Current Myths of Electricity: Part 1

Aug. 13, 2012

Electricity does kill people. Approximately 1,000 deaths per year in the United States are the result of electrical injuries. According to the National Institute of Occupational Safety and Health, occupational electrical hazards cause more than 300 deaths and 4,000 injuries each year. Statistics for home deaths and injuries are more difficult to acquire. Most sources for this type of information estimate 1,000 deaths, 12,000 shocks and burns, and 150,000 fires. Those are just the cases reported.

Probably almost everyone you know has felt the tingle or done the “110-volt Tango.” If you have done the tango or felt the tingle and you’re reading this article, the only reason you’re still alive is luck.

The following misunderstandings often lead us into trouble, says Bob LoMastro, president of LoMastro & Associates Inc. LoMastro draws upon a career spanning more than 40 years of teaching on safety- and health-related topics. He has developed a wide variety of electrical safety programs for OSHA and the National Fire Protection Association.

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Myth No. 1. It takes a lot of voltage to kill. The truth is you only need about 35 V to force current through human skin. Current and heat cause the damage.

Three things determine the outcome of contact with electricity:

  • The amount of current (the movement of electrons from one atom to the next) flowing through you
  • The path it finds through your body-When current flows from hand to hand, across the chest, the heart and lungs can be destroyed. One of my favorite experiments with students is running 2 milliamperes (mA) between their index and middle fingers. (This is a stringently controlled exercise; don’t try it at home.)
  • The amount of time the current flows-The longer the exposure, the more severe is the tissue damage. Once the voltage allows penetration, the current causes damage to the body. It takes very little current to cause injuries.

The average person can feel between 1 and 2 mA of alternating current (AC). The sensation level for direct current is about 5 mA. At somewhere between 3 to 8 mA, we cry out in pain. When current levels exceed 9 mA, our muscles go into contraction and often “lock” us to the tool or wire. Current at about 20 mA crossing the torso will cause the respiratory system to freeze and can quickly result in asphyxiation. Crank that up to between 60 and 100 mA, and the heart will go into fibrillation. Now at about 100 mA, the heart will freeze, and you die.

Why is this important? An ampere is 1,000 mA. Your typical receptacle or light circuit is 20 amps or 20,000 milliamps. In other words, that cord you’re trying to repair with electrical tape will be plugged into a 20,000 mA source.

But that’s not the whole story. In the typical household, your 20-amp circuit is powered through a 100- or 200-amp main supply panel, which will be powered by a transformer that can deliver up to 25,000 amps (25,000,000 mA).

When you make contact with an energized circuit, you can receive some or all of the power available from the transformer (25 million mA). The 20-amp circuit breaker won’t overheat until the current flow exceeds 20,000 mA. Anything over 100 mA reaching your heart will kill you, any body tissue that receives 300 mA or more will be cooked. That 120-V receptacle you thought you could replace (without shutting it down at the breaker) could kill you and 199 of your friends.

Most deaths and serious injuries result from contact with typical household or office circuits. The safety rules you’ve been taught concerning the use of extension cords-covering live parts, and using proper lockout procedures, for examples-are designed to protect you. Follow the safety rules. Don’t think it can’t happen to you. No one ever came into an emergency room with a story about a planned visit.

Myth No. 2. Electrical grounds will keep me safe. Most people misunderstand the primary purpose of grounds. System grounding and equipment grounding had separate histories and purposes. Edison began grounding the electrical systems in the 1890s. Many argued this was to save money by using the earth as a wire. In truth, the grounding system was implemented to allow stray current to use the earth as a path to complete the electrical circuit back to the generator and overload the newly created “fused links.” These fused links were designed as a means of fire protection for the system and the buildings provided with electrical power.

When the Tesla (alternating current) system was constructed, overhead wiring was used. The system’s grounding allowed lighting strikes to be dissipated into the earth, also reducing the risk of fires and equipment damage. The 1903 electrical code recommended grounding the electrical service in every building. By 1913, grounding certain electrical systems became mandatory. Receptacle grounding wasn’t required until 1947, and it was specific for laundry rooms.

The 1956 code extended that requirement to areas where a person might be in contact with the earth, such as basements, garages, and outdoor receptacles. These early grounding systems limited the time someone would be exposed to stray current, by allowing an increase in current flow through the ground path and melting the fuse. If you are in contact with the grounded materials it does not prevent shock or the occasional electrocution.

So why is this important? The function of receptacle and equipment grounding is primarily to allow a damaged circuit to take a path that will easily overload the fuse or circuit breaker. A tool that only allows 3 amps of current flow will not draw enough current to trip a breaker rated for 20 amps. If the wiring system is damaged, the short could exist for an indefinite period of time and result in fire or injury. So we connect all of the metal parts of our homes and equipment to the grounding electrodes (ground rods and water pipes). If the circuit is damaged and contacts the conductive materials, large amounts of current surge through the path and trip the breakers quickly.

While that has been effective in reducing the numbers of electrical fires, it means that anyone in contact with the metal parts is subject to receiving a shock. The likelihood of serious injury is reduced but not eliminated. That is why we now use the Ground Fault Circuit Interrupter (GFCI) protection in wet, potentially wet and temporary wiring services. The GFCI trips in fractions of a second, before an electrical injury occurs.

Myth No. 3. Power lines are insulated. Contacts with overhead power lines have resulted in thousands of deaths and tens of thousands of injuries since the hanging of electrical wires began in the 1880s. The policy of maintaining at least a 10-foot clearance between people and wires has been advocated for nearly 100 years. Long before OSHA, the power companies tried to educate the public of the need to maintain this minimum clearance.

Here is why. The typical adult can feel about 1 milliamp of current. In order to prevent us from feeling the current flowing through our extension cords and appliance wiring systems, the manufacturer covers the wires with an outer casing of material that will limit the amount of current to less than 1 milliamp. Our systems use 110 and 240 volts (nominal). To prevent us from feeling any current we need 1 ohm of resistance for every volt applied to the wire. An extension cord powered at 110 volts must have a covering providing 110,000 ohms to reduce the current to 1 milliamp.

To coat all overhead power lines with materials that could provide that level of protection would be astronomical in cost. The materials used to cover the millions of miles of overhead wiring are designed to protect the wires from damage-not as insulation to protect us from the current.

If you want proof of this “bleeding” of current from overhead wires, go outside after it rains and listen to the wires humming. That humming is current flowing between the wires because the covering does not have sufficient resistance to contain it. Always maintain the minimum distance of 10 feet between those suspended wires and anything conductive you’re in contact with.

Continue reading with Part 2, here.