What can cause the 50Hz supply waveform of equipment in electrical installations that's designed to switch the load at high-frequency to be modified to a complex waveform?
Harmonic Distortion
What are the 3 most likely causes of harmonics in A.C. Circuits?
Electronic Ballasts (Fluorescent Lighting)
Variable Frequency Drives
Inverters used with grid-connected PV equipment
What is installed to solve issues with 3rd order harmonics and their odd multiples?
A neutral conductor with a higher capacity than the active conductors.
What can visualise and determine harmonic waveforms and harmonic order?
A Power Analyser
What can reduce harmonics in an A.C. power system?
Inductive & Capacitive Filters
Resonance occurs in an A.C. circuit when ____ is equal with ____?
Inductive & Capacitive Reactance.
TRUE or FALSE?
Circuit impedance will be at a minimum and circuit current will be at a maximum in a series A.C. circuit.
True
TRUE or FALSE?
Circuit impedance will be at a maximum and circuit current will be at a minimum in a parallel A.C. circuit.
True
Why should you be careful when working on series resonant A.C. circuits?
There may be high voltages across inductive and capacitive components
To prevent control signals being absorbed by capacitors, where should blocking inductors be connected?
In series with the Capacitor
What type of electrical loads will cause harmonic distortion?
Non-linear loads (electronic switching devices)
In Australia, what would the 3rd harmonic frequency be?
150Hz
TRUE or FALSE
Harmonic distortion of a loads 'current' waveform, can distort the 'supply voltage' waveform
True
If frequency increases, what happens to capacitive reactance
It decreases
What is the fundamental frequency of the supply system in Australia?
50Hz
If frequency increases, what happens to inductive reactance?
It increases
A multiphase supply system is two or more single-phase A.C. voltages with what?
An electrical phase displacement
What colours are used to represent the phases of a three-phase electrical supply system in Australia?
Red
White
Blue
What is an advantage of a three-phase alternator over a single-phase alternator?
Two system voltages are available
To generate a three-phase supply, a rotating magnetic field cuts through three seperate phase windings that are arranged around the stator and displaced from each other by how many electrical degrees?
120 Electrical Degrees
A three-phase alternator generates a phase voltage with a maximum value of 325V. What is the RMS value of the waveform.
(Vrms = Vpeak x 0.707)
230 Volts
A sinusoidal current has a maximum value of 116A. What is the RMS value of the waveform.
(Irms = Ipeak x 0.707)
82 Amps
What is the phase relationship between the phase voltage and the line voltage of a Star connected system?
30°
TRUE or FALSE
A neutral conductor must be connected to a balanced three-phase system?
False
One of the phase windings of a Star connected alternator is accidently reversed during manufacturing. When the alternator is run up to the specified speed and tested with a multimeter what should the voltage range display?
Correct Phase voltages but incorrect Line voltages
TRUE or FALSE
Three-phase induction motors and three-phase commercial ovens are considered to as having a balanced load.
True
A three-phase consumer main is rated at 100A per phase. The installation consists mostly of linear single-phase loads which has very low harmonics. What should the minimum current rating for the neutral be?
(AS/NZS 3000 Clause 3.5.2)
100 Amps
The voltage drop in an unbalanced three-phase circuit can be conservatively calculated by assuming a balanced three-phase load while also ______?
(AS/NZS 3008 Clause 4.6)
Using the current in the heaviest load.
What is the phase relationship between the phase voltage and the line voltage of a Delta connected system?
0°
What is the phase relationship between the phase current and the line current of a DELTA connected system?
30°
An ‘open delta’ connection allows two single-phase transformers to be connected to a three-phase supply. While this provides a cheaper alternative to a three-phase transformer, by what percentage will the total power available compared to using three transformers be limited to?
57.7%
Be careful when connecting the secondary windings of a three-phase transformer in Delta. What can happen if one of the secondary windings is accidently reversed?
A high circulating current will flow within the secondary windings
Why does a kWh meter measure electrical energy?
To figure out how much to charge for electricity.
An existing electrical installation is to have a new submain installed to supply additional equipment. How should you check that the consumer mains have enough capacity for the extra load?
By measuring the maximum demand at the main switchboard
A true power measurement of a three-phase installation will indicate the power consumed by which component?
Resistive components
What is Reactive Power used for and what's it measured in?
For power factor correction equipment. Measured in Volt Amps Reactive (VAr)
The total power of a three-phase circuit can be measured using a single-phase wattmeter when?
The load is balanced
What does the ‘two wattmeter method’ measure?
The power consumed by an unbalanced three-phase circuit, as well as calculate the power factor
What does fault loop impedance refer to?
The circuit formed during a fault between Live parts and Earth
Fault-loop impedance is usually determined using AS/NZS 3000, however for larger cables (such as mains and submains) it can be determined using the relevant tables from AS/NZS 3008 provided that which values are found?
Resistance and Reactance
The earth fault-loop impedance of a lighting circuit needs to be tested when there is no supply available. To prepare for this test, which two conductors need to be shorted together?
Active & Earth
Which type of meter is used to measure fault loop impedance on a live socket outlet circuit?
An Earth fault-loop impedance tester
Fault-loop impedance is to be measured for a circuit. If supply is not available, which type of meter is used to measure the resistance of the active and earthing conductor for the circuit?
An Ohmmeter
When measuring the fault-loop impedance on a live circuit protected by a 20A Type 'C' circuit breaker, what is the maximum value permitted?
(AS/NZS 3000 Table 8.1)
1.5 Ω
When measuring the fault-loop impedance on a live circuit protected by a 63A Type 'B' circuit breaker, what is the maximum value permitted?
(AS/NZS 3000 Table 8.1)
0.9 Ω
Supply is not available and a circuit needs to be tested for fault loop impedance. An ohmmeter is used to measure the active-earth loop (Rphe). What is the maximum value of resistance permitted for a circuit run in 2.5mm2 TPS protected by a 16A Type 'C' circuit breaker?
(AS/NZS 3000 Table 8.2)
1.2 Ω
Supply is not available and a circuit needs to be tested for fault loop impedance. An ohmmeter is used to measure the active-earth loop (Rphe). What is the maximum value of resistance permitted for a circuit run in 2.5mm2 TPS protected by a 20A Type 'D' circuit breaker?
(AS/NZS 3000 Table 8.2)
0.6 Ω
Earth fault-loop impedance is to be tested for a circuit. If supply is not available, which table in AS/NZS 3000 can be used to determine the maximum allowable resistance value?
Table 8.2
When measuring fault-loop impedance on a LIVE circuit, which table in AS/NZS 3000 is used to determine the maximum allowable value?
Table 8.1