Electrical Accidents are often fatal. It takes very little electrical current 75-250 mA (1 mA = 1/1000 Amps) to disturb the heart’s electrical system and cause ventricular fibrillation. The heart beats in a rapid and uncoordinated manner. Once initiated, it is self-sustaining. Blood circulation ceases, and the person becomes unconscious in less than 10 seconds. In 3 to 6 minutes, irreversible brain damage occurs followed by death.
The purpose of safety codes and standards are to prevent loss of life and property.
I have had training in the above codes. I am a Past Chair of the IEEE Mississippi Section and a Life member of the IEEE. I am a member of NFPA.
Since the early 1990’s, the number of work-related electrical deaths in the U.S. has steadily declined. Still, there are around 175 work-related electrical deaths each year (2010). The majority of these (44%) are from contact with overhead power lines. Residential power lines (distribution lines) are often bare non-insulated conductors with a voltage in the neighborhood of 8,000 volts. At this voltage level, the resistance of the skin is negligible. The initial resistance of the human body from hand-to-hand or hand-to-foot is approximately 500 ohms. If we know the voltage and resistance, the current can be calculated via Ohms Law:
The percent body resistance between the hand and other large contact areas is given in Figure 1. The numbers in parentheses are for when both hands are joined and current flows between both hands and the indicated site.
For voltages below 600 volts, the skin resistance is significant. Table 1. gives the human resistance for various parts of the body.
|Dry Resistance||Wet Resistance|
|Hand Holding Wire||10-50K||3-6K|
|Hand Holding Pliers||5-10K||1-3K|
|Hand Around 1.5" Pipe|
or Drill Handle
|Two Hands Around 1.5" Pipe||0.5-1.5K||250-750|
Internal, Excluding Skin
Table 2. gives the resistance values for various materials.
|Rubber Gloves or Soles||> 20 M|
|Dry Concrete above Grade||1-5 M|
|Dry Concrete on Grade||0.2-1 M|
|Dry Leather Soles|
|Damp Leather Soles|
|Wet Concrete on Grade||1-5 K|
The human response to electricity is director proportional to the electric current. Table 3. gives these responses.
|Current (60 Hz)||Physiological Phenomena||Feeling or Lethal Incidence|
|< 1 mA||None||Imperceptible|
|Perception Threshold||Mild Sensation
|10 mA||Paralysis Threshold of Arm||Cannot Release Hand Grip.
May progress to higher current and be fatal.
|30 mA||Respiratory Paralysis||Stoppage of Breathing
|75 mA||Fibrillation Threshold 0.5%||Heart Action Discoordinated
|250 mA||Fibrillation Threshold|
99.5% > 5s
|4 A||Heart Paralysis Threshold|
|Heart stops for Duration of Current Flow.
For Short Stops, Heart Restarts.
|> 5 A||Tissue Burning||Not Fatal unless Vital Organs are burned.|
Actual Case #1. After a thunderstorm, a rice farmer let his son out of the truck to restart an irrigation pump. When he touched the metal enclosure of the motor starter, he was electrocuted. It was a three-phase 480 VAC motor (277 volts from any phase to ground). There was a rodent’s nest in the enclosure. The rodent had gnawed on a wire going to a surge protector. The wire was electrically welded to the enclosure door. The pump had been pumping water along with a little sand for over 20 years. The land around the pump was sinking. The boy was barefoot and standing in water. It is assumed that he touched the metal enclosure with his palm, and the enclosure was still wet.
R is the sum of the palm resistance, the body resistance, and the immersed foot resistance.
R = 1000 + 200 + 100 = 1300
I = V/R = 277/1300 = 213 mA
The fact that both feet were immersed in water would only slightly increase the current. The predominate factor is the palm resistance.
If the enclosure and his palm were dry, then
R = 3000 + 200 + 100 = 3,300.
I = V/R = 270/3300 = 82 mA.
This is near the lower limit for ventricular fibrillation.
1. Geddes, Leslie Alexander. “Electrical Properties of Living Tissues.” In Handbook of Electrical Hazards and Accidents. Boca Raton: CRC, 1995
Electrical Accidents Copyright 2016 - Dr. Ray Franco, PhD., PE