Grounding Electrodes and 25 ohms resistance

[Electron_Sam78]

According to 250.56 ground rods need to be resistance tested in reference to earth and not be more than 25 ohms. It says that if this standard is not met another rod should be installed.

Does this mean that if you can't test it you can just install two rods right off the bat and then be in compliance or are you required to test it and install rods until the 25 ohms is achieved? I thought it was the former until I was perusing the article the other day (and I was looking over the book from a grounding course I took a few years back) and saw the language in the section which seems to indicate the latter where it says , "Where multiple...electrodes are installed to meet the requirements of this section..." The grounding class I took taught that multiple electrodes were installed and tested until 25 ohms or less was achieved.

If the intent of the code was only 2 rods wouldn't it say "...two...electrodes..." instead of using the word multiple?

 

[dbuckley]

Many of the electricians on this forum just install two ground rods, and be done with it.

"ANother" in the code means just that. One more, besides the first. No retest necessary.

 

[roger]

The NEC only requires one extra rod if one rod does not achieve the mysterious 25 ohm requirement.

Two rods are all that is required by the NEC regardless of the ground resistance.

Roger

 

[iwire]

If there are no other electrodes available and you install two rods you are done regardless of the resistance.

If the site has only 1 CEE you use that and you are done.

 

[hardworkingstiff]

Many of the electricians on this forum just install two ground rods, and be done with it.

"ANother" in the code means just that. One more, besides the first. No retest necessary.

Count me in as one of the many.

 

[jumper]

Count me in as one of the many.

Ditto. The bloody meter to test is like 3000$, not worth it for average work.

 

[Electron_Sam78]

But how do you explain the language as I stated in the OP where is says multiple electrodes? Is it something that could use a formal interpretation from the NFPA?

 

[OTT2]

But how do you explain the language as I stated in the OP where is says multiple electrodes? Is it something that could use a formal interpretation from the NFPA?

I believe they are refering to the spacing of the "multiple" electrodes, not indicating there needs to be more than two.

 

[iwire]

But how do you explain the language as I stated in the OP where is says multiple electrodes? Is it something that could use a formal interpretation from the NFPA?

I would say you are taking it out of context. You are only required to add one but you are not prohibited from adding many.

250.56 Resistance of Rod, Pipe, and Plate Electrodes.

A single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified by 250.52(A)(4) through (A)(8). Where multiple rod, pipe, or plate electrodes are installed to meet the requirements of this section, they shall not be less than 1.8 m (6 ft) apart.

 

[Dennis Alwon]

I agree with OTTO who spells his name backwards.

Two rods can be multiple rods but sometimes it can be speced for more than 2 rods. So when you have multiple rods space them at least 6' apart is all that sentence is saying.

 

[Electron_Sam78]

ok I'm convinced. I'm not buying a ground resistance tester ​

 

[480sparky]

ok I'm convinced. I'm not buying a ground resistance tester ​

You will probably save money that way.

 

[roger]

Even owning the test set, it is normally cheaper and more cost efficient to drive the second rod and go home.

We only test when a performance criteria is specked or we are asked to do so and are paid a fee for doing it.

Roger

 

[brian john]

According to 250.56 ground rods need to be resistance tested in reference to earth and not be more than 25 ohms. It says that if this standard is not met another rod should be installed.

"[/B] The grounding class I took taught that multiple electrodes were installed and tested until 25 ohms or less was achieved.

If the intent of the code was only 2 rods wouldn't it say "...two...electrodes..." instead of using the word multiple?

Who taught this class and are you sure you understood fully?

 

[480sparky]

250.56 Resistance of Rod, Pipe, and Plate Electrodes.
A single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified by 250.52(A)(4) through (A)(8). Where multiple rod, pipe, or plate electrodes are installed to meet the requirements of this section, they shall not be less than 1.8 m (6 ft) apart.

I see this as saying if one rod is more than 25 ohms, you add one additional rod.

So I drive a rod. I look at it and say, "Gee, I dunno... maybe it's more than 25 ohms, maybe it's less."

Lacking proper testing equipment, I drive a second rod more than 6 feet away from the first one. Then I pack up and go home.

 

[Electron_Sam78]

Who taught this class and are you sure you understood fully?

I don't think I'll name the name of the company that gave the class but it definitely stated in the book (I just looked through it the other day)to keep driving ground rods until the resistance standard was achieved. It also said that a "computer quality" installation should be 5 ohms and an installation in a lightning prone area should not exceed 1 ohm. It did not say that the code required this however so from this discussion I can assume that the additional information is their opinion and not a code interpretation.

 

[dicklaxt]

Let me throw another scenario out on the table..

Gounding specs within an industrial plant proper usually will call for a grounding system of 25 ohms to be acheived.This could be a closed loop with at least two ties to adjacent loops and at remote requirements such as a floodlight pole where you may use one rod per pole independent of the loop.The closed loop would be a series of rods interconnected with a grounding conductor and individul taps leading to devices that required a ground.The requirement of closed loops and a minimum of two ties guarantees(hopefully) that a device remains grounded even tho the loop was cut/opened and would still be within the 25 ohn spec.

In the gulf coast areas of the US a spacing of 75 to 90 feet will normally achieve 25 ohms,moving inland would require more rods or all 20 foot long rods and even using deep well annode beds such as is used in high concentrations of shale and rock formations.

The system is made up of many rods some of which are called Ground Rods and some are called Ground Wells.The ground rods are what they seem to be,these are driven rods,,,,the ground wells are also driven rods but have a housing with a cover around them so that they are accessible in a pored concrete area as most plants are and they are used as test wells.If a loop is found to be above 25 ohms, the rod at a particular well is extended with the use of a rod coupling and then driven to a greater depth to bring the system into spec.If a system were still not within spec you would move on to another ground well within the same loop and extend that rod etc etc.It is not as simple as adding a rod and walking away,this may satisfy the code but not the spec.

dick

 

[Dennis Alwon]

I have said this before and I will say it again. Installation of a ufer will dramatically lower the resistance compared to a rod. In our area 2 rods will get you about 90 ohms whereas a ufer can get you to 13ohms.

My proof is really just from one jb that I did. I installed a ufer and compared it to the rod we had driven for the temporary pole. It was 98ohms , I believe- the second rod brought it down to 90 ohms.

 

[dicklaxt]

Right you are Dennis and industry is moving that way but its a slow transition,really slow I guess the old guys have to retire 1st:grin:

I think it has been close to 10 years now that I first got acquainted with it and its probably still not a plant requirement 5 % of the time.

dick

 

[zog]

I don't think I'll name the name of the company that gave the class but it definitely stated in the book (I just looked through it the other day)to keep driving ground rods until the resistance standard was achieved. It also said that a "computer quality" installation should be 5 ohms and an installation in a lightning prone area should not exceed 1 ohm. It did not say that the code required this however so from this discussion I can assume that the additional information is their opinion and not a code interpretation.

I think your instructor is a little confused.

IEEE Std 142 requires large commercial installations to be <5 Ohms and Generating or transmission stations to be <1 Ohm, it is common for the owner to require a lower spec knowing that the values may increase over time, specifically if chemical treatments are used, so 0.5 Ohms is not that odd for certian installations.