[TowerTalk] LIGHTENING--Ground or No Ground?

Fri May 28 15:36:55 PDT 2010

Some prominent and knowledgeable hams in the South have been arguing for
years that it is better to have NO GROUND at all at your station (other
than the electrical ground provided by the power company in your house's
electrical system), than to have a number of different grounds around
the station, tower, and house.

    One of them is a retired engineer for a telephone company and he
recites the story that, years ago, after any major electrical storm, the
phone company would lose a number of electronics boxes on telephone
poles--sometimes as many as 25-30 in a small town after a storm.  The
boxes were well-grounded with copper wire from the case of the box to a
ground rod beside the telephone pole.  He persuaded the phone company to
UN-GROUND all the boxes and they lost very few thereafter.

    The above advice cuts against the grain of everything I have ever
read from the ARRL and elsewhere.  Any comments?

				Dale   K4ROZ

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Today's Topics:

   1. lightning path vs current path (jimlux)
   2. Re: Lightning suppression through coax loops (Roger (K8RI))

----------------------------------------------------------------------

Message: 1
Date: Fri, 28 May 2010 08:08:06 -0700
From: jimlux <jimlux at earthlink.net>
Subject: [TowerTalk] lightning path vs current path
To: Towertalk Reflector <towertalk at contesting.com>
Message-ID: <4BFFDC56.6080504 at earthlink.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

This discussion of lightning, side flashes, etc, brings up some 
interesting points.. As a side note, about 15 years ago, I spent several

years making large (many meter) sparks follow predictable paths (or, at 
least trying to do so) as a theatrical effect.

It's important to distinguish between what Roger (K8RI) was describing, 
the lighting coming out the side of the tower and striking some distance

away, from the transient suppression/lightning protection thing, which 
is at short distances.

When talking about the large scale path that the lightning stroke takes,

that's pretty much all determined well before the kiloamp currents and 
big di/dt (for which inductance is important) even start.  The big 
currents are in the return stroke, which basically just follows back 
along the path established by the original leader.  The path the 
*leader* takes is pretty random, going in 50-100 meter jumps, generally 
following the electric field, but with a significant random component.

If you think about a leader propagating down from the clouds above, when

it reaches the top of the tower, it's still propagating, and there isn't

much current in the tower base yet, because the transient hasn't 
propagated there.  The transient front moves down the tower, but the 
circuit properties below the leading edge don't have a lot to do with 
what happens.. it's more about the local electric field, when the local 
field at the surface gets high enough to exceed breakdown.  A very 
smooth uniform tower will tend NOT to have the leader move away, because

there's no breakdown along the surface. A bumpy tower (or tree) will 
tend to branch off, because a localized field concentration can start 
the leader going a new direction.

The big current (and lightning damage) for which inductances are 
important, is the return stroke.  By then, the "circuit" is established,

and now, inductances are important.

------------------------------

Message: 2
Date: Fri, 28 May 2010 12:04:25 -0400
From: "Roger (K8RI)" <K8RI-on-TowerTalk at tm.net>
Subject: Re: [TowerTalk] Lightning suppression through coax loops
To: towertalk at contesting.com
Message-ID: <4BFFE989.3010005 at tm.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

On 5/28/2010 10:37 AM, jimlux wrote:
> Roger (K8RI) wrote:
>    
>> On 5/28/2010 1:39 AM, jimlux wrote:
>>      
>>> Uhhh... I'd kind of like to see a physics explanation of not like to
>>> travel in circles or bends and getting off and going somewhere else.
>>>
>>>
>>>        
>> Well, lightning likes to take the shortest path although that may not
be
>> a straight line
>>      
> A bit anthropomorphic isn't it..
>    
Wasn't intended to be.
> I think a fairer statement is that lightning is an electrical current,
> and it distributes according to the impedances.  "take shortest path"
> leads folks to believe that it ALL goes on the shortest path, and
> therein lies much trouble.
>
>    
Agreed
>    
>> Any conductor, straight, bent, circular, or crooked has inductance.
>> The rise time of a pulse creates a reverse EMF. The steeper the rise
>> time the greater the reverse EMF.
>>      
> Sure
>
>    
I forgot to add this is also dependent on the huge current within the 
stroke which may not be in the initial stroke.
>> A discontinuity in the coax such as a bend or coil increases the
>> inductance at that point.
>> In the case of a coil and a rapid rise time the shortest path may be
>> from turn to turn, to another cable, or to the tower rather than to
>> continue through the coil.
>>      
>
> Nope.. I think you have a non-physics definition of "shortest path"..
>
> What happens is this.. the voltage rises (because of the inductance)
and
> then, if and only if, that voltage is greater than the breakdown
> strength of the insulation, you get a breakdown to the adjacent
> conductor.  If we immersed your coil of coax in a tank of insulating
> oil, the "shortest path", mechanically, hasn't changed, but it won't
> breakdown the same as it would in air.
>
>    
Worded better than I did, but I think the potential between points can 
change rapidly which can also change the path
> Or, you could have coupling via the magnetic field (e.g. a transient
on
> the shield of one cable couples a transient to an adjacent cable)
>
>    
Again, worded much better than I did.
>> I've seen lightning strike the top of a tower and get off half or 3/4
of
>> the way down without following the guy wires. This was explained to
me
>> that the rise time created enough reverse EMF to make it easier for
the
>> lightning to get off the tower and jump to ground rather than proceed
>> straight ahead even though the tower provided a substantial
conductor.
>>      
> But what does "make it easier" mean here?  It's not like there's some
> resistance that changes as a function of rise time.  It's a breakdown
> phenomenon... if the voltage is above the breakdown voltage for the
gap,
> then it breaks down, an arc forms, and *now* you have a new path.
>
>    
True, although I'd still say it's due to the induced reverse EMF caused 
by the rise time but agree it takes less voltage to jump off the tower 
to the ground than overcome that reverse EMF.
> If you're talking about the stroke hitting the top and then coming out
> halfway down, and then forming another arc connecting to the ground,
> that's sort of a different matter.  That's more the chaotic nature of
> the breakdown in a very non-uniform field, and I doubt that small
> changes in inductance have much effect.  Lightning goes in jumps, with
> the prevailing theory going something like this..
>
>    
Quite possible.
> Just before a discrete jump, the spark channel is fairly conductive,
and
> current flows in the channel towards the end of the spark leader
> (because the surrounding electric field is oriented that way).
Agreed
>   As charge
> accumulates at the end of the leader, the voltage at the end starts to
> rise, increasing the Efield at the tip.. when the E field exceeds the
> breakdown strength of the air, a new section of channel starts to
form,
> in a pretty much random direction,
It certainly appears random, but a set of conditions must be favorable 
for the new leader to take that route.
>   although generally guided by the
> surrounding large scale field (i.e. sparks twist and turn, but
generally
> go in one direction).  That new channel grows as long as it can accept
> charge from the existing channel behind it.  The initial breakdown is
> fairly low current, and just ionizes the air, but once the ionization
is
> done, the resistance drops, so the current flow into the newly ionized
> area is high enough to heat the air, which reduces the resistance
more,
> etc.  It's sort of like connecting a piece of transmission line to the
> end of a longer piece, with the longer piece already having a high DC
> voltage on it.
>    
Agreed again.
> Same deal with hitting an object and jumping off.. current flows,
charge
> builds up, exceeds local breakdown strength, new spark leader forms,
etc.
>
>
>    
I think I said that, but again you have worded it much better.

Just woke up so I'll read more later, but I like your explanation better

than mine.

73

Roger (K8RI)
>
>    
>>> A loop has inductance. There will be some amount of voltage drop
across
>>> the inductance.  But a 1 turn loop doesn't have a heck of a lot of
>>> inductance (I think 6" diameter is like 0.5 uH)... about the same as
a
>>> half a meter of straight wire.
>>>
>>> Lightning has a rise time of about 1 microsecond, and peaks at say,
50kA
>>> (higher than most strokes, but makes the math easy).. so the voltage
>>> drop is  Ldi/dt is 0.5E-6*50E3/1E-6... about 25kV... that's big, but
not
>>> huge.. not going to leap feet, in any case.  Maybe an inch or so.
>>> Certainly would punch through the jacket on most coax and arc to any
>>> touching metal.  (hence the electrical code requirements to space
>>> conductors that might carry lightning away from other ones)
>>>
>>>
>>>        
>> Yet I've seen lightning bolts in both photos and in person, that
jumped
>> many feet, or rather 50 to 60 yards when they had only 50 to 100 foot
of
>> tower between them and ground.
>>      
>
> Yes.. but that's not because of a few microhenries of inductance.  The
> inductance of the stroke itself is probably greater than that of the
> tower.  It's all in the
> "stop/charge/jump/extend/stop/charge/jump/extend" stepwise extension
> process.. that "jump" phase when it starts a new step is essentially
random.
>
>
>
>    
>> The energy in lightning bolts is many times the stuff we were working
>> with, so I can understand the induced and reverse EMF values being
>> enough to jump quite a ways.
>>      
>
> Sure, the currents are high, but even with fairly high inductance, it
> can't get enough voltage to jump large distances.  To get meter scale
> jumps you need hundreds of kV AND fairly nonuniform electric fields.
>
>
>
>
>    
>> What do you get for a square steel tower, 2' on a side? Yet the
>> inductance was enough to cause the bolt to get off the tower and jump
>> sideways.
>>      
>
> fairly low.. probably half uH per meter.  The jumping off is likely
due
> to other factors than the inductance...
>
> That is, you could hit that same tower 100 times with the same
impulse,
> and a few of them will jump off one direction, a few will jump off
> another, and most will just go down the tower.
>
>
> It's like a big tesla coil or Van deGraaff generator.. you know there
> will be sparks, you know "generally" where they will go, but you can't
> predict the exact path.
>
> (the difference is that a Tesla Coil or Van deGraaff has very
different
> rise time properties: the energy source is much lower inductance than
> lightning, the rise time is very much faster, so the steps/branches in
> the spark development are at a finer scale.. current flows through the
> stepped leader to the new tip much quicker.)
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>
>
>
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