How many gorund rods is enough?

[bspettiford]

It's been a while since I've submitted in this forum first question what is a CCE or CEE?

Second question. Specification on a project requires 5ohm resistance or less for the grounding systems. Testing used was the fall of potential method at 62%. Three rods were installed initially. The ground resistance was 50.5ohms. The contractor installed 2 more rods and resistance went to 50ohms. We found out that the installation is at the top of a berm. (really rocky with dirt mixed in to fill the gaps). We went to the bottom of the berm approximately 50 to 75 feet away from the service, and installed a rod, the reading went to 36ohms. We pounded 3 additional rods and fiinal got a reading of 23.6ohms which meets the minimal requirment of the NEC. I contacted the AHJ for permission to walk away at this point which he agreed.

So the question is when should you walk away from something like this? The site is very small and there was no rebar installed so a UFER ground was not available. The soil in the area is really rocky and several of the rods could not be driven deep enough to meet the 8' requirements of the code.

A seasoned veteran in my office sugested that the NEC requires you to install one ground rod for your system. When measured, if the resistance is greater than 25ohms then all you need to do is install one supplemental rod and then walk away. Is this the correct interpertation?

 

[roger]

First question is "Concrete Encased Electrode"

Second question is not an NEC issue as you stated it was a spec, so the designer may not let you stop at the NEC requirement.

You may be able to use a Chemical Ground.

http://www.erico.com/products/ChemRod.asp

Roger

 

[bphgravity]

Just out of curiosity, what kind of building and what is the building being used for that would require 5-ohms ground resistance? And what state and region is the building in?

Thanks

 

[haskindm]

Requiring a 5-ohm resistance to ground indicates that the designer does not understand the purpose of grounding. That said, these are the specifications to which you agreed when you took the job, so you will need his permission to deviate from the specs, not the AHJ. This is not an NEC issue. You met the NEC requirement as soon as you drove the second rod.

 

[George Stolz]

When measured, if the resistance is greater than 25ohms then all you need to do is install one supplemental rod and then walk away. Is this the correct interpertation?

Yes, the reference is 250.56.

And I agree with the above posters, they're the designer's specs, not the AHJ. So the AHJ was satisfied at the second ground rod. The designer won't be happy until 5 ohms is reached.

Hal wrote:
Requiring a 5-ohm resistance to ground indicates that the designer does not understand the purpose of grounding.

Well, I don't know if you can be that harsh. I'm curious about the answer to Bryan's question to the OP.

 

[bspettiford]

Re: Ground Rods

Re: Ground Rods

Thanks guys for your responses. Maybe I didn't include enough information so here are answers to your questions.

The designer engineer used a City's Standard Guide Spec for the project. The City spec requires the 5-ohm resistance that is why I contacted them as AHJ. This is a requirement for all of their ground systems.

There are 10 sites in all. We only had this issue at one site. These sites are located in Arizona. Really rocky, dry, and hard ground. Trust me, blasting is not even an option in some areas.

I know this is an NEC Forum, but are you guys familiar with IEEE std 81? This standard references a 5-ohm resistance or less for a ground grid system (sub stations). I have seen a lot of engineers and municipalities in my area that require this same resistance and even less. In most of the inspections I've performed 5-ohms has been obtainable. So, please explain why the design engineer does not know what a ground system is for. I am not a design engineer and I'm trying to get a better understanding of the NEC and many design practices in this area. (I will be taking the FE in April.) In my area this seems to be a standard that is required across the board. From my understanding the lower the resistance the better the system and it will provide a more effective path to earth for fault currents.

Next question is how can two separate bodies have such a variance in their requirements? 5ohms, install additional rods until the resistance is met (IEEE) and then 25 ohm or one additional rod (NEC). One would think these values would be a little closer. As I stated in my previous post with 4 rods the resistance was 50ohms. I really had a hard time walking away with 23.6ohms because all I have seen is 5ohms or less on plenty sites. Why does the NEC even give the 25ohm reading? If one additional rod is all you need, why not just say all that is required is two grounding electrodes with 8ft of cover?

If the second ground rod is acceptable then why does the test set instruction manual discuss have a multiple rod system? Every piece of information I have run across eludes to installing additional ground rods and not just 1 rod and walking away. I understand that the NEC is the minimal requirements. And I guess what I'm trying to get to here is, when I started this everyone would tell me that the NEC is the minimal requirements. Then when I started reading specs and looking at designs and construction effort, all of those standards were more stringent than the NEC. Why have a standard, the NEC, whose minimal requirements does not measure up to industry standards? (As I wrote this I started thinking about what if there were no standards at all and how things would be, so no need to beat me down on this point. I left this last paragraph just to stir thought in anyone who may read it.) There just seem to be no consistency in this industry.

 

[roger]

I think we are looking at two different reasons for GES's here.

You seem to be looking for performance grounding verses the NEC grounding for Lightning, High voltage Surges, and some stabilization.

Substations and Sensitive Electronic installations may benifit from a GES at the levels you are after, but as far as providing safety to people or Fault clearing at our voltages, it means very little if anything.

Roger

 

[haskindm]

Even at 5-ohms resistance a ground system will not carry enough current to reliably trip a 20-amp circuit breaker in the event there is a 120-volt circuit directly shorted to the ground system. (120/5=24 amps, a 20-amp breaker will carry a 24-amp load for a long time) Many of these requirements are based on the mistaken assumption that electricity wants to return to earth. Current wants to return to its source, not earth. Earth is a very poor conductor, by achieving 5-ohms resistance to earth using a fall-of-potential meter, that does not guarantee a 5-ohm resistance for the current to return to the source. So if grounding is not useful for clearing faults at utilization voltages, why do we do it? At higher voltage faults there may be enough current travelling through the earth to open an overcurrent device, so there is some benefit there. There is also one source of electricity that is trying to get to earth, and that is lightning. At the tremendous voltages that are available in a lightning strike, I believe that the difference between a 5-ohm resistance to earth, and a 25-ohm, or even 100-ohm resistance would be negligible. I am not an engineer (and I don't even play one on TV) these are just my thoughts as an electrician. That is why I say a blanket requirement such as this shows a basic lack of understanding about the purposes of installing a grounding electrode system. There may be good reasons for requiring a very low resistance to ground in a specific installation, but to require a 5-ohm resistance with no justification makes little sense. Why not 10-ohms, or 1.5 ohms? At 1.5-ohms a 120-volt fault would develop 80 amps, which may eventually trip a 20-amp breaker, so that would be a justification.
I would be interested in learning about the reasoning behind the 5-ohm requirement.

 

[bphgravity]

The first thing that is important to understand is that grounding of premise wiring systems does nothing to ensure the clearing of ground faults. Sub-stations and distributions systems are a different story. Those systems rely on the earth as function of normal and fault current conditions.

The other major consideration is that the NEC is generally adopted as a statutory or municiple law, whereas IEEE standards are simply guidelines. Both provide limited or incorrect information as to why grounding systems must be of some predetermined impendance. The NEC's reference to 25-ohms is an old cross-over from telegraph installations when a man named Samuel Varley determine that 25-ohms or 1-siemens was sufficient ground resitance for establishing a reference point when troubleshooting telegraph systems. So this unit is nothing more than a performance issue for antique telegraph systems and has no real basis for today's electrical systems.

When grounding is broke down to its most basic function, lightning is the only real consideration. Again however, ground resistance plays only a small role in the reducing or limting of surge voltages on equipment when referneced to other conductive parts. In order for grounding to be effective in reducing damage from lightning events, adequate bonding of systems and equipment is the key, not the connection to earth.

All that being said, ground rods are going to be by far the most ineffective method for providing a low resistance grounding system. Everyday that a rod is in the Earth, it will become more impeding.. Ground rods have the shorest performance life and rely too heavily on soil conditions and other uncontrollable environmental conditions. You may have obtained 5-ohms in the past, but I would be willing to bet those same systems no longer or will not be able to maintain that level for any significant period of time. At least, not nearly as long as the other components of the electrical system.