ground rod

[pierre]

Re: ground rod

Don
I have read these posts, and there is some very informative information that is very useful.
My question still stands: based on the average soil conditions in this country and based on actual readings, I say it is a waste of money and time to install the second ground rod in most locations. I have taken readings and I have come up with 70 ohms and much higher even with the second ground rod. If I have 90 ohms reading and test for a second rod at 70 ohms, have I actually made that part of the system effective enough to warrant the cost and time of installation?
BTW - in our 'rocky' soil, sometimes it may take more than 1/2 hour to drive the rod and it will have an even higher ohmic value.
I am all for the Grounding Electrode System, I am not for wasted efforts. If they want 25 ohms to ground, than that I understand, but not this arbitrary installation of who knows what.

Pierre

 

[don_resqcapt19]

Re: ground rod

Pierre,
For a normal building service, I tend to agree that the addition of the second rod provides very little benefit. Maybe this a good section for 2008 proposals.
Don

 

[hurk27]

Re: ground rod

In reference to Ed's diagram:
If this person is at any distance farther than 2' from the "X" grounding electrode there will be a step potential and if there is any grounding to any of the branch circuits that this sub-panel might feed there will be a shock hazard too. and this GE would not prevent this. This GE would have to be lower than 2 ohms to open even a 60 amp main feeding this sub-panel.
I just think this diagram is a little misleading.
Or I'm I missing somthing?

 

[pierre]

Re: ground rod

Thanks Don, I think maybe you are right. Maybe even have a discussion about making concrete encased electrodes become mandatory in new services and ground rods would only be necessary in service upgrades if the concrete encased is not available.

Ed
I also do not see how that GE would help the soul standing where he is. Current takes all paths and is still going to travel through him.
Maybe I am missing something also?

Pierre

 

[Ed MacLaren]

Re: ground rod

I didn?t mean to imply that the proposed electrode (R7) would open the feeder OCPD. I agree that it will not.

Refer to sketch B below, and let's assume that the total earth path between the two buildings has a resistance of 1600 ohms. At 120 volts, that would support 75 milliamps, which is the ventricular fibrillation threshold in 5% of people.

Under dry conditions, the resistance offered by the human body may be as high as 100,000 Ohms. Wet or broken skin may drop the body's resistance to 1,000 Ohms. Even if the proposed electrode-to-earth connection (R7) had a resistance as high as 1000 ohms, it would still reduce the proportion of the 75 milliamps that flow through the victim.

Ed

[ February 01, 2004, 10:57 AM: Message edited by: Ed MacLaren ]

 

[scott thompson]

Re: ground rod

I would have to agree with Hurk and Pierre, as to the "Are We Missing Something" statements

As shown on Ed's drawing, the Panelboard where the Person is being Barbequed at (using the trade accepted slang term "Sub-Panel"), has a Ground Fault (L-G).

The EGC has become open between the Sub-Panel and the Panel feeding it.
I am "*** -U-Ming" the raceway of which the Sub-Panel's Feeders are run through, is Non-Metallic (PVC), or a Cable without an EGC... is this accurate?

Lastly, the Sub-Panel is not bonded to any sort of GES at this time, and the Grounded Center Tapped Conductor / Bus is not bonded to the Sub-Panel's Enclosure either.
Proposed GES is at Location ID "X", installation is pending at time of the victim's shocking experience.

OK, with the equipment setup as described above, this will result in the victim getting shocked (figuring the victim is standing directly on the soil, possibly even barefooted; plus victim is not wearing gloves).

Adding a Ground Rod, then Bonding the Sub-Panel's enclosure to it will not eliminate the shock hazard for the unfortunate, possibly barefoot, victim.

If the Circuit between the GES at the Service Panelboard, and the Ground Rod at the Sub-Panel (the path of current flow through the Earth Ground) was 10 Ohms, and figuring the Conductors going to the Electrodes at each Panel are 0.1 Ohms each, the resultant Current flowing through the Dirt will be just a little less than 12 Amps @ 120VAC.

Now if the victim is 10K Ohms and gets caught between the energized enclosure and the Earth Ground, the resultant Current flowing through the victim, through the Dirt, through the GES @ Main Service - then finally reaching the point where the GES is bonded to the system, will be just shy of 12 ma (milli amps, or 0.012 amps) @ 120VAC.

This should do the same Barbeque-like effects, as would occur without the Ground Rod at the Sub-Panel.

I'm overlooking something in the drawing, because I know, that you know, about the stuff I posted above.
(kinda sounds like Ralph talking to Ed Norton on the Honeymooners' - the "I know, that you know" statement)

Please let me know whaddaheck I'm missing.

TIA

Scott35

 

[dereckbc]

Re: ground rod

Ed, after looking at your first drawing it reminds me of something we do CO's. We install a suplemental ground to all distribution panels. The supplemental is usally connected between the panel enclosure and the nearest building steel if available or raised floor grid.. So if the EGC should fail (almost impossible as we use a EGC and conduit) the supplemental will provide low potential and fault path.

 

[don_resqcapt19]

Re: ground rod

Dereck,

So if the EGC should fail (almost impossible as we use a EGC and conduit) the supplemental will provide low potential and fault path.

How does this result in either a fault path or a low potential?
Don

 

[hurk27]

Re: ground rod

I think what I'm getting is Dereck is installing a EGC in the conduit with the feeders. so if the conduit was to come apart the EGC (wire) would still provide a fault path.

 

[Ed MacLaren]

Re: ground rod

Adding a Ground Rod, then Bonding the Sub-Panel's enclosure to it will not eliminate the shock hazard for the unfortunate, possibly barefoot, victim.

I agree that the proposed grounding electrode will not eliminate the shock hazard, but I believe it will reduce the hazard.

There are two ways of looking at the so-called "earth path" that the "shock current" would follow.
I suppose the truth depends on whether one believes that the earth (R9) between the two buildings actually has some resistance, or whether all of the "earth path" resistance, in addition to the victim's body resistance, is in the connections (electrode to earth- R1 and R2).

If indeed the "shock current" is limited by the earth's resistance, it seems obvious that the proposed electrode (R7) will divert some of it around the victim.

Ed

[ February 03, 2004, 10:53 AM: Message edited by: Ed MacLaren ]

 

[don_resqcapt19]

Re: ground rod

Ed,
I don't think that the additional rod at the subpanel will reduce the hazard unless the person is standing on the ground within a few inches of the rod. I think that the fault paths between the panel and the power source need to be treated separately. The addition on the grounding electrode does not reduce the current through the other path. The earth itself does not really limit the current as it has a huge cross sectional area, it is rather the connections to the earth and the small "shell" of earth around these connections that limit the current. When you create a second connection to the earth via the new ground rod, you have not reduced the current in the first path through the victim.
Don

 

[pierre]

Re: ground rod

Dereck
bonding to the steel at a panel downstream of the service in my opinion would not create a 'low potential path', because the steel is not run with the phase conductors.

Ed I also have to agree with Don, as the person introduces another path for the current to follow regardless of how many other paths there may be. Unless of course as you add enough paths that the current flow would open the OCPD, but at that rate the guy would be 'toast'.

Pierre

 

[hurk27]

Re: ground rod

Pierre

The building steel don't need to follow the circuit conductors to provide a low resistance path back to the neutral. It just hast to be connected at the other end somewhere to the GE,EGC,GEC or the neutral it self.

This is just like a water pipe it will trip breakers because the "other end is connected to the neighbor's house service neutral. But it dont run with the circuit conductors.

 

[don_resqcapt19]

Re: ground rod

Wayne,
While a remote fault return path will trip low power circuit faults, once you get circuits above 300 or 400 amps the inductive reactance of a remote fault return path can limit the current flow to a level that will not quickly open the OCPD.
Don

 

[dereckbc]

Re: ground rod

Originally posted by pierre:
Dereck
bonding to the steel at a panel downstream of the service in my opinion would not create a 'low potential path', because the steel is not run with the phase conductors. Pierre

Pierre, I respect your opinion. The environment I work in is not typical of your average residential and commercial applications. We go to great extremes to ensure PQ, safety, operation, and redundancy.

One of the measures we take is ensuring ground fault paths in all AC and DC circuits. As I stated earlier; We specify an EGC ran in conduit (either rigid or EMT) ran with all circuit conductors, bond all panels, transformer Xo, etc, to building steel and raised floor grid, or both. The building steel, cable racks, piping, raised floor grid, rebar, conduit, etc, are all bonded together to form a equipotential ground plane in the office with a minimum 1/0. In the unlikely event the EGC should fail, there are two other supplemental paths to take over. IMO the equipotential ground plane forms a lower impedance than any single EGC ran with phase conductors.

So in other words we are paranoid. It's like wearing suspenders with a belt to keep your pants from falling down.