Metal Roofs

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kwired

Electron manager
Location
NE Nebraska
My question is what do you bond a very thin piece of metal with that can take thousands of amps without destruction of the bonding point?

If lightning strikes the far corner of roof from where the bond is made you will also have potentially hundreds of pieces for current to travel through to get to the grounding conductor, each with poor electrical joints between them.

There will likely be burn marks at every joint, that is if you are lucky enough there was no fire afterwards and everything is still intact.

Metal roofs are not really that new - just a newer concept on dwellings. They have been around on farm buildings for years and have no more or less lightning incidents than asphalt or wood shingles AFAIK. I have seen more houses on farms with severe lightning damage than I have seen outbuildings over the past 20 years or so.

I do see other lightning damage on farms away from the house - there is just typically more in the house subject to damage. I remember a few years back lightning struck a customer side distribution pole on a farm. There was buldings with metal roofs around, some taller than the pole, yet the pole was obviously the direct hit as it was split pretty good.

There was damage of a few items here and there of electrical items in some nearby buildings. The house was one of the farther away structures yet had the most items effected. Somewhere it crossed to the telephone lines and about every phone and phone jack needed replaced. The phone companies surge protector on outside of house had cover completely blown off of it.
 

kwired

Electron manager
Location
NE Nebraska
Do you have a source for that? Most of the sources I find show air as have an impedance of 10^13 to 10^16 ohms.
ok, we'll disagree. actually dry air has an impedance of 377 ohms, not particularly high, especially when the voltage difference between the cloud and earth is in the millions of volts. so taking 0 ohms up the 25-30' above the surrounding ground seems significant to me. but when lightning strikes, the ionized air is no longer 377 ohms but 0. height above ground is one of the main reasons one place gets hit vs another.

I suspect that may be impedace once arcing begins to happen or something of that nature, otherwise using the formula E = I x Z, if we apply 480 volts to the terminals of a typical disconnect or circuit breaker with at least partially exposed lugs we will have 480 = 1.27 x 377 which means 1.27 amps will flow through the air between the terminals, that means about 610 KVA is flowing phase to phase through the air on all 480 volt systems and if three phase multiply that times 1.73 and you have about 1050 VA. This would be at every point where all three phases are exposed to air What about all the high voltage transmission and distribution lines that have no insulation on the conductors?:happysad:
 

iwire

Moderator
Staff member
Location
Massachusetts
I suspect that may be impedace once arcing begins to happen or something of that nature, otherwise using the formula E = I x Z, if we apply 480 volts to the terminals of a typical disconnect or circuit breaker with at least partially exposed lugs we will have 480 = 1.27 x 377 which means 1.27 amps will flow through the air between the terminals

That was pretty much my immediate thought when I read 377 ohms, we would have some serious issues if all air had an ultimate resistance of just 377 ohms.
 

iwire

Moderator
Staff member
Location
Massachusetts
Hopefully less than any of your choices, otherwise we should have a lot of problems with exposed conductive items that are closer than any of the choices you gave.


Exactly, my thought.


(BTW, this is another time where I can't tell if you are agreeing with me or just trying to say you disagree with my choices :?)
 

kwired

Electron manager
Location
NE Nebraska
Exactly, my thought.


(BTW, this is another time where I can't tell if you are agreeing with me or just trying to say you disagree with my choices :?)

Sorry, maybe a better reply would have been - "Exactly, but I would think you need some smaller dimension to have the correct answer in your choices"
 
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