by Stephen Fowler, Fowler Associates
Injuries from electrical shocks depend upon many
factors including the type and magnitude of current, the resistance
of the body at the point of contact, the current pathway, and
the duration of the current flow.
Type and magnitude of current
In general, direct current (DC) is less dangerous
than alternating current (AC). The effects of AC on the
body depend largely on the frequency; low-frequency currents,
50 to 60 Hz (cycles/sec), are usually more dangerous than high-frequency
currents, and three to five times more dangerous than DC of the
same voltage and amperage. DC tends to cause a convulsive
contraction, often forcing the victim away from further current
exposure. AC 60 Hz causes muscular contraction, often "freezing"
the hand to the circuit as the fist clenches the current source
and may result in prolonged exposure with severe burns if the
voltage is high. Generally, the higher the voltage and the
amperage, the greater the damage from either type of current.
Both AC and DC may affect the body either by altering physiologic
functions (involuntary muscular contractions and seizures, ventricular
fibrillation, respiratory arrest) or by producing thermal, electrochemical,
or other damage (burns, necrosis of muscle and other tissue, hemolysis,
coagulation, dehydration, vertebral and other skeletal fractures,
muscle and tendon avulsion). Electric shock often causes
a combination of these effects.
Threshold of perception
DC entering the hand is about 5 to 10 milliamperes
AC 60 Hz is about 1 to 10 mA.
The maximum current that can cause contraction
of the flexor musculature of the arm but still permit the subject
to release his hand from the current source.
DC - this value is about 75 mA
AC - this value is about 15 mA
and varies with muscle mass.
A low-voltage (110 to 220v) 60 Hz AC traveling
throughout the chest for a fraction of a second may induce ventricular
fibrillation at currents as low as 60 to 100 mA; about 300 to
500 mA of DC are required. If the current has a direct pathway
to the heart (e.g., via a cardiac catheter or pacemaker electrodes),
much lower currents (>1 mA, AC or DC) can produce fibrillation.
Body resistance (measured in ohms/sq. cm)*
is concentrated primarily in the skin and varies directly with
the skin's condition. Dry, well-keratinized, intact skin
has an average resistance of 20,000 to 30,000 ohms/sq. cm, whereas
the resistance of moist thin skin is about 500 ohms/sq. cm. If
the skin is punctured (from a cut or abrasion or by a needle)
or if current is applied to moist mucous membranes, the resistance
may be as low as 200 to 300 ohms/sq. cm. A thickly callused
palm or sole may have a resistance of 2 to 3 million ohms/sq.
cm. As current passes throughout the skin, if the resistance
is high, large surface burns can result at both the entry and
exit points with charring of tissues in between (heat = amperage
squared x resistance). Tissues are also burned internally
depending on their resistance; nerves, blood vessels and muscles
conduct electricity more readily than denser tissues such as fat,
tendon and bone. If the skin resistance is low, the victim
may have few, if any, extensive burns but may still suffer cardiac
arrest if current reaches the heart.
The pathway of current throughout the body can
be crucial in determining injury. Conduction from arm to
arm or between an arm and a foot at ground potential is much more
dangerous than contact between a leg and ground since the current
may traverse the heart. Electrical injuries to the head
may cause seizures, intraventricular hemorrhage, respiratory arrest,
ventricular fibrillation or asystole, or as a late effect, cataracts.
The most common entry point for electricity is the hand,
followed by the head. The most common exit point is the
The duration of current flow through the body
is important. While the heart is vulnerable to small currents
at relatively low voltages, in general the amount of injury to
the body is directly proportional to the duration of exposure
because tissue breakdown occurs with longer durations allowing
internal current flow. Heat is produced by current flow
through tissues, causing severe burns, protein coagulation, vascular
thrombosis, and tissue necrosis.
When a victim freezes to a circuit, he may suffer
** Note: Unlike material resistivity,
body resistance is measured in ohms/sq.cm, historically due to
the electrode configuration of 1cm x 1cm. This unit
has been chosen to show that for differing configurations, the
absolute resistance in ohms is dependent on size and spacing of
contacts. Please refer to the Merk Medical Manual for further