because lightning is not a 'small' event. when you think about lightning (and
also rf related 'grounding') you have to throw away the concepts
of resistors, capacitor, inductors, and anything else you can hold in your
hand... even an 8' ground rod is so small that it is basically insignificant
when talking about lightning. to fully appreciate lightning you have to look
at it from the cloud's perspective. the cloud is a BIG generator, similar to
how
a van de graff generator works except on the order of 25-30,000' tall. it
normally separates several coulombs of charge so that the negative charge
is near the bottom of the cloud and the positive is near the top (yes, it can
go the other way also but this is most common). while it is doing this
the large negative charge on the bottom of the cloud is forcing electrons on
the ground maybe 10,000' away to run away over a fairly large area,
the affected area on the ground may be a mile or more across and is not
becoming positively charged. Now is when you start to hear corona
static on high antennas as that charge migration has left everything in the
area positively charged and the higher and more conductive it is the
higher the electric field gets so your antennas start to go into corona... the
ground field at this point can be 25-30kv/m if I remember right. note, the
lightning
hasn't started yet!
at some point, and this is not well understood, some charge concentrates enough
in the cloud (yes, it can start on the ground also but is rarer) that it breaks
down the air and gets pushed down from the base of the cloud as an ionized
channel, by 150-300'... then it stops... charge from the cloud now flows down
that ionized channel until the electric field builds up enough then it randomly
jumps another 150-300'... then stops... this continues in those short steps
heading
toward the positively charged ground. remember this is starting 10,000' up,
looking down at your maybe 100' tower it is insignificant, and won't affect
the path or anything else until the leader is only a few hundred feet from the
ground. as this leader heads down it pushes more electrons away on the ground
and the positive charge increases on things closer to the bottom of the
leader... now any insulators on your guy wires are arcing, your hair stands on
end,
you see st elmo's fire from antennas, trees, your house, etc... TIME TO DUCK!
that st elmo's fire is the beginning of an upward streamer that when it
gets enough charge will make the final 150-300' ionized channel to connect to
the downward leader. but before that happens lets take a look at the
conditions on the ground and in the leader.
the downward moving leader has now accumulated negative charge all along its
length as it gets pushed down from the cloud by the even bigger
charge up there. it may contain 5 to 10 coulombs of charge for an average
stroke. on the ground this negative charge has pushed away electrons over
an area of thousands of feet or more leaving an essentially equal positive
charge concentrated under it but extending for quite a distance. if you
measured the
voltage from the top of a 100' tower to some point a long way away you could
measure potentially millions of volts. at this point electric fields near the
tower top, tree tops, house peak, etc could be on the order of 100-500kv/m or
more and everything is lighting up... remember, at this point you have not
seen the lightning yet, special cameras can capture it and make amazing high
speed images of the leader moving down.
NOW the streamer from something on the ground meets the downward moving leader
and the circuit is complete. so now those electrons that have
accumulated in that leader channel (now maybe 10,000' long and very conductive)
drain out of the channel and down your tower. this happens at near
the speed of light... the leader only grows at about 1/3c up to this point. so
you have 10,000' of a charged conductor suddenly with a connection to drain
to... that takes something like 33 micro-seconds to happen, but the way the
charge is distributed with more closer to the ground and less up near the cloud
because it keeps trying to get away from that big charged mass up there, the
peak current is in the first 2 microseconds or so. now remember, everything
above ground has been charged positively and this negative charge is suddenly
running in at a speed of about 300' per microsecond, it gets down to the
ground and spreads out at that same speed or less as now the charge is
spreading out into an ever larger area... now what does that ground at the tower
base do? well, it takes some of that downward moving charge and matches it up
with the positive charge that is really mostly near the surface because
it was being attracted upward towards the cloud, sure, some has migrated up
from below ground but that is a much slower process. think of the stories
of what happens to people clustered around a big tree when the tree gets hit by
lightning. people standing on the roots have been killed because the
charge came up out of the ground and through their shoes. side flashes from
trees or buildings also happen and can kill. anything above ground is
positively charged and that negative charge wants to get to it. voltages at
the base of a struck tower to the far earth can still be 250kv or more at the
peak
current even with a 'good' ground.
Now, how do your little wires react to this??? well, coming down the tower
all the unshielded wires from rotors or switches are going to see that huge
voltage change from positive to negative. but since they are relatively small
and close to the tower that is conducting most of the current the induced
voltages in them will make it possible for mov's in switches or the small air
gaps in connectors to pretty well equalize the voltage near the top, near the
bottom it may be harder where the wires move away from the tower as the current
in them won't have the tower in parallel with it... this is why lightning
arresters near the bottom of a tower with a long cable run can take more
stress, its the charge on the insulated cables that needs to be neutralized. as
the charge continues to spread out in all directions it will travel a bit
faster and more concentrated on good conductors. on buried conductors it will
be
a bit slower, maybe only .5c. at this point the current into the ground rod(s)
is ionizing the soil and spreading out also, being in this area is like being
under a tree, the ground voltage can change so much over a short distance that
it can come up and get you. when the negative charge reaches your spg
panel the mov's and other devices equalize the voltage between the conductors
from the tower and the power lines so that they all stay at near the same
voltage which prevents damage to your equipment.
However, if you don't have a proper spg what happens... negative current
flowing on the outside of the coax shield comes in and gets to your radio. from
the radio there is a nice positively charged power wire that goes through the
house to the service entrance and then out to the pole... remember, an area
of thousands of feet in diameter has been positively charged, including the
power line, the power line ground wires, the phone line, the cable tv line,
etc, and everything connected to them in your house... now you have introduced
that negative charge via coax, rotor cables, switch cables, etc... so it jumps
through
your equipment to the power line destroying the radio, rotor controller, switch
controller, etc, in the process and then proceeds to everything else in the
house and out the service entrance to all that charged power line beyond.
other devices in the house may or may not survive depending on how and where
they are connected to things outside and inside, as those who have experienced
a direct hit can testify its pretty random at that point.
your house.
Jan 13, 2016 07:45:50 PM, w3yy@cox.net wrote:
The latest posts about grounding, and finally some free time here, prompt me
to ask the following question.
Given lightning's desire to find the quickest way to ground, why doesn't it
expend itself in a single 8ft ground rod at the base of a tower, rather than
passing through another 250ft of transmission and control lines (also buried
in the ground) leading to the shack? I would think that by then it has had
plenty of opportunity to arc to ground itself.
I am not disagreeing with the experts on this subject, but I just don't
fully understand what is commonly recommended. With only a single 8ft
ground rod at the base of my 100ft and 120ft towers which are about 100ft
and 250ft from my house, I have only suffered two minor damages from a
lightning strike in over 40 years. And, I'm not sure that even had anything
do with the towers, but was just an unrelated power line surge.
73, Bob - W3YY
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of EZ
Rhino
Sent: Wednesday, January 13, 2016 6:05 PM
To: Towertalk Reflector
Subject: Re: [TowerTalk] Grounds, 'remote' towers, 'house' power system
I'm not in disagreement with you Jim, but then why doesn't NEC specify to do
things for lightning protection such as commonly followed by nearly all
commercial tower installations? Such as multiple ground rods, flat strap,
star grounds, etc? (Think Polyphaser's docs). We know that one ground rod
is woefully inadequate for a direct hit. If NEC is all about lightning, why
doesn't is specify using more than one? It sure seems like NEC is about the
bare minimum for AC protection and when it comes to RF and towers, it
doesn't really give much pertinent info at all.
Chris
KF7P
On Jan 13, 2016, at 15:49 , Jim Brown wrote:
On Wed,1/13/2016 2:35 PM, N3AE wrote:
The NEC is focused on electrical safety and not necessarily the most
effective system for lightning protection.
This is NOT true. The bonding required between your tower and power system
sub-panel is for LIGHTNING protection.
In general, proper bonding is critical for lightning protection, electrical
safety, fire safety, and to minimize hum, buzz, and RFI. Proper bonding is
described in
http://k9yc.com/GroundingAndAudio.pdf
I'm not going to repeat it here for those too lazy to study it.
BTW -- I TAUGHT courses on Power and Grounding for about ten years.
73, Jim K9YC
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