Grounding vs. Grounding Electrode Conductors

[tim janof]

Does anybody understand why the grounding electrode conductor sizing requirements are different than equipment grounding conductors? Why do we have two different tables? And why does the grounding electrode table max out at #3/0 whereas the equipment grounding conductors just keep on increasing in size.

Isn't one of the major functions of the grounding electrode conductor to conduct ground fault currents back to the source, so, in a sense, both conductor types have the same function in this case (ground fault current transport)?

Thanks for your help!

 

[iwire]

Re: Grounding vs. Grounding Electrode Conductors

Originally posted by tim janof:
Isn't one of the major functions of the grounding electrode conductor to conduct ground fault currents back to the source, so, in a sense, both conductor types have the same function in this case (ground fault current transport)?

I can give a little info and hopefully others can fill in some more info.

The first thing is no, the grounding electrode conductor is not for clearing faults inside the premise wiring system.

It simply can not do that, the resistance of the earth is to high for it to open any overcurrent devices operating at the voltages of a premise wiring system.

It can help open a utility overcurrent device at the voltages they operate at.

If you where to run a conductor right from a single pole 120 volt breaker to a ground rod driven in the ground and turn it on it would not trip the breaker.

Depending on soil conditions you might find 5 to 10 amps of current flowing on it.

So we have two different tables because they serve two different functions.

[ October 13, 2003, 04:23 PM: Message edited by: iwire ]

 

[hbiss]

Re: Grounding vs. Grounding Electrode Conductors

Because it is unlikely that the grounding electrode will have a low enough resistance to earth to be able to handle as much current as the equipment grounding conductor which has no such constraints.

 

[Guest]

Re: Grounding vs. Grounding Electrode Conductors

Originally posted by iwire:
<snip>
If you where to run a conductor right from a single pole 120 volt breaker to a ground rod driven in the ground and turn it on it would not trip the breaker.<snip>

By hearsay I can vouch for this statement. I didn't believe it at the time. I chalked it up to a DIY who was smoking something. I have never taken the time to verify it. It still does not make complete sense to me. Here goes:

A friend of a friend who was a draftsman built (& designed) his own house and did all his own electrical. He told me later that his meter was spinning all the time; he called the POCO; they came out and found that one of the service drop hot legs was grounded to earth.

Feel free to shred this story or to shed light on it. Until now I had forgotten about it and tucked it away in the Urban Legend category.

[I can see how a hot leg can be grounded to earth, but I'm having trouble visualizing the whole circuit and why other protections didn't kick in-- and how it got fouled up in the first place. It sounded too sloppy to be believed but I had no proof that it did not happen].

 

[tim janof]

Re: Grounding vs. Grounding Electrode Conductors

Oops, I should have said the "main bonding jumper," not grounding electrode conductor, which, per NEC 250.28(D), is to be sized per Table 250.66.

 

[ryan_618]

Re: Grounding vs. Grounding Electrode Conductors

Wayne, Use Ohm's law to verify it. 120 volts connected to a 25 Ohm ground rod. 120/25=4.8 amps. Not enough to open even a fifteen amp circuit breaker.

 

[Guest]

Re: Grounding vs. Grounding Electrode Conductors

I'm clear on that part. Just not the mechanics of the urban legend I described.

 

[pierre]

Re: Grounding vs. Grounding Electrode Conductors

Ryan
Your math is right on, but... that is with the thought that the 25 ohms to ground is what is at the GEC. In the area we live in the actual resistance that I have measured is 70 to 200 ohms.
Lets say it is 120 ohms, 120/120 = 1 amp of current flow
Lets take it one step further and say the resistance of the ground rod and ground electrode conductor is 5.5 ohms, The current flow would be about 22 amps. The 20 amp breaker may not trip for hours with only 22 amps of fault current, still a very dangerous situation.

For some reason there are more than less people in our industry who believe that the Grounding Electrode and Grounding Electrode Conductor are in place to conduct fault current to trip an overcurrent device. That is just not even close to the truth.

Pierre

 

[iwire]

Re: Grounding vs. Grounding Electrode Conductors

Originally posted by pierre:
For some reason there are more than less people in our industry who believe that the Grounding Electrode and Grounding Electrode Conductor are in place to conduct fault current to trip an overcurrent device.

I agree 100% and I think a lot of this confusion stems from calling everything grounding conductors.

Don's proposal to change to the way the Canadians do it makes a lot of sense to me.

Change Equipment grounding conductor to equipment bonding conductor.

IMO calling it grounding infers the "ground" or "earth" plays a role in the opening of the OCPD.

 

[tim janof]

Re: Grounding vs. Grounding Electrode Conductors

From what I can figure out, the primary purpose of the main grounding electrode is to dissipate transients on the utility lines caused by events such as lightning strikes. In other words, the grounding electrode is more for the utility company than for the building itself. Yes, it establishes a ground reference relative to the earth, but this doesn't do anything for breaker tripping inside the building. And grounding conductors and bonding jumpers (or conduits) provide the low impedance ground path, not the grounding electrode.

Does anybody have any insight to my original (but corrected) question? Why is the main bonding jumper sized differently than equipment grounds? One of the primary purposes of the bonding jumper is to conduct ground fault currents, so it seems to me that main bonding jumpers (either at main services or on transformer secondaries) should be sized the same as the equipment grounds, the latter being sized to withstand fault current for something like 5 cycles.

Thanks for your help!

 

[bennie]

Re: Grounding vs. Grounding Electrode Conductors

Many electricians do not care what the ground electrode conductor, or the equipment ground conductor performs in a system.

All they care about is the mechanics involved in the installation of the grounding system to pass inspection.

The term "Ground" is used on automobile, marine vessels, electronic equipment, and aircraft. The term "Bonding" is also used on the same equipment. The two words have different purposes and do not require any further defining.

Like I wrote on the other thread; Don't change the definition to meet the perception, change the perception to meet the definition.

 

[iwire]

Re: Grounding vs. Grounding Electrode Conductors

If the mis-perception is widespread it may make more sense to change the definition.

The mis-perception can lead to dangerous installations.

Changing the definition is not dangerous.

Ed seems to think it helped in Canada, and as you say we all deal with the same electrons.

 

[bennie]

Re: Grounding vs. Grounding Electrode Conductors

Why does an electrician have a problem with ground electrode conductors and equipment ground conductors? I think everyone I have encountered know the difference. But, so what. All they need to know is the mechanics of the connections, for a correct installation.

What will happen when an electrician is installing trolley rails? When he reads "install bonding jumpers between the rail joints". Is this for fault current?

Grounding is always bonding, but bonding is not always grounding.

I have posted this before; Bonding is shorting. Grounding is an inductor for electrostatic discharge. Two completely different applications.

 

[dereckbc]

Re: Grounding vs. Grounding Electrode Conductors

Originally posted by bennie:
I have posted this before; Bonding is shorting. Grounding is an inductor for electrostatic discharge. Two completely different applications.

I guess it depends on how you look at it Bennie and when you were taught.

Example: Visualize a radio transmitting tower and a equipment building. One side of the building facing the tower has a large copper plate attached to the building from the top of the building to below grade and welded to a ground ring. All entrance conductors (AC, Coax, Telephone, Alarm, etc) pass through the copper plate, and all grounded, ground conductors, and discharge units are bonded to the copper plate.

Is it grounded or bonded?


IMO it is a "equipotential ground plane".

[ October 13, 2003, 11:29 PM: Message edited by: dereckbc ]

 

[nvcape]

Re: Grounding vs. Grounding Electrode Conductors

You need to go back and look at the way utility services are connected. The wye is grounded at the transformer (source of power) with the phase and neutral conductors going the the service entrance board (no ground wire). The service entrance is grounded also. The fault current path is returned by the neutral and not earth. That's why even if you don't use the neutral, it's got a minimum size. Phase conductors are sized so as not to overheat, the intent is that the 3/0 will not carry current for long; how they came up with the exact size, I'm not sure. If you look at code minimum size for the electrode connection, I think it's still #8 at the rods, even for 3000 amp service! That's why it's bonded to the neutral at the service.

 

[tom baker]

Re: Grounding vs. Grounding Electrode Conductors

If you do the math, you'll find table 250.66 and 250.122 are both about 12.5 % of the ungrounded conductors. Remember table 250.66 is based on the size of the ungrounded conductors and 250.122 is on the size of the OCPD. On a service the OCPD is on the primary side of the POCO transformer, its a much longer fault current path.

 

[karl riley]

Re: Grounding vs. Grounding Electrode Conductors

Going back to the question of why so many electricians think that the ground rod is going to clear a fault, when V=IR is so simple.

I asked an English moderator (PaulUK) on an international Code forum (EC website) why electricians believe this myth. His opinion was that for some unknown reason their electrical logic kicks in when working on circuits but does not apply when they get to "earthing".

In rural buildings the OCPD operates directly from the current on the ground rod, BUT that is only because there is a sensing coil around the GEC which operates at only 30 mA to trip a relay to shut off power. Like a GFCI.

Karl

 

[matt]

Re: Grounding vs. Grounding Electrode Conductors

Correct me if I am wrong, but the main bonding jumper must be sized 12.5% of the ungrounded conductors, no maximum size.

 

[brian john]

Re: Grounding vs. Grounding Electrode Conductors

Being a bit cynicla but trying to be nice. In the areas I have worked in, with both union and non-union electricians, MOST (and a large majority of the MOST) electricians do not know the difference between the GEC, EGC, a grounded conductor and/or the neutral.

And a fair number of the MOST do not know the mechanics (how to do it right)of basic grounding. I see electrical work to support this almost every week.

Inspectors federal, local and engineers (being very careful here) fret and worry about the ground electrode they want a ufer and rods and more rods and testing (OK I MAKE SOME MONEY HERE) all in a concern over safety and while this is important they overlook. some of the important issues (IMO) regarding grounding; as it relates to safety. Grounds on the grounded conductor/neutral downstream from the main service, as part of our ground fault report we furnish this information and address it in our cover letters, typically it gets brushed aside as someone ask "How can we lower the resistance of our ground electrode system?"

 

[gwz2]

Re: Grounding vs. Grounding Electrode Conductors

"being a bit cynical" too.

It seems the people ' in power ' over the lowly electrical inspector etc. figure - "If the system works when the power is applied, It must meet the code".

Also,

Why do the job "right" when the inspector doesn't know what he/she is looking at anyway.


I sometimes think that a installation violation is like a rattlesnake, disturb it and it'll strike.

A VIOLATION of the adopted Electrical Code is for-ever a violation until corrected.