[TowerTalk] The Need for Grounding

Thu Jan 14 10:51:04 EST 2016

It's a bit more than semantics.  Neutral is where negative and positive 
charges are equal.  It matters not, if you remove electrons or add a 
positive charge, the area becomes positively charges in relation to that 
around it.

73

Roger  (K8RI)

On 1/14/2016 Thursday 8:49 AM, David Robbins wrote:
> 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 at 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 at 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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-- 

73

Roger (K8RI)

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