Physiological effects
of electricity
Most of us have experienced
some form of electric "shock," where electricity causes our
body to experience pain or trauma. If we are fortunate, the
extent of that experience is limited to tingles or jolts of
pain from static electricity buildup discharging through our
bodies. When we are working around electric circuits capable
of delivering high power to loads, electric shock becomes a
much more serious issue, and pain is the least significant
result of shock.
As electric current is
conducted through a material, any opposition to that flow of
electrons (resistance) results in a dissipation of energy,
usually in the form of heat. This is the most basic and
easy-to-understand effect of electricity on living tissue:
current makes it heat up. If the amount of heat generated is
sufficient, the tissue may be burnt. The effect is
physiologically the same as damage caused by an open flame
or other high-temperature source of heat, except that
electricity has the ability to burn tissue well beneath the
skin of a victim, even burning internal organs.
Another effect of electric
current on the body, perhaps the most significant in terms
of hazard, regards the nervous system. By "nervous system" I
mean the network of special cells in the body called "nerve
cells" or "neurons" which process and conduct the multitude
of signals responsible for regulation of many body
functions. The brain, spinal cord, and sensory/motor organs
in the body function together to allow it to sense, move,
respond, think, and remember.
Nerve cells communicate to
each other by acting as "transducers:" creating electrical
signals (very small voltages and currents) in response to
the input of certain chemical compounds called
neurotransmitters, and releasing neurotransmitters when
stimulated by electrical signals. If electric current of
sufficient magnitude is conducted through a living creature
(human or otherwise), its effect will be to override the
tiny electrical impulses normally generated by the neurons,
overloading the nervous system and preventing both reflex
and volitional signals from being able to actuate muscles.
Muscles triggered by an external (shock) current will
involuntarily contract, and there's nothing the victim can
do about it.
This problem is especially
dangerous if the victim contacts an energized conductor with
his or her hands. The forearm muscles responsible for
bending fingers tend to be better developed than those
muscles responsible for extending fingers, and so if both
sets of muscles try to contract because of an electric
current conducted through the person's arm, the "bending"
muscles will win, clenching the fingers into a fist. If the
conductor delivering current to the victim faces the palm of
his or her hand, this clenching action will force the hand
to grasp the wire firmly, thus worsening the situation by
securing excellent contact with the wire. The victim will be
completely unable to let go of the wire.
Medically, this condition of
involuntary muscle contraction is called tetanus.
Electricians familiar with this effect of electric shock
often refer to an immobilized victim of electric shock as
being "froze on the circuit." Shock-induced tetanus can only
be interrupted by stopping the current through the victim.
Even when the current is
stopped, the victim may not regain voluntary control over
their muscles for a while, as the neurotransmitter chemistry
has been thrown into disarray. This principle has been
applied in "stun gun" devices such as Tasers, which on the
principle of momentarily shocking a victim with a
high-voltage pulse delivered between two electrodes. A
well-placed shock has the effect of temporarily (a few
minutes) immobilizing the victim.
Electric current is able to
affect more than just skeletal muscles in a shock victim,
however. The diaphragm muscle controlling the lungs, and the
heart -- which is a muscle in itself -- can also be "frozen"
in a state of tetanus by electric current. Even currents too
low to induce tetanus are often able to scramble nerve cell
signals enough that the heart cannot beat properly, sending
the heart into a condition known as fibrillation. A
fibrillating heart flutters rather than beats, and is
ineffective at pumping blood to vital organs in the body. In
any case, death from asphyxiation and/or cardiac arrest will
surely result from a strong enough electric current through
the body. Ironically, medical personnel use a strong jolt of
electric current applied across the chest of a victim to
"jump start" a fibrillating heart into a normal beating
pattern.
That last detail leads us
into another hazard of electric shock, this one peculiar to
public power systems. Though our initial study of electric
circuits will focus almost exclusively on DC (Direct
Current, or electricity that moves in a continuous direction
in a circuit), modern power systems utilize alternating
current, or AC. The technical reasons for this preference of
AC over DC in power systems are irrelevant to this
discussion, but the special hazards of each kind of
electrical power are very important to the topic of safety.
Direct current (DC), because
it moves with continuous motion through a conductor, has the
tendency to induce muscular tetanus quite readily.
Alternating current (AC), because it alternately reverses
direction of motion, provides brief moments of opportunity
for an afflicted muscle to relax between alternations. Thus,
from the concern of becoming "froze on the circuit," DC is
more dangerous than AC.
However, AC's alternating
nature has a greater tendency to throw the heart's pacemaker
neurons into a condition of fibrillation, whereas DC tends
to just make the heart stand still. Once the shock current
is halted, a "frozen" heart has a better chance of regaining
a normal beat pattern than a fibrillating heart. This is why
"defibrillating" equipment used by emergency medics works:
the jolt of current supplied by the defibrillator unit is
DC, which halts fibrillation and and gives the heart a
chance to recover.
In either case, electric
currents high enough to cause involuntary muscle action are
dangerous and are to be avoided at all costs. In the next
section, we'll take a look at how such currents typically
enter and exit the body, and examine precautions against
such occurrences.
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REVIEW:
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Electric current is
capable of producing deep and severe burns in the body due
to power dissipation across the body's electrical
resistance.
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Tetanus is the
condition where muscles involuntarily contract due to the
passage of external electric current through the body.
When involuntary contraction of muscles controlling the
fingers causes a victim to be unable to let go of an
energized conductor, the victim is said to be "froze on
the circuit."
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Diaphragm (lung) and heart
muscles are similarly affected by electric current. Even
currents too small to induce tetanus can be strong enough
to interfere with the heart's pacemaker neurons, causing
the heart to flutter instead of strongly beat.
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Direct current (DC) is
more likely to cause muscle tetanus than alternating
current (AC), making DC more likely to "freeze" a victim
in a shock scenario. However, AC is more likely to cause a
victim's heart to fibrillate, which is a more dangerous
condition for the victim after the shocking current has
been halted.
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