Electrical Principles, Terminology, and Safety

1. Electrical Principles, Terminology, and Safety Instructional Materials Service Texas A&M University

2. Definition of Electricity • Layman’s Definition: a form of energy that can be converted to light, heat, sound & motion. • Electrical Engineer’s Definition: the flow of electrons from one atom to another. • This flow of electrons is controlled in an electric circuit. • The amount of energy produced depends on the number of electrons in motion.

3. Basics of an Atom • Nucleus: • Center of an atom • Contains positively charged protons • Electrically neutral (“Balanced State”): • Equal number of electrons & protons • Electrons: • Arranged in orbital layers around the nucleus • Negative charge • Can be forced to leave their outer orbital layer & attach to the outer ring of an adjacent atom. • Atom that receives extra electron becomes negatively charged. • Atom that gives up electron becomes positively charged.

4. Examples of Atoms • Hydrogen: • One proton • One electron • Good insulator • Copper: • 29 protons • 29 electrons • Electrons in outer orbit are loosely held; allows for exchange of electrons • Good conductor of electricity

5. How Electricity is Generated • Produced by generators run by water, steam or other energy forms. • Hydroelectric Generator: • Water is used as a source of power to turn generators • Less than 5% of electricity is produced this way in Texas • Water power uses principle of ancient “water wheel” • Water wheel is called a “turbine wheel” • Turbine wheel is constructed of metal, has an electric generator attached to turbine shaft • Water flows against turbine blades, turning turbine & generator • Coils of wire inside the generator turn through a magnetic field allowing electrons to flow along a conductor from the generating site.

6. Steam Power: • Produces approximately 85% of all electricity used in the U.S. • Water is heated in a boiler converting liquid to steam. • Steam at high temperatures & pressure causes the blades to rotate the turbine. • The turbine operates a generator producing electricity. • Also referred to as thermal-powered generators • Heat supplied from fossil fuels, nuclear fission, biomass, wood, wind & geothermal

7. How Electricity is Distributed • Electric energy is transported from the plant to the consumer in wires known as transmission or distribution lines. • Step-Up Transformers: necessary to move large quantities over long distances through transmission lines from the step-up substation to the distribution area. • Step-Down Transformers: allows for distribution to customers or consumers through distribution lines.

8. 69,000750,000 volts 15,000 volts 120 or 240 volts 7,200-14,000 volts

9. How Transformers Work • Transformer: used to increase or decrease the amount of voltage. • Voltage & Amperage are dependent on each other. • When voltage is stepped-up (increased), amperage is reduced proportionately. • When voltage is stepped-down (decreased), amperage is must be increased. • Basic Transformer: • Consists of two coils (windings): • primary coil & secondary coil • Both are made of wire • The coils are wound around an iron core. • One coil has more windings than the other • Coils & iron core are used to transform electrical energy into magnetic energy & then back into electrical energy.

10. Step-Up Transformer: • Power source (power plant) is connected to the coil with the fewest number of turns (primary coil). • Electricity is increased through induction in the secondary coil. • Increase in voltage due to greater number of turns in the secondary coil • Step-Down Transformer: • Power source (transmission lines) is connected to primary coil • Power from secondary coil is lower in voltage

12. How Electrical Energy Moves Through a Circuit • Circuit: path through which an electrical current flows. • Current: movement of electrons through an electrical conductor. • Generators produce & release current the moment it is needed. • Closed Circuit: complete electrical pathway from the power source through the load & back to the source. • Continuous flow of current as long as equipment is turned on and the circuit is closed • Open Circuit: an incomplete circuit. • When electrical equipment is turned off

13. Electrical circuits are made up of a hot and a neutral wire. • Hot wire: • A conductor that carries electrical pressure to the load • Will show voltage when measured with a voltmeter • Usually black or red • Neutral wire: • A conductor that carries current from the load back to the source • Not under electrical pressure (zero voltage) • Usually white • Ground wire: • Conducting wire that transmits stray electrical current to the earth to minimize the danger from electrical shock • Hot, neutral & ground wires are usually made of copper because copper is a good conductor.

14. Load: any device that converts electrical energy into another form of useable energy. • Ex: a light bulb converts electrical energy into light and heat. • Insulator: a material that will not conduct electricity because it will not release its own electrons. • Materials used as insulators: • Glass • Paraffin • Porcelain • Rubber • Silk • Cotton • Dry wood • Most plastics

15. Conductor: any material that allows electrons to move readily offering low resistance. • Best Conductors: 1. Gold (expensive) 2. Silver (expensive) 3. Copper (commonly used) 4. Aluminum 5. Bronze • DO NOT splice copper & aluminum together because it will result in deterioration and oxidation.

16. Measuring Electricity • Electric Current: • Amperes: unit of measure for the flow of electrical current. • Abbreviated as “A” or “I” (Intensity of Current) • May be referred to as amperage (or amp) • Amperage: the rate at which electric current flows through a conductor per second. • Ammeter: instrument used to measure this electrical current flow.

17. Electric Pressure: • Volt: unit of measure of electrical pressure or the pressure being applied to force electrons through a circuit. • Abbreviated as “V” or “E” (Electromotive Force) • Voltage: pressure available in energized circuits all the time whether electrical equipment is being used or not. • Common service voltages are 120 & 240 volts.

18. 120 volts: • One hot wire & one neutral wire • Circuits used for lighting & small appliances are typically supplied with 115 to 120 volts.

19. 240 Volts: • Two hot wires & one neutral wire • Each hot wire is one half of the total voltage • Used by larger equipment such as heating & air conditioning appliances, welders & motors ½ horsepower or larger. • If three incoming wires are present, both 120V & 240V service is available.

20. Volts can be measured with a voltmeter • Some voltage may be used trying to overcome the resistance in the conductors. • Happens when an appliance that requires a lot of current is started • Voltage Drop: the reduction of voltage. • Influencing factors: • Size of wire • Length of wire • # of amps flowing • May cause a loss of heat, light, or power output of a motor • Could cause motor burnout unless the motor is properly protected with items such as a time-delay fuse

21. Electric Power • Measured in watts(W) • Is the rate that electrical energy is transformed into some other form of energy such as light • May be referred to as wattage • Volts and amperes by themselves do not give a measure of the amount of power produced • Important Relationships: • Watts = Volts X Amperes • Volts = Watts/Amperes • Amperes = Watts/Volts

22. Kilowatt (KW) = 1,000 watts • Used in computing electrical energy consumed • Determined by dividing the # of watts by 1,000 • Horsepower (hp): the unit of mechanical power equal to 746 watts of electrical power (assuming 74.6% electric motor efficiency) • 1 hp & above motors are rated at 1,000 watts per hp • Motors below 1 hp are rated at 1,200 watts per hp

23. Resistance: • Any material that opposes the flow of electricity • Conductors have very low resistances • Insulators have very high resistances • Ohm: unit of measure of electrical resistance • Abbreviated “R” for resistance • Amount of resistance (ohm) is determined by: • Material conductor is made of • Size of the conductor • Length of the conductor • Resistance is proportional to the length & size of the conductor. • If length of wire is doubled, then resistance is doubled • If diameter of wire is reduced by half, then resistance is doubled.

24. Ohm’s Law: • The relationship between amps, volts & resistance • The Law Volts are equal to amperes times resistance. • The Equation  E = I x R • E = Volts • I = Amperes • R = Resistances • Equation can be rearranged to solve for any of the three values as long as the other two values are known • R = E/I (must know E & I) • I = E/R (must know E & R)

25. Multimeters are capable of measuring voltage, resistance & current flow in milliamperes. • Commonly called a volt-ohm-milliammeters (VOM)

26. Computing Electrical Energy Use & Cost • Watt-hour = use of 1 watt for 1 hour • Kilowatt-hour = use of 1,000 watts for 1 hour • This is the unit that electrical energy is measured and purchased in. • Formulas: • Watt-hours = Watts x hours of operation • Kilowatt-hours = Watt-hours/1,000 • Cost = kWh x local rate per kWh

27. Example: Cost problem using the Watts and Time Estimate method The nameplate data from an appliance indicates that it properly operates at 120 volts and 5 amps. The motor’s monthly hours of operation are 10 hours and the local rate per kWh used is 8 cents. The estimated cost of operation per month would be calculated as follows:

28. Step 1) Watts = Volts x Amperes = 120 x 5 = 600 Step 2) Watt-hours = Watts x hours = 600 x 10 = 6000 Step 3) kWh = Watt-hours/1,000 = 6000/1000 = 6 Step 4) Cost = kWh x rate = 6 x 8 = 48 cents

29. Types of Circuits • Basic Types: • Series • Parallel • Series-Parallel • Series-parallel circuit is a network of circuits that incorporates both series and parallel circuits.

30. Series Circuit: • All current must flow through each device in the circuit • Current has only one path to flow • Removing or opening any one device will stop the flow of current for the whole circuit • Same current or amperage flows through each load • Voltage is equal to the individual voltage drops from each load • Switches, fuses, and circuit breakers are always connected in series

31. Parallel Circuit: • Separate paths for the current to flow through • Provide the same voltage across each load (lights or appliances) • Removing one device has no effect on the other devices • Current flow is divided through each load on the circuit & is equal to the total current from the source

32. Types of Electric Current • Direct Current (DC): electrons always flow in one direction. • Examples: • Dry cell batteries • Batteries used in automobiles & tractors • Alternating Current (AC): electrons move back and forth, reversing their direction regularly. • Change direction of flow several times per second, depending on the direction the voltage forces it • Electric utilities produce alternating current • Examples: • Lights • Refrigerators • Other home equipment

33. Cycle: flow of electricity with voltage fluctuation. • Hertz (Hz): term for cycles per second • Power suppliers in the US control the power to 60 hertz. • This causes electric clocks to keep accurate time because most operate on 60 cycles. • European power is on a different cycle.

34. Type of Electric Power • Single-phase: • Most common type of electrical service or power available to consumers • One transformer is used between the distribution line and the meter • Usually three wires, two “hot” and one neutral, are installed to provide 120V & 240V single-phase service. • May also be supplied with three-phase service

35. Three-phase: • Designed especially for large electrical loads • More expensive due to installation of four wires & three transformers • Three wires are “hot” & one is neutral • Total electrical load can be divided among the three phases, requiring smaller wires and transformers

36. Circuit Failures • Many types of conditions can cause a circuit to fail. • Open Circuit: • Break or interruption in the electrical pathway that stops the current flow. • Example: turning off the switch for a light • Many times they are unintentional resulting from damage to the equipment or conductor • Example: cutting the wire

37. Overload: • Occurs whenever excess current flows through a circuit • Can cause shock or a fire • Examples: • Plugging too many products into a circuit • Using an extension cord that is rated 15 amps to supply electricity to a 20-amp load • The load(s) require the use of more amps than the circuit can safely supply

38. Short Circuit: • Occurs when current flows in an unintended path. • Unintended path usually offers a lower resistance path for the current to follow, bypassing the normal circuit load(s) • Can result from an insulation breakdown or a direct connection between conductors • Extremely dangerous • Example: • A drill fails to operate because wires in the cord are in direct contact with one another.

39. Circuit Protection Devices • Fuses and Circuit Breakers • Used to protect electrical equipment and wiring from overloads • When an overload occurs the circuit breaker trips or the fuse melts inside to prevent the wiring from over heating and causing a fire.

40. Fuses: • Contain a link made from a low-melting alloy designed to carry current up to the rating of the fuse. • Link melts when current exceeds amperage rating of fuse • This opens the circuit & stops the flow of electricity • Must be replaced once the link melts or blows inside the fuse • Common Fuses: • Fusetrons (time-delay) • Fustats (two-part time-delay) • Time-delay fuse has the ability to carry a temporary overload of current for a short duration without disengaging the fuse links • Types of Fuses: • Plug • Cartridge

42. Circuit Breakers: • Eliminate the replacement of fuses • Commonly used • Circuit breaker box cost more than a fuse box • Two Types: • Thermal: • Has two contacts held together by a bi-metal strip and latch • Overload causes bi-metal strip to heat & expand • Latch releases & the points spring open • After bi-metal strip cools, switch is reset & service is restored

44. Magnetic: • Has contacts that are held together by a latch, which is released by the action of an electromagnet • Amount of current flowing through the circuit determines size of electromagnet • Moving toggle switch to “on” position resets this type

46. Ground Fault Circuit Interrupters (GFCI): • Designed to protect humans, equipment, and/or electrical systems from injury or damage if there is a short circuit. • Very sensitive device • Compares current flowing out from source through the hot wire of a circuit with that returning back to the source through the neutral wire • Ground-fault: one wire is carrying less current than the other • Some of the current may take an alternate path back to the source

47. A difference in current flow as little as 5 milliamperes (mA) or greater will cause the GFCI to open the circuit, shutting off the power and eliminating any shock hazard. • Current flow as little as 10mA through the human body may be fatal under certain circumstances. • Once source of load imbalance has been identified and corrected, GFCI may be reset and tested • Current flow from a ground-fault may or may not trip a circuit breaker or blow a fuse, unless the current exceeds the amperage rating.

48. Some GFCI breakers will provide protection for everything on the circuit. • National Electric Code (NEC) requires GFCI’s for all 120V, single-phase, 15 and 20 amp receptacles installed outdoors, in bathrooms & in garages for residential buildings. • A GFCI is required at construction sites. • A variety of GFCI equipment is made for 120V & 240V circuits.

50. Electrical Safety • Organizations • National Fire Protection Association (NFPA) • Devoted to promoting and improving the science & methods of fire protection • Publishes the NEC which serves as a national standard for safe installation of electrical wiring and devices and is revised every three years • NEC is not law, but all wiring should meet its requirements as well as local building codes. • Underwriter’s Laboratory (UL) • Provides voluntary product testing of all types of wiring materials and electrical devices to determine if they operate safely and prevent electrical shock and /or fire