electrical safety understanding and protecting yourself from electrical shock humbly presented by...

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)