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Electrical Safety
Understanding 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
9
Spasm
Indirect Injury
Painful, Muscle Clamps
(Can’t Let Go)
76
16
Possibly Fatal
Severe Pain
Respiratory 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 (W)
Wet (W)
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 (fingerthumb 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 Safety
Based 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
•
•
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•
Commercial
Industrial
Medical
Military / Aerospace
A Remarkable Story…
•
•
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•
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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)