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?

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.

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.

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. 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

Ground Resistance Testers

Why Choose AEMC® Instruments Ground Resistance Testers?

We know it’s essential for you to have the ability to properly measure ground resistance to prevent costly downtime due to service interruptions caused by poor grounds. That’s why we offer one of the largest selections of easy-to-use ground resistance testers.

Hengfeng supply professional and honest service.

Pioneers of Innovation

We pioneered the use of clamp-on ground resistance testing and we regularly review and improve the features of our instruments so you can expect AEMC ground resistance testers to be the highest quality, the most complete package, and the simplest way to learn about and ensure ground integrity.

Our revolutionary Clamp-On Ground Resistance Testers will save you time and money with the ability to measure resistance without disconnecting the ground system.

We developed and introduced the only ground tester capable of testing energized transmission towers (Models /). It can also test ground resistance of individual power transmission tower legs (as well as total resistance) without disconnecting the overhead ground wire.

Widest Selection of Ground Resistance Testing Instruments

Whether you perform a simple point-to-point test, a clamp-on ground resistance test, or a more complete 3-or 4-Point Fall-of-Potential test, AEMC® Instruments manufactures the right instrument to fit your application.

Are you interested in learning more about insulation resistance tester? Contact us today to secure an expert consultation!

Accredited Ground Resistance Testing Seminars

Our experts understand the processes for earth resistance testing as referenced in the IEEE Standard No. 81. and we want you to confidently understand it too. We conduct accredited one day ground resistance technical training seminars throughout the country. Our course, comprised of both class and field training, will give you all the information you need to understand proper sizing and testing of grounding systems.

Private Seminars & Demonstrations

We also offer personalized private seminars. Have questions on how to use AEMC® Ground Resistance Testers? We're happy to provide a demonstration with our technical experts. Contact us at (800) 343- or us at

Tech Support

Our experts are available for field support in person or on the while you're at a job site.

AEMC provides complete technical support through our technical hot line 800-945- (ext.351), speak directly to one of our tech support team members. Or your clamp meter questions to our tech team

Ground Resistance Testers Selection Chart

We created this all-in-one comparison document to help you select a proper Ground Resistance Tester for your specific needs.