[TriEmbed] Current & Voltage

[Brian]

A few have responded, but I'll add my bit o' knowledge as well.

Pro tip: When working around dangerous voltages, keep your left hand in 
your pocket.  Why?  And what are "dangerous" voltages?  Read on.

The first thing to know is Ohm's Law (it's not just a good idea!).  It 
links current, voltage, and resistance; the three are meaningless apart 
from each other.  Ohm's Law in its typically-presented form is thus:

E = I*R

That is, the potential (voltage) is equal to the current times the 
resistance.  Any time you know two, you can solve for the third using 
algebra.  Consider a simple circuit of a resistor connected to a 
battery.  You know the potential (it's the battery's voltage) and you 
know the resistance (it's stamped on the resistor), so you can figure 
out how much current will flow.  For example, a 9-volt battery with a 
450-ohm resistor:

E = I*R
9V = (?)A * 450
(?)A = 9V / 450
? = 0.02 A

A 450-ohm resistor across a 9-volt battery will result in a current flow 
of 20 mA.

Now, back to the discussion at hand, concerning health and safety when 
handling electricity.

The adage about current being what kills you is mostly true, if a bit 
simplified; potential alone is simply separation of charge--it's when 
(and where) those charges start moving around (current) that problems 
occur.  Through the body, it typically takes less than 100 mA [1] of 
current to interfere with the heartbeat.  Sustained current through the 
body at 100 mA will be extremely painful and will cause involuntary 
muscle contraction (including the heart muscle).  If brief enough, body 
function may return to normal after the current flow is stopped.  As the 
current through the body increases, damage occurs much more quickly in 
much more permanent forms.  Much over 100 mA and the heart will enter 
ventricular fibrillation.  That means you die, unless you have a 
paramedic nearby with a defibrillator.

Pretty scary, no?  100 mA isn't much current!  Well, take heart.  While 
your wet, salty insides are pretty conductive, your skin (when dry) has 
a whole lot of resistance (in the 10,000s of ohms).  To get 100 mA of 
current to flow through 10 kOhms, Ohm's law tells us we need a potential 
of 1,000 volts!

Now, don't breathe that sigh of relief just yet.  I'm sure you've heard 
about how dangerous the voltage in your home's electrical outlets is. 
That's due to a couple reasons.  First, your skin is almost never 
completely dry.  You sweat, and your skin secretes oils.  These tend to 
lower the resistance; wet skin has a resistance closer to 1,000 ohms 
(only 100 V needed to hit 100 mA, less than your 110v wall outlet). 
Secondly, much lower currents (around 50 mA, only 50v across wet skin) 
will cause muscle contractions, meaning that if you grab a live wire you 
likely won't be able to let go.  The current, while lower, will be 
sustained and have time to do more damage.

If the skin is broken, that's a whole 'nother ball of wax.  Once you're 
through the skin, internal body resistance is generally in the 300-ohm 
range.  Now you only need 30 volts for quick fatality, and considerably 
less for severe discomfort and potential death.

Anyway, in summary, your car battery won't kill you because 12 volts 
across your 10-kilohm skin creates less than 2 mA of current internally 
(unless you are cut open).  Not even enough to feel a tingle.

Of course, it also matters where the current is flowing inside your 
body, and that leads to some safety practices you can adopt when working 
around dangerous voltages.  Your heart's only at risk if the current is 
flowing through it.  If current enters and exits your body just on one 
side, your heart will be reasonably safe [2].  If you stick your finger 
in a light socket, it will hurt like the dickens most of the way up your 
arm, but it probably won't kill you.

And that's why you keep your left hand in your pocket.  You're much less 
likely to create a current path that goes through your heart that way.

Hope this is helpful!
-Brian

[1] - There is a lot of disagreement concerning what constitutes 
dangerous / fatal current through body tissue.  Most of the sources I've 
read tend to specify currents in the 50 - 100 mA range.

[2] - Current flowing in a conductive medium spreads out and creates a 
field, but that's way beyond the scope of this discussion.

On 7/9/2015 11:00 AM, Grawburg via TriEmbed wrote:
> I know it's always, "it's the current that will kill you, not necessarily the voltage", but surely there is a reasonable way to determine what the "killing" combination is.  I've got a book that says 700 mA could do you in if it finds a path through your heart.  Is this really suggesting that if I take the 5VDC, 850 mA power supply I use for my Raspberry Pi, hook one lead to my big toe and the other lead to my ear lobe I may end up dead? When we discuss current & voltage in my RPi class next week I'd like some more information to pass on.
>
>
> Thanks,
> Brian Grawburg
>
>
>
>
>
> _______________________________________________
> Triangle, NC Embedded Computing mailing list
> TriEmbed at triembed.org
> http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
> TriEmbed web site: http://TriEmbed.org
>