Basic Question about Shock Current Path...

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hindusach

Joined Aug 30, 2006
1
I was reading through the all about circuits lecture on shock current path, and I had a basic questions that I was hoping someone could answer...

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

In the fourth example in the lecture, there is a man touching a high voltage line, whose feet are grounded to the earth. The circuit is not grounded to the earth. In the lecture he does not get shocked. I don't quite understand why. To my reasoning, the earth probably has a very low potential, the line he is touching has a high potential, and he seems to be a shorter path than the rest of the circuit to a point of low voltage?

If I understand the rest of the lecture, anytime that you and the power line share a ground to the earth at a high voltage point you don't get shocked, any time that you and the power line share a ground to the earth at a low voltage point, you don't get shocked. Anytime that you have a ground at either high or low voltage, and the power line has a ground at the point of opposite voltage, you get shocked...

But what I don't understand is, in an earth-grounded power line, where is the electricity actually flowing, back to the transformer, or back to the earth?
 

windy

Joined Apr 19, 2007
26
The earth is used as a conductor for the power grid. It is the common conductor. If there happens to be no return path for the power system in question there will be no shock.

In answer to the question about where the power travels, Transformers are an isolating device when there is a separat primary and secondary winding. If you are touching the secondary line and the secondary coil is not properly grounded you don't complete the system thru your body.

Hope this answers your question
 

mentaaal

Joined Oct 17, 2005
451
in the fourth example the guy is touching the circuit after the load so there is a definite potential difference but there is no conductor linking the guy to the other side of the load so no circuit is being completed. Where in the next example is exactly the same except that a tree had linked the circuit to ground and a circuit through the guy in formed through the tree and through the ground into the victim.
hope this helps
 

John Luciani

Joined Apr 3, 2007
477
http://www.allaboutcircuits.com/vol_1/chpt_3/3.html

In the fourth example in the lecture, there is a man touching a high voltage line, whose feet are grounded to the earth. The circuit is not grounded to the earth. In the lecture he does not get shocked. I don't quite understand why. To my reasoning, the earth probably has a very low potential, the line he is touching has a high potential, and he seems to be a shorter path than the rest of the circuit to a point of low voltage?
The power source is isolated from earth ground. Neither the positive or negative
terminal is connected to earth. If one of the terminals were connected there
would be a shock hazard.

(* jcl *)

---
www.luciani.org
 

subtech

Joined Nov 21, 2006
123
Concerning example #4, theory says that the person contacting the energised conductor would not be shocked. It is important to understand the principles.

In the real world however, the person contacting the overhead high voltage line would likely be killed or severely injured.
Although there may be no INTENTIONAL connections to earth from the overhead conductors, there is almost always some degree of capacitive coupling to earth which would allow for some amount of current to flow through the person contacting the energized conductor.
The only safe way to "touch" ANY energised conductor is to do so with the proper tools and using the correct protective equipment. It takes specialized training to know how to use this equipment properly.
I understand that we talking about a question posed in a textbook in this case, but let's not have anyone get the wrong idea about touching energized power lines...........
 

Winston

Joined Dec 25, 2007
22
There is a potential difference, but no return path.

Would someone at this point in the book know what "capacitive coupling" is?
 
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