Contributed by:
Tommy
Submitted September 9, 2003
The Physical Effects of Electricity
: Electrocution or electrical shock
occurs when an electric current I passes through the
body. The amount of current passing through the body is
determined by Ohm's Law:
I = E/R
I = Current Through the Body
E
= Voltage across the body
R = Resistance of
the Body
Body resistance is an important
variable when considering electrocution. There is a wide
variation in body resistance between people therefore
the same voltage level may result in different effects.
The typical human body has a hand to hand resistance (R)
somewhere between 1,000 and 2,000 ohms. Babies, Children
and some other people have less resistance.
The
current is the controlling factor for Electrocution and
Electrical Shock. The threshold for perception is about
100 microamps (0.0001 Amps). Also See Microshock
Electrocution Hazards for currents less than 100
microamps. The National Electrical Code (NEC) considers
5 milliamps (0.005 Amps) to be a safe upper limit for
children and adults hence the 5 milliamps GFI circuit
breaker requirement for wet locations. The normal
nervous system reaction to any perceptible electrical
shock may cause a person to injure themselves or others,
therefore the so called safe limit does not assure
freedom from injury.
The more serious
electrocution and shock hazards occur above the let go
limits. 99% of the female population have an let go
limit above 6 milliamps, with an average of 10.5
milliamps. 99% of the male population have an let go
limit above 9 milliamps, with an average of 15.5
milliamps. Prolonged exposure to 60 Hz. currents greater
than 18 milliamps, across the chest causes the diaphragm
to contract which prevents breathing and causes the
victim to suffocate. No data is available for females or
children but suffocation is presumed to occur at a lower
current level.
The frequency of the electrical
current is as important as magnitude when evaluating
electrocution and electrical shock injuries. Humans and
animals are most susceptible to frequencies at 50 to 60
hertz. The internal frequency of the nerve signals
controlling the heart is approximately 60 hertz.
Ventricular fibrillation occurs when 60 hertz current
from the electric shock interferes with the natural
rhythm of the heart. The heart loses its ability to pump
and death quickly follows. Ventricular fibrillation can
occur at current levels as low as 30 milliamps for a two
year old child and 60 milliamps for adults. Most adults
will go into ventricular fibrillation at hand to hand
currents below 100 milliamps (0.1 Amp).
Humans
are able to withstand 10 times more current at DC and at
1000 hertz than at 50 or 60 Hz.. Electro-Surgical
equipment operating above 100,000 Hertz pass high
currents through the body with no effect on the heart or
breathing of a patient. Do you think that Murphy's Law
had anything to do with the American power line
frequency being set at 60 Hertz and the frequency for
the rest of the world being 50 hertz? All of the current
limits referred to in the growfaq articles are based on
power line frequencies of 50 or 60 hertz.
Electrocution may or may not leave physical evidence
of the injury. The occurrence of burns or other skin
damage is dependent upon the current density at the
point where the current enters or leaves the body.
Electrocutions occurring at 110 VAC seldom cause skin
damage unless the point of contact is small or the
victim has delicate skin. When higher voltages are
involved, high currents pass through the body and there
is greater likelihood that skin damage will occur. At
higher voltages there are often, but not always entrance
and exit wounds.
