Re: [Phys-L] Current or Voltage is the thing that kills?

[John Denker <jsd@av8n.com>]

Paul Nord gets the prize for the best answer by far.

Let me add a couple of additional points:

*) The frequency of the current matters.  In particular,
 60 Hz AC is nearly the worst case.  In particular:

 60 Hz is bad because it mimics natural nerve signals.
 It is about the worst possible choice from a safety
 point of view, except for 50 Hz (as used in Europe)
 which is slightly worse.
  -- it can create a scenario where having grabbed a
   conductor you cannot let go, and
  -- it can stop your heart.

 Low frequency aka DC voltage|current|power is orders of
 magnitude less dangerous.  At some point it will cook
 you, but it's not likely to stop your heart.

 High-frequency current (either a single brief pulse or
 many cycles of RF) is also orders of magnitude less
 dangerous (compared to 50 or 60 Hz).  Nerves don't
 respond to it in the same way.

*) The geometry matters.  The path of the current matters.
 In particular, if you grab terminal A with one hand and
 terminal B with the other hand, you are inviting the
 current to flow through your chest, right where the
 heart is.  Not recommended.

 In contrast, if you touch terminal A with your left
 thumb and terminal B with your left pinkie finger, it
 might be painful or even injurious, but it's not likely
 to stop your heart.

 This can be used to advantage.  When working on live
 circuits, or something that might be live, it is common
 to see people working with one hand.  The other hand is
 tucked into their belt behind their back, to keep it out
 of the way.

*) After explaining the right answers, you can go back
 and show where the student's analysis went wrong.  In
 particular, you can discuss the OTBE fallacy.  People
 are often quick to assume "Other Things Being Equal"
 when that is not a safe assumption.  The fundamental
 fallacy can be illustrated using the following rigid
 lever:

	^
	^
	^           ?
        o=====o=====o=====o
	W     X     Y     Z

 Question:  Point W is moved upwards.  Will point Y move
 upwards or downwards?

 The answer very much depends on whether X or Z is the
 fulcrum!

 The same logic applies to Ohm's law.  You can infer the
 voltage from the current and vice versa, *assuming* the
 resistance is constant, which is not a safe assumption.
 In particular, the resistance of wet skin is an order
 of magnitude smaller than the resistance of unbroken
 dry skin.

 When you take this into account, you see that current
 is much more directly relevant than voltage.  Even then,
 there are other factors.

 This lesson about the OTBE fallacy is super-important.
 It applies to electrical safety and a thousand other
 things.  Remember: any time you are tempted to say
 "Other Things Being Equal" you are probably about to
 say something that is not reliably true.

On 12/18/18 8:43 AM, Paul Nord wrote:

> Working at the national lab in Los Alamos there were some signs which said,
> "Danger High Current."  This was on a 5 volt bus that supplied power to a
> few thousand detectors.  Sure, you could touch the terminals with your
> hand, but drop a wrench across those terminals and it would blow up in your
> face.
>
> This leads to another point on the hazards of electricity... there are
> multiple ways that it can kill you.
> 1) There is the obvious cooking way where lots of power is dissipated in
> your flesh, raises body temperature and chars important organs.  This seems
> to be the way that most of you are thinking about.
> 2) Arc Flash = a very brief flash from a breaking contact could simply set
> you on fire.  Then again it may produce a hot plasma cloud in your
> vicinity.  The human reflex to such a shock is to inhale.  Needless to say,
> hot plasma is bad for the lungs.  (Always inhale before you flip a circuit
> breaker.)
> 3) Wires or stuff simply exploding.
> 4) Enough current to cause your muscles to tense and freeze in place.  This
> might not burn you, but if one of the muscles is your diaphragm, your won't
> be able to breathe.
> 5) Stopping your heart.  I'm no expert on the physiology of cardiac shock,
> but I am certain that a typical AED does not provide sustained high
> current.  Whatever the voltage and pulse duration, it has a serious impact
> on the cardiac rhythm.  What other sources might cause this, I wouldn't
> venture to guess.
>
> That's probably not an exhaustive list, but it's enough to give you a
> healthy respect for electrical power.