Electrical Theory
The total resistance of a parallel circuit can be calculated by the ____ method.
(All o these) equal resistance product-over-sum reciprocal
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 ______.
(All of these) CEMF counter-electromotive force back-EMF
For ac circuits, the ac ______ of a conductor must be taken into consideration.
(All of these) eddy currents skin effect resistance
Improper wiring or mishandling of multiwire branch circuits can cause _____ connected to the circuit.
(All of these) overloading of the neutral conductors destruction of equipment because of overvoltage
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) ______
(All of these) short circuit ground fault
A GFCI is designed to protect persons against electric shock. It operates on the principle of monitoring the imbalance of current between the circuit's ______ conductor.
(All of these) ungrounded neutral
Factors that impact the available short-circuit current include transformer ______.
(All of these) voltage kVA rating impedance
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 ohms and current flow of 16A?
Cost per Year = Power for the Year in kWh * 0.08 Power per Hour = I^2 * R I = 16A R = 0.40 ohms Power per Hour = (16A * 16A) * 0.40 ohms Power per Hour = 102.40W Power for the Year in kWh = (102.40W * 24 hours * 365 days)/ 1,000 Power for the Year in kWh = 897 kWh Cost per Year = 897 kWh * $0.08 Cost per Year = $71.76
In a dc circuit, the only opposition to current flow is the physical resistance of the material. This opposition is called reactance and it is measured in ohms.
False
Lightning protection is intended to protect the building structure itself, as well as the electrical equipment on or inside the building structure.
False
The best conductors, in order of their conductivity, are: gold, silver, copper, and aluminum.
False
The motor FLA rating is used when sizing motor conductor size or branch circuit short-circuit and ground-fault protection.
False
The termination of the lightning strike is unlikely to ignite combustible materials.
False
To prevent fires and electric shock, the NEC specifies that neutral current can flow on metal parts of the electrical system.
False
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.
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.
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.
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.
True
If a conductor carrying current is next to another conductor carrying in the opposite direction, the electromagnetic field attempts to push the conductors apart.
True
Lightning frequently terminates to a point of elevation and it strikes nonmetallic as well as metallic objects with the same frequency.
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.
True
One major advantage of ac over dc is the ease of voltage regulation by the use of a transformer.
True
People become injured and death might occur when voltage pushes electrons through the human body causing the heart to go into ventricular fibrillation.
True
Providing a path to the earth often helps reduce electrostatic charge.
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.
True
How many 20A, 120V circuits are required for forty-two, 300W electric discharge luminaires (noncontinuous load) that have a power factor of 85 percent?
7 circuits VA per Circuit = Volts * Amperes VA per Circuit = 120V * 20A VA per Circuit = 2,400 VA VA per Luminaire = Watts/Power Factor VA per Luminaire = 300W/0.85 PF VA per Luminaire = 353 VA Lights per Circuit = 2,400 VA/353 VA = 6.80 Lights per Circuit = 6 Circuits = 42 luminaires/6 per circuit Circuits = 7
Magnetic lines of force can cross each other and they are called flux lines.
False
Nonmagnetic metals are ferrous, meaning they do not contain any iron, and cannot be magnetized.
False
According to the conventional current flow theory, electric current travels from positive to negative.
True
The opposition to current flow results in voltage drop.
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.
True
Typically, when a GFCI protection device fails, the switching contacts remain closed and the device will continue to provide power without GFCI protection.
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.
True
Factors that impact the available short-circuit current include circuit conductor ______.
(All of these) material size length
The purpose of overcurrent protection is to protect the conductors and equipment against excessive or dangerous temperatures because of overcurrent. Overcurrent is current is excess of the rated current of equipment or conductors. It may result from a(n) _____
(All of these) overload short circuit ground fault
To protect against electric shock or to prevent a fire, a dangerous _____ must be removed by opening the circuit's overcurrent device.
(All of these) overload short circuit ground fault
A balanced 3-wire, 120/240V, single-phase circuit is connected so that the ungrounded conductors are from different transformer lines (Line 1 and Line 2). The current on the grounded conductor will be _____ percent of the ungrounded conductor current.
0 (zero)
An electrical arc blast can approach ________, which vaporizes metal parts and produce an explosive and deadly pressure wave.
30,000 Degrees Fahrenheit
Three-phase, ______, wye-connected systems can overheat because of circulating odd triplen harmonic currents.
4-wire
Arcing is defined as a luminous discharge of electricity across an insulating medium. Electric arcs operate at temperatures between ______ and expel small particles of very hot molten materials.
5,000 and 15,000 Degrees Fahrenheit
What is the nameplate FLA for a 20 hp, 208V, three-phase motor with 90 percent power factor and 80 percent efficiency?
58A FLA = (hp * 746W)/(V * 1.732 * PF * Eff) FLA = (20 hp * 746W)/(208V * 1.732 * 0.90 PF * 0.80 Eff) FLA = 58A
When a motor starts, the current drawn is at least ______ times the motor FLA; this is known as motor locked-rotor current (LRC)
6
What is the voltage drop of two 12 AWG conductors ( 0.40 ohms ) supplying a 16A load, located 100 ft from the power supply?
E vd = I * R E vd = 16A * 0.40 ohms E vd = 6.40V
______ can be reduced by dividing the core into many flat sections or laminations.
Eddy currents
A GFCI protection 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.
False
A holding relay is primarily used for worker convenience.
False
AC inductive or capacitive reactive loads cause the voltage and current to be in-phase with each other.
False
Inverse time breakers operate on the principle that as the current decreases, the time it takes for the device to open decreases.
False
If the rotating part of the motor winding is jammed so that it cannot rotate, no CEMF will be produced in the motor winding. Result —- the motor operates at ______ and the windings will be destroyed by excessive heat.
LRC
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.
Magnetic
The electric motor works on the principle of the attracting and repelling forces of _____ fields.
Magnetic
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?
P = I * E I = 12A E = 120V * 3% E = 3.60V P = 12A * 3.60V P = 43.20W
What is the power loss in watts of a conductor that carries 24A and has a voltage drop of 7.20V?
P = I * E P = 24A * 7.20V P = 172.80W
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?
P = I^2 * R I = 16A R = 0.40 ohms P = (16A * 16A) * 0.40 ohms P = 102.40W
If the protection device is not rated to interrupt the current at the available fault values at its listed voltage rating, it could explode while attempting to clear the fault.
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.
True
In the 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.
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.
True
Kirchoff's Voltage Law states that in a series circuit, the sum of the voltage drops across all of the resistors will equal the applied voltage.
True
The current flowing on the neutral conductor of a multiwire circuit is called unbalanced current.
True
The energy transfer ability of a transformer is accomplished because the primary electromagnetic lines of force induce a voltage in the secondary winding.
True
The heating from harmonic currents is proportional to the square of the harmonic order.
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.
True
When power supplies are connected in parallel, the voltage remains the same, but the current or amp-hour capacity will be increased.
True
Three-phase ac generators have three equally spaced windings, ______ out-of-phase with each other.
120 degrees
If the primary phase voltage is 480V and the secondary phase voltage is 240V, the turns ratio is ______.
2:1
The _____ of a generator is forced to rotate while it is being subjected to the magnetic field of the stator.
Rotor
What is the approximate power consumed by a 10 kW heat strip rated 230 V, when connected to a 208V circuit?
The power of the heat strip will be less because the applied voltage (208V) is less than the equipment voltage rating (230V). To calculate this, we must determine the heat strip resistance rating at 230V, and then determine the power rating at 208V based on the heat strip resistance rating. P = E^2/R E = Applied Voltage = 208V R = Resistance of Heat Strip = E^2/P Heat Strip Voltage Rating = 230V Power Rating of Heat Strip = 10,000W Resistance of Heat Strip = 230V^2/10,000W Resistance of Heat Strip = 5.29 ohms P = E^2/R P = (208V x 208V)/5.29 ohms P = 43,624/5.29 ohms P = 8,178W/1,000 = 8.20 kW
______ 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.
Universal
The opposition offered to the flow of ac current by a capacitor is called capacitive reactance, which is expressed in ohms and abbreviated ______.
XC
Self-induced voltage opposes the change in current flowing in the conductor. This is called inductive reactance and is abbreviated ______.
XL
The abbreviation for impedance is _____.
Z
The formulas in the power wheel apply to _____
a) dc b) ac with unity power factor
Wasteful circulating ______ in the iron core cause(s) the core to heat up without any useful purpose.
eddy currents
Because of the dangers associated with an open grounded conductor, the continuity of the _____ conductor of a multiwire branch circuit cannot be dependent upon the receptacle.
grounded
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.
hysteresis
Transformers are rated in ______.
kVA
The ______ of an ac generator contains the electromagnetic field, which cuts through the stationary conductor coils.
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.
short-circuit
The effective value is equal to the peak value ______.
times 0.707
Swapping ______ of the line conductors can reverse a three-phase ac motor's rotation.
two
When describing the relationship between voltage and current, the reference waveform is always ______.
voltage
If the output is 1,600W and the equipment is 88 percent efficient, what are the input amperes at 120V
15A Input = Output/Efficiency Input = 1,600W/0.88 Eff Input = 1,818W Input Amperes = Watts/Volts Input Amperes = 1,818W/120V Input Amperes = 15.167A
What is the power consumed by a 10 kW heat strip rated 230V connected to a 115V circuit?
2.50 kW The power of the heat strip will be less because the applied voltage (115V) is less than the equipment voltage rating (230V). To calculate this, we must determine the heat strip resistance rating at 230V, and then determine the power rating at 115V based on the heat strip resistance rating. P = E2/R E = Applied Voltage = 115V R = Resistance of Heat Strip = E2/P Heat Strip Voltage Rating = 230V Power Rating of Heat Strip = 10,000W Resistance of Heat Strip = 230V2/10,000W Resistance of Heat Strip = 5.29 ohms P = E2/R P = 13,225/5.29 ohms P = 2,500W/1,000 P = 2.50 kW Note: Power changes with the square of the voltage. If the voltage is reduced to 50%, then the power consumed will be equal to the new voltage percent2 or 50%2, or 10,000 * (0.50 * 0.50 = 0.25 = 25%) = 2,500W
What size transformer is required for a 100A, 240V, single-phase noncontinuous load that has a power factor of 85 percent?
25 kVA Transformer kVA = (Volts * Amperes)/ 1,000 Transformer kVA = (240V * 100A)/ 1,000 Transformer kVA = 24 Note: Transformers should be sized using the apparent power, which is VA = E * I
What is the resistance of the circuit conductors when the conductor voltage drop is 7.20V and the current flow is 50A?
R = E/I R = 7.20V/50A R = 0.14 ohms
Ohmeters measure the ____ or opposition to current flow of a circuit or component.
Resistance
The megger is used to measure very high- _____ values, such as those found in cable insulation, or motor and transformer windings.
Resistance
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.
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.
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.
True
Available short-circuit current in amperes that is available at a given point in the electrical system.
True
The total opposition to current flow in ac circuits is called _____ and is measured in ohms.
impedance
A nonsinusoidal waveform is created when ______ loads distort the voltage and current sine wave.
nonlinear
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.
parallel, series
Because of the earth's high resistance to current flow, it cannot be used for the purpose of clearing a line-to-case ground fault for _____ wiring.
premises
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 ______.
skin effect
In a(n) ______ motor, the rotor is actually locked in step with the rotating stator field and is dragged along at the speed of the rotating magnetic field.
synchronous
Kirchoff's Current Law states that in a series circuit, the current is _____ through the transformer, the conductors, and the appliance.
the same
The _____ sensing element causes the circuit breaker to open when a predetermined calibration temperature is reached.
thermo
