APPENDIX A ELECTRICAL SAFETYELEC 103 ELECTRICAL SAFETY Page A -  1 ELECTRICAL SAFETY As an engineer, you will be required to work around electrical and electronic equipment in which dangerously high voltages may be present. Some work may be done in confined places and now is a good time to develop safe working procedures. Among the hazards that may be encountered in this type of work are electric shock, electrical fires, harmful gases that are sometimes generated by faulty electrical and electronic devices, and injuries caused by the improper use of tools. Because of these dangers, one should formulate safe and intelligent work habits since these are as important as the knowledge of electronics. One primary objective should be to recognize and correct dangerous conditions and unsafe acts. Electricity is extremely useful but may also be extremely dangerous, and should be treated with great respect and care. ELECTRIC SHOCK Electric shock may cause burns of varying degree, cessation of breathing and unconsciousness, ventricular fibrillation, or cardiac arrest. The following summarizes the approximate range of currents that produce this effect in a 70 kg man for a one to three second exposure to 60 Hz current applied across the hands, and will also apply anytime that current level is applied across the thorax. THRESHOLD OF PERCEPTION The threshold of perception is the minimum current that an individual can detect. When the local current density is large enough to excite nerve endings in the skin, the subject feels a tingling sensation. This threshold varies considerably among individuals and according to the measurement conditions. When someone with moistened hands grasps small copper wires, the lowest thresholds are about 0.5 mA for 60 Hz, and 2 to 10 mA for dc currents. Both levels will be accompanied by a slight warming of the skin. LET−GO CURRENT For higher levels of current, the nerves and muscles are vigorously stimulated eventually resulting in pain and fatigue. Involuntary contractions of muscles or reflex withdrawals by a subject experiencing any current above threshold may cause secondary physical injuries such as falling off a ladder. As the current increases further, the involuntary contractions of the muscles can prevent the subject from voluntarily withdrawing. The let−go is defined as the maximum current from which the subject can withdraw voluntarily. For men, the 50 percentile for let−go current is 9.5 mA. RESPIRATORY PARALYSIS Still higher currents cause involuntary contraction of respiratory muscles severe enough to cause asphyxiation if the current is not interrupted. During let−go experiments, it was observed that respiratory arrest occurs at 18 to 22 mA. Strong involuntary contractions of the muscles and stimulation of the nerves can be painful and cause fatigue if there is long exposure. VENTRICULAR FIBRILLATION The heart is susceptible to electric current in a special way that is particularly dangerous. Part of the current passing through the chest flows through the heart. If the magnitude of the current is sufficient to excite only part of the heart muscle, then the normal propagation of electrical activity in the heart muscle is disrupted. Once activity in the ventricles is desynchronized, the pumping action of the heart ceases, and death occurs in minutes.ELEC 103 ELECTRICAL SAFETY Page A -  2 This desynchronzation of the cardiac muscle is called fibrillation. Unfortunately, it does not stop when the current that triggered it is removed. Ventricular Fibrillation is the major cause of death due to electric shock. The threshold for ventricular fibrillation for the average sized man varies from 75 to 400 mA. Normal rhythmic activity will return only if a brief, high current pulse from a defibrillator is applied to simultaneously depolarize all the cells of the heart muscle. After the cells relax together, a normal rhythm usually returns. SUSTAINED MYOCARDIAL CONTRACTION When the current is high enough, the entire heart muscle contracts. Although the heart stops beating while the current is applied, the normal rhythm resumes when the current is interrupted, just as in defibrillation. Data from ac fibrillation experiments on animals show that minimum currents for complete myocardial contraction are in the range from 1 to 6A. No irreversible damage to the heart is known to result from short exposure to these currents. BURNS AND PHYSICAL INJURY Very little is known of the effects of currents in excess of 10A, particularly for currents of short duration. Resistive heating causes burns, usually on the skin at the entry points, because the skin resistance is high. Voltages greater than 240V can puncture the skin. The brain and other nervous tissue lose all functional excitability when high currents are passed through them. Also, excessive currents may force muscular contractions that are strong enough to pull the muscle attachment away from the bone. The above effects are for a 70kg man and for 60 Hz current applied for 1 to 3 seconds to moistened hands grasping a No. 8 copper wire. All values vary widely with body weight and frequency of the current. It is important to remember that current rather than voltage is the criterion of shock intensity. It should be clearly understood that resistance of the body varies greatly. If the skin is dry and unbroken, the body resistance will be quite high, on the order of 150kΩ to 500kΩ. If the skin is moist or broken, the body resistance may drop to as low as 300Ω. Under these conditions, a voltage of 30V may be fatal.ELEC 103 ELECTRICAL SAFETY Page A -  3 WORKING ON ENERGIZED CIRCUITS 1. Never hurry. Haste may cause accident. 2. Food, beverages, and smoking are prohibited. Smoking is not allowed in any building. Brookdale is a smoke free environment, 3. Make sure that you have adequate illumination. 4. Where practical, use only one hand. 5. If it appears that someone is being shocked, turn off power immediately. Pull the plug on the power cord or do what is necessary to remove power. Do not touch the person until you are sure the power is off. 6. Do not wear loose or flapping clothing. If possible, wear shoes with rubber or nonconducting soles. 7. Do not work on a circuit with wet clothes or hands. 8. All rings, watches, necklaces, and bracelets should be removed before working on equipment. Do not wear clothing that contains exposed zippers or metal buttons. Cutoff shirts