25 ohms resistance measurement

[tom baker]

The real desire for grounding is to get a low enough resistance to trip a breaker during a ground fault. .

!!This statement is incorrect. The purpose of grounding is given in 250.4(A)(1)!!

At 25 ohms resistance, a ground rod will have 4 amps at 120 volts, in a fault where the earth is used as the return path to trip a breaker.

What I tell electricians in my classes is your number one job is bonding, which is creating the electrically conductive path to clear a fault. In a line to case fault, electrons go back to the source, not earth.

Unless you are dealing with a telecom site, dispatch center, etc, the resistance of the ground rods does not matter. Use a ufer ground, you get a low resistance at virtually no cost, If you need ground rods, drive two and go home.

 

[kwired]

The real desire for grounding is to get a low enough resistance to trip a breaker during a ground fault. Some places this is not difficult depending on location and season. Other places it is virtually impossible. In my opinion, housing couldn't absorb the cost of installing ground rings around very house that had high resistance soil, and there isn't a manufacturer who would make enough money to lobby for it. So, they now require the UFER (concrete encased electrode) for new installations (which I think is a really good thing) and give the installer the out of two ground rods maximum in an existing installation.

Even the Ufer will seldom yield low enough resistance to allow enough fault current to trip overcurrent protection.


The purpose of grounding is to give the system a reference to earth.

Bonding is what ensures a low resistance to help facilitate overcurrent device operation when there is a ground fault.

 

[gadfly56]

I think not. Unless you are dealing with medium voltage systems, planet Earth is never going to play a role in getting a breaker to trip. (What, never? No never! What, never? Well, hardly ever!) Indeed, the last sentence in 250.4(B)(4) tells us never to count on it doing so.

Maybe if you polish up the brass so carefully?

 

[meternerd]

I always assumed the ground rods were for lightning protection. Am I wrong? I never was taught that they had anything to do with ground faults. As far as the 25 ohms, how many AHJ inspectors are even qualified to interpret ground resistance readings. I know I'm not. We did tests in substations, but that was to verify ground grid continuity.

 

[user 100]

I always assumed the ground rods were for lightning protection. Am I wrong? I never was taught that they had anything to do with ground faults.

Ground rods are supposed to help with lightning and or unintended power surges ( think primary drops on secondary, etc)-whether they would be effective in mitigating damage from a direct or very close strike is highly debatable-the number of amps recorded in many "positive" strikes has exceeded 50,000 and a gec (or 2 or 3), probably isn't going to help that much.

 

[kwired]

I always assumed the ground rods were for lightning protection. Am I wrong? I never was taught that they had anything to do with ground faults. As far as the 25 ohms, how many AHJ inspectors are even qualified to interpret ground resistance readings. I know I'm not. We did tests in substations, but that was to verify ground grid continuity.

I agree with user100 - they help with lightning protection. Too many say they are for lighting protection which leads to misunderstanding in that they are more of a lightning protection system, which they are not.

Plus all acceptable grounding electrodes provide same function not just ground rods.

 

[iwire]

I agree with user100 - they help with lightning protection. Too many say they are for lighting protection which leads to misunderstanding in that they are more of a lightning protection system, which they are not.

And I don't think they do much as providing a 'reference to ground' that reference tapers off quickly as you move away from the electrode.

I am not convinced they do much at all for systems under 600 volts.

 

[kwired]

And I don't think they do much as providing a 'reference to ground' that reference tapers off quickly as you move away from the electrode.

I am not convinced they do much at all for systems under 600 volts.

On an isolated system correct, if you have a bond to the utility system you have a huge network of electrodes all tied together to make a pretty solid reference.

Now that doesn't mean earth will be a suitable circuit conductor or fault carrying conductor, but you can still measure full voltage from an ungrounded conductor to "ground" most any distance from your service grounding electrodes.

Someone on this site once said earth is a good conductor, but is often difficult to make a low impedance connection to it with limited sized electrodes.