The Science of Safety

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Alicia Garcia’s recent blog post about the importance of safety in the workplace and my own post about the importance of scientific literacy reminded me of a common misconception about electricity. Electricity is a phenomenon that stands at that intersection of both topics because it must be understood through the lens of science in order to be used safely. The common fallacy to which I refer is embodied by the common adage, “It’s not the volts that kill you, it’s the amps!” This is misleading and dangerous.

In order to understand why it’s misleading and dangerous we need to know some of the basic units used to quantify electricity. Electricity is the continuous flow of free electrons through a material. Current (measured in amps) indicates how many electrons pass a given point in a circuit each second. Voltage (measured in volts) is a measure of the force that pushes those electrons around a circuit. Resistance (measured in Ohms) tells how strongly a substance will resist or impede the flow of current.

Now, it’s true that the flow of electrons (current) is directly responsible for causing physical harm. Each electron carries a small electric charge that together adds up to the energy necessary to power all of our electronic devices. The higher the number of electrons passing through you each second (amps) the worse off you will be. However, current is not the whole story.

The amount of current in a circuit depends on the amount of voltage available to make the electrons move. If you have a lot of voltage and very little current, as with static electricity, the danger is minimal. For example, as I drive, my foot rubs against the carpeted floor mat of my car, causing a lot of electrons to flow from the carpet to my body. As soon as I get out of the car and make contact with the car door, my excess of electrons rushes over to the door and I get zapped. The electrons were in quite a hurry (high voltage), but there simply weren’t enough of them (amps) to do any damage. The flow of electrons is also quite momentary. A finite number of electrons go from point A to point B and then it’s over.

The 120 volt electrical socket in your home, however, is another story. It has relatively low voltage (the electrons are not being pushed very hard), and an unlimited flow of electrons (they just keep coming and coming). If exposed to 120 volts of electricity, a person with dry hands will be subjected to a barely perceptible 1 milliamp of current because human skin offers a resistance of 100,000 ohms. However, under wet conditions (including sweaty or greasy hands) the body’s resistance is lowered to such a degree that the same 120 volts of electricity will deliver a very lethal 120 milliamps of current. Thus, in this case the adage is correct. The amps will kill you.

Dry Conditions: Current = Volts/Ohms = 120/100,000 = 1mA
a barely perceptible level of current

Wet conditions: Current = Volts/Ohms = 120/1,000 = 120mA
sufficient current to cause ventricular fibrillation

The problem is that high voltage has the same effect as wet or greasy hands. It can quickly break down the resistance offered up by your skin and allow what is considered to be a relatively low amount of current to flow through the highly conductive water and blood inside your organs. That is why you see DANGER: HIGH VOLTAGE signs on a multitude of devices. Sure, it’s still the current that kills, but it would never have gotten to you if not for the high amount of voltage. That is why it’s misleading to say that only amps are dangerous.

The moral of the story is that amperage is not the only factor to consider when talking about electrical safety. The severity of an injury due to electric shock depends on the current’s voltage, the amount of current (amperage), the type of current (direct or alternating), the body’s resistance to the current, the current’s path through the body, and the length of time the body remains in contact with the current. The interplay of these factors can produce effects ranging from a mild sting to instant death.

If you want to learn more about safety in the workplace, including the why behind many of the rules and regulations, check out the Safety in Welding seminar at American Welding Online. This comprehensive online course presents the science behind various workplace hazards in an engaging and easy-to-understand format.

Carlos Plaza