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Name: Cameron
Status: student
Age: 18
Location: N/A
Country: N/A
Date: 2/26/2003


Question:
If a human being grabs onto an electric fence, they interrupt the circuit, and get electrocuted, that is, the current chooses to flow through the human's bloodstream towards the ground rather than continuing on through the wire. If a human being is just a huge resistor, why would the current choose to flow through the greater resistance of the human, rather than continue through the wire?

In broader form; Why do humans get electrocuted?


Replies:
Cameron,

It is hoped that one is not electrocuted if one touches an electric fence. The circuitry that powers the fence is designed to deliver a very unpleasant shock to the person or animal unlucky enough to touch it -- it is not supposed to deliver enough current to kill (electrocute). The intent is to impart a high voltage, low current shock that for the most part stays on the surface of the body rather than flowing though its internals.

The human body is not much of a resistor. Rather, it is a fairly good conductor of electricity because the fluids in our bodies are solutions of salts and ions which readily enable electricity to flow. When the voltage and current are sufficient, electrical flow through the body can fatally interrupt the electrical circuitry of the heart. Then things get deadly.

Regards,
ProfHoff 590


Cameron,

I am not certain how an electric fence works, but I do not believe it has a high current to begin with. A good design for such a fence would be one high voltage wire that is not a complete circuit. When a person touches both the fence and the ground, a complete circuit is formed. Most electric circuits have a ground wire that can feed current into the water within the ground and then back to the generator through the earth. One pole of the generator is attached to the electric fence wire. One pole is attached to the ground. This is not necessarily how all electric fences work, but it is how I would design one.

Another such example is the electric wires in the air. If a bird lands on one, nothing happens. If a bird touches two at the same time, the bird is dead.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


Cameron -

Electric fences do not have current flowing through them. In fact, if you go to the farm store, they sell them as "fence chargers." The wire on the fence has a charge compared to ground. This charge takes advantage of a path through a cow, horse... or human... who by touching it provides a path to the ground. Most electric fences are designed to interrupt the flow when they are touched thereby decreasing the likelihood of electrocution. However, if you come in contact with an uninterrupted source (like the wires feeding your house) the results could be deadly.

Larry Krengel


When a person grabs the fence, the person becomes a resistance in parallel with the fence circuit to ground, so that: 1/R = 1/Rf + 1/Rp where R is the total resistance, Rf is the resistance of the fence, and Rp is the resistance of the person. It is not an "either or" situation it is a "both and" situation.

Vince Calder


I will answer your questions in reverse order. Why humans get electrocuted primarily is because an external electrical current flowing through the body disrupts the electrical polarity of the nerve and muscle cells of the heart, ruining the heart's triggering mechanism. If the heart does not beat, not much else goes on.

In an electric fence, the circuit is ordinarily open. Current does not flow through the wire; instead, the wire has an electrical potential with respect to the ground. When a person, a cow, or anything else provides a route for the electrons to move between the fence and the ground, THEN the current flows.

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois


Two things:

1. You may not be a good conductor but you are a conductor, so if you make yourself part of any electrical circuit PART of the current will flow through you.

2. An electric fence has a potential applied between the fence and ground. The wire in the fence does not complete the circuit since it never touches the ground. Normally the resistance between the wire and ground, which would complete the circuit, is very high (an air gap of several inches to a few feet). Compared to that you are a great conductor.

Greg Bradburn



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