[rescue] I'm not an EE Q - clarification

[James Lockwood]

On Fri, 19 Jul 2002, Kris Kirby wrote:

> Until the battery's internal resistance catches up, a gell cell can dump
> an insane amounts of current. 48V x 50A = 2400W = crispy fried in your own
> juices. All it takes is a blink of an eye, and gell-cells are best known
> for short term, high current capabilities -- car batteries take this to an
> extreme, however.

Ah, a favorite subject of mine.

You are never going to get this low of an impedance through the human
body, even with skin contact electrodes and conductive gel.  More typical
body impedance is in the neighborhood of a few kilohms, dipping down to
the 500 ohm area in ideal (worst case?) circumstances.  This (500) is
roughly the internal impedance of human tissue, the increase in total body
impedance usually observed is due to the skin interface.  High voltage
punches through the interface.  See NIOSH (98-131), "Worker Deaths by
Electrocution".

>From my standards collection, IEC479-2 (1987) "Effects of current passing
through the human body":

Table 1:

Total Body Impedance

                        Values for the total
                        body impedance (ohms)
                        that are not exceeded
                        for a percentage
                        (percentile rank) of
Touch Voltage           5%      50%     95%
(V)                     of the population

25                      1750    3250    6100
50                      1450    2625    4375
75                      1250    2200    3500
100                     1200    1875    3200
125                     1125    1625    2875
220                     1000    1350    2125
700                     750     1100    1550
1000                    700     1050    1500
Asymptotic value        650     750     850

"The values of the total body impedance given in Table 1 are valid for
living human beings and a current path hand to hand or hand to
foot for large contact areas (50cm^2 to 100cm^2) and dry conditions.

"At voltages up to 50V, values measured with contact areas wetted with
normal water are 10% to 25% lower than in dry conditions
and conductive solutions decrease the impedance considerably down to half
the values measured in dry conditions.

"At voltages higher than approximately 150V the total body impedance
depends only slightly on humidity and on the surface area of
contact."

In a nutshell, you're highly unlikely to have a contact impedance of under
a kilohm at 48V.  This gives you a current of 50mA or so, enough to kill
under worst-case circumstances, but the increased current capacity of the
gelcells has no real significance here.  You're only dissipating 2.5W, not
enough to have significant thermal effects.  High amp capacity doesn't
start to make a difference until you get to the multi-kilovolt range.

Does anyone know if there were any deaths from the old 90V B batteries
like the B126 (usually used in tube applications to supply plate voltage)?
I don't remember how much current they could flow.

-James