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?
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.
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
Dr. Ken Mellendorf
Illinois Central College
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.
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
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
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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.
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Update: June 2012