Ground Rod Resistance

[jdunlop]

I do not understand the logic behind the following. A single ground rod is required to have a resistance to the earth of less than 25 ohms. If it is over 25 ohms it must be augmented with an additional rod not less than 6 feet away. And if the resistance of the two rods still exceeds 25 ohms no additional rods are required. Why, if a single rod must be less than 25 ohms, can by adding a second rod make it OK to exceed 25 ohms? The logic would then follow that it doesn't matter what the resistance of the rods are as long as you install two, and if you do why even bother to test the resistance. What am I missing here?
Thanks for any help you can provide.

 

[Dennis Alwon]

It is an odd ball rule, IMO. The idea is to try and augment the first rod a bit but short of making us install 10 or more rods the code has accepted one extra rod. In most cases the rods are worthless anyway. The rods are suppose to help with lightning and surges and that's it.

 

[tom baker]

Its called drive two and go home.
Adding the second rod reduces the resistance by 60%, the third by a bit less, and so on. So two ground rods is the most effective.
The most efficient way to parallel ground rods is to install them 2x their length apart or 16 ft.

 

[ToolHound]

Its called drive two and go home.
Adding the second rod reduces the resistance by 60%, the third by a bit less, and so on. So two ground rods is the most effective.
The most efficient way to parallel ground rods is to install them 2x their length apart or 16 ft.

Any merit in this idea? The good effect of concrete-encasement of a GES electrode leads to me wonder about the possible merit of encasing even rod-type electrodes in concrete. Instead of driving the grounding rod into the ground, it would seem a fair bet that grounding rod conductivity-to-earth could be improved by auger'ing or otherwise digging a big hole--something like 10 feet deep by 12-18 inches diameter. And burying most of the rod-type electrode in concrete. Obvioulsly this approach could be somewhat at odds with cost control. But I could imagine any otherwise anemic rod-type electrode conductivity-to-earth would be beefed up by this approach. Probably more electrode conductivity to earth, right. ???? Just a thought...especially for anyone living in Lightning-ville, USA, or for anyone who especially values additional, robust GES conductivity to earth.

 

[mivey]

Any merit in this idea? The good effect of concrete-encasement of a GES electrode leads to me wonder about the possible merit of encasing even rod-type electrodes in concrete. Instead of driving the grounding rod into the ground, it would seem a fair bet that grounding rod conductivity-to-earth could be improved by auger'ing or otherwise digging a big hole--something like 10 feet deep by 12-18 inches diameter. And burying most of the rod-type electrode in concrete. Obvioulsly this approach could be somewhat at odds with cost control. But I could imagine any otherwise anemic rod-type electrode conductivity-to-earth would be beefed up by this approach. Probably more electrode conductivity to earth, right. ???? Just a thought...especially for anyone living in Lightning-ville, USA, or for anyone who especially values additional, robust GES conductivity to earth.

Absolutely has merit and would be a version of using ground enhancement material (GEM).