Grounding Question
Grounding Question
(OP)
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
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






RE: Grounding Question
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.
RE: Grounding Question
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
RE: Grounding Question
I now handle all copper wires just like I handle an unloaded gun. Assume they're hot!
<tg>
RE: Grounding Question
I am not sure if the same concept applied in nonsubstation grounding.
RE: Grounding Question
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.
RE: Grounding Question
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.
RE: Grounding Question
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
RE: Grounding Question
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
RE: Grounding Question
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RE: Grounding Question
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