Ground rods... what are they for?

[mbrooke]

What functions do ground rods provide at the service?


I keep hearing from computer engineers and electronic techs ground rods absorb static, radio interference and common node noise. Having extra ground rods and meshes fixes these problems.

An example: "Lost earth ground or high impedance to earth ground can cause problems, even in residential electronics. If you cannot divert the common mode noise to ground that is produced by most home electronics, all you're going to do is pass it around. If you lose your ground, you will have a lot of noise running around your electrical on into your electronics."

"A poorly grounded system cannot allow the electronics to divert noise through the ground, so it generally hangs around and gets into other circuits."


Is there any truth to these claims frequently made?

 

[GoldDigger]

Very little.
Proper single point grounding and keeping current off the ground wires (EGCs) does much more than getting a good earth contact.

On the other hand a weak neutral that causes current to flow in the GES can seriously damage cable and other communications systems regardless of how good the earth connection is.

Tapatalk!

 

[mbrooke]

Very little.
Proper single point grounding and keeping current off the ground wires (EGCs) does much more than getting a good earth contact.

On the other hand a weak neutral that causes current to flow in the GES can seriously damage cable and other communications systems regardless of how good the earth connection is.

Tapatalk!

Thanks!

But just wondering don't ground rods help with common mode noise/EMI interference?

 

[GoldDigger]

Thanks!

But just wondering don't ground rods help with common mode noise/EMI interference?

Common mode interference rejection is helped by common mode noise immunity in inputs, and can also be reduced by filters, usually involving capacitors to "ground" from both hot and neutral wires at the power cord entry into the equipment.
Since the filter connection is typically made to the same point as the chassis ground and internal wiring ground, the common mode noise rejection will not be greatly affected by whether that single point is well earthed or not.
But if there is no good earth connection then other equipment which is connected by non-balanced signal paths to that equipment may see the common mode noise sent to ground by the filter as signal.
Once again, the way to reduce that problem is by proper ground/shield/signal connections among the pieces of equipment, not by changing the resistance from that common point to "earth".

 

[mbrooke]

Common mode interference rejection is helped by common mode noise immunity in inputs, and can also be reduced by filters, usually involving capacitors to "ground" from both hot and neutral wires at the power cord entry into the equipment.
Since the filter connection is typically made to the same point as the chassis ground and internal wiring ground, the common mode noise rejection will not be greatly affected by whether that single point is well earthed or not.
But if there is no good earth connection then other equipment which is connected by non-balanced signal paths to that equipment may see the common mode noise sent to ground by the filter as signal.
Once again, the way to reduce that problem is by proper ground/shield/signal connections among the pieces of equipment, not by changing the resistance from that common point to "earth".

Very well explained! Im learning! Thanks!

 

[ritelec]

Still trying to understand why it is to "just" drive another ground rod and be done with it.

Reading through Mike Holt's illustrated guide to "chances to the nec 2011".

It's mentioned the forms of an electrode.

One being the water ground 10 ft or more can serve as grounding electrode.
250.53 mentions it must be supplemented ... but then it says in the EX. if the single rod, pipe, plate has earth contact of 25 ohms or less the supplemental rod isn't required.

So, for years people have been using the water ground and rod and as of lately it's been common practice to install a 3rd electrode (rod) to not have to worry about meeting the 25 ohm rule.

Shouldn't that have been accomplished with the 1st (supplemental) ground rod.

Where does it say to drive 2 supplemental and be done with it.

It's also interesting how it mentions the electrodes to be 6' or more apart, but Mike Holts Analysis mentions twice the length of the rod (16')



What if you considered the rod your electrode and your water pipe the supplemental, then twice the water pipe length might put you in the next town?

 

[GoldDigger]

Still trying to understand why it is to "just" drive another ground rod and be done with it.

Reading through Mike Holt's illustrated guide to "chances to the nec 2011".

It's mentioned the forms of an electrode.

One being the water ground 10 ft or more can serve as grounding electrode.
250.53 mentions it must be supplemented ... but then it says in the EX. if the single rod, pipe, plate has earth contact of 25 ohms or less the supplemental rod isn't required.

So, for years people have been using the water ground and rod and as of lately it's been common practice to install a 3rd electrode (rod) to not have to worry about meeting the 25 ohm rule.

Shouldn't that have been accomplished with the 1st (supplemental) ground rod.

Where does it say to drive 2 supplemental and be done with it.

It's also interesting how it mentions the electrodes to be 6' or more apart, but Mike Holts Analysis mentions twice the length of the rod (16')



What if you considered the rod your electrode and your water pipe the supplemental, then twice the water pipe length might put you in the next town?

The NEC only covers the interactions of apples with apples. It does not attempt to look at the interactions between a CEE or a water pipe and a ground rod, although they will be significant.

If your ONLY ground is rod and that rod has a measured resistance over 25 ohms, then you drive a second rod.
If you have a CEE, then no matter what its resistance is (and it almost certainly will be less than 25 ohms) you do not have to drive even one rod.

The separation of the rods is based on having the minimum required length rod (8 feet in the ground) and does not really meet the design goal that Mike states based on the physics of ground conduction. It is a bare minimum, as is the requirement for a second rod in the first place.

If you have a water pipe ground, it does not get the same presumption of adequacy that a CEE does (and for good reason, since the state of the water pipe after it ducks below the earth is not known. It may transition to plastic pipe as soon as it hits the street.)

 

[kwired]

Still trying to understand why it is to "just" drive another ground rod and be done with it.

Reading through Mike Holt's illustrated guide to "chances to the nec 2011".

It's mentioned the forms of an electrode.

One being the water ground 10 ft or more can serve as grounding electrode.
250.53 mentions it must be supplemented ... but then it says in the EX. if the single rod, pipe, plate has earth contact of 25 ohms or less the supplemental rod isn't required.

So, for years people have been using the water ground and rod and as of lately it's been common practice to install a 3rd electrode (rod) to not have to worry about meeting the 25 ohm rule.

Shouldn't that have been accomplished with the 1st (supplemental) ground rod.

Where does it say to drive 2 supplemental and be done with it.

It's also interesting how it mentions the electrodes to be 6' or more apart, but Mike Holts Analysis mentions twice the length of the rod (16')



What if you considered the rod your electrode and your water pipe the supplemental, then twice the water pipe length might put you in the next town?

A water pipe is generally a better electrode then a 8 foot rod. I think the main reason a water pipe is required to be supplemented is not because the pipe is a questionable elecrode, but rather it is because should the pipe ever be changed to something non conductive you lose the electrode and you at least still have the supplemental electrode.

NEC says 6 feet apart as a minimum, but twice the length of the rod will likely give you about the lowest possible resistance you will get from the conditions, though there may not be all that much difference in resistance between the six feet and the sixteen in many cases.

 

[FionaZuppa]

does the 25ohm rule apply everywhere? i mean, in some places the ground is very very dry thus raising the ohms. and even if one could dry a rod down deep enough to reach water, the ohms path between ground surface and rod will remain high.

does one measure ohms from service provider gnd and the gnd spike?

 

[Dennis Alwon]

does the 25ohm rule apply everywhere? i mean, in some places the ground is very very dry thus raising the ohms. and even if one could dry a rod down deep enough to reach water, the ohms path between ground surface and rod will remain high.

does one measure ohms from service provider gnd and the gnd spike?

Technically if you measure 25 ohms at the time the inspector sees it then that is all that is necessary. You are correct that when you install a rod on a rainy day you are more likely to get a lower reading then during a dry spell. This is partly why most people just install 2 rods and then they don't have to worry about the ohm reading.

If you do an ohm test then you need to do a 3 point fall of potential test or you could use a clamp on meter tester that test the resistance. Each of these uses a different method of testing

 

[brian john]

does the 25ohm rule apply everywhere? i mean, in some places the ground is very very dry thus raising the ohms. and even if one could dry a rod down deep enough to reach water, the ohms path between ground surface and rod will remain high.

does one measure ohms from service provider gnd and the gnd spike?

Which really does not mean anything, because there is no real average day, the ground is either wet, or in some phase of drying then there is a drought, followed by freezing of the soil. plus the average rain barely soaks in very deep and the driven electrodes are 8,10,12???..feet in the ground.

Typically when I am told that the engineer, EC or end user wants a new ground electrode system and then they argue about a triad over a square, 8 foot rods or 10 foot rods, I ask if we can do some investigations and determine the real cause, because it is not the Earth.

When you test a ground electrode system you are supposed to isolate the electrode under test (assuming you are utilizing a 3-point ground test). In my experience most ground test are not performed properly, I have looked at many test reports completed by our competitors and based on what I have read they skimped on the test. Did not go out far enough with their test probes, or only did a single test not the minimum 9 measurements, or utilized a clamp on test measuring in the wrong location.

And on top of all this (and we make decent money performing ground electrode test) I am not sure how important this is.

We were hired to do a ground test, site had a concrete slab with a ufer ground and an exterior ground ring around the building, #2 tinned copper driven 10' - copper clad 5/8" rods every 20 feet. Building was 200' x 600 (more than 80-10' rods), all tied to the structural steel at ever column around the building. I tried to explain we could not go out far enough to obtain an acceptable reading, we could not isolate the system (site was on line) and even if the reading was high what were they going to do? They had the best made electrode possible. They hired another firm and got a reading they were not only happy with but stuck it in my face at our next meeting