Nikola Tesla Crossword

Electrical Hazard

by Stephen Fowler, Fowler Associates

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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 (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 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 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.

**  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 details.