drumguy1384 said:
I was just illustrating the fact that A.C. currents are far
less dangerous than D.C. per volt.
Of course if you actually did grab a 220VAC line you
would almost certainly take a trip to the emergency room,
but it's far less dangerous than grabbing 12VDC (say, a
car battery for instance)
The reasoning goes like this ... since A.C. is constantly
changing state it is only at that 220 rated volts for a very
small amount of the time, in fact, most of the time it is far
below that. With D.C. you are hit with a constant one-way
voltage, which is far more dangerous to your heart (which
operates on electrical pulses ... not constant voltages) than
an oscillating signal.
Both are dangerous, don't get me wrong ... but 12VAC is
incredibly less dangerous than 12VDC.
Maybe my 220VAC analogy was a little over the top,
but from my own experience 220VAC is less dangerous
than 110VAC. While 220VAC has enough "kick" to throw
you off of the line 110VAC will just "hold on to you"
causing longer contact with the dangerous voltage levels
and making things worse.
A little background: AC voltages are measured not from
peak-to-peak (i.e. not from maximum positive to maximum
negative), nor from zero to maximum, but as the
root-mean-square of the voltage over a complete cycle.
That means mathematically taking the average of the square
of the voltage at small incremental values of time over the
full cycle of positive and negative and then adding those
squares up and then dividing by the number of time incre-
ments (i.e. no. of mesurements) and then taking the square
root of the result to give the "root-mean-square", i.e the
square root of the average square of the voltage value over
the complete cycle. (For the root-mean-square of a sine
wave, see the web page at URL:
<
http://scienceworld.wolfram.com/physics/Root-Mean-SquareVoltage.html>
While it may seem esoteric to the layperson, it's a way of
expressing the power-producing equivalent of a Direct
Current. In other words, 10 root-mean-square ("RMS")
volts across a 100 ohm load would give the same 1 watt
of heat as 10 volts of DC applied over the same 100 ohm
load. Note that the value of the RMS voltage is lower
than the zero-to-peak value of the voltage. And it is this
RMS value that is referenced when one says that house
voltage is 120 volts, *not* that the maximum excursion
is 120 volts positive above neutral (zero) or 120 volts
negative below neutral. Practically speaking, that means
that a 100 watt light bulb will burn as brightly using 120
RMS AC volts as it would with 120 DC volts. Notice
that the true maximum voltage excursion is really somewhere
around 120 x (square root of 2), or about 120 x 1.42 = 170
volts from neutral given a standard sine wave-shaped voltage
(i.e. that of normal power grids). And that means that any
insulation for wires carrying such voltage must withstand at
*least* 170 volts for sine wave AC voltage rated at
"120 volts" because "120 volts" in that case really means
"120 volts RMS", not "a maximum voltage of 120 volts".
The reason usually given for high voltages (i.e. several
hundred volts) being less deadly than house current voltages
has to do with its effect on the heart and whether the heart
will go into a protective spasm or continue to beat in an
unsychronized way (fibrillation). I have read that voltages
lower, somewhere around 90 volts to 75 volts are actually
more dangerous for this reason. And, as DrumGuy has stated,
at the same voltage, DC is more dangerous than AC because
it puts the heart into a fixed spasm rather than a series
of on-and-off spasms. But 12 volts - either AC or DC -
is way below the dangerous value.
Here is an excerpt from the web page at the following URL:
http://www.colorado.edu/physics/phys5430/phys5430_sp01/Course Information/Laboratory Safety.htm
"A good standard practice when working with exposed
high voltage is to always keep one hand in your pocket
to avoid the possibility of a current entering your body through
one hand and leaving through the other, because it is closest
will pass through your heart and start it fibrillating. Most
accidental electrocutions are due to fibrillation. For this reason
the most lethal voltage levels are around 400-500 V, since this
is most likely to lead to currents which cause fibrillation.
Higher voltages (for example, 2000 V), while still dangerous,
tend to cause the heart muscle to simply spasm, but beating will
usually resume if the voltage is removed quickly. High voltage,
high current sparks frequently cause very deep burns which
take a long time to heal. The 120 V line power is relatively safe,
but can still can provide painful shocks and can be quite
dangerous if your body is a relatively low resistance path, for
example if your hands are wet when you touch the wire.
Voltages below 50 V usually involve little danger."
In short, you don't have to worry about 12 volts. Inside the
power supply (and inside a CRT monitor) is the only place
in a PC that you'll find voltages higher than that. If you stay
out of the power supply, your only worry is killing a chip,
not killing yourself.
*TimDaniels*
