Electrical
Hazard
From The Merck Manual
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
are usually more dangerous than high-frequency currents and 3 to 5 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, 60Hz, 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,
etc.). Electric shock often cause a combination of these effects.
Threshold of perception
DC entering the hand is about 5 to 10 milliamperes (mA)
AC, 60 Hz , is about 1 to 10 mA.
"let-go" current
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 current
s 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 (eg., via a cardiac catheter
or pacemaker electrodes), much lower currents (>1 mA, AC or DC) can
produce fibrillation.
Body resistance (measure 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 20,00 to 30,000 ohms/ sq. cm,
whereas the resistance of moist thin skin is about 500ohms/sq. cm. If
the skin is punctures (eg., 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/sec squared. A thickly callused palm or
sole may have a resistance of 2 to 3 million ohms/sec squared. As current
passes throughout the skin, much energy may be dissipated at the surface
if the skin resistance is high, and large surface burns can result at
both the entry and exit points with charring of tissues in between (heat
= amperage2 x resistance). Tissues are also burned internally, depending
on their resistance; nerves, blood vessels, and muscles conduct electricity
more readily than denser tissues, eg., fat, tendon, and bone. If the
skin resistance is low, the patient may have few, if any, extensive
burns but may still suffer cardiac arrest if current reaches the heart.
The pathway of current throughout he 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 foot.
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 severe burns.
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