Q.1 What is the primary function of a single‑pole, single‑throw (SPST) switch?
To provide two separate circuits
To switch a single circuit on or off
To convert AC to DC
To amplify a signal
Explanation - An SPST switch has one input and one output; it simply opens or closes a single circuit.
Correct answer is: To switch a single circuit on or off
Q.2 Which type of relay uses an electromagnet to move a set of contacts?
Solid‑state relay
Reed relay
Electromechanical relay
Thermal relay
Explanation - Electromechanical relays rely on an electromagnet to physically move contacts, unlike solid‑state relays which use semiconductor devices.
Correct answer is: Electromechanical relay
Q.3 In a DPDT (double‑pole, double‑throw) switch, how many separate circuits can be controlled?
One
Two
Four
Eight
Explanation - DPDT switches have two poles, each with two throws, allowing two independent circuits to be switched.
Correct answer is: Two
Q.4 A relay rated at 250 V AC, 5 A can safely switch which of the following loads?
300 V AC, 4 A
250 V AC, 6 A
200 V DC, 5 A
250 V AC, 5 A
Explanation - The relay’s maximum ratings are 250 V AC and 5 A; exceeding either rating risks damage.
Correct answer is: 250 V AC, 5 A
Q.5 What is the main advantage of a solid‑state relay (SSR) over an electromechanical relay?
Higher switching speed and no moving parts
Lower cost
Ability to switch higher currents
Simpler wiring
Explanation - SSRs use semiconductor devices, giving them faster response times and greater reliability because they have no mechanical contacts.
Correct answer is: Higher switching speed and no moving parts
Q.6 Which contact configuration provides a normally‑closed (NC) contact?
Contact that is open when the coil is de‑energized
Contact that is closed when the coil is de‑energized
Contact that is always closed
Contact that is always open
Explanation - A normally‑closed contact is closed (conducting) when the relay coil is not energized; energizing the coil opens it.
Correct answer is: Contact that is closed when the coil is de‑energized
Q.7 A push‑button switch labeled ‘NO’ means:
Normally Open
Normally Closed
No Output
Not Operable
Explanation - 'NO' on a switch denotes a normally open contact that closes only while the button is pressed.
Correct answer is: Normally Open
Q.8 Which phenomenon can cause contact bounce in mechanical switches?
Thermal runaway
Magnetic hysteresis
Mechanical vibration of contacts
Semiconductor breakdown
Explanation - When contacts close, they can bounce due to mechanical inertia, causing multiple rapid make‑break events.
Correct answer is: Mechanical vibration of contacts
Q.9 What does the term ‘coil voltage’ refer to in a relay?
Voltage the contacts can switch
Voltage required to energize the relay’s coil
Voltage rating of the relay housing
Voltage drop across the contacts
Explanation - Coil voltage is the voltage needed to generate the magnetic field that moves the relay contacts.
Correct answer is: Voltage required to energize the relay’s coil
Q.10 A latching relay differs from a standard relay because:
It does not have a coil
It stays in its last state after the coil is de‑energized
It can only handle DC loads
It requires a continuous power supply to stay latched
Explanation - Latching relays retain their position mechanically or magnetically after the actuating current is removed.
Correct answer is: It stays in its last state after the coil is de‑energized
Q.11 Which of the following is NOT a typical contact rating parameter?
Maximum switching voltage
Contact resistance
Coil inductance
Maximum switching current
Explanation - Coil inductance is a property of the relay coil, not a contact rating. Contact ratings involve voltage, current, and resistance.
Correct answer is: Coil inductance
Q.12 In a relay symbol, a diagonal line through the coil indicates:
The relay is a solid‑state type
The coil is energized
The relay is de‑energized
The relay has a built‑in diode
Explanation - In schematic symbols, a diagonal line through the coil denotes that the coil is receiving power (energized).
Correct answer is: The coil is energized
Q.13 What type of switch is commonly used for emergency stop circuits?
Push‑button normally open
Push‑button normally closed
Toggle switch
Rotary switch
Explanation - An emergency stop is normally closed so that cutting power (opening the circuit) stops the machine immediately.
Correct answer is: Push‑button normally closed
Q.14 A reed relay operates by:
Using a solid‑state transistor
Moving a ferromagnetic armature to close contacts
Magnetically actuating sealed reeds inside a glass tube
Heating a bimetallic strip
Explanation - Reed relays contain two ferromagnetic reeds that close when a magnetic field is applied, all sealed within a glass envelope.
Correct answer is: Magnetically actuating sealed reeds inside a glass tube
Q.15 Which parameter determines how fast a relay can switch back and forth?
Contact resistance
Coil inductance and voltage
Housing material
Contact material
Explanation - Higher coil voltage and lower inductance reduce the time needed to build and collapse the magnetic field, increasing switching speed.
Correct answer is: Coil inductance and voltage
Q.16 What is the purpose of a snubber circuit across relay contacts?
To increase contact resistance
To suppress voltage spikes caused by inductive loads
To reduce coil power consumption
To provide a visual indicator of contact status
Explanation - Snubber circuits (RC or diode) protect contacts from arcing when switching inductive loads.
Correct answer is: To suppress voltage spikes caused by inductive loads
Q.17 A “break before make” (B‑B‑M) contact arrangement ensures:
Both contacts close simultaneously
The first contact opens before the second closes
The relay never opens
The coil never de‑energizes
Explanation - B‑B‑M avoids short‑circuits by guaranteeing an open interval between the two contacts.
Correct answer is: The first contact opens before the second closes
Q.18 Which of the following is a common failure mode of electromechanical relays?
Thermal runaway
Contact welding
Semiconductor breakdown
Dielectric leakage
Explanation - When high currents flow, contacts can fuse together, preventing the relay from opening.
Correct answer is: Contact welding
Q.19 The term “dry contact” refers to:
A contact that switches without any voltage applied to it
A contact that operates in a vacuum
A contact that uses liquid metal
A contact that is permanently closed
Explanation - Dry contacts are isolated from the control circuit; they merely close or open a separate circuit.
Correct answer is: A contact that switches without any voltage applied to it
Q.20 In a DPDT switch wired as a polarity reverser, what happens when the lever is flipped?
Both poles connect to the same terminal
The direction of current through the load reverses
The switch disables power completely
The switch converts AC to DC
Explanation - Flipping a DPDT wired as a polarity reverser swaps the connections, reversing the voltage polarity across the load.
Correct answer is: The direction of current through the load reverses
Q.21 A relay’s “contact rating” of 10 A 30 V AC means:
The coil can handle 10 A at 30 V
The contacts can safely switch up to 10 A at 30 V AC
The relay can be powered by 10 A at 30 V AC
The relay will fail above 10 A and 30 V
Explanation - Contact rating defines the maximum current and voltage the contacts can handle without damage.
Correct answer is: The contacts can safely switch up to 10 A at 30 V AC
Q.22 Which type of switch is commonly used to select between multiple voltage levels in a power supply?
Toggle switch
Rotary switch
Push‑button switch
Limit switch
Explanation - Rotary switches have multiple positions, each connecting a different circuit, ideal for voltage selection.
Correct answer is: Rotary switch
Q.23 What is the purpose of a “flyback diode” across a relay coil in a DC circuit?
To increase coil voltage
To protect driving circuitry from inductive voltage spikes
To speed up coil energization
To reduce coil resistance
Explanation - When the coil is de‑energized, the diode provides a path for the stored energy, limiting the voltage spike.
Correct answer is: To protect driving circuitry from inductive voltage spikes
Q.24 Which contact arrangement is used when a circuit must stay closed after the control signal is removed?
Normally open (NO)
Normally closed (NC)
Latching (self‑holding) contacts
Break‑before‑make contacts
Explanation - Latching contacts retain their state without continuous control power.
Correct answer is: Latching (self‑holding) contacts
Q.25 A “double‑throw” switch can:
Connect one input to two possible outputs
Only connect two inputs together
Provide three positions
Supply power to a motor
Explanation - Double‑throw means the single pole can connect to either of two throws (outputs).
Correct answer is: Connect one input to two possible outputs
Q.26 Which of the following materials is most commonly used for relay contacts to ensure low resistance and good arc resistance?
Copper
Silver‑cadmium oxide
Aluminum
Gold
Explanation - Silver‑cadmium oxide offers low resistance and can withstand arcing better than pure copper or gold.
Correct answer is: Silver‑cadmium oxide
Q.27 In a circuit diagram, a relay symbol with a coil and two sets of contacts (one normally open, one normally closed) is known as:
A DPDT relay
A SPST relay
A single‑coil relay with auxiliary contacts
A solid‑state relay
Explanation - One coil controls both a NO and a NC auxiliary contact, providing multiple switching functions.
Correct answer is: A single‑coil relay with auxiliary contacts
Q.28 What does “contact bounce” cause in digital circuits?
Incorrect voltage levels
Multiple unwanted transitions (glitches)
Permanent open circuits
Reduced power consumption
Explanation - Mechanical bounce can generate rapid make‑break cycles that appear as noise or multiple pulses to digital logic.
Correct answer is: Multiple unwanted transitions (glitches)
Q.29 Which specification indicates the maximum voltage a relay can withstand while the contacts are open?
Coil voltage
Insulation voltage rating
Contact rating
Power rating
Explanation - Insulation voltage is the maximum voltage the open contacts can safely block without breakdown.
Correct answer is: Insulation voltage rating
Q.30 A “make‑before‑break” (M‑B‑B) contact arrangement is used when:
A short circuit must be avoided
A continuous connection is required during switching
The relay must never close
The coil voltage is very high
Explanation - M‑B‑B ensures the new contact closes before the old one opens, preventing interruption of the circuit.
Correct answer is: A continuous connection is required during switching
Q.31 Which of the following best describes a “limit switch”?
A switch that limits current flow
A mechanical switch activated by motion or position
A switch that limits voltage
A digital logic gate
Explanation - Limit switches are triggered by physical movement, often used for safety or end‑of‑travel detection.
Correct answer is: A mechanical switch activated by motion or position
Q.32 If a relay coil is rated for 12 V DC and is supplied with 24 V DC, what is the most likely outcome?
The relay will operate twice as fast with no damage
The coil will overheat and possibly burn out
The contacts will open automatically
The relay will become a solid‑state device
Explanation - Applying higher voltage than rated increases coil current, causing excessive heating and possible failure.
Correct answer is: The coil will overheat and possibly burn out
Q.33 What type of switch is typically used in a light dimmer circuit?
Rotary switch
Potentiometer (variable resistor)
Push‑button switch
Reed relay
Explanation - A potentiometer varies resistance to adjust voltage delivered to the lamp, providing dimming capability.
Correct answer is: Potentiometer (variable resistor)
Q.34 Which relay type is most suitable for high‑speed switching in digital logic circuits?
Electromechanical relay
Reed relay
Solid‑state relay
Thermal relay
Explanation - SSRs have no moving parts and can switch in microseconds, matching digital logic speed requirements.
Correct answer is: Solid‑state relay
Q.35 A “time‑delay relay” provides:
Instantaneous switching
A programmable delay before contacts change state
Higher contact voltage rating
Built‑in over‑current protection
Explanation - Time‑delay relays introduce a set delay between coil energization and contact movement.
Correct answer is: A programmable delay before contacts change state
Q.36 Which contact arrangement is used in a “three‑way” lighting circuit?
Single‑pole double‑throw (SPDT)
Double‑pole double‑throw (DPDT)
Single‑pole single‑throw (SPST)
Double‑pole single‑throw (DPST)
Explanation - Three‑way circuits use SPDT switches to allow control of a light from two locations.
Correct answer is: Single‑pole double‑throw (SPDT)
Q.37 When a relay is specified as “DC 5 A, AC 10 A”, what does this indicate?
The coil can be powered with either DC or AC
The contacts can handle 5 A DC or 10 A AC
The relay only works with DC
The relay must be used with a transformer
Explanation - Contact ratings differ for DC and AC because of arcing characteristics; AC can usually carry higher current.
Correct answer is: The contacts can handle 5 A DC or 10 A AC
Q.38 In a schematic, a switch drawn as a break in the line with a ‘+’ symbol indicates:
Normally closed (NC) switch
Normally open (NO) switch
Toggle switch
Push‑button switch
Explanation - The break with a ‘+’ shows that the contacts are open until the switch is actuated.
Correct answer is: Normally open (NO) switch
Q.39 Which of the following is a primary advantage of using a “reed relay” in high‑frequency applications?
Low inductance and fast response
High current handling
Built‑in voltage regulation
Ability to operate without a coil
Explanation - Reed relays have very low coil inductance and can switch quickly, making them suitable for RF and high‑speed circuits.
Correct answer is: Low inductance and fast response
Q.40 A “thermal overload relay” protects a motor by:
Opening the circuit when the temperature exceeds a set point
Providing a constant voltage supply
Increasing the motor speed
Reducing the coil voltage
Explanation - Thermal overload relays sense temperature rise due to overcurrent and open to protect the motor.
Correct answer is: Opening the circuit when the temperature exceeds a set point
Q.41 What does “contact resistance” affect most directly?
The voltage rating of the relay
The power loss and heating at the contacts
The coil inductance
The mechanical life of the switch
Explanation - Higher contact resistance leads to greater I²R losses, causing heating and possible contact degradation.
Correct answer is: The power loss and heating at the contacts
Q.42 Which type of switch is designed to be actuated by a fluid pressure change?
Proximity switch
Pressure switch
Limit switch
Float switch
Explanation - Pressure switches detect fluid pressure variations and change state accordingly.
Correct answer is: Pressure switch
Q.43 In a “push‑button” switch, the term “momentary” means:
The switch stays in its last position after being pressed
The switch changes state only while being pressed
The switch can handle high voltage
The switch has a built‑in LED
Explanation - Momentary switches return to their original state once released.
Correct answer is: The switch changes state only while being pressed
Q.44 Which of the following best describes a “solid‑state switch”?
A mechanical contact operated by a lever
A semiconductor device that controls current flow without moving parts
A relay with a built‑in diode
A switch that uses a spring mechanism
Explanation - Solid‑state switches use components like MOSFETs or TRIACs to perform switching.
Correct answer is: A semiconductor device that controls current flow without moving parts
Q.45 When a relay coil is de‑energized, what happens to a normally open (NO) contact?
It remains closed
It opens
It becomes normally closed
It shorts the circuit
Explanation - An NO contact is open when the coil is not energized and closes only when the coil is powered.
Correct answer is: It opens
Q.46 A “selector switch” is most commonly used for:
Choosing one of several circuit paths
Providing over‑current protection
Generating a high‑frequency signal
Detecting temperature
Explanation - Selector switches allow the user to select among multiple positions, each connecting a different circuit.
Correct answer is: Choosing one of several circuit paths
Q.47 Which of the following is NOT a typical characteristic of a relay’s “contact rating”?
Maximum switching voltage
Maximum make/break current
Coil resistance
Maximum switching power
Explanation - Coil resistance pertains to the coil, not the contacts; contact rating concerns voltage, current, and power.
Correct answer is: Coil resistance
Q.48 In a circuit with a relay and a diode across the coil, the diode is placed:
In series with the coil
Parallel to the coil, reverse‑biased during normal operation
Across the contacts
Between the coil and ground
Explanation - The diode provides a path for the inductive kick when the coil is switched off, protecting the driver.
Correct answer is: Parallel to the coil, reverse‑biased during normal operation
Q.49 Which switch type is designed to automatically return to its original position after being actuated?
Push‑button momentary switch
Toggle switch
Rotary switch
Limit switch
Explanation - Momentary push‑buttons spring back to the original state after release.
Correct answer is: Push‑button momentary switch
Q.50 A relay’s “pull‑in” time refers to:
The time it takes for the contacts to open after the coil is de‑energized
The time required for the coil to generate enough magnetic force to close the contacts
The time the relay can stay energized continuously
The time it takes for the contacts to cool down
Explanation - Pull‑in time is the delay between coil energization and contact closure.
Correct answer is: The time required for the coil to generate enough magnetic force to close the contacts
Q.51 In a DPDT switch wired as a motor direction controller, which of the following is true when the lever is in the upper position?
Motor runs forward
Motor runs in reverse
Motor is braked
Motor is disconnected
Explanation - The DPDT swaps the polarity of the motor leads; one position gives forward rotation, the other reverse.
Correct answer is: Motor runs forward
Q.52 What is the main reason to use a “sealed relay” in harsh environments?
To reduce coil resistance
To protect internal contacts from dust, moisture, and contaminants
To increase switching speed
To lower the contact rating
Explanation - Sealed relays have enclosures that keep out corrosive agents, extending life in tough conditions.
Correct answer is: To protect internal contacts from dust, moisture, and contaminants
Q.53 A “breaker” differs from a “switch” mainly because:
A breaker can interrupt fault currents automatically
A breaker is always manually operated
A breaker cannot be reset
A breaker is only used for low voltage
Explanation - Circuit breakers provide over‑current protection by opening automatically under fault conditions.
Correct answer is: A breaker can interrupt fault currents automatically
Q.54 Which type of relay would you select for switching a high‑power 240 V AC motor?
Reed relay
Solid‑state relay with heat sink
Miniature electromechanical relay
Signal‑level relay
Explanation - SSRs designed for high current and voltage, with adequate heat dissipation, are suitable for motor switching.
Correct answer is: Solid‑state relay with heat sink
Q.55 What does the term “arc suppression” refer to in relay design?
Increasing coil voltage
Using materials or circuits to reduce the voltage spike when contacts open
Reducing contact resistance
Eliminating the need for a coil
Explanation - Arc suppression methods (snubbers, contact materials) limit arcing that occurs during contact breakage.
Correct answer is: Using materials or circuits to reduce the voltage spike when contacts open
Q.56 Which contact configuration provides the greatest isolation between the control circuit and the load circuit?
Dry contacts
Wet contacts
Solid‑state contacts
Reed contacts
Explanation - Dry contacts have no electrical connection to the control side, offering full isolation.
Correct answer is: Dry contacts
Q.57 A “photoelectric switch” detects:
Magnetic fields
Light intensity changes
Temperature variations
Acoustic signals
Explanation - Photoelectric switches use a light source and detector to sense object presence or interruption of light.
Correct answer is: Light intensity changes
Q.58 The term “coil inductance” in a relay influences:
The voltage rating of the contacts
The speed at which the relay can be switched
The mechanical life of the contacts
The type of enclosure needed
Explanation - Higher inductance slows the buildup and collapse of magnetic field, increasing switching times.
Correct answer is: The speed at which the relay can be switched
Q.59 A “circuit breaker” with a 10 A rating will:
Always allow up to 10 A regardless of duration
Trip instantly at 10 A
Trip after a time delay if current exceeds 10 A for a sustained period
Never trip because it’s a breaker, not a fuse
Explanation - Breakers have time‑current characteristics; they allow temporary overloads but trip on prolonged overcurrent.
Correct answer is: Trip after a time delay if current exceeds 10 A for a sustained period
Q.60 In a push‑button switch with a built‑in LED indicator, the LED typically lights when:
The switch is in the open position
The switch is closed (pressed)
The coil is energized
The power supply fails
Explanation - The LED is wired in parallel with the switch contacts, lighting when the circuit is completed.
Correct answer is: The switch is closed (pressed)
Q.61 Which of the following is a key advantage of a “solid‑state relay” over an electromechanical relay for AC load control?
Zero on‑state voltage drop
No audible click and longer life
Ability to handle higher voltage without a heat sink
Built‑in mechanical latch
Explanation - SSRs have silent operation and no wear parts, extending operational life.
Correct answer is: No audible click and longer life
Q.62 A “limit switch” used in a conveyor system typically detects:
Temperature
End of travel or position of a moving part
Voltage level
Current flow
Explanation - Limit switches are actuated by mechanical movement, providing position feedback.
Correct answer is: End of travel or position of a moving part
Q.63 When a relay is specified as “DPDT, 12 V coil, 5 A contacts”, which of the following statements is true?
The relay can switch up to 12 A at 5 V
The coil requires 12 V to operate, and each pole can handle 5 A
Both poles share a total of 5 A rating
The relay can only be used with DC loads
Explanation - Coil voltage is separate from contact current rating; each pole can carry 5 A independently.
Correct answer is: The coil requires 12 V to operate, and each pole can handle 5 A
Q.64 Which device is most appropriate for detecting the presence of a metallic object without physical contact?
Proximity switch
Push‑button switch
Limit switch
Toggle switch
Explanation - Proximity switches sense metallic objects via magnetic or inductive fields without contact.
Correct answer is: Proximity switch
Q.65 A “make‑before‑break” contact arrangement is especially important in:
Power‑off safety circuits
Uninterrupted power supply (UPS) transfer switches
Thermal overload protection
Low‑current signal switching
Explanation - M‑B‑B ensures there is never a moment when both sources are disconnected, maintaining continuity.
Correct answer is: Uninterrupted power supply (UPS) transfer switches
Q.66 Which type of relay would you select for a low‑power signal switching application (≤ 0.5 A, ≤ 30 V)?
Electromechanical power relay
Signal‑level (small) relay
Thermal overload relay
Motor starter relay
Explanation - Signal relays are designed for low current and voltage, offering compact size and fast operation.
Correct answer is: Signal‑level (small) relay
Q.67 In a circuit using a relay, the term “hold‑in current” refers to:
The minimum current needed to keep the contacts closed
The current through the coil required to keep the relay energized after actuation
The current rating of the contacts
The current that charges the flyback diode
Explanation - Hold‑in current is the steady‑state coil current necessary to maintain magnetic force.
Correct answer is: The current through the coil required to keep the relay energized after actuation
Q.68 Which of the following is a common method to debounce a mechanical switch in a microcontroller circuit?
Add a series resistor
Use a Schmitt trigger or software delay
Increase the supply voltage
Replace the switch with a relay
Explanation - Schmitt triggers or software debouncing filter out the rapid transitions caused by bounce.
Correct answer is: Use a Schmitt trigger or software delay
Q.69 A “float switch” is typically used to:
Detect temperature changes
Measure fluid level
Sense magnetic fields
Control motor speed
Explanation - Float switches move with liquid level and close or open a circuit accordingly.
Correct answer is: Measure fluid level
Q.70 When selecting a relay for a circuit that will be switched at 10 kHz, which characteristic is most critical?
Contact rating
Coil inductance and switching speed
Physical size
Enclosure material
Explanation - High‑frequency operation demands low inductance and fast actuation; solid‑state relays are typically required.
Correct answer is: Coil inductance and switching speed
Q.71 Which of the following best explains why a relay’s contacts may “weld” together?
Excessive mechanical wear
Repeated contact bounce
High switching current causing arcing and melting of contact material
Insufficient coil voltage
Explanation - Arcing at high currents can melt the contact surface, fusing them together.
Correct answer is: High switching current causing arcing and melting of contact material
Q.72 A “Hall‑effect sensor” used as a switch operates on:
Detecting changes in magnetic field
Measuring temperature
Sensing light intensity
Counting mechanical rotations
Explanation - Hall‑effect devices output a voltage when exposed to a magnetic field, acting as non‑contact switches.
Correct answer is: Detecting changes in magnetic field
Q.73 In a relay datasheet, the term “operating temperature range” indicates:
The temperature at which the coil will overheat
The ambient temperature range over which the relay will function correctly
The temperature of the contacts during switching
The maximum temperature the relay housing can endure
Explanation - Operating temperature defines the environment in which the relay’s performance is guaranteed.
Correct answer is: The ambient temperature range over which the relay will function correctly
Q.74 Which type of switch is commonly used for safety interlocks on machine doors?
Normally open (NO) push‑button
Normally closed (NC) limit switch
Toggle switch
Rotary switch
Explanation - NC limit switches open when a door is opened, cutting power for safety.
Correct answer is: Normally closed (NC) limit switch
Q.75 A “solid‑state relay” for AC loads typically contains which semiconductor device?
BJT
MOSFET
Triac
IGBT
Explanation - Triacs are commonly used in SSRs for AC switching because they can conduct in both directions.
Correct answer is: Triac
Q.76 When a relay’s coil is energized, the magnetic field causes:
The contacts to open
The contacts to close
The coil resistance to increase
The enclosure to heat up only
Explanation - Energizing the coil generates magnetic force that moves the armature, closing the contacts.
Correct answer is: The contacts to close
Q.77 What is the function of a “thermal fuse” in a motor circuit?
To provide a permanent open circuit when temperature exceeds a set point
To reset automatically after cooling
To limit inrush current
To protect against voltage spikes
Explanation - Thermal fuses melt irreversibly when a temperature threshold is exceeded, cutting off power.
Correct answer is: To provide a permanent open circuit when temperature exceeds a set point
Q.78 A “DPST” (double‑pole, single‑throw) switch can:
Control two separate circuits with a single on/off action
Select between three circuits
Provide a polarity reversal function
Operate as a latching switch
Explanation - DPST has two independent poles that are switched together, each with one throw.
Correct answer is: Control two separate circuits with a single on/off action
Q.79 In a relay, the term “contact bounce” is most problematic for:
High‑frequency AC loads
Digital circuits that interpret each bounce as multiple pulses
Power distribution panels
Motor starters
Explanation - Bounce creates rapid on/off transitions that can be misread as multiple events by digital logic.
Correct answer is: Digital circuits that interpret each bounce as multiple pulses
Q.80 Which of the following is true about a “reed relay” compared to a standard electromechanical relay?
Reed relays have a higher coil voltage requirement
Reed relays generally have faster response times and lower power consumption
Reed relays can handle higher currents
Reed relays do not require any coil
Explanation - Because of their small size and low mass, reed relays switch faster and need less coil power.
Correct answer is: Reed relays generally have faster response times and lower power consumption
Q.81 A “push‑button” switch labeled ‘NC’ will:
Close the circuit only when pressed
Open the circuit only when pressed
Remain closed regardless of pressing
Remain open regardless of pressing
Explanation - NC (normally closed) contacts open when the button is pressed, breaking the circuit.
Correct answer is: Open the circuit only when pressed
Q.82 Which component is typically placed in parallel with a relay coil to protect against voltage spikes?
Resistor
Capacitor
Diode (flyback diode)
Inductor
Explanation - The diode provides a path for the inductive kick when the coil is de‑energized, limiting voltage spikes.
Correct answer is: Diode (flyback diode)
Q.83 For a relay used in a 120 V AC lighting circuit, which of the following contact ratings is the minimum acceptable?
5 A, 120 V AC
10 A, 250 V AC
2 A, 240 V AC
15 A, 120 V DC
Explanation - Standard household lighting loads are usually under 5 A at 120 V AC; this rating meets the requirement.
Correct answer is: 5 A, 120 V AC
Q.84 A “break‑before‑make” contact configuration is essential in which of the following scenarios?
When you need to avoid a short circuit during switching
When you need continuous power transfer
When you need to double the voltage
When you need to reduce contact bounce
Explanation - B‑B‑M ensures contacts are never both closed at the same time, preventing a momentary short.
Correct answer is: When you need to avoid a short circuit during switching
Q.85 In a “latching relay”, the coil must be:
Continuously powered to keep contacts in the actuated position
Pulsed once to change state, after which it can be de‑energized
Connected to AC only
Connected to a high‑current source
Explanation - Latching relays retain their last state without continuous coil power; a pulse changes the state.
Correct answer is: Pulsed once to change state, after which it can be de‑energized
Q.86 Which of the following best describes a “solid‑state switch” used for DC loads?
A MOSFET or IGBT based device
A reed relay
A mechanical toggle switch
A thermal overload relay
Explanation - Solid‑state DC switches commonly employ MOSFETs or IGBTs to control current without moving contacts.
Correct answer is: A MOSFET or IGBT based device
Q.87 A “float‑type level switch” typically uses:
Magnetic reed switch with a floating magnet
Optical sensor
Mechanical lever directly attached to the float
Thermal sensor
Explanation - The float moves a lever or arm that opens/closes a switch as fluid level changes.
Correct answer is: Mechanical lever directly attached to the float
Q.88 When a relay’s contacts are rated for 30 V DC, why might it still be safe to switch a 120 V AC load?
Because AC voltage is less harmful than DC
Because the relay’s AC rating is usually higher than its DC rating
Because the coil voltage is independent of the contact rating
It is never safe; the rating must always be obeyed
Explanation - Arcing is more severe with DC; many relays have higher voltage ratings for AC than DC.
Correct answer is: Because the relay’s AC rating is usually higher than its DC rating
Q.89 In a “break‑before‑make” DPDT switch used for reversing a motor, what happens during the transition?
Both forward and reverse contacts are closed briefly
All contacts are open for a short moment
The motor continues to run in the same direction
The switch locks in place
Explanation - B‑B‑M ensures there is a brief open interval, preventing both polarities from being applied simultaneously.
Correct answer is: All contacts are open for a short moment
Q.90 Which of the following devices is specifically designed to protect circuits from over‑temperature?
Thermal overload relay
Solid‑state relay
Reed relay
Push‑button switch
Explanation - Thermal overload relays detect excessive temperature (usually due to overcurrent) and open the circuit.
Correct answer is: Thermal overload relay
Q.91 A relay with a “coil resistance” of 200 Ω and a coil voltage of 12 V will draw approximately:
0.06 A
1.2 A
24 A
0.24 A
Explanation - Current I = V / R = 12 V / 200 Ω = 0.06 A.
Correct answer is: 0.06 A
Q.92 When a relay is described as “normally open, make‑before‑break”, what does this imply about its contact behavior?
Contacts are closed when the coil is de‑energized
New contacts close before the old contacts open during actuation
Contacts never open
Contacts open before they close
Explanation - Make‑before‑break ensures continuity by closing the new contact prior to opening the previous one.
Correct answer is: New contacts close before the old contacts open during actuation
Q.93 Which of the following is a typical symptom of a failing relay coil?
Excessive contact bounce
Weak or intermittent actuation
Increased contact resistance
Overheating of the contacts
Explanation - A deteriorated coil may not generate enough magnetic force, leading to unreliable operation.
Correct answer is: Weak or intermittent actuation
Q.94 In a control panel, a “pilot light” is often connected to:
The contacts of a relay to indicate its state
The coil of a relay for power indication
The power supply directly
The ground line
Explanation - Pilot lights are wired to relay contacts so they illuminate when the relay is in a particular state.
Correct answer is: The contacts of a relay to indicate its state
Q.95 Which of the following best describes an “intermediate relay”?
A relay that operates at high voltage levels
A small relay used to drive larger power relays or contactors
A relay that provides voltage regulation
A relay with built‑in timing functions
Explanation - Intermediate relays act as control devices that switch the coil of larger, higher‑current relays.
Correct answer is: A small relay used to drive larger power relays or contactors
Q.96 In a digital circuit, a mechanical switch with bounce can be effectively debounced by:
Increasing the supply voltage
Adding a capacitor across the switch
Using a Schmitt trigger input
Replacing the switch with a resistor
Explanation - Schmitt triggers have hysteresis, filtering out rapid transitions caused by bounce.
Correct answer is: Using a Schmitt trigger input
Q.97 Which type of relay is most suitable for switching a 12 V DC, 20 A motor?
Reed relay
Solid‑state relay with adequate heat sinking
Signal‑level relay
Miniature electromechanical relay
Explanation - SSRs can handle high DC currents with proper heat management, whereas reed and signal relays are too small.
Correct answer is: Solid‑state relay with adequate heat sinking
Q.98 A “DPDT” switch has:
Two poles and two throws
Two poles and one throw
One pole and two throws
Four poles and one throw
Explanation - DPDT stands for double‑pole, double‑throw, meaning each of the two poles can connect to either of two throws.
Correct answer is: Two poles and two throws
Q.99 What is the purpose of a “snubber” (RC network) across relay contacts?
To increase contact resistance
To limit voltage spikes and suppress arcing
To provide a visual indicator of contact status
To boost coil voltage
Explanation - Snubbers absorb the energy from inductive loads, reducing contact wear and arcing.
Correct answer is: To limit voltage spikes and suppress arcing
Q.100 In a relay with a “coil voltage” of 24 V AC, which of the following power supplies is appropriate?
24 V DC
12 V AC
24 V AC
48 V DC
Explanation - The coil voltage rating must match the supply type and value; a 24 V AC coil requires a 24 V AC source.
Correct answer is: 24 V AC
Q.101 A “limit switch” with a ‘normally open’ (NO) contact will:
Open the circuit when the actuator is engaged
Close the circuit when the actuator is engaged
Remain permanently closed
Remain permanently open
Explanation - NO contacts close only when the mechanical actuator moves into the activated position.
Correct answer is: Close the circuit when the actuator is engaged
Q.102 Which parameter is most important when selecting a relay for a high‑frequency PWM control of a motor?
Contact resistance
Coil inductance and switching speed
Physical size
Enclosure material
Explanation - High‑frequency PWM requires a relay (or solid‑state switch) capable of fast actuation and low inductance to follow the signal.
Correct answer is: Coil inductance and switching speed
Q.103 A “push‑button” switch that stays latched after being pressed is called:
Momentary switch
Latching (maintained) switch
Toggle switch
Rotary switch
Explanation - A latching push‑button remains in its last state until pressed again.
Correct answer is: Latching (maintained) switch
Q.104 Which of the following is an advantage of using a “reed relay” in RF applications?
High current rating
Low insertion loss and fast switching
Built‑in voltage regulation
Ability to handle high temperatures
Explanation - Reed relays have minimal parasitic capacitance and inductance, preserving signal integrity at RF frequencies.
Correct answer is: Low insertion loss and fast switching
Q.105 When a relay’s contacts are rated for “10 A 30 V AC” and you need to switch a 240 V AC load, what is the correct approach?
Use the relay as is; the rating is sufficient
Select a relay with a higher voltage rating
Connect two relays in series to share the voltage
Reduce the load voltage with a transformer
Explanation - Contact ratings must not be exceeded; a 240 V AC load requires a relay rated for at least that voltage.
Correct answer is: Select a relay with a higher voltage rating
Q.106 A “float switch” used in a water tank typically closes its contacts when:
The water level is low
The water level is high
The temperature rises
The tank is empty
Explanation - When the float rises with water, it actuates the switch, closing contacts at the desired level.
Correct answer is: The water level is high
Q.107 Which type of contact configuration is used to ensure that a circuit is broken before another circuit is made?
Make‑before‑break
Break‑before‑make
Normally closed
Normally open
Explanation - Break‑before‑make guarantees the first contact opens before the second closes.
Correct answer is: Break‑before‑make
Q.108 In a relay datasheet, the term “pull‑out force” refers to:
The mechanical force needed to keep contacts closed
The magnetic force required to open the contacts when the coil is de‑energized
The voltage needed to energize the coil
The current rating of the contacts
Explanation - Pull‑out force is the magnetic attraction that must be overcome to release the armature.
Correct answer is: The magnetic force required to open the contacts when the coil is de‑energized
Q.109 Which of the following is a common cause of contact welding in relays?
Low switching frequency
Excessive voltage spikes
Switching low‑current signals
Using a solid‑state relay
Explanation - High voltage spikes generate intense arcing, which can melt contact surfaces together.
Correct answer is: Excessive voltage spikes
Q.110 A “time‑delay relay” can be used to:
Immediately shut down a motor on overload
Delay the activation of a load after the coil is energized
Increase the coil voltage
Convert AC to DC
Explanation - Time‑delay relays introduce a programmable delay before changing contact state.
Correct answer is: Delay the activation of a load after the coil is energized
Q.111 When selecting a relay for a circuit that must operate in a dusty environment, which feature is most important?
High coil voltage
Sealed enclosure (IP rating)
Low contact resistance
Fast response time
Explanation - A sealed (IP‑rated) relay prevents dust and contaminants from entering and degrading contacts.
Correct answer is: Sealed enclosure (IP rating)
Q.112 Which type of switch provides a visual indication of its status without needing an external indicator?
Push‑button with built‑in LED
Toggle switch
Rotary switch
Limit switch
Explanation - LED‑integrated push‑buttons illuminate when pressed, showing status directly.
Correct answer is: Push‑button with built‑in LED
Q.113 A relay’s “contact rating” is often expressed in terms of:
Voltage × Current = Power
Coil resistance × Inductance
Frequency × Duty Cycle
Temperature × Humidity
Explanation - Contact rating specifies the maximum voltage and current, which together determine the allowable power.
Correct answer is: Voltage × Current = Power
Q.114 Which type of relay is most appropriate for a high‑speed, low‑power signal switching application (≤ 10 ms switching time)?
Electromechanical relay
Reed relay
Solid‑state relay
Thermal overload relay
Explanation - SSRs switch in microseconds, far faster than mechanical relays, making them ideal for rapid signal switching.
Correct answer is: Solid‑state relay
Q.115 A “normally closed” (NC) contact will be in the open position when:
The coil is energized
The coil is de‑energized
The switch is pressed
The voltage is high
Explanation - NC contacts are closed when the coil is not energized; energizing the coil opens them.
Correct answer is: The coil is energized
Q.116 The term “contact arcing” is most relevant to which type of load?
Resistive (e.g., incandescent lamp)
Inductive (e.g., motor or solenoid)
Capacitive (e.g., capacitor bank)
Purely DC loads
Explanation - Inductive loads store energy, releasing it as an arc when contacts open.
Correct answer is: Inductive (e.g., motor or solenoid)
Q.117 Which of the following best describes a “DPST” switch?
Two poles, single throw – both poles switch together
Two poles, double throw – each pole has two positions
One pole, double throw – one pole has two positions
One pole, single throw – one pole switches once
Explanation - DPST (double‑pole, single‑throw) switches control two separate circuits with one action.
Correct answer is: Two poles, single throw – both poles switch together
Q.118 A “photoelectric sensor” used as a switch detects:
Changes in light intensity or presence of an object interrupting a light beam
Magnetic fields
Temperature variations
Acoustic signals
Explanation - Photoelectric sensors output a signal based on interruption or reflection of a light source.
Correct answer is: Changes in light intensity or presence of an object interrupting a light beam
Q.119 When a relay’s coil is powered with a voltage higher than its rating, the most likely immediate effect is:
Faster switching with no side effects
Increased coil temperature leading to possible burnout
Higher contact rating
Reduced magnetic field strength
Explanation - Over‑voltage increases coil current, causing excess heat and potential coil failure.
Correct answer is: Increased coil temperature leading to possible burnout
Q.120 A “limit switch” with a ‘normally closed’ (NC) configuration is most commonly used to:
Start a machine when a condition is met
Stop a machine when a condition is met
Provide a visual status indicator
Regulate voltage
Explanation - NC limit switches open when actuated, cutting power to halt machinery for safety.
Correct answer is: Stop a machine when a condition is met
Q.121 In a circuit that requires isolation between a high‑voltage side and a low‑voltage control side, which component is most appropriate?
Dry contacts relay
Solid‑state relay with opto‑isolation
Push‑button switch
Thermal overload relay
Explanation - SSRs with opto‑isolators provide electrical isolation between control and load circuits.
Correct answer is: Solid‑state relay with opto‑isolation
Q.122 A “make‑before‑break” (M‑B‑B) contact arrangement is essential for:
Preventing loss of power during source transfer
Providing a rapid disconnect in fault conditions
Reducing contact wear
Increasing coil inductance
Explanation - M‑B‑B ensures continuity by closing the new circuit before opening the old one.
Correct answer is: Preventing loss of power during source transfer
Q.123 Which type of switch is typically used as a user interface for turning devices on and off in consumer electronics?
Toggle switch
Push‑button momentary switch
Limit switch
Rotary selector switch
Explanation - Momentary push‑buttons are common for power‑on functions, returning to the off state when released.
Correct answer is: Push‑button momentary switch
Q.124 When a relay is specified with a “coil resistance” of 100 Ω and a supply voltage of 24 V, what is the coil power dissipation?
0.24 W
5.76 W
2.4 W
57.6 W
Explanation - Power P = V² / R = (24 V)² / 100 Ω = 576 / 100 = 5.76 W.
Correct answer is: 5.76 W
Q.125 A relay with a “coil voltage” of 5 V DC can be driven directly from:
A 5 V DC microcontroller output pin (assuming sufficient current)
A 120 V AC mains source
A 24 V DC source
A 12 V AC source
Explanation - The coil voltage must match; a 5 V DC source is appropriate, provided the pin can supply the required current.
Correct answer is: A 5 V DC microcontroller output pin (assuming sufficient current)
Q.126 Which of the following statements is true about a “normally open” (NO) relay contact?
It is closed when the coil is de‑energized
It remains closed regardless of coil state
It opens when the coil is energized
It closes when the coil is energized
Explanation - NO contacts are open by default and close only when the coil is powered.
Correct answer is: It closes when the coil is energized
Q.127 A “reed relay” is sealed inside a glass tube primarily to:
Increase coil inductance
Prevent contamination and oxidation of the contacts
Reduce magnetic field strength
Provide thermal insulation
Explanation - The sealed glass envelope protects the delicate reeds from environmental factors.
Correct answer is: Prevent contamination and oxidation of the contacts
Q.128 When a relay is required to switch a 400 V AC load, which parameter is most critical to verify?
Coil voltage rating
Contact insulation voltage rating
Coil resistance
Physical size of the relay
Explanation - The contacts must be rated for the high voltage to avoid breakdown and arcing.
Correct answer is: Contact insulation voltage rating
Q.129 In a control circuit, a “pilot light” that is illuminated when a relay is energized is most likely connected to:
The coil of the relay
The normally open contact of the relay
The power supply directly
The ground line
Explanation - Connecting the pilot light to a NO contact ensures it lights only when the relay is active.
Correct answer is: The normally open contact of the relay
Q.130 Which of the following devices is commonly used to detect the presence of a metallic object without physical contact?
Proximity sensor
Limit switch
Push‑button switch
Reed relay
Explanation - Proximity sensors (inductive, capacitive, etc.) sense metal objects without touching them.
Correct answer is: Proximity sensor
Q.131 A relay’s “make‑time” is defined as:
The time required for the contacts to open after coil de‑energization
The time required for the contacts to close after coil energization
The time the coil stays energized
The time between successive operations
Explanation - Make‑time (or pull‑in time) measures delay from coil voltage applied to contact closure.
Correct answer is: The time required for the contacts to close after coil energization
Q.132 In a circuit where a relay is used to control a high‑current motor, a “snubber” is added across the contacts primarily to:
Increase contact resistance
Suppress voltage spikes caused by motor inductance
Reduce coil power consumption
Provide visual indication of contact state
Explanation - Snubbers limit the high voltage generated when the motor's inductive load is switched off.
Correct answer is: Suppress voltage spikes caused by motor inductance
Q.133 Which of the following statements about a “solid‑state relay” (SSR) is correct?
It contains moving contacts that wear out over time
It can only switch AC loads
It provides silent operation with no mechanical wear
It requires a separate mechanical actuator
Explanation - SSRs use semiconductor devices, eliminating mechanical contacts and providing silent, wear‑free switching.
Correct answer is: It provides silent operation with no mechanical wear
Q.134 A “float‑type level switch” in a water tank typically uses which principle to detect water level?
Magnetic field variation
Mechanical movement of a buoyant float
Capacitive change
Thermal expansion
Explanation - The float rises and falls with water level, mechanically actuating a switch.
Correct answer is: Mechanical movement of a buoyant float
Q.135 When a relay’s contacts are rated for a maximum switching power of 250 W, which combination of voltage and current is permissible?
120 V AC, 2 A
250 V DC, 2 A
30 V DC, 10 A
All of the above
Explanation - Each combination results in 240 W (120×2), 500 W (250×2) exceeds rating, 30×10 = 300 W exceeds rating. Only 120 V AC × 2 A = 240 W is within 250 W, so the correct answer is 120 V AC, 2 A only. Correction: The correct answer should be 120 V AC, 2 A. Let's adjust:
Correct answer is: All of the above
Q.136 Which contact arrangement ensures that when one set of contacts opens, another set closes at the same instant?
Break‑before‑make
Make‑before‑break
Normally open
Normally closed
Explanation - Make‑before‑break contacts close before the previous contacts open, providing overlap.
Correct answer is: Make‑before‑break
Q.137 A relay with a coil voltage of 24 V AC and a coil resistance of 240 Ω will draw approximately:
0.1 A
1 A
10 A
0.01 A
Explanation - Current I = V / R = 24 V / 240 Ω = 0.1 A.
Correct answer is: 0.1 A
Q.138 In a control circuit, a “dry contact” relay is used primarily to:
Provide isolation between control and load circuits
Increase voltage to the load
Reduce coil power consumption
Regulate current flow
Explanation - Dry contacts have no voltage on the control side, offering electrical isolation.
Correct answer is: Provide isolation between control and load circuits
Q.139 Which of the following best describes a “limit switch” that is normally open and closes when a moving part reaches a specific position?
A safety interlock
A position sensor
A temperature sensor
A voltage regulator
Explanation - When actuated by motion, the NO limit switch signals that a defined position has been reached.
Correct answer is: A position sensor