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The Fatal Current
Electrocution occurs when a small, specific amount of electrical current flows through the heart for 1 to 3 seconds. 0.006-0.2 Amps (that's 6-200mA milliamps) of current flowing through the heart disrupts the normal coordination of heart muscles. These muscles loose their vital rhythm and begin to fibrilate. Death soon follows. To provide an example of how small an amount of current it takes to kill; a 15 Watt night light draws about 125mA.
Animation by Paddy Morrissey
A potential "ground fault" exists when an ungrounded (hot) conductor comes in contact with the metal frame or chassis of the equipment it is supplying. In fig. above, the hot conductor has frayed where it enters the toaster. *Toasters are not grounded. There is now a potential for electricity to flow between the chassis of the toaster and any grounded surface that it might become connected to. In other words, it is waiting(ominously) to go to ground. A copper wire is conductive. It can conduct electricity. The earth is conductive. It can conduct electricity. The human body is conductive. It can conduct electricity. In the above example, our ill fated human has provided a conductive path between the "hot"(energized) toaster chassis and ground. A GROUND FAULT is occurring. There is current flowing from the chassis to the victims left hand, arm , heart, right arm, and right hand which is contacting the grounded metal plumbing system. The current then flows into the ground and finds its way back to the source of energy for this circuit, the utility's transformer. *Toasters are intentionally not grounded so that when a person starts forking around for a stuck piece of toast and they make contact with the nichrome heating element, they won't provide a fault path from the energized fork, hand, chest, hand and over the grounded metal enclosure. In part, this is the reason all countertop receptacles are now required to be GFCI protected.

Subject: Re: Electrocution in the Bathtub -- Resistance of distilled
water
Date: Thu, 15 May 1997 02:01:07 -0700
From: "K. Jones" <Mahkjq@RohmhaasNOSPAM.com>
Organization: Rohm and Haas Company
Newsgroups: misc.consumers.house , alt.home.repair

Gary Slusser wrote:
>
> Don Klipstein <don@Misty.com> wrote in article
> <5l8akh$ddc$2@news.misty.com>...
> > Rick Matthews wrote:
> > > Just filled my coffee cup with distilled water. Not the cleanest
> > > cup, and not the purest water, but still the resistance from one
> > > side to the other was 2 Megs.
> > > If 120 volts were applied across 2 Megs, a current of only
> > > 60 microamps would result.
> >
> > I could explain how things can get worse:
> >snip>
> > 3. One's body and metal bathtub parts and fallen electric appliances
> have
> > more contact area than ohmmeter leads, to such an extent as to outweigh
> > the greater distances in a bathtub than in a coffee cup. (I am guessing
> > the 2 meg reading was with ohmmeter leads or wires with maybe 1 square
> cm.
> > of contact area.)
> >
> > - Don Klipstein (don@misty.com)
>
> I assume the distance between the probes in the cup makes a difference in
> the 2 megs. I don't know the standard but my conductivity meter has about
> 1/8" dia. probes about 1/2" apart in the bottom of the cup. Only about 20
> m/l of water is used for the test and the reading is in ppm TDS. The reason
> I mention this is because 18 meg water is about the best there is and I'm
> sure there is a test standard set so everyone is one the same page.
>
> Gary
>

"Conductivity" meters are basically ohm meters. Conductivity for water
is measured in micro "mhos" (ohms backwards, and are the reciprocal of the
ohm) or microsiemens. TDS (mmhos per centimeter times 0.55 (low salt)) is
approximated from the reading taken. (It depends what you are testing though)
i.e. 2 mmhos = 1 ppm in low salt solutions, 7 mmhos = 1 ppm in high salt
soln

Conductivity depends on the temperature, 20degC is a standard ref temp.
A conductivity meter measures across a distance of 1 cm.

Our tapwater has a conductivity of about 250 mmhos, I'm guessing bath
water would be about 1000 mmhos (which by chance = 1000 ohms).
120V / 1000R = .12A - more than enough to kill.

Please don't throw any appliances in my bath, thanks.

--
K. Jones
wearing flame-proof threads, and shades
The opinions expressed are not necessarily my own,
they are not those of my employer,
and anyone else can speak for themselves.

Subject: Re: Electrocution -- other stuff for the website
Date: 15 Oct 1997 03:35:16 GMT
From: don@Misty.com (Don Klipstein)
Organization: Thomas' Consulting
Newsgroups: misc.consumers.house

David E. Buxton (davidbu@eng.tm) wrote:
: Some things to consider that I think would be worth covering on the web : site:
:
: * Typical resistance of dry caloused hands. How much current would flow : at 120 VAC. Sweaty caloused hands. Dry office hands. Sweaty office : hands.

Small contact point on dry caloused hands - a megohm or two. Dry "office hands" - Last time I tested that one, I got 300K ohms. Sweaty hands - sometimes 20K to 40K ohms for small contact area, sometimes just a few thousand ohms. Large contact area on sweaty hands - a few thousand ohms, sometimes as low as several hundred. Beware - electric shock can stimulate sweat glands. Enough current to cause electric shock may also heat up parts of your skin - and the resistance decreases wildly and sometimes unpredictably with increasing temperature. Higher voltages can break through the dryest, outermost region of the skin - and your resistance drops precipitously!

: * One artical I read stated that in the context of a hospital Intensive : Care Unit, using IV tubes with saline solution connected to needles into : the blood stream . . . . that the resistance can be very low, low enough : that a 1.5 volt battery can kill a patient. 1.5VDC / 0.05 A = 30 ohms. : May 50 mA wouldn't kill a healthy person and a lot less than that could : kill somebody in an ICU who is on the brink of death anyway. Perhaps : somebody could measure the resistance of a saline solution, perhaps even : some real blood and see what resistance is measured to be.

Please note some facts and figures on how much current can kill, if conducted from one arm through the chest cavity and to the other arm! This is for 50 and 60 Hz AC.

The currents most likely to cause fatal heart disturbances are in the range of 100 milliamps to 1 amp. Although higher currents are surprisingly less likely to cause fatal heart trouble, they are obviously dangerous and often enough deadly. Lower currents are sometimes fatal! Currents as low as 13 mA are truly dangerous, and currents as low as 5 mA have a slight chance of being fatal!

DC is less likely to cause electrocution, but sometimes still does. DC has no special ability to make you unable to let go of a conductor - AC does this also. DC also has no special ability to burn - AC does this also. Horror stories about DC abound mostly because the victims lived to tell them. However, BEWARE - DC has about 20 to 30 percent as much of the ability to cause fatal heart disturbances as AC does, and sometimes does kill.
Pulsating DC is pretty bad. People have killed themselves from accidentally discharging high voltage capacitors with their bodies.

: * Safety precautions when working with 120 VAC circuits. I've worked on : such circuits live while making sure I was not grounded -- wooden floor : with carpet. Using only one hand so no current path through my heart, : etc.
:
: * Also a consideration is that when working with 120 VAC, you may be 100% : careful not to touch the stuff, and then drop a screw driver across some : high amperage capacity connections and have molten metal explode into your : eyes. So, wear safety glasses, even if you think the circuit is dead.

- Don Klipstein (don@misty.com)