Year 1 - Mike Holt - Basic Electrical Theory (Final Exam)
The effective value is equal to the peak value ______. (16.12)
x 0.707
The induced voltage in a conductor carrying alternating current opposes the change in current flowing through the conductor. The induced voltage that opposes the current flow is called "______." (18.2)
"Counter-Electromotive Force" (CEMF), or "back-EMF"
What is the resistance of the circuit conductors when the conductor voltage drop is 7.20V and the current flow is 50A? (9.7)
0.14 Ohms
The total circuit resistance of two 12 AWG conductors (each 100 ft long) is 0.40 ohms. If the current of the circuit is 16A, what is the power loss of the conductors in watts. (9.11)
100W?
Three-phase ac generators have three equally spaced windings, ________ out-of-phase with each other. (21.3)
120°
If the output is 1,600W and the equipment is 88 percent efficient, what are the input amperes at 120V? (19.9)
15A
What is the power loss in watts of a conductor that carries 24A and has a voltage drop of 7.20V? (9.8)
175W
What is the power consumed by a 10 kW heat strip rated 230V connected to a 115V circuit? (9.13)
2.5 kW
If the primary phase voltage is 480V and the secondary phase voltage is 240V, the turns ratio is _____. (22.7)
2:1
An electrical arc blast can approach ________, which vaporizes metal parts and produces an explosive and deadly pressure wave. (4.4)
30,000°F
Three-phase, _____, wye-connected systems can over-heat because of circulating odd triplen harmonic currents. (22.5)
4-wire
What is the conductor power loss in watts for a 120V circuit that has a 3 percent voltage drop and carries a current flow of 12A? (9.11)
43W
How many 20A, 120V circuits are required for forty-two, 300W luminaries (noncontinuous load) that have a power factor of 85 percent? (19.7)
5 Circuits
What is the approximate power consumed by a 10 kW heat strip rated 230V, when connected to a 208V circuit. (9.9)
5 kW
Arcing is defined as a luminous discharge of electricity across an insulating medium. Electric arcs operate at temperatures between _______ and expel small particle of very hot molten material. (15.16)
5,000° - 15,000°
What does it cost per year (at 8 cents per kWh) for the power loss of a 12 AWG circuit conductor (100 ft long) that has a total resistance of 0.40 ohm and current flow of 16A. (9.12)
50$
What is the nameplate FLA for a 20 hp, 208V, three-phase motor with 90 percent power factor and 80 percent efficiency? (20.5)
58A
What is the voltage drop of two 12 AWG conductors (0.40 ohms) supplying a 16A load, located 100 ft from the power supply? (9.7)
6.40V
What size transformer is required for a 100A, 240V, single-phase noncontinuous load that has a power factor of 85 percent? (19.7)
?
The total resistance of a parallel circuit can be calculated by the _____ method. (11.3)
Any of these
Kirchoff's Current Law states, "In a series circuit, the current is ______ through the transformer, the conductors and the appliance." (10.2)
Consistent
Wasteful circulating _____ in the iron core cause(s) the core to heat up without any useful purpose. (22.4)
Eddy Currents
_____ can be reduced by dividing the core into many flat sections or laminations. (22.4)
Eddy Currents
A GFCI-protection device contains an internal monitor that prevents the device from being turned on if there is a neutral-to-case connection downstream of the device, but this only occurs if there is a load on the circuit. (15.12) True False
False
A holding relay is primarily used for worker convenience. (6.4) True False
False
AC inductive or capacitive reactive loads cause the voltage and current to be in-phase with each other. (19.3) True False
False
According to Kirchoff's Current Law, the total current provided by the source to a parallel circuit will equal the sum of the currents of all of the branches. (11.2) True False
False
Inverse time breakers operate on the principle that as the current decreases, the time it takes for the device to open decreases. (15.2) True False
False
Lightning protection is intended to protect the building itself, as well as the electrical equipment on or inside the structure. (1.8) True False
False
Magnetic lines of force can cross each other and they are called flux lines. (3.9) True False
False
Nonmagnetic metals are ferrous, meaning they do not contain any iron, and cannot be magnetized. (3.5) True False
False
The best conductors, in order of their conductivity, are gold, silver, copper, and aluminum. (9.3) True False
False
The motor FLA rating is used when sizing motor conductor size or circuit protection. (20.4) True False
False
The termination of the lightning strike is unlikely to ignite combustible materials. (1.8) True False
False
To prevent fires and electric shock, the NEC specifies that neutral current can flow on metal parts of the electrical system. (14.4) True False
False
Because of the earth's ____ resistance to current flow, it cannot be used for the purpose of clearing a line-to-case ground fault. (14.5)
High
As current flows through the transformer, the iron core is temporarily magnetized. The energy required to realign the core molecules to the changing electromagnetic field is called "_____" loss. (22.4)
Hysteresis
The total opposition to current flow in an AC circuit is known as ____. (18.4)
Impedance
If the rotating part of the motor winding is jammed so that it cannot rotate, no CEMF is produced in the motor winding. As a result, the motor operates at _____ and the windings will be destroyed by excessive heat. (20.8)
Locked-Rotor Current
The electric motor works on the principle of the attracting and repelling forces of ______ fields. (6.2)
Magnetic
A clamp-on ac ammeter has a coil that is clamped around the conductor and detects the rising and falling __________ field being produced due to the ac flow through the conductor. (6.1)
Magnetic Field
Factors that affect the available short-circuit current include circuit conductor _____. (15.7)
Material, Size, and Length
Because of the dangers associated with an open neutral conductor, the continuity of the ____ conductor cannot be dependent upon the receptacle. (13.8)
Neutral
A nonsinusoidal waveform is created when ____ loads distort the voltage and current sine wave. (16.6)
Nonlinear
To protect against electric shock or to prevent a fire, a dangerous ____ must quickly be removed by opening the circuits overcurrent protection device. (15.2)
Overload, Short Circuit, Or Ground Fault
The purpose of overcurrent protection is to protect conductors and equipment against excessive or dangerous temperatures because of overcurrent. Overcurrent is current in excess of the rated current of equipment or conductors. It may result from a(n) _____. (15.1)
Overload, Short Circuit, or Ground Fault
Improper wiring or mishandling of multiwire branch circuits can cause _____ connected to the circuit. (13.7)
Overloading of the neutral conductors and destruction of equipment because of overvoltage.
Dual-voltage ac motors are made with two field windings. The field windings are connected in _____ for low-voltage operation and in _____ for high-voltage operation. (20.2)
Parallel, Series
For ac circuits, the ac _____ of a conductor must be taken into consideration. (18.3)
Resistance
Ohmmeters measure the _____ or opposition to current flow of a circuit or component. (6.1)
Resistance
The megger is used to measure very high- ______ values, such as those found in cable insulation, or motor and transformer windings. (6.1)
Resistance
The _______ of a generator is forced to rotate while it is being subjected to the magnetic field of the stator. (6.3)
Rotor
The ________ of an ac generator contains the electromagnetic field, which cuts through the stationary conductor coils. (21.2)
Rotor
Equipment must have a(n) ____ current rating that permits the protection device to clear a short circuit or ground fault without extensive damage to the components of the circuit. (15.9)
Short Circuit
The magnetic time-delay circuit breaker operates on the solenoid principle where a movable core, held with a spring, is moved by the magnetic field of a(n) ___. (15.5)
Short Circuit or Ground Fault
When a motor starts, the current drawn is at least _____ times the motor FLA; this is known as motor "locked-rotor amperes" (LRA). (20.6)
Six
The expanding and collapsing magnetic field within the conductor induces a voltage in the conductors (CEMF) that repels the flowing electrons toward the surface of the conductor. This is called "_______." (18.3)
Skin Effect
In a(n) _______ motor, the rotor is locked in step with the rotating stator field and is dragged along at the speed of the rotating magnetic field. (20.14)
Synchronous
The ____ sensing element causes the circuit breaker to open when a predetermined calibration temperature is reached. (15.5)
Thermal
According to the Conventional Current Flow Theory, electrons travel from positive to negative. (7.3) True False
True
According to the Electron Current Flow Theory, electrons flow away from the negative terminal of the source, through the circuit and load, toward the positive terminal of the source. (7.2) True False
True
An AFCI-protection device provides protection from an arcing fault by recognizing the characteristics unique to an arcing fault and by functioning to de-energize the circuit when an arc fault is detected. (15.19) True False
True
Available short-circuit current is the current in amperes available at a given point in the electrical system. (15.7) True False
True
Because of conductor resistance, flux leakage, eddy currents, and hysteresis losses, not all of the input power is transferred to the secondary winding for useful purposes. (22.6) True False
True
Circuit breakers and fuses are intended to interrupt the circuit, and they must have an ampere interrupting rating (AIR) sufficient for the available short-circuit current. (15.8) True False
True
Electrons leaving a power supply are always trying to return to the same power supply; they are not trying to go into the earth. (14.1) True False
True
Even when power is removed from the circuit, capacitors can store large amounts of energy for a long period of time. They can discharge and arc if inadvertently shorted or grounded out. (17.3) True False
True
If a conductor carrying current is next to another conductor carrying current in the opposite direction, the electromagnetic field attempts to push the conductors apart. (5.3) True False
True
If the protection device is not rated to interrupt the available fault current at its listed voltage rating, it can explode while attempting to clear the fault. (15.8) True False
True
If the ungrounded conductors of a multiwire circuit are not terminated to different phases, this can cause the neutral current to be in excess of the neutral conductor rating. (13.3) True False
True
In a dc circuit, the only opposition to current flow is the physical resistance of the material. This opposition is called "reactance" and is measured in ohms. (9.6) True False
True
In an ac induction motor, the stator produces a rotating magnetic field that induces current in the rotor windings. The rotor current generates a magnetic field in opposition to the magnetic field of the stator, thereby causing the rotor to turn. (20.13) True False
True
It is not the force of the magnetic field through a conductor that produces electricity; it is the relative motion of the field to the electrons within the conductor that produces the movement of electrons. (4.2) True False
True
Kirchoff's Voltage Law states, "In a series circuit, the sum of the voltage drops across all of the resistors will equal the applied voltage." (10.2) True False
True
Lightning frequently terminates to a point of elevation and strikes nonmetallic as well as metallic objects with the same frequency. (1.7) True False
True
Metal parts of premises wiring must be bonded to a low-impedance path designed so that the circuit protection device will quickly open and clear a ground fault. (14.5) True False
True
People become injured and death occurs when voltage pushes electrons through the human body causing the heart to go into ventricular fibrillation. (4.4) True False
True
Providing a path to the earth often helps reduce electrostatic charge. (1.5) True False
True
Severe electric shock or death can occur if a person touches the ungrounded and the neutral conductors at the same time, even if the circuit is GFCI protected. (15.13) True False
True
The current flowing on the neutral conductor of a multi-wire circuit is called "unbalanced current". (13.5) True False
True
The energy transfer ability of a transformer is accomplished because the primary electromagnetic lines of force induce a voltage in the secondary winding. (22.1) True False
True
The heating from harmonic currents is proportional to the square of the harmonic current. (22.5) True False
True
The major advantage of ac over dc is the ease of voltage regulation by the use of a transformer. (9.2) True False
True
The opposition to current flow results in voltage drop. (10.2) True False
True
The severity of an electric shock is dependent on the current flowing through the body, which is impacted by circuit voltage and contact resistance. (4.4) True False
True
Typically, when a GFCI-protection device fails, the switching contacts remain closed and the device will continue to provide power without GFCI protection. (15.14) True False
True
Unsafe arcing faults can occur in one of two ways, as series arcing faults or as parallel arcing faults. The most dangerous is the parallel arcing fault. (15.17) True False
True
Voltage induced in the secondary winding of a transformer is dependent on the number of secondary turns as compared to the number of primary turns. (22.2) True False
True
When power supplies are connected in parallel, the voltage remains the same, but the current or amp-hour capacity will be increased. (11.5) True False
True
Swapping ______ of the line conductors can reverse a three-phase ac motor's rotation. (20.15)
Two
A GFCI is designed to protect persons against electric shock. It operates on the principles of monitoring the imbalance of current between the circuit's ____ conductor. (15.11)
Ungrounded and Neutral Conductors
_______ motors are fractional horsepower motors that operate equally well on ac and dc and are used for vacuum cleaners, electric drills, mixers, and light household appliances. (20.14)
Universal
When describing the relationship between voltage and current, the reference waveform is always _____. (16.11)
Voltage
Factors that affect the available short-circuit current include transformer _____. (15.7)
Voltage, kVA rating, and Impedance
Self-induced voltage opposes the change in current flowing in the conductor. This is called "inductive reactance" and it is abbreviated _________. (18.8)
XL
The opposition offered to the flow of ac current by a capacitor is called "capacitive reactance" which is expressed in ohms and abbreviated _______. (17.6)
Xc
The abbreviation for impedance is _________. (18.4)
Z
The formulas in the formula wheel apply to _____. (9.9)
dc or ac circuits
Transformer are rated in ________. (22.8)
kVA