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SACA C-201 ELECTRICAL SYSTEMS STUDY GUIDE
Solved 100% Correct
define and test a ground point in circuit connected to the earth. how to test = use DMM, if resistance is higher than 0, may indicate ground does not provide protection. electrical safety rule always use a circuit tester / voltage detector to check that a circuit is de-energized before working on it. electrical safety rule never touch electrical wiring or equipment w/ wet hands or while standing on a wet surface. electrical safety rule do not operate electrical equipment that is damaged, frayed, or otherwise in poor condition. electrical safety rule use the correct fuse or circuit breaker for the circuit you are working on. electrical safety rule never remove or bypass the ground prong on a plug. electrical safety rule do not use extension cords or multiple outlet adapters that are not rated for the equipment you are using. electrical safety rule do not overload circuits by plugging in too many devices or using high powered equipment.
electrical safety rule do not use electrical equipment in wet or damp conditions unless it is specifically designed for that purpose. electrical safety rule use properly rated and properly installed surge protectors for sensitive electronic equipment. electrical safety rule always follow manufacturer's instructions for using electrical equipment. purpose of a lockout / tag out to prevent unexpected energization or startup of machinery or equipment while it is being services or repaired. two types of electrical current AC = alternating current (flows in one direction first, then alternates to the other) DC = direct current (electricity flows in only one direction) how to use a circuit tester
- locate the circuit breaker or fuse that controls circuit
- using tester, touch the red lead to the live wire and black to neutral / ground functions of four basic components of an electrical circuit power source
- source of electrical energy for circuit. can be a battery, generator, or power outlet. the power supply provides electrical current that flows thru the circuit. functions of four basic components of an electrical circuit
(n.c.) normally closed switch transformer motor
1. knife switch
- manually operated electrical switch used to make and break electrical circuits. it had hinged blade used to open/close contacts, allowing flow of electricity 2. pushbutton switch
- pushbutton switch
- manual switch actuated by pressing a button. when pressed, switch is turned on, and when it is pressed again, switch is turned off. 3. selector switch
- manual switch that allows user to select one of several different circuit paths/positions. typically actuated by lever/knob that is rotated 5 types of output devices (resistor)
- limits flow of electrical current. it is one of the most commonly used components in electrical circuits 5 types of output devices (lamp)
- lamp or indicator light that gives off light when electrical current flows thru its resistive (usually wire) element 5 types of output devices (buzzer)
- causes vibration to occur inside housing when electricity is applied 5 types of output devices (solenoid)
- has an armature or plunger that moves in a linear motion when electrical current is applied to its coil 5 types of output devices (motor)
- produced rotary mechanical motion
- when electricity is applied, magnetic field is set up inside it. this magnetic field causes a shaft inside to start rotating. whatever is attached to the shaft also rotates define voltage the force that causes electrons to flow in a circuit
- Volts (V) how to use voltmeter analog = red (+) lead to higher potential point, and black (com) lead to lower potential point digital = most auto detect polarity (+/-), so for most DMM it's not critical to attach red lead to high potential, and black lead to low potential voltage characteristics in a series circuit while current thru each component is the same, the voltage drops across each load is not. as electricity passes thru each load, the voltage decreases voltage characteristics in a parallel circuit while current flowing thru each branch is different, the voltage across each branch is the same
- opposite of series define current the movement of electrons is called current
- Amps (A) how to use an ammeter
how to measure continuity where there is a continuous or complete path for current to flow
- use DMM
- display shows very small resistance value (ex. 0.01 Ohms)
- continuity tester
- should beep if continuity is found interpret metric prefixes on DMM Mega Ohms
- (1,000,000 x value) Kilo Ohms
- (1,000 x value) Milliamps
- (1,000 mA = 1A) Microamps
- (0.000001 A = 1 Microamp) define resistance
- measurement of components ability to resist flow of current in a circuit
- when current passes thru a component that has resistance, the current decreases
- resistance = ohms calculate total resistance in series Rt = R1 + R2 + R calculate total resistance in parallel 1/Rt = 1/R1 + 1/R2 + 1/R Ohms Law (solve for Voltage) E = I x R
Ohms Law (solve for Current) I = E/R Ohms Law (solve for Resistance) R = E/I Kirchhoffs Voltage Law for series KVL (SC)= total voltage in a SC is = to the sum of the individual voltage drops in the circuit Vt = Vr1 + Vr2 + Vr define power power = measure of energy consumed by a circuit measured = Watts (W)
- one watt of energy is consumed when one volt pushes one amp thru a circuit calculate total power If Voltage and Current are known: P = I x E If Current and Resistance are known: P = I^2 x R calculate total power in series circuit calculated by adding power used by each load Pt = Pr1 + Pr2 + Pr Kirchhoffs Current Law
- Circuit Breaker
- can be reset and used again. just as a fuse blows, a cb pops or trips when an excess current is present to test:
- manually switch the breaker off and confirm that current flow is interrupted
- deliberately overload the breaker and confirm that it switches off automatically to reset:
- flip switch back to on position to reset a breaker identify two types of circuit breakers and how to reset
- thermally triggered circuit breaker
- has long time delay before it trips since the temperature must increase enough to trip the breaker
- either a level switch or a pushbutton switch to reset identify two types of circuit breakers and how to reset
- magnetically triggered circuit breaker
- trips immediately when a surge of excessive current is present
- however, a slow increase in current may not cause it to trip
- this is why it is best to have a circuit breaker that uses both
- either a level switch or a pushbutton switch to reset explain how electromagnetism works
- when electrons flow thru a wire, an invisible force is created around the wire. this invisible force is called a magnetic field & can be used to do many useful tasks such as operate an electric motor
- the creation of a magnetic field by the flow of electric current is referred to as electromagnetism define inductance
- characteristic of electromagnetic devices that opposes any changes in current flow measurement: the unit "henrys" (H)
- a henry is a fairly large unit, so you will usually see inductance listed in millihenrys (1/1, henry) 1mH = 0.001H effect of an inductor in a DC circuit inductance effects a DC circuit only when the current is first turned on, turned off, or when there is a change in the load resistance. These 3 conditions are the only times the current is changing in a DC circuit effect of an inductor in a AC circuit
- since the current in an AC circuit is continuously changing, an inductor has a different effect in an AC circuit than a DC circuit
- in an AC, the induced voltage across the inductor always resists this change. this constant resistance is called inductive reactance inductive reactance formula XL = 2πfL
- where XL is the inductive reactance in ohms, f is the frequency of the AC current in hertz, and L is the inductance of the inductor in henrys calculate total inductance and inductive reactance for series
- when inductors are connected in series, the total inductance = the sum of the individual inductances
- polarized/electrolytic (polarity must be observed mainly in DC circuits)
- usually have markings to let you know the polarity of their leads. the (-) lead is negative and (+) is positive 3 types of capacitors
- variable (can be adjusted)
- like variable resistors, variable capacitor values can be adjusted by turning a knob how to safely discharge a capacitor
- large capacitors should be discharged thru a resistor to prevent damaging device
- to discharge a small capacitor, touch connection posts with a lead wire. if there is any charge, you should notice a spark when the wires touch the posts how to test a capacitor with a DMM
- can be tested by measuring resistance
- if the capacitor is good, the display with drop to a fairly low resistance reading and then start to climb rapidly until it reaches OL. if the resistance value is close to 0 and does not change, the dielectric is shorted, and capacitor is bad effect of a capacitor in a DC circuit
- capacitor charges to a voltage that is equal to the source voltage
- since the dielectric is an insulator and the voltage potential across the capacitor is = to the source, a charged capacitor acts like an open and will not allow current to flow in the circuit effect of a capacitor in an AC circuit
- a capacitor has a different effect in an AC circuit than a DC
- the current in a DC circuit is blocked because the capacitor charges to the level of the source voltage and current can't go thru the dielectric calculate the total capacitance and capacitive reactance for series
series capacitance formula Ct = 1/ (1/C1 + 1/C2 + 1/C3) series capacitive reactance formula Xct = Xc1 + Xc2 + Xc calculate the total capacitance and capacitive reactance for parallel parallel capacitive formula Ct = C1 + C2 + C parallel capacitive reactance formula Xct = 1/ (1/Xc1 + 1/Xc2 +1/Xc3) capacitive reactance formula
- the apparent resistance to ac current is called capacitive reactance (expressed as Ohms) Xc = 1/2(pi)fc pi = 3. f = ac freq (hertz) c = capacitance (farads) calculate the time to charge and discharge a capacitor
- RC time constant formula T = R x C
- RC circuit charge / discharge time formula Tcd = T x 5 functions of capacitors and inductors in an electric power supply
- connect an ohmmeter across the main line
- disconnect each branch one at a time state and apply three steps for troubleshooting an open circuit
- disconnect the power supply
- connect the ohmmeter across the main line
- disconnect each branch one at a time explain the operation of a transformer
- a transformer uses the principle of mutual induction to create an AC electrical voltage in the secondary coil
- the principle of mutual inductance says that when 2 electric coils are placed near each other, AC electric current flowing in one coil induces an AC voltage in the other coil
- this occurs because the electric current creates a magnetic field around the first coil which in turn induces a voltage in the secondary coil how to calculate output voltage of a transformer
- input voltage
- turns ratio turns ratio formula: TR = NP/NS (NP = # of turns in primary coil) (NS = # of turns in secondary coil) transformer output voltage formula: VS = VP/TR (TR = turns ratio) (VP = primary voltage (V))
(VS = secondary voltage (V)) how to troubleshoot a transformer with a DMM
- measure input / output voltages
- if voltages are reasonably close to theoretical values, the transformer is good
- check transformer with an ohmmeter
- can be used to check for open coils, shorts between coils or coils shorted to the core without any power applied to the transformer
- to test for these conditions, check:
- opens in coils
- shorts between primary and secondary coils
- coils shorted to the core size a transformer given the voltage and load of the circuit
- input voltage available
- output voltage desired
- output current required, both in rush and steady state steady state VA formula (volt-amperes) VA = Vs x Is (Is = steady state 2nd current during operation (Amps)) (Vs = 2nd voltage (Volts)) (VA = steady state volt-amperes rating (Volt-Amperes)) max in rush VA formula VAmax = Vs x Is max (Is max = max in rush current during load startup (Amps))
- calculate actual current load Ip = VAp/Vp (Vp = primary voltage (Volts)) (VAp = primary volt-amperes (Volt-Amperes)) Ip = primary current (Amps)) state two basic categories of transformers
- isolation transformers
- has primary and secondary windings that are physically and electrically isolated. the isolation transformer is magnetically coupled, not electrically coupled
- autotransformers
- uses only one coil for the primary and secondary. it uses special connections on the coil called taps to produce different ratios and voltages explain the function of a tap on the secondary of a transformer
- many transformers have a secondary coil that has an extra lead attached to it known as a tap
- the tap is connected at points between the ends of the coil so that you can get a different output voltage from the transformer
- the tap allows a different voltage to be produced because it uses a reduced # of turns in the secondary coil
