Every now and then one of our electric-related articles will surface an old debate: what’s really dangerous: voltage or amperage? The most recent post that raised the issue was last week’s Frigidaire wall oven heating issue where I warned readers to turn off the breaker because “220 volts can be lethal.”
One of our commenters, Katharine, chimed in saying, “Voltage isn’t lethal, amperage (current) is.”
So what really is the dangerous component, voltage or amperage?
The answer is both. It’s actually the combination of voltage and (available) amperage that are dangerous.
To illustrate, let me use one of my favorite analogies for electricity: a flowing river. While not a perfect analogy, it provides a simplified view of electricity in a way we can all understand. If an electrical circuit is a river, voltage is the steepness of the river, while amperage is the amount of water that flows across a section of the river for some set period of time.
So, if voltage is very high, but amperage very low, then a very small amount of water would be flowing down an extremely steep slope like a thin waterfall. If voltage is low and amperage is high, a large amount of water is flowing very slowly, like an almost-stagnant, yet wide river.
If either voltage or amperage are very low, it’s clear that the situation isn’t dangerous. A tiny waterfall or a massive slow moving river probably won’t do you much harm. On the other hand, if both are high (a large, raging waterfall, for example), it would indeed be very dangerous.
Now, the river analogy breaks down because rivers don’t follow Ohm’s law, which says that current and voltage are related by the equation V=IR, where V is Voltage, I is Current, and R is Resistance. This equation tells us that the amount of current that passes through your body (a resistor) is directly proportional to the voltage, because the “R” in the equation (essentially your chest, limbs, etc) is a constant. If you bridge two wires, one in each hand, the amount of current that passes through your body will vary directly with the voltage across the lines.
So what is a dangerous amount of current to flow? According to a great article over at All About Circuits, about 6 miliamps (6/1000 of an amp). That’s when severe pain kicks in but you can still control muscles. 100 miliamps is the point at which heart fibrillation is likely to occur.
It is worth noting here that tissue-damaging currents are measured in milliamps (mA), while over-current protection provided by breakers is measured in amps. In other words, if you bridged 240 volts from one hand to the other (through your chest), you’ll likely be dead long before a breaker trips. This is one reason that GFCIs add safety to circuits, because usually when a circuit is interrupted, the current goes to ground and a GFCI trips before tissue-damaging current can flow.
What is a dangerous amount of voltage? Shock danger generally begins around 30-40 volts and escalates as voltage increases. However, even lower voltages can be dangerous in situations with lower resistance (e.g. when hands are sweaty, or more skin surface area is in contact with the voltage source.) For dry skin with minimal surface contact, 30 volts is the amount of potential necessary for any current to flow through your body.
Generally speaking, this is why you don’t get shocked if you you have dry hands and grab both terminals on a car battery, even though such batteries can deliver up to 300 amps of cranking power to start a car. 12 volts isn’t enough potential to complete the circuit through your body.
(Photo Credit: Frustrated Writer)
I’m surprised no one commented on this. It’s pretty useful information, distilled down to the basics that can help homeowners.
To further the amperage angle- cardiac monitors use very few mA (as few as 30-40 mA) to pace a heart- that is, to take over the electrical stimulation when the heart’s own electrical signals are too slow or out of sync. Internal pacemakers use even less, because their wires are installed directly on the chambers of the heart, and have almost no real resistance to combat. The skin, when dry, is a pretty good resistor.
I wish I knew what the amperage on the defibrillators is, but their output is measured in joules, a measurement of energy, like calories. Even 120 V AC is enough to cause fibrillation in the heart, particularly if the discharge happens at exactly the right spot in the electrical activity of the heart. 220 is particularly dangerous, but people survive lightning strikes, which are an average of 30 kA and 500 megajoules. Still better to not get shocked.
It was a little surprising to us when we first ran it too. I thought maybe it was just so clear that people were speechless 🙂
Alot of surviving shocks has to do with whether you can get out of the way. I’ve heard of people dying in an attic because they were working on electric, got trapped on a bare wire and simply couldn’t move. That’s a horrible way to go.
Uuum, you know that joules and Watt-seconds are the same thing, right?
So e.g. given 30mA × 120V = 3.6W
then 3.6W × 3s = 7.2J
I’ve heard the same stories. I’ve had coworkers that have had to remove those bodies. Seems like electricity and attics make for a poor combination.
Thats bullshit . What ever he said is wrong
So I have a transformer set up to convert the 120 or so volts from an outlet to About 1-5 volts but a huge amount of amps, i don’t know exactly but I think over 500
Would I be able to touch the two terminals safely??
I would suggest not, for safety reasons, in case something went wrong, but you should be able to. Even a small amount of water on your skin can mess it up though, or if you have a cut that’s even worse.
That is all correct and clear thanks. Just one correction the water flow analogy doesnt brake down fluid flow has the same qualitative properties as electrical flow. Voltage difference is equivalent to pressure difference. Current is equivalent to flowrate. Ohms law is equivalent to Poiseuille law.