# You're Grounded! (Everything you wanted to know about ...

Youre 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 2 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 (Ohms Law)

3 4 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. 5

Current The rate of charge flow. In order for current to flow, there must be a closed circuit! Its the volts that jolts, but its 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! 6 Power Power = Voltage * Current The rate at which electrical energy (q*V) is converted into heat.

7 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 its dry) Normally, the skin will resist voltage differences up to 30-40V. The interior of the body is a pretty good conductor 8 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! 9 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

10 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. 11 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 12 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 180 out of phase One neutral lead (V~0 w.r.t. ground) Industrial / Commercial: typically three phase Three hot leads 120 out of phase

One neutral lead Mendel has three phase power! 13 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) 14 120V and 240V outlets (US) 120 V two prong outlet (no ground connection!)

Hot lead, hot lead ground lead 15 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! 16 Whats Wrong with this Picture? 17

Whats Wrong with this Picture? 18 Whats Wrong with this Picture? 19

Whats Wrong with this Picture? 20 Whats Wrong with this Picture?

21 22 23 24 Check wattage of devices ensure youre not overdrawing!

Rule of thumb: dont 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! 25 In case of electrical

emergencies. Blown fuse / circuit breaker Electric shock / electrocution Electrical fire Exposed / downed lines 26 27

28 29 30 Questions?