Presentation on theme: "Electrical Safety Thomas G. Cleaver University of Louisville Department of Electrical and Computer Engineering July 11, 2011 2011 Thomas G. Cleaver. All."— Presentation transcript:
Electrical Safety Thomas G. Cleaver University of Louisville Department of Electrical and Computer Engineering July 11, Thomas G. Cleaver. All rights reserved.
Are these birds safe? Image found at HTTP: Current requires an entrance point and an exit point.
Current Flow in the Body
I Sing the Body Electric Walt Whitman, Leaves of Grass, 1900 Muscles and nerves are electro-chemical tissue. This includes skeletal muscle, the heart, the spinal cord, and the brain. Externally applied electricity can cause muscles to contract and nerves to conduct.
Effect of 60 Hz Current on the Body CurrentEffect 20 APermanent brain damage 5 ARespiratory arrest 2 ACentral nervous system damage 1 ABurns 80 mAVentricular fibrillation 50 mAAsphyxia 9 mAMuscles frozen 1 mAPain 0.2 mAThreshold of perception 0No effect
Electrical Resistance of the Body Internal resistance of the body between any two points is about 500. Resistance of 1 cm 2 of skin is about 200 k. The skin is the first line of defense.
Effects of Voltage on the Body Voltage less than 40 V is probably safe. Voltage greater than 240 V causes skin puncture.
Effects of Frequency on the Body DC is safer than 60 Hz AC. 60 Hz is quite dangerous to the heart, as it provides 120 chances/second for the heart muscle to depolarize. If frequency is greater than 1000 Hz, the skin acts like a capacitor. High frequencies (approximately 500 kHz) are used for electrosurgery.
Lightning Lightning accounts for about 1000 deaths per year in the US. Lightning tends to strike the tallest object in the vicinity. Dont stand under the only tree. Lightning rods do not attract lightning.
Three-Phase Power Distribution Drawing by Dr. Donald J. Scheer
Distribution Transformer Drawing by Dr. Donald J. Scheer
Types of Wiring Conduit – Thin walled pipe through which conductors are pulled. Good mechanical protection. BX – Flexible spiral-wound metal cable. Contains black (hot), white (return) and green or bare (ground) conductors. Plastic-coated cable (NM or Romex ) – Provides minimal mechanical protection. May be used in wet environments.
Outlet Box Connection Details
Various Wall Sockets
Procedure for Replacing a Receptacle Turn off power. Test that power is off. Remove the face plate and the old receptacle. Install the new receptacle as follows: –Connect the black wire to the brass screw. –Connect the white wire to chrome screw. –Connect the green or bare wire to the green screw. Screw the new receptacle into the box and replace the face plate Check ground/return continuity. Turn power on. Check the voltage.
Properly Grounded Appliance
3-Prong to 2-Prong Adapter
Ground Fault Circuit Interrupter
Safety Hints When changing fuses (or working on any circuit you suspect may be "hot") keep one hand in your pocket. When working on circuits, remove jewelry, including rings, necklaces, bracelets, and watches with metal watch bands. Use the "buddy system." Have somebody stand by the breaker box to make sure nobody turns it back on while you are working. Never use a drill (or other power tool) with the third prong removed. Especially when working outdoors, make sure your tools are properly grounded. Do not overload circuits. Beware of appliances with metal cabinets if you have wet hands or feet. Don't put extension cords under rugs.