HOW CAN ELECTRIC CURRENT NOT SHOCK YOU?
2004 Bill B.
The numerical answer: Voltage is like electrical pressure, voltage causes
charges to flow, and when charges flow through our bodies at a low rate,
we can't even feel a thing. But when they flow at more than a certain
rate, bad things happen.
To stay safe, we should only play with voltages
which are way less than 45 volts. Don't mess with plug-in appliances,
they use 120 volts. As long as the electric charges flow through your
body at a rate that is lots less than 1/1000 of an ampere (one milliamp),
they're not dangerous. You can't even feel them. To stay safe, only
send electrical current
through your body at a rate less than 1/1000 ampere, and do this by
staying away from high voltage. But this is numerical, so it's not that
The "electricity" which flows through human bodies is charge, but it's
not electrons. Instead it's charged particles: potassium ions, sodium
ions, chloride, etc. Since these particles are always inside our bodies,
we can't say that "electricity" is dangerous. Instead, it is the FLOW of
charges which causes problems. Think like this: human beings are partly
composed of movable "electricity" or charges, but these charges normally
don't move very much. If something makes them move, abnormal things can
happen. When someone gets a shock from touching wires, electricity WAS
NOT put into their body from outside. Instead, the electric voltage on
the wires pushed upon the charges already inside their body, which then
moved forward, causing an electric current to appear inside. During
electrocution, human beings become part of an electric circuit, and the
circuit causes their own internal "electricity" to begin flowing fast.
During electrocution, the large abnormal electric current (ion currents)
causes nerves to fire, which causes muscles to tense up, and this can
include your heart muscle and your lungs' diaphragm (electric current can
stop the heart and prevent breathing.) Also, the suddenly-tensed muscles
can throw your body across a room hard enough to break bones or cause
concussions. Also, electric currents directed through your heart can
trigger fibrillation, which is a type of fast, quivering heartbeat which
does not pump blood. Once fibrillation is triggered, it might not stop by
itself. And finally, large electric currents can cause heating which
As long as the value of electric current within tissues is small, it will
not cause significant pain, heat, or muscle contraction. For example, if
you touch the metal terminals of a 9 volt battery with wet fingertips from
each hand, the voltage does push a measurable electric current in a
circular path through your arms and chest! Yet the only result is that
your fingertips become full of corroded metal from the electrochemistry
occuring where the wet skin touches metal. In other words, 9 volt
batteries are relatively safe. Now if you did the same thing with a 90
volt photographer's photoflash battery, the voltage-push is ten times
higher, and the value of current would be ten times higher. You'd feel
violent pain, your arms would jerk away, and some people would even go
into cardiac fibrillation or "heart attack." The 9v battery creates a
small current which you can't even feel. The 90v battery creates a larger
current which is painful and dangerous.
Voltage is not the only thing which determines whether you get a shock.
ELectrical resistance of your skin is also important. If the resistance
of your skin is low, then a smaller voltage will create a larger current.
For example, if you have tough, thick, dry skin on your hands, you might
be able to touch even a 90-volt battery without harm (don't try it
though!). Your higher skin resistance only allowed the battery to create
a small current through your body. On the other hand, if you have thin,
moist skin, then the same battery voltage will push a much higher current
through you. If your skin is thin enough, then even a 9v battery might
cause a larger current, which gives your fingers a slight tingle.
What is "safe?" I've heard that the threshold for feeling an electric
current is around 1/1000 ampere. Present UL regs say that voltage below
45 Volts is not an electrocution hazard. These are general rules of thumb
for everyday situations. However, voltages less than 45v aren't always
safe. If you poked a 40 volt battery into a big cut on your chest, I
wouldn't guarantee that you'd survive that experiment! The cut is a gap
in your skin; the wet meat inside your skin has low electrical resistance,
and your heart area is more sensitive than other parts of your body. On
the other hand, if you accidentally touched the two prongs of an AC plug
while plugging them in, the current might flow only through your fingers.
This hurts a lot, but won't cause death.
One way to be slightly safer: touch a 900 volt battery instead! A very
large flow of charge will appear inside you, which will fire all muscles
including your heart muscle. This large current usually force your hand
to jerk away from the wires, and it will will automatically defibrillate
your heart. Make no mistake, this is very painful and extremely
dangerous. But because of the fibrillation vs. defibrillation issues,
there is a medium range of currents which are MORE dangerous than low
currents or high currents. The medium currents trigger heart fibrillation.
High currents do not. Still, high currents can quickly cook flesh because
of heating effects, so very high currents are still extremely dangerous.
The safety of Alternating current isn't much different from direct current
if the AC frequency is low, like the 60 cycles per second on household
lines. But for very high frequencies (like above 20,000 cps), the ions
don't really flow within the body, instead they sit still and quiver. This
doesn't fire nerves, so the pain, muscle contractions, and cardiac
problems go away. All that's left is the danger of cooking your flesh.
I've accidentally touched power supply terminals having 200,000 cycles per
second frequency, which caused my fingertips to become warm, and it made
my sholder joints ache. If the same power supply ran at 60 cps instead, I
would have felt a painful shock.
> And connected to this question, what makes a "ground" and what
> happens to the electricity when it goes into the ground?
"Ground" has several different meanings. For electrical safety, "ground"
is important because part of the electric company generators are connected
to the earth via metal ground rods driven deeply into the dirt. When
you stand barefoot on the dirt, your body is connected to the electric
company generators! When you touch metal pipes which lead into the earth,
same situation. Why? Lightning protection, mostly. See "Why Three
Prongs" article for lots more about this.
Electricity doesn't really flow into the ground. That's not the way to
think about it. Instead, imagine that the ground is just another wire.
All wires are full of movable charges, they are full of "electricity." If
the two terminals of an electric generator were connected to the ground,
the generator would act as an "electricity pump", and the charges would
flow out of the ground, into one wire, through the generator, out through
the other wire, and back to the ground. In this way the generator causes
the "electricity" of the ground to begin flowing. But the generator
doesn't inject "electricity" into the ground, since the ground was already
full of movable charges to begin with.