Electrician Talk - Professional Electrical Contractors Forum Can someone explain
 Register Electrical Photos FAQ Members List Social Groups Contractor Talk Search Today's Posts Mark Forums Read

 06-13-2012, 07:07 PM #1 Senior Member     Join Date: May 2012 Location: Newyork Posts: 138 Can someone explain 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?

Warning: The topics covered on this site include activities in which there exists the potential for serious injury or death. ElectricianTalk.com DOES NOT guarantee the accuracy or completeness of any information contained on this site. Always use proper safety precaution and reference reliable outside sources before attempting any construction or remodeling task!
06-13-2012, 07:16 PM   #2
Senior Member

Join Date: Dec 2011
Location: NC
Posts: 184

Quote:
 Originally Posted by 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."
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

Quote:
 Originally Posted by Sunny 1 First, I thought the earth had little resistance? If not, then why are we grounding to begin with?
Not small portions of earth and your question has been pondered by many but, it is basically for HV surges and Lightning strikes.

Quote:
 Originally Posted by Sunny 1 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?
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

Last edited by Roger.; 06-13-2012 at 07:19 PM.

06-13-2012, 07:19 PM   #3
976-EVIL

Join Date: Feb 2009
Location: State of Euphoria
Posts: 13,408

Quote:
 Originally Posted by 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?
Ohms law . Simple math 20 ohms at 120 volts would never open an ocpd.

 The Following User Says Thank You to mcclary's electrical For This Useful Post: Mike D (08-31-2012)
 06-13-2012, 07:25 PM #4 Senior Member   Join Date: Jul 2010 Location: IL Posts: 1,435 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).
06-13-2012, 07:49 PM   #5
Senior Member

Join Date: May 2012
Location: Newyork
Posts: 138

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

06-13-2012, 08:16 PM   #6
Senior Member

Join Date: Dec 2011
Location: NC
Posts: 184

Quote:
 Originally Posted by 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?
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

 06-13-2012, 08:31 PM #7 Senior Member     Join Date: Mar 2011 Location: Vermont Posts: 11,540 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~
06-13-2012, 08:36 PM   #8
Senior Member

Join Date: May 2012
Location: Newyork
Posts: 138

Quote:
 Originally Posted by 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
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?

 06-13-2012, 08:46 PM #9 animal lover /rat bastard     Join Date: Apr 2009 Location: central east coast us Posts: 7,044 low impedance path to the source (the breaker) __________________ As President Roosevelt said: "We have nothing to fear but fear itself. And Chuck Norris. And 滿口胡言. And Grabthar's hammer. And Gort. and 江南 Style. and rotting in the street and Zombies . . . and Wayne Griffen "
 06-13-2012, 08:46 PM #10 Chief Electron Relocator     Join Date: Sep 2007 Location: Cornpatch USA Posts: 31,283 Get a copy of Soare's Book on Grounding and Bonding. __________________ In winter, why do we try to keep the house as warm as it was in summer when we complained about the heat?
 The Following 2 Users Say Thank You to 480sparky For This Useful Post: chicken steve (06-14-2012), Mike D (08-31-2012)
06-13-2012, 09:21 PM   #11
Senior Member

Join Date: Dec 2011
Location: NC
Posts: 184

Quote:
 Originally Posted by 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?
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

06-13-2012, 09:34 PM   #12
Senior Member

Join Date: May 2012
Location: Newyork
Posts: 138

Quote:
 Originally Posted by 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
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

06-13-2012, 09:39 PM   #13
Senior Member

Join Date: May 2012
Location: Newyork
Posts: 138

Quote:
 Originally Posted by 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
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"

06-13-2012, 09:45 PM   #14
Senile Member

Join Date: Jan 2007
Location: Honolulu
Posts: 7,393

Quote:
 Originally Posted by 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~
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.
__________________
Proof and exoneration http://www.latimes.com/news/politics...,7305052.story

 The Following 2 Users Say Thank You to macmikeman For This Useful Post: BBQ (06-14-2012), user4818 (06-14-2012)
06-13-2012, 10:14 PM   #15
Registered Member

Join Date: Feb 2008
Location: Atlanta, Ga/Hamilton, Al
Posts: 4,868

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)

 The Following User Says Thank You to InPhase277 For This Useful Post: bruce6670 (08-31-2012)
06-14-2012, 05:40 AM   #16
Senior Member

Join Date: Mar 2011
Location: Vermont
Posts: 11,540

Quote:
 Originally Posted by 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.

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~

06-14-2012, 05:46 AM   #17
Senior Member

Join Date: Dec 2011
Location: NC
Posts: 184

Quote:
 Originally Posted by 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"
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

 06-14-2012, 05:55 AM #18 Senior Member     Join Date: Mar 2011 Location: Vermont Posts: 11,540 worth a 1000 words Rog.... ~CS~
 06-14-2012, 06:12 AM #19 Senile Member     Join Date: Jan 2007 Location: Honolulu Posts: 7,393 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...... __________________ Proof and exoneration http://www.latimes.com/news/politics...,7305052.story
 The Following User Says Thank You to macmikeman For This Useful Post: bruce6670 (08-31-2012)
 06-14-2012, 06:35 AM #20 Moderator     Join Date: May 2009 Location: Chapel Hill, NC Posts: 12,030 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 __________________

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are Off Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post jhall.sparky General Electrical Discussion 11 08-13-2011 08:34 AM stuiec General Electrical Discussion 34 08-04-2011 06:26 PM uber stein NEC Code Forum 53 10-02-2010 11:18 PM Musky Hunter General Electrical Discussion 43 01-23-2010 07:04 PM zappy NEC Code Forum 5 04-18-2009 08:38 AM

Top of Page | View New Posts

All times are GMT -4. The time now is 05:10 PM.