So your not best friend is over looking at your E type. Just for fun, you ask him/her to grab onto the spark plug wire while you do a test. You burst into laughter as the friend jumps back ten feet. “What are you trying to do, kill me?” You reply, no worries, it won’t hurt you, it is the amps that will get you, not the volts.
I have seen lots of (OSHA required) signs warning of high voltage, non warning of high amperage. So what is the scoop?
It’s the amperage that will get you. Anything over about .01 amps between the left arm and right leg is getting in danger territory. The skin is basically an insulating layer over a conductive salt core. The voltage is needed to punch through the skin layer. You can get a shock from 12V if you have wet skin and make a circuit with your body. DC is more dangerous than AC because with DC you can’t let go. High voltage from a spark coil has enough voltage to punch through insulating layers like skin and shoes but not enough energy to kill you. I used to work with up to 10KV in industrial processes. In 37 years I had a couple of shocks where I needed to sit down for awhile. You learn quick after that.
(Author of the Book, former owner of an '83 XJ-S H.E.)
It’s the volts that will electrocute you, meaning it can stop your heart if you’re not careful – but it’s also volts they use to re-start it. It’s amps that will burn you, as in char your innards into well-done flank steak.
There’s a movie coming out called “The Current War” about Edison vs. Westinghouse vs. Tesla in developing a standard for setting up an electric utility. Edison argued that AC current was dangerous, and to illustrate his point he invented the electric chair which was used to execute condemned prisoners for decades thereafter.
Specifically it’s the voltage Vm developed across the cell membranes of cardiac myocytes (muscle cells) that kills you by depolarizing them. That voltage is developed by a certain current I that flows through the heart–Vm = I x Rm where Rm is the cell membrane resistance. To get sufficient Vm you need sufficient I. That’s provided by the shock voltage Vs and the total body resistance Rb between the location of the shock and its effective return, I = Vs/Rb. If you grab wires with each hand, there’s a direct path through the heart. If you grab only one wire, body capacitance comes into effect. With the simpler example of two wires, Rb comprises skin resistance (pretty high, less if sweaty) and the resistance of the paths inside the arms and chest, all added together in series. Skin R is the highest. In fact, to get sufficient Vm (about 50 mV) you need an I of about 10 mA. Skin R might require a Vshock of, say, 100V to get the 10 mA. So if Vshock is less than that, you live.
But the Vshock source must be able to provide the 10 mA. Many high voltage sources can’t do that, so in that sense it’s the current that kills you regardless of how high the voltage is. IMHO. AC vs DC is complicated–avoid both!
My outlook on the topic… not to be taken as against anything factual!
Everyone has walked up to a doorknob and been shocked. Typically 10,000+ volts. Never hurt. Most have been shocked by 5000+ volts spark plug wire. Hurt, but not bad. So apparently it is not the volts. Many of you have welded. Often having 150 amps going through a bare conductor in a bare hand. And never got hurt. (I know, the amps did not go through you, but the amps were right there- the voltage was too low to push the amps into your body. It takes about 60 volts to go through the typical body.) So it seems to me neither the volts alone OR the amps alone are the problem, but it is the combination that does it. Volts time amps is the power that will hurt you. Maybe the signs should read “warning, high power!”
Ohio State University seems to disagree with you. Go back and read the article I linked to a few posts back. It is absolutely the current, induced by voltage, that is the danger. Even 12V, properly applied, can mess with your heartbeat. It is, fortunately, not easy to properly apply such a low voltage to cause any real harm. But it is entirely possible.
You might also want to read up on, for example, Van de Graaff generators:
Even small ones are capable of generating hundreds of thousands of volts, but at extremely low currents. You’ll have a very hard time burning, much less killing, anyone with one. Science museums around the world routinely have visitors come up and touch them during demonstrations.
And DC, properly applied, is every bit as capable of killing you as AC.
Let’s not forget the role grounding plays in getting shocked.
If you were floating in the air, not touching anything else you won’t get shocked if you grab a “hot” wire. That’s how birds can land on high voltage lines and not be killed; they are touching nothing else.
These guys, too, with a wee bit’o help from Messr Faraday!
(Author of the Book, former owner of an '83 XJ-S H.E.)
I knew a guy who once barehanded a 500KV high-tension power line. The power company uses insulated bucket trucks so you’re not grounded. Don’t touch two of them, but one is fine. He says your hair stands up, much as it does when you have your hand on the Van de Graaff generator.
What about capacitors ? I readily admit I am an electrical idiot but I will still dabble with it on the cars. Twenty five years ago I took care of the ground equipment parts at my airline. We had a shelf where the mechanics would put unused parts for re-stocking. One day as a joke they left a charged capacitor on the shelf. I picked it up and got the shock of my life. That arm hurt for days. Now I am scared of them even though I know they can be safely discharged. The big start capacitor on my house AC compressor likes to blow every few years but I always call the professional for fear of being blown across the yard if I make a mistake.
Another issue, I believe, is that electricity can hurt humans in different ways, depending on many factors. Some harm is caused when the heart is stopped, typically a more voltage related injury. Burns seem to often be caused by high current situations. Severe shocking can be so intense it causes the body fluids to boil, typically a result of high voltage and current.
An example is a capacitor that is built into the CRT (picture tube) of all TVs and monitors before flat screen became the norm. It is the filter capacitor for the second-anode potential that accelerates electrons so that they smash into the screen, thereby emitting light. If you’re uninitiated, when you stick your hands into an unplugged TV, there’s a good chance you will touch the circuit that contains that capacitor. You have lots of time to discover it, as it doesn’t discharge, at least for a good hour after you’ve unplugged it.
When I began working on TVs they were all black-and-white. That circuit was several thousand volts. It smarted, but a bigger problem was that your hand would jump and invariably get cut by whatever sharp sheet metal edge it banged into. Despite that I often failed to remember to discharge it, and got zapped several times.
Then came colour TV. That was an engineering feat; the second anode voltage, as I l eventually learned, had to be at least 25,000 V for it to work. The first time I encountered that was a completely different experience! Very painful, and emotionally taxing like being in a car wreck. I was very careful…although I think I hit it at least a time or two more.
The picture tube itself acts like a giant capacitor. After disconnecting the anode lead from the tube, you ALWAYS wanted to discharge the anode, lest you accidentally stick your finger in the hole and get a good zap! The anode supply on a large color CRT was typically ~35kV.