Information about electric shocks
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Working with electricity is a part of our daily lives, from using your toaster in the morning, charging up your iPod, or pulling your computer apart, you will no doubt have an interaction with an electrical device at some stage.
In order to use electricity safely you need to have a basic awareness of how it works and what you need to do to avoid being hurt. While most people would know not to mix water with electricity, and that using a knife to get a piece of toast out when the toaster is plugged in is most likely a bad idea, there is some confusion about where the limits are - for example, can a 240V shock really kill me? What about the ignition system in my car? This article will attempt to answer those questions and more.
How electric shocks happen
Put simply an electric shock is unwanted current passing through the human body created by some external voltage source. The human body is conductive as the body works via electrical signals from the brain to all the various muscles around the body in order for the heart to beat, lungs to breathe, and muscles to move body parts. These signals are very small in magnitude and can be severely interrupted by relatively modest external voltage sources.
In order to have a passage of current through the human body there needs to be a potential difference (or voltage) from one point of the body to another. For example if you were to touch the positive terminal of your car battery with your left hand, and the negative terminal with your right, you would have 12V of potential between your hands. Another example is in your house if you were to put your knife in the toaster while it was turned on, whilst holding it with your hand. This example is a bit more complex as you need to account for ground impedances but it is possible that you would have a maximum of 240V of potential between your hand and your feet. In both examples you will have some current passing through your body, what effect that will have on you is explained in the next section.
What dictates the severity of a shock
The answer to this question is quite complex, as it depends on a number of variables, namely -
- The current path through the human body
- The touch potential
- The duration of the current flow
- The frequency of the supply
- The degree of moisture in the skin
- The surface area of contact
- The amount of pressure exerted against the electrode
- The temperature
- The individuals own body impedance
- Any additional external impedances in the current path
From the above it is easy to see that the severity cannot be simply derived by an Ohm's Law calculation. Another factor that is commonly overlooked is that body impedance is not constant for a range of applied voltages, and in fact is an inverse relationship, such that as the applied voltage is increased the body impedance decreases. The reason for this phenomena is unknown.