1. Electrical Safety Lightning Electric Shock Electromagnetic Hazards
Arc Flash Hazards
The National Electrical Code

2. Lightning

3. (5% of all strikes to earth) (95% of all strikes to earth)
Lightning Mechanism
“Positive Lightning”
(5% of all strikes to earth)
“Negative Lightning”
(95% of all strikes to earth)

4. Lightning Information
Just about a perfect independent current source
(injection of current with no associated voltage)
Very few strikes to earth are less than 10kA
Very few strikes to earth are greater than 100kA
Average strike (to earth) intensity is 30kA
(50 kA in Florida)

5. Lightning Information
Rise time: 50 kA in 10 s = 5  109 A/s
Suppose a typical “Florida” lightning stroke hits a power line with a grounding impedance of 5 ohms of resistance and 100 microhenrys of inductance. What will be the voltage rise as the lightning current flows to earth?

7. Isokeraunic Map of United States

8. Isokeraunic Map of World

9. Electric Shock

10. Electric Shock Current, not voltage, poses the risk of electric shock
Current Effect on Human Body
< 1 mA none
1 mA perception threshold
1-10 mA mild to painful sensation
10 mA paralysis threshold –
cannot release hand grip
30 mA respiratory paralysis
mA fibrillation
4-5 A heart stops; organs burn

11. Electromagnetic Hazards

12. Electromagnetic Hazards
High Frequency
RADAR: RAdio Detection And Ranging
The idea of using microwave energy to cook food was accidentally discovered in 1945 by Percy LeBaron Spencer of the Raytheon Company when he found that radar waves had melted a candy bar in his pocket.
Higher frequencies (~1017 Hz) constitute ionizing radiation (gamma, X, and cosmic radiation) which damages DNA and can lead to cancer.

13. Electromagnetic Hazards
Low Frequency
Power Transmission
Voltages as high as 765kV and currents as high as 3000A are used for power transmission. These waveforms are considered extremely low frequency (50-60 Hz).
Voltage → Electric Field
Current → Magnetic Field
Some states / countries have standards governing power line EMF; most do not (Florida does).
FL: 2 kV/m at edge
of right-of-way
FL: 200 mG at edge
of right-of-way
Earth’s magnetic field is about 500 milligauss

14. Arc Flash Hazards

15. Arc Flash Hazards
Arc flash hazards are due to thermal energy release (burns)
Energy = I2 t
Per NFPA 70E,
Category 1 5 calories/cm2
Category 2 8 cal/cm2
Category 3 25 cal/cm2
Category 4 40 cal/cm2

16. National Electrical Code

17. National Electrical Code
20A Polarized Receptacle
neutral
hot (120V)
ground (green screw)
green wire
gold screws
black wire
silver screws
white wire
15A Polarized Receptacle
(Fig on P. 658 of your text is INCORRECT)

18. National Electrical Code
Ground Fault Circuit Interrupter (GFCI) Receptacle

19. National Electrical Code
Wire Ampacities

20. Never Take Chances! If you’re not 100% sure, DON’T DO IT!
Don’t fear electricity; RESPECT it.
Assume that every circuit is live.
DON’T EVEN THINK ABOUT short-cutting safety procedures!!
Be safe and require everybody around you to do the same.