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
