by : Lawrence Kamm, Expert Witness
Every year approximately 400 people in the U.S. are killed by electric shock. More are injured by being startled by a shock, losing their balance, and falling off a ladder.
(Peopleare injured by electric shock, electrocution, electric flash burn, and electrical fires.)
The mechanism of a lethal shock is ventricular fibrillation, heart muscle fibers thrown out of synchronism so that they stop cooperating to pump blood. (The fibers are normally triggered by very small electric currents from the nervous system. Pacemakers provide such currents when the normal system becomes unreliable.) Fortunately the heart is sensitive to shock damage during only about 1/7 of its normal cycle so many shocks do not kill. The author once picked up 440/60 because he forgot to open a switch but was only quite upset as a result. Current which does not pass through the heart may damage other organs or only cause a burn.
Treatment for fibrillation is by a second electric shock by a defibrillator, but unless it is administered within a minute or two there will be brain damage and then death.
It is current, not voltage, which injures. High electrostatic voltage may make your hair stand on end, but if there is no low resistance circuit there is negligible current. Lethal current is usually between 100 and 150 milliamperes and always above 60 milliamperes. An average value of body resistance is approximately 1000 ohms but it varies widely. Most of the body's resistance is in the skin; your interior resistance is that of your blood system which has the resistance of sea water. There are horny handed electricians who test for the presence of voltage by touching the wires. If you are not positive that a conductor is dead, touch it with the back of your hand. If you get a shock your galvanic response will be to pull away; otherwise it might be to grasp the conductor tighter.
Circuits through the body may be closed by other means than metal conductors. All electric power circuits are grounded somewhere - for good reasons - so a shock circuit is often completed through a ground connection. A man was once electrocuted via a stream of urine reaching a grounded metal plate. Many people have been electrocuted via the water in their baths. Water pipes and faucets, structural metal, and electrical conduit are all paths to ground. Metal enclosures of electronic equipment are grounded to prevent a shock from the enclosure to ground. That third contact on an electric plug is for a ground path to a metal case. The wide blade on and electric plug is for the grounded side of the voltage supply.
The rules for safe wiring are given in the National Electrical Code (NEC), but it is not retroactive so you should not assume that it protects you. Even in new constructions, electricians make mistakes.
A remarkable safety device, now specified by the NEC and now installed by the millions, is the Ground Fault Interrupter, GFCI. It is a circuit breaker sensitive to current to ground, such as shock current, independent of current to load. It will trip open within 1/10 second if the ground current exceeds 6 milliamperes.
Every year approximately 400 people in the U.S. are killed by electric shock. More are injured by being startled by a shock, losing their balance, and falling off a ladder.
(Peopleare injured by electric shock, electrocution, electric flash burn, and electrical fires.)
The mechanism of a lethal shock is ventricular fibrillation, heart muscle fibers thrown out of synchronism so that they stop cooperating to pump blood. (The fibers are normally triggered by very small electric currents from the nervous system. Pacemakers provide such currents when the normal system becomes unreliable.) Fortunately the heart is sensitive to shock damage during only about 1/7 of its normal cycle so many shocks do not kill. The author once picked up 440/60 because he forgot to open a switch but was only quite upset as a result. Current which does not pass through the heart may damage other organs or only cause a burn.
Treatment for fibrillation is by a second electric shock by a defibrillator, but unless it is administered within a minute or two there will be brain damage and then death.
It is current, not voltage, which injures. High electrostatic voltage may make your hair stand on end, but if there is no low resistance circuit there is negligible current. Lethal current is usually between 100 and 150 milliamperes and always above 60 milliamperes. An average value of body resistance is approximately 1000 ohms but it varies widely. Most of the body's resistance is in the skin; your interior resistance is that of your blood system which has the resistance of sea water. There are horny handed electricians who test for the presence of voltage by touching the wires. If you are not positive that a conductor is dead, touch it with the back of your hand. If you get a shock your galvanic response will be to pull away; otherwise it might be to grasp the conductor tighter.
Circuits through the body may be closed by other means than metal conductors. All electric power circuits are grounded somewhere - for good reasons - so a shock circuit is often completed through a ground connection. A man was once electrocuted via a stream of urine reaching a grounded metal plate. Many people have been electrocuted via the water in their baths. Water pipes and faucets, structural metal, and electrical conduit are all paths to ground. Metal enclosures of electronic equipment are grounded to prevent a shock from the enclosure to ground. That third contact on an electric plug is for a ground path to a metal case. The wide blade on and electric plug is for the grounded side of the voltage supply.
The rules for safe wiring are given in the National Electrical Code (NEC), but it is not retroactive so you should not assume that it protects you. Even in new constructions, electricians make mistakes.
A remarkable safety device, now specified by the NEC and now installed by the millions, is the Ground Fault Interrupter, GFCI. It is a circuit breaker sensitive to current to ground, such as shock current, independent of current to load. It will trip open within 1/10 second if the ground current exceeds 6 milliamperes.
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