LightningElectric ShockElectromagnetic HazardsArc Flash HazardsThe National Electrical Code

Electrical Safety

Lightning

Lightning Mechanism

“Negative Lightning”(95% of all strikes to earth)

“Positive Lightning”(5% of all strikes to earth)

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)

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?

)/105)(10100()50)(5( 96 sAHkADt

dILRIV

kV750000,500000,250

Isokeraunic Map of United States

Isokeraunic Map of World

Electric Shock

Electric ShockCurrent, not voltage, poses the risk of electric shock

Current Effect on Human Body

< 1 mA none1 mA perception threshold1-10 mA mild to painful sensation10 mA paralysis threshold –

cannot release hand grip30 mA respiratory paralysis75-250 mA fibrillation4-5 A heart stops; organs burn

Electromagnetic Hazards

Electromagnetic HazardsHigh 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.

Electromagnetic HazardsLow 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).

r

1 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

Arc Flash Hazards

Arc Flash HazardsArc 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

National Electrical Code

National Electrical Code

15A Polarized Receptacle

(Fig. 15.13 on P. 658 of your text is INCORRECT)

silver screwswhite wire

gold screwsblack wire

neutral hot (120V)

ground (green screw)green wire

20A Polarized Receptacle

National Electrical Code

Ground Fault Circuit Interrupter (GFCI) Receptacle

National Electrical Code

Wire Ampacities

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.