Electrical Accidents

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.

The National Electrical Safety Code (NESC), or IEEE C2, is overseen by the Institute of Electrical and Electronic Engineers (IEEE). It governs the utility power industry. It safeguards persons against electrical hazards during the installation, operations and maintenance of electric supply and communications lines.
The National Electric Code (NEC), or NFPA 70 is overseen by the National Fire Protection Association (NFPA). It governs the safe installation of electrical wiring and equipment in industrial and residential locations.
The Standard for Electrical Safety in the Workplace, or NFPA 70E, covers employee protection from electrical hazards including shock, arc blasts, explosions initiated by electricity and outside conductors. It is the “how” to comply with OSHA’s Code of Federal Regulations (CFR) 20 Part 1910 Subpart S and Part 1926 Subpart K.
The Standard for Industrial Machinery, or NFPA 79, provides safeguards or industrial machinery to protect operators and equipment form electrical hazards and fires.

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.

Physiological Effects of Electricity

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:

$\begin{equation*} \text{I} = \frac{\text{V}}{\text{R}} = \frac{8000}{500} = 16 \text{ Amps.} \end{equation*}$

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.

Human Body Resistance

For voltages below 600 volts, the skin resistance is significant. Table 1. gives the human resistance for various parts of the body.

Table 1. Body Resistances

	Dry Resistance	Wet Resistance
Finger Touch	40K-1M	4-15K
Hand Holding Wire	10-50K	3-6K
Finger-Thumb Grasp	10-30K	2-5K
Hand Holding Pliers	5-10K	1-3K
Palm Touch	3-8K	1-2K
Hand Around 1.5" Pipe or Drill Handle	1-3K	0.5-1.5K
Two Hands Around 1.5" Pipe	0.5-1.5K	250-750
Hand Immersed		200-500
Foot Immersed		100-300
Human Body Internal, Excluding Skin		200-1000

Table 2. gives the resistance values for various materials.

Table 2. Resistances of Various Materials

Material	Resistance
Rubber Gloves or Soles	> 20 M
Dry Concrete above Grade	1-5 M
Dry Concrete on Grade	0.2-1 M
Dry Leather Soles including Foot	0.1-0.5 M
Damp Leather Soles including Foot	5-20 K
Wet Concrete on Grade	1-5 K

The human response to electricity is director proportional to the electric current. Table 3. gives these responses.

Table 3 – Human Responses to Electric Current

Current (60 Hz)	Physiological Phenomena	Feeling or Lethal Incidence
< 1 mA	None	Imperceptible
1 mA 1-3 mA 3-10 mA	Perception Threshold	Mild Sensation Painful 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 Frequently Fatal.
75 mA	Fibrillation Threshold 0.5%	Heart Action Discoordinated Probably Fatal
250 mA	Fibrillation Threshold 99.5% > 5s
4 A	Heart Paralysis Threshold No Fibrillation	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