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Sci.Electronics.Basics -> Human Electrocution: How is the resistance not ridiculously high?
There are 28 messages in this thread.
You are currently looking at messages 20 to 28.
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Author: ChuckDate: 20:37 03-04-08
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On Thu, 3 Apr 2008 09:05:36 -0700 (PDT), Tomás Ó hÉilidhe
<toe@lavabit.com> wrote:
>
>I've been doing electronics for three years now but I don't understand
>how a person can be electrocuted by touching one part of the circuit
>in a mains supply.
>
>If I hold one lead of an ohmmeter in my left hand, and the other in my
>right hand, it registers the resistance to be approximately 2
>megaohms, which is ridiculously high.
>Now if I hold one lead in my hand, and dig the other into the grass,
>it doesn't even register -- I may as well be holding the leads apart
>in thin air.
>
>Current = Voltage divided by Resistance
>
>Current = 230 volts divided by 2 megaohms = 115 microamperes
>
>115 microamperes is nowhere near enough to electrocute someone.
>
>So lets say I stick a metal rod into the socket on the wall. The
>current has to flow thru my hand, down to my foot, thru my cotton
>sock, thru my shoe, thru the wooden floorboards, thru the concrete,
>thru the clay down to the metal rod we call ground. Now excuse me, but
>is that not a RIDICULOUS amount of resistance, up in the gigohms
>somewhere?
>
>It may sound like I'm denying that people get electrocuted -- I'm not,
>I realise that people do get electrocuted. But I can't for the life of
>me understand how enough current can flow, given the massive
>resistances that are involved.
>
>Can anyone enlighten me?
Probably not I.
But have you considered the difference in potential between your head
and your feet when you stand outside? A somewhat different situation
that may cause you to pose the same questions.
Chuck
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Author: stanDate: 21:41 03-04-08
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David L. Jones wrote:
> On Apr 4, 5:12 am, Tomás Ó hÉilidhe <t...@lavabit.com> wrote:
>> Michael A. Terrell:
>>
>> > Then stick your hands into a live AC power line, after making sure
>> > your will is up to date, and your insurance is paid up. You know just
>> > enough to be dangerous. The resistance also depends on contact area,
>> > and what part of the body. Loose the attitude, or someone will find you
>> > dead from your ignorance.
>>
>> If you'll read my original post, I explicitly express that I _don't_
>> deny that people get electrocuted, so your accusations of ignorance
>> and "attitude" are illformed. What I'm questioning is the science of
>> it, as we understand it today. I've never, ever, not once, heard a
>> single valid explanation of how someone can get electrocuted by
>> touching a 230 V mains terminal.
>>
>> I'm no denying that it happens. In fact I'm acknowledge that it
>> happens, and I also acknowledge that I don't understand how it
>> happens, and so I'm inviting people here to discuss the science of it.
>>
>> I myself know about DC, AC, resistivity, resistance, capacitance,
>> inductance, impedance... but none of these things explain how 230 V is
>> enough to kill me if I grab the positive terminal.
>>
>> Here's the path of the current:
>> 1) From my hand to my foot: About 2 megohms.
Several people have tried to explain that a human body is not very much
like a typical carbon resistor. I suggest that you start by trying to
wrap your mind around that concept. Think of a cap for a second.
Frequency is one factor that impacts the impedance. In a somewhat
similar way there are many factors that affect the resistance of a human
body. Your trusty multimeter is incapable of giving you meaningful
information about a capacitor. Likewise it's incapable of giving you any
real info about the human body. Life is more complicated than we like
sometimes.
>
> On your multimeter at 1V. It will vary a LOT under all sorts of
> conditions.
> That has very little correlation to the "resistance" at much higher
> voltages.
>
>> 2) From my foot thru to the other side of my sock: Probably in the
>> megohms, if not gigaohms.
I'd be very surprised if your feet don't sweat at least a tiny bit. I'm
sure you know that salt waater conducts pretty well.
>
> Same as above.
>
>> 3) From my sock to the inside of my shoe, to the sole of my shoe:
>> Probably in the megohms, if not gigaohms.
Would depend a lot on the shoe. For a street shoe and some boots which
use nails or tacks to help hold the sole on I think you might be way
off.
>
> Same as above.
>
>> I just can't understand how 230 V is enough to push anything other
>> than a negligible current thru that gargantuan resistance.
On a good day it won't. The problem is that other day when a shoe
becomes compromised. Even when the shoe isn't the path all it takes is
an entry and an exit. Techs are taught habits that reduce the chances
you will get into a bad situation. Removing watches and rings, keeping
one hand in a pocket when reaching into a live circuit, and others. At
really high power at rf frequencies even providing a sharp point can be
a bad thing. Techs are taught not to point at anything while in the
transmitter room at LORAN stations. Pointing with a single finger can
privide an opportunity for an arc that won't be any fun.
Anyway, the resistance of a human body is very complex and bears almost
no relation to the common and much simpler carbon resistor. You're going
to have to shift your thinking here.
>
> You just don't get it.
> The "resistance" varies a LOT based on all sorts of factors, not least
> of which is voltage.
> The figures you have measured on your multimeter are of very little
> indication at all.
> That is why "high voltage" resistance meters ("meggers") are
available
> and commonly used to test insulation resistance at mains voltages.
Being retired military we always had a lot of meggars around. Guys would
play around with them once in awhile. It was always amazing how heavy
those test leads got so fast. I was also surprised that apparently
meggars seem to turn people into geneologists. Soon after dropping the
heavy leads they would start questioning ancestry almost without
exception. I can't explain the brain chemistry behind it, but but it was
clearly a statistically significant observation. :)
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Author: John GDate: 21:56 03-04-08
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"Tomás Ó hÉilidhe" <toe@lavabit.com> wrote in message
news:151ecf10-14fb-4ff8-b72c-0a3012f21fc3@u36g2000prf.googlegroups.com...
> Michael A. Terrell:
>
>> Then stick your hands into a live AC power line, after making sure
>> your will is up to date, and your insurance is paid up. You know just
>> enough to be dangerous. The resistance also depends on contact area,
>> and what part of the body. Loose the attitude, or someone will find you
>> dead from your ignorance.
>
>
> If you'll read my original post, I explicitly express that I _don't_
> deny that people get electrocuted, so your accusations of ignorance
> and "attitude" are illformed. What I'm questioning is the science of
> it, as we understand it today. I've never, ever, not once, heard a
> single valid explanation of how someone can get electrocuted by
> touching a 230 V mains terminal.
>
> I'm no denying that it happens. In fact I'm acknowledge that it
> happens, and I also acknowledge that I don't understand how it
> happens, and so I'm inviting people here to discuss the science of it.
>
> I myself know about DC, AC, resistivity, resistance, capacitance,
> inductance, impedance... but none of these things explain how 230 V is
> enough to kill me if I grab the positive terminal.
>
> Here's the path of the current:
> 1) From my hand to my foot: About 2 megohms.
> 2) From my foot thru to the other side of my sock: Probably in the
> megohms, if not gigaohms.
> 3) From my sock to the inside of my shoe, to the sole of my shoe:
> Probably in the megohms, if not gigaohms.
>
> I just can't understand how 230 V is enough to push anything other
> than a negligible current thru that gargantuan resistance.
No body has said much about AC and Ground.
Most everything that is not a live wire is connected to GROUND in one way or
another and so the whole environment you are in is one side of a capacitor
and so if you touch a live wire then you are the other side of this
capacitor and some current will flow because the power system is Alternating
Current.
No measurements with your punny little multimeter will mean anything in this
case and depending on a myriad of facts already discussed you could be dead.
John G.
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Author: Paul E. SchoenDate: 02:46 04-04-08
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"John G" <green@ozemail.com.au> wrote in message
news:47f58ad8$0$13241$5a62ac22@per-qv1-newsreader-01.iinet.net.au...
>
> "Tomás Ó hÉilidhe" <toe@lavabit.com> wrote in message
> news:151ecf10-14fb-4ff8-b72c-0a3012f21fc3@u36g2000prf.googlegroups.com...
>> Michael A. Terrell:
>>
>>> Then stick your hands into a live AC power line, after making sure
>>> your will is up to date, and your insurance is paid up. You know just
>>> enough to be dangerous. The resistance also depends on contact area,
>>> and what part of the body. Loose the attitude, or someone will find
>>> you
>>> dead from your ignorance.
>>
>>
>> If you'll read my original post, I explicitly express that I _don't_
>> deny that people get electrocuted, so your accusations of ignorance
>> and "attitude" are illformed. What I'm questioning is the science of
>> it, as we understand it today. I've never, ever, not once, heard a
>> single valid explanation of how someone can get electrocuted by
>> touching a 230 V mains terminal.
>>
>> I'm no denying that it happens. In fact I'm acknowledge that it
>> happens, and I also acknowledge that I don't understand how it
>> happens, and so I'm inviting people here to discuss the science of it.
>>
>> I myself know about DC, AC, resistivity, resistance, capacitance,
>> inductance, impedance... but none of these things explain how 230 V is
>> enough to kill me if I grab the positive terminal.
>>
>> Here's the path of the current:
>> 1) From my hand to my foot: About 2 megohms.
>> 2) From my foot thru to the other side of my sock: Probably in the
>> megohms, if not gigaohms.
>> 3) From my sock to the inside of my shoe, to the sole of my shoe:
>> Probably in the megohms, if not gigaohms.
>>
>> I just can't understand how 230 V is enough to push anything other
>> than a negligible current thru that gargantuan resistance.
>
> No body has said much about AC and Ground.
>
> Most everything that is not a live wire is connected to GROUND in one way
> or another and so the whole environment you are in is one side of a
> capacitor and so if you touch a live wire then you are the other side of
> this capacitor and some current will flow because the power system is
> Alternating Current.
>
> No measurements with your punny little multimeter will mean anything in
> this case and depending on a myriad of facts already discussed you could
> be dead.
>
> John G.
There is some more information at
http://van.physics.uiuc.edu/qa/listing.php?id=6793, where it states that
the external human body reistance is about 1k to 100k Ohms, and the
internal resistance is 300 to 1000 ohms. Only a thin layer of dry skin
separates the internal resistance from an external object.
The human body capacitance to a far ground is 100-200 pF, which is really a
minimum value. This correlates to an impedance of about 13 MegOhms at 60
Hz, which corresponds to a minimum of 9 uA at 120 VAC to ground. This is
enough to be sensed and used for capacitively operated light dimmers.
Here is a way to measure your body capacitance:
www/Capacitance.html" target=_blank>http://web.mit.edu/Edgerton/www/Capacitance.html
The inside of your body can be considered a conductor, and thus if you
place your hand flat on a metal plate, you will form a capacitor with an
area of perhaps 15 square inches, with a thin (maybe 0.005") insulating
layer of dry skin, which will form a capacitor much higher in value than
the 200 pF stated above. According to a formula in
http://www.sayedsaad.com/fundmental/11_Capacitance.htm, this would be C =
0.2249 * k * A / d = 1350 pF, (assuming k for skin is 2, about like dry
paper). This will be an impedance of about 2 MegOhms, and current of 60 uA.
This is still below the normal threshold of sensation, and still far below
the usual safe current levels of 1 to 5 mA.
The actual thickness of the epidermis (per
http://dermatology.about.com/cs/skinanatomy/a/anatomy.htm) varies from 0.05
mm (0,002") for eyelids to 1.5 mm (0.06") for palms and soles, but the
actual outer layer of the epidermis that is a good insulator is composed of
flat, dead cells, which is much thinner. So the capacitance could be much
higher than the quick estimate above.
Probably the main reason for electrical current to reach levels high enough
for electrocution to occur (6 to 200 mA for 3 seconds, according to
http://www.codecheck.com/ecution.htm), is when skin becomes sweaty or
otherwise loses its dry protective layer, which quickly exposes the
underlying 1000 ohms or less, which will conduct 120 mA at 120 VAC.
There are safe ways to measure the body's resistance and capacitance using
realistic higher voltages, skin conditions, and contact surfaces, but I'm
not going to suggest anyone try it. Suffice it to say that ohmmeter
readings are misleading, and any carelessness around any kind of voltage
source can be dangerous.
For very high voltages, there are standard minimum distances that must be
maintained between a worker and an energized line:
http://www.dir.ca.gov/oshsb/rubberglove.html. I found this on a search for
rubber glove testing. My previous company manufactured oil and glove
insulation breakdown testers.
The field intensity near high voltage lines is so great that it might be
fatal to touch them even if you were suspended in free air. You may notice
that birds can sit on lower voltage transmission lines which are 5kV to 50
kV or so, but not on 200kV+ lines.
Paul
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Author: =?ISO-8859-1?Q?Teodor_V=E4=E4n=E4nen?=Date: 11:00 04-04-08
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Chuck skrev:
> On Thu, 3 Apr 2008 09:05:36 -0700 (PDT), Tomás Ó hÉilidhe
> <toe@lavabit.com> wrote:
>
>> I've been doing electronics for three years now but I don't understand
>> how a person can be electrocuted by touching one part of the circuit
>> in a mains supply.
>>
>> If I hold one lead of an ohmmeter in my left hand, and the other in my
>> right hand, it registers the resistance to be approximately 2
>> megaohms, which is ridiculously high.
>> Now if I hold one lead in my hand, and dig the other into the grass,
>> it doesn't even register -- I may as well be holding the leads apart
>> in thin air.
>>
>> Current = Voltage divided by Resistance
>>
>> Current = 230 volts divided by 2 megaohms = 115 microamperes
>>
>> 115 microamperes is nowhere near enough to electrocute someone.
>>
>> So lets say I stick a metal rod into the socket on the wall. The
>> current has to flow thru my hand, down to my foot, thru my cotton
>> sock, thru my shoe, thru the wooden floorboards, thru the concrete,
>> thru the clay down to the metal rod we call ground. Now excuse me, but
>> is that not a RIDICULOUS amount of resistance, up in the gigohms
>> somewhere?
>>
>> It may sound like I'm denying that people get electrocuted -- I'm not,
>> I realise that people do get electrocuted. But I can't for the life of
>> me understand how enough current can flow, given the massive
>> resistances that are involved.
>>
>> Can anyone enlighten me?
>
> Probably not I.
>
> But have you considered the difference in potential between your head
> and your feet when you stand outside? A somewhat different situation
> that may cause you to pose the same questions.
Another thing that occured to me while reading this thread is the
question of what kind of ohmmeter the OP used to get 2 Megohms...
My interests apart from electronics have occationally caused me to
experiment with skin galvanic response (the resistance of the skin and
how it drops and rises under certain circumstances (wikipedia it)).
One thing I noticed during those experiments is that I get a much higher
reading with Digital Multimeters than with old-school Analog ones. IIRC,
the main difference between the two types of meter is that the digital
ones tend to measure the voltage drop over the resistor when connected
to a constant current source, while analog ones tend to to be a battery
connected in seriers with the meter (through a variable resistor to be
able to null the meter).
I do not claim to know the cause of this, but my hunch is that the
amplifier (OP or Transistor) picks up hum from the body, making the
reading be a little off. I do not know if making the amplifier
insensitive to AC voltages is a priortiy for the designers, but it
wouldn't surprise me if they save money on not making it so.
Oh, BTW, the readings I've got with my analog meter have been 50k to
150k -ish, which puts it near the area of being dangerous with regards
to current passing through your body. Factors I've noted that affect
skin resistance is your emotional state, area of contact (2 leads when
compared to, say, two washer with leads soldered to them), to name a few...
And the traditional "my worst zap story":
I've accindentally connected myself to a 235VAC/50Hz power grid, phase
in on hand, neutral in the other (don't ask). I actually heard the 50Hz
hum in my ears, and my ticker occationally still bitches about it, a
whole 14 (sic!) years later... Yes, I do respect the national power grid
a lot more these days :-)
Just my $.02 worth,
/Teo.
--
Teodor Väänänen | Don't meddle in the affairs of wizards,
<teostupiditydor@algonet.se> | for you are good and crunchy with
http://www.algonet.se/~teodor/ | ketchup.
Remove stupidity to reply. |
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Author: Don KlipsteinDate: 02:41 05-04-08
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In <97de30be-74bd-456b-96ca-ba2ce8b96024@b5g2000pri.googlegroups.com>,
Tomás Ó hÉilidhe wrote:
>I've been doing electronics for three years now but I don't understand
>how a person can be electrocuted by touching one part of the circuit
>in a mains supply.
>
>If I hold one lead of an ohmmeter in my left hand, and the other in my
>right hand, it registers the resistance to be approximately 2
>megaohms, which is ridiculously high.
I do find readings like this for this situation to be common.
Now, for some factors to complicate this:
1. Skin being just a little on the moist side - due to body chemistry,
mood, recent past activity, body response to ambient temperature and
humidity - it's a little common for this to be a few hundred K-ohms rather
than a couple megohms. Occaisionally this kind of reading can get down to
50K ohms or so.
2. Current in the roughly-1-milliamp range or more can do a few things
to make the resistance decrease:
a) Stimulate sweat glands - especially if the current is AC or
pulsating DC of frequency probably anywhere in/near the lower half of the
audio range, especially 50/60/100/120 Hz
b) Cause electrolysis that results in a decrease in contact resistance
over time.
Try holding tightly two bare wires coming from a DC power supply of
voltage of whatever voltage is low enough for you to assume is safe and
not have yourself or next of kin sue me over if things go wrong in any
way, with a milliammeter or microammeter in series with the current path.
If that voltage is around/above 12 volts, see if that current stays
steadily low or starts increasing.
Imagine what could happen at 120 volts.
c) At/near 120 volts or more, localized heating could occur at skin
contact points. Skin and body fluids generally have negative
temperature coefficients for their resistance, especially skin.
===========================
Other things to consider:
1. You may get accidentally shocked or shocked by malfunctioning
equipment with skin contact area larger than that typical with handling of
ohmmeter leads.
2. You could get such a shock if sweaty or otherwise wet.
3. The most-widely-mentioned "fatal range" of current, for causing
ventricular fibrillation, is 100 mA to 1 amp for an arm-to-arm or
arm-to-leg shock with 50-60 Hz AC. (Increase of current past 1 amp has
fatality rate less than that of .1-1 amp, in case of arm-to-arm shock with
"power line frequency AC", but there is still some fatality rate from
outright cardiac arrest - plus risk of vital organs getting outright
cooked.)
This is merely a "most deadly range", with the "deadliness" not
dropping
to zero at 99 or 90 mA. Some sources say 50 mA is the lower end of the
range of having a fairly significant chance of causing ventricular
fibrillation from an arm-to-arm shock, and a small number of sources say
that 30 milliamp neon sign transformers (which have current-limiting
means, unlike most transformers that are not "lamp ballasts") have a bit
of a body count!
For that matter, I have seen one bit saying that there is some chance of
fatality at currents as low as around 5 mA - from someone being paralyzed
by the shock, with paralysis including paralysis of breathing muscles.
Keep in mind that shock causing someone to involuntarily maintaining a
position that maintains exposure to the shock is widely said to be worse
with DC, but is actually worse with AC (or pulsating DC as opposed to
steady DC). Steady DC is "less-shocking", since most effects of electric
shock result from variation of current.
The horror stories from people receiving severe electrical burns on (and
also inside) their bodies mostly involve those zapped with either DC or
radio frequencies - so that they survive to tell the horrors!
======================
Keep in mind that electrocution can get unreliable. The "Electric
Chair" appears to me designed to rely on the jolt either cooking vital
organs, and/or paralyzing breathing muscles (and preferably also the
heart) long enough to have the brain deprived of oxygen severely enough to
be unable to restart breathing when the jolt stops.
Sometimes the condemned is subjected to more than one jolt.
As unreliable as electrocution is, lack of fatality from electric shock
is similarly unreliable.
======================
The human body is a 470K-ohm 1/4 watt resistor with tolerance of
+5000/-98 % and a negative temperature coefficient!
(I don't know who started this, and I could easily be "off" with the
numbers somewhat for that one)
- Don Klipstein (don@misty.com)
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Author: Don KlipsteinDate: 02:47 05-04-08
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In article <pprfc5-l21.ln1@invalid.net>, stan wrote:
>John Popelish wrote:
>> Tomás Ó hÉilidhe wrote:
>>> I've been doing electronics for three years now but I don't understand
>>> how a person can be electrocuted by touching one part of the circuit
>>> in a mains supply.
>>>
>>> If I hold one lead of an ohmmeter in my left hand, and the other in my
>>> right hand, it registers the resistance to be approximately 2
>>> megaohms, which is ridiculously high.
>>> Now if I hold one lead in my hand, and dig the other into the grass,
>>> it doesn't even register -- I may as well be holding the leads apart
>>> in thin air.
>>>
>>> Current = Voltage divided by Resistance
>>>
>>> Current = 230 volts divided by 2 megaohms = 115 microamperes
>>>
>>> 115 microamperes is nowhere near enough to electrocute someone.
>>>
>>> So lets say I stick a metal rod into the socket on the wall. The
>>> current has to flow thru my hand, down to my foot, thru my cotton
>>> sock, thru my shoe, thru the wooden floorboards, thru the concrete,
>>> thru the clay down to the metal rod we call ground. Now excuse me, but
>>> is that not a RIDICULOUS amount of resistance, up in the gigohms
>>> somewhere?
>>>
>>> It may sound like I'm denying that people get electrocuted -- I'm not,
>>> I realise that people do get electrocuted. But I can't for the life of
>>> me understand how enough current can flow, given the massive
>>> resistances that are involved.
>>>
>>> Can anyone enlighten me?
>>
>> Do the calculation again but with the other hand on the
>> water tap or the grounded case of an appliance.
>>
>
>How much current do you think it takes o cause death? Do you think it
>matters if it's AC or DC?
AC is worse. Pulsating DC is worse than steady DC.
DC gets some worse reputation due to higher survival rate of those
getting zapped with current that causes horrific burns, especially to
internal tissue that heals slowly or never heals right.
Being jolted into maintaining body position that has you getting zapped
is often said to be worse with DC, but that is actually worse with AC (or
pulsating DC). The horror stories come more from survivors than from
those who don't live to tell horror stories.
- Don Klipstein (don@misty.com)
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On Apr 4, 10:47=A0pm, d...@manx.misty.com (Don Klipstein) wrote:
<snip>
> =A0 AC is worse. =A0Pulsating DC is worse than steady DC.
>
> =A0 DC gets some worse reputation due to higher survival rate of
those
> getting zapped with current that causes horrific burns, especially
to
> internal tissue that heals slowly or never heals right.
> =A0 Being jolted into maintaining body position that has you getting
zapped
> is often said to be worse with DC, but that is actually worse with
AC (or
> pulsating DC). =A0The horror stories come more from survivors than
from
> those who don't live to tell horror stories.
>
> =A0- Don Klipstein (d...@misty.com)
Has Tom=E1s =D3 h=C9ilidhe stopped responding because he offed himself ?
GG
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