To: | kd4e@verizon.net, towertalk@contesting.com |
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Subject: | Re: [TowerTalk] Re: Floating Ground Steel Building? |
From: | Jim Lux <jimlux@earthlink.net> |
Date: | Fri, 02 Jul 2004 13:57:55 -0700 |
List-post: | <mailto:towertalk@contesting.com> |
At 03:51 PM 7/2/2004 -0400, doc wrote:
> The slab itself is grounded, isn't it... This is what UFER ground 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 _______________________________________________ See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA. _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk |
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