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Electrical Theory Exam 2024/2025 Questions With Completed & Verified Solutions.
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Electrical Theory Exam 2024/2025 Questions With Completed & Verified Solutions.
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Electrical TheoryState the permissible test result for an insulation resistance test for the electrical installation with
ALL the permanently-connected electrical equipment DISCONNECTED - ANS 1 Mega Ohm
State the permissible test result for an insulation resistance test for the electrical installation with
ALL the permanently-connected electrical equipment CONNECTED - ANS 0.01 Mega Ohms
A three-phase induction motor normally runs on full-load. State ONE reason why this motor
should not be run LIGHTLY LOADED when it runs on full load. - ANS - The power factor of the
motor will be poor
- The motor will be less efficient
- running costs rise
A three-phase induction motor has been OPERATING NORMALLY ON FULL-LOAD. The motor
starts to over-heat even though it is drawing the normal full load current.
State ONE reason why this situation would occur - ANS - The motor ventilation is restricted in
some manner
- The cooling fan has become detached from the motor shaft
A clip-on ammeter is being used to test the integrity of the insulation of a repaired Class I
electrical appliance, while the appliance is live. Where on the appliance would the clip-on
ammeter be used? Also state the maximum test result that would allow the appliance to return
to service. - ANS Clamped around the protective earthing conductor and 5mA
Explain what is meant by the term INVERSE TIME-CURRENT CHARACTERISTIC in relation to
protective devices - ANS as current through the protective device increases, the time taken to
operate decreases
State TWO components that are typically used to smooth the d.c output of a rectifier - ANS -
Capacitors
- Inductors
- Resistors
The output circuit of an autotransformer MUST BE insulated for the highest input or output
voltage of the autotransformer. State ONE reason for this requirement - ANS An auto
transformer can fail in such a way so the primary voltage appears across the secondary
An isolator for 230V electrical equipment has been replaced. During the work the phase (active)
and neutral were accidentally transposed at the supply side of the isolating switch.
, What potential danger would this transposition create? - ANS The internal circuitry of the
electrical equipment would be live when the isolating switch is in the OFF position.
What to the initials BJT mean when applied to semiconductors - ANS A bi-polar junction
transistor
State the function of a transducer - ANS A device that changes energy from one form to
another
State TWO requirements for good illumination - ANS Any TWO of:
● Provides clear vision without eye strain
● Sufficient light intensity
● Minimum glare
● Uniform light intensity throughout the illuminated area
● Suitable colour rendering
XC - ANS Capacitive Reactance (ohms)
Explain why REWIREABLE FUSES are classed COARSE EXCESS CURRENT protection -
ANS There is no certainty that the fuse will operate within 4 hours at 1.5x it's rated current
Explain why gG type HRC fuses and MCBs are classed as close current protection - ANS The
HRC fuse or MCB will operate with certainty within 4 hours at 1.5x its rated current
The total time taken for a HRC fuse to operate is defined as the total operating (or clearance)
time. The total clearance time comprises the pre-arcing time and the arcing time.
1. Define the term pre-arcing time
2. Define the term arcing time - ANS 1. The pre-arcing time is the time taken for a fault current
large enough to operate the fuse. Or the time between the fuse elements beginning to melt and
the initiation of the arc
2. The arcing time is the time between the initiation of the arc and the extinguishing of the arc
Describe how correct discrimination is achieved between protective devices in an electrical
installation - ANS Only the protective devices closest to the fault operates
A MEN, low voltage electrical installation operates at standard low voltage
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