Two identical 100 kVA transformers of the same impedance are used tosupply separate industrial electrical installations. In the first installation the100kVA transformer is located 20 metres from the boundary of theinstallation.In the second installation the 100kVA transformer is located within theinstallationState the main reason why the second electrical installation will have alarger prospective short circuit current than the other electrical installation.(2 marks)
The impedance is lower because the cable run is shorter
Describe how the prove-test-prove method of testing is carried out.
● The test instrument is checked tobe operating correctly on a knownlive source.● The equipment is tested toconfirm (or otherwise) that it isisolated.● The test instrument is againchecked on a known live source toensure is still operates correctly.
You are doing a polarity test to find out if the supply to a threephasemain switchboard has been isolated.Between which terminals would you test to clearly establish thatisolation (or otherwise) has taken place.
● Test between each phase andearth.● Test between each phase andneutral.
The three-phase PVC flexible cable supplying a 400V three-phase motor hasbeen replaced with a three core neutral-screened flexible cable.State TWO advantages that a neutral-screened flexible cable has whencompared to a PVC cable.
Any TWO of:● Better mechanical protection.● Provides increased safety whenhandled live.● Provides electrical shielding.
For which type of protective device is the term tripping factor used.
A circuit breaker
Define the term tripping factor.
It is the ratio of the MCB minimumtripping current to the MCB currentrating.
A three-phase TPS cable is to be installed as the mains to a three-phaseelectrical installation.State TWO reasons why the loads in the installation should be balanced (asmuch as possible) over the three phases
Minimise cable size● Reduce neutral current● Reduce heat● To prevent nuisance tripping on theheaviest loaded phase.
State the recommended maximum residual current rating of an RCDused for socket-outlet final subcircuits in a kindergarten?
An RCD protects a circuit and a fixed-wired Class I electrical appliance.The residual current device (RCD) has operated. What has the RCDdetected that would cause it to operate?
Earth leakage current
State TWO different methods that are used to provide the starting torquefor a single-phase induction motor
Capacitor● The start and run windings havedifferent impedances.● The shading ring on a shaded polemotor.
State the easiest method of reversing the direction of rotation of athree-phase induction motor controlled by a DOL starter.
Reverse any two phases
State the easiest method of reversing the direction of rotation of athree-phase induction motor controlled by a star/delta starter.
Reverse any two phases on the supplyto the starter.
There are three methods by which a lamp can produce light.State TWO of those methods
Any TWO of:● Current through a filament.● Current through a light-emitting diode● Current through a gas.
A voltage test for isolation is being carried out on an oven that may or maynot be live.A multi-function test instrument set on the voltage scale is being used.State the main safety issue that would occur if the instrument probes wereinadvertently connected across the terminals of an open control switch
If the circuit controlled by the switchwas not continuous the meter wouldshow a 0V reading.(1 mark)● Even though the oven may still be live
Briefly explain why fuses are not installed in the secondary of a CT(current transformer).
If a fuse opens (blows) or the fusecarrier removed high voltages coulddevelop.(1 mark)● Which could damage the CT windings.
Fluorescent fittings are installed in final subcircuits across three phases in afactory.State TWO methods of reducing the stroboscopic effect of the fluorescentfittings.
In double-tube fittings, a capacitorconnected in series with one ballast.orInstall lead/lag fittings● Fittings are connected on differentphases.● Incandescent lighting is interspersedwith the fluorescent lighting.
An electrician used a voltmeter to test for isolation on the supply side of athree-phase isolator using the following procedure:● A test on an independent live source confirmed that the meter workedcorrectly.● A test between each phase showed no reading on the meter.● Another test on an independent live source confirmed that the meterstill worked correctly.The electrician dismantled the isolator and received an electric shock.Explain why the testing was ineffective.
It was ineffective because no test wascarried out between phase and earth orbetween phase and neutral.
Explain the term sensitivity as it applies to a residual current device(RCD).
The current imbalance (earth leakage)required to trip the RCD.
State the maximum residual current rating for a residual currentdevice (RCD) installed in a patient treatment area
A three-phase motor has six terminals and three windings rated at230V.State how this motor is connected to a 400V three-phase supply.
Explain why three phase induction motors run smoother and quieterthan single-phase induction motors.
They have a constant rotating magnetic field
State TWO reasons why a run capacitor would be fitted to a single-phasesplit-phase induction motor
Any TWO of:● Quieter running● Better running torque● Less current drawn● Increased efficiency
Name TWO components that are typically used to smooth the d.c. outputof a rectifier.
Any TWO of:● Capacitors● Inductors● Resistors● Transistor● Zenor Diode● Voltage regulator
Explain what is meant by the term inverse time-current characteristicin relation to protective devices?
● As the current through the protectivedevice increases.(1 mark)● The time taken to operate decreases
A two-core MIMS cable is to be re-connected to a single-phase fuel pumpmotor.State TWO important mechanical precautions specific to an MIMS cablethat should be taken when doing the reconnection.
Any TWO of:· Avoid damage to metal sheath· Maintain loop or "S" bend in cable.· Do not disturb pot seal· Do not over-flex the solid conductors· Ensure gland is clean and tight● Connections must be tight