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eHam Forums => Station Building => Topic started by: KI4HNN on January 22, 2012, 08:59:03 PM



Title: Station Grounding / Lighting Protection
Post by: KI4HNN on January 22, 2012, 08:59:03 PM
I know that subject has been beaten to death but I really need some help getting my equipment setup. 

I was wondering if anyone could offer a suggestion as i am trying to get my equipment setup but just don't have all the details that I need.  I currently have a fold-over mast buried into the ground 4feet that I would like to install a HF dipole antenna on, a vhf/uhf antenna and a scanner antenna on at the current moment.  I would like to add to this later and install a small tower.  What I have in mind is the following and would like to get your opion on this.  At the base of the tower i would like to install a box on a pole and run all my coax wire into this making this my single point ground for my feedlines.  inside of here I would like to install a copper piece of angle, drill holes in it and attach bulkhead adapters.  The angle will be bolted to a piece of copper strip about 2 inches wide and about say 1 to 2 feet long.  This would be then attached to a ground rod via a heavy guage wire.  Would this be ok for lighting protection?  From this box and single point ground location I would then like to run coax from this to a window feedthough panel.  The window feedthrough panel will have bulkhead adapters on it to attach the coax from the grounding box and then short jumpers will connect to the station equipment. To ground the station equipment will be grounded to the outside of the room via a feedthrough insulator attached to a ground rod.  My question is does this setup sound like a good idea, i would like to keep things simple if possible.  I do have one other question, i would I go about grounding 450ohm feedline through the grounding panel?  In the event of a lighting storm, i always  lower my dipole anyway but just wondering? If possible any pictures would be great!! I am more of a visual learner. 


Thanks,

Joey

KI4HNN


Title: RE: Station Grounding / Lighting Protection
Post by: AD4U on January 23, 2012, 06:19:12 AM
Joey, I am sure there are people on this site who will adamantly disagree with me, but there is nothing that the typical HAM can do ($$$) as far as grounding is concerned that will GUARANTEE that lightning will not "hit" your antenna or if it does that you will not have any damage to the antenna and / or to your equipment.  

IMO the only way to guarantee that lightning will not damage your equipment is to disconnect it from EVERYTHING when not in use .  Disconnect it from the AC line, the antenna feedline, and even from the station ground.

I suggest that you google "polyphaser" and read what they have to say about grounding.  They have more to say about grounding than anybody could ever cover in this forum.

The problem in most areas of the US is that soil conductivity is very poor.  This is because over the eons rainwater has washed away most of the minerals in the soil.  Plain dirt (even damp dirt) and rain water are poor conductors of electricity.  That means that even though you have a ground rod at the base of your mast driven into typical poor soil in the US, where does the lightning go if it hits your antenna?

About the best thing a typical HAM can do (in addition to what you have already done) is to run a heavy gauge (#6 or bigger) copper wire in a straight line from the base of your antenna mast and ground rod to the ground rod where the electric power meter attaches to your house.   Use the proper connectors from the ground rods to the #6 copper wire.  Don't skimp here.

Dick  AD4U


Title: RE: Station Grounding / Lighting Protection
Post by: KH6AQ on January 23, 2012, 07:31:00 AM
Your proposed system provides a path for lighting charge through the house AC wiring.

This can be avoided by running the coaxial lines to the AC power ground first and then to the shack. See the ARRL antenna book for how to povide protection for the ladder line. It is ok to ground the lines between the antenna and the AC power ground.


Title: RE: Station Grounding / Lighting Protection
Post by: W6RMK on January 23, 2012, 07:53:29 AM


About the best thing a typical HAM can do (in addition to what you have already done) is to run a heavy gauge (#6 or bigger) copper wire in a straight line from the base of your antenna mast and ground rod to the ground rod where the electric power meter attaches to your house.   This is a requirement by the NEC.  Use the proper connectors from the ground rods to the #6 copper wire.  Don't skimp here.


I'm not sure NEC actually requires from "base of antenna" to service entrance.  It requires bonding of the coax shield "from point of entry to the building" to the system grounding point.

for what it's worth, many houses these days don't have a ground rod.  They have a "Concrete Encased Grounding Electrode" (aka Ufer ground), which addresses a lot of the issues raised by AD4U about poor soil properties (concrete is uniform, holds moisture, is a good conductor, etc, and has a large contact area with the surrounding soil, compared to a skinny rod shoved into the dry dirt)

In any case, there will be somewhere near the service entrance that is the place to which you bond (using that AWG6 wire) your cable shields, protectors, etc.


Title: RE: Station Grounding / Lighting Protection
Post by: K0ZN on January 23, 2012, 12:04:49 PM

Hi, Joey.

This is a common question, and if you look back into the archives on here you will find TONS of comments and guidelines. As we get into
spring, this becomes an even more common topic.

If you live in an area with lightning, it is well worth your time and money to put some effort into protection and it sounds like you are
aware of that, which is good.  Over a period of time the odds are pretty high your set up will get hit. There is a lot of data to support this.
The name of the game is to keep the energy off your rig and out of your house wiring, electronics and appliances, etc.
The energy IS going to ground....the only question is HOW it get there. By doing your homework, you can reduce the risk of damage a lot.

FYI:  One ground rod is nowhere near enough. You need MANY ground rods in an effective system.

For whatever it is worth:  I live in an area with extremely strong storms and heavy lightning during the spring severe weather season. Lightning protection
is not an option; you are begging for thousands of dollars in damage without it !~ 

MY program.....
doesn't mean it is right or better.... just what I do.... is DISCONNECT everything OUTDOORS and ground it OUTDOORS to an extensive ground system.
Yes, it is kind of a pain to do.

I the summer and spring, my "normal" mode is disconnected and grounded. I hook the antennas up to get on the air. I have seen what severe lightning
can do, and I am probably a little more paranoid that some other people. I have real "religion" when it comes to lightning.
When those house shaking thunder claps are hitting nearby, I just feel a LOT better knowing that my rig and antennas are totally and completely separated.
I have 14 rods in my ground system connected with AWG # 2. Even then, that # 2 wire seems awfully SMALL when you see some really heavy strikes hit.


73,  K0ZN


Title: RE: Station Grounding / Lighting Protection
Post by: K9KJM on January 23, 2012, 10:59:26 PM
The advice from AD4U to "disconnect" is not only bad, But very DANGEROUS!   Simply "disconnecting" equipment without providing a good ground to the disconnected lines is an invitation to disaster!

Commercial radio stations, Repeaters, Police, Fire, Cellular towers, etc, Along with many ham radio towers including mine take DIRECT lightning strikes most every storm with no damage to equipment. None of them "disconnect" anything.

My interpretation of what you propose to do sounds pretty good overall, But remember, Your "Single Point" ground panel should be somewhere near where your coax feedlines enter the building.  Your window panel entrance would be an ideal location for it.
At your mast, All that is important is that the shield of the coax be grounded at that point. No need for fancy plates, etc.
The most important point of lightning protection is to properly BOND all grounds together.
#6 copper is mentioned lots of times as a good conductor, That is the MINIMUM size to use.....  A lower cost better choice can be some soft copper tubing sold in rolls that is much larger in diameter.  Flat copper strap as used for roof flashing of about .022" or so thick also works well.
Whatever you use, Make sure there are no sharp bends or kinks in it.

For some tips on how to do it on a budget:
 
http://www.scribd.com/anon-849269/d/14868226-lightning-protectiontaming-thors-thunderon-a-budget

(Give that site plenty of time to load)


Title: RE: Station Grounding / Lighting Protection
Post by: K1CJS on January 24, 2012, 06:21:41 AM
Quote
Your proposed system provides a path for lighting charge through the house AC wiring.

This can be avoided by running the coaxial lines to the AC power ground first and then to the shack. See the ARRL antenna book for how to povide protection for the ladder line. It is ok to ground the lines between the antenna and the AC power ground.

If the ground rods are bonded with #6 grounding cable, that is the path that a lightning charge would take.  If you take a direct strike, you would have damage inside your house from the effects of the strike no matter what you may do.  The amount of voltage in a typical lightning bolt is going to do damage--unless you have a complete professionally done grounding system in place, because those sites that some claim to have taken a direct hit and still function has exactly those type grounding systems in place.  You would be surprised at how many steel water well casings are sunk for ground systems--and not for wells.


Title: RE: Station Grounding / Lighting Protection
Post by: W6RMK on January 25, 2012, 02:41:49 PM


FYI:  One ground rod is nowhere near enough. You need MANY ground rods in an effective system.


Actually, ground *rods* are pretty ineffective, no matter how many you have. They just don't have much contact area with the surrounding soil.  A ground ring or a concrete encased grounding electrode are MUCH better, and probably easier to install.  There's a reason Herb Ufer developed better grounding techniques than those pioneered by Ben Franklin a century and a half before.  And there's a reason why a lot of jurisdictions don't allow rods to be sole grounding means anyway.

The name of the game in energy spreading and dissipation is area and volume. So burying a AWG6 wire that's 100 feet long around your house is much better than driving a dozen 8 foot rods, which you'd then have to connect with AWG6 anyway. I'd rather dig a trench and fill it with concrete and a suitable wire than drive a pile of rods and fool with OneShots or clamps.

As others have pointed out, it's more about paying attention to "keep everything at the same voltage".  There's always going to be a voltage rise, because of inductance, which you can't do much about, so make sure everything rises together.


Title: RE: Station Grounding / Lighting Protection
Post by: K9KJM on January 25, 2012, 11:16:16 PM


FYI:  One ground rod is nowhere near enough. You need MANY ground rods in an effective system.


Actually, ground *rods* are pretty ineffective, no matter how many you have. They just don't have much contact area with the surrounding soil.  A ground ring or a concrete encased grounding electrode are MUCH better, and probably easier to install.  There's a reason Herb Ufer developed better grounding techniques than those pioneered by Ben Franklin a century and a half before.  And there's a reason why a lot of jurisdictions don't allow rods to be sole grounding means anyway.

The name of the game in energy spreading and dissipation is area and volume. So burying a AWG6 wire that's 100 feet long around your house is much better than driving a dozen 8 foot rods, which you'd then have to connect with AWG6 anyway. I'd rather dig a trench and fill it with concrete and a suitable wire than drive a pile of rods and fool with OneShots or clamps.

As others have pointed out, it's more about paying attention to "keep everything at the same voltage".  There's always going to be a voltage rise, because of inductance, which you can't do much about, so make sure everything rises together.

All true.   Simple facts that lots of hams have a hard time understanding.  The "UFer" ground IS one of the very best ways to do it..........

A rule of thumb to remember, Ground rods should be spaced about twice the distance apart as the depth. (8 foot deep rods spaced about 16 feet apart)
And the good bonding off all grounds IS the most important part.

http://www.psihq.com/iread/ufergrnd.htm


Title: RE: Station Grounding / Lighting Protection
Post by: KH6AQ on January 26, 2012, 08:40:50 AM
Quote
Your proposed system provides a path for lighting charge through the house AC wiring.

This can be avoided by running the coaxial lines to the AC power ground first and then to the shack. See the ARRL antenna book for how to povide protection for the ladder line. It is ok to ground the lines between the antenna and the AC power ground.

If the ground rods are bonded with #6 grounding cable, that is the path that a lightning charge would take.  If you take a direct strike, you would have damage inside your house from the effects of the strike no matter what you may do.  The amount of voltage in a typical lightning bolt is going to do damage--unless you have a complete professionally done grounding system in place, because those sites that some claim to have taken a direct hit and still function has exactly those type grounding systems in place.  You would be surprised at how many steel water well casings are sunk for ground systems--and not for wells.

If the coaxial cables are routed to the shack instead of being routed to the AC service ground first, and then to the shack with a #6 wire provided from the shack to the AC service ground, we have two paths for the lightning current to the AC service ground: thru the house wiring and thru the #6 wire. The house wiring from the shack to the AC service ground consists of three #12 wires (ground, neutral and line) which together are equivalent to a single #7 wire. When hit by a 300k amp lightning strike there will be 130k amps thru the house wiring and 170k amps thru the #6 wire.

To break the path thru the house we can route the coaxial cables to the AC ground then to the shack (we are free to add all the ground rods we want between the antenna/coax and the AC service ground). Now the house is not in a ground loop but is hung off the side of one; hence no lightning current thru the house.


Title: RE: Station Grounding / Lighting Protection
Post by: K1CJS on January 27, 2012, 05:11:36 AM
If the lightning strike isn't direct, static charges will take the easiest path to ground--the #6 cable.  If the lightning strike IS direct, no amount of trying to keep the charges on one single ground point is going to help.  There WILL BE damage done one way or another, even if all the grounds are "hung off to one side.".


Title: RE: Station Grounding / Lighting Protection
Post by: AD4U on January 27, 2012, 06:36:09 AM
The advice from AD4U to "disconnect" is not only bad, But very DANGEROUS!   Simply "disconnecting" equipment without providing a good ground to the disconnected lines is an invitation to disaster!

My reply to disconnect the rig from EVERYTHING was aimed at protecting the original poster's rig and the eqpuipment inside the shack.  I did not try to explain about the danger of ungrounded (open) feedlines dangling inside the shack. 

That is why in my original post I suggested he visit the POLYPHASER site and spend a day reading for additional info.

In the limited space provided here, NOBODY can adequately cover the subject of grounding.  Commercial installations are hit by lightning and they are damaged by lightning very often.  Many of these damaged systems had very expensive grounding systems - much better grounding systems than in the typical HAM installation.  I know.  I am "responsible" for many such sites.  We have spent (and we continue to spend) many tens of thousands of dollars on grounding, and we have lightning damage every year.

IMO a magazine on a table or a coffee cup inside a kitchen cabinet has just as much chance of being "hit" by lighting as does a HAM radio inside the same house, if the HAM radio has been disconnected from EVERYTHING (AC power line, antenna feedline, station "ground").  Yes if the antenna feedline is disconnected from the rig inside the shack it should be securely "tied" to the station ground.

Does lightning hate HAM radios so much that it crashes through the roof and ignores the magazine and the coffee cup and seeks out the HAM radio to damage it.

I don't think so.

Dick  AD4U


Title: RE: Station Grounding / Lighting Protection
Post by: KG7MWW on January 30, 2012, 05:35:03 PM
Lots of opinions on this subject.  So for my installation, my antenna mast system's base plate is about 12' from my electrical panel.  Should I even bother driving a 8' ground rod near the mast base, or just connect #6 wire or larger, directly from the mast base to the ground wire coming up from the slab, in what appears to be a UFer, in concrete slab, electrical ground at my house?  My station is located about 80' away from the antenn base plate, so should I just use #6 wire or flexible copper pipe, to create another ground point there for all the station grounds, or do that plus drive 8' ground rods every 16' from the antenna base to the ham station entry point?  I also bought a lightning arrestor for the base of my antenna system that I had planned to connect to the ground rod at the antenna mast base, in order to protect the coax shields.  Should I still do this or is just a bulkhead connection connected to the ground run going to the electrical panel ground sufficient?  If ground rods aren't good due to low surface area, what is best if the station is located a good distance from the electrical panel ground?  I don't think a trench with wire and concrete to create a ground loop will be feasible where I am, so what is the next best option?

My situation is also somewhat unique in that my system is "temporary" where the antenna is raised and lowered and dis-assembled in sections when not in use to avoid HOA issues (Yea its a pain, but you gotta do what you gotta due to get on the air).  So assuming I don't operate when a rare lightning storm is approaching, do I just ground the antenna mast base to keep stray static voltage down per my antenna's installation guide (cushcraft R5)?  Would a simple ground rod outside the wall of where the station is be sufficient for a temporary setup, or does running the #6 wire to the electrical panel ground rule still apply independant of whether the antenna setup is temporary or not?


Title: RE: Station Grounding / Lighting Protection
Post by: K9KJM on January 30, 2012, 10:38:19 PM
Each part of a system should go to it's own ground rod(s) FIRST, Then be bonded with the other grounds, So Yes, You should drive at least one ground rod at the base of the mast.
(It is very difficult to pull out installed ground rods. But at only about 10 bucks each for a commercial rated 1/2 or 5/8" X 8 foot long heavy copperclad at most discount home supply stores, Just remove the clamp and wire and drive the rod deeper and leave it in place if you have to move.)

Installing a lightning "arrestor" is actually about the very least important part of a lightning protection system.  Good bonding of all grounds (Mast, Telco, Electric entrance Catv, etc) is the most important part.

Some tips on doing it on a low budget:
http://www.scribd.com/anon-849269/d/14868226-lightning-protectiontaming-thors-thunderon-a-budget
(Give that site plenty of time to load)


Title: RE: Station Grounding / Lighting Protection
Post by: K1CJS on January 31, 2012, 11:43:47 AM
Each part of a system should go to it's own ground rod(s) FIRST, Then be bonded with the other grounds, So Yes, You should drive at least one ground rod at the base of the mast....

Yes!  That is the way to do it.  Not run everything to one ground point, and not try to make due with what you've got.  A ground rod at your mast, a ground rod at the coax entry point to the shack, and of course, the ground rod for the house electrical system.  Bond them ALL together with number six cable.  That is what the national electrical code calls for, and that is the RIGHT way to do it.  73!


Title: RE: Station Grounding / Lighting Protection
Post by: KG7MWW on February 05, 2012, 10:47:07 PM
Ok, so using a neumatic hammer, I drove one 8' ground rod at the antenna mast base, and another 8' ground rod near where the coax will enter the shack.  About 80' separate the two, so do I need to put in more every 16' or so, or just connect these two rods to the service ground coming up through the slab using #6 wire?   


Title: RE: Station Grounding / Lighting Protection
Post by: K1CJS on February 06, 2012, 06:35:15 AM
You can put in additional ground rods if you wish to, it may improve the ground system--then again, it may not.  Connecting or 'bonding' all the ground rods you have in now is the minimum you need to do, and it has to be with at least #6 cable. 

If I were you, I would use the ground cable that is used for grounding electrical services.  It has no insulation and would make a fine continuous ground if you were to bury it--execept at the ends where you clamp it to the ground rods.  Of course, if you were to cadweld the cable to the ground rods, you can bury those connections too.

Good luck, and 73!


Title: RE: Station Grounding / Lighting Protection
Post by: K9KJM on February 06, 2012, 10:35:59 PM
Ok, so using a neumatic hammer, I drove one 8' ground rod at the antenna mast base, and another 8' ground rod near where the coax will enter the shack.  About 80' separate the two, so do I need to put in more every 16' or so, or just connect these two rods to the service ground coming up through the slab using #6 wire?   

If this were mine, I would add another ground rod about 20 feet from each end, And then one more in the middle.   A total of three more rods.  Total of 5 rods for the entire run.


Title: RE: Station Grounding / Lighting Protection
Post by: KH6AQ on February 07, 2012, 08:14:11 AM
The rule for multiple ground spacing is two times the ground rod depth.


Title: RE: Station Grounding / Lighting Protection
Post by: K9KJM on February 07, 2012, 09:12:25 PM
The rule for multiple ground spacing is two times the ground rod depth.

That is a "rule" of thumb for "average" soil types, If there is such a thing.  The more rods the better, But when you start installing them too close to each other, You are "wasting" effectivness, So is poor economy.


Title: RE: Station Grounding / Lighting Protection
Post by: K1CJS on February 08, 2012, 05:23:14 AM
Oh, come on now boyze--stop fighting.  There are no requirements about distance apart from each other as far as ground rods go.  As I said, the way I stated is the minimum requirement. 

And it is true that putting the ground rods (8 footers) closer than 16 feet apart is redundant, but it doesn't 'waste' effectiveness.  It simply overlaps the recommended discharge area of installation.  The only thing it actually 'wastes' is the money you would spend for the extra ground rods that you're using.


Title: RE: Station Grounding / Lighting Protection
Post by: W6RMK on February 08, 2012, 08:03:19 AM

If this were mine, I would add another ground rod about 20 feet from each end, And then one more in the middle.   A total of three more rods.  Total of 5 rods for the entire run.

What engineering basis do you have for recommending more rods?  As you point out "installing them too close... is poor economy", and I think the same would apply to number as well as spacing.

Not that more rods isn't better (it is, in general), but where is the cost/benefit tradeoff? 

Let's look at the physics.  There's three reasons to connect to the soil:
1) RF ground for an antenna where the soil is part of the radiating system
2) Electrical safety, in case a power line or other live conductor comes in contact with the antenna
3) Lightning energy dissipation

Ground rods in the middle of the bonding run do nothing for #1, because the AC resistance of the wire connecting them is sufficiently high at amateur HF frequencies that it's like hooking a big resistor between the "ground" and the antenna.

Ground rods in the middle of the bonding run do nothing for #2, because the DC/60Hz resistance of AWG6 (<0.5 ohms in 100 ft) is so low that the voltage drop along the wire is small in the event of a fault, so that all the equipment stays close to "bare feet" potential. (one of the reasons for NEC grounding)

Ground rods in the middle of a bonding run don't do much for #3, because the inductance of the 20 feet of wire to that middle rod is about 6-7 microhenries.  At the nominal 1 MHz for lightning, that's an impedance of 18 ohms, and at, say, 1 kA, that's a voltage drop of 18 kV.  Buzzing along to the next rod, the current is less, but the inductance is more... The net result is that the added rods don't add much to the effectiveness of a lightning ground.    If you're worried about reducing the impedance of your lightning ground, drive more rods near the antenna, or even better, bury some radials, or, if the antenna has a concrete base, make sure it's well connected to the grounding system.

Take home message:

The bonding rules aren't for lightning or RF.  They're for line frequency faults and shock hazard.  From that standpoint, it turns out that the bonding really doesn't need to be connected to "earth ground" very well at all. (and in fact, the NEC code makers are gradually changing the wording from grounding to bonding)

Long wires (unless buried) make terrible additions to RF grounding system.

Lightning is all about keeping the strike energy somewhere other than delicate stuff,  and making sure that all equipment goes up and down together.  The currents are so high and the rise time so fast that trying to keep the voltage "low" is impossible.

Ground rods worked great for Ben Franklin, because they replaced *nothing*.  But there's a lot of better ways to do it now, and we know a LOT more about the underlying physics and applications.



Title: RE: Station Grounding / Lighting Protection
Post by: K9KJM on February 08, 2012, 09:11:45 PM
You are correct, Ground rods do close to nothing for RF grounding. (But that interconnecting wire DOES add to RF ground)
I agree, Just a single rod is plenty for a DC/safety ground.
However,
The extra rods DO MAKE A  BIG difference to dissipate a direct lightning strike, And are effective at least out to 75 or so feet away from the strike location.
And the original post question is about lightning protection.

The cost benefit tradeoff?   As long as you are running that ground wire anyhow, for a mere 30 bucks more you can add a lot to the potential of surviving a direct lightning strike.  

The bonding IS for good lightning protection!   In fact, Good bonding is the main part of a good lightning protection system.

Read Motorola R56 if you want to better understand lightning protection systems, And read the Polyphaser notes about installing ground rods, Along with spacing.


Title: RE: Station Grounding / Lighting Protection
Post by: KF4WXD on February 10, 2012, 12:04:00 PM
I would urge you all to find a copy of the May/June 1998 edition of "Power Quality Assurance" magazine.  Perhaps a note to the publisher will yield a copy or a .pdf of the article "Principles of Lightning Protection for Telecommunications Facilities" by A. J. Surtees.  My copy is on the bookshelf in my shack next to the "Handbook" and the "operating Manual".

This article covers, in excrutiating detail, how telecom professionals and companies plan and execute lightning protection and grounding in outside plant facilities, equipment huts and central-office type buildings.  Having worked for a couple of different telephone companies, Mr. Surtees' article agrees completely with what I saw in the field as "best suggested practices".

That being said, as a telecom engineer, I am a AMATEUR radio operator and there is NO need for me to be operating when there is electricity in the air.  My station antennas and grounding are in compliance with sections 250 and 800 of the NEC and the coax is connected firmly to ground and the equipment is disconnected from power when I am not operating.  If lightnng stikes your antenna, tower or house, there will be loss, possibly catastophic.  We can only minimize it and comply with the code so that insurance investigators will have a reduced ability to deny your claim.  I have this argument with auto hobbiests as well.  Modifying your car with aftermarket wheels and tires can cause your insurer to deny a claim.

If you have doubts about implementing grounding and coax entrance, hire a pro, pull permits and do it the right way.

Russ - kf4wxd


Title: RE: Station Grounding / Lighting Protection
Post by: KH6AQ on February 10, 2012, 12:19:04 PM
See these MIL documents:

MIL-STD-188-124B:       http://webbooks.net/freestuff/MILSTD188.pdf (http://webbooks.net/freestuff/MILSTD188.pdf) 

MIL-HDBK-419:             http://webbooks.net/freestuff/MILSTD188.pdf (http://webbooks.net/freestuff/MILSTD188.pdf)






Title: RE: Station Grounding / Lighting Protection
Post by: W7ETA on February 10, 2012, 03:48:27 PM
http://www.w8ji.com/ground_systems.htm