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Re: [TowerTalk] CATV & Phone grounds

To: "K8RI on Tower talk" <>,<>
Subject: Re: [TowerTalk] CATV & Phone grounds
From: Jim Lux <>
Date: Tue, 11 Jul 2006 15:49:40 -0700
List-post: <>
At 01:44 PM 7/11/2006, K8RI on Tower talk wrote:

>I don't have the figures at hand for the pressure inside the initial bolt
>but it is quite high.  Temperatures are phenomenal.

Around 20,000K, as it happens.  Determined from spectroscopic measurements 
of the lightning.

>If you've ever used a "plasma torch" for cutting metal you have at  least
>the beginnings of an  idea as to the temperature inside a lightning bolt.
>The typical "torch" uses low pressure air and a relatively low power arc to
>generate a plasma of roughly 30,000 degrees.

Interestingly, a plas cutter is hotter than lightning.

>  This is hot enough to vaporize
>thin sheet steel so quickly I have cut painted "barn metal" without
>scorching the paint either side of the cut.  Even with a smaller torch and
>using a guide I've cut 1/4" steel plate more smoothly than I could have done
>in a band saw and I'm a rank amateur when it comes to the torch.

Truly a useful tool..

>   However
>IIRC lightning can produce temperatures many times this. I believe some
>strikes are capable of producing something on the order of one to two orders
>of magnitued more which is hotter than the surface of the Sun.

Not really.. There's a fundamental limit on how fast the air in the spark 
channel can heat.  The channel has some inductance, so the current rise 
rate is limited, and the channel expands fairly quickly.  A free burning 
stable arc in air at sea level is about 7000K.  Lightning is hotter because 
it has a fast enough di/dt to heat the air before it can move out of the way.

In laboratory equipment, one can heat the air much faster, thereby getting 
higher temperatures, but it's for a very short arc and requires careful low 
inductance and/or clever transmission line systems. The Sandia Z machine is 
essentially a specialized version of this.

> >
> > In the case of lightning, the enormous voltage present rips the air
> > molecules and atoms apart, creating the plasma, which then provides a
> > current path for the lightning strike.

It actually doesn't take much voltage *at the tip of the leader*.  What's 
important is the field, which has to be greater than around 3MV/meter, but, 
with a 1 cm diameter spark, that's only 30kV.

In lightning (and in long sparks in general) the field is highly 
nonuniform, which is why the spark "grows" as opposed to breaking down 
simultaneously everywhere.  This causes problems for folks designing HV 
transmission lines (1 MV will easily jump 4-5 meters) and is of great 
benefit to folks building Tesla Coils (you only need a couple hundred kV to 
get the spark going, but power gives you spark length)  With a 5-10kW 
supply it's pretty easy to get 10 foot sparks, even if the peak voltage is 
never bigger than 300 kV.  Compare to a uniform field gap, where 300kV 
would just bearly breakdown a 10cm gap, and even in a rod gap, it would be 
around 50 cm.


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