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Chapter 25: PRINCIPLES OF ELECTRICITY Summary This chapter is the first of several that deal with building electrical systems. As noted by the authors, electricity is the most prevalent form of energy in today’s buildings and knowledge of its properties, delivery, and usage is beneficial to the building designer. Electricity is characterized as a form of energy that is not terribly useful in its natural forms (lightning and galvanic action), but which has proven invaluable in its manufactured forms. Electricity is comprised of electrons flowing in a circuit. The flow is called a current (measured in amperes). Direct and alternating current (dc and ac) systems are available, with alternating current predominating in building applications. DC systems are encountered when batteries or photovoltaic cells are used. The tendency for electrons to flow is termed potential or voltage (measured in volts). Resistance to current flow is termed resistance (in dc circuits) or impedance (in ac circuits) and is measured in ohms. Ohm’s Law defines the relationship between voltage, current, and resistance in a circuit. Circuits may be arranged in series (with elements connected one after another) or in parallel (with elements connected via branches to/from the same points). Parallel circuits are standard for all building wiring. In a parallel circuit loads are additive with respect to current, and each load experiences the same voltage. A short circuit occurs if an inadvertent connection appears across the circuit, with potentially damaging results. AC electricity can be generated at different frequencies; 60 Hertz (cycles per second) is the standard in North America. AC power generation is briefly discussed. The difference between power and energy is emphasized and noted as a commonly misunderstood distinction. Units of measurement for power and energy are described and the concept of power factor introduced. Sample calculations for power, energy, and electricity costs are provided. Load factor and demand charge are explained. Typical electric demand control approaches are reviewed. The distinction between demand control and energy management is emphasized. Meters and measurements for electric systems are discussed. Chapter Outline 25.1 Electric Energy 25.2 Unit of Electric Current—The Ampere 25.3 Unit of Electric Potential—The Volt 25.4 Unit of Electric Resistance—The Ohm 25.5 Ohm’s Law 25.6 Circuit Arrangements (a) Series circuits (b) Parallel circuits 25.7 Direct Current and Alternating Current 25.8 Electric Power Generation—DC 25.9 Electric Power Generation—AC 25.10 Power and Energy 25.11 Power in Electric Circuits 25.12 Energy in Electric Circuits 25.13 Electric Demand Charges 25.14 Electric Demand Control (a) Level 1—load scheduling and duty-cycle control (b) Level 2—demand metering alarm (c) Level 3—automatic instantaneous demand control 1 (d) Level 4—ideal curve control (e) Level 5—forecasting systems 25.15 Electrical Measurements Key Concepts electricity (as the preeminent energy source for buildings) electric current (describing the flow of electricity) alternating and direct current (as two forms of electricity) voltage or potential (as the driving mechanism for electricity) conductor or insulator (as opposing sets of properties) Ohm’s Law (as it relates current, voltage, and resistance) parallel and series circuits (as distribution options) energy and power (as critical concepts) demand control (as a design strategy—and distinct from energy management) Terminology and Metrics Important Terminology electric current (direct or alternating; dc or ac) electric potential or voltage electric resistance conductor insulator Ohm’s Law electric circuit (parallel, series) short circuit cycle frequency wavelength battery generator (dc; ac = alternator) commutator kWh meter energy power reactance power factor (pf) vectorial sum overall load factor demand charge electric demand control time-of-day (variable) utility rate interruptible and uninterruptible loads demand control (shedding, automated load, peak demand, programmable load controls) duty cycle load comparator controller rate control ideal curve forecasting meter (amperage, voltage, watt-hour) 2 Important Metrics A (ampere; electric current) V (volt; electric potential) R (resistance; DC resistance, in ohms) Z (impedance; AC resistance, in ohms) Hz (Hertz; same as cycles per second) electron-volt (a measure of energy at the atomic scale) W (watt; electric power; also kW) PF (power factor; dimensionless) volt-ampere (wattage equivalent in a resistive circuit) Wh (watt-hour; electric energy; also kWh) load factor (a measure of peak power demand versus average power demand; dimensionless) Links to Resources Why is electricity so hard to understand? [not recommended for the faint of heart]: http://www.eskimo.com/~billb/miscon/whyhard1.html 3