Jeremy P. Carlo Department of Physics

1. You’re Grounded!(Everything you ever wanted to know about electrical safety but were afraid to ask.) Jeremy P. Carlo Department of Physics

2. Outline • Basic Electrical Theory • Household Wiring • Safe Working with Electrical Equipment • Response to Electrical Emergencies

3. Basic Electrical Quantities • Voltage (V) (Electric Potential): Energy per unit charge • Units: Volts (V). 1 V = 1 Joule per coulomb of charge • Current (I): The rate at which charge flows • Units: Amps (A). 1 A = 1 coulomb of charge per second • Power (P): The rate at which electrical energy is expended • Units: Watts (W). 1 W = 1 joule of energy per second • P = dU/dt = V*I (power = voltage * current) • Resistance (R): How much a material or object resists the flow of current • Units: Ohms (). 1  = 1 volt per amp • V = I * R ; R = V / I ; I = V / R (Ohm’s Law)

5. Voltage • The amount of energy per unit charge. • How much the charge is being “pushed.” • How much capacity to do work ispossessed by each charge. • Important: Voltage is only meaningful as a difference between two points! • Define “ground” as V = 0.

6. Current • The rate of charge flow. • In order for current to flow, there must be a closed circuit! • “It’s the volts that jolts, but it’s the mils (milliamps) that kills!” • A shock on a door knob can be thousands of volts, but harmless. • But as little as I = 6 mA, even at relatively low voltage, can stop the heart!

7. Power • Power = Voltage * Current • The rate at which electrical energy (q*V) is converted into heat.

8. Resistance • Limits the flow of current. • If R  0, current I becomes “yuuuuge:” “short circuit” • Is water a good conductor? • Resistance of the human body: • The skin is a pretty good resistor ~ few hundred k (as long as it’s dry) • Normally, the skin will resist voltage differences up to 30-40V. • The interior of the body is a pretty good conductor

9. Alternating vs. Direct Current • Very similar but… • Big advantage of AC is the abilityto use transformers to “transform” voltages up & down.This is why almost all household and industrial electrical distribution uses AC!

10. Electrical Distribution: Wiring considerations • All conductors have some resistance. • The four “good” conductors: Cu, Ag, Al, Au • Wire thickness: • American Wire Gauge (AWG) • The lower the number, the thicker the wire • The thicker the wire, the more current it can safely handle (can find tables online) • Insulated Wire vs. bare wire • Stranded vs. solid wire

11. Electrical Distribution: Overcurrent Protection • If you have a “short circuit,” currentcan be enormous  wires overheat fires! • The “canary in the coal mine”Fuses Circuit Breakers Breakers are typically installed in a power distribution panel. Individual equipment items often have fuses installed.

12. Household Electrical Power • Transmitted long distances at veryhigh voltages (>100kV) on“high tension lines” • At a substation voltage is converted down to (typically) 1.2 kV

13. Household Electrical Power • 1.2 kV power is down-converted at a transformer to 120V for distribution to residences and businesses.Residential: typically “two phase” • Two “hot” leads 180º out of phase • One “neutral” lead (V~0 w.r.t. ground) • Industrial / Commercial: typically “three phase” • Three “hot” leads 120º out of phase • One “neutral” lead • Mendel has three phase power!

14. Two-phase power Color code: • The “hot” leads are red and black • The “neutral” lead is white • An additional “ground” wire (green or bare copper) comes from an “earth ground” (rods driven into the soil) • Black-white or red-white: 120V • Black-red: 240V • Three phase: additional “hot” wire(usually blue) 120V between “hot” and “neutral” 208V between two “hot” leads • Industrial settings can have other voltagestoo, such as 480V/277V (Mendel lighting)

15. 120V and 240V outlets (US) 240 V outlet Hot lead, hot lead ground lead 120 V grounded outlets (“three prong”) ground lead Hot lead, neutral lead 120 V “two prong” outlet (no ground connection!) hot lead, neutral lead

16. Why is the ground lead important? • All outlets at VU should be 3-prong! • “Cheater plugs:” not allowed at VU! • Ground Fault Circuit Interrupt (GFCI) • Detects “imbalance” between leads • Much faster reaction than circuit breaker • CB: protects the house against fire • GFCI: protects you against electrocution!

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25. Check wattage of devices – ensure you’re not overdrawing! • Rule of thumb: don’t exceed 75% of max rating • Never overload a socket with too many plugs • Never daisy-chain cords together • Refrain from using extension cords • Should not run across floor – tripping hazard! • And they should never be used as long-term replacements for a nearby outlet • A hot outlet or cord can be a sign of overloading • Keep cords tidy and secure, and unplug unused appliances • Keep appliances away from water • Ensure space around equipment for air circulation • Always follow manufacturer recommendations! • Never defeat safety features!

26. In case of electrical emergencies…. • Blown fuse / circuit breaker • Electric shock / electrocution • Electrical fire • Exposed / downed lines

31. Questions? • ElectroBOOM: https://www.youtube.com/watch?v=W0idCh8AqhU