Grounding Question

[Dobber1978]

On site we have construction going on and a question was posed to me today and would like other peoples options on it.

They are going to be digging around the existing 4/0 bare copper ground cable that runs around the main building and I was asked if there is a fault to ground if there any potential that someone gets shocked off this bare ground cable. I think that there is potential, pretty small but if a guy is standing in the hole and happens to be touching the ground cable at the time of fault he may feel something.



Let me know what your thoughts are on this.

Thanks
Jeff

 

[dpc]

The probability may be low, but it is a definite saftey hazard. If someone is in contact with the bare ground conductor during a high current ground fault, he just had a really bad day. If the fault current is high enough, there **could** be enough potential difference between the conductor and whatever his feet or (worse) other hand is touching to kill him.

This is a concern in electrical substation where fault currents and potential rise can be high. In other facilities, the risk may be lower, but it can't be totally ignored. Anyone delibrately making contact the bare ground wire should be wearing insulated gloves.

 

[resqcapt19]

Assuming that the system is a grounded system, I don't see any more danger touching the ground ring than I do touching any metallic item that is bonded to the electrical system. Under fault conditions, until the fault is cleared, all conductive items that are bonded to the electrical system will have an elevated voltage with respect to earth. This voltage will be equal to the voltage drop on the grounded conductor between the utility transformer and the main bonding jumper. This voltage will only exist until the fault is cleared by the OCPD. This voltage could be high enough to be a shock hazard.
If it is an ungrounded system, a single fault should not produce any voltage on the grounding system as there will not be enough current flow to create a voltage drop.
Don

 

[telecomguy]

Got a jolt from a ground rod one time while working on a water leak. Found out the ground wire coming to the meter box was dangling up on the power pole. The building was running on a floating ground. That helped explain why the florescent lights kept acting up and the UPS on the computer kept sounding an alarm. ;o)

I now handle all copper wires just like I handle an unloaded gun. Assume they're hot!

<tg>

 

[bacon4life]

In a substation application, the grounding system consists of both the ground grid and the substation yard rock. The shock potential would be higher standing in a damp hole than standing on insulative crushed rock.

I am not sure if the same concept applied in nonsubstation grounding.

 

[waross]

In the event of a ground fault in the building the majority of the ground current never sees the ground grid. It flows through the grounding conductors to the neutral bonding jumper and back to the source through the neutral.
There are several ways that the ground grid can become energized.
In the case of an open neutral the neutral current returns to the distribution transformer through the ground rods and/or grid and the distributed multiple earthing of the utilities neutral conductor.
In the case of an open neutral and a missing phase, the neutral current returning through ground can be about the same as the current drawn by the remaining single-phase loads.
In the case of two missing phases, the neutral current returning through ground will equal the phase current.
Continuous ground currents tend to warm and dry the ground and raise the resistance. The higher the ground resistance, the greater the touch or step potential at a given current level.
Another way the ground grid can become energized is if a vehicle hits a power pole and a primary conductor drops onto the neutral.
Most of these things never happen in the life of a building.
When they do happen, the duration is usually very short (except for the open neutral).
Summary;
Note: - In all cases the hazard increases directly as the current to ground increases.
1> For a fault inside the building the fault current should follow the grounding conductors and the bonding jumper to the neutral.
The hazard at the ground grid should be negligible.
2> With a missing neutral bonding jumper there would be a definite hazard in the event of a ground fault in the building. The hazard at the ground grid would range from considerable in the case of a single ground rod to negligible in the case of an extensive grounding system if only a small part of the ground grid is exposed.
3> Open neutral. The ground grid will be passing the neutral current.
The hazard will be proportional to the neutral current.
The hazard with a single ground rod will be fairly high.
The hazard will probably be negligible in the case of an extensive grounding system if only a small part of the ground grid is exposed.
4> Nearby lightning strikes. High voltage contact with the utility's neutral on the poles.
Hazard unknown, see "Act of God".
The actual risk is quite low, but there is an exremely small chance that there could be an incident.
The odds of a serious incident under item 4 may be something like The total number of seconds in the life of 100 buildings, each 85 year old, divided by a 5 second fault duration, and multiplied by the plate number of the car that hit the pole.
There is a very small chance of a problem but none of us will say that there is no problem.
It may be wise to call an electrician or engineer to verify that the ground to neutral bonding jumper is properly installed and in good order. A bad bonding jumper will be the most likely problem and the most likely to cause an incident.
respectfully
And I agree with all the previous posts.

 

[alehman]

I think the answer depends on how good the earth contact is for what's left in the earth. Poor or small contact area means high impedance means high touch potential.

If the remaining portion of the grounding electrode is adequate then resqcapt would be correct. If the remaining system is so weak that there is danger in touching the ground ring, then there is danger in touching any equipment in the system and it should be shut down.

 

[resqcapt19]

alehman,
The condition of the grounding electrode system would have nothing to do with this issue. The fault current does not flow to the earth, it flows back to the source via the main bonding jumper.
Don

 

[Dobber1978]

To add a little about our system.

1. System is resistance grounded with monitors that alarm first then trip.

2. Lightning arrestors on the main incoming (115kv)and on secondary (13.8kv)

3. Remote ground grid to give our low resistance connection to ground

4. System is less then 2 years old.

Jeff

 

[RalphChristie]

If resistance grounded the probability of a shock is less than with a solidly connected ground. During a ground-fault a part of the voltage will be over the resistors(amount will depend on the resistance) and thus the step and touch potentials will be reduced.

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[waross]

Hello Dobber1978
I think that most of us were visualizing a smaller system with a solidly grounded neutral.
Your grounding impedance/resistance should limit any ground currents from an internal building fault to a low level. You probably have negligible hazard from internal ground faults.
Faults external to your facility would still create the same touch and step potentials on your system.

The hazard will probably be the same as the hazard present if you were standing on moist ground and touching a grounded fence when an external ground fault occured.
respectfully