[TowerTalk] exploding foundations and semantic quibbles regarding Ufer & "Single Point" grounds

[Gary Schafer]

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.

73
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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