by Tony R. Kuphaldt
extracted from "Lessons
In Electric Circuits, Volume I – DC"
October 18, 2006
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
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
The brain, spinal cord, and sensory/motor organs in the
body function together to allow it to sense, move, respond, think,
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
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
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.
How AC affects the body depends largely on frequency.
Low-frequency (50- to 60-Hz) AC is used in US (60 Hz) and
European (50 Hz) households; it can be more dangerous than
high-frequency AC and is 3 to 5 times more dangerous than DC of the
same voltage and amperage.
Low-frequency AC produces extended
muscle contraction (tetany),
which may freeze the hand to the current's source, prolonging
exposure. DC is most likely to cause a single convulsive
contraction, which often forces the victim away from the current's
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 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
Electric current is capable of
producing deep and severe burns in the body due to power
dissipation across the body's electrical resistance.
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."
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.
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.