Electric shock precautions.
There are not many safety hazards in the Pchemlab, but every experiment involves electrical and electronic apparatus. Sometimes during the course of an experiment, students need to fiddle with the apparatus, which brings up the remote possibility of electric shock hazard. Nobody has yet been injured by a shock in our lab, but since electrical and electronic devices are a ubiquitous part of everyday life, a brief summary of electric shock precautions is given below.
Experience shows that more than 98% of all difficulties experienced with balky Pchemlab apparatus involve two problems: (1) equipment which isn't plugged in properly or which isn't turned on; (2) pens which don't write.
Offhand, it would seem that a shock of 10,000 volts would be more deadly than 100 volts. That is not necessarily so! Individuals have been electrocuted by appliances using ordinary house supplies of 110 volts and by electrical apparatus in industry using as little as 42 volts direct current. The real measure of a shock's intensity lies in the amount of current (amperes) forced through the body, and not the voltage. Any electrical device used on a house wiring circuit can, under certain conditions, transmit a fatal current.
It's the electrical current that does the damage. Current equals voltage divided by resistance (I = V/R), but the resistance of the human body varies so widely it is impossible to state that one voltager is "dangerous" and another is "safe".
The actual resistance of the body varies depending upon the condition of the skin at the points of contact (moist or dry). The skin resistance may vary from 1000 ohms for wet skin to over 500,000 ohms for dry skin. However, once the skin is broken through (for example by the burning away of skin) the body presents no more than 500 ohms resistance to the current.
The path through the body has much to do with the shock danger. A current passing from finger to elbow through the arm may produce only a painful shock, but that same current passing from hand to hand or from hand to foot may well be fatal.
Therefore, the practice of using only one hand (keeping one hand behind your back) while working on high-voltage circuits and of standing or sitting on an insulating material is a good safety habit.
The Physiological Effect of Electric Shock
Electric current damages the body in three different ways: (1) it harms or interferes with proper functioning of the nervous system and heart; (2) it subjects the body to intense heat; and (3) it causes the muscles to contract.
(1) Chart 1 shows the physiological effect of various currents. Note that voltage is not a consideration. Although it takes a voltage to make the current flow, the amount of shock-current will vary, depending on the body resistance between the points of contact.
Figure 1
As shown in Figure 1, shock is relatively more severe as the current rises. At values as low as 20 milliamps, breathing becomes labored, finally ceasing completely even at values below 75 milliamps.
As the current approaches 100 milliamps, ventricular fibrillation of the heart occurs-an uncoordinated twitching of the walls of the heart's ventricles.
Above 200 milliamps, the muscular contractions are so severe that the heart is forcibly clamped during the shock. This clamping protects the heart from going into ventricular fibrillation, and the victim's chances for survival are good.
(2) AC is said to be four to five times more dangerous than DC. For one thing, AC causes more severe muscular contractions. For another, it stimulates sweating that lowers the skin resistance. Along that line, it is important to note that resistance goes down rapidly with continued contact. The sweating and the burning away of the skin oils and even the skin itself account for this. That is why it's extremely important to free the victim from contact with the current as quickly as possible before the climbing current reaches the fibrillation-inducing level.
The frequency of the AC has lots to do with the effect on the human body. Unfortunately, 60 cycles is in the most harmful range. At the house voltage frequency, as little as 25 volts can kill. On the other hand, people have withstood 40,000 volts at a frequency of a million cycles or so without fatal effects.
A very little current can produce a lethal electric shock. Any current over 10 ma. will result in serious shock.
Summary
Voltage is not a reliable indication of danger because the body's resistance varies so widely it is impossible to predict how much current will be made to flow through the body by a given voltage.
The current range of 100- to 200-ma, is particularly dangerous because it is almost certain to result in lethal ventricular fibrillation. Victims of high-voltage shock usually respond better to artificial respiration than do victims of low-voltage shock, probably because the higher voltage and current clamps the heart and hence prevents fibrillation.
AC is more dangerous than DC, and 60-cycle current is more dangerous than high-frequency current.
Skin resistance decreases when the skin is wet or when the skin area in contact with a voltage source increases. It also decreases rapidly with continued exposure to electric current.
Prevention is the best medicine for electric shock. That means having a healthy respect for all voltage, always following safety procedures when working on electrical equipment.
In case a person does suffer a severe shock, it is important to free him from the current as quickly as can be done safely and to apply artificial respiration immediately. The difference of a few seconds in starting this may spell life or death to the victim. And keep up the artificial respiration until a physician pronounces the victim dead!
Mouth-to-mouth method of resuscitation. Safety experts prefer the use of mouth-to-mouth resuscitation because it moves the largest volume of air into and out of the victim's lungs. The steps of application are described below:
1. The victim should be laid on his back with his head turned to one side and his mouth cleared of any saliva or foreign objects (e.g. dental plates) by insertion of the operator's thumb. Waste no time!
2. His head is then placed as far back as possible so that his neck is extended, chin toward ceiling. Mouth should drop open.
3. The operator uses one hand to support the victim's neck to keep head in proper position and then closes the victim's nose with his other hand (Figure 2).
Figure 2
4. After taking a deep breath, the operator places his mouth completely over the victim's mouth with airtight contact. The victim's mouth should not be held open too wide as it must be totally covered by the operator's lips.
5. The operator then breathes or blows into the victim's mouth, forcefully for adults and gently for children. The victim's chest should be watched, and as soon as it rises, the blowing should be stopped and the operator's mouth quickly removed from the mouth of the victim, allowing him to exhale passively.
6. The jaw must be held in an elevated position on both the inspiration and expiration phases, preferably by supporting victim's neck.
7. If the chest does not rise, the position of the head and jaw should be improved and the blowing done more forcefully. If the victim's lungs are still not ventilated, his airway may be obstructed. Try the Heimlich maneuver described on the wall card.
8. The cycle of inflation and exhalation should be repeated 12 times per minute for adults.
9. If the victim's stomach swells during resuscitation, air may be entering it. This may be corrected by the operator gently pressing on the victim's stomach during exhalation.
Link to National Institute of Health's National Library of Medicine
