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Re: [John Muir Trail] Lightening reminder

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  • Allen Freeman
    ... Lightening? Is somebody about to give birth? Traipsing around the Sierra in the last week of pregnancy probably isn t a great idea. ;-)
    Message 1 of 47 , Jul 10, 2013
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      On Sun, Jul 7, 2013 at 8:12 PM, Jack Young <trail2nowhere@...> wrote:
      > Lightening can be a major problem in the Sierra.

      Lightening? Is somebody about to give birth? Traipsing around the
      Sierra in the last week of pregnancy probably isn't a great idea. ;-)
    • straw_marmot
      Thanks, John. On further reflection overnight, I think my mistake was in my conception of how big the ground current really is. After all - when you consider
      Message 47 of 47 , Jul 12, 2013
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        Thanks, John.

        On further reflection overnight, I think my mistake was in my conception of how big the ground current really is. After all - when you consider a typical lightning rod, it's not all that substantial, yet it's able to carry the entire current from a strike without melting. The local ground current will be some decreasing fraction of the total current.

        So contrary to my initial intuition, your EE friend was probably correctly assuming that a large proportion of the local ground current can be safely carried by an object as small as a Bearikade if it's conducitivity is reasonable.

        So we've gone from an assumption that being near any metal objects is incredibly dangerous, to your friend's expertise now suggesting that the safest place might be to be standing/sitting on a metal box! It does highlight how much uncertainty there is about strategies for ground current mitigation.

        I think I'm still convinced that our plan should be to focus 99% of our energies on the thing that we KNOW works - consider local terrain and trees to find the safest possible place where strikes are unlikely. Then when the storm's near, just get in a good basic crouch/sit with the recommended single point of contact with the ground, and not obsess over tentpoles, airbeds, bearcans and all these other uncertain details.

        Ralph



        --- In johnmuirtrail@yahoogroups.com, John Ladd <johnladd@...> wrote:
        >
        > I'm not really qualified to speak on this, but I think the point is that
        > the difference in ground voltage from one side of the metal plate bottom of
        > the Bearikade is not all that great as it is relatively small area, but
        > that the metal plate, because conductive will end up with causing
        > *same*voltage (the average voltage of the ground under the can)
        > passing up
        > *through all sides* of the less-conductive carbon fiber ring that comprises
        > the sidewalls of the can. So the metal *top* surface of the can should have
        > no voltage differential at all from one side of the circle to the other. If
        > your butt is all at one voltage (the average voltage of the ground under
        > the can bottom) and each foot is touching the ground very near the can, the
        > voltage differential might be something like
        >
        > left foot 601k volts
        > my butt 602k volts
        > right foot 603k volts
        >
        > Enough to feel it, but not enough to be fatal.
        >
        > And maybe sitting on the can allows you to put extra insulation under each
        > foot (e.g., empty water bottles) since your feet are for balance, not for
        > taking weight.
        >
        > Do i know that's right? No. But there is a logic to it.
        >
        > John Curran Ladd
        > 1616 Castro Street
        > San Francisco, CA 94114-3707
        > 415-648-9279
        >
        >
        > On Thu, Jul 11, 2013 at 11:25 PM, straw_marmot <ralphbge@...> wrote:
        >
        > > **
        > >
        > >
        > > Thanks for this John,
        > >
        > > I'd like to understand the physics of what your electrical engineer friend
        > > is saying. He talks about a highly conductive object forming a "near
        > > unipotential surface" and says in particular "Even with the Bearikade on
        > > bare ground, its conductivity will reduce voltage differential between
        > > opposite sides".
        > >
        > > I don't understand why this is so. Take an idealized case - instead of a
        > > bear can, imagine a highly conductive copper plate placed directly on the
        > > (much less conductive) soil, through which ground current is flowing. Now,
        > > it's surely true that each point on the copper plate will always be at the
        > > same potential as the point on the soil that it's touching. Is he saying
        > > that such a small object could affect the potential gradient locally in the
        > > soil underneath? That seems odd. I would have thought that, conversely, the
        > > ground current would be overwhelming large, and continue to flow through
        > > the soil, with the potential gradient across the soil therefore maintained
        > > across the copper plate, resulting in a massive current flow through the
        > > plate, heating and melting of the plate. What am I missing?
        > >
        > > Ralph
        > >
        > >
        > > --- In johnmuirtrail@yahoogroups.com, John Ladd <johnladd@> wrote:
        > > >
        > > > A follow-up on this post and the suggestion that sitting on a bearcan
        > > might
        > > > be better than trying to maintain a squat position during a storm
        > > >
        > > > On Wed, Jul 10, 2013 at 8:22 AM, John Ladd <johnladd@> wrote:
        > >
        > > > >
        > > > > On Tue, Jul 9, 2013 at 9:02 PM, Oz <trailtickles@> wrote:
        > > > >>
        > > > >> But out of curiosity, could you not sit on your bear canister with
        > > your
        > > > >> feet together? Or would it make it worse?
        > > > >
        > > > > Interesting thought. From the logic presented in the MythBusters video,
        > > > that
        > > > > seems to make abundant sense. Maybe not on a Bearikade, but on a
        > > plastic
        > > > > bearcan it sounds like an excellent ides (with an insulator like a
        > > > > closed-cell sleeping pad (if available) under the can. Lots more
        > > > comfortable
        > > > > than the oft-pictures lightning position squat, which I've never
        > > thought I
        > > > > could hold very long.
        > > >
        > > >
        > > > I don't know enough to evaluate the following logic, but it makes some
        > > > sense. From an Electrical Engineering consultant re a carbon composite
        > > > bearcan as a stool for sitting on in a storm:
        > > >
        > > > "The conductivity of carbon composite material has been of interest to me
        > > > regarding its EMI shielding properties. Unfortunately, the data varies
        > > > widely (range of 100 to 1) because of the dependence upon carbon to epoxy
        > > > ratio. Conductivity depends upon what % of fibers make electrical
        > > > connection with each other. However, a specific number is not needed to
        > > > make some lightning safety related inferences. We may assume conductivity
        > > > will be much less than a metal but much greater than an insulating pad.
        > > > The Bearikade, being more conductive than an underlying pad, will result
        > > > in a near unipotential surface. This is ideal assuming one's feet are
        > > very
        > > > close to the base of the Bearikade. Even with the Bearikade on bare
        > > > ground, its conductivity will reduce voltage differential between
        > > opposite
        > > > sides but that is where some specific numbers are needed. Values for
        > > ground
        > > > current and soil conductivity enter the calculations and it gets messy.
        > > >
        > > > A nonconductive cylinder would be less protective, although if on an
        > > > insulating pad, the distinction is small. A nonconductive cylinder on
        > > bare
        > > > ground would offer less protection than the Bearikade due to lack of the
        > > > unipotential effect but still better than standing on the ground. This
        > > > case becomes dependent on where one's feet are.
        > > >
        > > > This is all based on minimizing differential ground voltage as portrayed
        > > in
        > > > the NOLS video and presumes one is not doing something foolish to
        > > attract a
        > > > direct strike such as standing upright on a high knoll with no
        > > surrounding
        > > > taller growth. Because ground current spreads with distance, the voltage
        > > > gradient decreases directly with distance which is reason to get as far
        > > as
        > > > possible from the most likely target for a direct hit. An interesting
        > > > situation surrounds a well protected metal structure such as a metal
        > > > transmission line tower. A direct lightning strike will hit the tallest
        > > > point (often containing lightning protection) offering protection to
        > > people
        > > > below without the danger of an exploding tree trunk, but ground current
        > > > risk is unaffected. Standing on a metal base plate might be best. But
        > > > then, transmission towers are scarce in the back country."
        > > >
        > > >
        > > > As I say, I can NOT vouch for the correctness of this theory. As many
        > > have
        > > > said, the big safety item is avoiding bad locations. Just what you do,
        > > when
        > > > caught in the wrong place, is less important. But kind interesting to
        > > think
        > > > about.
        > > >
        > > > Again, all these theories are dependent on keeping all your points of
        > > > contact with the ground as close to each other as possible so that one
        > > foot
        > > > (or other body part) does not encounter a voltage different from the
        > > other
        > > > foot. It's the differential *between* the contact points that drives the
        > >
        > > > current thorough the body. Point was made very well in the NOLS video.
        > > >
        > >
        > >
        > >
        >
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