Grounding Description in Vol I Ch 3 -- Huh?!?!

Discussion in 'General Electronics Chat' started by wtfhuh, Oct 9, 2014.

1. wtfhuh Thread Starter New Member

Oct 9, 2014
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I'm thoroughly confused by the grounding description in Vol I Ch 3. I've found a few similar threads on this forum, but none of them really got to the meat of the issue for me.

See this pic:

Why is there no current along the pink line?

I suspect some people will mention something about the hand connection and the foot connection being electrically common or whatever. Well, if I'm not mistaken, the two black dots on the bottom wire are also electrically common, yet that doesn't prevent current between them.

2. tshuck Well-Known Member

Oct 18, 2012
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The path through the person is highly resistive, where the node between the hand and ground has no resistance (in an idealized case, a comparatively small resistance otherwise).

In the idealized case, no current will flow through the person because the ground node has no resistance and carries all of the current.

3. alfacliff Well-Known Member

Dec 13, 2013
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because the person is grounded, and the line he is touching is grounded.

4. studiot AAC Fanatic!

Nov 9, 2007
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Exactly.

Both the line and person are in good contact with somethings that are at the same potential

No current can ever flow between any two points in any circuit that are at the same potential.

5. Lestraveled Well-Known Member

May 19, 2014
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In the second figure, the mans foot is connected to ground and his hand is connected to ground. They are connected together. Neither the hand nor foot is connected to the "hot" side.

6. wtfhuh Thread Starter New Member

Oct 9, 2014
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My immediate question was going to be "How come the highly resistive path conducts in one case (shocked person) but not in the other (non-shocked person)?" That was something else I'd been bothered by in reading comments.

But I think I see the source of my confusion now. When comparing non-shocked-person-path with hand-ground-node (the bottom wire), I was looking at the similar voltage but not at the different resistance. And when comparing non-shocked-person-path with shocked-person-path, I was looking at the similar resistance but not at the different voltage.

7. studiot AAC Fanatic!

Nov 9, 2007
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That would actually have been a very good question, because it doesn't actually matter how resistive the path is or isn't.
And sometimes the ground resistance is very high.

So long as both points are at the same potential the idiot that grabs onto power lines cannot be shocked.

Dec 31, 2010
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Pat - W9ZO

9. tshuck Well-Known Member

Oct 18, 2012
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...and that same potential is entirely dependent on the impedance of the path - both directly to ground and through the person.

This is only valid for ideal circuits and superconductors.

10. wtfhuh Thread Starter New Member

Oct 9, 2014
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Thanks for the answers, everyone. They've given me a few light-bulb moments

11. studiot AAC Fanatic!

Nov 9, 2007
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Can't agree with this.

If one end was grounded in New York and the other in Beijing, the result would be the same.

And there's thousands of miles of granite, conductivity zilch, between NY and Bj, not the mention the even denser political heads.

A ground is a body that does not change its potential regardless of the current it sinks or sources. Good old Mother Earth is the best and nearest thing we have to this ideal.

12. tshuck Well-Known Member

Oct 18, 2012
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In a non-ideal case, there is resistance in the return path, shown as $R_{rtnpath}$ in the image below:

So, you are telling me that if $R_{rtnpath}$in the above image is comparable, let alone larger than the resistance of the person, still the person will not be shocked?

13. t_n_k AAC Fanatic!

Mar 6, 2009
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There is another related situation known as Earth Potential Rise which can occur in the vicinity of substantial ground faults in sub stations or transmission lines.
See Wikipedia ...

14. alfacliff Well-Known Member

Dec 13, 2013
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unless you are standing on dry solid rock, sometimes even then, earth ground can be concidered very low resistance. even in the days of the old telegraph, one overhead wire and ground were the circuit. thats with batteries supplying up to 100 volts to the circuit, sometimes less, and conductance in the tens of milliamperes., over hundreds of miles. huge mass is the difference.

15. wtfhuh Thread Starter New Member

Oct 9, 2014
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IDEAL WORLD: no potential difference between hand and foot, hence no current through body. (In I = V/R, with V = 0, R is irrelevant.)
REAL WORLD: negligible potential difference between hand and foot with high resistance, hence negligible current through body.

Is that a fair way of describing ground safety?

16. #12 Expert

Nov 30, 2010
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Most of the time, yes.
If you went around sticking a ground rod in the soil and testing grounds on your household outlets, you would probably find a few millivolts on several places in the house. I just measured myself from hand to hand and I was 5 million ohms about ten minutes after washing my hands. If I grounded out 10 millivolts, I would pass a current of 2 nanoamps. Way below the level of perception.

ps, I only said, "most of the time" because I can't think of every possible scenario and my first rule is, "Never bet your life (that the electricity is safe, even if you just measured it)".

Sep 9, 2010
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18. #12 Expert

Nov 30, 2010
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Hmmm...hadn't thought of that. Thanks for the suggestion.

19. wayneh Expert

Sep 9, 2010
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It is like dancing on the 3rd rail.

20. tshuck Well-Known Member

Oct 18, 2012
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There is more to it than that - all voltages and resistances are important in this, but, for the sake of simplicity, no current flows through the person because the person is in parallel with a node of zero resistance.