Sunny 1

Hey guys!!! I'm confused and hope someone can make light of this. I'm sitting here reading article 250 in my Mike Holt "Understanding The NEC" book, and stumble upon this:

"Danger: Because the resistance of the earth is so high, very little current returns to the electrical supply source via the earth. If a ground rod is used as the ground-fault current path, the circuit overcurrent protection device will not open and metal parts will remain energized."

First, I thought the earth had little resistance? If not, then why are we grounding to begin with?

Also, I know it's a violation to use the building's grounding system as a "Effective ground fault current path", but why wouldn't the breaker trip if this system was used as such?

Roger.

Dirt itself has very high resistance and using ohms law E/R = I figure out how much current will flow if your GES had a resistance of 25 ohms

Not small portions of earth and your question has been pondered by many but, it is basically for HV surges and Lightning strikes.

Ok using the formula above for a 125 volt circuit 125 / 25 = 5 amps, that won't do much to trip a 15 or 20 amp breaker.

Roger

mcclary's electrical

Ohms law . Simple math 20 ohms at 120 volts would never open an ocpd.

don_resqcapt19

The earth itself has a huge cross sectional area and the resistance of the earth itself approaches zero ohms. The problem is the connection between the grounding electrode and the earth. That is where the high resistance comes in.

As far as why we are grounding the system, take a look at 250.4(A)(1).

Sunny 1

Does the ground rods have resistance of 25 ohms each= 50 ohms together? Or is it 25 ohms between the 2 of them?

Roger.

25 is just a random number (and the required resistance if using one rod) you could have infinity (well, infinity on a scale) with more than one rod and be legal.


Adding rods in theory is to lower resistance not increase it.


The point is, earthing at the voltages we as electricians deal with will do very little for equipment bonding puposes.


Roger

chicken steve

The usual confusion lies in how a GEC and EGC differ.

A GEC provides premisis sine wave stability & mitigates lightning strikes

An ECG deals with faults back to the OCPD's

~CS~

Sunny 1

Thank Roger! You have made things a lot clearer now. But with that being said, I have just one last question. Why is it that when a live/phase conductor comes in contact with a grounded neutral conductor, does the breaker then trip? Can you clarify this for me? I mean what's the difference?

wildleg

low impedance path to the source (the breaker)

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480sparky

Get a copy of Soare's Book on Grounding and Bonding.

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Roger.

The term "grounded conductor" is simply a description of the second side of an AC circuit in a grounded system, we call it a neutral which is not really accurate in most cases and in reality, this conductor does not even have to be grounded to complete the circuit.

What happens when you touch the first side of the circuit to the second side of the circuit with an extremely low resistance you have very high current flow.

Roger

Sunny 1

Your loosing me roger. The definition I have of the grounded neutral conductor is the conductor connected to the bar that is intentionally connected to earth

Sunny 1

I feel like I'm running in circles now. I thought we established that the grounded neutral conductor had to be grounded to create the "Effective ground fault path"

macmikeman

Please provide some solid proof of the above statement in red. I see it in print all over , but short in the solid evidence department. I came here to learn, but I do manage to have a few opinions I realize. One of them is that there isn't a whole lot of proof of the above , more like speculations.

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InPhase277

Sunny, a "grounded neutral" is simply a neutral that happens to be grounded. A hot wire that touches a neutral trips the breaker because there is a complete circuit for current to flow. Grounding one wire does nothing to change short circuits in the line.

Look at the pic of the two batteries I drew. One is grounded, the other isn't. There is no difference as far as line shorts are concerned.

Attached Images

short pic.png (8.8 KB, 352 views)

chicken steve

I have no proof Mac, i'm simply repeating what i've been taught

in fact, i've questioned the efficacy of GEC's install methods quite a lot

~CS~

Roger.

Sunny, the "Grounded Conductor" is the Effective Fault Clearing Path in that it completes the circuit back to the source ahead of the service equiment. Grounding (earthing) has nothing to do with this function of the conductor, see InPhases drawings in post # 15






The reason this conductor is grounded is explained in 250.4 as Don pointed out.




The reason for bonding is to clear faults by taking the current back to the source, this is acheived by the EGC carrying the fault back to the neutral at the service equipment. In the illustration below take note of the Equipment Grounding Conductor carrying the fault current back to the Neutral conductor at the meter.







Roger

chicken steve

worth a 1000 words Rog....

~CS~

macmikeman

Except I have witnessed enough of crispy critter interior wiring systems after lightning strikes nearby to dwellings that causes me to still question the whole theory presented in the drawing and "teachings" about how it "limits" the strain on the system wiring. From up here on planet macmikeman it looks more like it contributes to the impact on the interior wiring of buildings than it does to lessen it. As for part two- high voltage imposed from the utility- make em run a bonding wire back to their equipment instead of making our cows jittery...

P.S. I also have a bumper sticker that says question authority......

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Dennis Alwon

Mac if there is a direct lightning hit then no grounding system will help, IMO. Lighting is incredibly strong and if it hits the gec can only do so much.

The gec will help with smaller surges from poco and lightning

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