sporkme
drop the crantini and move it, sister
Premium,MVM
join:2000-07-01
Morristown, NJ
 ·Optimum Online
| reply to DaDogs
Re: Will burying a feed line ground the CPE?
said by DaDogs  :Yep, lightening is some crazy stuff. That current was INDUCED current, and there was plenty of it too. People I think tend to think too often of lightning "strikes". An actual hit to your structure or equipment is probably MORE rare than having an induced current cause damage. It's a hard concept to wrap your head around, but there's a few examples that I can think of:
-a transformer, even a lowly wall-wart uses induced current to step down the 120V to 12V. The 120V side DOES NOT have conductivity to the 12V side. You simply have two coils of wire wrapped together in a specific ratio for a sort of "controlled inductance".
-you can string a length of wire underneath a high-voltage transmission line (the sort that have the big metal towers), and if you run it parallel to the above power lines, you can measure a current in that wire that is simply lying parallel to the high voltage line. Again, no direct physical connection.
Chew on that and then think about a lightning strike within a few hundred feet of some equipment with a cable run. What do you suppose happens in that cable when the strike occurs? It's not a direct hit, but nearby there is a surge of voltage/amperage that is more powerful than anything we can replicate for a brief moment. I don't even know how much juice a lightning strike is, but let's say that your cabling gets even 1/10000th of that power induced in it... Boom! |
|
public
join:2002-01-19
Santa Clara, CA
 ·DSL EXTREME
| reply to wifi4milez
said by wifi4milez :If I were to set up an AP in a house, and wanted to mount the antenna outside would this create a lot of grounding issues? I was thinking about mounting an omni on a pole or large tree 75 feet or so from the house. My idea was to simply bury the cable that runs from the antenna and then bring it up right near the house (I am assuming this would ground it??). Is this possible or would lighting stay in the cable and enter the house, thus frying the AP and starting a fire?  What you propose is very bad, but it can be improved. First of all use an air terminal above the omni to prevent a strike to the omni. Mount the terminal on your tower, and groung the tower with three 8 foot rods. install a surge suppressor at the antenna and at the house entrance. Same thing if you use poe cat5. |
|
public
join:2002-01-19
Santa Clara, CA
·DSL EXTREME
| reply to sporkme
said by sporkme : I don't even know how much juice a lightning strike is, but let's say that your cabling gets even 1/10000th of that power induced in it... Boom! Average lightning current is 30000A (30kA) with rise time about 10 us and duration 300 ms. You can assume unlimited voltage. |
|
IntraLink
Premium,MVM
join:2002-08-14
Utah Valley
| reply to wifi4milez
I four years we haven't had very little damage from lightning because of two things I think:
1. Make sure there is a short to ground around your equipment so the potential does not build up and attract lightning/electricity. In case of an incident this same grounding can create a "safer" path for the energy to travel.
2. Make sure you have arrestors at least at the entry point of the wire into the building. This helps de-couple the current during an incident.
Essentially proper grounding should make your equipment invisible to the bright hand of lightning. But that won't help when it strikes nearby and creates a "cloud" of engery penetrating objects at mysterious places, creating large amounts of inducted amps/voltage. That is why you should have multiple surge arrestors and lightning suppresors. |
|
nboxmedia
join:2004-07-10
Mount Olive, NC
| reply to wifi4milez
Let's say I have my APs and antennas together at the top of a 175-foot tower, just below the very top of the tower. This tower is constructed of steel and various metal. I have my antennas just below the tip-top in hopes that lightning will strike the top of the tower and not my equipment. Is this smart?
What other precautions should I take lightning and grounding-wise to help out? |
|
IntraLink
Premium,MVM
join:2002-08-14
Utah Valley
| If the tower is grounded properly the only thing you need to worry about is making sure anything that runs from the tower to another structure is on the same grounding potential.
Theoretically the tower itself is the shortest path to ground to drain the potential and current from a strike.
But lightning is not predictable and you should make sure you have arrestors at the antenna and at the entry into your equipment shed that are also grounded to the SAME potential.
In short:
If you look at your tower and don't see a ton of thick wires/cables going from the feet into the ground or into grounding rods then you have something to worry about.
If you look in your equipment shed and don't have lots of bare copper wire running around the ceiling and floor that is all connected to each other then you have something to worry about. |
|
John Galt
Forward, March
Premium
join:2004-09-30
Happy Camp
·CenturyLink
| reply to public
said by public : --diagram-- This diagram is inaccurate...for several reasons.
It shows two ground rods. These ground rods MUST be bonded together with a #6 or larger wire.
It also fails to show that those ground rods MUST be bonded to the main electrical service entrance grounding electrode (most likely a ground rod, but could be a copper water pipe or concrete-encased rebar in the footing, or building steel)with a #6 or larger wire.
If this diagram had these elements included, it would be essentially correct.
--
A is A |
|
Beebe
join:2002-10-25
Beebe, AR
| reply to sporkme
I remember something from physics class. It takes twenty thousand volts for electricity to jump one centimeter through the air. The reason static doesn't kill you is that although it's loads of volts, it's not many amps so it won't do much damage.
How many centimeters does a lightning bolt travel through air? Thousands. So the voltage is millions of volts - again, not a whole lot of amperage, but a whole lot more wattage than a typical static shock. |
|
public
join:2002-01-19
Santa Clara, CA
·DSL EXTREME
2 edits | said by Beebe :How many centimeters does a lightning bolt travel through air? Thousands. So the voltage is millions of volts - again, not a whole lot of amperage, but a whole lot more wattage than a typical static shock. 30kA on average is a fairly high current. Even the magnetic forces can break small gauge wires, arc and start fires.
Typical air breakdown is 1kV/mm, so that you need 10kV for 1 cm gap.
said by IntraLink :I four years we haven't had very little damage from lightning because of two things I think: 1. Make sure there is a short to ground around your equipment so the potential does not build up and attract lightning/electricity. In case of an incident this same grounding can create a "safer" path for the energy to travel. Never assume that anything you do will prevent lightning.
You can only redirect the lightning current to ground when a strike occurs. |
|
public
join:2002-01-19
Santa Clara, CA
·DSL EXTREME
| reply to nboxmedia
said by nboxmedia :Let's say I have my APs and antennas together at the top of a 175-foot tower, just below the very top of the tower. This tower is constructed of steel and various metal. I have my antennas just below the tip-top in hopes that lightning will strike the top of the tower and not my equipment. Is this smart? What other precautions should I take lightning and grounding-wise to help out? Using the tower top as a lightning air terminal is good practice. You should also measure your ground resistances and inductances so that you can calculate voltage differences your equipment will experience during the strike. |
|
public
join:2002-01-19
Santa Clara, CA
·DSL EXTREME
1 edit | reply to bito
said by bito :Next to a beer, this is one of the best $3 you can spend: » www.radioshack.com/product.asp?c···=61-2144I was in the shop one night last month when we took a direct strike to our omni rig on the roof. It instantly fused the POS arrestor we had on our coax run and blew the radio, but it wasn't done yet...the whole damn thing when critical and the cables at the AP EXPLODED. Also, it blew the top off the omni and VAPORIZED a full foot of the metal element inside. It blew the damn connectors off of the omni side of the LMR. Blown off, as in loosened and then blown off and found on the ground. Needless to say, the next day is when we started looking for better protection  Moral of the story: Take as many precautions as you can when it comes to lightning protection. Lightning is some crazy sh!t, and it hates your equipment  Caleb This is a graphic example of a design error. The omni should have been protected by an air terminal. Most of the current would then be diverted into ground instead into the equipment inside.
It is incomprehensible why some operators in lightning prone areas mount an omni at the very top and thus use it as a lightning air terminal.
With a little foresight path of a lightning discharge when a strike occurs can be fairly predictable. Just allow for 10-150kA current, with 30kA average for up to 100ms. |
|
bito
Premium
join:2001-10-08
Atlanta, GA
| No kidding. Hindsight is wonderful though. It should have been ok (at least for the equipment), had our inline arrestor not fused shut and instead of diverting to ground it continued to zip through the coax. It was an old piece of junk evidently. |
|
public
join:2002-01-19
Santa Clara, CA
·DSL EXTREME
| said by bito :No kidding. Hindsight is wonderful though. It should have been ok (at least for the equipment), had our inline arrestor not fused shut and instead of diverting to ground it continued to zip through the coax. It was an old piece of junk evidently. What was used probably was a surge suppressor, not a lightning arrestor. The only real arrestor used on power lines is a large spark gap.
Other equipment or structures need to be protected with a grounded air terminal for the direct strike, and surge suppressors for voltage rise effects due to the lightning current in the main protection circuit. |
|
