Grounded TV antenna on chimney, house more likely to burn down?

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Spinnor
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We have no longer have cable TV but receive free local broadcast via a homemade coat-hanger TV antenna, see:

http://www.google.com/images?hl=en&...=UTF-8&source=og&sa=N&tab=wi&biw=1024&bih=581

The antenna was fastened to the ceiling with good results. The antenna was stuck out the nearest window (which faces most of the local stations) with better results, but cold air came in as I could not close the window all the way. A hole was drilled in the ceiling and the antenna was moved to the attic, much better results. Now I'm thinking why not move the antenna outside and get better results still. My only problem with this is if the antenna is outside the house it must be properly grounded, see for example:

http://www.channelmaster.com/v/vspfiles/assets/images/cm-3000a_instruction%20sheet_12july10_web.pdf [Broken]

My question and worry, if I set up an antenna outside and attached to a chimney, and it is properly grounded would this increase or decrease the chance that our house would burn down due to a lighting strike?

On the one hand one might think that sticking a grounded metal conductor some 18 feet up in the air will increase the chance that a lighting strike will occur near our house and some of the lighting strikes energy might go into starting our house on fire. On the other hand in our attic running along the ridge beam we have electrical wires which include a ground and if lighting were to strike these wires I'm guessing there is a good chance that such a lighting strike would start a fire.

So if I set up a properly grounded antenna outside do I increase or decrease the risk of a lighting started fire? Any thoughts on this matter would be appreciated!

Edit: I should have mentioned that we live on a slight ridge and have many tall trees around us.
 
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  • #2
dlgoff
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The best way to have an out side antenna for receiving all the good free HD signals is to use a proper antenna (VHF and UHF) mounted on a mast attached to the chimney then attach a ground wire from the mast down to a deep driven rod. What happens is that the charge that is built up on the roof/house top is due to rain bringing it down there The charge will be discharged to a ground potential making a lighting strike very unlikely. You'll be safe and will be able to get all the available HD channels.

Good Luck.
 
  • #3
davenn
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The best way to have an out side antenna for receiving all the good free HD signals is to use a proper antenna (VHF and UHF) mounted on a mast attached to the chimney then attach a ground wire from the mast down to a deep driven rod. What happens is that the charge that is built up on the roof/house top is due to rain bringing it down there The charge will be discharged to a ground potential making a lighting strike very unlikely. You'll be safe and will be able to get all the available HD channels.
Good Luck.
Dont confuse rain static (a build up of charge on an antenna) with a lightning discharge
2 very different beasties. Yes a charge built up from rain will be grounded with that grounding cable. But thats NOT going to stop the possibilities of a lightning strike. The charges building up between the cloud base and the ground below it are many orders of magnitude larger. The ground and higher objects, regardless of if they are metallic or not will build up a charge during the storm at an opposite polarity to the charge on the base of the cloud and regardless of if there is rain in that area or not.

There is going to be a higher build up of charge in areas where the ground or anything on it, and the cloud base is closer together. In the absense of highpoints over relatively flat ground, where the lightning strikes will be somewhat more random.

Note in the drawing, the displayed positions + and - charges can be reversed depending on what is happening within the different parts of the storm.
A couple of times during my storm chasing, I have been able to video ground to cloud strikes.

Dave
 

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davenn
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No form of grounding cable to earth rods is going to protect your property from a direct strike. The voltages and currents involved are just so huge. Your antenna mast pole coax and grounding cable will all most likely end up being molten blobs of metal.
In a former life (read... previous job) I did installations of antenna on comms towers, wooden and steel lattice. We would go to extrodinary lengths to try and mitigate lightning strikes 2.5 - 5cm diameter 2 metre long galvanised tubes bolted to the top of the structure , above all the antennas The grounding straps from the tubes to the ground were 2.5 cm wide .5cm thick copper bands which would be connected to large earth mats buried in the ground around the mast.
Even with all that we would still get comms equip totally fried and blown to bits.
We deemed all that protection only really valid for close by strikes.

Look at it this way... what thickness of copper cable are you going to use for your grounding lead from the mast to the earth rod ? 3-4mm diameter ?
it will be as effective as a 1 amp fuse in a 200 amp cct :)

it all boils down to if you have a direct strike, the best protection is a VERY good insurance policy :)

Dave
 
  • #5
Spinnor
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...

it all boils down to if you have a direct strike, the best protection is a VERY good insurance policy :)

Dave
Guess I should talk to our insurers. %^)

There is probably data out there, number of home fires from lighting strikes with and without antennas. If we also knew ratio of homes with and without antennas then we might figure out the relative safety?

Thanks!
 
  • #6
dlgoff
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No form of grounding cable to earth rods is going to protect your property from a direct strike. The voltages and currents involved are just so huge. Your antenna mast pole coax and grounding cable will all most likely end up being molten blobs of metal.
In a former life (read... previous job) I did installations of antenna on comms towers, wooden and steel lattice. We would go to extrodinary lengths to try and mitigate lightning strikes 2.5 - 5cm diameter 2 metre long galvanised tubes bolted to the top of the structure , above all the antennas The grounding straps from the tubes to the ground were 2.5 cm wide .5cm thick copper bands which would be connected to large earth mats buried in the ground around the mast.
Even with all that we would still get comms equip totally fried and blown to bits.
We deemed all that protection only really valid for close by strikes.

Look at it this way... what thickness of copper cable are you going to use for your grounding lead from the mast to the earth rod ? 3-4mm diameter ?
it will be as effective as a 1 amp fuse in a 200 amp cct :)

it all boils down to if you have a direct strike, the best protection is a VERY good insurance policy :)

Dave
Funny. I've lived on a hill top for about 30 years well above the neighbors roof mounted antenna mast and have never had a lighting strike whereas the neighbors gets several direct strikes every summer. It's all a matter of potential. Ground the dang thing and you should be well protected.
 
  • #7
Spinnor
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Funny. I've lived on a hill top for about 30 years well above the neighbors roof mounted antenna mast and have never had a lighting strike whereas the neighbors gets several direct strikes every summer. It's all a matter of potential. Ground the dang thing and you should be well protected.
So it seems from your account that the neighbors antennas must have increased the chance of a strike "near" their house. But it sounds like even with good lighting protection sometimes some of the energy goes where we don't want it.

So is it better to crouch in a lighting storm or stand in a metal suit of armor %^) ?

Am I safe in a tank?
 
  • #8
davenn
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So it seems from your account that the neighbors antennas must have increased the chance of a strike "near" their house. But it sounds like even with good lighting protection sometimes some of the energy goes where we don't want it.

So is it better to crouch in a lighting storm or stand in a metal suit of armor %^) ?
Am I safe in a tank?
both are going to produce a faraday shield around you... would help is the individual plates of the suit of armour are fully connected :)

Have a look on youtube at some guys playing with the huge arcing HV generators and the wire mesh suits they wear etc.
 
  • #9
davenn
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I have seen excellent pic of where lightning doesnt go for the highest point even on a single structure. One of my USA stormchaser mates photo'ed lightning hitting 1/2 way down that tall monument in Washington DC ( sorry dunno what its called)

why it didnt go for the shortest path, to the top, who knows

Dave

PS.... something that didnt occur to me earlier as we were so engrossed to direct strikes and antennas.
Do you have overhead mains power or telephone lines into your home or are they an underground feed ?

Of much greater risk of fire/general home damage ... if overlead lines, think of the miles and miles of those power lines running around your neighbourhood. The chances of a strike on them and very high voltages coming down those lines into your home is much, much higher.
 
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dlgoff
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... if overlead lines, think of the miles and miles of those power lines running around your neighbourhood. The chances of a strike on them and very high voltages coming down those lines into your home is much, much higher.
You do know most power poles are individually grounded.

As far as my neighbors house goes, they don't have their antenna mast grounded; hence the strikes.
 
  • #11
davenn
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You do know most power poles are individually grounded.

As far as my neighbors house goes, they don't have their antenna mast grounded; hence the strikes.
doesnt matter if the pole is grounded or not ! :) you cant ground the Hot/Phase wire
its going to carry the excess voltage right into your home and blow the crap out of everything it goes into

I have had to work with the results of that for many yrs in various occupations

the lightning is MORE likely to strike a high point that is grounded than something not grounded (but from a lightning static charge point of view, anything touching the earth is grounded buildings, poles, masts etc etc)
look at the videos of lightning hitting radio transmission towers, lightning rods on tops of tall buildings repeatedly

having a grounding strap from the top of the mast to the earth is just supplying a lower impedance path for the lightning to follow and hjopefully keep it out of the building

Dave
 
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  • #12
dlgoff
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doesnt matter if the pole is grounded or not ! :) you cant ground the Hot/Phase wire
its going to carry the excess voltage right into your home and blow the crap out of everything it goes into

I have had to work with the results of that for many yrs in various occupations

Dave
It does shield a lot of direct strikes however.
 

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