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