Electrical System Ground Rods

Hot diggy dog! I've waited years for anything useful to do with my IEEE Green Book. Here's the straight poop.

"The 25Ω value noted in the National Electrical Code...applies to the maximum resistance for a single electrode. ....There is no implication that 25Ω per se is a satisfactory level for a grounding system."

So, basically the code says 25Ω is how you check the quality of a single ground rod. If it's greater than that, you have to add parallel rods. But, even if it's less than 25Ω, that doesn't mean the grounding system is good enough. That depends on how much current you're planning on dumping in the ground.

I don't know nothing 'bout no water pipes. We're not allowed to use them for ground 'round these parts. But, I'm told that, where they do use 'em, they're considered to be 3Ω if they're continuous metal (How the devil do I know if they're continuous? I'm not gonna dig 'em up.)

Now, for the measurement. You can't use a Megger or an Ohmmeter. There are some commercial units made that use four (I think) small electrodes. For DIY, the basic idea is that you drive another test rod (sometimes two) and you either measure series resistance or you measure the voltage drop for a given current. If you can find an old copy of Fink and Carroll, Standard Handbook for Electrical Engineers, it's got the methods.

Quote "NFPA 70 (National Electrical Code) Article 250.56 Requires the resistance of Ground Rods to be 25 OHMs or less. Otherwise we are required to drive a second rod and this satisfies the Code.? The NEC does not require any minimum value of resistance for a grounding electrode(ground rod). It merely says that if the resistance is higher than 25 ohms, then you must add a supplemental electrode.(250.56) This goes for all electrodes. It is my understanding that the 25 ohm value evolved from old telegraph line specs. There is nothing magic about 25 ohms. The NEC says that the spacing of the additional electrode be at least 6 ft. This is far from the best spacing for minimum rod resistance. Yet I know of some inspectors that will not pass a job if the spacing is not 6 ft.

The grounding electrode is not required for the operation of an electrical system. Any system will work without it. The reason electrodes are installed is pointed out in the NEC. A system that is Effectively Grounded must be connected to the earth or something that serves in place of the earth thru a ground connection of sufficiently low impedance(no definition of low impedance) and having sufficient ampacity to prevent the buildup of voltages that may result in undue hazards to equipment or to persons. These voltage buildups may result from lightning or from electrical utility conductors that make earth contact. By installing a ground electrode, we establish a zero potential reference point. The entire electrical system is bonded to this point so that should any metal surface associated with the electrical system become energized accidentally, the fault will have a path of return and the over current device will operate and remove the fault. The bonding system is designed to maintain a low touch potential to maximize safety to humans.

Quote TVP45 "So, basically the code says 25Ω is how you check the quality of a single ground rod. If it's greater than that, you have to add parallel rods. But, even if it's less than 25Ω, that doesn't mean the grounding system is good enough. That depends on how much current you're planning on dumping in the ground.

The grounding electrode is not installed to provide a path for a ground fault current. With a resistance of 25 ohms, a 120 volt fault would only produce 120 volts/ 25 ohms equals almost 5 amps. I doubt a breaker would trip with that amount of current. In other words we do not plan to dump any fault into the earth because it won't conduct electricity very well.

For ground resistance testers, Google ground resistance meters

http://www.testequipmentdepot.com/newequipment/groundresistancetesters.htm?gclid=COqWg8H4xJsCFQZeswod5hf2Aw

With the growing use of non-metallic pipes for water supply it is no longer possible to ensure a decent earth with a water connection.

Wareagle: The NEC requires only rod, pipe or plate electrodes to meet the standard of 25 ohms or less, it says nothing about the resistance of buried water lines, concrete encased electrodes, ground rings, well casings, or the metel frames of buildings. Ref: 250.56 OK so now we drive our rod and measure >25 ohms. 250.56 then requires us to install one additional electrode and it "shall be not less than 1.8 m (6 ft) apart".

There is no requirement to retest or measure the resistance again after the second electrode has been installed. This makes me wonder what is so magical about the 25 ohm requirement. And why 25? How did this number become the standard? And if it is so important why wouldn't the NEC require us to simply build a grounding electrode system that measures 25 ohms or less?

We have measured ground resistance of electrodes and found that the ground resistance will vary quite a bit in different times of the year due to the moisture content of the soil and other environmental factors.

I'm trying really hard to make sense of this requirement