electrical safety understanding and protecting yourself from electrical shock humbly presented by...
TRANSCRIPT
Electrical SafetyUnderstanding and Protecting Yourself from
Electrical Shock
Humbly Presented
By
Prof. Bitar
Factors Involved in Electrical Shock
• The Amount of Current (amps)
• The Current Path
• The Duration
Q. So what about the Voltage?
What Does the Amount of Current Depend On?
• The amount of current that flows depends on the voltage applied as well as the body resistance. (ie. Ohm’s Law: I = V / R )
Dangerous Current Levels( Based on a Body Weight of 150 Lbs. )
Effect / Feeling Current (mA) DC AC(60Hz)
Incident Severity
Slight Sensation 1 0.4 None
Perception Threshold 5.2 1.1 None
Shock Not Painful 9 1.8 None
Shock Painful 62 9Spasm
Indirect Injury
Painful, Muscle Clamps(Can’t Let Go) 76 16 Possibly Fatal
Severe PainRespiratory Arrest
(Can’t Breathe)170 30 Frequently Fatal
Heart Fibrillation(after 3 seconds) 500 100 Probably Fatal
Source: Environment, Safety, and Health Manual, Volume II, Part 16.1, U.S. Dept. of Energy at University of California Lawrence Livermore National Laboratory, Contract W-7405-ENG-48, Feb 1996.
Body Resistance (Ohms)Contact Points Dry ( Wet (
Wire Touched by Finger 40,000 – 1,000,000 4,000 – 15,000
Hand Holding Wire 15,000 – 50,000 3,000 – 5,000
Finger Thumb Grasp 10,000 – 30,000 2,000 – 5,000
Hand Holding Pliers 5,000 – 10,000 1,000 – 3,000
Palm Touch 3,000 – 8,000 1,000 – 2,000
Hand Around Pipe 1,000 – 3,000 500 – 1,500
Two Hands Around Pipe 500 – 1,500 250 - 750
Source: Environment, Safety, and Health Manual, Volume II, Part 16.1, U.S. Dept. of Energy at University of California Lawrence Livermore National Laboratory, Contract W-7405-ENG-48, Feb 1996.
Scenario #1
• An electrician with sweaty hands picks up a live 120VAC 60Hz circuit (finger-thumb grasp)…
Q1: How much current will flow?
Q2: Is this amount of current fatal?
Scenario #2
• A lab power supply is set to +/- 30V DC with wires all over the bench. A coke is accidentally spilled, and a student wipes up the mess with paper towels…
Q1: How much current will flow?Q2: Is this amount of current fatal?
Where is the Danger?
• How many volts are involved in a typical electrostatic shock? Isn’t this dangerous?
• If I grab the terminals of a 12V car battery, will I be electrocuted? What is the danger?
• Which is more dangerous, DC or AC?
Safe Working Habits
• One Hand...• Never Alone…• Safety Glasses…• Shut Power Off…• Unplug Equipment…• Remove Obstacles…• Be Neat…• Discharge Capacitors…
Four Phases of Electrical SafetyBased on Electrical Safety Training at Massachusetts Electric Co.
• Unconsciously Unsafe
• Consciously Unsafe
• Consciously Safe
• Unconsciously Safe
What About Protecting Components and Equipment?
1. Do Not Exceed…• Voltage Ratings (Volts)• Current Ratings (Amps)• Power Ratings (Watts)
2. Use a Reasonable Safety Margin• Commercial• Industrial• Medical• Military / Aerospace
A Remarkable Story…
• 20,000 Volts 60Hz AC• Estimated Current (50-100mA)• Grabbing with Both Hands• A Critical Path• Longer than 3 Seconds
“It is good to learn from one’s mistakes… but it is better to learn from the mistakes of others.”
“There, but for the grace of God go I !”George Whitefield (1714-1770)