# Can the smallest electric shock kill you?

I remember being told over and over again throughout school that an electric charge, no matter how small, can still kill a human being if it coincides precisely with the electric pulse controlling heartbeat.

Whats the script with this? Surely by now there must have been at least a few people who died by taking off their jumpers. I have never heard of anything like this happening though, so can only conclude that it is either pure bunk or else 99.99% improbable.

I would guess this is bunk. With the hundreds (maybe thousands) of static electric shocks I’ve received in my lifetime one would guess that the odds are sooner or later I’d have killed myself in this was true. Even if I’m somehow lucky multiply my doing it by roughly 6 billion people in the world and one would suppose you’d hear about this more if it were true.

Additionally, I don’t think people would risk their kids rubbing ballons on their head or shuffling across carpets to shock your friend as good fun if there was even a remote chance of killing someone.

It is true that you can be killed by what would otherwise be a minor strike to the chest if it coincides with a very particular moment in your heartbeat. This happened to some kid around Chicago a few years ago while he was playing Little League baseball. He got hit in the chest by the ball at the precisely wrong moment and died. The at any other time the kid would have likely shrugged of the hit from the baseball and continued playing with nothing worse than a possible bruise.

As little as one tenth of an amp, for short duration, at 60Hz, may cause fibrillation, but maybe not, too.

Look here for more detail.

Really, it’s the amperage that kills you. The voltage of a power source is meaningless in and of itself. That’s the explanation that’s always given, anyway.

I = E/R

That’s ohm’s law.
I is current. Let’s say 100mA kills you…
R is you touching ground. Let’s say you’re 10000 ohms…
E is a bare wire; let’s call it 120VAC.

So, you touching a wire gives you 12.0mA - uh oh – you’re safe, but maybe not comfortable.

Luckily the human body has a LOT more resistance to ground under normal circumstances. Standing in a puddle of water in your socks, let’s say you’re 1000 ohms. Thats 120mA, and you’re dead.

So you can see why higher voltage gives the appearance of being more dangerous – it lets flow more current. But conversely, lower voltages can be deadly if the circumstances are right.

Of course, the numbers are examples – I think I’ve read 100mA can kill a guy, and I don’t know typical human-to-ground resistance, nor how long the shock has to last. So, if you kill yourself, kindly haunt your survivors and ask them not to sue me

Absolutely, a static electric charge from some kid rubbing her sneakers on the carpet can kill you if she touches you. So when a 10 year old girl comes up to you looking like she might touch you, run screaming from the room as if your life depended on it, because it might. It works for me, I’m still alive at 39 years of age, 6 feet tall and 235 pounds. And others tell me that they are quite amused at the sight of me screaming and running for my life.

biomed engineer checking in here: very small currents when timed correctly can disruct the electrical cycle of the heart and induce fibrillation. this current must pass across the heart at the point when cells are repolarizing. some cells will be in their refractory periods and some will not. this allows some (post refractory) cells to fire. the spread of electrical current in the heart will then be disorderly as it spreads from cell to cell and this induces uncoordinated sontraction among myocardial cells -> fibrillation -> death due to no flow of blood.

now this is not to say that we should fear every source of electricity. the human skin (especially the outermost epithelial layer) is an excellent insulator against electric current. the only real way to deliver the current necessary to induce fibrillation is either with very large voltage/current or by bypassing the outer skin layer. this is possible in many settings including IV lines and various other hospital type equipment. this also means very stringent FDA requirements for leakage current etc in medical equipment.

(and there is reasonably credible testimony) that one bright boy managed to electrocute himself with a 9-volt ‘transistor’ battery.

No, I am not going to say how - some bozo would end up trying it.

out of curiosity, what’s a transistor battery? (screw the bozos… i say let natural selection work its magic)

I was afraid of this…

you know those little rectangular batteries with the ‘cap’ terminals?

Used in smoke detectors, calculators, whatever?

They were originally used to power small (by 50’s standards) ‘transistor’ radios. These radios came to be called ‘transistors’ (they used transistors instead of radio tubes - quite revoluntary).

Hence, their batteries came to be called ‘transistor’ batteries.

god, I feel old. Very, very old…

And here I thought they were called 9-volt batteries…

Here’s a list that my dad brought home from work. It’s from the Industrial Safety and Hygiene manual, chapter 3, section 9, Electrical Safety.

10 Amps to 4 Amps- Severe Burns, Not fatal unless vital organs burned. (I thouht it seemed odd that this is not fatal unless the organs burn)

4 Amps to 250 mA- Heart stops during shock. May restart if current is removed before death.

250 mA to 75 mA- Heart Fibrillation in 1-2 seconds. Usually Fatal.

75 mA to 30 mA- Breathing Stops, often fatal.

30 mA to 10 mA- Cannot let go, current may increase to fatal level.

10 mA to 3 mA- Painful Sensation

3 mA to 1 mA- Mild Sensation

1 mA to 500 uA (500 is the U.L. Limit for consumer products)- Imperceivalbe

500 uA to 100 uA (100 uA is the U.L. limit for hospital equipment not connected directly to the body)

`` So according to this chart the lowest fatal level is 10 mA, and 30 mA to 250 mA are the highest danger levels.  Unfortunatly I just have a copy of the chart and not the text that goes with it so I don't know any of the details or exceptions to the information.``

Just a nitpick, here. The human body is not governed by Ohm’s Law. Ohm’s Law expresses a time-invariant linear relationship between voltage and current. In animal tissue, the relationship is more complex than that.

Of course, we still talk about “resistance” in such situations, but that’s a pretty gross approximation of the actual relationship.

Everything (in the DC world) is goverened by ohm’s law, because ohm’s law simply states a relationship based on at least one constant. I = E/R simply means that current & voltage are directly related, given a constant resistance; and that current & resistance are inversely related, given a constant voltage. So, given that a human body’s resistance is equal to a constant value of R, the current I will be equal to the potential difference V divided by R.

If you meant that ohm’s law is difficult to apply and therefore not a very useful tool where skin contact resistance is concerned, I’ll agree with that because skin contact resistance can vary from a few tens of Kohms to a few hundred Kohms depending on skin moisture levels & the force of the contact. I don’t count reactance since the OP seems to be talking about DC only (“charge”).

Um… not quite.

There are many complex things which do not have a simple linear relationship between voltage and current. Ohm’s law is only valid for things which have a simple linear relationship, like graphite, copper, aluminum, etc. Semiconductor diodes quite clearly do not have a linear relationship (it’s exponential). And, as was already mentioned, animal tissue doesn’t behave this simply either.

You will find that with the human body, your “constant” varies with the voltage, and a constant that varies isn’t a constant now is it?

What I said was that there is nothing wrong with using ohm’s law assuming that there is at least one constant. You seem to be saying that semiconductors don’t have a constant resistance and therefore ohm’s can’t be aplied (to get a straight line on a graph), so I don’t see where we have said anything different.

No news here.

>> I = E/R simply means that current & voltage are directly related, given a constant resistance

Attrayant, I believe the point engineer_comp_geek is trying to make is that animal tissue does not present a constant resistance but rather varies with a number of factors and therefore Ohm’s lwaw, which presupposes constant resistance, is not applicable.

Yes, that is what I mean in my very first post when I said

I was puzzled to see ECG come back with “but wait, skin contact resistance can vary!” Yeah, I already acknowledged that.

I’m not sure where the disagreement lies. We may just be quibbling over the ambiguous phrasing like “ohm’s law is not applicable under condition x”, I would prefer "ohm’s law is not a useful formula under condition x. That’s why I was careful to exclude condition x (i.e. non-linear loads) in my first reply.

My purpose was to clarify because JT’s post seemed to be saying that Ohm’s law should be “I is sometimes equal to E/R”. My first reply was just to affirm that I is always equal to E/R.

Enough with the hijack. he answer to the OP seems to be a unanimous NO.

not a unanimous no.
i think the OP was asking if there is a specific point in heart’s electrical cycle where a applying minor shock not normally dangerous could result in death. the answer in this case is yes. quick search on google turned this up:
http://www.eelab.usyd.edu.au/ELEC3801/notes/Safety.htm

So the answer is yes, but not through the skin?

Well looks like my old physics teacher was not so daft after all.

Incidentally, this brought back to me the story of another old teacher at my old old school (before my time). It seems he liked to use bare wires connected to a mains supply to flick unruly students with. Could be an urban myth, but you learned never to underestimate anyone at that school…

The problem I have with writing Ohm’s law as “V = I R” is that it assumes R is constant. In other words, it assumes R is not a function of voltage and/or current.

This is rarely the case. To be more precise, V = I R(V[sub]R[/sub], I[sub]R[/sub]).

Now you can write Ohm’s Law another way that is always valid:

R = V[sub]R[/sub] / I[sub]R[/sub]

In the above equation were basically defining R, i.e. R is the ratio of the instantaneous voltage to the instantaneous current.