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Re: [TowerTalk] exploding foundations and semantic quibblesregarding Ufe

To: "'Jim Lux'" <>
Subject: Re: [TowerTalk] exploding foundations and semantic quibblesregarding Ufer & "Single Point" grounds
From: "Gary Schafer" <>
Date: Fri, 14 Sep 2007 17:25:29 -0500
List-post: <>
The chart from Jim is good to keep in mind when considering employing your
verticals radial system as part of your lightning ground system. I see many
worry that they should use large wire for the radials if they are to be
depended upon for lightning dissipation. As you can see even #16 wire can
carry a lot of current. Also consider that you will normally have several
radials that will share the current so no one wire will have that much
current on it.

Gary  K4FMX

> Here's a table for solid copper wire in free air, 1 millisecond pulses,
> using the equation from Onderdonk.
> AWG   Fusing Current (kiloAmps)
> 4     193.9
> 8     77.2
> 10    48.7
> 12    30.7
> 14    19.4
> 16    12.2
> Most lightning strokes are MUCH shorter than 1 millisecond (50
> microseconds per strokes, but there could be 4-5 strokes in a single
> hit, so you want some margin, because the wire's not going to cool much
> in the few tens/hundreds of milliseconds between strokes)
> Heavy wire is used for another reason: the continuing current after the
> main stroke can be several hundred amps, and can last for seconds.
> While an AWG 16 wire can take 12.2 kA in a 1 ms pulse, if the pulse
> lasts 5 seconds, the fusing current is less than 200A. There's also a
> mechanical ruggedness consideration, and an allowance for conductor and
> connection degradation over time.  The electrical and lightning
> protection codes recognize that these kinds of things are installed and
> forgotten for the next 20-30 years.
> I think the literature shows it's pretty well understood that any
> examples of explosion or spalling are not due to the wire failing, but
> due to water filling a gap between conductor and surrounding concrete,
> getting heated and flashing into steam.  While it might take 17 kJ to
> raise a meter of AWG16 to 1083C, it only takes 1400 J to get to 100C to
> boil water.  So there's another reason why heavier wire is better.. more
> thermal mass to take the heat pulse (and a reduced thermal load in the
> first place because the resistance is lower)
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