Electrical Troubleshooting MCQ Quiz — Part 5 of 5

Real-World Troubleshooting Scenarios | 50 Practice Questions (Q201–Q250 of 250) | For ITI Electrician, Wireman, Diploma Electrical/EEE and industrial electrical maintenance trainees

Topic Focus: Real-World Troubleshooting Scenarios
Practical fault-diagnosis scenarios: motor-won't-start diagnosis, breaker-tripping patterns, voltage-drop diagnosis, short-vs-open circuit testing, loose-connection heating and panel-level troubleshooting sequence.
This is Part 5 of a 5-part, 250-question Electrical Troubleshooting practice series covering real-world fault-diagnosis scenarios.

Question Palette (tap any number to jump)

Source basis: Standard industrial electrical maintenance principles and troubleshooting methods as taught in ITI/Diploma electrical curricula, with reference to recognised practice such as IS 3043 (earthing), IS 732 (wiring colour code) and IEC 60947-4-1 (overload relay trip classes) where applicable. Prepared for practice and skill-building purposes; always follow your equipment manufacturer's manual and workplace safety procedures.

About this practice set

This page is Part 5 of 5 in the Electrical Troubleshooting MCQ Quiz series. It contains 50 multiple-choice questions (Q201 to Q250 of the 250-question set) on Real-World Troubleshooting Scenarios. The questions are written for ITI, diploma and B.Tech trainees and for anyone preparing for a maintenance or fault-finding interview. Each question has four options; the quiz above checks your answer and shows an explanation, and the full answer key is printed below so you can also revise without running the quiz.

How to use this quiz

Attempt the questions in one sitting without searching for answers.
Read the explanation shown after each answer, even when you were right.
Write down the questions you missed and the reason in your own words.
Open the answer key below to revise those questions.
Retake the quiz after a day or two and compare your score.

Sample of what is covered

A three-phase motor hums loudly but does not rotate when the start button is pressed. What is the FIRST likely cause a technician should check?
A megger test on a motor winding gives a very low insulation resistance reading (well below the acceptable minimum). This most likely indicates:
An MCB protecting a lighting circuit trips intermittently with no obvious overload. The most likely causes to investigate first are:
A three-phase motor runs but with visibly reduced speed and very weak torque under load, without fully stalling. A likely cause is:
A technician feels a mild tingling sensation when touching a motor's metal frame while it is running. This is a serious warning sign of:
Measuring the DC resistance of each of the three windings of a healthy three-phase motor with an ohmmeter should show:
A three-phase motor develops a rhythmic growling/vibrating noise during running and eventually trips on overload, and inspection reveals no obvious external mechanical issue. A likely internal cause is:
PLC or control equipment in a panel resets or misbehaves intermittently, and investigation reveals a long, lightly loaded control cable run with a noticeably loose neutral terminal at the source end. This is most likely causing:

Answer key with explanations (50 questions)

Q1. A three-phase motor hums loudly but does not rotate when the start button is pressed. What is the FIRST likely cause a technician should check?

A. Single-phasing (one phase missing) or a mechanically jammed shaft preventing rotation ✓ Correct answer

B. The motor's paint colour has faded

C. The motor is too efficient for the load

D. The supply frequency is exactly 50 Hz

Explanation: A motor that hums without turning is typically energized on only two of three phases, or is mechanically locked, since in either case it cannot develop enough torque to start rotating.

Q2. A motor fails to start and there is absolutely no hum, noise, or vibration at all from the motor. This points most strongly to:

A. A dead control circuit — likely a blown control fuse, tripped overload, or open contactor coil preventing power reaching the motor at all ✓ Correct answer

B. A single blown fuse on only one of the three power phases

C. A perfectly healthy motor with an overloaded mechanical load

D. A capacitor bank that is oversized

Explanation: Complete silence with no hum usually means no power at all is reaching the motor windings, pointing to the control circuit (fuse, overload trip, coil) rather than a partial power issue.

Q3. An MCB trips the INSTANT it is switched on, before any load draws current. This behavior most strongly suggests:

A. A short circuit or very low resistance fault downstream, rather than a simple overload ✓ Correct answer

B. The connected load is perfectly balanced across all phases

C. The circuit is drawing exactly its rated current

D. The breaker's trip curve is set too high

Explanation: Instantaneous tripping on closing, before any load current can even build up, is the classic signature of the breaker's magnetic (short-circuit) element detecting a direct fault, not a gradual thermal overload.

Q4. A breaker trips only after the motor has been running for 20-30 minutes under load. This pattern is most consistent with:

A. A thermal overload condition, such as sustained overcurrent, poor ventilation, or high ambient temperature ✓ Correct answer

B. An instantaneous short circuit fault

C. A completely open circuit somewhere in the wiring

D. A capacitor bank discharge event

Explanation: Delayed tripping after a period of running matches the thermal (time-delayed) trip characteristic responding to sustained overcurrent or heat buildup, rather than the instantaneous magnetic trip used for short circuits.

Q5. Equipment at the far end of a long cable run shows visibly lower voltage than at the source panel. The most direct way to confirm this is genuine voltage drop (not a supply fault) is to:

A. Measure voltage at the source panel and at the equipment simultaneously under load, and compare the difference against the cable's expected drop ✓ Correct answer

B. Assume the supply transformer itself is faulty without any further testing

C. Replace the equipment with a new unit immediately

D. Increase the circuit breaker's current rating

Explanation: Comparing voltage at both ends of the same cable under actual load current isolates whether the shortfall is due to cable voltage drop (undersized conductor/long run) rather than a problem at the source.

Q6. To distinguish a short circuit from an open circuit using a multimeter on a de-energized circuit, a technician should primarily use:

A. The continuity/resistance function — near-zero resistance suggests a short, infinite resistance suggests an open circuit ✓ Correct answer

B. The AC voltage function only

C. The frequency measurement function

D. The capacitance measurement function on the suspect wire

Explanation: With power isolated, measuring resistance across the suspect path reveals a short circuit as a very low (near-zero) reading, while a break in the conductor shows as an open circuit (infinite resistance/OL).

Q7. A megger test on a motor winding gives a very low insulation resistance reading (well below the acceptable minimum). This most likely indicates:

A. Insulation failure, moisture ingress, or contamination in the winding that requires drying out, cleaning, or rewinding ✓ Correct answer

B. A perfectly healthy winding with excellent insulation

C. The megger's test voltage was set too high and damaged nothing

D. The motor is drawing too much starting current

Explanation: A low IR reading means current is leaking through degraded, wet, or contaminated insulation instead of staying confined within the winding, signaling a genuine insulation health problem needing corrective action before re-energizing.

Q8. A panel terminal is found discolored (browned/blackened) with signs of heat damage, even though the load current measured is within the rated capacity of the cable. This is most likely due to:

A. A loose connection at that terminal creating high contact resistance and localized overheating ✓ Correct answer

B. The cable being significantly oversized for the load

C. Correct torque having been applied during installation

D. The overload relay tripping too frequently

Explanation: A loose connection increases contact resistance at just that joint, so even normal load current generates excess localized heat (I²R at the joint) leading to discoloration, without the overall circuit current being abnormal.

Q9. When systematically fault-finding a dead circuit with a multimeter, a common and efficient approach is to:

A. Check for the presence of expected voltage at successive points working from the source toward the load, until the point where voltage is unexpectedly absent is found ✓ Correct answer

B. Only check voltage at the very last point in the circuit without testing anywhere else

C. Replace every component in the circuit at once without testing individually

D. Assume the fault is always at the load end without testing

Explanation: Working systematically from source toward load (or using a half-split method) and checking for expected voltage at each point quickly narrows down exactly where the circuit is broken.

Q10. A clamp meter reading shows one phase of a three-phase motor at 0A while the other two phases show significantly higher-than-rated current. This is a classic signature of:

A. Single-phasing — the motor is trying to run/start on only two of its three phases ✓ Correct answer

B. A perfectly balanced healthy three-phase supply

C. A capacitor bank correction that is working properly

D. Normal behavior during a soft-start ramp

Explanation: Zero current in one phase combined with elevated current in the remaining two phases is the textbook clamp-meter signature of single-phasing, where the motor is being forced to operate on an incomplete supply.

Q11. A sound general sequence for panel-level electrical troubleshooting, after ensuring safety, is:

A. Visual inspection → check for expected voltage presence/absence → continuity/resistance tests on suspect components → repair → re-test before re-energizing ✓ Correct answer

B. Immediately replace the entire panel without any diagnosis

C. Only check voltage once, then declare the panel faulty regardless of findings

D. Skip visual inspection and go straight to disassembling every component

Explanation: A structured approach — starting with a visual check for obvious damage, then voltage checks, then resistance/continuity tests on suspect parts — narrows down the fault efficiently and safely before any repair is attempted.

Q12. A motor runs noticeably hotter than usual but the overload relay never trips. The most likely explanation is:

A. The overload relay is set too high relative to the motor's actual rated current, or ventilation to the motor is blocked ✓ Correct answer

B. The overload relay is definitely faulty and set too low

C. The motor is running perfectly within design limits

D. The supply voltage is too high, which never causes overheating

Explanation: If the relay is set above the motor's true rated current, or if airflow to the motor is restricted (dirty fins, blocked fan), the motor can overheat without the current ever exceeding the relay's (too-high) set point.

Q13. An MCB protecting a lighting circuit trips intermittently with no obvious overload. The most likely causes to investigate first are:

A. A loose neutral connection, an accumulating overload from added fixtures, or a faulty lamp/fixture causing intermittent short leakage ✓ Correct answer

B. The bulbs being too dim for the circuit

C. The circuit being perfectly balanced across all phases

D. The MCB being oversized for the load

Explanation: Intermittent nuisance tripping on a lighting circuit is commonly traced to a loose neutral causing fluctuating current paths, gradual overloading as more fixtures were added, or a specific faulty fixture with intermittent leakage.

Q14. An RCCB trips consistently only when one specific appliance is switched on, while all other circuits remain fine. This strongly indicates:

A. That specific appliance has an internal earth leakage fault and should be isolated and tested/repaired ✓ Correct answer

B. The RCCB itself is definitely faulty and must be replaced immediately without further checks

C. The building's main earthing system has completely failed

D. The supply voltage is too low for that appliance

Explanation: Since only that appliance triggers the trip, the leakage current causing the RCCB to operate is most likely originating inside that appliance's wiring or insulation, and it should be isolated and tested separately.

Q15. A suspect switch, when tested with a multimeter set to AC voltage across its terminals while the circuit is live and the switch is OFF, shows full supply voltage. This is:

A. Normal and expected behavior for a healthy open switch in the OFF position ✓ Correct answer

B. A sign the switch is short-circuited internally

C. Proof the switch is definitely faulty and must be replaced

D. Evidence of a blown fuse upstream

Explanation: An open (OFF) switch should show the full supply voltage across its open contacts, since it is blocking current flow; this reading actually confirms the switch is functioning correctly as an open circuit.

Q16. To test whether a suspected blown fuse is actually open circuit, the safest and most reliable method is to:

A. Isolate the circuit from supply, then use a multimeter's continuity/resistance function directly across the fuse ✓ Correct answer

B. Leave the circuit energized and touch both fuse terminals with bare hands

C. Assume it is fine if the panel lights are on

D. Replace the entire distribution board without testing

Explanation: With the circuit safely isolated, a continuity or resistance check directly across the fuse element will clearly show an open circuit (infinite resistance) if the fuse element has blown, without risking contact with live parts.

Q17. A capacitor bank repeatedly trips its protective breaker shortly after being switched in. A likely cause to investigate is:

A. A shorted or failed capacitor unit within the bank drawing excessive current ✓ Correct answer

B. The capacitor bank being significantly undersized for the load

C. The power factor already being at unity before the bank was switched in

D. The breaker being rated far higher than the bank's current

Explanation: An internally shorted capacitor can draw abnormally high current the instant the bank is energized, tripping the protective breaker almost immediately, and requires locating and replacing the faulty capacitor unit.

Q18. A VFD trips on overcurrent shortly after a previously smooth-running motor starts making a new grinding noise. The most likely combined cause is:

A. A developing bearing failure or seized rotor causing increased mechanical drag that raises the motor's current draw ✓ Correct answer

B. The VFD's carrier frequency being set correctly

C. A perfectly healthy motor with no mechanical issues

D. The motor cable being too short for the installation

Explanation: A failing bearing or partially seized rotor increases the mechanical torque (and hence current) needed to turn the shaft, which combined with the new grinding noise points strongly to a developing mechanical fault rather than a purely electrical one.

Q19. A three-phase motor runs but with visibly reduced speed and very weak torque under load, without fully stalling. A likely cause is:

A. Running on reduced/unbalanced voltage or partial single-phasing, reducing available torque ✓ Correct answer

B. The motor operating at exactly its rated nameplate conditions

C. A perfectly matched capacitor bank correcting power factor

D. The overload relay being set correctly at nameplate current

Explanation: Reduced or unbalanced supply voltage lowers the torque a motor can develop (torque falls with voltage squared), so the motor struggles under load, runs slower than normal, and draws higher current trying to compensate.

Q20. To isolate which specific final circuit is causing a main RCCB to trip in a distribution board with multiple circuits, a practical method is to:

A. Switch off all individual circuit MCBs, then reset the RCCB and switch each MCB back on one at a time to identify which one causes the trip ✓ Correct answer

B. Replace the main RCCB immediately without further testing

C. Assume the fault is in the circuit that was most recently installed with no verification

D. Leave all circuits on and randomly disconnect appliances one at a time from every room simultaneously

Explanation: Sequentially reconnecting one circuit at a time after isolating them all lets the technician pinpoint exactly which branch circuit's leakage current is causing the main RCCB to operate, without guesswork.

Q21. An intermittent electrical fault that only appears when a nearby machine vibrates heavily is most likely caused by:

A. A loose wire connection or cracked solder/crimp joint that momentarily breaks contact under vibration ✓ Correct answer

B. A perfectly tightened and stable termination unaffected by vibration

C. The supply frequency changing due to vibration

D. An oversized cable that vibration cannot affect

Explanation: Vibration can intermittently open a marginal or loose connection, so gently tapping or flexing suspect wiring/terminals while monitoring continuity or voltage often reproduces and helps pinpoint this type of intermittent fault.

Q22. Visible sparking is observed at a wall socket whenever a plug is inserted or removed. This is most commonly caused by:

A. Worn, loose, or corroded contacts within the socket creating poor connection and arcing ✓ Correct answer

B. The appliance being plugged in being perfectly rated for the socket

C. The socket's earth pin functioning correctly

D. Normal and acceptable behavior for all sockets

Explanation: Worn or loosened socket contacts increase resistance and allow small arcs to form as the plug pins make or break contact, which is a fire and shock hazard and means the socket should be replaced.

Q23. A motor's terminal box is found badly burnt and discolored, but the overload relay never tripped before this happened. The most probable root cause is:

A. A loose terminal connection inside the box causing localized arcing and heat buildup, independent of the overall current level ✓ Correct answer

B. The motor cable being significantly oversized for the load

C. The overload relay having been set too low

D. Correct torque having been applied to all terminal screws

Explanation: Localized heating from a loose terminal connection can build up and cause severe damage right at that joint without ever raising the overall line current enough to trip an overload relay, which only monitors total current, not joint temperature.

Q24. A cable feeding a load is found running unusually hot along its length, and the fuse protecting it is rated well above the cable's actual current-carrying capacity. The correct corrective action is to:

A. Replace the fuse with one correctly matched to the cable's rated capacity, and verify/upsize the cable if it is genuinely undersized for the load ✓ Correct answer

B. Simply fit an even higher-rated fuse so it stops nuisance blowing

C. Ignore the heat since the fuse has not blown yet

D. Wrap the hot cable in extra insulation to hide the heat

Explanation: An oversized fuse fails to protect the cable itself; the fix is to correctly coordinate the protective device rating with the cable's ampacity, and if the load genuinely exceeds the cable's rating, install a properly sized cable.

Q25. A technician feels a mild tingling sensation when touching a motor's metal frame while it is running. This is a serious warning sign of:

A. Insulation leakage allowing a voltage to appear on the earthed frame, requiring immediate isolation, megger testing, and earthing verification ✓ Correct answer

B. Perfectly normal static electricity with no safety concern

C. The motor's cooling fan malfunctioning

D. The motor drawing slightly less current than rated

Explanation: A healthy, properly earthed motor frame should be at zero potential; any perceptible voltage indicates insulation breakdown allowing leakage current to the frame, and the motor must be isolated and tested immediately before continued use.

Q26. During a star-delta start, the motor trips on overcurrent right at the moment of transition from star to delta. A likely cause is:

A. The timer is set too short, switching to delta before the motor has accelerated close enough to running speed ✓ Correct answer

B. The timer is set far too long, allowing the motor to reach full speed before transition

C. The overload relay is set exactly at the motor's rated current

D. The star contactor and delta contactor are correctly interlocked

Explanation: If the transition happens too early, the motor is still running well below full speed when suddenly switched to full delta voltage, causing a second high inrush current similar to a fresh DOL start, which can trip protection.

Q27. A motor draws unusually high no-load (running with the load disconnected) current compared to its typical baseline. Possible causes to investigate include:

A. Mechanical friction from bearing drag, shaft misalignment, or undervoltage causing excess magnetizing current ✓ Correct answer

B. Perfectly aligned coupling and correctly lubricated bearings

C. The motor being run at exactly rated voltage with a healthy rotor

D. A properly balanced three-phase supply with no other issues

Explanation: Elevated no-load current can result from increased mechanical drag (worn/misaligned bearings) requiring more torque even unloaded, or from abnormal voltage conditions increasing magnetizing current, both of which merit investigation.

Q28. A machine stops suddenly during operation with no obvious cause. A logical first troubleshooting step is to:

A. Check for tripped protective devices (MCB, MCCB, overload relay, RCCB) at the panel before investigating further inside the machine ✓ Correct answer

B. Immediately begin disassembling the motor without checking upstream protection

C. Assume the incoming utility supply has failed without checking locally

D. Replace the motor immediately as a precaution

Explanation: Since protective devices are specifically designed to interrupt supply on a fault, checking whether any breaker or relay has tripped locally is the fastest way to identify why power stopped reaching the machine.

Q29. A newly replaced fuse on a circuit blows again within seconds of being switched back on, even though it is correctly rated for the circuit. This strongly suggests:

A. A genuine, persistent fault (such as a short circuit) still exists in the circuit and must be located and repaired before the fuse is replaced again ✓ Correct answer

B. The fuse itself was simply defective and a third replacement will solve the issue without further checks

C. The circuit is functioning normally and this is expected behavior

D. The fuse rating should simply be increased until it stops blowing

Explanation: A correctly rated fuse blowing immediately and repeatedly is a strong indicator of an ongoing fault condition (like a short circuit) downstream that must be found and fixed, rather than continually replacing fuses or increasing the rating, which would remove protection.

Q30. To check whether a motor winding has developed an internal short to the frame, the appropriate test is:

A. An insulation resistance (megger) test between each winding and the motor frame/earth ✓ Correct answer

B. Measuring the supply voltage at the motor terminals only

C. Timing how long the motor takes to reach full speed

D. Checking the motor's nameplate current rating

Explanation: A megger test specifically measures the resistance between the winding conductors and the earthed frame, directly revealing whether insulation has broken down and created a leakage/short path to the frame.

Q31. Measuring the DC resistance of each of the three windings of a healthy three-phase motor with an ohmmeter should show:

A. Very close to equal resistance values across all three windings ✓ Correct answer

B. Significantly different resistance values, which is considered normal

C. Zero resistance on all three windings

D. Infinite resistance on at least one winding

Explanation: In a healthy motor, all three windings are manufactured with the same wire gauge, turns, and length, so their DC resistances should closely match; a noticeably unequal reading on one phase indicates a developing winding fault.

Q32. A single-phase capacitor-start motor hums but fails to start, and a visual check shows the run capacitor is slightly bulged and leaking. This most likely indicates:

A. A failed/failing capacitor that needs to be replaced with one of the correct rating ✓ Correct answer

B. A perfectly healthy capacitor requiring no action

C. An issue entirely unrelated to the starting problem

D. A problem with the supply voltage magnitude only

Explanation: A bulged or leaking capacitor case is a clear physical sign of internal failure (loss of capacitance or a short), which removes the phase shift needed to start a single-phase motor, explaining the humming-but-not-starting symptom.

Q33. When a breaker refuses to reset and immediately trips again on every attempt, the correct practice is to:

A. Investigate and clear the underlying fault first rather than repeatedly forcing the breaker to hold ✓ Correct answer

B. Hold the breaker handle firmly in the ON position by force until it stays closed

C. Tape the breaker handle in the ON position to bypass the trip mechanism

D. Replace the breaker with one of a much higher current rating so it stops tripping

Explanation: A breaker that will not hold is doing its job correctly by detecting a real fault; defeating or forcing it closed removes vital protection and can lead to fire or equipment destruction, so the actual fault must be found and repaired first.

Q34. A distinct burning smell is noticed coming from an energized control panel. The correct immediate response is to:

A. Safely isolate/de-energize the panel if possible and investigate the source before any further operation ✓ Correct answer

B. Ignore it if no smoke is visible yet

C. Continue normal operation and monitor it over the next shift

D. Open all panel doors while it remains fully energized to look inside

Explanation: A burning smell is a strong early warning of overheating insulation or an active fault; the safe response is to de-energize the equipment as soon as it can be done safely and investigate before any further use, rather than risking fire or further damage.

Q35. In a dead control circuit fault-finding scenario, after confirming supply is present at the panel incomer, the next logical step is typically to:

A. Check the control transformer output and control circuit fuse, then work progressively toward the contactor coil ✓ Correct answer

B. Immediately replace the motor without checking any control components

C. Assume the earthing system has failed with no further testing

D. Bypass the overload relay permanently

Explanation: Since the control circuit is usually fed via a step-down control transformer and fuse before reaching switches, relays, and the contactor coil, checking these in sequence from the transformer output toward the coil systematically narrows down where the control path is broken.

Q36. Room lights visibly dim for a moment every time a large motor starts nearby on the same supply. This is best explained by:

A. A significant voltage drop caused by the motor's high starting inrush current interacting with the supply's source impedance ✓ Correct answer

B. The motor operating at a leading power factor

C. The lighting circuit being on a completely separate, unrelated supply transformer

D. The motor drawing less current during starting than while running

Explanation: The large inrush current during motor starting causes a temporary voltage drop across the supply network's impedance, which is shared with other loads like lighting on the same feeder, causing the visible dimming.

Q37. A three-phase motor develops a rhythmic growling/vibrating noise during running and eventually trips on overload, and inspection reveals no obvious external mechanical issue. A likely internal cause is:

A. Broken or cracked rotor bars causing pulsating torque and current, a classic squirrel-cage rotor fault ✓ Correct answer

B. A perfectly healthy squirrel cage rotor with no defects

C. The overload relay being incorrectly set too high

D. A correctly balanced three-phase voltage supply

Explanation: Broken rotor bars disrupt the smooth rotating field interaction with the rotor, producing a characteristic pulsating noise/vibration and current fluctuation at twice slip frequency, eventually leading to overheating and tripping.

Q38. To locate the approximate section of a cable/feeder where an earth fault exists in a system with several isolatable sections, a practical troubleshooting method is to:

A. Isolate and megger test each section individually, comparing insulation resistance readings to identify the specific faulty section ✓ Correct answer

B. Megger test the entire system as one continuous length only, with no sectioning

C. Assume the fault is always at the far end of the cable without testing

D. Skip insulation testing entirely and rely only on visual inspection

Explanation: By isolating and testing each cable/feeder section separately, the section showing an abnormally low insulation resistance reading can be pinpointed as containing the fault, rather than testing the whole run as one unit.

Q39. Before beginning any hands-on troubleshooting inside a control panel, the essential safety step is to:

A. Confirm isolation using a proven voltage tester and apply lockout-tagout where the circuit can be de-energized ✓ Correct answer

B. Assume the panel is safe because the main switch appears to be off

C. Start testing live components immediately to save time

D. Rely solely on a co-worker's verbal assurance that power is off

Explanation: A visual switch position or a colleague's word is not a reliable guarantee of a de-energized state; a proven voltage tester confirms the actual absence of voltage, and lockout-tagout prevents accidental re-energization during the work.

Q40. In a three-phase four-wire system, an unusually high current is measured flowing in the neutral conductor even though each phase current appears moderate. This points to:

A. Significant unbalance between the single-phase loads connected across the three phases ✓ Correct answer

B. A perfectly balanced three-phase load with no single-phase loads

C. The neutral conductor being oversized for the installation

D. A healthy earthing system with zero leakage

Explanation: In a balanced system the neutral carries very little current since phase currents largely cancel; a high neutral current indicates the single-phase loads distributed across the three phases are significantly unequal (unbalanced).

Q41. After a lightning storm, an MCB is found tripped on a circuit feeding sensitive equipment. Before simply resetting the breaker, the recommended practice is to:

A. Inspect the connected equipment and wiring for possible surge damage or insulation breakdown before re-energizing ✓ Correct answer

B. Immediately reset the breaker without any inspection, since storms rarely cause damage

C. Replace the breaker with a much higher-rated one to prevent future tripping

D. Disconnect the earthing system before resetting

Explanation: A surge from a nearby lightning strike can damage insulation or components even if the breaker successfully tripped to protect the circuit, so checking the equipment before blindly re-energizing helps avoid further damage or unsafe conditions.

Q42. A clamp meter check during a suspected single-phasing event shows two phases reading normal load current and the third phase reading exactly zero, while the motor is still connected and attempting to run. The correct action is to:

A. Immediately stop and isolate the motor to prevent winding damage, then trace back to find the open phase (fuse, contactor pole, or connection) ✓ Correct answer

B. Continue running the motor since two phases are still supplying current

C. Increase the overload relay setting so the motor can keep running

D. Swap any two phases at the motor terminals while it is still running

Explanation: Confirmed single-phasing should prompt immediate shutdown to prevent overheating damage to the windings, followed by systematically checking upstream fuses, contactor poles, and terminal connections to locate the open phase.

Q43. PLC or control equipment in a panel resets or misbehaves intermittently, and investigation reveals a long, lightly loaded control cable run with a noticeably loose neutral terminal at the source end. This is most likely causing:

A. Fluctuating voltage at the equipment due to the loose neutral connection, disrupting sensitive control electronics ✓ Correct answer

B. Complete and total loss of all power with no possibility of intermittent operation

C. Improved stability of the control circuit voltage

D. No effect at all on the connected equipment

Explanation: A loose neutral connection creates variable contact resistance, causing the voltage delivered to fluctuate unpredictably, which is a common cause of intermittent resets or erratic behavior in sensitive electronic control equipment.

Q44. For comprehensive field fault-finding on an electrical installation, the combination of instruments most commonly used together includes:

A. A digital multimeter, a clamp meter, an insulation resistance tester (megger), and where available a thermal imaging camera ✓ Correct answer

B. Only a single fixed-range analog voltmeter, with no other instruments

C. A stopwatch and a measuring tape exclusively

D. Only a visual inspection with no instruments at all

Explanation: Effective troubleshooting typically draws on a multimeter for voltage/continuity, a clamp meter for current without breaking the circuit, a megger for insulation health, and thermal imaging to spot hot spots from loose connections or overload.

Q45. A megger test on a piece of equipment shows an extremely high (near-infinite) insulation resistance reading. In general, this result should be interpreted as:

A. A good sign, indicating healthy, intact insulation with minimal leakage current ✓ Correct answer

B. A definite sign of insulation failure requiring immediate replacement

C. Proof that the equipment is not actually connected to supply

D. Irrelevant information with no bearing on equipment condition

Explanation: A very high insulation resistance reading means very little leakage current flows through the insulation under test voltage, which is the expected and desirable result for healthy, dry, undamaged insulation.

Q46. A motor's terminal box feels warm to the touch, but a clamp meter confirms the current draw is within the motor's rated nameplate value. This combination suggests:

A. A loose or poor-quality terminal connection causing localized resistive heating, rather than a genuine overload condition ✓ Correct answer

B. The motor is definitely overloaded and needs immediate load reduction

C. The clamp meter reading must be incorrect since the box is warm

D. Normal expected behavior requiring no further attention

Explanation: Since the measured line current is normal, the localized warmth is best explained by a high-resistance joint at the terminal (loose lug, corrosion) generating heat right at that point rather than an overall overload of the motor.

Q47. A three-phase exhaust fan runs but blows air in the reverse direction from what is expected after recent panel maintenance. The most likely cause and fix are:

A. Phase reversal from swapped wiring during maintenance; fix by interchanging any two of the three supply phase connections ✓ Correct answer

B. A faulty capacitor that must be replaced, as in a single-phase motor

C. The overload relay tripped and needs resetting

D. The fan blades are physically installed backward and cannot be electrically corrected

Explanation: If two phase conductors were accidentally swapped during maintenance, the rotating field direction reverses, spinning the motor (and fan) the wrong way; simply swapping any two phases back corrects the rotation direction.

Q48. A protective device trips more frequently during the hottest part of the day than in cooler mornings, on a circuit with no change in connected load. This pattern often points to:

A. A loose or marginal connection whose resistance/heating is worsened by higher ambient temperature, or a thermal overload relay influenced by ambient heat ✓ Correct answer

B. A completely unrelated coincidence with no technical explanation

C. The circuit drawing less current when it is hot outside

D. Definite proof of a short circuit fault only

Explanation: Higher ambient temperature can push an already marginal loose connection or a temperature-sensitive thermal overload relay closer to its trip threshold, explaining why tripping becomes more frequent during the hottest periods even with unchanged load.

Q49. To identify which individual core of a multi-core control cable has developed a fault (with both cable ends accessible and the cable isolated), a practical method is to:

A. Test continuity and insulation resistance on each core individually, with the others left open, to isolate the specific faulty conductor ✓ Correct answer

B. Test all cores bundled together simultaneously as one group only

C. Assume the fault is randomly distributed with no specific core responsible

D. Skip testing individual cores and replace the entire cable without diagnosis

Explanation: Testing each core separately for continuity (to check for a break) and insulation resistance (to check for a short to another core or earth) systematically isolates exactly which conductor within the multi-core cable is faulty.

Q50. As a final, essential step after completing any repair on a de-energized electrical panel, before returning it to normal service, a technician should:

A. Perform appropriate tests (continuity/insulation as relevant) and a controlled test energization, then verify correct operation before removing lockout ✓ Correct answer

B. Immediately remove all locks and tags and walk away without any verification

C. Skip testing since the repair was visually inspected as complete

D. Leave the panel de-energized indefinitely with no further action

Explanation: Verifying the repair with appropriate tests and a controlled, supervised re-energization confirms the fault is genuinely resolved and no new hazard has been introduced, before the lockout is removed and normal operation resumes.

Frequently asked questions

Who is this Electrical Troubleshooting quiz for?

It is for ITI, diploma and B.Tech students, apprentices and working technicians who want to test fault-finding knowledge. The explanations are written to teach the reason, not just the answer.

How should I use the answer key?

Attempt the quiz first without looking. Then open the answer key, read the explanation for every question you missed, and retake the part after a day to see whether the explanation stuck.

Do I need to finish all five parts?

No, but the parts build on one another: fundamentals first, then components, then circuits and real-world scenarios. Part 5 is the best test of whether you can apply the earlier parts to a live fault.

More practice

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