ONE LINE DIAGRAM Power systems are extremely complicated electrical networks that are geographically spread over very large areas. In fact, the power systems are so complex that a complete conventional diagram showing all the connections is impractical. Yet, it is desirable, that there is some concise way of communicating the basic arrangement of power system components. This is done by using Single Line Diagrams (also called One Line Diagrams.
ANALYSIS OF A ONE LINE DIAGRAM Equivalent Transmission and Generation Grid AC VOLTAGE 138,000V CIRCUIT BREAKER STEP DOWN TRANSFORMER 4160 V STEP DOWN TRANSFORMER 240V Arrows represent Houses Distribution Grid
CIRUCIT DIAGRAM vs. ONE LINE DIAGRAM Which lines could fail to produce greatest outages and least outages 10 -2 ohms 10 ohms 12 V
CURRENT AND RESISTANCE If a fault occurs in one of the lines. What will happen to the overall resistance of the grid? What effect will this have on current?
RESISTIVITY Conductive wires which make up circuits also offer resistance to current. The conducting ability of a material is often indicated by its resistivity. The amount of resistance a wire offers depends on the length, cross sectional area, and material the wire is made of ρ is the static resistivity (measured in ohm meters) R is the electrical resistance of a material (measured in ohms) l is the length (measured in meters) A is the cross-sectional area (measured in square meters) http://en.wikipedia.org/wiki/Resistivity ρ = RA/l
RESISITIVITY VALUES Material Resistivity(ohmmeter) Silver 1.59 x 10-8 Copper 1.7 x 10-8 Gold 2.4 x 10-8 Aluminum 2.8 x 10-8 Tungsten 5.6 x 10-8 Iron 10 x 10-8 Platinum 11 x 10-8 Lead 22 x 10-8 Nichrome 150 x 10-8 Carbon 3.5 x 10 5 Polystyrene 10 7 - 10 11 Polyethylene 10 8 - 10 9 Glass 10 10 – 10 14 Hard Rubber 10 13
CIRCUIT BREAKERS The circuit breaker is an important safety mechanism, both on the grid and in your home. Whenever there is too much current flowing through it, these simple machines cut the power until somebody can fix the problem. Without circuit breakers household electricity would be impractical because of the potential for fires and other mayhem resulting from simple wiring problems and equipment failures. http://static.howstuffworks.com/gif/circuit-breaker-intro.jpg
GROUNDING Electricity always follows the path of the least resistance. You do not want to be that path!!! You do not want an exposed point of very high voltage (relative to what? relative to ground!)in your home. If a person touched that point, while their feet were touching the ground a large voltage would be across their body, driving a large current and shocking them. GROUNDING is to electrically attach a circuit to the lowest "electrical potential"...mostly referred to as 0 Volt (zero) potential. If a circuit begins to The ground wire will provide the path of least resistance should the circuit become charged, thus safeguarding against electrical shock.
ELECTRICAL SAFTEY Electrocution or electrical shock occurs when an electric current passes through the body. The amount of current passing through the body is determined by Ohm's Law: I = V/R I = Current Through the Body V = Voltage across the body R = Resistance of the Body
THE ROLE OF VOLTAGE IN SHOCK electricity requires a complete path (circuit) to continuously flow Birds can safely rest on high- voltage power lines without getting shocked: they make contact with the circuit at only one point. Many times, one side of a power system will be intentionally connected to earth ground, and so the person touching a single wire is actually making contact between two points in the circuit (the wire and earth ground):
THE ROLE OF CURRENT IN SHOCK The level of voltage is not a direct guide to the level of injury or danger of death, despite the common misconception that it is. A small shock from static electricity may contain thousands of volts but has very little current behind it due to high internal resistance.voltagecurrent
EFFECT OF CURRENT ON HUMAN BODY Magnitude of CurrentEffect 1 to 8 mAPerceptible, but not painful 8 to 15 mA Painful shock, but muscular control is not lost 15 to 20 mA Painful shock, Muscular control affected, cant let go 20 to 50 mA Severe Muscle contractions, Breathing difficult 50 to 100 mA Serious contraction of heart muscle, Possible death, No known remedy 200 mA and above Severe burns, Stoppage of heart, Death certain
THE ROLE OF RESITANCE IN SHOCK Will the 120 volt common household voltage produce a dangerous shock? It depends! If your body resistance is 100,000 ohms, then the current which would flow would be: But if you have just played a couple of sets of tennis, are sweaty and barefoot, then your resistance to ground might be as low as 1000 ohms. Then the current would be: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/shock.html
Because electric current must pass through the boot and the body and the glove to complete its circuit back to the battery, the combined total (sum) of these resistances opposes the flow of electrons to a greater degree than any of the resistances considered individually. http://www.allaboutcircuits.coml
AVOIDING ELECTRIC SHOCK Indoor safety: Check to make sure all electric cords are free of fraying or cracking. Replace any cords that may be damaged. When unplugging appliances, always pull from the plug, not the cord. Keep all electric appliances away from sinks, baths, and other water sources. If an appliance falls into water, never reach in to get it. to retrieve it. Instead, call your local electric company for assistance. http://www.puco.ohio.gov/PUCO/Consumer/information.cfm?doc_id=1262
AVOIDING ELECTRIC SHOCK Outdoor safety: Downed power lines are often caused by thunderstorms or other severe weather. Always treat a downed power line as if it were live, and do not touch it or any objects around it. If you see a downed power line, contact the police department, fire department, or your electric company immediately. Don’t use electric powered lawn tools in wet conditions. Always wear rubber-soled shoes or boots when using electric lawn tools. Never operate an electric tool barefoot. Never allow children to climb trees that are touching electric lines. Never allow children to fly kites, balloons, or other toys near power lines. http://www.puco.ohio.gov/PUCO/Consumer/information.cfm?doc_id=1262