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Electrical SafetyThomas G. Cleaver
University of LouisvilleDepartment of Electrical and Computer Engineering
July 11, 2011
2011 Thomas G. Cleaver. All rights reserved.
Are these birds safe?Image found at HTTP: http://davin.ws/~krishen/blog/pictures/birdonawire.jpg
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
Current Effect
20 A Permanent brain damage
5 A Respiratory arrest
2 A Central nervous system damage
1 A Burns
80 mA Ventricular fibrillation
50 mA Asphyxia
9 mA Muscles frozen
1 mA Pain
0.2 mA Threshold of perception
0 No effect
Electrical Resistance of the Body
• Internal resistance of the body between any two points is about 500 Ω.
• Resistance of 1 cm2 of skin is about 200 kΩ.
• 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.
• Don’t stand under the only tree.
• Lightning rods do not attract lightning.
Lightning Arresters
Three-Phase Power DistributionDrawing by Dr. Donald J. Scheer
Distribution TransformerDrawing 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
Ungrounded Appliance
3-Prong to 2-Prong Adapter
Ground Fault Circuit Interrupter
Isolation Transformer
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.
Darwin Award Candidates