1. Jeremy P. Carlo Department of Physics
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 is possessed 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 ability to 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,” current can 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 very high voltages (>100 kV) 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) V between “hot” and “neutral” V between two “hot” leads
Industrial settings can have other voltages too, 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: