Sanity Check Needed ? Theory

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charlie b

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I will soon be giving a presentation of basic electrical theory to non-engineers. There is something I want to say that I know they won?t believe, at least at first. Two things, actually. I would like confirmation that I have these two things right. Opinions, contradictions, and controversy are welcome.

Scenario: A person is using a portable tool with a metal case, but has it plugged into a non-grounded, non-GFCI receptacle. It has an internal short that energizes the case. The person receives a shock. I want to say that the path for current flow is from the panel, via the receptacle, via the power cord, into the inside of the tool, via the short circuit to the case, from there through the person?s hands, arms, torso, legs, and feet to the floor, from there into the ground, via the dirt to the ground rod, up the GEC to the panel?s neutral bar, which completes the circuit (i.e., the current has returned to its source). It sounds crazy, but it is the truth. Call this a "senior confidence moment," but I will ask anyway. Am I right?

Second question: Am I right in believing that utility companies use planet Earth as a neutral return path for transmission and distribution systems, from which it can be inferred that there is current flowing through the dirt all the time?
 

stevebea

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Location
Southeastern PA
I will soon be giving a presentation of basic electrical theory to non-engineers. There is something I want to say that I know they won’t believe, at least at first. Two things, actually. I would like confirmation that I have these two things right. Opinions, contradictions, and controversy are welcome.

Scenario: A person is using a portable tool with a metal case, but has it plugged into a non-grounded, non-GFCI receptacle. It has an internal short that energizes the case. The person receives a shock. I want to say that the path for current flow is from the panel, via the receptacle, via the power cord, into the inside of the tool, via the short circuit to the case, from there through the person’s hands, arms, torso, legs, and feet to the floor, from there into the ground, via the dirt to the ground rod, up the GEC to the panel’s neutral bar, which completes the circuit (i.e., the current has returned to its source). It sounds crazy, but it is the truth. Call this a "senior confidence moment," but I will ask anyway. Am I right?

Second question: Am I right in believing that utility companies use planet Earth as a neutral return path for transmission and distribution systems, from which it can be inferred that there is current flowing through the dirt all the time?

Scenario #1 I would say yes

Scenario #2 I would say no unless the POCO is using SWER. Wikipedia says SWER is used in Australia, New Zealand, Brazil and parts of the upper Midwest for what it's worth.
 
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al hildenbrand

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Electrical Contractor, Electrical Consultant, Electrical Engineer
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I think you said it just right.

And as for the PoCo use of earth, any wired return conductors are constantly regrounded, so the earth is a parallel path for any unbalance current to travel in.
 
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hurk27

Senior Member
First question I would say yes you have it right in a basic sense not to get into all the parallel paths that current could take back to the source.

As for the second question, SWER systems are in use in many countries, and there are some HVDC SWER systems used even in our country, to add to this is the parallel current that is also flowing through the Earth from all the grounding of the electric system as a whole, but most of this current would not have any locally potential reference since it is being injected at points afar, if you can remember the Gary thread, "Time To Eat Crow" his test showed how low the over all impedance of all the parallel paths in Earth is very low, and current will take all paths in all directions back to source.

Why is this current in Earth not a problem?
Simple, on a much smaller scale, think of it like this, lay down a 1" thick copper plate 4' x 10' (size is not critical) apply a 100 amp current across this plate, now go stand in the middle, will you be shocked?

Nope, because the voltage potential between the two points where your feet would make contact would be very little, this is no different then the current flowing through Earth.

Why does a larger wire handle more current then a smaller wire, same reason, more parallel paths through the wire.
 

hurk27

Senior Member


Al, great diagram but the problem in the above diagram, is that your applying the neutral resistance to the equation, I believe that the question in the OP was the hot will make contact with the drill case with out the fault to the neutral which would most likely be cleared by the breaker, without this fault then the potential across the 500 ohm resistor (person), and the 512 ohm resistor (Earth) would be 120 volts minus the loss in the hot conductor, the out come would be much different.;)
 

don_resqcapt19

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Illinois
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...
Second question: Am I right in believing that utility companies use planet Earth as a neutral return path for transmission and distribution systems, from which it can be inferred that there is current flowing through the dirt all the time?
There are some articles that say that the utilities undersize their grounded conductors and 25% or more of the grounded conductor current flows in the earth. Even if the grounded conductors are of the correct size, there will be grounded conductor current in the earth as their grounded conductor is connected to a grounding electrode every 1200'.
 

al hildenbrand

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Minnesota
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Electrical Contractor, Electrical Consultant, Electrical Engineer
. . . the potential across the 500 ohm resistor (person), and the 512 ohm resistor (Earth) would be 120 volts minus the loss in the hot conductor, the out come would be much different.;)
Yes, you are absolutely correct.

I thought of this set of diagrams because of the illustration of the earth path, not the exact copy of the OP scenario. Charlie B's scenario would give almost all of the open circuit voltage to the person holding the portable tool.

What's interesting to me, is that the EGC, in my diagram, is intact. The touch voltage on the case of the drill, solidly connected to the 150' of SJ cord EGC, in a ground fault condition, results in a voltage divider that still hammers the person, especially because of how well connected the person is to the earth and the tool.
 

72.5kv

Senior Member
At the substation the transformer neutral is bonded to the station ground grid. The neutrals for each distribution feeder originates from the ground grid which is then brought out with is respective phase conductors. The same hold true at transmission sites,the static or shield wire at the very top of the tower is bonded to the station ground grid.
 

hurk27

Senior Member
Yes, you are absolutely correct.

I thought of this set of diagrams because of the illustration of the earth path, not the exact copy of the OP scenario. Charlie B's scenario would give almost all of the open circuit voltage to the person holding the portable tool.

What's interesting to me, is that the EGC, in my diagram, is intact. The touch voltage on the case of the drill, solidly connected to the 150' of SJ cord EGC, in a ground fault condition, results in a voltage divider that still hammers the person, especially because of how well connected the person is to the earth and the tool.

So very true and is one of the best examples to why we must not use the neutral to ground equipment to after the main breaker.
 

hurk27

Senior Member
What's interesting to me, is that the EGC, in my diagram, is intact. The touch voltage on the case of the drill, solidly connected to the 150' of SJ cord EGC, in a ground fault condition, results in a voltage divider that still hammers the person, especially because of how well connected the person is to the earth and the tool.

Ok I didn't see the above statement in my first read, I agree if you rename the neutral to a EGC in you diagram, yes your right if fault is to the EGC, but the more common fault/overload to the neutral would apply no voltage to the EGC.
 

al hildenbrand

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Minnesota
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Electrical Contractor, Electrical Consultant, Electrical Engineer
Ok I didn't see the above statement in my first read, I agree if you rename the neutral to a EGC in you diagram, yes your right if fault is to the EGC, but the more common fault/overload to the neutral would apply no voltage to the EGC.
Whether it's more common or not, I don't know. I thought of this set of diagrams, also, because of the metal body tool that gets an energized conductor up against it, like Charlie B's OP. The difference is that my diagram's tool is bonded to the EGC in the 150' of SJ cord.

If the GFI receptacle/breaker is faulty and if the overcurrent protection doesn't respond quickly, the hammering of the person holding the metal body tool continues.

In Charlie B's scenario, say, using a cheater adapter to plug a grounded three conductor tool cord to an existing two wire ungrounded receptacle, the fault energized metal tool body is truly hazardous for any person likely to be connected to any ground path (while holding the tool).
 

kingpb

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SE USA as far as you can go
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Engineer, Registered
cb, you did not state whether the cord on the tool has a ground prong or not. You might want to add the plug configuration to your description.

Also, how would that change the outcome?
 

charlie b

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cb, you did not state whether the cord on the tool has a ground prong or not. You might want to add the plug configuration to your description.

Also, how would that change the outcome?
I will be describing the incident two ways: one without a grounded receptacle outlet, and one with. My intent is to show how the presence of the ground wire creates a high current condition that forces the breaker to trip and thereby terminate the event.
 

charlie b

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To all responders: Thanks. I feel more nearly sane now. Perhaps when I finish my coffee, I might even be awake before the presentation begins. :grin:
 

Dennis Alwon

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Chapel Hill, NC
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I would think it would be very hard to get a shock thru the earth when wearing insulated boots-- of course, it would depend on how wet the boots were and how wet the earth was at the time. Certainly bare foot would do it but that is not likely- I hope. Now if the worker comes in contact with something else that is grounded, while holding the energized tool then yeah he/she will feel it.
 

don_resqcapt19

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Illinois
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I will be describing the incident two ways: one without a grounded receptacle outlet, and one with. My intent is to show how the presence of the ground wire creates a high current condition that forces the breaker to trip and thereby terminate the event.
But there still may be a shock hazard until the OCPD clears the circuit. Under fault conditions there can be a substantial voltage drop on the EGC and that voltage drop is a potential to earth at the point of fault.
 
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