Electrical General 1 Flashcards
(244 cards)
1
Q
What is a capacitor?
A
A capacitor is an electronic component that stores electrical energy in an electric field
2
Q
What is the basic construction of a capacitor?
A
The basic structure of a capacitor consists of two conductive plates separated by a dielectric material
3
Q
What does the word dielectric refer to?
A
a dielectric refers to an insulating material that is placed between the conductive plates of a capacitor
4
Q
What is voltage?
A
It is a measure of the electric potential or the force that drives electric current through a conductor. It can be easily described as pressure.
5
Q
What is current?
A
the flow of electric charge in a conductor. It is the rate at which electric charges move through a circuit
6
Q
What is resistance ?
A
the opposition offered by a material or component to the flow of electric current. It is a measure of how effectively a material resists the passage of electrical charges
7
Q
What is the basic calculation for Resistance?
A
V/I
8
Q
What is the basic calculation for Current?
A
V/R
9
Q
What is the basic calculation for Voltage?
A
I x R
10
Q
What is the unit for Current?
A
Amperes
11
Q
What is the unit for Resistance?
A
Ohms Ω
12
Q
What is the unit for Voltage?
A
Volts ( V )
13
Q
What is the relationship between Volts, Current and Resistance?
A
The relationship between voltage (V), current (I), and resistance (R) is described by Ohm’s Law, which states that the current flowing through a conductor is directly proportional to the voltage applied across it and inversely proportional to the resistance. Mathematically, Ohm’s Law can be expressed as V = I × R or I = V / R.
14
Q
What is an electron?
A
An electron is a subatomic particle that carries a negative electric charge. It is one of the fundamental particles that make up atoms
15
Q
What is a proton?
A
A proton is a subatomic particle that is one of the fundamental building blocks of matter. It carries a positive electric charge and is found in the nucleus of an atom
16
Q
What is a Neutron?
A
A neutron is a subatomic particle that is one of the fundamental constituents of an atom. It is electrically neutral, meaning it carries no net electric charge
17
Q
How many Watts are in 1 Horsepower?
A
One horsepower is equivalent to approximately 745.7 watts
18
Q
What is a diode?
A
A diode is an electronic component that allows current to flow in one direction while blocking it in the opposite direction. It is a two-terminal device with a characteristic called the “p-n junction,” which is formed by combining two differently doped semiconductor materials.
19
Q
What is a Centrifugal switch?
A
A centrifugal switch is an electromechanical device used in single-phase induction motors. It operates based on the centrifugal force generated by the motor’s rotation, opening and closing the switch contacts at specific speeds. The centrifugal switch helps control the starting and stopping of the motor, improving its efficiency and protecting it from damage.
The purpose of a centrifugal switch is to disconnect certain motor components from the power supply once the motor reaches a predetermined speed
20
Q
What is a Zener diode?
A
A Zener diode is a specialized type of diode that is designed to operate in the reverse breakdown region of its voltage-current characteristic curve. Unlike regular diodes that are primarily used for rectification or blocking current flow, Zener diodes are specifically engineered to provide precise voltage regulation and voltage reference in electronic circuits.
The key characteristic of a Zener diode is its ability to maintain a nearly constant voltage across its terminals, called the Zener voltage (VZ), when it is reverse biased and operated in the breakdown region. The breakdown voltage of a Zener diode can be precisely controlled during manufacturing, allowing it to be selected for specific voltage requirements.
21
Q
What is Valence?
A
Valence refers to the number of electrons that an atom can gain, lose, or share.
Valence: Outer electrons affect electricity flow.
Few = good conductor (metal);
Many = poor conductor (insulator).
22
Q
What is Covalence?
A
Covalence, also known as the covalent bond, refers to the sharing of electron pairs between atoms in a chemical bond. It is a type of chemical bond characterized by the mutual sharing of electrons to achieve a more stable electron configuration for the participating atoms.
In a covalent bond, two or more atoms share one or more pairs of electrons. This shared electron pair occupies the overlapping region between the atomic orbitals of the bonded atoms. By sharing electrons, the participating atoms can achieve a more complete or stable valence shell, similar to that of a noble gas configuration
23
Q
What is doping ?
A
The deliberate introduction of impurities into a semiconductor material to modify its electrical properties. Semiconductors are materials that have an intermediate level of electrical conductivity, between conductors (such as metals) and insulators (such as nonmetals).
24
Q
Are primary cells of batteries rechargeable?
A
No,
primary cells, are non-rechargeable. Once the chemical reactions within a primary cell are completed and the reactants are consumed, the cell cannot be recharged or restored to its original state. Primary cells are designed for single-use applications and are commonly found in devices such as remote controls, flashlights, and disposable electronic devices.
25
Are secondary cells of batteries rechargeable?
Yes,
rechargeable batteries, also known as secondary cells, are designed to be recharged multiple times. They can undergo reversible chemical reactions, allowing them to be discharged (used) and then recharged by applying an external electrical energy source. This recharging process replenishes the reactants within the battery, restoring its capacity to deliver electrical energy.
Rechargeable batteries are widely used in various applications, including portable electronics, electric vehicles, renewable energy systems, and many other devices where repeated charging and discharging are required.
26
What is a motor?
A motor is a device that converts electrical energy into mechanical energy. It is a machine that generates motion and is widely used in various applications, ranging from industrial machinery to household appliances.
27
What is an alternator?
An alternator is a type of electrical generator that converts mechanical energy into electrical energy. Unlike a motor that uses electrical energy to produce mechanical motion, an alternator operates in the reverse manner. It takes mechanical power, typically provided by an engine or a turbine, and converts it into alternating current (AC) electrical power.
28
What is the function on an RCD?
The primary function of an RCD is to monitor the flow of electrical current in a circuit and quickly interrupt the power supply if it detects a leakage or imbalance of current
29
What is the function of a MCB, in the way it protects circuits.
The primary function of an MCB is to detect abnormal electrical conditions, such as excessive current flow, and quickly interrupt the circuit to prevent damage to electrical equipment, fire hazards, or electrical accidents.
Overload Protection: MCBs have a built-in thermal trip mechanism that responds to sustained overcurrent conditions. If the current exceeds the rated limit of the MCB for a specified period, the thermal trip mechanism heats up and trips the MCB, cutting off the power supply to the circuit.
Short Circuit Protection: MCBs also provide protection against short circuits, which occur when a high amount of current flows through a circuit due to a fault. MCBs have a magnetic trip mechanism that quickly detects the sudden rise in current and trips the MCB to isolate the faulty circuit.
30
What is an EFLI?
Earth Fault Loop Impedance (EFLI) is a term used in electrical to describe the impedance or resistance to the flow of fault current in the earth path of an electrical circuit. It is a crucial parameter used to assess the effectiveness of the earthing system and the ability to clear a fault in a timely manner.
31
Describe a short circuit.
A short circuit refers to an abnormal electrical condition that occurs when a low-resistance path or direct connection is established between two points in an electrical circuit that are intended to have different potentials. In simpler terms, a short circuit bypasses the normal load or resistance in a circuit, allowing an excessive amount of current to flow.
32
What is synchronous speed?
Synchronous speed refers to the speed at which the magnetic field of a synchronous motor rotates. It is determined by the frequency of the power supply and the number of magnetic poles in the motor.
In an AC (alternating current) synchronous motor, the rotation of the magnetic field is synchronized with the frequency of the AC power supply. The synchronous speed (Ns) of the motor is given by the following formula:
Ns = (120 * f) / P
Where:
Ns is the synchronous speed in revolutions per minute (RPM).
f is the frequency of the power supply in hertz (Hz).
P is the number of magnetic poles in the motor
33
What is frequency?
Frequency refers to the number of occurrences of a repeating event per unit of time. It is a fundamental concept in physics.
In the context of time-varying phenomena, such as waves or oscillations, frequency measures how rapidly the event or cycle repeats within a given time frame. It is typically measured in hertz (Hz), which represents the number of cycles or oscillations per second.
34
What is known as an overload in a circuit containing cables.
an overload refers to a situation where the current flowing through a circuit exceeds its designed capacity or the rated load it can handle. It occurs when the amount of electrical current drawn by the devices or appliances connected to the circuit exceeds the safe operating limit
35
What is a potential cause of an overload?
An overload can happen due to various reasons, including:
Excessive Load: Connecting too many electrical devices or appliances to a single circuit can cause an overload. If the combined power consumption of the devices exceeds the circuit's capacity, it leads to an excessive current flow.
Malfunctioning Equipment: Faulty or malfunctioning equipment can draw more current than usual, causing an overload in the circuit it is connected to.
Short Circuits: Short circuits, which create a direct connection between the live and neutral wires or between the live wire and ground, can result in an overload condition as a large amount of current flows through the fault path.
Inadequate Wiring: Using insufficiently sized wires for the electrical installation can limit the current-carrying capacity of the circuit, leading to an overload if the demand exceeds the wire's capacity.
36
What is the ratio of ladder safety?
Horizontal:Vertical
1:4
37
What is maximum demand?
Maximum demand, in the context of electrical power systems, refers to the highest amount of power or electrical load that is drawn from a system or a specific circuit during a specific period. It represents the peak power demand experienced over a defined time interval.
The maximum demand is typically measured in kilowatts (kW) or kilovolt-amperes (kVA), and it is an important factor in designing, sizing, and managing electrical systems. Knowing the maximum demand helps in determining the capacity requirements of power generation, distribution equipment, and wiring systems.
38
What is close protection?
Close protection devices operate within 4 hours at 1.5 times its rated current.
39
What is coarse protection?
Coarse protection devices operate outside a 4 hour period at 1.5 times the rated current.
40
What is minimum depth of an earth stake?
1.8m
41
What is the minimum depth of a buried earth conductor
500mm
42
What is the equation for adding resistance in parallel?
When resistors are connected in parallel, the total resistance of the combination can be calculated using the following equation:
1/R_total = 1/R1 + 1/R2 + 1/R3
where:
R_total is the total resistance of the parallel combination.
R1, R2, R3, ... Rn are the individual resistances connected in parallel.
43
An ammeter should always be connected in
Series
44
A voltmeter should always be connected in
Parallel
45
What is the purpose of the main earth conductor?
the main earth cable, often referred to as the earthing or grounding conductor, is designed to provide a low-resistance path for electrical fault currents to flow safely into the ground. The resistance of the earth cable is crucial to ensure the effectiveness of the grounding system in maintaining electrical safety.
46
What is the maximum resistance a main earthing cable can be in ohms?
0.5 Ohms
47
What is a class one appliance?
Class I appliances have a provision for an earth wire or grounding conductor. This wire connects the metallic parts of the appliance to an earth terminal or ground, thereby providing a path for fault currents to flow safely to the ground
48
What is a class 2 appliance?
A Class II appliance, also known as a Class 2 appliance, refers to an electrical device or equipment that is designed with double insulation or reinforced insulation as a safety measure. These appliances are built to provide an enhanced level of protection against electric shocks without relying on an earth connection
49
What is an isolating transformer?
An isolating transformer, also known as an isolation transformer, is a type of transformer that is specifically designed to provide electrical isolation between the input and output sides of the transformer. It is used to separate the primary (input) circuit from the secondary (output) circuit, ensuring that there is no direct electrical connection between them
50
Why don’t isolating transformers typically have an earth cable?
If an isolating transformer were to have an earth connection, it would introduce a potential path for electrical current to flow between the primary and secondary sides. This would compromise the isolation and defeat the purpose of the transformer. The earth connection could create a conductive path for faults or surges to bypass the transformer's isolation barrier, potentially causing electric shock hazards or damaging equipment
51
What is Peak inverse voltage as applied to a semi conductor device?
The maximum voltage a semi conductor device can withstand in a reverse bias.
52
How do you regulate voltage on a power transformer with varying loads
With the use of a tap changer.
53
In a big installation, how many boards should contain a MEN link?
One. Normally the one closest to the supply.
54
What are two main functions of the ballast in a fluorescent light fitting
Provide a starting voltage.
Limit current in the circuit.
55
A main earth tag must always be located at…
The main earthing conductor.
56
What are Lumens?
Lumens are a unit of measurement used to quantify the total amount of visible light emitted by a light source. In simpler terms, lumens measure the brightness of light as perceived by the human eye. The higher the number of lumens, the brighter the light source appears.
57
What starters control the speed of a three-phase a.c induction motor?
Electronic variable speed drive
Secondary resistance starter
58
What components are used to filter the pulsating output of a rectifier?
Inductors
Resistors
Capacitors
59
What is the definition of impedance as applied to an a.c circuit
The total opposition in ohms to current flow in an ac circuit
60
What is a diac?
A diac, short for "diode alternating current," is a two-terminal electronic device that conducts electrical current in both directions.
The main purpose of a diac is to provide a trigger or gate signal to the thyristor when the voltage across it reaches a specific level. When the voltage exceeds the diac's breakover voltage, it rapidly switches from a high-resistance state to a low-resistance state, allowing current to flow through it. This sudden change in resistance triggers the thyristor into conduction, allowing a larger current to flow through the circuit.
61
What is a Triac?
A triac is a three-terminal semiconductor device that is widely used for controlling alternating current (AC) power.
The term "triac" is derived from its structure, which combines elements of a thyristor and a diac. It consists of two thyristor structures connected in parallel and triggered by a single gate terminal. The main purpose of a triac is to enable the control of AC power by regulating the flow of current in both directions.
By controlling the triggering of the triac at different points in each half-cycle of the AC waveform, the conduction angle can be varied, allowing for the control of power flow to connected devices. This makes triacs suitable for applications such as lamp dimmers, motor speed controllers, heating control systems, and AC power switches.
One important characteristic of triacs is that they can conduct current in either direction, making them suitable for AC circuits where the current periodically changes direction. This bidirectional conduction capability sets triacs apart from other thyristors, such as silicon-controlled rectifiers (SCRs), which can only conduct current in one direction.
62
How do Dimmers work?
Dimmers are devices used to control the brightness or intensity of electric lights, allowing users to adjust the level of illumination according to their preference. They work by altering the amount of electrical power supplied to the light source, effectively varying its brightness.
Traditional dimmers typically use a technique called phase control or phase-cut dimming, which is based on altering the waveform of the alternating current (AC) supplied to the light source. Here's how it works:
Triac-based Dimming: The most common type of dimmer is based on a triac, as mentioned earlier. The dimmer is connected in series with the light source, interrupting the flow of current to the light. By adjusting the point in each half-cycle of the AC waveform at which the triac is triggered, the amount of power reaching the light can be controlled.
Leading Edge Dimming: In leading-edge dimmers, the triac is triggered near the beginning of each half-cycle of the AC waveform. By doing so, a portion of the waveform is cut off, reducing the amount of power delivered to the light source. This method is commonly used for incandescent bulbs.
Trailing Edge Dimming: In trailing-edge dimmers, the triac is triggered near the end of each half-cycle of the AC waveform. This method is typically used for dimming LED and CFL (compact fluorescent lamp) bulbs. Trailing-edge dimmers often employ additional electronic circuitry to provide a smoother dimming experience and prevent flickering.
63
Define:
Alternating Current.
Alternating current (AC) is an electric current that periodically changes direction. It is the most common type of electrical power used in homes, businesses, and industrial settings. AC current is characterized by its sinusoidal waveform, which represents the variation of voltage or current over time.
In an AC circuit, the flow of electric charge alternates back and forth, reversing its direction at regular intervals. This is in contrast to direct current (DC), where the flow of electric charge is unidirectional.
64
Define Direct Current
DC stands for direct current. It refers to the steady flow of electric charge in a single direction. Unlike alternating current (AC), which periodically changes direction, direct current maintains a constant polarity and magnitude.
In a DC circuit, the electric current flows continuously from the positive terminal (anode) to the negative terminal (cathode) of a power source, such as a battery or a DC power supply. The flow of electrons is unidirectional, meaning it does not reverse its direction over time.
65
What is a resistor?
A resistor is an electrical component that is used to restrict or control the flow of electric current in a circuit. It is a passive two-terminal device that dissipates energy in the form of heat when an electric current passes through it.
The main function of a resistor is to provide resistance to the flow of electrons. Resistance is the property that opposes the flow of current and is measured in ohms (Ω). A higher resistance value indicates a greater opposition to current flow.
Resistors are typically made from materials with high resistivity, such as carbon composition, metal film, or wire-wound materials. They come in various shapes and sizes, including cylindrical, rectangular, or surface-mount packages, depending on the specific application.
66
A 4 pole 50Hz induction motor has 4% slip. The motor speed would be?
120xF = 6000
6000/Number of poles = 1500
1500 x 0.96 ( 4% Slip ) = 1440 RPM
67
True or false.
Is switching off the light a safe method to use before replacing the light fitting in a domestic electrical installation?
False
68
How is the direction of a three-phase delta connected ac induction motor changed?
Interchange any two motor supply connections
69
A 230V/230V portable isolating transformer designed for personal protection has been repaired and tested to AS/NZS3760. What insulation resistance test value would permit the isolating transformer to be returned to service
>1 MΩ
70
In New Zealand, what type of RCD is required to be voltage dependent
Portable Residual Current Device
71
In what form is the voltage output from a PWM converter?
A series of rectangular pulses
72
Define the term Peak Inverse Voltage as applied to a semiconductor device?
Maximum voltage semiconductor device can withstand in reverse bias.
73
What is the standard maximum secondary voltage for a voltage Transformer
110V
74
What is the relationship between the available torque at a given speed and the voltage at an induction motor terminals?
Torque is proportional to voltage squared
75
What is torque?
Torque is a physical quantity that describes the rotational force applied to an object. It is a measure of the turning or twisting force that can cause an object to rotate around an axis. Torque is commonly represented by the symbol "τ" (tau) and is measured in units of newton-meters (N·m) or foot-pounds (ft·lb).
76
State the main reason why interlocks are used in a star/delta starter.
To prevent the star and delta contactors closing simultaneously
77
Thermistors are fitted to the windings of a three-phase induction motor. How do they provide protection to the motor when a mechanical overload occurs?
The thermistor detects a rise in temperature in the windings. It opens the control circuit when a predetermined temperature has been exceeded
78
What is the typical tripping current of an RCD used for personal protection?
30mA
79
What is the maximum permitted rated residual current of an RCD installed for Personal protection in a domestic installation
30mA
80
What is the maximum permitted rated residual current of an RCD installed in a medical electrical installation
10mA
81
What is the maximum permitted rated residual current of an RCD installed for the protection of property
300mA
82
State the minimum Insulation Resistance of an electrical installation that does not contain MIMS heating elements
1 MΩ
83
What is the formula for calculating the frequency of an alternator?
F = NumberOfPoles/60
F = NP/60
84
What is the main purpose of a fault loop impedance test at the main switchboard of a low voltage electrical installation?
Confirm that the protective device can safely interrupt the installation PSCC
85
What does a PSCC stand for, and what is it?
A prospective short circuit current refers to the calculation or assessment of the potential electrical current that could flow in the event of a short circuit in an electrical system. It is also known as a short circuit current or fault current
Knowing the prospective short circuit current helps in selecting and setting up protective devices that can quickly detect and interrupt the excessive current, preventing potential damage or hazards. It is an important consideration in electrical system design, maintenance, and safety planning.
86
An earth loop impedance test result of 0.32Ω has been recorded on a 230V final sub-circuit supplying socket outlets in a low voltage electrical installation.
Calculate the fault current that will flow if a short circuit occurs on the final sub-circuit
230V/0.32Ω
= 718.75 A
87
For it to operate correctly, how should a Zener Diode be connected?
Reversed biased.
88
What is a transducer?
A transducer is a device or instrument that converts one form of energy into another. It takes input in one form, such as electrical, mechanical, or acoustic energy, and transforms it into a corresponding output in a different form. For example, a microphone is a transducer that converts sound waves (mechanical energy) into electrical signals, while a speaker is a transducer that converts electrical signals back into sound waves. Transducers are widely used in various applications, including sensors, speakers, microphones, and more, to facilitate the conversion of energy between different forms.
89
Why are Transformer cores laminated?
Reduces Eddy current losses
90
What are Eddy Currents?
Eddy currents are circulating currents that are induced in conductive materials when exposed to a changing magnetic field. These currents flow in closed loops and create their own magnetic fields, which oppose the original magnetic field that generated them. Eddy currents are primarily a result of electromagnetic induction
Efforts are often made to minimize the effects of eddy currents in various applications. Techniques such as laminating or using materials with low electrical conductivity (such as laminated iron cores in transformers) are employed to reduce the magnitude of eddy currents and their associated energy loss.
91
What are Hysteresis losses?
Hysteresis losses in a transformer refer to the energy losses that occur due to magnetic hysteresis in the transformer's core material. Transformers use a core made of ferromagnetic material (such as iron or steel) to efficiently transfer electrical energy from one coil to another through magnetic induction. However, during the operation of the transformer, the changing magnetic field induces hysteresis losses in the core material.
Hysteresis losses contribute to the overall inefficiency of a transformer. They lead to energy wastage and generate heat in the transformer's core. To mitigate hysteresis losses, transformer cores are often made from materials with low coercivity and low hysteresis loss properties, such as transformer-grade silicon steel. Additionally, the design of the transformer is optimized to reduce the magnetic field variations and minimize the hysteresis losses, improving the transformer's efficiency.
92
What is Pull-Out Torque as it applies to an AC induction motor?
The maximum value of torque the motor can produce.
93
What could be the consequences when carrying out an insulation resistance test on electrical equipment containing semi-conductor devices with the tester set on 500V DC?
The voltage of the test instrument may damage the semi-conductor device.
94
What does the term discrimination mean as applied to protective devices?
The protective device closest to the fault will operate first
95
In a star connected Electrical system state the angle between line voltage and phase voltage.
30 degrees
96
What is a ballast?
A ballast refers to a device used in lighting systems to regulate the electrical current flowing through a gas discharge lamp, such as fluorescent lamps, high-intensity discharge (HID) lamps, or neon lamps. The primary purpose of a ballast is to provide the lamp with the necessary voltage, current, and starting conditions for proper operation.
97
Define the term 'clearance time'
Total time for any fuse to operate and clear a fault
98
define the term luminous efficacy
The ratio of luminous flux emitted by a lamp to the power consumed by the lamp.
99
Define the term synchronous speed as applied to an a.c. motor
Speed of the rotating magnetic field
100
What is the stroboscopic effect?
the stroboscopic effect in electrical terms refers to the synchronization between the frequency of a flashing light source and the apparent motion of a rapidly moving object, leading to the perception of stationary or slowed-down motion due to the persistence of vision
101
What do the initials BJT stand for as applied to semiconductors?
Bi-Polar Junction Transistor
102
Why are induction motor rotor bars normally skewed?
To give smoother quieter running and more uniform torque
103
Define the term 'luminous flux'.
the quantity of light emitted by a light source
104
How are split - phase motors reversed?
Reverse either the run or the start windings
105
What does the term inverse time-current characteristic mean?
Inverse time tripping is a characteristic of circuit breakers in which the breaker trips in more time with lower overcurrent, and less time with higher overcurrent
106
True or false,
An isolating transformer protects the user from a phase to neutral shock.
False
107
Transmission lines are typically made of?
Aluminium
108
As a conductor length decreases, resistance..
Decreases
109
Who issues the trainee licenses in New Zealand?
EWRB
110
A fellow apprentice has held his trainee licence for 5 years. Is it still valid?
No
111
An inductive load causes the current flowing in a circuit to _____ the voltage.
Lag
112
What can be the result of capacitive reactance of the corrective capacitor becoming equal with the inductive reactance of the load?
Resonance can occur
113
What is an advantage of an HRC fuse compared to an MCB?
It has a higher rupturing capacity
114
What does the sensitivity of an RCD relate to?
The minimum difference in phase and neutral current required to trip the RCD.
115
What is “electrical power”
The rate at which electrical energy is expended
116
You have tested the earthing conductor of a Class 1 appliance and the result is 0.7Ω
Is this acceptable?
Yes.
117
A Semi-conductor crystal having an excess of electrons after doping is known as..?
N-Type Semiconductor
118
What percentage charge is a capacitor said to hold (in relation to full charge) after ONE time constant?
63%
119
What is the minimum Category rating your test instrument must have when measuring voltages at the main switchboard of an electrical installation?
CAT III
120
Eddy currents in a transformer can be minimised by?
Laminating the core of the transformer
121
What type of fire extinguishers should you use on an electrical fire ?
CO2
Dry Powder
122
What happens to cable current ratings when they are bundled together, i.e tied along a catenary wire?
It decreases
123
What is the minimum distance that extra low voltage cables (such as data or telephone cables) should be installed near low voltage cables in an installation without a barrier in between?
50mm
124
According to AS/NZS 3000 can an earthing conductor be used as an “active” (Phase) conductor?
No.
125
In regards to testing, describe the prove - test - prove method
• The test instrument is proved that it works on a known supply
• The test is carried out
• The test instrument is proved that it still works on a known
supply
126
Why is the prove test prove method used?
To ensure the test instrument works prior to carrying out the test and then retested to ensure it has not failed during the test
127
Why are incandescent lamps so inefficient (poor efficacy)?
most of the energy is converted to heat instead of light
128
Would changing the dielectric material in a capacitor effect its characteristics?
Yes
129
Why should conductive surfaces such as benchtops have equipotential bonding when close (within arms-reach) to electrical equipment such as a class one appliance?
To ensure there is no rise in potential voltage between the two which may cause a shock
130
Refer to Section 5 of AS/NZS 3000 and state the minimum length for New Zealand of a strip type earth electrode.
7.5M
131
What is the main purpose of the HVDC system in New Zealand?
To Link the supply of electricity between the North and South Island
132
Does the North or South Island produce more power than it consumes?
South Island
133
Can rewireable fuses be installed in a new installation?
No.
134
What is the typical breaking capacity (KA rating) of a rewireable fuse?
1KA
135
Can a RCD be used instead of a circuit breaker or fuse?
No.
136
When designing control circuits, emergency stops are always connected in ____________.
Series.
137
Motor overloads should always be set to what load current of the motor.
The full load current of the motor.
138
What changes the output colour of fluorescent lights?
the phosphorus coating in the tube
139
Why do single phase induction motors require a start winding?
a start winding is used to simulate a rotating magnetic field to help start the motor
140
What is the unit for battery capacity?
Ampere hour
141
How many windings does an Autotransformer have?
One
142
What is an auto transformer?
An autotransformer is an electrical transformer with a single winding that serves as both the primary and secondary winding. It differs from a conventional transformer, which has separate primary and secondary windings. In an autotransformer, a portion of the winding is shared by both the primary and secondary circuits.
The autotransformer has at least three terminals: a common terminal, which is connected to a portion of the winding shared by both circuits, and two additional terminals for the primary and secondary connections. The common terminal is typically connected to a tap along the winding, creating a voltage step-up or step-down function.
143
What are auto transformers used for?
Autotransformers are used for a variety of applications, including voltage regulation, impedance matching, and providing variable voltage for equipment testing. They are often used in power systems to step up or step down voltage levels, as they offer advantages such as compact size, improved voltage regulation, and cost-effectiveness compared to conventional transformers.
144
What is the electrical risk of using an auto transformer?
due to the shared winding, there is a direct electrical connection between the primary and secondary circuits, which means that a fault in the autotransformer could lead to a higher risk of voltage and current transference between the two circuits. Therefore, appropriate safety measures should be taken when working with autotransformers to ensure safe operation.
145
What is an SCR?
A SCR, or Silicon-Controlled Rectifier, is a type of semiconductor device widely used in power control and switching applications.
The SCR has three main terminals: the anode (A), cathode (K), and gate (G). It operates as a rectifier, allowing current flow in only one direction, similar to a diode. However, unlike a diode, an SCR can also be controlled to conduct current or block it completely by applying a gate signal.
146
When transformer windings are carrying current, they heat up causing a loss of energy. What is this loss of energy known as?
Copper losses.
147
If the losses in a transformer increase, the efficiency will:
Increase or decrease?
Decrease.
148
If the area of a capacitor’s plates is increased, what will happen to the capacitance?
The capacitance will increase.
149
What might be the danger of having a very high value of earth fault loop impedance?
The fault current would be too low to operate the protective devices in a fast-enough time.
150
Will an RCD protect a circuit against overcurrent?
No.
151
What two ‘protective’ components does an MCB contain? And what do they do.
Magnetic coil and bi-metal strip
Magnetic coil = Short circuit
Bi metal strip = Overload
152
A type of RCD that has the addition of overcurrent protection is called?
An RCBO
153
What is a Bi Polar Junction Transistor?
A bipolar junction transistor (BJT) is a three-terminal electronic device that is widely used in both analog and digital circuits. It is a type of transistor that consists of two pn-junctions, hence the term "bipolar." The BJT has three layers of semiconductor material, namely the emitter, base, and collector.
The BJT has two predominant types: NPN (Negative-Positive-Negative) and PNP (Positive-Negative-Positive). In an NPN transistor, the emitter is made of an n-type semiconductor, and the base and collector are made of p-type semiconductors. In a PNP transistor, the emitter is p-type, while the base and collector are n-type.
The operation of a BJT relies on the flow of majority charge carriers (electrons or holes) across the different regions. The emitter-base junction is forward-biased, while the base-collector junction is typically reverse-biased. The flow of charge carriers in the BJT is controlled by the base current.
154
What is reversed bias?
When a diode or a junction is reverse biased, it means that we connect the positive terminal of a voltage source to the N-type material and the negative terminal to the P-type material. This is the opposite of the usual forward bias where the positive terminal is connected to the P-type material.
When a diode is reverse biased, it creates a region called the depletion region, where there are no free charge carriers. This happens because the bias voltage pushes the majority carriers (electrons and holes) away from the junction.
In reverse bias, the diode has very high resistance and does not allow much current to flow. Only a tiny leakage current, known as the reverse saturation current, flows due to some minority carriers that are thermally generated.
155
What is forward bias?
Forward bias refers to the application of a voltage across a diode or a junction in such a way that the positive terminal of the voltage source is connected to the P-type material and the negative terminal is connected to the N-type material.
When a diode is forward biased, it becomes conductive and allows electric current to flow easily through it. This happens because the bias voltage helps overcome the natural resistance of the diode.
In forward bias, the positive terminal attracts the majority carriers (electrons in the N-type material and holes in the P-type material) toward the junction. This creates a low-resistance path for the current to flow through the diode.
The forward bias condition is commonly used in electronic circuits to allow current to flow through diodes, which can be utilized for rectification (converting alternating current to direct current), signal amplification, and other purposes.
156
What is a depletion region/zone?
A depletion region refers to a region near the junction of a semiconductor diode or transistor where the concentration of charge carriers (electrons and holes) is significantly reduced, creating a region depleted of free charge carriers. This depletion region is formed when a p-n junction is created by joining a P-type semiconductor material (with an excess of holes) and an N-type semiconductor material (with an excess of electrons).
When the P-type and N-type materials are brought together, the majority carriers from each side (holes from the P-type and electrons from the N-type) diffuse across the junction, leading to a recombination of carriers. As a result, the region around the junction becomes depleted of charge carriers.
The depletion region acts as a barrier to the flow of electric current because it lacks free carriers to conduct electricity. It forms a potential barrier, known as the built-in potential, due to the presence of immobile ions that create an electric field across the junction.
Under reverse bias, the depletion region widens as the applied voltage increases, increasing the barrier for current flow. Under forward bias, the depletion region narrows, allowing current to flow more easily through the diode.
The presence of the depletion region is a crucial characteristic of p-n junctions and diodes, as it affects their electrical behavior and functionality.
157
What happens to resistance when the cross sectional area increases?
The Resistance decreases.
158
What happens to the resistance of a cable if the ambient temperature increases?
The Resistance of the cable increases.
159
What happens to the resistance of a cable if the ambient temperature decreases?
The resistance decreases.
160
Why is the secondary terminal voltage of a transformer less at full load, than no load?
When current flows, it causes an internal volt drop due to the impedance of the windings.
161
If the burden of a transformer being used is increased, what will be the affect on the primary current of the transformer?
There will be no affect. The VA rating remains constant.
162
What is considered Extra Low Voltage according to AS/NZS3000
Voltage not exceeding 50VAC or 120VDC
163
Name the 2 constant losses you expect to find in a transformer
Eddy current losses
Hysteresis losses
164
In an alternator, frequency is determined by what?
Rotation speed of the armature
165
What is the calculation for efficiency?
Output/Input x 100
166
Frequency is measured in…?
Hertz
167
Which of the following elements of electrical engineering cannot be analyzed using Ohm’s law?
a) Capacitors
b) Inductors
c) Transistors
d) Resistance
C) Transistors.
168
How many cycles will an AC signal make in 2 seconds if its frequency is 100 Hz?
200
169
Which of the following is correct about the power consumed by R1 and R2 connected in series if the value of R1 is greater than R2?
a) R1 will consume more power
b) R2 will consume more power
c) R1 and R2 will consume the same power
d) The relationship between the power consumed cannot be established
a) R1 will consume more power.
Power can be expressed as P = I2*R. When two resistors are connected in series the current flowing through the resistors is the same and thus, power consumed by the larger resistor will be more
170
What will happen in a transformer when the number of secondary turns is less than the number of primary turns?
The voltage gets stepped down.
171
What is the number of primary turns in a 200/1000 V transformer if the emf per turn is 10V?
20
172
Who defined electric current and devised a method to measure current?
Andre-Marie Ampere
173
What is responsible for the current to flow?
a) Protons
b) Electrons
c) Nucleus
d) Protons and Electrons
b) Electrons
174
To obtain a high value of capacitance, the permittivity of dielectric medium should be?
High
The capacitance of a capacitor is directly proportional to the permittivity of the medium used as dielectric. Therefore to obtain a high value of capacitance, the permittivity of dielectric medium should be high
175
Four capacitors each of 40 µF are connected in parallel, the equivalent capacitance of the system will be?
160 µF
The impedance of a capacitor is inversely proportional to its capacitance value. Reciprocal of equivalent impedance of parallel connected circuit elements is sum of reciprocal of impedance of each of the elements.
176
Five capacitors each of 5 µF are connected in series, the equivalent capacitance of the system will be?
1 µF
When numbers of circuit elements are connected in series, the impedance of equivalent combination is sum of impedance of all elements in series. Again, capacitance is inversely proportional to impedance. Hence, when capacitors are connected in series
177
1 F is theoretically equal to?
ratio of 1 C to 1 V
1 Farad capacitance is defined as the capacity of dielectric medium to store 1 Coulomb charge when 1 Volt potential difference is applied across it. Thus, capacitance is expressed as the ratio of charge to voltage (Q = CV).
178
Magnetic flux has the unit of?
Weber
179
If P is the power of a star connected system then what will be power of an equivalent delta connected system?
P
The power in both of the connections is same either it is connected in Star or Delta.
180
What is power dissipation?
Power dissipation is the process of converting electrical energy into heat energy within an electronic component or a circuit. It occurs when the energy that flows through a component, such as a resistor or a semiconductor, is not entirely used for performing useful work (like producing light or motion) but instead is lost as heat.
181
What is an inductor?
An inductor is a passive electronic component that stores and releases energy in the form of a magnetic field when an electric current flows through it. It consists of a coil of wire wound around a core, often made of materials like iron or ferrite. Inductors are commonly used in circuits to control the flow of current, filter signals, and store energy. They have applications in various electronic devices, including power supplies, transformers, and radio frequency circuits.
182
An inductor does not allow the sudden change of?
Current
183
A capacitor does not allow the sudden change of?
Voltage
184
What should be measured by a voltmeter ?
Voltage
185
What should be measured with an ammeter?
Amps
186
Which quantity consists of a unit 1KWh ?
Energy
187
Is the emf induced into a dc generator armature winding AC or DC?
AC
188
A commutator in a DC generator is used for?
converting the AC armature current into DC
189
A DC generator without a commutator is a?
AC generator
190
What is permittivity?
Permittivity is a measure of how easily the electric field "penetrates" or "passes through" a material. Some materials let the electric field lines pass through them easily, while others resist it more.
Materials with higher permittivity can increase the capacity of a capacitor to store electric charge and energy.
191
Why doesn’t an inductor allow a sudden change of current?
An inductor resists sudden changes in current due to its inherent property known as "inductance." When the current through an inductor changes, it induces a voltage that opposes that change. This phenomenon is described by Faraday's law of electromagnetic induction.
This property is useful in various electrical applications, such as in smoothing out voltage fluctuations and controlling the rate of current changes in circuits.
192
Why doesn’t a capacitor allow a sudden change of voltage?
A capacitor resists sudden changes in voltage due to its ability to store and release electrical energy in the form of electric charge. When the voltage across a capacitor changes suddenly, the capacitor resists that change by either absorbing or releasing charge, which in turn affects the rate of voltage change.
This property makes capacitors useful in applications like filtering noise and stabilizing voltage levels in electronic circuits.
193
Describe how an AC motor works.
An AC motor consists of two main parts: a stationary part called the stator, which generates a rotating magnetic field when AC voltage is applied, and a rotating part called the rotor, which is placed inside the stator and interacts with the magnetic field to produce rotational motion. This design allows for the conversion of electrical energy into mechanical motion in various applications.
194
Describe how a DC motor works.
A DC motor typically consists of a stator, which includes a stationary set of magnets or electromagnets, and a rotor, which is the rotating part. The rotor contains coils of wire that carry electric current. When direct current (DC) is supplied to the coils, a magnetic field is generated that interacts with the stator's magnetic field, resulting in rotational motion. Brushes and a commutator are used to transfer electrical current to the rotor coils as it turns, ensuring continuous rotation. This basic design allows the conversion of electrical energy into mechanical motion in a DC motor.
195
Name the 3 main types of single phase motors.
Universal motors.
Shaded pole motors.
Split phase induction motors:
-standard split phase motors
-capacitor start
- permanently split
- capacitor start capacitor run.
196
What is a stator?
The overall collection of stationary parts of an AC motor.
Foot/flange/mounting plate.
Casing/frame.
End shields.
Fan cowl.
Bearings.
Yoke.
Field poles.
Field windings, start, aux, run, main.
Brush holders and brushes.
Capacitor.
Centrifugal switch integrated over temperature protection.
197
Universal motors are used because they are:
Compact
Powerful for their size
Economical to make
Easy to reverse
Cheap and easy to vary speed
198
Typical uses for universal motors.
Drills
Skill saws
Angle grinders
Food mixers
Vacuum cleaners
Electric weed whackers
199
Another name for a universal motor?
Series motor.
200
Why are shaded pole motors used?
Compact.
Quiet.
Cheap to make.
Require little to no maintenance.
Very reliable.
Speed controlled easily be altering motor voltage.
Doesn’t draw excessive current when stalled.
201
Disadvantages of shaded pole motors
Low starting torque
Speed varies as the motor load increases
Direction of motor cannot be reversed easily
The efficiency of the motor is very low
202
Typical applications for standard split phase motors
Washing machines
Dryers
Bench grinders
Fans and blowers
Light duty woodworking machinery
Rotary pumps
203
On a split phase motor what is the standard percentage when the centrifugal switch disconnects the start winding?
75% full speed
204
Typical applications for a capacitor start motor?
Larger blowers
Small conveyors
Concrete mixers
Reciprocating pumps
Vacuum pumps
205
What are the windings called in a permanent split capacitor motor?
The main windings
The auxiliary windings
206
Typical applications for a permanently split capacitor motor?
HVAC blowers and fans
Water pumps
Refrigeration pumps
Office equipment ( because of quiet running )
207
Reversing a permanently split capacitor motor?
Reverse the polarity of either the run or auxiliary winding.
208
Applications for cap start cap run motors
Air compressors
Refrigeration compressors
Loaded conveyors
High pressure pumps
Floor polishers
209
How do you reverse a cap start cap run motor
Reverse the polarity on either the start or run windings.
210
What is a VFD?
A variable frequency drive is an electronic device that takes an AC supply, coverts it into DC and then uses that DC to artificially produce an AC output of any chosen frequency to run the motor at the required speed.
211
Typical applications for multi speed split phase motors?
Air conditioning fans
Pumps
Conveyor belts
212
Typical applications for a shaded pole motor?
Blowers for heaters and dehumidifiers.
Extractor fans and exhaust fans.
Hair dryers.
Washing machine and dishwasher. Water pump motors.
Electric timer motors.
Record players.
Refrigeration circulation fans.
Servo motors and damper controllers.
213
What are the installation requirements of a switchboard?
Easy access and safe exit path.
Must not be in a restricted location.
Location must be safe from external damage, weather or other environmental influences.
Switchboard must be arranged to allow accessibility for operation, inspection, maintenance and repair.
Interns metal parts bonded.
214
What are the two basic types of switchboards?
Main switchboards.
Distribution boards.
215
How must you connect the phase and neutrals on an Edison screw lamp holder?
The neutral must be connected on the outer contact, and the phase on the centre contact at the bottom.
216
What is the definition of ‘Earthed’?
Connected to the general mass of earth.
217
What are the six essential parts to an earthing system?
Protective earthing conductors.
Main earthing conductor.
Main earth bar.
MEN connection.
Earth electrode.
Equipotential bonding.
218
Within an installation, where must the main earthing tag be located.
At the connection point between the main earth conductor and the main earthing cable. ( at the earth electrode )
219
What is the minimum cable size that can be used a main earthing conductor?
4mm2
10mm2 conductor = 4mm2 cable
16mm2 conductor = 6mm2 cable
25mm2 conductor = 6mm2 cable
35mm2 conductor = 10mm2 cable.
220
What are some advantages of using LEDs in lighting?
Low power consumption
Small size
Fast switching
Physically robust
Long lasting
221
What is the formula for power factor?
True power/Apparent power.
222
What are the factors that influence the inductance of a coil?
Number of turns
Length of magnetic coil
Core permeability
Cross sectional area of the core
223
How do you calculate the RMS voltage?
Vrms = Vmax x 0.707
224
What 3 components make up the fire triangle?
Flammable material
Oxygen
Ignition energy
225
What are some examples of damp situations?
Water containers ( baths, showers )
Swimming, paddling or spa pool and tubs.
Fountains and water features.
Saunas.
Refrigeration rooms.
Sanitizations and hosing down operations.
226
What converts chemical energy into electrical energy?
Cells and batteries
227
What’s converts mechanical energy into electrical energy by means of electromagnetic induction?
Generators
228
What converts heat energy into electrical energy?
Thermocouples
229
What converts light energy into electrical energy?
Photo-electric
230
What converts mechanical energy into electrical energy by means of pressure on crystals?
Piezoelectric
231
What is it called when friction between two materials produces electrical energy?
Static electricity
232
What is electromotive force ? (EMF)
The electrical force which causes electrons to flow in a conductor.
It is defined as “the force which causes current to flow in electric circuits”
233
What is an anode?
An anode is the positively charged electrode in an electrical circuit or an electrochemical cell. It typically attracts negatively charged particles or ions and undergoes oxidation reactions
234
What is a cathode?
A cathode is the negatively charged electrode in an electrical circuit or an electrochemical cell. It usually attracts positively charged particles or ions and undergoes reduction reactions. In batteries, during discharge, the cathode accepts electrons to complete the circuit and allow the device to function.
235
Why shouldn’t fuses be installed into the secondary side of a current transformer?
If a fuse opens ( blows ) or the fuse carrier is removed high voltages could develop.
This could damage the current transformer windings.
236
What are two methods that reduce the stroboscopic effect?
Install lead/lag fittings.
In double tube fittings wire a capacitor in series with one of the ballasts.
Fittings are connected on different phases.
Incandescent lighting is interspersed with the fluorescent lighting.
237
Explain the term sensitivity as it applies to an RCD.
The current imbalance (earth leakage) required to trip the RCD.
238
What is the maximum residual current rating for an RCD installed in a patient treatment area
10 mA
239
A three-phase motor has six terminals and three windings rated at 230V.
State how this motor is connected to a 400V supply.
In star.
240
Why do three-phase induction motors run smoother and quieter than a single phase induction motor?
Because they have a rotating magnetic field.
241
State two factors that affect the severity of an electric shock upon the human body.
Voltage applied
Current level
Contact duration
Body resistance
Current path
The frequency of the voltage
242
State one device that could be used instead of a centrifugal switch in single phase a capacitor start, capacitor run motor.
Solid state current relay
External starting relay
Timer
243
What are two safety features an MEN system provides for an electrical installation?
The voltage to earth on any part of the installation is limited to 230V.
The voltage potential between neutral and earth is about 0V
The low impedance main neutral ensures that protective devices operate effectively
244
What are some potentially hazardous situations that could occur when a MEN system supplies an electrical installation?
Because of the low impedance you can get dangerously high fault currents.
A phase/ neutral transposition on the mains.
Removal of MEN link.
Not installing the MEN link.
