Electricity.pdf

Three factors determine whether or not an electrical shock is fatal: How much current (amperage) passes through your body. How long (duration) the current passes through your body. Whether the current passes through your heart. A milliampere (mA) is 1/1000 of an ampere. About 100 mA to 2...

Three factors determine whether or not an electrical shock is fatal: How much current (amperage) passes through your body. How long (duration) the current passes through your body. Whether the current passes through your heart. A milliampere (mA) is 1/1000 of an ampere. About 100 mA to 200 mA of current will kill a person. This is about 1/10 of the current required to operate a 100 W bulb. Safe Current Values 1 mA — Causes no sensation, nothing felt. 1 mA to 8 mA — Shock sensation, but not painful. No loss of muscle control. Unsafe Current Values 8 mA to 15 mA — Painful shock. No loss of muscle control. 15 mA to 20 mA — Painful shock. Hand muscles contract. Cannot let go. 20 mA to 75 mA — Painful shock. Severe muscle contractions. Breathing is extremely difficult. 100 mA to 200 mA — Painful shock and ventricular fibrillation of the heart. This is like a pacemaker telling the heart to beat 3600 times per minute! It is a fatal heart condition for which there is no known remedy or resuscitation. It usually means death because sophisticated equipment is required to quickly restore normal heartbeat. Over 200 mA — Severe burns. Muscular contractions so severe that chest muscles clamp the heart and stop it for the duration of the shock, preventing ventricular fibrillation. Artificial respiration should be administered immediately. In most cases, the victim can be revived if removed from the circuit quickly. An ohmmeter is an instrument for accurately checking resistance. Ohmmeters are available in analog or digital. An ohmmeter can be used to check continuity. Continuity is a term that describes a complete circuit (path) for electron movement from one point to another. An ohmmeter measurement is taken and low to no resistance is checked. A continuity check can be used to prove that wires are not broken, fuses are not blown, or switches are not open in an electrical circuit (path of flow). See Figure 23-18. When checking continuity, the adjustable-type ohmmeter is set to the lowest resistance scale (R × 1). Do not use the meter multipliers (R × 10, R × 100). Electrical circuits normally contain minimal resistance to electron flow. Very low resistance cannot be determined unless the meter is calibrated to multiply the reading. However, most digital type (LED) ohmmeters automatically multiply until a small decimal (fractional) resistance reading is obtained. Electrons cannot flow in an open circuit (a circuit that is not complete or that contains a break in the path). The ohmmeter indicates an open circuit by registering unlimited resistance, because resistance is too high for the meter to measure it. Unlimited resistance is indicated by the symbol for infinity (∞). An analog meter shows unlimited resistance when the needle goes all the way over to the peg (called pegged out). A digital meter shows unlimited resistance by displaying the letters OL, or “overloaded,” because the meter cannot read such a large number (infinity). An open circuit indicates an open switch, blown fuse, broken wire, or disconnection, among other possible failures. A failure disconnects the circuit, preventing electrons from flowing across the opening. A drawbridge is a good analogy. When the bridge opens (infinite resistance), traffic (electron flow) stops. The electrons must wait until the drawbridge recloses (zero resistance) before continuing their flow. Voltage drop is a term that describes the loss of emf (voltage) between each of the two wires of the electrical source or across the electrical load. Voltage drop can be caused by an electrical load, a loose connection, or an undersized conductor. All conductors have some resistance to the flow of electrons. The type, size, and length of wire selected for a conductor is used to minimize the voltage drop through the length of the conductor. If the wires are too small, voltage drop within a conductor will cause the conductor to get hot and prevent the load from receiving its designed operating voltage. Any voltage lost between the power supply and the electrical load is voltage unavailable to the equipment. All loads contain a specific resistance and are designed to operate within a specific voltage range. Most loads are designed to operate prop- erly within ±5% of the rated voltage. Voltage drop should occur across the resistance of the load. Excess voltage drop is easily detected by an accu- rate multimeter.