Applied Electricity: EEE 102 Lecture 6 PDF

APPLIED ELECTRICITY: EEE 102 PROTECTIVE DEVICES AND ELECTRICAL SAFETY Protective Devices Fuses and circuit breakers are used to deliberately create an open circuit when the current exceeds a specified number of amperes due to a malfunction or other abnormal condition in a circuit. Th...

APPLIED ELECTRICITY: EEE 102 PROTECTIVE DEVICES AND ELECTRICAL SAFETY Protective Devices Fuses and circuit breakers are used to deliberately create an open circuit when the current exceeds a specified number of amperes due to a malfunction or other abnormal condition in a circuit. The fundamental difference between a fuse and a circuit breaker is that when a fuse is “blown,” it must be replaced, but when a circuit breaker opens, it can be reset and reused repeatedly. 2 Modified: June 2024 Protective Devices Cont’d Both devices protect against damage to a circuit due to excess current or prevent a hazardous condition created by the overheating of wires and other components when the current is too great. Several typical fuses and circuit breakers, along with their schematic symbols, are shown in Figure 6.1. 3 Modified: June 2024 Protective Devices Cont’d Two basic categories of fuses in terms of their physical configuration are cartridge type and plug type (screw-in). Cartridge-type fuses have various-shaped housings with leads or other types of contacts, as shown in Figure 6.1(a). A typical plug-type fuse is shown in Figure 6.1(b). 4 Modified: June 2024 Protective Devices Cont’d Figure 6.1: Typical fuses and circuit breakers and their symbols. 5 Modified: June 2024 Protective Devices Cont’d Typical circuit breakers are shown in Figure 6.1(c), and the symbol is shown in part (e). Generally, a circuit breaker detects excess current either by the heating effect of the current or by the magnetic field it creates. In a circuit breaker based on the heating effect, a bimetallic spring opens the contacts when the rated current is exceeded. 6 Modified: June 2024 Protective Devices Cont’d Once opened, the contact is held open mechanically until manually reset. In a circuit breaker based on a magnetic field, the contacts are opened by a sufficient magnetic force created by excess current and must be mechanically reset. 7 Modified: June 2024 Electrical Safety Safety is a primary concern when working with electricity. The possibility of an electric shock or a burn is always present, so caution should always be used. You provide a current path when voltage is applied across two points on your body, and the current produces an electrical shock. 8 Modified: June 2024 Electric Shock Current through your body, not the voltage, is the cause of electrical shock. It takes voltage across a resistance to produce current. When a point on your body encounters a voltage and another point encounters a different voltage or the ground, such as a metal chassis, there will be current from one end to the other. 9 Modified: June 2024 Electric Shock Cont’d The path of the current depends on the points across which the voltage occurs. The severity of the resulting electrical shock depends on the amount of voltage and the path that the current takes through your body. The current path through the body determines which tissues and organs will be affected. 10 Modified: June 2024 Effects of Current on the Human Body The amount of current is dependent on voltage and resistance. The human body has a resistance that depends on many factors, which include body mass, skin moisture, and points of contact of the body with a voltage potential. Table 6.1 shows the effects of various current values in milliamperes. 11 Modified: June 2024 Effects of Current on the Human Body Cont’d Table 6.1: Physical effects of electrical current. Values vary depending on body mass. 12 Modified: June 2024 Body Resistance Resistance of the human body is typically between 10 kΩ and 50 kΩ and depends on the points between which it is measured. The moisture of the skin also affects the resistance between two points. The resistance determines the voltage required to produce each effect listed in Table 6.1. 13 Modified: June 2024 Safety Precautions Avoid contact with any voltage source. Turn the power off before you work on circuits when touching circuit parts is required. Do not work alone. A telephone should be available for emergencies. Do not work when tired or taking medications that make you drowsy. 14 Modified: June 2024 Safety Precautions Cont’d Remove rings, watches, and other metallic jewelry when you work on circuits. Do not work on equipment until you know proper procedures and know potential hazards. Make sure power cords are in good condition, and grounding pins are not missing or bent. 15 Modified: June 2024 Safety Precautions Cont’d Keep your tools adequately maintained. Make sure the insulation on metal tool handles is in good condition. Handle tools properly and maintain a neat work area. Wear safety glasses when appropriate, particularly when soldering and clipping wires. 16 Modified: June 2024 Safety Precautions Cont’d Always shut off power and discharge capacitors before you touch any part of a circuit. Capacitors can be discharged with a unique capacitor discharge tool that releases the charge in a controlled manner. Know the location of the emergency power-off switch and emergency exits. Never try to override or tamper with safety devices like an interlock switch. 17 Modified: June 2024 Safety Precautions Cont’d Always wear shoes and keep them dry. Do not stand on metal or wet floors when working on electrical circuits. Stand on a rubber mat if possible. Never handle instruments when your hands are wet. Never assume that a circuit is off. Double-check it with a known good meter before handling. 18 Modified: June 2024 Safety Precautions Cont’d Set the limiter on electronic power supplies to prevent currents larger than necessary to supply the circuit under test. When making circuit connections, always connect to the point with the highest voltage as your last step. Avoid contact with the power supply terminals. 19 Modified: June 2024 Safety Precautions Cont’d Always use wires with insulation and connectors or clips with insulating shrouds. Keep cables and wires as short as possible. Connect polarized components properly. Be aware of and follow all workplace and laboratory rules. Do not have drinks or food near equipment. 20 Modified: June 2024 Safety Precautions Cont’d If another person cannot let go of an energized conductor, switch the power off immediately. If that is impossible, use nonconductive material to separate the body from the contact. Use a lockout/tagout procedure to avoid someone turning the power on while you are working on a circuit. 21 Modified: June 2024