Caution: Power Supplies!!

Scott Gravenhorst chordman at
Wed Mar 25 00:30:06 CET 1998

On Tue, 24 Mar 1998 23:12:19 +0100, "Prof. Antonio Guerrieri"
<ga026sci at> wrote:

>>Speaking from a guy that works daily with 12,500 to 345,000 volts, I can
>>tell you that more people are killed from 110-480 volts that from any of
>>the higher voltages.  The reason--lack of respect for the danger.
>I agree but.....for what I know it is more dangereous a high CURRENT value
>than an high voltage... I mean very probably it is more dangereous to
>handle a circuit with few volts but supplying some ampers than one
>delivering i.e. some kvolts and few picoamps.

I = E / R.

This equation describes the current in a circuit.

I've won several bets with people because they didn't understand this.
Here's the bet:

"I'll bet I can place my hands, one on each of the battery terminals
of my car, (NOT RUNNING) and suffer NO ill effects."

This is a 12 volt battery capable of pushing several hundred _amperes_
of current.   ....  Given a low enough resistance load.  !!!

What the other guy doesn't understand is that my body's electrical
resistance is too high too allow sufficient current at 12 volts.
Current is the *real* killer, but only if it can actually flow at a
lethal level. The actual current is defined by the above equation.
Commercial "mains" power distribution circuits always have the
capability of pushing enormous current and are lethal because the
body's resistance is low enough to allow lethal levels of current to
flow.  Conversely, a jolt from a flyback transformer might not be
lethal because the impedance of the power circuit plus your own body's
resistance may not allow lethal current levels.  Painful, but not
usually lethal.

So, one must consider both the voltage available AND the ability to
supply lethal current.  If the voltage of the supply is low enough,
the body's resistance protects against the flow of lethal current.

-- Scott Gravenhorst

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