Transcript Developing Team Skills - University of Louisville

Electrical Safety
Thomas G. Cleaver
University of Louisville
Department 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
5A
Respiratory arrest
2A
Central nervous system damage
1A
Burns
80 mA
Ventricular fibrillation
50 mA
Asphyxia
9 mA
Muscles frozen
1 mA
Pain
0.2 mA
0
Threshold of perception
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 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
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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