When current flows through the human body, the person is subjected to
an electrical shock. Electrical shock can range from a slight tingling
sensation to a feeling of a severe blow. The amount of current flow
through the body and the duration of the current flow affects the body
in different ways. Have you ever wondered why you can touch both posts
of a 12-volt car battery that can produce an electrical current
of several hundred amperes, and not feel any sensation of electrical
shock The answer is resistance.
Dry skin has a resistance level of
100,000 to 600,000 Ω. Recall from Ohm's law that current flow is
calculated by dividing the voltage level by the total resistance. If we
divide the battery voltage (12V) by the lower figure of skin resistance
(100,000 Ω), we find that the current flow is .00012 amperes. Although
there is current flow, it is of such a small value that it is not felt.
'''Figure 2''' below shows typical body resistances and the expected
current flow through various paths through the human body when the
voltage level is 100 volts.
'''Figure 2: Typical Body Resistances and Current Flows'''
CURRENT
EFFECTS
| 1 milliampere or less | No sensation. |
| 1 to 3 milliamperes | Some sensation of shock, but not painful. Individual can let go at will as muscular control is not lost. |
| 3 to 15 milliamperes | Painful shock. Sufficient magnitude to prevent about 3 percent of people from letting go at will. |
| 15 to 30 milliamperes* | Painful shock. Local muscle contractions. Sufficient magnitude to cause freezing to the circuit for about 50 percent of the people. |
| 30 to 75 milliamperes | Painful shock. Severe local muscle contractions. Breathing difficult. Can cause unconsciousness or asphyxiation. |
| 75 to 200 milliamperes |
Possible ventricular fibrillation of the heart.**
200 to 300 milliamperes Certain ventricular fibrillation of the heart.** |
| >300 milliamperes | Severe burns and muscle contractions. The contractions can be so severe that the chest muscles clamp around the heart and stop if for the duration of the shock (this prevents ventricular fibrillation). |
| *There is evidence that skin resistance decreases as the duration of current flow increases (through blistering). If a victim is frozen to a circuit by 15 milliamperes, the decrease in resistance with time might be sufficient to increase the current to 30 milliamperes. | **Ventricular fibrillation of the heart is a condition in which the heart beats rapidly and out of rhythm. It is usually fatal within a few minutes. |
As shown by the chart, the threshold of pain is between 3 and 15
milliamperes. Currents as low as 30 milliamperes are capable of causing
death. General thumb rules for electrical shocks are as follows:
- 1 milliampere - no sensation
- 10 milliamperes - painful sensation of shock
- 50 milliamperes - loss of muscle control
- 100 milliamperes - death if received for more than 1 second
0 التعليقات:
إرسال تعليق