At 10:43 AM 7/4/2006, Roger D Johnson wrote:
>JThe NEC is definitely more focussed on dissipating transient energy (from
>near strikes, etc.) and fault currents, not on dissipating a direct
>stroke, so the 20ft encased in concrete thing might be insufficient for
>lightning protection.
>
>I did find a site which said that 20 ft of rebar in concrete is capable
>of absorbing an 8 kiloamp surge. Since the average lightning strike is
>around 15 Kamps, it's obvious that these mini-Ufers don't offer much
>protection from lightning strikes. I've noticed that most of the sites
>that promote a Ufer ground made up of the tower base and rebar also
>advocate the use of a radial system. My feeling is that the radial
>system actually provides the lions share of the protection.
or, given that most rebar reinforced slabs have a lot more than 20 ft of
rebar in them, it works out ok.
I used to have some equipment in a shed like metal building about 20 ft
square out at White Sands Test Facility near Las Cruces, a place where
lightning is an extremely common occurance. They had 8 rods sticking up
above the roof about 2 feet around the perimeter, a AWG2 (or bigger) wire
coming down from each rod to the foundation where it was run into the
concrete. There was also a peripheral ring connecting all the rods at the
roof level. The building structure was the usual steel I beam type frames
bolted to the foundation with corrugated sheet metal as the skin and roof.
The frames were also bonded to AWG2 wire which disappeared into the
foundation. There was a fair amount of rebar in the foundation, as well.
So.. while this building was clearly grounded with a Ufer style ground, it
was a heck of a lot more than the NEC required 20 ft of conductor. I
suspect that in "lightning country" the wise builder does this as a matter
of course.
An interesting sidelight on this might be the increasingly common Post
Tension Slab style of construction, where there's no rebar in the
slab. Instead, there are several steel cables on roughly 5 foot spacings
that are tightened after the concrete is poured to keep the slab in
tension. The cables run through a plastic sleeve, and I'd imagine that they
aren't connected to the grounding system. Furthermore, on most PTS
systems, there's a vapor barrier (read, plastic sheet) between slab and
ground, although from an RF standpoint, the capacitance across the sheet is
huge so the RF reactance is low. (This is also the case on most slab
construction (post tension or not), in residential use, anyway). In
California, for seismic reasons, there are also turndowns at the edges of
the slab (to transfer shear loads to the soil), and I'm pretty sure there's
rebar in those.
Jim
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