[TowerTalk] Re: Floating Ground Steel Building?

[Jim Lux]

At 03:51 PM 7/2/2004 -0400, doc wrote:
> > The slab itself is grounded, isn't it... This is what UFER ground
> > depend on. There may be a moisture barrier between the slab and some
> > of the ground, but the footings around the edges should be in contact > 
> with the soil, and there's a fairly large contact area there.
>
>As I understand the UFER system either the rebar is extended
>beneath the slab well down into the soil or a second metal mat
>is laid beneath the slab -- I don't recall the UFER ground depending
>only on the marginal conductivity of moist concrete.

Nope.. UFER grounds (aka "concrete encased grounding electrode) are just a 
wire embedded in the concrete, or a suitably bonded rebar cage.



>I am particularly concerned about the likely concentrated discharge at the 
>anchor/mounting bolt shattering the slab.

Why would there be a discharge? Shouldn't the fault current be carried by 
the conductor which might get hot (that 25-30kA current)(or, are you 
concerned about side flash and such?)

The current is high in a lightning stroke, but not that high, and it's a 
short pulse.  Consider that fault currents in a "bolted short" in 
industrial feeders can run in the 10s of kA (most residential 
circuitbreakers are rated at 10kA interrupting capacity or greater).  The 
implication is that the wire can take it for a short time until the 
overcurrent protection opens.

But, to put some real numbers to it...
What's the fusing current for a AWG10 copper wire subjected to a pulse of 
100 microseconds (which is about twice as long as the typical lightning 
impulse of 50 microseconds, and even more conservative, since 50 
microseconds is the time to half current).. using the Onderdonk equation, 
we get that the fusing current is about 156 kA... (The Onderdonk equation 
is used for short pulses, as opposed to the other popular forumala, due to 
Preece, which is for constant currents)


The wire's not going to melt.  This is why the NEC requires a particular 
gauge for lightning protection, which seems quite small.  Recall, though 
that the NEC is concerned about not burning down the house or causing 
physical damage, not blasting your delicate electronics.  The latter reason 
may inspire you to put in a somewhat lower resistance (or, more important, 
lower inductance) path, to limit the peak voltages.

This is why GOOD connections (tight clamps, exothermic welding) are needed. 
You do not want an arc forming




>I like this method for its simplicity:
>http://www.scott-inc.com/html/ufer.htm
>
> > It's Called a UFER ground...google it.  I placed my ground rods
> > every 6 feet around my qounsit hut...ten in all, each tied to the
> > building with a 2ft #6 copper jumper.
>
>Have you observed any dissimilar metals reactivity between your copper
>jumper and your steel quonset hut?
>
>I'm sure I can get the dissimilar metal connectivity kits from Polyphaser, 
>Texas Tower, or other sources but was wondering what
>your experience has been.
>
>Also, are you relying on mechanical connections only or did you
>bond your straps to the hut and the ground rods using the explosive
>weld method (I forget the brand name)?
>
>Sounds as though I will need 12-15 rods in addition to the three
>20 foot rods I have planned for the entry point.  Time to start
>the search for an affordable source!
>
>Thanks! & 73, doc kd4e
>_______________________________________________
>
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>
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