video

https://www.youtube.com/watch?v=DuNLm284m80

You can hear the sound of / see (indirectly) the flash of the spark, but in this case the lightning is in the nearby, I can expect that could induce a lot of EMF; in my case lightnings are a lot far away (also > 10 seconds between lightning and thunder)

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    \$\begingroup\$ yes, in a suburban environment (flat land) a nearby strike can audibly (and simultaneously) produce an audible "tick" as wall-mounted switches and/or power plugs arc over - thunder occurs a few seconds later. A high location could easily provide an electric field environment that extends the range. \$\endgroup\$ – glen_geek Aug 6 '18 at 14:49
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    \$\begingroup\$ How good is the ground? In dry, rocky locations, it may be difficult to achieve a good ground. See ecmweb.com/content/achieving-acceptable-ground-poor-soil Also, in the open, there are strong inductive and capacitive effects, causing induced voltages in long conductors. Thanks for posting your interesting anecdotes, BTW. \$\endgroup\$ – DrMoishe Pippik Aug 7 '18 at 0:34
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    \$\begingroup\$ I've never heard of this effect before, it's quite interesting. If you're certain of the quality of your system grounding, are you certain that each of these metal bodies is grounded? Just to rule out that this could be at least partly capacitive? Just because your house itself is grounded does not mean that each metal object in it was properly bonded together. \$\endgroup\$ – K H Aug 7 '18 at 2:39
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    \$\begingroup\$ Hard to tell if your interior cabin wiring itself is acting as antenna, or if power line to your cabin is the source. Even with modern 3-wire (hot, neutral, ground) interior wiring, I've heard the "tick" arcs from nearby strikes. No damage done - no fires started. \$\endgroup\$ – glen_geek Aug 7 '18 at 2:42
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    \$\begingroup\$ A good test for if it's charge built up in unintended capacitors, you can either ground the object manually and see if it stops the sparking (it almost certainly will) and/or use a nonconductive string and hang a piece of wire close to but not touching one of these earthen walls. \$\endgroup\$ – K H Aug 7 '18 at 2:43
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Short answer, lightning carries so much energy in the form of electromagnetic waves, it can create current in objects a few km's away.

Lightning can create electric fields that are more than 100kV/m directly, shown from data taken from a plane-strike.

enter image description here Source: http://www.atmo.arizona.edu/students/courselinks/spring15/atmo589/lecture_notes/apr03_2015.html

These taper off with near field and far field rules. If the lightning were obeying far field rules the electric field would taper off with the distance squared. These fields can still can be quite high even only 1km ( or a few kms) away from the strike, a 1m conductor would experience capacitive coupling of a few hundred volts a few kilometers from the strike.

I expect that the stove pipe is a little longer than this.

Here is the fields from a simulated strike. Even the magnetic fields are in the A/m range. Every conductor every wire could also have current inductively coupled into it.

enter image description here Source: Analysis of lightning electromagnetic field propagation in mountainous terrain and its effects on ToA‐based lightning location systemsenter link description here

At even a trace of 1cm could experience a few volts across it, which may be why your LED's are lighting up.

Apparently nearby mountains amplify electric fields as shown in this simulation (although the wave and the mountain in the simulation are not on the same scale, perhaps this is leading to even more amplification of the EM-waves):

enter image description here Source: Analysis of lightning electromagnetic field propagation in mountainous terrain and its effects on ToA‐based lightning location systemsenter link description here

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  • \$\begingroup\$ Thank you very much! I think capacitive coupling is the most likely thing, but it's not easy to understand who is responsible. I have some updates that bring some extra elements. Before I heard a spark bursting at the same time as a distant lightning (about 12 km), proof of how the phenomenon is NOT absolutely connected to direct or secondary discharges. Now there was a thunderstorm right here and I noticed that only a few lightning strikes the described effects, it seems to depend more on direction than distance. \$\endgroup\$ – SpecialFx Aug 17 '18 at 22:51
  • \$\begingroup\$ So far I have only considered two possibilities for capacitive coupling: some elements of the house (e.g. the chimney) the long underground cable that connects us to the electricity grid In reality there is a third possibility: the metal pipe that connects us to the aqueduct. To better visualize the situation I prepared an image: snag.gy/5z2NC4.jpg In yellow there is the (presumed) cable route. in blue there is the path of the pipe to the well The red X indicates where the earth post is installed. \$\endgroup\$ – SpecialFx Aug 17 '18 at 22:51
  • \$\begingroup\$ (the slope increases to the south, the bottom; the level of the land in the south is about 4 meters higher than in the north) \$\endgroup\$ – SpecialFx Aug 17 '18 at 22:52
  • \$\begingroup\$ BONUS During today's thunderstorm I managed to resume both effects, spark and led: youtube.com/watch?v=GKW2Awuv63k I verified that the pipe, from the water heater up to where the spark is triggered, is connected to the ground; from there on, no. youtube.com/watch?v=g5G9EKJeUXo the spotlight has 6 LEDs; the number of LEDs that light up depends on the intensity of the phenomenon; they are NOT reflected. \$\endgroup\$ – SpecialFx Aug 17 '18 at 22:56
  • \$\begingroup\$ I believe that the capacitive coupling with the aqueduct tube is the most likely hypothesis: on the one hand you have the ground of the house, on the other you have the cold water tube directly connected to the aqueduct tube. What do you say about it? Thank you very much to everyone! \$\endgroup\$ – SpecialFx Aug 17 '18 at 22:57

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