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Human Body Capacitance and Resistance

W

Winfield Hill

Jan 1, 1970
0
A repost of a comment I made elsewhere, for discussion here.

The subject is ESD Human Body model values. I'm enamored by
a 1989 symposium paper by Richard Fisher, of Sandia Nat'l Labs,
where he created a "Severe Human ESD Body Model." His model
had worst-case numbers meant for use in electrostatic-discharge
circuit-protection analysis, etc.

Fisher's Severe Body Model consists of two parts, the body and an
arm with hand reaching out to zap something. The body part has
400pF of capacitance in series with 250 ohms and 0.5uH. Then the
arm and hand part bridges the body terminals with 10pF, and finally
we have another 110 ohms and 0.1uH in series to complete the model
and connect it to the poor real-world victim. The body capacitance
is higher than you may see elsewhere first because the body is
sitting down, and second because it's a worst-case body. We won't
go further into what that means. :)

You charge the 400pF capacitor to a voltage of your choosing.
20kV is a nice high number. During discharge we get a fast spike
of current from the 10pF, with sub-ns risetime to dangerous levels,
with up to 5A peak current, and lasting up to 5ns into the "load."
This is followed by a slower discharge of the 400pF capacitance,
lasting up to 200ns.

This would be followed by, ahem, a postmortem.

As for the effect of high moisture and humidity, as said, these can
affect things, e.g., lowering resistances to the low levels we see
in Fisher's Severe Model, but it also means the maximum electrostatic
voltage developed on the 400pF capacitor is likely to be much lower.
I suspect Fisher would prefer to take the dry-air high voltage with
the moist-skin low resistances for his Severe case.
 
P

Paul Keinanen

Jan 1, 1970
0
As for the effect of high moisture and humidity, as said, these can
affect things, e.g., lowering resistances to the low levels we see
in Fisher's Severe Model, but it also means the maximum electrostatic
voltage developed on the 400pF capacitor is likely to be much lower.
I suspect Fisher would prefer to take the dry-air high voltage with
the moist-skin low resistances for his Severe case.

Those resistances seem to be quite low for the resistance of the skin.
If the resistance would be that low, you would get severe burns each
time you touched the 230 V mains.

Those resistances in the order of a few hundred ohms would make sense,
if we assume that the spark penetrates the skin and the current
propagates in fluids under the skin.

RF burns from touching an antenna connected to a 100 W transmitter can
be quite painful, put it can be hard to detect, where the RF current
penetrated the skin.

With the component values given, it would form a lossy resonant
circuit with a resonant frequency in the upper HF range, thus
expecting a few cycles at that frequency, until the oscillation dies
down.
 
A

Archimedes' Lever

Jan 1, 1970
0
A repost of a comment I made elsewhere, for discussion here.

The subject is ESD Human Body model values. I'm enamored by
a 1989 symposium paper by Richard Fisher, of Sandia Nat'l Labs,
where he created a "Severe Human ESD Body Model." His model
had worst-case numbers meant for use in electrostatic-discharge
circuit-protection analysis, etc.

Fisher's Severe Body Model consists of two parts, the body and an
arm with hand reaching out to zap something. The body part has
400pF of capacitance in series with 250 ohms and 0.5uH. Then the
arm and hand part bridges the body terminals with 10pF, and finally
we have another 110 ohms and 0.1uH in series to complete the model
and connect it to the poor real-world victim. The body capacitance
is higher than you may see elsewhere first because the body is
sitting down, and second because it's a worst-case body. We won't
go further into what that means. :)

You charge the 400pF capacitor to a voltage of your choosing.
20kV is a nice high number. During discharge we get a fast spike
of current from the 10pF, with sub-ns risetime to dangerous levels,
with up to 5A peak current, and lasting up to 5ns into the "load."
This is followed by a slower discharge of the 400pF capacitance,
lasting up to 200ns.

This would be followed by, ahem, a postmortem.

As for the effect of high moisture and humidity, as said, these can
affect things, e.g., lowering resistances to the low levels we see
in Fisher's Severe Model, but it also means the maximum electrostatic
voltage developed on the 400pF capacitor is likely to be much lower.
I suspect Fisher would prefer to take the dry-air high voltage with
the moist-skin low resistances for his Severe case.

The skin is the big unknown.

Once opened by a wound, etc., the resistance is VERY low.

Open heart defib is only 2mA on the paddles.

Arm-to-arm with open wounds is only 10mA as opposed to 40mA on dry skin
to cause a fibrillation.

Area of contact is also a factor in lowering skin resistance (lowering
of the interface resistance into the salty blood conductor of our inner
body).

As for an electrostatic event causing a fibrillation, it is not very
likely until one gets up into the lightning bolt voltages. The time of
stroke becomes a factor.
 
L

legg

Jan 1, 1970
0
A repost of a comment I made elsewhere, for discussion here.

The subject is ESD Human Body model values. I'm enamored by
a 1989 symposium paper by Richard Fisher, of Sandia Nat'l Labs,
where he created a "Severe Human ESD Body Model." His model
had worst-case numbers meant for use in electrostatic-discharge
circuit-protection analysis, etc.

Fisher's Severe Body Model consists of two parts, the body and an
arm with hand reaching out to zap something. The body part has
400pF of capacitance in series with 250 ohms and 0.5uH. Then the
arm and hand part bridges the body terminals with 10pF, and finally
we have another 110 ohms and 0.1uH in series to complete the model
and connect it to the poor real-world victim. The body capacitance
is higher than you may see elsewhere first because the body is
sitting down, and second because it's a worst-case body. We won't
go further into what that means. :)

You charge the 400pF capacitor to a voltage of your choosing.
20kV is a nice high number. During discharge we get a fast spike
of current from the 10pF, with sub-ns risetime to dangerous levels,
with up to 5A peak current, and lasting up to 5ns into the "load."
This is followed by a slower discharge of the 400pF capacitance,
lasting up to 200ns.

This would be followed by, ahem, a postmortem.

As for the effect of high moisture and humidity, as said, these can
affect things, e.g., lowering resistances to the low levels we see
in Fisher's Severe Model, but it also means the maximum electrostatic
voltage developed on the 400pF capacitor is likely to be much lower.
I suspect Fisher would prefer to take the dry-air high voltage with
the moist-skin low resistances for his Severe case.

So0me references, if you don't already have them:

http://www.aecouncil.com/Papers/aec1.pdf
http://www.globalsmtindia.in/documents/ESD_DAMAGE_MODELS_AND_CHEMICAL_KINETICS-PART_I.pdf
http://www.barefoothealth.com/science/body_voltage_study.pdf

Combining dry ait high voltage with moist skin low resistance sounds
like a typical solutiion, when a committee avoids the use of it's
individual brains.

RL
 
S

Shaun

Jan 1, 1970
0
Archimedes' Lever said:
The skin is the big unknown.

Once opened by a wound, etc., the resistance is VERY low.

Open heart defib is only 2mA on the paddles.

Arm-to-arm with open wounds is only 10mA as opposed to 40mA on dry skin
to cause a fibrillation.

Area of contact is also a factor in lowering skin resistance (lowering
of the interface resistance into the salty blood conductor of our inner
body).

As for an electrostatic event causing a fibrillation, it is not very
likely until one gets up into the lightning bolt voltages. The time of
stroke becomes a factor.



Once again Dimbulb is wrong, "always wrong".

Don't you understand Ohms Law, if resistance goes up (dry skin), current
goes down, YOU IDIOT!.
Also it take the same amount of current through the body (dry skin, moist
skin or open wounds) to cause their heart to fibrillate. The closer you are
to the heart the less current it takes because more of that current will
flow directly though the heart.

Resistance goes down for open wounds(assuming the current goes into the open
wound) because there is a direct connection to the internal fluids of the
body, hence less voltage is needed to cause fibrillation. With dry skin
(high resistance) it takes more voltage to cause dangerous amounts of
current to flow.


The current levels that you specified are also wrong!

20 uA of current is considered hazardous to the Heart if it passes directly
through the heart.

500 uA of current is the maximum amount of leakage current allowed in
medical equipment through ground and if the ground connection was broken,
you touch the case of the device and ground, that current will flow though
you.

These specs are for patients at high risk, poor health condition.
 
A

Archimedes' Lever

Jan 1, 1970
0
Don't you understand Ohms Law, if resistance goes up (dry skin), current
goes down, YOU IDIOT!.


You are the idiot. Every statement I made was about OPEN skin. You
know, intimate access to the blood pathway.

Learn to read, dipshit.

Get a clue while you are at it. Then... grow the **** up.
 
S

Shaun

Jan 1, 1970
0
Archimedes' Lever said:
You are the idiot. Every statement I made was about OPEN skin. You
know, intimate access to the blood pathway.

Learn to read, dipshit.

Get a clue while you are at it. Then... grow the **** up.

No it wasn't all about open skin. You said dry skin take 40mA to cause
fibrillation, that is WRONG, like you always are. You know nothing you
fucking idiot. Why do you even post on the usenet, you should know by now
that you get shot down every time. Just give up! and do us all a favour.

You were WRONG on the mA ratings that you posted and your theory about
current required to cause fibrillation was wrong. You are always wrong.

Just give up.
 
A

Archimedes' Lever

Jan 1, 1970
0
Also it take the same amount of current through the body (dry skin, moist
skin or open wounds) to cause their heart to fibrillate.



Wrong. The key is the pathway, and how much of the total current
actually flows through that part of the pathway that is also comprised of
the heart. The heart only needs a couple milliamps through it to
fibrillate or defibrillate. As one gets further and further from the
heart as far as the current source and exit is concerned, the amount of
current needed to get the heart at 2mA increases, because the pathway is
millions of parallel resistors of which only a few thousand relate to
current flowing through the heart. The current in is one value, and the
current out is that same one value, but the current in each of the
millions of parallel resistors differs for each and not all contribute to
any flow that would relate to fibrillation or pass through the heart. So
entry point and egress point are very important. There are pathways that
would not cross the heart at all. Mode of entry is also important. Dry
skin has a high resistance. Blood does not. If punctures by the voltage
or by other means is involved, the current to cause fibrillation lowers
because the skin is not longer in the model.

You lose. Again.
 
A

Archimedes' Lever

Jan 1, 1970
0
hence less voltage is needed to cause fibrillation.


NOT voltage, idiot. CURRENT causes fibrillation.

You lose, again.
 
A

Archimedes' Lever

Jan 1, 1970
0
20 uA of current is considered hazardous to the Heart if it passes directly
through the heart.


I made HV medical device supplies. I know about what the limits are
and what the design constraints are.

Operating room defib paddles run at around 2mA during a cycle.

Grow the **** up.
 
A

Archimedes' Lever

Jan 1, 1970
0
500 uA of current is the maximum amount of leakage current allowed in
medical equipment through ground and if the ground connection was broken,

Those numbers are for AC powered devices.

Give us the handheld numbers, obsolete boy.
 
Archimedes' Lever said:
[snip]

As for an electrostatic event causing a fibrillation, it is not very
likely until one gets up into the lightning bolt voltages. The time of
stroke becomes a factor.

Correct. 400pF doesn't store much energy at a few kV. At 3kV its 1.8 mJ.
Getting stuck to a 3kV DC source is a different matter.

According to http://en.wikipedia.org/wiki/Defibrillation, it takes
hundreds of Joules to defibrillate a heart.

300J from chest to back, the best path possible. BTDT, burned a bit.
 
W

Winfield Hill

Jan 1, 1970
0
Robert Baer wrote...
Let me point out that I was thinking of a component postmortem.
I have known of two extreme cases of skin resistance: one person
could feel a slight "tingle" of he placed his hands between 120VAC
(himself in middle), and the other got killed when he accidentally got
himself in series with a SIX VOLT car battery ("good old days") - the
current thru the chest was more than sufficient to do the job.

JEDEC standard JESD22-A114D spells out the familiar HBM ESD
test model: 100pF in series with 1500 ohms.

Fisher's Severe HBM was created from hundreds of measurements
reported in the literature. His interest was in the worst-case
observations. His 360-ohm value of body resistance is lower
than you observe with simple ohm-meter measurements, etc., but
keep in mind it's a high-voltage measurement. Is it reasonable
to hope that our outer-skin-layer insulation can withstand a
say 20kV discharge and maintain high-resistance?
 
D

Dirk Bruere at NeoPax

Jan 1, 1970
0
A repost of a comment I made elsewhere, for discussion here.

The subject is ESD Human Body model values. I'm enamored by
a 1989 symposium paper by Richard Fisher, of Sandia Nat'l Labs,
where he created a "Severe Human ESD Body Model." His model
had worst-case numbers meant for use in electrostatic-discharge
circuit-protection analysis, etc.

Fisher's Severe Body Model consists of two parts, the body and an
arm with hand reaching out to zap something. The body part has
400pF of capacitance in series with 250 ohms and 0.5uH. Then the
arm and hand part bridges the body terminals with 10pF, and finally
we have another 110 ohms and 0.1uH in series to complete the model
and connect it to the poor real-world victim. The body capacitance
is higher than you may see elsewhere first because the body is
sitting down, and second because it's a worst-case body. We won't
go further into what that means. :)

You charge the 400pF capacitor to a voltage of your choosing.
20kV is a nice high number. During discharge we get a fast spike
of current from the 10pF, with sub-ns risetime to dangerous levels,
with up to 5A peak current, and lasting up to 5ns into the "load."
This is followed by a slower discharge of the 400pF capacitance,
lasting up to 200ns.

This would be followed by, ahem, a postmortem.

Well, since people regularly charge themselves up to at least 30kV (a 1
cm spark) I would say not.
 
W

Winfield Hill

Jan 1, 1970
0
Dirk Bruere at NeoPax wrote...
Well, since people regularly charge themselves up to at least
30kV (a 1 cm spark) I would say not.

Right you are. As I clarified earlier, I was thinking of a
component postmortem.
 
M

Mycelium

Jan 1, 1970
0
Well, since people regularly charge themselves up to at least 30kV (a 1
cm spark) I would say not.


Except that he was referring to the chip. D'oh!
 
D

Don Klipstein

Jan 1, 1970
0
Well, since people regularly charge themselves up to at least 30kV (a 1
cm spark) I would say not.

30 KV per centimeter is the breakdown gradient of air. But that number
only correlates to spark gap length when the eectric field is even just
before breakdown.

In the usual case of people charging themselves up by shuffling their
shoes on carpet, they make sparks in gaps with uneven electric field. So,
less than 30 KV can make a 1 cm spark. If one end of the spark gap is a
sharp point or the tip of a wire maybe AWG 22 (approx. .63 mm) or smaller,
then a 1 cm spark can occur from about 11 KV, easily from 12KV.

Also, these sparks often appear bigger than they are. I find 1 cm to be
uncommon, but I find 8 mm fairly easy to achieve with favorable shoes and
a fairly favorable carpet and favorable humidity. So, I think 10 KV is
common but much more is not.

However, I remember experiencing one apartment with one exceptionally
favorable carpet and I somewhat remember making 15 mm, possibly 18 mm
sparks (corresponding to probably about 17 to possibly 20 KV).
 
S

Shaun

Jan 1, 1970
0
If you had read my hole post you would have seen that I mentioned that right
after quote above. See below

Quote: "Don't you understand Ohms Law, if resistance goes up (dry skin),
current
goes down, YOU IDIOT!.
Also it take the same amount of current through the body (dry skin, moist
skin or open wounds) to cause their heart to fibrillate. The closer you are
to the heart the less current it takes because more of that current will
flow directly though the heart.

Resistance goes down for open wounds(assuming the current goes into the open
wound) because there is a direct connection to the internal fluids of the
body, hence less voltage is needed to cause fibrillation. With dry skin
(high resistance) it takes more voltage to cause dangerous amounts of
current to flow.

end quote

Wrong. The key is the pathway, and how much of the total current
actually flows through that part of the pathway that is also comprised of
the heart. The heart only needs a couple milliamps through it to
fibrillate or defibrillate. As one gets further and further from the
heart as far as the current source and exit is concerned, the amount of
current needed to get the heart at 2mA increases, because the pathway is
millions of parallel resistors of which only a few thousand relate to
current flowing through the heart. The current in is one value, and the
current out is that same one value, but the current in each of the
millions of parallel resistors differs for each and not all contribute to
any flow that would relate to fibrillation or pass through the heart. So
entry point and egress point are very important. There are pathways that
would not cross the heart at all. Mode of entry is also important. Dry
skin has a high resistance. Blood does not. If punctures by the voltage
or by other means is involved, the current to cause fibrillation lowers
because the skin is not longer in the model.

You lose. Again.

I never lost in the first place, but you always lose!

You must have taken that paragraph right out of a book, bravo! I know you
don't have the brains to come up with that your self or even put together a
paragraph like that.
 
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