Grounding and bonding

[bphgravity]

Re: Grounding and bonding

When did I ever say or even imply that it assists in clearing the fault? I am well aware that it plays no role in this!!!!

 

[Ed MacLaren]

Re: Grounding and bonding

This sketch might help to emphasize how important a low-impedance equipment grounding conductor (egc) is in the event of a line-to-enclosure fault.

Any excess resistance in the fault-clearing path (R3 - egc) will permit more current through the victim.

In fact, in the situation shown, it can be demonstrated by calculations that the lower the resistance of the grounding electrode-to-earth connections (R1 and R2), the more current flows through the victim (R4).

Also, in this situation, a grounding electrode connected to the faulted enclosure would seem to be advantageous to the victim.

Ed

[ May 17, 2003, 10:38 PM: Message edited by: Ed MacLaren ]

 

[don_resqcapt19]

Re: Grounding and bonding

Bryan,

When did I ever say or even imply that it assists in clearing the fault? I am well aware that it plays no role in this!!!!

I never said that you said the grounding electrode has a part in the fault clearing process, but you did say that;

I am not claiming an Earth return path is going clear any faults, I am just saying without a doubt, that touch voltage can be reduced by lowering Earth resistance to the electrode.

and

When a conductor or metal equipment is connected to an Earth electrode, it is forced to take the same ZERO potential as the Earth. Any attmept to raise the voltage of a grounded object results in current passing over the connection until the potential of the object and the Earth are equal.

My point is that the grounding electrode system does not provide any real reduction in the touch voltage under fault conditions. Look at the numbers in my post of 12:16 pm. Look at the very small change in touch potential that results from reducing the grounding electode resistance from 25 ohms to 1 ohm. A change in touch potential of less than one volt!!! The grounding electrode system simply does not provide any real increase in safety under fault conditions.
Don

[ May 17, 2003, 11:01 PM: Message edited by: don_resqcapt19 ]

 

[don_resqcapt19]

Re: Grounding and bonding

Ed,
Thanks for the drawing.
I don't think that I agree with your statement; "Also, in this situation, a grounding electrode connected to the faulted enclosure would seem to be advantageous to the victim."
It would be helpful to the victim only if he was standing on the grounding electrode. If he is more than foot or so from the grounding electrode he will be subjected to a voltage that is equal to the voltage drop in the fault return path. In this case that will be the total of the voltage drop on the EGC back to the main bonding jumper plus the voltage drop on the grounded conductor back to the utility transformer. The only protection that the victim has is a low impedance fault clearing path so that the OCPD opens quickly.
Don

 

[Ed MacLaren]

Re: Grounding and bonding

Also, in this situation, a grounding electrode connected to the faulted enclosure would seem to be advantageous to the victim.

Just out of curiosity, I assigned some resistance/impedance values, and made a few calculations with regard to my proposed grounding electrode.

Sketches B and C are schematic equivalents of Sketch A, before and after connecting the proposed grounding electrode to the faulted enclosure. It appears that the lower it’s electrode-to-earth resistance, the lower the victim’s touch voltage.

I suppose it all comes down to whether we assign a resistance value to the earth itself, or not. I've seen convincing arguments pro and con in this forum.

The values used are arbitrary of course, and could be argued, but should serve for comparison purposes. Perhaps someone could check my numbers for errors.

Ed

 

[bennie]

Re: Grounding and bonding

This verifies my theory that Peg Leg Pete can lift his good leg and work on energized systems.

 

[bennie]

Re: Grounding and bonding

Seriously though these are excellent illustrations Ed. Good work.

Bennie

 

[don_resqcapt19]

Re: Grounding and bonding

Ed,
I don't agree. I think that in both cases, the victim receives a 60 volt shock. While I agree that there will be a total of 60 volts dropped across the series/parallel circuit created by the 3 grounding electrodes, there will also be 60 volts dropped across the victim. The drawing makes it appear that the human and R7 are in parallel with each other and then in series with paralleled R1 and R2. I don't think that is correct. What we really have is the 37.5 ohm series/parallel resistance of the R1, R2, and R7, in parallel with the human. This is a parallel circuit and the voltage across each of the legs must be the same, so he will still receive a 60 volt shock.
The only way to reduce the voltage across the victim is to reduce the impedance of the fault return path enough to reduce the voltage drop on this path. If you place a second EGC in parallel with R3, you would have a 1600 amp fault, a drop of 56 volts in the transformer, 24 volts in the feeder and 40 volts for the two parallel EGCs.
Don

 

[bennie]

Re: Grounding and bonding

I agree after a nodal analysis the voltage at the point of human contact is 60 volts. The ground rod makes no difference.

However the ground contact, and impedance of the surface area, is often at or near an open circuit. There would be no potential difference to create a shock in most cases.

Surface resistance in some locations is as high as 2000 to 8000 Ohms. The ground rod resistance at 8 feet deep may be 8 to 20 Ohms at the same location.

The amount of current possible on the ground rods will not open a thermal/magnetic breaker.
The current will open an internal logic breaker. My point is the ground current flow is important for clearing faults when proper sensors are in use.

Positively declaring a ground system will not clear a ground fault is technically wrong. The statement must be qualified with details.

 

[bphgravity]

Re: Grounding and bonding

I tend to agree that the most pertinent reasons for service grounding is for external faults and conditions.

The effects of lightning, transformer insulation break down, or high voltage crossover is best reduced with low impedance electrode systems.
Internal faults should not and generally do not rely on Earth connection to clear the fault or protect equipment from the fault.

The same issue is covered by both Mike Holt and Tom Henry in published work. Each seem to have a conflicting idea on grounding electrode systems, however, both consider their mentor and greatest teacher Mr. Joe McPartland.

I would be interested to see what ideas he has published himself concering touch voltage and ground rods.

I think both Don and Ed have good mathematical and fundemental understanding of this subject. I appreciate both views tremendously. I am still somewhere in the middle and realize I need to obtain a greater knowledge of Earth grounding.

Great topic!

 

[Ed MacLaren]

Re: Grounding and bonding

The drawing makes it appear that the human and R7 are in parallel with each other and then in series with paralleled R1 and R2.

I believe that to be the case, if we assume that the earth is a common "node" with no resistance of it's own, the resistance all being in the electrode's connections to the earth.

Sketches D and E are arranged so that this is easier to visualize. The portion of fault current that follows the earth can only get there through the "victim" and R7 in parallel, then through the earth, and must return to the transformer neutral through the two system grounding conductors, R1 and R2 in parallel.

If the R7 resistance is lowered (sketch E), the voltage across the victim is lowered.

Isn't this the reason a grounding electrode connected to the enclosure is required at a sub-service in a second building, even when the feeder has an EGC?

Ed

 

[bennie]

Re: Grounding and bonding

The path for the human would be across the line, on top of the dirt, bypassing the other resistors.

Only two ground rods should be counted. One at the service and one at the panel. Parallel resistors are not relevant, only series ones affect voltage drop across the body.
The path for R-4 is in parallel with R-3. The voltage across R-3 will be felt by the person.

An example...Apply 120 volts across a 1 megohm resistor. Touch each end. You will get fried.
Now place yourself in series with a 1 megohm resistor. Nothing is felt.

Apply 120 volts across a 10 Ohm resistor. Touch each end. You will get fried.

Now place yourself in series with a 10 Ohm resistor, you still get fried

[ May 19, 2003, 01:36 AM: Message edited by: bennie ]

 

[don_resqcapt19]

Re: Grounding and bonding

Ed,

The drawing makes it appear that the human and R7 are in parallel with each other and then in series with paralleled R1 and R2.(don) I believe that to be the case, if we assume that the earth is a common "node" with no resistance of it's own, the resistance all being in the electrode's connections to the earth. (Ed)

The only way that the voltage across the victim could change is if the earth that the victim is standing on would be raised to 20 volts above "true earth". While there is voltage gradient around the electrode, it doesn't extend very far, at this type of voltage, before the voltage is very close to "true earth". I believe that if the victim is more than a foot or 2 from the electrode he will be subjected to almost the full 60 volts.
Don

 

[don_resqcapt19]

Re: Grounding and bonding

Ed,

Isn't this the reason a grounding electrode connected to the enclosure is required at a sub-service in a second building, even when the feeder has an EGC?

In my opinion the reason for a grounding electode at the second building it for the same reasons as at the service as listed in 250.4(A)(1).

250.4(A)(1) (1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.

While the chance of contact with a high voltage system would normally be remote for a second building feeder, both above and underground feeders can be hit by lightning.
Don

 

[bennie]

Re: Grounding and bonding

The earth is not a bond. There is variable resistance vertically, as well as laterally.

Current is the car, resistance is the road.

 

[Ed MacLaren]

Re: Grounding and bonding

In my opinion the reason for a grounding electode at the second building it for the same reasons as at the service as listed in 250.4(A)(1).

Don, I was referring to the case where a "four-wire" feeder that includes an equipment grounding conductor is run to a second building.
The required grounding electrode is connected only to the enclosure and branch circuit EGCs, not to the feeder grounded (neutral) conductor.

Ed

 

[don_resqcapt19]

Re: Grounding and bonding

Ed,
In the case of the 4 wire second building feeder, I don't see any reason, other than the code requirement, to install a grounding electrode system. Other than helping to shunt a lightning strike to earth, it serves no real function. I do not see it playing any part in making the system safer under ground fualt or open neutral conditions.
Don

 

[bennie]

Re: Grounding and bonding

Don: To have any credibility in the code, are we entitled to a sensible explanation of the intent of a section?

[ May 19, 2003, 11:58 PM: Message edited by: bennie ]

 

[don_resqcapt19]

Re: Grounding and bonding

Bennie,
The only place that I know of that has that information is the ROPs (TCRs) and the ROCs (TCDs), but the problem is that you must have the one when the section was placed into the code or changed. Sometimes things do get into the code without any real technical proof.
don