Grounding For Remote Communications Trailer

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ShockedOneinAZsun

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Yuma, Arizona
I set up remote communications trailers 65 mile down a dirt road in the Arizona desert. These communications trailers, complete with a variety of sensitive electronics and antennas, are powered with 1 phase, 2P, 120/240 volt power from a diesel operated, portable 35 KW generator. My problem, however, is grounding them to protect the equipment, and operators from static, stray voltages, harmonics, etc.

I have driven an eight foot grounding rod into this extremely dry, rocky, sandy soil and tested. With a single grounding rod, 5/8" by eight foot, with a #04 bare grounding conductor going to a 125 ampere panel, I get at best 564 ohms to ground!

Should I drive a second grounding rod and just connect it to the generator? Is this giving me adequate protection at all?
 
Assuming you measured your resistance to ground correctly, yes you need to drive a second ground rod... to meet code anyway. If you are really concerned about getting the resistance to earth ground a lot lower, you will have to utilize one or more of the various methods available.

I have a suspicion you are not adept at the reasons for grounding and bonding...

In as simple terms as I can get it, earth grounding is mostly for dissipation of lightning surges. Equipment and non-current carrying metal grounding is for the other types of protection you seek and accomplished by bonding such items to the system ground.
 
090141-2000

ShockedOneinAZsun:

You need to define why you need a ground. What is the purpose of this ground?

There are many people with various sorts of noise problems that will to told by someone that a good ground will solve their problem. No definition of what good ground means. Which if any of these resistances to ground would you consider a good ground, if any? 100, 25, 10, 1, 0.1, 0.01, 0.001 ohms. And then from a circuit point of view why?

Do you want a ground plane for a vertical monopole antenna? Do you want to avoid shocks? Or lightning protection? How about noise reduction?

When you can define what you want and why, then you can work on how to accomplish that goal.

.
 
If it was my "sensitive electronic equipment" I'd want a 0 ohm reading. Especially since the rod(s) will be your only earthing point.

Driving a ground rod and connecting it to the generator is not the answer. Is the neutral bonded to the generator frame? All non-current carrying metal parts on the generator should be bonded to the generator frame.

The grounding electrode connection needs to be made at the same point where the system bonding jumper is connected. 250.30(A)(1)

Follow the grounding rules in NEC 810.

Doubling the depth of the ground rod will lower the resistance much better than driving multiple rods. It will also eliminate any potential ground loops. Better get a bigger hammer
 
HotConductor said:
If it was my "sensitive electronic equipment" I'd want a 0 ohm reading. Especially since the rod(s) will be your only earthing point.
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0 Ohm?

I will assume you just meant close to 0?

But how is 1 ohm going to be better then 500 ohms at protecting this "sensitive electronic equipment" that is already entirely isolated from any other influences other then lightning?

And if lightning was to smack this equipment no amount of earth connection is going to save it.

Doubling the depth of the ground rod will lower the resistance much better than driving multiple rods. It will also eliminate any potential ground loops.
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'Ground loops' between rods is not going to mean a thing to the equipment.

Unless as Gar mentioned there is some sort of antenna that requires exceptional grounding means for it's functionality I would drive two 8' rods and get out of the heat. :)
 
iwire said:
0 Ohm?

I will assume you just meant close to 0?

But how is 1 ohm going to be better then 500 ohms at protecting this "sensitive electronic equipment" that is already entirely isolated from any other influences other then lightning?

And if lightning was to smack this equipment no amount of earth connection is going to save it.



'Ground loops' between rods is not going to mean a thing to the equipment.

Unless as Gar mentioned there is some sort of antenna that requires exceptional grounding means for it's functionality I would drive two 8' rods and get out of the heat. :)
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With all due respect, what if it wasn't a direct hit? What about a static charge? Just driving two ground rods and forgetting about it is not protecting this sensitive electronic equipment... in my humble opinion.
 
this seems more like an engineering issue that should be addressed by the people who designed the trailer.

Unless the plans or instructions call for something else, I would put in the NEC minimum and point out that is what you did in the absence of other instructions.

anything else you do has the potential to come back to bite you. you are probably not qualified to decide just what a proper ground is beyond what the NEC requires. By attempting to take on the task of defining what is proper, you bring the liability of future problems on to yourself.

static is mostly a bonding and not a grounding issue. if all the metal pieces (current carrying and non-current carrying) are bonded together, and static mats are put inside the trailer, there is no place for static to build up. you can pound a ground rod to the center of the earth and it will have very little effect on static buildup.

I would also not be advertising the results of the ground tests. They are meaningless even if they were done correctly, other than meaing you have to pound another rod, which you should have done anyway rather than waste the time on ground tests, unless that was required by the instructions or plans.
 
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HotConductor said:
With all due respect, what if it wasn't a direct hit?
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I don't see that making a difference, whenever someone posted some math on this subject it becomes clear for lightning protection a very low resistance connection to earth is not needed.


From another thread Stray Voltage
dereckbc said:
You are assuming that 100 ohms is a lightning impedance it is not, it is a power frequency measurement.

Look at it another way take a 10 foot piece of 750 MCM conductor. Would you say that is a sufficient size and low impedance conductor for a GEC? Well if we are talking about DC and power frequencies it would be low impudence. But at lightning rise and fall times that 10 foot piece of 750 is around 2.6 K ohms. So now put that in series with your so called 100 GES and tell me what the impedance is. In my book it is still 2.6 K ohms or higher. Reality is that 100 ohm GES you are thinking of is much higher at lightning frequencies, several hundred time higher.
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What about a static charge?
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It takes very little to drain away a static charge, certainly two rods will do that, I bet a piece of bare 18 AWG laid on the ground and connected to the frame would do it.


From another thread. Static Grounding for U/G Fuel Tanks
don_resqcapt19 said:
The IEEE Green Book says that a path of one million ohms or less will prevent the build up of static charges.
Don
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Just driving two ground rods and forgetting about it is not protecting this sensitive electronic equipment... in my humble opinion.
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I am just asking for some references for that opinion.
 
090201-1256 EST

ShockedOneinAZsun:

You need to come back and tell us what kind of problem you are trying to solve or prevent.

Additional comments:

Assume your trailer outer body and frame are all conductive and electrically connected together (bonded), all equipment within the trailer and generator system is also connected to this trailer body and frame, then everything within is inside of a Faraday shield and essentially at an equipotential level. There could be a million volts or zero to earth outside and the equipment would not care.

Assuming you have equipment external to the trailer ( this at least includes the antennas mentioned, but may include a number of sensors ), then this external stuff needs electrical isolation to minimize effects on your equipment. Where possible this should be fiber optic. I have optical isolation products, but no fiber optic. Fiber optic will give you the greatest isolation. Your RF stuff needs to couple thru isolated RF transformers designed for the maximum voltage you might expect.

Actually I would suggest that the transmitters be external to the trailer and considered as expendable. Then use fiber optic for communication with the transmitters.

The value of the trailer and what is inside of it determines the extent to which you go to protect it. It is a value balancing act.

If you avoid putting currents in the earth, then two ground rods, even of high resistance to earth, are probably adequate to prevent step potential problems, and may provide some protect from lightning currents. Provide humidification internally to reduce internal static problems.

If your trailer has one of a kind equipment and is worth 100,000,000 dollars, then you take measures quite different than if it is a 50,000 dollar package.

.
 
ShockedOneinAZsun said:
IShould I drive a second grounding rod and just connect it to the generator? Is this giving me adequate protection at all?
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Caution: Following is strictly personal opinion.

Driving a second ground rod is an AHJ/NEC issue - that has nothing to do with providing adequate protection.

Keep in mind: The NEC is not a design document. It is a set of minimum installation specs that may or may not work well.

As others have already discussed, providing adequate protection is a design issue.

Take a look at IEEE 1100, Powering and Grounding Sensitive Electronic Equipment. That is the best reference I have found.

Soares Grounding is another good reference.

As others have said, figuring out just what you wish to protect against is the first step. I could not even hazard a guess about what you need to do without you figuring this out first. These two references should help you decide that.

cf
 
Thank you for giving me some things to think about. I am, too obviously, swimming in new waters with this project, so, I will only proceed within my limitations

gar said:
090141-2000

ShockedOneinAZsun:

You need to define why you need a ground. What is the purpose of this ground?

There are many people with various sorts of noise problems that will to told by someone that a good ground will solve their problem. No definition of what good ground means. Which if any of these resistances to ground would you consider a good ground, if any? 100, 25, 10, 1, 0.1, 0.01, 0.001 ohms. And then from a circuit point of view why?

Do you want a ground plane for a vertical monopole antenna? Do you want to avoid shocks? Or lightning protection? How about noise reduction?

When you can define what you want and why, then you can work on how to accomplish that goal.

.
Click to expand...
 
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