Electrical Troubleshooting MCQ Quiz — Part 3 of 5

Control Components | 50 Practice Questions (Q101–Q150 of 250) | For ITI Electrician, Wireman, Diploma Electrical/EEE and industrial electrical maintenance trainees

Topic Focus: Control Components
Covers relays vs contactors, timers, MCB/MCCB/ELCB/RCCB (including 30 mA sensitivity and MCB B/C/D trip curves), HRC fuses, ladder logic and start-stop/interlocking circuits.
This is Part 3 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 3 of 5 in the Electrical Troubleshooting MCQ Quiz series. It contains 50 multiple-choice questions (Q101 to Q150 of the 250-question set) on Control Components. 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 relay, in general electrical terms, is best described as:
The coil voltage rating of a contactor and the current rating of its main contacts are:
An RCCB (Residual Current Circuit Breaker) operates by detecting:
In control panel wiring practice, keeping the power circuit wiring and the control circuit wiring physically separated is important mainly to:
A contactor that audibly 'chatters' (rapidly makes and breaks) when energized is most likely experiencing:
A Type B MCB trip curve, with an instantaneous trip range of about 3-5 times rated current, is best suited for:
Compared to traditional hard-wired relay logic, a PLC (Programmable Logic Controller) offers the main advantage of:
If a fuse is deliberately fitted with a rating much higher than the cable's safe current-carrying capacity, the likely consequence is that:

Answer key with explanations (50 questions)

Q1. A relay, in general electrical terms, is best described as:

A. An electrically operated switch that uses a small control current to open or close contacts controlling another circuit ✓ Correct answer

B. A device that only measures voltage

C. A device that only converts AC to DC

D. A type of fuse used for overload protection

Explanation: A relay uses an energized coil to mechanically operate a set of contacts, allowing a low-power control signal to switch another (often higher-power) circuit.

Q2. The main practical difference between a relay and a contactor is that:

A. Contactors are built to switch higher power/current loads (like motors), while relays typically handle lower-power control and signal circuits ✓ Correct answer

B. Relays can only be used on DC circuits

C. Contactors have no coil, only mechanical contacts

D. Relays are always larger in physical size than contactors

Explanation: Contactors have heavier-duty contacts and arc-quenching designed for switching motor and heater loads, while relays are generally rated for smaller currents in control/signal applications.

Q3. A power contactor is fundamentally constructed from:

A. An electromagnetic coil that, when energized, pulls in an armature to close a set of main power contacts ✓ Correct answer

B. A bimetallic strip alone with no coil

C. A set of purely mechanical contacts with no electrical actuation

D. A capacitor bank connected across the load

Explanation: Energizing the contactor's coil creates a magnetic field that pulls the armature and attached contacts closed, allowing current to flow to the load; de-energizing releases them via spring action.

Q4. A Normally Open (NO) contact, in its de-energized (rest) state, is:

A. Open, meaning no current flows through it until the associated coil/actuator is energized ✓ Correct answer

B. Closed, meaning current always flows through it

C. Permanently welded shut

D. Only used in three-phase circuits

Explanation: By definition, a normally open contact remains open at rest and only closes when the device (relay/contactor) it belongs to is actuated.

Q5. A Normally Closed (NC) contact, in its de-energized (rest) state, is:

A. Closed, meaning current flows through it until the associated coil/actuator is energized, which then opens it ✓ Correct answer

B. Open, meaning no current ever flows through it

C. Only found in three-phase power circuits

D. Identical in function to a fuse

Explanation: A normally closed contact conducts at rest and opens once the relay/contactor coil is energized, the opposite behavior of a normally open contact.

Q6. Auxiliary contacts fitted on a contactor are primarily used to:

A. Provide additional NO/NC contacts for control/interlocking/indication functions, separate from the main power contacts ✓ Correct answer

B. Carry the full motor load current in place of the main contacts

C. Replace the need for an overload relay

D. Increase the coil's operating voltage

Explanation: Auxiliary contacts are lower-current contacts mechanically linked to the main contactor, used for holding circuits, interlocking, and indicator lamps without loading the main power contacts.

Q7. The coil voltage rating of a contactor and the current rating of its main contacts are:

A. Two independent ratings; the coil operates on a control voltage while the main contacts switch a much higher load current ✓ Correct answer

B. Always numerically identical

C. Interchangeable specifications with no distinction

D. Only relevant for DC contactors

Explanation: A contactor's coil (e.g., 230V or 24V) simply needs to be energized to operate the mechanism, which is a separate specification from the main contacts' rated current-carrying/switching capacity for the load.

Q8. An ON-delay (TON) timer relay, once its input signal is applied, will:

A. Wait for the preset time delay before its output contacts change state ✓ Correct answer

B. Change its output contacts instantly with no delay

C. Change its output contacts only after the input signal is removed

D. Never change state regardless of input

Explanation: A TON timer starts timing as soon as it receives power/input, and only switches its output contacts after the set delay period has elapsed.

Q9. An OFF-delay (TOFF) timer relay changes its output contacts back to the rest state:

A. After a preset delay following removal of the input signal ✓ Correct answer

B. The instant the input signal is applied

C. Only when manually reset by a technician

D. Immediately when input is removed, with no delay

Explanation: A TOFF timer's output switches immediately when the input is applied, but reverts back to its normal state only after a delay once the input signal is removed.

Q10. A Miniature Circuit Breaker (MCB) is generally used for:

A. Protecting low-current circuits (typically up to about 100A) against overload and short circuit in domestic/light industrial wiring ✓ Correct answer

B. Protecting circuits rated in the thousands of amperes only

C. Only earth leakage protection with no overcurrent function

D. Boosting voltage in a distribution panel

Explanation: MCBs are compact thermal-magnetic devices designed for lower current ranges typical of final distribution circuits, offering both overload (thermal) and short-circuit (magnetic) protection.

Q11. A Moulded Case Circuit Breaker (MCCB) differs from an MCB mainly in that it:

A. Handles higher current ratings and often offers adjustable trip settings for larger feeders and industrial loads ✓ Correct answer

B. Can only be used on single-phase circuits

C. Cannot provide short-circuit protection

D. Is always smaller in physical size than an MCB

Explanation: MCCBs are designed for higher current capacities than MCBs and frequently include adjustable thermal and/or magnetic trip settings, making them suitable for main incomers and larger feeders.

Q12. Which statement correctly distinguishes an ELCB from an RCCB?

A. An ELCB (voltage-operated type) senses voltage on the earth conductor, while an RCCB senses the current imbalance between line and neutral directly, making RCCBs more reliable ✓ Correct answer

B. ELCB and RCCB are simply two different names for the exact same technology with no difference

C. RCCBs only work on DC circuits, ELCBs only work on AC circuits

D. An ELCB provides overload protection, while an RCCB provides only short circuit protection

Explanation: Older voltage-operated ELCBs depend on a separate earth electrode voltage rising above a threshold, which can fail to detect some faults; modern RCCBs directly compare line and neutral current and trip on any imbalance, which is why RCCBs are now preferred.

Q13. An RCCB (Residual Current Circuit Breaker) operates by detecting:

A. An imbalance between the current flowing in the line conductor and the current returning in the neutral conductor ✓ Correct answer

B. A rise in supply frequency above 50 Hz

C. A drop in supply voltage below rated value

D. The total power factor of the connected load

Explanation: Under normal conditions, current out on the line equals current back on the neutral; any leakage to earth creates a difference (residual current) that the RCCB's internal current transformer detects, tripping the device.

Q14. For protection against electric shock to personnel, RCCBs used on final circuits commonly have a rated residual operating current of:

A. 30 mA ✓ Correct answer

B. 300 mA

C. 3 A

D. 30 A

Explanation: A 30 mA sensitivity is widely specified for additional protection against electric shock on socket outlets and similar circuits, since this level is low enough to prevent dangerous prolonged shock to a person.

Q15. An important limitation of an RCCB is that it:

A. Does not provide protection against overload or short-circuit current by itself and should be paired with an MCB/fuse ✓ Correct answer

B. Provides complete protection against all types of electrical faults on its own

C. Cannot be used in any three-phase system

D. Automatically corrects the power factor of the circuit

Explanation: An RCCB only responds to earth leakage (residual) current; it does not sense pure overload or short-circuit conditions, so it is typically installed alongside an MCB which handles those fault types.

Q16. When selecting the current rating of a fuse for a given circuit, the fuse should generally be rated:

A. Slightly above the normal full load current of the circuit, but below the safe current-carrying capacity of the cable ✓ Correct answer

B. Far below the normal running current, so it blows frequently

C. Exactly equal to the short-circuit fault current expected

D. At the maximum current the supply transformer can deliver

Explanation: A correctly rated fuse allows normal load current to flow continuously without nuisance blowing, while still operating fast enough to protect the cable before it is damaged by overcurrent.

Q17. HRC stands for, in the context of fuses, and refers to:

A. High Rupturing Capacity, a fuse type designed to safely interrupt very high fault currents ✓ Correct answer

B. High Resistance Cable, a special type of conductor

C. High Reactive Current, a measure of inductive load

D. Heavy Rated Contactor, a type of switching device

Explanation: HRC fuses use a sand-filled ceramic body designed to safely clear very high fault currents without violent rupture, unlike older rewireable fuse wire designs.

Q18. Compared to a semi-enclosed (rewireable) fuse, an HRC fuse offers the key advantage of:

A. More accurate, consistent, and faster operation under fault conditions, without risk of an incorrectly rated fuse wire being fitted ✓ Correct answer

B. Being infinitely reusable without replacement

C. Requiring no enclosure or housing

D. Operating only on DC circuits

Explanation: HRC fuses have a factory-calibrated element and cannot be tampered with (unlike rewireable fuses where an incorrect wire gauge can be fitted), giving more reliable and predictable protection.

Q19. In control panel wiring practice, keeping the power circuit wiring and the control circuit wiring physically separated is important mainly to:

A. Reduce interference, avoid accidental damage to low-current control wiring from high-current faults, and simplify safe troubleshooting ✓ Correct answer

B. Reduce the total copper used in the panel

C. Eliminate the need for any overload protection

D. Increase the panel's overall physical size unnecessarily

Explanation: Separating power and control wiring reduces electromagnetic interference on sensitive control signals and makes it safer and easier for technicians to trace and troubleshoot each circuit independently.

Q20. In a ladder logic (ladder diagram) representation of a control circuit, each horizontal line (rung) typically represents:

A. One complete control circuit path between the two vertical power rails ✓ Correct answer

B. A single physical wire with no logical function

C. The main three-phase power feeder only

D. A mechanical drawing of the panel's physical layout

Explanation: Ladder diagrams show control logic as rungs between two vertical rails, with each rung representing a distinct logical path that energizes an output when its contact conditions are satisfied.

Q21. In standard ladder diagram symbols, a Normally Open contact is typically drawn as:

A. Two parallel vertical lines with a gap between them ( | | ) ✓ Correct answer

B. Two parallel vertical lines joined by a diagonal slash through them

C. A circle with a cross inside

D. A rectangle with a diagonal line

Explanation: The standard NO contact symbol shows two short parallel lines representing an open gap, which closes (completes the circuit) only when actuated.

Q22. In a standard motor start-stop push button control circuit, the 'seal-in' (holding) contact is used to:

A. Keep the contactor coil energized after the momentary start button is released, using an auxiliary NO contact wired in parallel with the start button ✓ Correct answer

B. Permanently disconnect the stop button from the circuit

C. Provide overload protection instead of a thermal relay

D. Increase the voltage supplied to the motor

Explanation: Once the contactor picks up, its own auxiliary NO contact closes in parallel with the momentary start button, maintaining the coil circuit (latching it) even after the operator releases the button.

Q23. In a typical start-stop station, the Normally Closed stop button is wired:

A. In series with the coil circuit, so pressing it breaks the circuit and de-energizes the contactor ✓ Correct answer

B. In parallel with the start button

C. In series with the overload relay's heater element only

D. Directly across the motor's main terminals

Explanation: Because the stop button is NC and wired in series, pressing it interrupts the control circuit path, de-energizing the contactor coil and stopping the motor, regardless of the seal-in contact.

Q24. Electrical interlocking between a forward contactor and a reverse contactor in a reversing starter is essential to:

A. Prevent both contactors from closing simultaneously, which would cause a direct line-to-line short circuit ✓ Correct answer

B. Allow both directions to run at the same time for faster reversal

C. Eliminate the need for an overload relay

D. Increase the motor's starting torque

Explanation: If forward and reverse contactors closed together, two supply phases would be connected directly together through the contacts, causing a severe short circuit; interlocking (electrical and/or mechanical) prevents this.

Q25. A contactor that audibly 'chatters' (rapidly makes and breaks) when energized is most likely experiencing:

A. A loose or faulty coil connection, or supply voltage too low to fully seat the armature ✓ Correct answer

B. Perfectly normal operation requiring no attention

C. An oversized coil rating for the application

D. A fault in the connected motor's rotor only

Explanation: Chattering typically results from an unstable or insufficient coil voltage (loose wiring, sagging supply, or a failing coil) that cannot maintain a strong enough magnetic pull to hold the armature firmly closed.

Q26. Welded or stuck main contacts on a contactor are most commonly caused by:

A. Repeated switching under high current/short-circuit conditions or excessive switching frequency beyond the contactor's duty rating ✓ Correct answer

B. Using the correct auxiliary contacts for interlocking

C. Setting the overload relay to the nameplate current

D. Proper periodic maintenance and cleaning

Explanation: Arcing during switching under abnormally high current (such as a downstream short circuit) or excessive operating frequency can locally melt the contact surfaces, causing them to fuse together and stick closed.

Q27. If a motor control circuit's fuse blows, the typical symptom observed at the starter is:

A. The contactor will not energize at all when the start button is pressed, since the control circuit is dead ✓ Correct answer

B. The motor runs continuously and cannot be stopped

C. The overload relay trips instantly regardless of load

D. The motor runs at reduced speed only

Explanation: A blown control fuse removes power from the entire control circuit, so no command signal reaches the contactor coil and it cannot energize, even though the main power circuit upstream may be healthy.

Q28. A contactor coil that has failed 'open circuit' (burnt out) will result in:

A. The contactor failing to pick up/energize even when the start command is correctly given ✓ Correct answer

B. The contactor remaining permanently closed regardless of commands

C. The overload relay tripping immediately

D. No effect at all on the circuit's operation

Explanation: If the coil winding itself is open, no current can flow through it even with correct control voltage applied, so the magnetic field needed to pull in the contacts never develops.

Q29. When troubleshooting a motor that fails to start, using a multimeter to check continuity of the control circuit wiring (with power isolated) helps identify:

A. Breaks, loose connections, or failed components in the low-current control path before power is restored ✓ Correct answer

B. The exact running speed of the motor

C. The power factor of the load

D. The insulation resistance to earth

Explanation: A continuity check on the de-energized control circuit quickly isolates whether the fault lies in the push buttons, interlocks, wiring, or coil path, without exposing the technician to live circuits.

Q30. MCB trip curve types (B, C, D) are primarily selected based on:

A. The inrush/starting current characteristics of the connected load, to avoid nuisance tripping while still protecting against faults ✓ Correct answer

B. The colour of the wiring insulation used

C. The physical size of the distribution panel

D. The brand of the circuit breaker only

Explanation: Different loads have different inrush current profiles; matching the MCB curve type to the load's characteristics ensures the breaker does not trip on normal startup surges but still trips promptly on genuine faults.

Q31. A Type B MCB trip curve, with an instantaneous trip range of about 3-5 times rated current, is best suited for:

A. Resistive loads such as lighting and heating circuits with minimal inrush current ✓ Correct answer

B. Large motors with high starting inrush current

C. Welding transformers with very high inrush

D. X-ray machines with extreme inrush current

Explanation: Type B breakers trip at a relatively low current multiple, making them ideal for loads like lighting and general power circuits that do not experience significant inrush surges.

Q32. A Type C MCB trip curve, with an instantaneous trip range of about 5-10 times rated current, is commonly used for:

A. Moderately inductive loads such as small motors and transformers that have some starting inrush ✓ Correct answer

B. Purely resistive heating elements only

C. Precision instrumentation circuits with zero inrush

D. Battery charging circuits exclusively

Explanation: Type C breakers tolerate a higher inrush current multiple than Type B, suiting loads such as small motors and transformers that briefly draw several times their rated current on starting.

Q33. A Type D MCB trip curve, with an instantaneous trip range of roughly 10-20 times rated current, is typically chosen for:

A. Highly inductive loads with very high inrush, such as transformers, welding equipment, and X-ray machines ✓ Correct answer

B. General lighting circuits in an office

C. Simple resistive heater circuits

D. Low-power control circuits only

Explanation: Type D breakers tolerate the highest inrush multiples, making them appropriate for equipment with very heavy starting surges that would nuisance-trip a Type B or C breaker.

Q34. Protective device 'discrimination' (selectivity) between an upstream MCCB and a downstream MCB means that:

A. Only the downstream device nearest the fault trips, isolating just the faulty section while the rest of the installation stays powered ✓ Correct answer

B. Both devices always trip together for any fault anywhere in the system

C. Neither device will ever trip regardless of fault severity

D. The upstream device always trips first regardless of fault location

Explanation: Good discrimination is designed so that for a fault on a final circuit, only the local protective device closest to the fault operates, minimizing the disruption to the rest of the healthy installation.

Q35. In star-delta starter panels, the timing function for the star-to-delta transition can be implemented using:

A. An electronic or pneumatic timer relay set to the required starting duration ✓ Correct answer

B. Only a manually operated stopwatch with no relay involved

C. An overload relay repurposed as a timer

D. A fuse with a built-in delay mechanism

Explanation: A dedicated timer relay (electronic being most common today, pneumatic in older installations) is set to hold the star connection for enough time for the motor to accelerate before switching to delta.

Q36. An auxiliary (control) relay is often added to a circuit to:

A. Multiply the number of available switching contacts beyond what the original device provides, for driving multiple outputs from one signal ✓ Correct answer

B. Directly switch a three-phase motor's full load current

C. Replace the need for any contactor in the motor circuit

D. Provide overload protection to a motor

Explanation: Control relays are inserted where more contacts (NO/NC) are needed than the initiating device has, allowing one signal to simultaneously energize indicator lamps, interlocks, and other circuits.

Q37. Compared to traditional hard-wired relay logic, a PLC (Programmable Logic Controller) offers the main advantage of:

A. Control logic can be modified through software/programming changes rather than rewiring physical relays ✓ Correct answer

B. It eliminates the need for any input/output field wiring

C. It can never be used to control motor starters

D. It requires no power supply to operate

Explanation: A PLC executes the control logic in software, so changing the sequence of operation is a matter of reprogramming rather than physically rewiring relay contacts, offering flexibility and easier troubleshooting.

Q38. A loose terminal connection inside a control panel commonly leads to which symptom during operation?

A. Intermittent operation, localized heating, and possible arcing at that terminal over time ✓ Correct answer

B. Permanently stable and fault-free operation

C. Immediate blowing of the main incoming fuse only

D. An instant improvement in power factor

Explanation: A loose connection increases contact resistance, causing localized heating and voltage drop that can cause intermittent circuit behavior and, over time, visible discoloration, arcing, or a burnt terminal.

Q39. Periodic tightening of terminal connections to the manufacturer's specified torque during preventive maintenance is important because:

A. Connections can loosen over time due to thermal cycling and vibration, increasing resistance and fire risk if not checked ✓ Correct answer

B. Terminals never loosen once installed correctly

C. Over-tightening has no negative effect on any connector

D. It is only relevant for control wiring, never for power wiring

Explanation: Repeated heating/cooling cycles and vibration can gradually loosen even correctly torqued connections, so scheduled re-tightening checks help prevent the resistive heating that can lead to failures or fires.

Q40. An indicator (pilot) lamp on a control panel, typically wired across a contactor's auxiliary contact, is used to:

A. Give a visual indication of the status (running/stopped/tripped) of the associated equipment ✓ Correct answer

B. Provide the main power supply to the motor

C. Replace the function of the overload relay

D. Act as a fuse for the control circuit

Explanation: Pilot lamps give operators a quick visual status of equipment (for example, motor running, tripped, or ready) by being wired to switch on or off along with specific control conditions.

Q41. A momentary (spring-return) push button, as commonly used for Start/Stop functions, differs from a maintained (latching) selector switch in that it:

A. Automatically returns to its rest position once released, whereas a maintained switch stays in the position it is set to ✓ Correct answer

B. Cannot be used in any control circuit

C. Always carries the full motor load current directly

D. Requires no seal-in contact ever in the circuit

Explanation: A momentary push button only holds its actuated position while pressed and springs back when released, which is why a seal-in (holding) contact is needed to keep a motor running after the start button is released.

Q42. A selector switch in a control panel (e.g., Hand-Off-Auto) is used to:

A. Allow the operator to choose between different modes of operation for the equipment being controlled ✓ Correct answer

B. Provide short-circuit protection for the power circuit

C. Measure the current drawn by the motor

D. Replace the function of a contactor entirely

Explanation: A selector switch mechanically selects between different predefined control circuit states (such as manual local control, off, or automatic/remote control), directing how the equipment is operated.

Q43. If a fuse is deliberately fitted with a rating much higher than the cable's safe current-carrying capacity, the likely consequence is that:

A. The fuse may fail to blow before the cable overheats and its insulation is damaged, creating a fire risk ✓ Correct answer

B. The cable will automatically be protected regardless of the fuse rating

C. The fuse will blow instantly on normal load current

D. Nothing changes since fuse rating has no relation to cable size

Explanation: Overcurrent protection must be coordinated with the cable's rated ampacity; an oversized fuse can allow a sustained current beyond what the cable can safely carry without tripping, risking insulation breakdown and fire.

Q44. Using an undersized cable for a given load current, even with correctly rated overcurrent protection, primarily risks:

A. Excessive voltage drop and overheating of the cable under normal continuous loading ✓ Correct answer

B. Improved power factor of the circuit

C. Reduced starting current of connected motors

D. No risk at all if the fuse rating is correct

Explanation: A cable sized below the actual load current will run hotter than intended and experience larger voltage drop, degrading insulation life and equipment performance even if the fuse/breaker never trips.

Q45. In a control panel, which device is specifically responsible for fast disconnection during a genuine short-circuit fault, rather than a sustained overload?

A. The MCB/MCCB (or fuse) via its magnetic/instantaneous trip element ✓ Correct answer

B. The thermal overload relay's bimetallic strip alone

C. The indicator pilot lamp

D. The selector switch

Explanation: The magnetic (instantaneous) trip mechanism in an MCB/MCCB, or a fuse's fast-clearing element, is designed to interrupt very high short-circuit currents within milliseconds, distinct from the slower thermal response used for overloads.

Q46. A control transformer stepping down panel control circuit voltage (e.g., to 24V or 110V) from the main supply is used mainly to:

A. Reduce shock hazard risk to personnel working on control wiring, push buttons, and relays ✓ Correct answer

B. Increase the motor's running torque

C. Eliminate the need for any earthing in the panel

D. Boost the main power circuit voltage

Explanation: Operating control devices like push buttons and pilot lamps at a lower, safer voltage significantly reduces the risk of serious electric shock to operators and maintenance staff who frequently handle this wiring.

Q47. Clearly numbering and labelling wires and terminals in a control panel (wire marking) is a good practice mainly because it:

A. Greatly speeds up and simplifies fault tracing and troubleshooting against the circuit/wiring diagram ✓ Correct answer

B. Is purely decorative and has no practical benefit

C. Eliminates the need for a wiring diagram altogether

D. Reduces the current-carrying capacity required of the wire

Explanation: Consistent wire and terminal numbering that matches the schematic diagram lets a technician quickly locate and test specific circuit points during troubleshooting, saving significant diagnostic time.

Q48. Nuisance (unwanted) tripping of an RCCB is most often caused by:

A. Genuine but low-level earth leakage from aging insulation, moisture ingress, or an incorrect neutral-earth connection downstream ✓ Correct answer

B. The RCCB being of too high a current rating for the circuit

C. The MCB installed alongside it being of the wrong trip curve

D. Using copper conductors instead of aluminium

Explanation: Small but real leakage currents from degraded insulation, damp equipment, or an inadvertent neutral-to-earth link downstream of the RCCB can accumulate and exceed its trip threshold, causing what appears to be a nuisance trip.

Q49. A small RC (resistor-capacitor) snubber network connected across an AC contactor coil is fitted mainly to:

A. Suppress the voltage spike generated when the coil is de-energized, reducing arcing at the switching contact and EMI ✓ Correct answer

B. Increase the coil's holding current permanently

C. Replace the need for an auxiliary contact

D. Provide the main power switching for the load

Explanation: When an inductive coil is suddenly de-energized, its collapsing magnetic field induces a high voltage spike; a snubber across the coil absorbs this energy, reducing contact arcing on the switching device and limiting electrical noise.

Q50. Under IEC 60947-4-1 utilization categories for contactors, category AC-3 specifically covers:

A. Starting of squirrel-cage induction motors and switching them off only after they have reached running speed ✓ Correct answer

B. Switching of purely resistive, non-inductive loads only

C. Plugging and inching duty involving frequent reversal of a running motor

D. Only DC motor control applications

Explanation: AC-3 is the standard utilization category for contactors that start squirrel-cage motors and interrupt them during normal running, a common industrial duty distinct from AC-1 (resistive loads) and AC-4 (plugging/inching).

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

Electrical Troubleshooting MCQ Quiz — Part 1 of 5 Electrical Troubleshooting MCQ Quiz — Part 2 of 5 Electrical Troubleshooting MCQ Quiz — Part 4 of 5 Electrical Troubleshooting MCQ Quiz — Part 5 of 5 Electrical Troubleshooting Q&A (70 questions)Motor Troubleshooting Q&A (70 questions)