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Electrical Theory Questions With Correct Detailed Answers
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Electrical Theory Questions With Correct Detailed Answers
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Electrical Theory Questions With CorrectDetailed Answers
State the permissible test result for an insulation resistance test for the electrical
installation with ALL the permanently-connected electrical equipment DISCONNECTED
- ANSWER- 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 -
ANSWER- 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. - ANSWER- - 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 - ANSWER- - 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. - ANSWER- Clamped around the protective earthing conductor and
5mA
Explain what is meant by the term INVERSE TIME-CURRENT CHARACTERISTIC in
relation to protective devices - ANSWER- 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 -
ANSWER- - 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 -
ANSWER- 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? - ANSWER- 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 - ANSWER- A bi-polar
junction transistor
State the function of a transducer - ANSWER- A device that changes energy from one
form to another
State TWO requirements for good illumination - ANSWER- 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 - ANSWER- Capacitive Reactance (ohms)
Explain why REWIREABLE FUSES are classed COARSE EXCESS CURRENT
protection - ANSWER- 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 -
ANSWER- 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 - ANSWER- 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
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