getting zapped

[Webb]

Ed Mellinger wrote:
> 
> Hi Folks,
> 
> I work with 1600 volts on the deck of a ship, so I was motivated to do a
> bit of reading about electrical safety.  Here's the gist of what I
> learned.  I can provide further references if anyone's interested...
> it's a bit off the EFI topic.
> 
> DISCLAIMER:  I'm forwarding this to the list as general background
> material on an important subject.  It is based on my personal
> interpretation of things I've read in the literature.  This material has
> not been reviewed by any safety agency, is not guaranteed to be
> accurate, and must not be used for design or other human safety
> purposes.  'Nuff said.
> 
> [Intro stuff deleted]
> 
> "Though we speak of hazardous voltages, the real villain is current
> forced through the body, and in particular through the heart, by the
> voltage in question.  For a given voltage the current depends on the
> skin resistance, which varies by orders of magnitude depending upon the
> individual and the presence of water or perspiration.  The physiological
> effects of current flow also vary, but typical examples are the
> following:
> 
>                                              DC        AC
> threshold of perception                       5   mA    1   mA
> painful shock                                30         6
> muscular control lost ("let-go threshold")   50        10
> ventricular fibrillation (3 sec)            500       100."
> 
> "Since average skin resistance is between 1,000 and 10,000 ohms, it is
> easy to see that any voltage over 100 V should be regarded as
> potentially lethal, and 1600 V as almost instantly so.  According to the
> literature there is very little middle ground in human shock cases:
> there is either a temporary stunning effect and a great story to tell
> later, or the heart stops beating and the victim is found dead."
> 
> Obviously your ignition's 25 kV will pretty much have its way in forcing
> current through your body, but fortunately this multi-kV source is both
> current and energy limited, to the extent that I've never heard of
> anyone being electrocuted by their car.  BUT there's plenty of current
> for involuntary muscle contraction, and as has been noted, if that
> throws you into the fan or a v-belt, you could easily be hamburger
> anyway.
> 
> Hope this is interesting... now go home and make sure the GFIs in your
> garage work!
> 
> Ed Mellinger

My career started with RADAR, so my motivation for understanding this
matter has been high as well. I've survived a non current limited
400VDC, and I've seen 5KV survived- but I've seen 7.5KV kill. Of course
the ignition on a car can top 50KV, but that is current limited, so when
you put yourself in the circuit (a big ugly resistor) the voltage seen
across your body is much lower. 

The above post was a good summary of my understanding of the
situation-with a couple of small exceptions.

1) It neglects the time factor. 500 mA through the heart for a small
enough time period (say 1 nanosecond) obviously won't kill ya. It's
total energy, not just instantaneous current.

2) It also concerns me because it's easy to misinterpret. The notion
that "It's the current that kills you, not the voltage" is about the
oldest misconception around - even among people who should know better. 

The above post has the situation correct, but anyone who has not had
Ohm's law, Watt's law, and Thevnin's(sp?) theorem beaten into them will
then conclude that the 12V car battery is dangerous, because it can
supply high levels of current. 

The battery demands respect. It can cause sparks that can burn. It can
produce enough current to vaporize a screw driver with ohmic heating. It
can cause high current through an inductive load (the horn for instance)
that can generate inductive kick (a way of generating high voltage) and
get your attention. It can supply the current for the ignition system
and zap you that way... 

But it will not electrocute you - or even exceed the threshold of
feeling. Ohm's and Watt's laws are still the way it is.

A better way is to rate the fatal energy (in joules) delivered to te
heart. A Joule is a Watt-second. The level that will put the heart into
fibrillation(sp?) I have seen listed at about 40 Joules delivered to the
heart.

Ron Webb