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I'm contemplating using an MCU sitting in my garage to read data from a couple of sensors, one of which may be as much as 10m distant. I'm not using analog sensors, as this question asks, instead, I'll be using Maxim OneWire devices (used in non-parasitic power mode).

I believe the actual digital signal and power should be fine at these distances (I'll probably use something like CAT5, and I can tie multiple conductors together if I really need to), but I'm concerned about other issues, mainly EMI, and at the top of my list of worries is exactly what happens when lightning strikes nearby? Doesn't this wire act as a large antenna, and could pick up meaningful voltages when lightning strikes?

It may or may not be relevant to state that the wire(s) in question would be run inside the building (in the attic), and not out in the open air. I'm not worried so much about a lightning strike directly on the wire, as I am about the large EMI voltages that could (would?) be generated in the wire upon a nearby strike.

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    \$\begingroup\$ 1-wire is supposedly good at over 100 meters with normal telephone wire. Also, protecting against lightning strikes is really hard. Do you have good reason to do so - Many lightning hits, expensive sensors/micros, minimum uptime, etc. ? \$\endgroup\$
    – Kevin Vermeer
    Commented Sep 2, 2011 at 16:52
  • \$\begingroup\$ No, just normal I-live-in-the-desert storm lightning. This is a one-off system designed by a hardware-illiterate software guy (me). Am I likely to lose the sensor (only a buck or two) or the MCU (~$40) or the entire system (~$100 or more) during the first storm? \$\endgroup\$
    – Mark
    Commented Sep 2, 2011 at 17:09
  • \$\begingroup\$ (1) you don't have a $40 MCU. You likely have a board on which a $40 MCU is mounted. Socket a DIP micro, or use a hot-air gun on a SMD micro - Neither should take more than 5 minutes to change out the micro. (2) It's difficult to predict what will happen when lightning strikes; it depends on where it hits. See the answers below for more. \$\endgroup\$
    – Kevin Vermeer
    Commented Sep 2, 2011 at 18:23
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    \$\begingroup\$ @Kevin: that's extremely unhelpful advice. It's like saying changing your oil isn't a big deal, because you don't have a $2500 piston ring, you have a $1 piston ring, and if it blows, just swap it out. For those of us that have neither the skills nor the equipment to do SMD soldering, the MCU board IS the MCU, for all intents and purposes. \$\endgroup\$
    – Mark
    Commented Sep 2, 2011 at 18:46
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    \$\begingroup\$ @Mark: Kevin's advice is actually quite reasonable. An Arduino is basically a $3 (ish) micro mounted on a board. The MCU is socketed for easy change (and I've recently fixed a few "broken" Arduinos by replacing the micros, a couple of minutes work). Bigger question is: is it worth the time/money/effort to lightning proof your system? You'll probably spend more on expensive cable/filtering/snubbers than just swapping the micro should it fail. \$\endgroup\$
    – Al Bennett
    Commented Sep 5, 2011 at 10:58

3 Answers 3

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I have had personal experience with network cables melting during a lightning storm. In almost all cases of failure due to lightning it is because of bad power or ground. If you end up having 2 devices far apart from each other and each has their own ground, it is possible for the ground potential of one to change because of the lightning and then the small gauge wire in the Ethernet line ends up taking on a lot of current which results in the insulation melting.

The same thing can happen if you have a surge on the power of one device; it can cause the communication wire to pass a lot of current down it.

If your "external" device has its own ground then you might need to worry a bit, but it probably won’t be that big of deal (especially at 10m).

You will need to worry about noise. The longer your run is the greater chance you have of picking up noise. This may not be an issue for what ever it is that you are doing, but you do need to keep it in mind.

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    \$\begingroup\$ In simplest terms, I would say that if all the devices on your cable will have the same unified power and ground connection, cable length won't make you particularly susceptible to lightning strikes, but if the things at either end of your cable might have separate grounds, a lightning strike could fry you even if your cable is three feet long. \$\endgroup\$
    – supercat
    Commented Sep 5, 2011 at 15:31
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I would find it hard to believe that a nearby lightning strike could cause a large enough (or even measurable) inductive spike at the I/O pin connected to a really long wire to do damage to a microcontroller. I think 1/R^2 is on your side here... also your house is probably a pretty decent insulator.

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  • \$\begingroup\$ It seems so, but I've heard (third- or fourth-hand) all sorts of horror stories about lightning-induced destruction of ethernet networks run between buildings, or even to sheds in the backyard. Are the stories just that, urban myths? \$\endgroup\$
    – Mark
    Commented Sep 2, 2011 at 17:29
  • \$\begingroup\$ Certainly not myths. I worked at a company that replaced an entire small office of gear after a lightning strike took out every Ethernet-connected device in the building. Exploded chips, etc. The lightning supposedly hit very close to a buried Ethernet cable. \$\endgroup\$
    – darron
    Commented Sep 2, 2011 at 18:54
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    \$\begingroup\$ The killer is not what's induced in the cable, but rather what's directly driven into the ground. It's normal for different buildings to have ground potential differences which aren't very many volts, but can supply many amps. It's less normal for the voltages to exceed a thousand volts, but in case of a lightning strike near either building it can definitely happen. \$\endgroup\$
    – supercat
    Commented Sep 5, 2011 at 15:35
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Can't you use a shielded cable, like cable TV coax, and ground the shield? If not coax, then some kind of shielded twisted-pair ... I'm sure you can find shielded CAT3 (telephone wire).

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  • \$\begingroup\$ Certainly possible, but necessary? \$\endgroup\$
    – Mark
    Commented Sep 2, 2011 at 19:55

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