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

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

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)

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.

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!

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

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

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!

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

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.

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

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!

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 voltages too, such as 480V/277V (Mendel lighting)

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

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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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!

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

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