[time-nuts] Lightning and grounds...

[Don Latham]

Right, Magnus.
Right, Phil. The first leader from a common strike starts in the cloud and
steps downward. The result resembles a charged piece of coax. As it nears
the earth, an actual breakdown occurs, shorting the coax (sort of). It's
this short (on the order of 10 m) breakdown that's really got a fast
risetime.  Flashes starting from the ground (leaders originating at the
ground)are known, and occur usually from very tall towers or mountaintops.
Following strokes don't branch, but the process is similar. The ground
terminus for subsequent strokes in the flash don't necessarily have to be
at the same spot on the ground...

Could be, Jim. I'd take them down, as in a moderate field, if the brush
points are still sharp, they'll generate a lot of noise!

Yes, Chuck, they're wrong. I'll bet $ to donuts that a plant manager of
some kind was sold a bill of goods, and the physicists were not even asked
( bitter experience).

Don

Magnus Danielson
> Don Latham wrote:
>> Hi Burt and all:
>> Only a reminder: lightning is a pulse, and especially at the ground
>> terminus can have frequency components in excess of tens of MHz. Think
>> in
>> terms of impedance rather than resistance.
>
> I think the use of frequency components is a bit of missguiding when
> talking about a pulse. If concept as risetime is used instead, then a
> suitable comparision to sine frequency can be made.
>
> A similar thing is found in ESD, where the finger has a risetime of
> about 0,7-1 ns... to say 8 kV, then the hand is discharged and the
> inductor (arm) finally conducts it will sent a second pulse as it
> discharges the body capacitance. The second pulse has the energy, the
> first pulse has the risetime. The hand pulse can act as a leader to the
> main bolt.
>
> The return stroke of a ligthning has about 200 ns of risetime, can have
> current derivates peaking at 100 GA/s (induction!) and a charge transfer
> at 20 Coloumb. After the main dump a longer period having several
> hundred amps gets conducted in the ionised channel as the cloud
> redistribute charge in more conduction rather than inductive manner.
> Peak current can be some 20 kA.
>
> The spark-gap (or com-gap) goes down to about 3 ns trigger time, unless
> you salt them with some ionating radiation in which they is about 1 ns
> of trigger time. It is prefered to have balanced ones, as one side can
> trigger the other... which is a good thing since if you use two single
> com-gaps and only one triggers you have just converted the jolt from
> common mode to diffrential mode... which defeats some of the
> protections. A problem with spark-gaps is that they may not be triggered
> since the rising edge is too slow. Thus, one wants to ensure that there
> is a low-inductance path through the spark-gap so that the bolt takes
> that path and also triggers it.
>
> One has to recall that protection goes in stages in which over-voltage
> and over-currents gets smaller and smaller.
>
> I haven't looked at any modern research on this, so I guess people have
> learned more. So the above may be outdated, but it should be enougth to
> give a ballpark hint on the properties.
>
> The main difference between ESD and ligthning is the scale of things.
>
> Trying to use a flask-brush to emit ions towards the clouds in hope to
> de-charge them in advance is meaningless if done on a single tower.
> Clouds typically move quickly on thunderstorms so you need to put alot
> of brushed up in a rather large circumference around whatever is
> sensitive stuff in order to have a main effect. I'm a sceptic here.
> The leader goes down from the clouds and meet the streamer from the
> ground... the comes the return strike. A brush or ligthning rod just
> aids in selecting where the streamer from ground goes up and thus where
> the return strike hits.
>
> I can't grasp how the brush-method significantly changes that basic
> behaviour.
>
> Cheers,
> Magnus
>
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-- 
Dr. Don Latham AJ7LL
Six Mile Systems LLP
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