How Can Electric Current Not Shock You?22 Dec 2011
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 anything. When they flow at more than a certain high 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), 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 meaningful.
The “electricity” which flows through human bodies is 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 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 “electricity” to begin flowing fast.
During electrocution, the large abnormal electric current (ion currents) cause nerves to fire, which causes muscles to tense up, and this can include your heart muscle and your diaphragm (electric current can stop the heart and prevent breathing.) 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 cooks tissue.
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.
The article from: http://amasci.com/amateur/elsafe.html