PHYS345 Class 10

Quote of the Day -- Question of the Day

Electrical Safety
This is presented as a public service. Please inform me if you find any errors...

CURRENT KILLS (not voltage)

Current effects on human body, current through chest
(A, amps)
<0.01	tingling or imperceptible
0.02	painful, cannot let go
0.03	breathing disturbed
0.07	breathing very difficult
0.10	death due to fibrillation
>0.20	no fibrillation, but severe burning, no breathing
Also please see Electrical Hazards from the MIT Safety Office

Effects of different current levels through human body

• Current passing through victim is determined by resistance.
• Dry skin resistance is typically ~500,000 ohms.
• Wet skin resistance falls to ~1000 ohms.
• Internal resistance of body is only 100 to 500 ohms.
• Most household electrical injuries occur in the bathroom as a consequence.
• Our skin, when dry, normally protects us from electrocution if we make inadvertent contact with 115V household voltages. Not so when wet...
• GFCIs (Ground Fault Circuit Interrupters) are essential in the kitchen and bathroom
  • How a GFCI works, from Fuses, Circuit Breakers, and GFCI page of Electrical Safety Awareness web-based training at Lawrence Livermore National Lab
  • Recent Success Stories from SMS Generators

Fibrillation and resuscitation

Fibrillation is the fine, rapid, erratic, movements that replace the normal contraction of the vetricular muscle of the heart Fibrillation can be stopped by application of another controlled electrical shock, known as defibrillation, often seen in movies and on TV... A stopped heart can often be resuscitated with CPR techniques (cardiopulmonary resuscitation), but seldom a fibrillating heart.

Dielectric breakdown and sparking

• Air suffers dielectric breakdown at ~3,000,000 volt per meter.
  -- Demonstration of Tesla coil
• 115 V source (wall socket) will not "reach" for you with a spark, but a 50,000 V power line will spark over before you actually contact (touch) it.
• Contacting voltages higher that ~230 V often results in current puncturing skin, compromising the protection offered by its dry resistance.
• Sometime a person contacts a wire that has sufficient current to contract hand muscles onto wire. The resulting current through the body may not be lethal initially, but skin resistance may drop with time, under these conditions, until a lethal current level is achieved. Remove person as soon as possible without endangering self!

graphic from City of Austin Electric Utility

STAY OUT!!! Electrical Safety from the MIT Safety Office Using Electricity Safely from Ohio Edison

Quiz 4: ac Circuit Analysis

"http://www.physics.udel.edu/~watson/phys345/class/10.html"
Last updated Sept. 30, 1999.
Copyright George Watson, Univ. of Delaware, 1999.