Re: [Phys-l] Electrocution in Bathtub - Hollywood style

[John Denker <jsd@av8n.com>]

On 10/17/2007 03:36 PM, Scott Goelzer wrote:

> A demo can be set up to help feel this on a small scale.
>
> Probably catch some negative comments, 

Yup.

> but during my internship I was  
> taught a demo that I now call the electric fish tank of death.  
> Dramatic and effective.
>
> Set up 10-20 gallon tank filled halfway with fresh water.
>
> Stand metal plates at each end and attach the hot and neutral of 120V  
> AC line voltage to the plates through a GFI.

I have lots of doubts about the margin of safety here.

Whenever serious safety is required, you want multiple *layers*
of safety.  Having two layers of safety doesn't make the risk
two times smaller;  it makes it thousands of times smaller.  Do
the math.  See also:
  http://www.av8n.com/how/htm/decision.html#sec-layers-safety

> Standing on a non-grounded floor with rubber shoes, 

Rubber shoes won't do you a bit of good if the victim puts both
hands into the water.

The GFCI won't do you a bit of good if the victim puts both hands
into the water.

The *only* layer of safety I see along this line is the warning

> Place left hand on top of head and leave hand there while tank is plugged in.

but that is IMHO a rather slim reed to lean on.  By way of
contrast:
 -- it would be better to put the other hand in a thick 
  lineman's glove.
 -- it would be better to put the other hand on a momentary-on
  switch, so that any attempt to move the hand would cut the
  power.
 -- or both
 -- but these contrasts are not to be considered sufficient
  improvements.

Another whole category of contrasts revolves around using
something like an electric fence charger rather than the AC
line.
 -- it is designed to be intermittent, so that the victim has
  a chance to let go
 -- it is designed to be a high-impedance low-current source
  (very unlike the AC line)
 -- it is designed to be nowhere near 60 Hz [which is physiologically
  about as bad as it could be (except for the 50 Hz they use in
  Europe, which is worse)].
 -- again, these contrasts are not to be considered sufficient
  improvements.

Also:  I wouldn't do anything remotely like this unless I had a 
defibrillator and somebody who knew how to use it.
		
Summary:  The foregoing is less than 1% of what I would consider 
an adequate safety analysis for this demo.  My analysis is *not* 
sufficient to create a safe version of this demo.  I stopped my
analysis after it became clear that the cost of making it safe 
exceeds the value of the demo.

I have problems with this demo on two levels
 *) The obvious level is that somebody might get hurt or killed 
  right before your eyes.
 *) The less-obvious level is that students might take away the
  idea that the level of safety provided in this demo is an
  adequate level of safety.


Bottom line:  I wouldn't do anything remotely like this at all,