Modern electric fences, used in animal enclosures, are now pulsed on and off to allow people who touch them to get free, rendering them less lethal than in the past. Greater currents may affect the heart. The threshold for ventricular fibrillation is between and mA. At about mA and above, the shock can cause burns, depending on the concentration of current—the more concentrated, the greater the likelihood of burns.
Very large currents cause the heart and diaphragm to contract for the duration of the shock. Both the heart and breathing stop. Interestingly, both often return to normal following the shock. The electrical patterns on the heart are completely erased in a manner that the heart can start afresh with normal beating, as opposed to the permanent disruption caused by smaller currents that can put the heart into ventricular fibrillation.
The latter is something like scribbling on a blackboard, whereas the former completely erases it. TV dramatizations of electric shock used to bring a heart attack victim out of ventricular fibrillation also show large paddles.
These are used to spread out current passed through the victim to reduce the likelihood of burns. Current is the major factor determining shock severity given that other conditions such as path, duration, and frequency are fixed, such as in the table and preceding discussion.
The same person soaking wet may have a resistance of When wet, salts go into ion form, lowering the resistance significantly. The interior of the body has a much lower resistance than dry skin because of all the ionic solutions and fluids it contains. If skin resistance is bypassed, such as by an intravenous infusion, a catheter, or exposed pacemaker leads, a person is rendered microshock sensitive.
Stringent electrical safety requirements in hospitals, particularly in surgery and intensive care, are related to the doubly disadvantaged microshock-sensitive patient. The break in the skin has reduced his resistance, and so the same voltage causes a greater current, and a much smaller current has a greater effect. Figure 4. The lower the value, the more sensitive the body is at that frequency.
Factors other than current that affect the severity of a shock are its path, duration, and AC frequency. Path has obvious consequences. For example, the heart is unaffected by an electric shock through the brain, such as may be used to treat manic depression. And it is a general truth that the longer the duration of a shock, the greater its effects.
Figure 4 presents a graph that illustrates the effects of frequency on a shock. The curves show the minimum current for two different effects, as a function of frequency. The lower the current needed, the more sensitive the body is at that frequency. Ironically, the body is most sensitive to frequencies near the or Hz frequencies in common use.
At higher and higher frequencies, the body becomes progressively less sensitive to any effects that involve nerves. This is related to the maximum rates at which nerves can fire or be stimulated. At very high frequencies, electrical current travels only on the surface of a person.
Thus a wart can be burned off with very high frequency current without causing the heart to stop. Do not try this at home with Hz AC! See Figure 5.
Electrical safety devices and techniques are discussed in detail in Electrical Safety: Systems and Devices. Figure 5 Is this electric arc dangerous? This is well above the perception level of 1 milliamp, and slightly below the 15 milliamp "let go" threshold. We feel it, but we can let go and have no lasting physical damage. If we are wet or standing in water, we become a much better conductor, thereby offering less resistance.
The current flow is again found by dividing the voltage, , by the lowered resistance of 1, ohms, which yields 0. This is easily enough current to send the heart into fibrillation and cause electrocution. Depth at which the electrode is embedded d.
Quality of coal dust and charcoal in the earth electrode pit. For a rod type earth electrode made of steel, the diameter value should not be less than? According to CPWD guidelines on electrical work of , the plate-type copper earth electrode should be sized.
In case of small power installations, the number of GI of copper plates required for earthing is:. Suggested Test Series. Suggested Exams. More Estimation and Costing Questions Q1. If I were working in some hot, dirty, industrial environment, the resistance between my hands would likely be much less, presenting less opposition to deadly current, and a greater threat of electrical shock.
But how much current is harmful? The answer to that question also depends on several factors. Individual body chemistry has a significant impact on how electric current affects an individual.
Some people are highly sensitive to current, experiencing involuntary muscle contraction with shocks from static electricity. Others can draw large sparks from discharging static electricity and hardly feel it, much less experience a muscle spasm. Despite these differences, approximate guidelines have been developed through tests which indicate very little current being necessary to manifest harmful effects again, see the end of the chapter for information on the source of this data.
Keep in mind that these figures are only approximate, as individuals with different body chemistry may react differently. It has been suggested that an across-the-chest current of only 17 milliamps AC is enough to induce fibrillation in a human subject under certain conditions. Most of our data regarding induced fibrillation comes from animal testing. Obviously, it is not practical to perform tests of induced ventricular fibrillation on human subjects, so the available data is sketchy.
Suppose I were to place my two hands across the terminals of an AC voltage source at 60 Hz 60 cycles, or alternations back-and-forth, per second. How much voltage would be necessary in this clean, dry state of skin condition to produce a current of 20 milliamps enough to cause me to become unable to let go of the voltage source? Far less would be required to cause a painful shock! Also keep in mind that the physiological effects of any particular amount of current can vary significantly from person to person, and that these calculations are rough estimates only.
Bear in mind this is only with one finger of each hand contacting a thin metal wire. Recalculating the voltage required to cause a current of 20 milliamps, we obtain this figure:. In this realistic condition, it would only take volts of potential from one of my hands to the other to cause 20 milliamps of current.
0コメント