Life and death - Practical
Perspective:


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Unformatted text preview: Practical
Perspective:
 Human
Body
Resistance;
Life
and
Death
in
the
World
of
Electricity
 
 Now
we
can
explain
the
resistance
measurements
discussed
in
the
beginning
 of
this
chapter
and
point
out
a
typical
blunder.
First
of
all,
let
us
see
how
an
 ohmmeter
works.
As
you
already
learned,
the
resistance
R
of
a
circuit
 component
can
be
calculated
as
the
ratio
of
the
voltage
VProbe
across
and
the
 current
IProbe
through
this
component.
This
is
exactly
what
happens
in
an
old‐ fashioned
ohmmeter:
it
applies
a
small
voltage
(such
as
from
a
1.5‐V
battery)
 across
the
terminals,
to
which
you
connect
the
resistor
whose
resistance
you
 want
to
measure,
and
measures
the
current
through
these
terminals
with
the
 V built‐in
ammeter;
the
unknown
resistance
is
obtained
as
their
ratio
 R = Probe 
 I Probe (see
Figure
2‐33).
 
 
2.6.
Practical
perspective:
How
to
avoid
 blunders
in
measurements
of
electric
 resistance?
 
 
 Figure
2‐33.
An
ohmmeter,
which
is
built
the
old
way,
applies
a
 small
voltage
VProbe
to
the
resistor
whose
resistance
you
are
 measuring
and
measures
the
current
IProbe
through
it.
The
 equivalent
resistance
R
is
calculated
as
the
ratio
(VProbe
/IProbe).
 Note
that
only
the
resistor
R
is
connected
to
the
instrument’s
 terminals,
which
are
shown
as
small
circles.

 ©
2010
Alexander
Ganago

 Page
1
of
12
 File:
Life
and
death
 Practical
Perspective:
 Human
Body
Resistance;
Life
and
Death
in
the
World
of
Electricity
 You
have
also
learned
that
the
unknown
resistance
can
be
calculated
from
 formulas
for
voltage
division
(see
Application
#1
in
section
2.3.5,
the
diagram
 in
Figure
2‐20
and
the
equations
derived
for
this
circuit).
This
idea
is
used
in
 a
more
modern
ohmmeter
that
applies
VProbe
~
1
V
to
a
series
combination
of
 the
unknown
resistance
R,
which
you
connect
to
its
terminals,
and
the
 internal
reference
resistance
RRef
;
the
built‐in
voltmeter
measures
the
 voltage
across
RRef
;
the
unknown
resistance
is
calculated
as
we
did
in
section
 2.3.5
and
displayed
for
the
user
(see
Figure
2‐34).

 
 The
correct
way
to
measure
resistance
is
sketched
in
Figure
2‐1
at
the
 beginning
of
this
chapter.
It
applies
to
any
type
of
ohmmeter.
The
essence
is
 that
only
the
resistor
whose
resistance
you
need
to
measure
should
be
 connected
to
the
instrument’s
terminals.
The
circuit
involves
only
the
 resistor
R
to
be
measured
and
the
internal
circuitry
of
the
ohmmeter
(Figure
 2‐33
or
2‐34).
The
wrong
way
to
measure
resistance
is
sketched
in
Figure
2‐ 4
and
repeated
with
explanations
in
Figure
2‐35:
distortion
takes
place
when
 the
user
touches
the
connectors
of
the
resistor
during
measurement.
 
 
 Figure
2‐34.
A
more
modern
ohmmeter
applies
a
small
voltage
 VProbe
to
a
series
combination
of
the
resistor
whose
resistance
 you
are
measuring
and
the
reference
resistor
RRef
,
measures
 the
voltage
across
RRef
,
and
calculates
the
unknown
resistance
 R
from
voltage
division
as
we
did
in
section
2.4.4.
Again,
only
 the
resistor
R
is
connected
to
the
instrument’s
terminals,
which
 are
shown
as
small
circles.

 ©
2010
Alexander
Ganago

 Page
2
of
12
 File:
Life
and
death
 Practical
Perspective:
 Human
Body
Resistance;
Life
and
Death
in
the
World
of
Electricity
 Evidently,
due
to
this
finger
touch,
the
human
body
becomes
part
of
the
 circuit:
the
resistance
of
human
body
RHB
is
connected
in
parallel
with
the
 resistor
R
as
emphasized
in
Figure
2‐35.
 
 
 
 Figure
2‐35.
When
the
user
touches
the
resistor’s
connectors
 during
resistance
measurement,
part
of
the
probe
current
from
 the
ohmmeter
flows
through
the
human
body
(HB),
which
 becomes
connected
in
parallel
with
the
resistor
R.

 
 Figure
2‐36
presents
the
diagram
for
the
circuit
that
involves
the
human
 body.

 
 ©
2010
Alexander
Ganago

 Page
3
of
12
 File:
Life
and
death
 Practical
Perspective:
 Human
Body
Resistance;
Life
and
Death
in
the
World
of
Electricity
 
 
 Figure
2‐36.
When
the
user
touches
the
resistor’s
connectors
 during
resistance
measurement,
current
division
takes
place
 between
the
resistor
R
and
the
human
body,
which
are
 connected
in
parallel.

 
 Touching
the
resistor,
which
is
connected
to
the
ohmmeter,
is
a
 commonplace
blunder
of
amateurs:
you
have
already
learned
that
the
 measured
value
gets
smaller
than
R
due
to
the
parallel
connection
between
 the
resistor
R
and
the
human
body.
Put
to
use
your
studies
in
this
chapter:
 avoid
the
amateur’s
blunder
and
measure
the
resistances
correctly.

 
 <Text
box
on
the
margin>
See
it
on
the
web:
file
“resistors
 01.mp4”
shows
that
when
I
touched
the
connectors
of
a
1.15‐ MΩ
resistor,
the
ohmmeter’s
reading
dropped
to
about
830
kΩ
 when
my
fingertips
were
dry
and
to
180
kΩ
when
they
were
 wet.
The
correct
reading
was
restored
as
soon
as
I
moved
my
 hands
away
from
the
resistor
being
measured.


 
 You
can
take
advantage
of
new
learning
and
turn
the
table:
using
the
 equations
for
parallel
connection
and
the
two
readings
on
the
ohmmeter,
 calculate
the
human
body
resistance!

 
 
 Examples
and
Practice
Problems
 ©
2010
Alexander
Ganago

 Page
4
of
12
 File:
Life
and
death
 Practical
Perspective:
 Human
Body
Resistance;
Life
and
Death
in
the
World
of
Electricity
 
 Example
2‐11
 
 While
measuring
a
1.15
MΩ
resistor,
I
touched
its
connectors
with
my
fingers
 and
read
180
kΩ
on
the
ohmmeter.
Calculate
the
electric
resistance
of
my
 body.
Assume
that
the
voltage
applied
to
ohmmeter’s
terminals
equals
1
V
 and
calculate
the
current
IHB
through
my
body.
 
 Solution

 
 Denote:
 R
–
the
actual
resistance
of
the
resistor,

 Req
–
the
equivalent
resistance
measured
by
the
ohmmeter
when
the
resistor
 is
in
parallel
with
the
human
body,
and

 RHB
–
resistance
of
the
human
body.

 
 Due
to
the
parallel
connection:
 R ⋅ Req R ⋅ RHB 

 
 
 [equation
2‐27]
 Req = thus RHB = R + RHB R − Req 
 Substitute
and
obtain:
 RHB
=
213.4
kΩ.
 
 Assuming
that
the
ohmmeter
applied
1
V
across
my
body
(from
hand
to
 hand),
obtain
the
current
IHB
through
my
body
from
Ohm’s
law:
 1 V I HB = = 4.686 ⋅ 10 −6 A = 4.686 µ A 
 213.4 kΩ 

 
 Practice
problem
2‐11
(Easy)
 
 When
I
touched
the
connectors
of
a
1.15‐MΩ
resistor
and
my
fingertips
were
 dry,
the
equivalent
resistance
was
read
as
830
kΩ.
What
was
the
electric
 resistance
of
my
body
in
this
measurement?

 
 Answer:
2.98
MΩ.

 
 
 
 ©
2010
Alexander
Ganago

 Page
5
of
12
 File:
Life
and
death
 Practical
Perspective:
 Human
Body
Resistance;
Life
and
Death
in
the
World
of
Electricity
 Pay
attention
to
these
numbers:
the
electric
resistance
of
a
human
body
is
 not
a
constant!
Due
to
wetting
the
fingertips,
it
can
be
reduced
by
more
than
 an
order
of
magnitude,
in
this
case,
from
2.98
MΩ
to
213
kΩ.
In
the
next
 section
you
will
learn
that,
dealing
with
electricity,
the
skin
condition
can
 become
an
issue
of
life
and
death!

 
 
 
 Human
body
is
an
electric
machine:
tiny
electric
pulses
in
the
nervous
 system,
which
are
caused
by
currents
of
ions,
mostly
Na+
and
K+,
control
the
 motion
of
each
muscle,
including
the
heart!
Some
of
these
electric
signals
are
 within
our
control:
for
example,
I
can
choose
whether
to
raise
my
arm
or
turn
 my
head.
At
the
same
time,
many
functions
of
vital
importance
are
beyond
 my
control:
for
instance,
I
voluntarily
stop
breathing
when
I
walk
through
a
 room
filled
with
smoke,
but
after
a
minute
or
two
my
body
disobeys
my
will
 and
resumes
breathing.
Similarly,
ordinary
people
cannot
control
their
 heartbeat.
Such
are
the
mechanisms
by
which
nature
protects
our
lives.

 
 
 However,
as
you
just
learned
from
the
amateur’s
blunder
in
resistance
 measurements,
electric
current
from
an
external
source
such
as
an
 ohmmeter
can
enter
the
human
body
(Figures
2‐35
and
2‐36).
The
good
 news
is
that
the
currents
caused
by
an
ohmmeter
are
harmless
because
they
 are
extremely
small:
our
estimate
in
Example
2‐11
is
~5
µA.
The
bad
news
is
 that
larger
currents
caused
by
external
sources
are
life
threatening,
even
in
 the
milliamp
range.

 
 
The
big
picture
is
that
electric
currents
caused
by
external
sources
–
not
the
 voltages
–
determine
the
effects
of
electricity
on
the
human
body.
These
 external
currents
can
interrupt
vital
functions
of
the
body
such
as
breathing,
 heart
beat,
motion,
etc.
Even
relatively
small
currents
can
do
harm.
Table
2‐2
 summarizes
their
effects.
Exact
numbers
depend
on
the
individual
responses,
 health
conditions,
etc.,
thus
various
sources
may
provide
slightly
different
 data.

 
 Table
2‐2
 2.7
Practical
perspective:
Life
and
death
in
the
world
 of
electricity:
How
to
escape
electrocution
at
home
 and
on
the
job
 ©
2010
Alexander
Ganago

 Page
6
of
12
 File:
Life
and
death
 Practical
Perspective:
 Human
Body
Resistance;
Life
and
Death
in
the
World
of
Electricity
 
 Effects
of
external
electric
currents
on
the
human
body
 
 Current
 Effects
 Below
1
mA
 Barely
perceptible
 1
–
3
mA

 Pain

 7
–
10
mA

 Let‐Go
threshold
 Above
15
mA
 Respiratory
paralysis
 Above
50
mA
 Ventricular
fibrillation
 Above
50
mA

 Muscle
paralysis
 Above
500
mA

 Can
stop
the
heart
 
 Currents
of
1
–
3
mA
cause
pain.Currents
about
7
–
10
mA
can
cause
your
 muscle
disobey
you:
this
is
the
so‐called
Let‐Go
threshold.

 
 The
Let‐Go
threshold
is
a
very
serious
and
partly
counterintuitive
condition
 that
deserves
a
special
discussion.
The
human
body
can
get
rid
of
some
 sources
of
danger:
for
example,
if
I
accidentally
grab
a
hot
handle
of
a
frying
 pan,
my
hand
muscles
will
automatically
(before
I
give
it
a
thought)
open
my
 fist
to
let
go
of
the
dangerous
object.
But
it
is
all
opposite
in
the
case
of
 electricity.
If
I
accidentally
grab
a
hot
wire
(a
bare
wire
under
voltage),
the
 muscles
of
my
palm
will
automatically
contract:
against
my
will,
I
will
keep
 holding
that
wire,
unable
to
let
go
of
it.
This
happens
if
the
external
electric
 currents
exceed
the
Let‐Go
threshold.
Such
external
currents
defeat
the
 protection
mechanisms
of
the
human
body,
leading
to
dire
consequences.


 
 At
about
15
mA,
external
currents
can
cause
respiratory
paralysis,
or
 difficulty
breathing.
Currents
above
50
mA
lead
to
serious
malfunction
of
the
 heart,
called
ventricular
fibrillation
–
a
potentially
fatal
series
of
very
rapid,
 ineffective
contractions
of
the
ventricles
caused
by
chaotic
electric
impulses.
 Figure
2‐37
shows
two
electrocardiograms
–
of
the
heart
in
ventricular
 fibrillation
and
of
the
normal
heartbeat.

 
 ©
2010
Alexander
Ganago

 Page
7
of
12
 File:
Life
and
death
 Practical
Perspective:
 Human
Body
Resistance;
Life
and
Death
in
the
World
of
Electricity
 Figure
2‐37.
Electrocardiograms
of
the
heart
in
ventricular
 fibrillation
(top)
and
of
the
normal
heartbeat
(bottom).
Source:
 http://www.merck.com/mmhe/sec03/ch027/ch027h.html

 
 
 Currents
above
1
A
can
stop
the
heart,
cause
severe
burns
of
the
internal
 organs
and
the
skin.
Electrically
caused
burns
are
usually
of
the
3rd
degree,
 because
the
body
can
hardly
heal
itself
and
skin
grafting
is
needed.

 
 Why
then
we
are
not
hurt
every
day?
An
old
C‐type
battery
can
produce
 currents
of
about
1
A:
how
can
it
be
safe
to
replace
it
with
bare
hands?
 Common
toasters
and
hair
dryers
use
more
than
10
A:
is
it
really
prudent
to
 use
them
without
wearing
rubber
gloves?
The
answer
is,
thanks
to
Ohm’s
 law.
Remember:
electric
current
is
the
ratio
of
voltage
and
resistance;
if
 electric
resistance
of
the
human
body
is
high,
the
external
currents
through
 the
body
remain
small
and
harmless,
and
this
is
indeed
our
everyday
best‐ case
scenario.

 
 The
human
body
is
complex;
thus
its
electric
resistance
depends
on
many
 factors
(according
to
some
reports,
the
resistance
drops
when
the
blood
 alcohol
content
grows);
as
usual,
we
look
at
the
big
picture.
A
very
crude
 
 ©
2010
Alexander
Ganago

 Page
8
of
12
 File:
Life
and
death
 Practical
Perspective:
 Human
Body
Resistance;
Life
and
Death
in
the
World
of
Electricity
 circuit
model
in
Figure
2‐38
shows
the
total
body
resistance
as
the
sum
of
 resistances
due
to
the
skin
and
due
to
the
internal
organs.

 
 Also
important
is
the
path
of
electric
current
through
the
human
body:
 currents
passing
from
hand
to
hand
or
from
hand
to
foot
can
go
through
the
 heart,
lungs,
and
other
vital
organs,
while
currents
passing
from
finger
to
 thumb
are
relatively
less
harmful.
This
is
why
the
professionals
working
with
 high
voltages
habitually
hold
one
hand
behind
the
back:
if
they
accidentally
 get
an
electric
shock,
the
path
of
current
will
be
limited
to
one
hand.
In
the
 following
discussion
we
look
at
the
worst‐case
scenario
that
involves
 currents
passing
through
the
vital
organs.
 
 
 
 Figure
2‐38.
The
electric
resistance
of
the
human
body
can
be
 presented
as
the
sum
of
resistances
due
to
the
skin
and
due
to
 the
internal
organs.

 
 The
numerical
values
of
electric
resistances
RSkin
and
RInternal
are
grossly
 different.
The
internal
organs
are
wet
and
full
of
ions:
they
easily
conduct
 electric
currents;
their
effective
resistance
is
estimated
at
~200
Ω.
On
the
 contrary,
the
resistance
of
healthy,
dry
skin
can
be
very
high,
in
the
mega‐ ohm
range
for
a
finger
touch.
This
is
food
for
serious
thought.
Apply
a
few
 volts
to
the
internal
organs
unprotected
by
the
skin
–
and
obtain
serious
 conditions
of
pain
and
distorted
function
of
muscles,
possibly
threatening
the
 ©
2010
Alexander
Ganago

 Page
9
of
12
 File:
Life
and
death
 Practical
Perspective:
 Human
Body
Resistance;
Life
and
Death
in
the
World
of
Electricity
 heartbeat,
breathing,
etc.
It
is
Ohm’s
law
–
and
a
good
skin
–
that
protect
us
 from
dangers
of
the
world
where
electricity
is
all
around!

 
 But
beware:
the
electric
resistance
of
the
human
skin
is
not
a
rock‐solid
 constant!
In
the
resistance
measurements
discussed
above,
wetting
my
 fingertips
led
to
a
drop
of
my
body
resistance
from
~3
MΩ
to
about
200
kΩ
 causing
the
current
through
my
body
increase
by
about
a
factor
of
15.
 Fortunately,
even
this
higher
current
remained
within
the
safe
range
because
 of
two
reasons:
the
voltage
remained
at
~1
V,
and
the
electric
contact
was
 limited
to
the
finger
touch.

 
 Wetting
the
skin
dramatically
decreases
electric
resistance
of
the
human
 body:
see
Table
2‐3
for
details.
 
 Table
2‐3
(from
Electrical
Safety
Handbook)
 
 In
the
extreme
case
of
a
hand
or
foot
immersed
in
water,
the
skin
resistance
 drops
to
~200
Ω,
similar
to
the
resistance
of
internal
organs!
Then
the
 
 ©
2010
Alexander
Ganago

 Page
10
of
12
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Life
and
death
 Practical
Perspective:
 Human
Body
Resistance;
Life
and
Death
in
the
World
of
Electricity
 currents
caused
by
120
V
from
a
household
outlet
can
grow
to
dangerous
 levels,
well
above
the
threshold
for
ventricular
fibrillation:

 120 V 120 V I HB = = = 200 mA 

 RSkin + RInternal + RSkin ( 200 Ω ) + ( 200 Ω ) + ( 200 Ω ) 
 
 
 This
is
why
many
thousand
people
get
hurt
–
and
killed
–
by
electricity
at
 their
homes
and
workplace:
commonplace
voltages
present
a
lethal
danger
 when
the
skin
resistance
decreases
due
to
sweating,
moisture,
etc.,
but
the
 greatest
hazard
is
blatant
neglect
of
safety
rules.


 

 
 The
safety
rules
provide
straightforward
 guidance
such
as:
 ‐ Know
how
to
turnoff
the
voltage
that
powers
 household
appliances
and
equipment
at
work:
learn
 where
the
circuit
breakers
are
and
how
to
operate
 them
 ‐ Always
to
turn
off
the
voltage
before
trying
to
repair
 appliances
or
replace
their
parts

 ‐ Never
touch
a
bare
wire
which
can
be
under
voltage

 ‐ Never
try
to
plug
in
a
bare
wire
into
a
wall
outlet
 ‐ Do
not
replace
a
light
bulb
until
you
make
sure
the
 socked
is
not
powered

 ‐ Do
not
attempt
to
repair
an
appliance,
which
can
be
 under
voltage
 ©
2010
Alexander
Ganago

 Page
11
of
12
 File:
Life
and
death
 Practical
Perspective:
 Human
Body
Resistance;
Life
and
Death
in
the
World
of
Electricity
 ‐ 
Do
not
open
covers
of
any
electronic
equipment:
it
 may
contain
charged
capacitors
at
high
voltage,
even
 if
the
unit
had
been
disconnected
from
the
power
 grid

 ‐ Be
especially
careful
with
appliances
that
can
get
 wet
–
in
the
kitchen,
bathroom,
etc.

 ‐ Never
use
a
hair
dryer
in
the
bathtub
 ‐ Be
especially
careful
when
your
hands
can
get
wet
 or
moist
–
and
you
have
to
work
with
electricity
 ‐ Do
not
operate
power
tools
such
as
electric
drills
 when
you
stand
on
a
wet
floor
in
wet
footwear.
 
 You
have
just
learned
why
these
rules
make
sense
and
can
really
save
your
 life.
This
is
the
practical
value
of
studying
Electrical
Engineering
regardless
of
 the
career
you
choose.


 
 
 ©
2010
Alexander
Ganago

 Page
12
of
12
 File:
Life
and
death
 ...
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