"It's not the volts that kill you, it's the amps"

Uploaded by Afrotechmods on 20.07.2011

Every few days I get a comment on my videos saying
"It's not the volts that'll kill you, it's the amperes."
While there's some truth to it, it's basically a load of crap and dangerously
So in this video I'm going to put on my beret and walrus mustache and bust
this myth very thoroughly.
Let's start out with the origin of this belief.
In high school physics, everybody learns about static electricity.
The classic thing the teacher shows you is a balloon charged up with static
electricity and sticking to a wall, and the balloon is charged up to a couple of
thousand volts with respect to the wall.
Or maybe you get the example where you rub your shoes on a carpet,
charge yourself up to a few thousand volts,
touch a grounded piece of metal, a spark flies, and yet you still don't die.
At this point the students get confused. How can there be thousands of volts and
yet no one is dying?
Then the teacher delivers the infamous line:
"It's not the volts that kill you, it's the amperes!"
Now if you interpret that statement literally, it's technically true.
The static charges are a few thousand volts in magnitude
and it's ultimately the current going through your body that determines
whether you live or die.
Here is one of the standard safety guides showing you how much damage a few
milliamps can do.
Now here's the problem. Static electricity charges are completely
incomparable to everyday high voltage situations
when you're working with electronics.
Let's start out by modeling a static electricity situation
with a capacitor and resistor.
Balloons and human beings are capable of holding electric charge, but their
equivalent capacitance is tiny; Somewhere in the picofarad range.
And I'm using a high value series resistance in the model because
obviously a rubber balloon or a person's skin is going to have much higher
resistance than a piece of copper wire.
100 picofarads and 1.5 kilohms are a fairly standard human
static discharge model.
Now let's say that your body is charged twelve thousand volts.
It could be higher than that but 12000V is pretty common.
The equation to find out the energy stored in a capacitor is
energy equals 0.5 x capacitance x voltage squared.
So the total energy stored in the form of a static charge here is
7.2 millijoules.
That's tiny.
That amount of energy can't possibly do any damage to your body.
Also using this model I can show you what the current flowing through your
body will be.
Here's the equation that gives you the
amperes flowing through you versus time.
If you plug in our values you get this graph.
As you can see the theoretical peak current is 8 amps
which is extremely dangerous.
However, as I mentioned before, the total energy stored is so low,
that the current flowing through you decays so quickly,
it drops to almost zero in literally a microsecond.
So that explains why when you experience a static shock,
it's painful,
but since it's over so quickly it doesn't damage you.
Okay so now you know exactly why a
high voltage static charge won't kill you.
It's because of a tiny amount of capacitance,
a large amount of equivalent series resistance,
and low total stored energy.
Now let's compare that to situations you would encounter when working with electronics.
A high voltage power supply can deliver lethal amounts of current continuously,
or in one big burst when the output capacitor is discharged.
A model for power supply like this is a constant voltage source
with a low series resistance.
In fact the series resistance is usually so low, we might as well pretend it doesn't exist.
Okay so let's say you have someone who is not very intelligent who is touching the wires
of a 300 volt power supply
because he believes that the volts won't kill you, it's only the amps that will.
Well, what determines the amount of current?
First, it's a common mistake to think that just because a constant voltage
power supply is capable of delivering, for example, one ampere,
that means it will push one ampere through everything it is connected to.
(Otherwise you could kill people with a phone charger.)
The ampere rating on power supply only tells you
the maximum current the power supply is capable of handling.
The thing that determines the amount of current flowing through the circuit is
the "load".
The load only draws as much current as it needs at a given voltage.
The equation we use here is very simple - It's is just Ohm's law.
Take the voltage divided by the resistance of whatever you are connecting it
to and you will get the current.
The resistance of the human body is somewhere in the one kilohm
to thirty kilohm range depending on how wet your hands are.
For now I'm going to use three kilohms as an example.
And by the way... you can't reliably measure body resistance with a simple multimeter.
So that's why you won't get the same values if you
measure yourself at home.
Okay, so let's apply Ohm's law with our friend over here.
Three hundred volts divided by three kilohms equals a hundred milliamps.
If you look that up in the safety chart, you can see that that's enough current
to damage your heart and nervous system.
So he shouldn't be touching those wires.
Ohm's law is a linear relationship, meaning that for any given body resistance,
the more volts you have, the more amperes you will have.
And that's why it's misleading to say
"It's not the volts that will kill you, it's the amps."
The volts directly determine the amperes! There is no way around it!!
Also, the resistance of the human body actually decreases as voltage increases,
because as you start penetrating the outer layers of dead skin,
your inner organs and blood will have a resistance of
a few hundred ohms instead of a few kilohms.
So things just get worse and worse with higher voltages.
Large high voltage capacitors are equally dangerous.
In my capacitor replacement tutorial video, I mentioned that a two hundred
volt capacitor could kill you.
And some people didn't believe me!
Well let's look at the math.
This is a real capacitor here, not a static charged person.
And we've got a capacitance of 560 microfarads.
Since this is a large capacitor used in a power supply I can estimate that the
equivalent series resistance is going to be very low.
I might as well ignore it completely.
And let's assume that the capacitor is charged to its maximum
of two hundred volts.
Using our stored energy equation again, we can see that this capacitor can hold up to
11.2 joules of energy.
That's over 1500 times more energy than a typical static shock.
You don't need to be good at math to know that's going to hurt a lot!
A young man would probably survive it with a burn mark,
but if you've got a weak heart it'll kill you.
Now what might surprise you is that you don't even have to get into the
hundreds of volts range
for things to start being painful.
Here I have my bench power supply set to thirty volts and I'm measuring the
current flowing through my body.
It's roughly one milliamp and I can already feel it.
It's a slightly annoying tingle.
From personal experience I can tell you that fifty volts is where things
start getting really painful.
Remember what I said: As the voltage increases, the effective resistance of
your body decreases.
So the danger gets exponentially worse.
And if your skin gets pierced by a sharp wire or a high voltage arc,
the resistance of your body drops dramatically...
so you don't even have to have hundreds of volts
before things start getting dangerous.
A good rule of thumb is that voltages higher than just thirty volts
are dangerous,
and you should take safety precautions and make sure you don't touch anything.
And with the exception of static electricity,
you should treat every high voltage source as if it could kill you.
One exception to the rule is taser circuits which produce
hundreds of thousands of volts
but they deliberately have resistors inside them to limit the current to
nonlethal levels.
But as we all know... that doesn't always work out as planned...
so you should still treat them as deadly.
In conclusion, in almost every situation,
the volts directly determine the current flowing through your body.
If anyone tells you that volts won't kill you, they are an idiot!