Re: Safety FAQ is here --

While surfing the web for info on a lightning research station near my
"home" town, I found this interesting page on current and the human
body at:


I posted it because of the section on frequency.


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When the human body becomes a part of an electrical circuit, it then
follows Ohm's Law. Current flow through the body,
resulting in severe shock, depends on several factors. The amount of
current the human body can withstand has been
determined and verified by many tests. A voltage difference across the
human body is necessary for shock. It is the resultant
current flow that does the damage. Human-body response to alternating
current flow can be classified into three levels, i.e.,
perception current, let-go current, and lethal current. Body
resistance, voltage, amplitude, and frequency also enter into the
severity of the shock.

Body resistance. The human body essentially acts as a resistor at 60
Hz. Minimum resistance of the body, between the major
extremities, has been generally defined as 500 ohms. The chief initial
current-limiting factor is the surface or contact resistance,
which varies between different parts of the body and between
individuals. Dry skin resistance varies from 100 to 300,000
ohm-cm^2. Wet skin has only 1% of the resistance of dry skin. If shock
current flows for a few seconds, blisters develop
which further decrease surface resistance.

Voltage amplitude. The voltage necessary to produce shock current
depends on contact resistance and total circuit
impedance. When contact resistance is low, commercial 120V AC is
lethal. High voltage contact resistance is not a factor,
since voltages in excess of 240V AC puncture the skin upon contact.
Under this condition only the internal body resistance
limits the shock current.

Frequency. Body impedance at 60 Hz is essentially resistive. As the
frequency increases, body impedance begins to act as a
resistor-capacitor network and becomes non-linear. At about 50 kHz
body impedance may decrease more than 50%, and
contact resistance is negligible. Frequency also affects perception,
let-go, and fibrillation currents. A current of 7mA at 5kHz is
required to produce tingling. Between 100 to 200 kHz perception
changes from tingling to heat. At 5kHz let-go current
increases threefold over the 60 kHz value. The current required to
cause fibrillation also increases at higher frequencies.

Perception current. Perception or reaction current is that level of
alternating current that produces a slight tingling sensation.
The startling effect from this sensation could produce involuntary
muscular reaction that might cause injury. Laboratory tests
show that the lowest perceivable current at 60 Hz varies in different
individuals. Less than 1% can sense current levels as low
as 0.3 mA. However, the mean value of perception current in men is
about 1.1 mA. The mean perception level for women is
about 0.7 mA. These levels do not damage human tissue. As a result,
perception current has been established at 0.5 mA. 

Let-go-current. Let-go current is the maximum current at which an
individual grasping a conductor can release it by using
muscles directly affected by the current. Let-go current, averaging
about 16 mA for men and 10.5 mA for women, also does
not damage human tissue. 

Lethal current. Lethal currents begin at only slightly higher levels
than let-go currents. When a current in excess of 18 mA
flows through the chest cavity, the chest muscles contract to stop
breathing. Maintained current flow results in loss of
consciousness and, eventually, death. Ventricular fibrillation is
another potentially lethal result of shock in which the heart ceases
its rhythmical pumping action, and instead, feebly quivers to
effectively stop blood circulation. The heart rarely recovers
spontaneously from this condition. Extrapolations from data compiled
from experiments on animals show that current levels
necessary for producing fibrillation in humans depend on shock
duration, body weight, current-flow path, flow duration, and
current magnitude. It is generally believed that the heart of a normal
adult is likely to fibrillate when a shock current in
milliamperes exceeds 116/t^0.5, where t is shock duration in seconds.

     Shock current that greatly exceeds the level necessary to produce
fibrillation may completely stop the heart
     action, seriously burn body tissues, damage the nervous system,
and stop breathing--all potentially lethal

     In order to understand the preceeding better, assume the
resistance of the human body to be 700 ohms and a
     percentage distribution of resistance as shown in Biegelmeier and
Rotter (1971).

Note: Thanks to Gil Sharick and abc Teletraining, Geneva, IL for the
original authorship of this article. [Sharick, G: 1981,
Grounding and Bonding, abc Teletraining, Geneva, IL. Tel.

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