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Earthing & Lightening Protection for a Home on a Mountain Top

  1. Oct 26, 2011 #1
    G'Day guys,

    We have been looking around and feel this would be an appropriate forum full of good knowledgeable people to ask a few challenging questions.

    The missus an I are about to build a house on a mountain top that is the highest peak for at least 1-2 km's (1 mile or more). Obviously because of the position we will need to take lightening protection very seriously and I feel we should probably get this done before we even start on the house. Also I tend to 5x over-engineer things just to be sure, and using second-hand industrial waste (cheap) wherever possible.

    The mountain is covered in trees, dense in some places and spaced out in other parts. On the very top is a small, cleared, flat plateau of about 30-40 meters (32-43 yards) diameter. We will take down most of the small bushy trees near the top to open up the view, but we have decided to keep a circle of the biggest & tallest trees around the top edge to keep some shade and to help obscure the house into the skyline. Without the small trees, the occasional big trunks won't block our views too much. I believe that as long as these big trees all remain higher than the house (which will be roughly in the center of the flat plateau), they should help with some degree of lightening protection.

    The house is likely to be constructed primarily of a heavy gauge steel frame and roughly in a conical or dome shape as we are expecting occasional strong winds. This frame would be anchored into the ground in at least 15-30 places (internal steel posts), so I imagine the whole house will be somewhat similar to a faraday cage, and although lower than the tree tops, it will be a very low resistance target for lightening. Given that the decent sized trees will be relatively close to the home I would prefer not to encourage the lightening to hit any of them and explode the timber / send a big tree crashing down onto the house.
    I don't mind how much work goes into setting this up properly but I only want to do this once, not fixing and maintaining the system every few years, digging up and replacing rusty metal in the ground etc...

    I have come up with 2 ideas to address this situation, 1 simple, 1 more complex.



    Idea 1 (Simple):

    Attach a lightening rod to the very top of the house that will ensure that it casts a greater than 45 degree shadow over the entire home (and preferably further). The only restriction here is that there will be a height limit. If the tower was to poke up higher than the tree tops (or even above 10m from the ground), the council (local government) will definitely get grumpy and we'll probably have to get some difficult permission / have to attach a flashing beacon for planes etc...
    So long as it was lower than the trees I think we will be ok, but then the trees will be the highest point all around not the tower so it should protect the house but may not encourage lightening away from the trees.

    scan0001_Simple_Side.jpg scan0001_Simple_Top.jpg

    If we were to go this way I have 2 questions:

    Q A1:
    The steel frame house by it's nature will be very grounded. The Lightening tower/rod will also be very grounded. Should I electrically connect the 2 together or keep the rod/cable/ground rod insulated from the house frame?
    My reasoning here is that if they were insulated from each other, although both grounded, the lightening will likely go for the much higher tower/rod, then releasing most of the power directly into the ground, maybe only some power will flow from the ground back up into the house frame? Alternatively, if they were both directly electrically connected, the power would run down the rod and into the frame as well before it goes to ground. Would this increase the chances that if someone was near or touching the house frame or anything connected to it, that they could get a boot?
    If we do connect the rod electrically to the frame is there any point running a separate cable / ground rod when the power can flow through the entire very heavy steel frame into ground through the many posts?

    Q A2:
    Is there more we can do to increase the electrical conductivity of the soil/cement around the buried house frame posts / earth rods given that the soil is likely to be fairly dry? (the moisture at the top tends to drain down into the mountain). I would prefer not to use salt or anything that would speed up corrosion, and I believe most additives need moisture to be effective anyway. The steel posts are thick 'C' section and galvanised, but I'm guessing concrete is not the best conductor? Should we make a mud/clay mix of our own? Perhaps we could add a heap of metal shavings / drill swarf into the mix (good until it all rusts)? Other suggestions?



    Idea 2 (More complex):

    Snag a heap of second hand 2" to 4" diameter aluminium irrigation pipe (like a flag pole). Weld sections together to form a long pole close to the same height as the trees. Attach each pole along each tree trunk but not buried in the ground (not supposed to bury aluminium for lightening ground rods?). This would provide around 10-15 poles in a circle around the mountain top and the poles could be hidden from view by placing them on the side of the tree trunk away from the house. A thick copper cable (insulation stripped off), or a decent sized copper tube (thick water pipe?) would then be buried in sections, each end connecting to the aluminium poles to form a closed ring connecting all the poles at the bottom in the circle around the plateau. Possibly even sink in a copper ground rod between each pole and attached to the copper cable/tube for more ground contact.

    Ideally, having the poles rise well above the tree tops would help insure the poles get hit before the trees, but again we face the council / aeroplane warning lights / eyesaw issues so we need to keep the pole tops below the tree tops.
    I also thought of having the poles between the trees instead of leaning right up against them, but then we would have no tree support to hold them up and have to firmly anchor each pole in the ground and possibly provide a cable support at the top.
    We really want to get away from making this thing seem like we are living in a cage.

    I imagine that having 10-15 aluminium poles about 10-15 meters long each, and all connected together by a copper loop that is buried in the ground will provide an excellent, low resistance and very attractive path for lightening, even without the additional copper ground rods inserted between each pole for extra ground earthing to be sure.
    However I wonder wether it will be enough, as right next to each pole is a very large electrical collector network (tree branches), though it will then have to go through a much higher resistance cable (tree trunk), but again is able to distribute the power into the ground through a very large earth rod network (tree roots).
    If we could provide a way to electrically couple the tree to the pole, may they be able to help each other? This page seems to suggest that the lighting would probably hit the tree first, go down the branches then jump to the pole (possibly just exploding the top part of the tree?): http://cr4.globalspec.com/comment/71350

    Also, I am aware that special connectors will be required to join copper to aluminium, and that copper should not be placed above aluminium due to incompatibility of the two metals leading to accelerated corrosion?

    One extra benefit of this much higher cost and effort design is that instead of using cable buried for the in-ground loop, we could run buried pipes to connect all the vertical poles and put sprinklers on top of each pole effectively doubling the system's use as a bushfire sprinkler system that surrounds the house. With the pipes full of water they are only likely to decrease the electrical resistance and this fire protection would help justify the effort.

    scan0001_Complex_Side.jpg scan0001_Complex_Top.jpg

    This idea presents several questions:

    Q B1:
    Would this approach do more to prevent lightening strikes as it might provide more ability for built up static to dissipate to ground before it reaches a critical mass (more metal exposed to air and spread around the site)?

    Q B2:
    Would the lightening more likely still go for the trees because they are still higher than the poles and have larger collectors, or would it more likely prefer the poles because they offer lower resistance to ground?

    Q B3:
    Should we electrically insulate the poles from the trees (tie the poles with rope?) to further discourage the trees from getting hit, or should we do our best to electrically connect the poles and trees (tie the poles with metal cable/strapping in several places?) so any power that does hit the tree tops will better flow into the pole offering an easier path to ground than just wood and hopefully reducing damage to the tree?

    Q B4:
    Should we add another cable loop (aluminium) connecting all the poles at their tops as well, to provide an in-air collector ring to further attract lightening rather than just 10-15 pole tops slightly below the tree tops? (Would prefer not to do this, would like to keep the whole system as hidden as possible).
    Going further with this, we could even criss-cross a cable (or 2) from the top of one pole across to the top of a pole on the other side providing a hanging cable (simple net) well above the home but directly over it? (again, really would prefer not to have cables in view if possible).

    Q B5:
    I would like to weld a cover over the top of each pole so rain doesn't get in them. I have seen some lightening rods with a ball on top, I see other tops as just a single sharp pointy stick, and sometimes even a prickly porcupine of 5 or more sharp sticks pointing in various directions. Given that there is likely to be tree top branches hanging around near the top of the poles, would any of these pole tops make any difference?

    Q B6:
    Again both the house frame and this outer pole/cable loop shield will be connected to ground, but would there be any advantage to directly electrically connect them together with a metal conductor, or would it be better that they are only connected through the ground (provide some isolation for the people in the home)?

    Q B7:
    Is a round tube as good a conductor as a solid rod with the same cross-sectional area of metal?
    (Do flag poles ever melt / get damaged from lightening strikes?)

    Q B8:
    One of the advantages of living high up is the reception (transmission and receiving of frequencies). At what point does this crude faraday cage begin to affect this? I assume this would be dependant upon the wavelength of the frequency and the size of the gaps between the poles and cables?


    So many questions...
    ...answer as many or as few as you feel you can contribute to.

    I hope this is an interesting challenge for you guys. My lady and I would appreciate any thoughts you have on these two approaches, or any other ideas to help us safely continue to enjoy the light show.

    I am sorry this is so long, again I like to be thorough and hope this will provide some excellent advice/direction for others planning a similar build.
     
  2. jcsd
  3. Oct 27, 2011 #2
    A simple lightning arrestor on the highest point on the roof-top should suffice. In the 2nd case, the tree branches could outgrow the length of the vertical poles and reduce the effectivemness of the design. One would need to keep trimming the branches regularly to ensure effectiveness of protection. I personally feel the 2nd idea is a bit impractical! I think effective protection could be provided by methods much less extravagent then the ones suggested above...

    P.S. Maintain earth resistance below 1 ohm.

    Regards,
    Shahvir
     
    Last edited: Oct 27, 2011
  4. Oct 27, 2011 #3

    jim hardy

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    Here's an interesting report that cites Franklin's work from mid-1700's.

    www.lightningsafety.com/nlsi_lhm/conventionalLPT.pdf

    it was he who figured out the air terminals should be pointed.

    your objective is to keep lightning current from flowing through your house structure.
    in my power plant the lightning system had its own separate "ground" rods which were driven twice as deep as the power system ground rods.
    The two systems were connected only by Mother Earth.
     
  5. Oct 27, 2011 #4
    b.shahvir:
    Thanks for taking the time to read the thread and for the reply.
    I wasn't thinking we would keep trimming the trees on design #2, if they kept growing then oh well, lightning may hit the top part of the tree first, then from what I understand would jump to the pole (lower resistance) to finish the journey to ground.
    We might loose some of the top of a tree, but the main part of the trunk wouldn't take the whole electrical load.
    Although many times more work and cost, the second design is appealing if we can double it as a bushfire defence system (the sprinklers would be spraying right into the tree canopy!). However if from a lightening point of view there is no benefit or is potentially even worse than 1 simple central rod over the house, then we could design a more appropriate separate bushfire system for less money and effort.
    Thanks for the tip on the 1 ohm goal.

    Cheers,
    David.
     
  6. Oct 27, 2011 #5
    Hi Jim,
    Cheers, I will try to have a read on the weekend.
    You believe I should not try to isolate the house structure from the ground, but I should completely isolate any "here I am, come and get me" lightening rods from the house structure? So the only way the 2 metal systems are connected is through the ground? That seems intuitive.

    But I read in a large lightening protection manual that if your house / building steel frame was substantial enough, that you could connect the whole system together as there would be so many paths to ground through thick steel that it would all support each other for an even easier path to ground. But that made me wonder "what if I was touching the frame or was close to it when a strike hit"? Maybe the thick grounded steel is just too tempting for the lightening? Or maybe I could still get a wallop?

    The bushfire threat is also a very real concern, the district back burns every year and the grass fire flames come right up to the house, so we have to build with thick steel not timber.

    If it is true that we should keep the steel frame isolated from the lightening systems (only connected through ground), then I wonder why the lightening strike would prefer the rod (even if it is a bit higher), when it only has 1 or 2 rods into the dry ground, whereas the house is a huge steel structure (collector?) that may have anywhere from 15 to 50 steel beams going into the ground?


    Thanks again for your time and effort,
    David
     
  7. Oct 28, 2011 #6
    You welcome!

    Well, I still feel it's risky as during moist conditions the electrical conditions could be vice-versa and the sap could explode!

    Bond all plumbing together solidly & connect it to earth. Ensure all appliances are un-plugged from their recepticles when away from home for long. This might not be normally necessary, but safe if the location is lightning sensitive.

    Regards,
    Shahvir
     
  8. Oct 28, 2011 #7
    Here is the U.S. Forest Service recommendation for lightning protection on lookout towers:

    "Evaluating Lightning Protection on Lookouts
    and Communication Facilities"

    http://www.firelookout.org/lightning-protection.pdf [Broken]

    Many years ago my brother and I built a cabin on a hilltop. Lightning rods were placed on the cabin, and all the appliances, plumbing, and electrical wiring were grounded. Several large ponderosa pines surrounded the cabin. The lower branches blocked the view. They were too high to reach by ladder, so we used a 30-30 to trim them off.
     
    Last edited by a moderator: May 5, 2017
  9. Oct 31, 2011 #8
    Cheers, sounds like good advice!!!
     
  10. Oct 31, 2011 #9
    That was a good read, cheers for that.
    Wow, the cabin sounds awesome, so rewarding building your own.
    I had no idea what a 30-30 was. We don't really have the gun culture here in Oz. I googled it and now understand you blasted the branches off with a gun. That's pretty funny and a much more peaceful use for weapons =)
    Cheers again for your feedback.
     
    Last edited by a moderator: May 5, 2017
  11. Oct 31, 2011 #10

    jim hardy

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    well i have to confess my prejudice about keeping lightning out of the house structure was based on premise the extremely high current can be destructive to the structure, as in wood.

    maybe you have huge structural steel bridge beams or something - i dont know. i assume it's that light steel they use for house frames. if so,,,

    i would not invite lightning currents to flow in my structural steel either.
    instead encourage them to flow around the outside through cables dedicated to that purpose.

    you do not want to be touching anything metallic while a lightning strike to it is in progress. (I base that claim on personal experience.)

    think about it for a second - for duration of the stroke that metal is connected directly to the bottom of a lightning bolt. Charge moves out of the bolt and and along the metal on its way from the sky into the earth.
    There is substantial voltage drop along the metal.
    Indeed the voltage drop continues even across the ground itself, as that charge makes its way outward from point of contact(ground rod) and dissipates into surrounding earth.
    You do not to be an alternate path for that charge. As you said, intuitive.


    google term "faraday cage" -
    imho you'd be better off making your house into a steel 'faraday cage' that does not have to handle the kiloamps of the lightning stroke itself , only the smaller induced currents caused by its proximity.

    So i'd do a little overkill on the lightning rod grounding electrodes and keep an eye toward encouraging the lightning bolt currents to go around and away from the structure.

    most lightning is in the range of ten to a few hundred amperes and doesn't even bother thin aluminum aircraft skin.
    thirty kiloamps is a whopper but not unusual in severe thunderstorms
    and there's monsters in hundreds of kiloamps.

    disclosure - my wood house has no lightning rods. But it's not on a hilltop either and is surrounded by tall trees.

    old jim
     
    Last edited: Oct 31, 2011
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