Ground fault power line

  • Thread starter RedX
  • Start date
  • #1
970
3

Main Question or Discussion Point

If a hot power line hits the ground, and you are touching the neutral line with your hands and the ground with your feet, would you get shocked?

Also, same question, except the lines are coming out of a grounded portable generator instead of the electric company.
 

Answers and Replies

  • #2
dlgoff
Science Advisor
Gold Member
3,828
1,759
This depends on how high the potential of the line is, soil material, soil moisture ...

In electrical engineering, earth potential rise (EPR) also called ground potential rise (GPR) occurs when a large current flows to earth through an earth grid impedance. The potential relative to a distant point on the Earth is highest at the point where current enters the ground, and declines with distance from the source. Ground potential rise is a concern in the design of electrical substations because the high potential may be a hazard to people or equipment. The potential gradient (drop of voltage with distance) may be so high that a person could be injured due to the voltage developed between two feet, or between the ground on which the person is standing...
http://en.wikipedia.org/wiki/Earth_potential_rise" [Broken]
 
Last edited by a moderator:
  • #3
256bits
Gold Member
3,175
1,192
Dlgoff has summed it up quite nicely.

Standing in one spot you might not get a shock. Standing 10 feet away you could be shocked, due to the potential gradient.

And also beware what neutral or ground means. It can be common or reference potential or return path of the current and also not necessarily 0v depending on a particular circuit.
 
  • #4
970
3
In power lines, the neutral wire is tied to ground, causing a hazard if you touch the ground and the hot wire simultaneously.

It seems it would be much better not to ground the neutral wire, so that you only get shocked if you touch the hot wire and the neutral wire simultaneously.

An explanation I heard for grounding the neutral wire is that a fault can happen, either the neutral or hot wire making a connection to ground. Then if you're holding the wire that's not touching the ground, you'll get shocked. So there is a 50/50 chance you're holding the wrong wire.

The point of tying neutral to ground was that then you're guaranteed that you can always touch the neutral and ground and be safe. But I don't see how that's true, since if the hot wire touches the ground then there's a circuit from the hot wire, through the ground, through you, and back through the neutral wire. The chance of the hot wire falling to the ground versus the neutral wire is still 50/50.

I find it interesting that if your feet are spread apart a lot then the voltage drop through you can be really high because of the resistance of the soil. But I think the situation I described is a little different because you're actually holding a low resistance neutral wire. So the voltage between your feet developed by a potential drop from currents flowing under the earth may still shock you, but in the situation I described current has a direct path from ground, through you, through the neutral wire.

Also, it doesn't have to be that the hot wire falls to the ground. Lightning can ionize the air and also break down any dielectrics shielding the line (if there are any - I don't know), creating a path from the hot wire through the ionized air to the ground.
 
  • #5
dlgoff
Science Advisor
Gold Member
3,828
1,759
This should clear up your misunderstanding of why there's a Safety Ground, which is the fault currents path as opposed to your body.

gfault.gif


http://hyperphysics.phy-astr.gsu.edu/hbase/electric/bregnd.html" [Broken]
 
Last edited by a moderator:
  • #6
970
3
That makes sense. I was reading this website:

http://www.allaboutcircuits.com/vol_1/chpt_3/3.html

and they make no mention about circuit breakers at all! It begins in the middle of the page with the paragraph:

"Circuit grounding ensures that at least one point in the circuit will be safe to touch. But what about leaving a circuit completely ungrounded? Wouldn't that make any person touching just a single wire as safe as the bird sitting on just one? Ideally, yes. Practically, no. Observe what happens with no ground at all..."

It just didn't make sense to me how grounding makes one point in the circuit safe to touch, but with a circuit breaker that's true.
 
  • #7
dlgoff
Science Advisor
Gold Member
3,828
1,759
It just didn't make sense to me how grounding makes one point in the circuit safe to touch, but with a circuit breaker that's true.
If you ground a point in a circuit (0 volt reference) and you touch this point (Vfinger=0) while your feet are grounded (Vfeet=0) there will be no current through your body resistance (Rbody measured in Ωs) since there's no potential difference between your finger and feet. i.e. I=(Vfinger-Vfeet)/Rbody=(0-0)/Rbody= 0 amps
 
  • #8
970
3
If you ground a point in a circuit (0 volt reference) and you touch this point (Vfinger=0) while your feet are grounded (Vfeet=0) there will be no current through your body resistance (Rbody measured in Ωs) since there's no potential difference between your finger and feet. i.e. I=(Vfinger-Vfeet)/Rbody=(0-0)/Rbody= 0 amps
Right, but if you don't ground that point in the circuit, and touch the circuit, then you're just as safe. In this case, Vfinger=Vfeet, since current can't flow out of your feet since the system is not connected to the ground.
 
  • #9
dlgoff
Science Advisor
Gold Member
3,828
1,759
Right, but if you don't ground that point in the circuit, and touch the circuit, then you're just as safe. In this case, Vfinger=Vfeet, since current can't flow out of your feet since the system is not connected to the ground.
This is correct and you are safe like a bird on a wire. But, like in your link,
...this could all change with an accidental ground, such as a tree branch touching a power line and providing connection to earth ground...
 
  • #10
5,439
7
Good morning electricalele,

Safety of electrical apparatus and sytems is only one reason for using earths. There are others.

A safety earth plays no part in the normal operation of an electrical circuit and may not even be connected to it. It is only brought into action in fault conditions.

Other uses of earthing do play a part in normal circuit operation. These have been discussed extensively here at PF.

go well
 
  • #12
241
0
It seems to me the only reason the "earth" anything would be to dissipate static electricity (Jim has a real life story in my link).

Grounding/bonding internally, like dlgoff stated in post #5, makes perfect sense.
I agree with you that bonding all the metal stuff together in our structures and tying them to nuetral and grounding them is a good idea to protect us against shocks from faults... And dissipating static electricity will most definitely protect our equipment, as in Jim's example.

But I'm not following you on the rest of this... Talking strictly about the electrical grid, are you saying that an ungrounded distribution system would be just as safe as a grounded one? What about the unintended ground fault? Or, for that matter, the example where two people were touching the conductors, thinking it was safe and creating a parallel path?
 
  • #13
32
2
I think the grounded grid is invaluable for its static electricity dissapation and would not change a thing.

But, lets say, there was no such thing as static electricity seeking a path to ground. I dont see how an ungrounded system would be un safe. The unintentional ground fault caused by a tree touching the "Hot" wire would supposedly make the return "unsafe". Well I see trees touching the "Hot" wire now right next to a grounded pole and its not shorting out, maybe the trees and dirt are poor conductors. Two people touching opposite wires and getting shocked , I think those two people would need to be really close together to have this affect.

A good practice anyway is to ensure the wires are not "hot" before touching them

I am not completely married to this idea, so I am open to the possibility I am wrong, I am interested in some discussion on the matter. This is just what makes sense to me.
 
  • #14
5,439
7
Why the narrow view of earthing?

Most of the metal that is earthed is not ever intended to be part of electrical apparatus or an electric circuit.
 
  • #15
jim hardy
Science Advisor
Gold Member
2019 Award
Dearly Missed
9,839
4,877
A system that's earthed through a high resistance (or impedance) has this advantage:

when something's insulation fails and it shorts to earth,,
the circuit breaker does not trip because the impedance limits the current through the failed insulation to a low value.
So the faulted piece of equipment can continue to operate for a while.

That gives you time to start a backup machine , which can avoid shutting down a complex and expensive process like a factory or nuke power plant.

such power systems are equipped with "ground fault" detection so you'll know that insulation has failed and you need to fix it.

That's differeent from your house where the system is earthed through a solid wire connection. At home a failed insulation passes high current which trips the circuit breaker rendering the faulted machine or wire safely de-energized.

The completely un-earthed system would have a potential to earth that is completely unknown.
No static electricity (or some other effects) - no problem, but it's not good design to "just take your chances" .

"Well I see trees touching the "Hot" wire now right next to a grounded pole and its not shorting out, maybe the trees and dirt are poor conductors. " that's the answer.
High enough voltage will shoot sparks and set limbs afire.

BTW those drawings Digoff posted are Just Great !!!
I truly admire (and envy) you guys who can do those wonderful graphics.
 
  • #16
241
0
The completely un-earthed system would have a potential to earth that is completely unknown.
No static electricity (or some other effects) - no problem, but it's not good design to "just take your chances"
Why is static electricity being introduced to this discussion?

Does a normally operating loop (circuit) that is (let's just say) hovering in mid-air, somehow create a build up of charge that would be attracted to the earth below?

I don't really understand this because it seems to me that whatever leaves the source, has to come back on the return... If every electron that left the source has to return, which ones are being attracted to the earth? Where did they come from?
 
  • #17
jim hardy
Science Advisor
Gold Member
2019 Award
Dearly Missed
9,839
4,877
Why is static electricity being introduced to this discussion?
because it's a real phenomenon that can cause trouble, and because it's mentioned in earlier posts.
Does a normally operating loop (circuit) that is (let's just say) hovering in mid-air, somehow create a build up of charge that would be attracted to the earth below?
Not so long as things are going well. But rotating machinery like motors moves a lot of air to cool itself and static electricity can build up wherever there's moving parts and insulators. Consider resemblance of a conveyor belt to Van de Graff generator.


I don't really understand this because it seems to me that whatever leaves the source, has to come back on the return... If every electron that left the source has to return, which ones are being attracted to the earth? Where did they come from?
You're restating Kirchoff's Current law. Kirchoff describes electricity flowing in a closed system.

Static electricity appears to violate Kirchoff. But i told my students to think of it this way -
for the case of static electricity Kirchoff will accept a temporary delay. Static means it's temporarily not flowing.

Example - Lightning is electrons that were carried up from the earth's surface by water or dust molecules. It's returning to earth with a bang. Same thing when you walk across a carpet in wintertime up north and touch a doorknob - electrons pulled out of the carpet by your leather shoe soles make a spark to the doorknob, then work their way back via the hinges and doorjamb to the carpet, giving you that shock.

The electrons do get back, eventually.

So your question is valid - where does the potential come from?
It comes from either an excess or a shortage of electrons on the surface of the un-earthed loop. Its value is literally undefined.
That potential should be small, but Murphy's Law says if you are depending on it to be small you will get embarassed. What if your circuit recently suffered a lightning stroke that deposited a zillion electrons there?

So we design to make sure it's small.

that's my attempt at a word picture.

In any capacitor Q = CV

and your loop has some capacitance to earth
so potential to earth in volts V is equal to excess (or shortfall) of charge Q divided by capacitance to earth C.
Usually Q is small and so is V.
But dont bet your life on it.

ever wonder about earth itself?
Does earth have an excess of shortfall of electrons?
What is earths potential with respect to the sun?

Earth has an electric field a hundred volts per meter or so.. search on earth potential gradient

It has a magnetic field. As if a current were flowing around equator.
Would excess charge riding on the surface give those two effects?

Maybe that's discussed in astronomy forum i dont know, just i have always been curious.
Will peruse over there.

old jim
 
Last edited:
  • #18
32
2
The completely un-earthed system would have a potential to earth that is completely unknown.
No static electricity (or some other effects) - no problem, but it's not good design to "just take your chances" .
Why would we care about any potential to earth if the electricity is not trying to go there?

I definitely see the benefit of grounding/bonding in the home, as digoff demonstrated. In the case of insulation breaking down inside a conduit, having the green/bare wire bonded to the metal fixtures, and then bonded to the neutral in the PDP makes perfect sense, or else you would energize your metal fixtures. But, on the power poles I'm not sure that is the case.
 
  • #19
jim hardy
Science Advisor
Gold Member
2019 Award
Dearly Missed
9,839
4,877
oops lost another long post

""Why would we care about any potential to earth if the electricity is not trying to go there?""

the answer is real un-sophisticated and has noting to do with circuit theory.

It's a maintenance and reliability matter.

Since as you observe electricity is not trying to go there, a single failure of the insulation would have no effect other than to fix the potential to earth. That'd be zero on the node with faulted insulation, and in accordance with Kirchoff's voltage law at all other nodes. Since there's no other effect you wouldn't even notice.

When your SECOND failure comes along at a different node now you have pyrotechnics at both faults.
If you'd known about the first one you might've fixed it before second one appeared.

In house wiring the first fault trips a breaker to announce its presence, and put the appliance or whatever in a safe state.

In heavy industrial wiring the power system is usually arranged so there's symmetric voltage to earth and the first fault upsets that symmetry but doesn't pass very much current. The asymmetry is detected and sounds an alarm so you know you need to fix something. In many systems that doesn't trip the breaker.

Reason it doesn't trip a breaker is the faulted circuit might feed something very important and that gives you time to start a backup machine.

The big power lines you see outdoors are a solidly earthed system. In S Florida the neutral wire runs along the top to act as a lightning rod and is earthed frequently for that reason.
 

Related Threads on Ground fault power line

  • Last Post
Replies
6
Views
1K
  • Last Post
Replies
10
Views
1K
  • Last Post
Replies
9
Views
4K
  • Last Post
Replies
2
Views
11K
  • Last Post
Replies
16
Views
3K
  • Last Post
Replies
10
Views
2K
  • Last Post
Replies
2
Views
658
Replies
6
Views
1K
Replies
38
Views
2K
Replies
5
Views
2K
Top