Electrical General 1 Flashcards
What is a capacitor?
A capacitor is an electronic component that stores electrical energy in an electric field
What is the basic construction of a capacitor?
The basic structure of a capacitor consists of two conductive plates separated by a dielectric material
What does the word dielectric refer to?
a dielectric refers to an insulating material that is placed between the conductive plates of a capacitor
What is voltage?
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.
What is current?
the flow of electric charge in a conductor. It is the rate at which electric charges move through a circuit
What is resistance ?
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
What is the basic calculation for Resistance?
V/I
What is the basic calculation for Current?
V/R
What is the basic calculation for Voltage?
I x R
What is the unit for Current?
Amperes
What is the unit for Resistance?
Ohms Ω
What is the unit for Voltage?
Volts ( V )
What is the relationship between Volts, Current and Resistance?
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.
What is an electron?
An electron is a subatomic particle that carries a negative electric charge. It is one of the fundamental particles that make up atoms
What is a proton?
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
What is a Neutron?
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
How many Watts are in 1 Horsepower?
One horsepower is equivalent to approximately 745.7 watts
What is a diode?
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.
What is a Centrifugal switch?
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
What is a Zener diode?
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.
What is Valence?
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).
What is Covalence?
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
What is doping ?
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).
Are primary cells of batteries rechargeable?
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.
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.
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.
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.
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
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.
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.
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.
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
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.
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
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.
What is the ratio of ladder safety?
Horizontal:Vertical
1:4
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.
What is close protection?
Close protection devices operate within 4 hours at 1.5 times its rated current.
What is coarse protection?
Coarse protection devices operate outside a 4 hour period at 1.5 times the rated current.
What is minimum depth of an earth stake?
1.8m
What is the minimum depth of a buried earth conductor
500mm
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.
An ammeter should always be connected in
Series
A voltmeter should always be connected in
Parallel
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.
What is the maximum resistance a main earthing cable can be in ohms?
0.5 Ohms
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
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
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
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
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.
How do you regulate voltage on a power transformer with varying loads
With the use of a tap changer.
In a big installation, how many boards should contain a MEN link?
One. Normally the one closest to the supply.
What are two main functions of the ballast in a fluorescent light fitting
Provide a starting voltage.
Limit current in the circuit.
A main earth tag must always be located at…
The main earthing conductor.
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.
What starters control the speed of a three-phase a.c induction motor?
Electronic variable speed drive
Secondary resistance starter
What components are used to filter the pulsating output of a rectifier?
Inductors
Resistors
Capacitors
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
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.
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.
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.
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.
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.
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.
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
True or false.
Is switching off the light a safe method to use before replacing the light fitting in a domestic electrical installation?
False
How is the direction of a three-phase delta connected ac induction motor changed?
Interchange any two motor supply connections
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
In New Zealand, what type of RCD is required to be voltage dependent
Portable Residual Current Device
In what form is the voltage output from a PWM converter?
A series of rectangular pulses
Define the term Peak Inverse Voltage as applied to a semiconductor device?
Maximum voltage semiconductor device can withstand in reverse bias.
What is the standard maximum secondary voltage for a voltage Transformer
110V
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
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).
State the main reason why interlocks are used in a star/delta starter.
To prevent the star and delta contactors closing simultaneously
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
What is the typical tripping current of an RCD used for personal protection?
30mA
What is the maximum permitted rated residual current of an RCD installed for Personal protection in a domestic installation
30mA
What is the maximum permitted rated residual current of an RCD installed in a medical electrical installation
10mA
What is the maximum permitted rated residual current of an RCD installed for the protection of property
300mA
State the minimum Insulation Resistance of an electrical installation that does not contain MIMS heating elements
1 MΩ
What is the formula for calculating the frequency of an alternator?
F = NumberOfPoles/60
F = NP/60
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
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.
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
For it to operate correctly, how should a Zener Diode be connected?
Reversed biased.
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.
Why are Transformer cores laminated?
Reduces Eddy current losses
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.
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.
What is Pull-Out Torque as it applies to an AC induction motor?
The maximum value of torque the motor can produce.
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.
What does the term discrimination mean as applied to protective devices?
The protective device closest to the fault will operate first
In a star connected Electrical system state the angle between line voltage and phase voltage.
30 degrees
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.
Define the term ‘clearance time’
Total time for any fuse to operate and clear a fault
