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Hazard get shock from high Voltage or Current electric?

Read this explaination.

(from Book Practical Electronics for Inventors by Paul Scherzs



Your body is a complex system that is controlled by electro chemical signals that are sent to and from your brain. If you screw these signals up by introducing an external flow of electrons, vital organs may cease to function properly, which ultimately may lead to death. Applying a current of about 1 mA through your body will have the effect of providing a tingling feeling or mild sensation. A current of about 10 mA will result in a

shock of sufficient intensity to cause involuntary loss of muscle control. A current of 100 mA lasting for more than 1 second can result in crippling effects and may result in death. Beyond 100 mA, extreme shock occurs, which may result in ventricular fibrillations (irregular heartbeat) that could easily lead to death. The resistance of a human body to current flow is between 1 MΩ when it is dry and a few hundred ohms when it is wet. To figure out the amount of current flow through a body that is connected to an ideal voltage source, simply use Ohm’s law (I = V/R body). For example, say you attach wires from your hand and foot to the terminals of a 6-V battery. If your pretend that your internal resistance is 300,000 Ω (you are perspiring that day), the amount of current that would flow from one arm to the other would be 6 V/300,000 Ω = 20 nA. This amount of current is well within the safety limits, and you probably would not feel a thing. However, if you were unfortunate enough to drop a 120-V ac-powered dryer into the bathtub (when you are in it), the amount of current that would flow through your body—assuming for now that a wet body has a resistance of 1000 Ω—would be 120 V/1000 Ω = 0.12 A. This amount of current most likely would be fatal.


Now, you have probably heard the saying, “It’s not the voltage that will kill you, it’s the current.” But according to Ohm’s law, it appears the voltage term (V) sets the current term (I), so the two would appear to have an equal part to play. What’s going on? Is this statement true, or what? Well, this is where it is important to understand what exactly is meant by voltage, at least in terms of using it in Ohm’s law, since this is how we are determining the currents. When using Ohm’s law, it is always assumed that voltage sources are ideal. As you learned in the theory section, an ideal voltage source is a device that maintains a fixed voltage no matter what size load is attached to it. For an ideal voltage source to maintain its voltage no matter the resistance placed between it, it must be able to supply varying amounts of current. If you were to treat all sources of power as ideal voltage sources, then the saying, “It’s not the voltage, it’s the current that kills you” does not make sense. The problem with this approach is that in the real world you are often dealing with less than ideal voltage sources—ones that can only produce a limited output current. For such cases, blindly applying Ohm’s law does not work. A good example to demonstrate this point is to consider the static electric charge that accumulates on a brush when you are combing your hair. During this simple operation, it is possible that as many as 1010 electrons will be stripped from your hair and deposited onto the brush, resulting in a voltage (relative to ground) of 2000 V. If you plug this voltage into Ohm’s law, taking R body to be, say, 10,000 Ω, you would get a result of 0.2 A—a potentially lethal current. But, wait a minute! How many people do you know who have been killed by statically charged combs? There has got to be something wrong. The problem lies in the fact that you are dealing with a classic non ideal voltage source. Unlike an ideal voltage source, the number of charges needed to make up a current runs out very quickly. You can make a very rough calculation of the time required for these charges to run out by finding the initial charge on the comb. If there are 1010 electrons, each of which have a charge of 1.6 . 10−19 C, then you get a net charge of 1.6 . 10−9 C. Next, using the definition of current I = ∆Q/∆t, set ∆Q = 1.6 . 10−9 C and I = 0.2 A, and then solve for the time ∆t. Doing this calculation, you get ∆t = 8 . 10−9 s, or 8 ns. Being exposed to such a short pulse of current is not going to do you any harm, unlike the case where you stick a fork into an ac outlet. In the fork in the ac outlet case, you would be receiving a continuous flow of current, frying your body tissues in the process. A more intuitive explanation as to what the statement, “it’s not the voltage that will kill you, it’s the current,” refers to is given by the following analogy: Suppose that you drop a grain of sand from a second-story window onto a pedestrian. As long as the grain of sand does not hit the individual in the eye, there is little chance that any permanent damage will be done. If you drop the same grain from a tenth-story window onto the same person, the sand may prick the top of the individuals head with a little more intensity but still will not kill the individual. However, if you repeat this experiment but this time replace the single grain of sand with a hundred-pound bag of sand, the results, as you can image, will be drastically different. The sand in this analogy represents electrons (current), the height from which the sand is drop represents the voltage, and the pedestrian represents the internal organs and tissue of a human being.



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