SB: FWD Re: Lightning strikes

[Robert W Tom <archilogic@juno.com>]

--------- Forwarded message ----------
From: Mark Piepkorn <duckchow@greenbuilder.com>

"Ted Shelton" <ted655@hotmail.com> wrote:
>After our third extended lightning storm, with many cloud 
>to ground strikes. It has occurred to me the potential 
>danger in building something without a safe grounded path.
>Given all the natural materials in the three pigs house, 
>what happens when lightning strikes them?


        I'm not sure why this question moved me to delurk momentarily...
rousting
me from my invisible archive-browsing to actually sign on, post, and sign
back off again. (A task at which I have failed, incidentally, and have
thrown up my hands about.)

        The following article is from The Last Straw #24 (Winter 1998).
There's so
much stuff that's been printed in that journal over the last - what, it's
been about eight years now, right? Stuff that's not in any of the books,
not on any of the websites, not on any list. Stuff that's nowhere else.
I'm
still amazed at how many people interested in SB who want to build
themselves (or worse, their customers) the best house they possibly can
don't read it.

        I was the editor of TLS for a couple years, but that ended a year
ago. I
was also the behind-the-scenes listmonkey of the CREST SB list for a
couple
years (which ended about four years ago now, I guess), and I still
cheerlead for that, too.

        Obviously, a person won't learn *everything* about building with
bales
from any one book, or email list, or newsjournalmagazinething, or person.
Get all the info ya can from as many sources as possible, that's my
advice.
Use yer own smarts and everybody else's too.

        What the following article doesn't speak to is the movement away
from
metal lathing (and cement stucco), which has really grown rapidly over
the
last couple years. Perhaps the cautious individual could implement
lighting
protection along with things like steel strapping (in seismic and
high-wind
country particularly) or steel cable tie-downs.

        Or do what most people do: nothing.

The Last Straw info - http://www.strawhomes.com

-----

Observations of Stucco-Finished Homes Struck by Lightning
Leon Byerley; Tucson, Arizona

In September 1997 I had an opportunity to carefully examine three
wood-frame, stucco-finished homes that were struck by lightning. What I
found is relevant to builders and owners of straw-bale homes.

The houses in question are nearly identical. The neighborhood association
has strict rules forbidding roof-mounted equipment. The houses have
multilevel pitched roofs covered in concrete tile. Each house that was
struck showed clear evidence of lightning attachment at the peak end of
one
of the lower levels of the roof. At this place on each stricken house,
the
concrete cap roof tile was blown off and some splintering of the wood
beam
underneath was evident. Aside from miscellaneous damage to electrical
appliances and equipment in each stricken house, the loss of one roof
tile
was the only external physical damage found.

There was no evidence of any lightning arcing or flashover around the
base
perimeter of the houses or at the AC service entrance. Inside the houses,
the electrical damage was remarkably low considering these structures
have
no thought given to lightning protection. Why did these three houses
stand
up to direct lightning so well?

In the stucco-finished houses I examined, the minimal lightning damage
appears to be due to several aspects of construction which worked
together
to provide the elements of an effective lightning protection system.

1. On the exterior walls, at each roof peak, the metal lath and chicken
wire rise to a point just below the cap roof tile and this place serves
as
an air terminal (a place from which lightning currents can be
"launched").

2. The steel lath and chicken wire used to hold stucco make essentially
continuous "sheet metal walls" extending from near the ground to just
under
the eaves. These extensive flat metal sheets have very low impedance for
fast lightning currents and serve the purpose of main and down
conductors.
If we race a current pulse that changes abruptly down a broad flat sheet
of
metal and a round cable, the current in the flat sheet will win every
time.

3. These houses, unlike most homes but like most commercial and
industrial
buildings, have a "Ufer Ground" instead of the more typical (and less
effective) "driven rod" as an earth electrical contact. Ufer grounds use
the steel reinforcing rods in the concrete footers and foundations to
make
extensive and effective electrical contact with the earth. (It is
important
to note that concrete is about as conductive as the first meter of
topsoil
in Southern Arizona.)

The combination of exterior lath walls and Ufer ground electrodes appear
to
have enabled lightning currents to pass with relative ease from the roof
comer peak of the lath, through the lath and chicken wire, to the earth
through the extensive network of rebar in the foundation. The surge
impedance of the sheet metal walls and ground electrode together were low
enough that when lightning struck, the voltage rise in the lath was
minimized, and flashover into house wiring and plumbing was minimal or
non-existent. The neighborhood restrictions on roof-mounted equipment
kept
lightning currents from entering the homes via conductors on the roof.

The relatively light electrical damage seen inside the homes can be
attributed partly to the exceptional ground electrode but also to the
fortuitous co-location of all electrical conductors (power, phone, cable
TV, satellite TV, etc.) at the AC power service entrance of each house.
This "bulkhead" approach to cable entry in buildings and facilities is a
fundamental rule of electrical equipment lightning protection.

Houses that are constructed with exterior walls of lath and stucco, Ufer
ground electrodes, absence of electrical conductors on the roof and
proper
bulkheading of conductors at the service entrance appear to give
remarkably
good resistance to lightning damage. It appears likely that significant
lightning protection benefits resulting from the combination of more or
less common building practices may be available to straw-bale builders at
a
marginal cost near zero.

Leon Byerley
Lightning Protection Technology
Tucson AZ
520 326-1129
<byerley@azstarnet.com> 
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