electrical safety thomas g. cleaver university of louisville department of electrical and computer...

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