RETA Programmable Logic Controllers (PLC) in Refrigeration
Exam Practice Questions And Correct Answers (Verified
Answers) Plus Rationale | Instant Download Pdf - 177 Questions
and Answers Already Graded A+ Premium Exam Tested And
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Subject Area Refrigeration Automation & PLC Control Systems
Description This exam assesses mastery of programmable logic controllers (PLCs) as applied
to industrial refrigeration systems, including programming (ladder logic, function
block), hardware configuration, troubleshooting, and integration with sensors,
actuators, and safety protocols. Questions require multi-step reasoning and
application of industry standards (ASHRAE, IEC 61131).
Expected Grade A+
Total Questions 177
Duration 3 hours
Learning Outcomes 1. Design and interpret ladder logic for refrigeration control sequences (defrost,
compressor staging, condenser fan control).
2. Analyze PLC hardware configurations and select appropriate I/O modules for
refrigeration applications.
3. Troubleshoot complex faults using PLC diagnostic tools and logic analysis.
4. Apply safety and redundancy principles (e.g., high-pressure cutouts,
pump-down cycles) in PLC programs.
5. Evaluate communication protocols (Modbus, Ethernet/IP) for refrigeration
system integration.
Accreditation Meets RETA (Refrigerating Engineers & Technicians Association) competency
standards and aligns with US university engineering technology curricula.
Page 1
,1. A refrigeration system uses a PLC-controlled pump-down cycle. The suction
pressure transducer fails low (0 psig). Which PLC logic behavior is most likely to
occur?
A. The compressor will short-cycle on low-pressure cutout.
B. The liquid line solenoid will close permanently, starving the evaporator.
C. The compressor will run continuously until a high-pressure safety trips.
D. The PLC will initiate an emergency shutdown due to loss of signal.
Answer: A. The compressor will short-cycle on low-pressure cutout.
A failed-low transducer signals suction pressure below the cutout setpoint, causing the
PLC to de-energize the compressor contactor. Once pressure recovers (if at all), the
compressor restarts, leading to short cycling. Option B is incorrect because the solenoid
valve is typically controlled by thermostat, not suction pressure directly. Option C
would occur only if the transducer failed high. Option D is not typical unless a 4-20 mA
loop fault is detected.
2. In a PLC program for an ammonia refrigeration system, a defrost termination
thermostat (DTT) is wired to a normally closed (NC) input. The ladder logic uses an
XIC (examine if closed) instruction for the DTT. If the DTT contacts open when the
coil temperature reaches 50°F, what is the state of the instruction during defrost?
A. True before defrost, becomes false when termination temperature is reached.
B. False before defrost, becomes true when termination temperature is reached.
C. Always false because the input is NC and the instruction is XIC.
D. Always true because the input is NC and the instruction is XIC.
Answer: A. True before defrost, becomes false when termination temperature is
reached.
With a NC input, the field device is closed (conducting) at normal temperatures. The
XIC instruction evaluates true when the input is closed. When the DTT opens at 50°F,
the input becomes open, so the XIC becomes false. Thus the instruction is true before
defrost and false at termination. Option B would be correct if the input were NO or the
instruction were XIO. Options C and D ignore the dynamic behavior.
Page 2
,3. A PLC controls multiple evaporators in a cold storage warehouse. Each
evaporator has a liquid line solenoid and a defrost heater. The defrost sequence is
initiated by a time clock and must be staggered to avoid excessive electrical demand.
Which programming method best ensures that only one evaporator defrosts at a
time?
A. Use a single timer to sequence defrost starts sequentially.
B. Program each evaporator's defrost start on a unique time-of-day schedule.
C. Employ a mutual exclusion (interlock) using one-shot bits and a shared resource bit.
D. Connect all defrost contactors in series with a single PLC output.
Answer: C. Employ a mutual exclusion (interlock) using one-shot bits and a shared
resource bit.
A mutual exclusion algorithm using a shared 'defrost active' bit ensures that only one
evaporator can enter defrost at a time, regardless of timing variations. Option A may
cause overlap if timers drift. Option B works but is less robust to clock changes and
program modifications. Option D would force all evaporators to defrost simultaneously,
defeating the purpose.
4. A technician observes that a condenser fan controlled by a VFD via PLC analog
output (0-10 V) runs at full speed regardless of the pressure setpoint. The PLC
program uses a PID instruction. Which of the following is the most likely cause?
A. The pressure sensor is scaled incorrectly in the PLC.
B. The PID output is limited to 100% by a clamp.
C. The VFD is configured for 4-20 mA input instead of 0-10 V.
D. The integral gain is set too low, preventing error correction.
Answer: C. The VFD is configured for 4-20 mA input instead of 0-10 V.
If the VFD expects a 4-20 mA signal but receives 0-10 V, it may interpret the voltage as
a constant full-speed command (e.g., 10 V equals 20 mA, but 0 V might be seen as a
fault or full speed depending on configuration). Option A would cause offset but not
necessarily full speed. Option B would cap output but not force full speed if PID is not
saturated. Option D would cause sluggish response, not full speed.
Page 3
, 5. In a PLC-based refrigeration system, a high-pressure cutout (HPCO) is wired to a
safety input that immediately stops the compressor via a hardwired interlock. The
PLC also monitors the HPCO status. Which of the following describes the correct
relationship between the hardwired safety circuit and the PLC program?
A. The PLC program must duplicate the hardwired logic for redundancy.
B. The hardwired circuit overrides the PLC output regardless of program state.
C. The PLC program can bypass the hardwired circuit during startup.
D. The hardwired circuit is only active when the PLC is in RUN mode.
Answer: B. The hardwired circuit overrides the PLC output regardless of program
state.
Hardwired safety circuits are independent of the PLC and provide a fail-safe that
overrides PLC outputs. The PLC cannot bypass this circuit. Option A is not required;
redundancy is achieved by the hardwired circuit itself. Option C violates safety
standards. Option D is false; hardwired circuits operate regardless of PLC mode.
6. A PLC program for a refrigeration plant uses a function block for compressor
capacity control based on suction pressure. The block outputs a 4-20 mA signal to a
slide valve actuator. If the analog output module is configured for 0-20 mA but the
actuator expects 4-20 mA, what will happen when the PLC demands minimum
capacity (0% output)?
A. The actuator will move to the fully closed position.
B. The actuator will remain at its last position.
C. The actuator will move to the fully open position.
D. The actuator will fault due to underrange.
Answer: D. The actuator will fault due to underrange.
A 0% output from the PLC corresponds to 0 mA, which is below the 4 mA minimum
for the actuator. Most 4-20 mA devices will detect a loop fault (underrange) and may go
to a failsafe state or alarm. Option A (fully closed) would occur only if the actuator
interprets 0 mA as a valid signal, which is unlikely. Options B and C are not typical.
Page 4
Exam Practice Questions And Correct Answers (Verified
Answers) Plus Rationale | Instant Download Pdf - 177 Questions
and Answers Already Graded A+ Premium Exam Tested And
Verified
Subject Area Refrigeration Automation & PLC Control Systems
Description This exam assesses mastery of programmable logic controllers (PLCs) as applied
to industrial refrigeration systems, including programming (ladder logic, function
block), hardware configuration, troubleshooting, and integration with sensors,
actuators, and safety protocols. Questions require multi-step reasoning and
application of industry standards (ASHRAE, IEC 61131).
Expected Grade A+
Total Questions 177
Duration 3 hours
Learning Outcomes 1. Design and interpret ladder logic for refrigeration control sequences (defrost,
compressor staging, condenser fan control).
2. Analyze PLC hardware configurations and select appropriate I/O modules for
refrigeration applications.
3. Troubleshoot complex faults using PLC diagnostic tools and logic analysis.
4. Apply safety and redundancy principles (e.g., high-pressure cutouts,
pump-down cycles) in PLC programs.
5. Evaluate communication protocols (Modbus, Ethernet/IP) for refrigeration
system integration.
Accreditation Meets RETA (Refrigerating Engineers & Technicians Association) competency
standards and aligns with US university engineering technology curricula.
Page 1
,1. A refrigeration system uses a PLC-controlled pump-down cycle. The suction
pressure transducer fails low (0 psig). Which PLC logic behavior is most likely to
occur?
A. The compressor will short-cycle on low-pressure cutout.
B. The liquid line solenoid will close permanently, starving the evaporator.
C. The compressor will run continuously until a high-pressure safety trips.
D. The PLC will initiate an emergency shutdown due to loss of signal.
Answer: A. The compressor will short-cycle on low-pressure cutout.
A failed-low transducer signals suction pressure below the cutout setpoint, causing the
PLC to de-energize the compressor contactor. Once pressure recovers (if at all), the
compressor restarts, leading to short cycling. Option B is incorrect because the solenoid
valve is typically controlled by thermostat, not suction pressure directly. Option C
would occur only if the transducer failed high. Option D is not typical unless a 4-20 mA
loop fault is detected.
2. In a PLC program for an ammonia refrigeration system, a defrost termination
thermostat (DTT) is wired to a normally closed (NC) input. The ladder logic uses an
XIC (examine if closed) instruction for the DTT. If the DTT contacts open when the
coil temperature reaches 50°F, what is the state of the instruction during defrost?
A. True before defrost, becomes false when termination temperature is reached.
B. False before defrost, becomes true when termination temperature is reached.
C. Always false because the input is NC and the instruction is XIC.
D. Always true because the input is NC and the instruction is XIC.
Answer: A. True before defrost, becomes false when termination temperature is
reached.
With a NC input, the field device is closed (conducting) at normal temperatures. The
XIC instruction evaluates true when the input is closed. When the DTT opens at 50°F,
the input becomes open, so the XIC becomes false. Thus the instruction is true before
defrost and false at termination. Option B would be correct if the input were NO or the
instruction were XIO. Options C and D ignore the dynamic behavior.
Page 2
,3. A PLC controls multiple evaporators in a cold storage warehouse. Each
evaporator has a liquid line solenoid and a defrost heater. The defrost sequence is
initiated by a time clock and must be staggered to avoid excessive electrical demand.
Which programming method best ensures that only one evaporator defrosts at a
time?
A. Use a single timer to sequence defrost starts sequentially.
B. Program each evaporator's defrost start on a unique time-of-day schedule.
C. Employ a mutual exclusion (interlock) using one-shot bits and a shared resource bit.
D. Connect all defrost contactors in series with a single PLC output.
Answer: C. Employ a mutual exclusion (interlock) using one-shot bits and a shared
resource bit.
A mutual exclusion algorithm using a shared 'defrost active' bit ensures that only one
evaporator can enter defrost at a time, regardless of timing variations. Option A may
cause overlap if timers drift. Option B works but is less robust to clock changes and
program modifications. Option D would force all evaporators to defrost simultaneously,
defeating the purpose.
4. A technician observes that a condenser fan controlled by a VFD via PLC analog
output (0-10 V) runs at full speed regardless of the pressure setpoint. The PLC
program uses a PID instruction. Which of the following is the most likely cause?
A. The pressure sensor is scaled incorrectly in the PLC.
B. The PID output is limited to 100% by a clamp.
C. The VFD is configured for 4-20 mA input instead of 0-10 V.
D. The integral gain is set too low, preventing error correction.
Answer: C. The VFD is configured for 4-20 mA input instead of 0-10 V.
If the VFD expects a 4-20 mA signal but receives 0-10 V, it may interpret the voltage as
a constant full-speed command (e.g., 10 V equals 20 mA, but 0 V might be seen as a
fault or full speed depending on configuration). Option A would cause offset but not
necessarily full speed. Option B would cap output but not force full speed if PID is not
saturated. Option D would cause sluggish response, not full speed.
Page 3
, 5. In a PLC-based refrigeration system, a high-pressure cutout (HPCO) is wired to a
safety input that immediately stops the compressor via a hardwired interlock. The
PLC also monitors the HPCO status. Which of the following describes the correct
relationship between the hardwired safety circuit and the PLC program?
A. The PLC program must duplicate the hardwired logic for redundancy.
B. The hardwired circuit overrides the PLC output regardless of program state.
C. The PLC program can bypass the hardwired circuit during startup.
D. The hardwired circuit is only active when the PLC is in RUN mode.
Answer: B. The hardwired circuit overrides the PLC output regardless of program
state.
Hardwired safety circuits are independent of the PLC and provide a fail-safe that
overrides PLC outputs. The PLC cannot bypass this circuit. Option A is not required;
redundancy is achieved by the hardwired circuit itself. Option C violates safety
standards. Option D is false; hardwired circuits operate regardless of PLC mode.
6. A PLC program for a refrigeration plant uses a function block for compressor
capacity control based on suction pressure. The block outputs a 4-20 mA signal to a
slide valve actuator. If the analog output module is configured for 0-20 mA but the
actuator expects 4-20 mA, what will happen when the PLC demands minimum
capacity (0% output)?
A. The actuator will move to the fully closed position.
B. The actuator will remain at its last position.
C. The actuator will move to the fully open position.
D. The actuator will fault due to underrange.
Answer: D. The actuator will fault due to underrange.
A 0% output from the PLC corresponds to 0 mA, which is below the 4 mA minimum
for the actuator. Most 4-20 mA devices will detect a loop fault (underrange) and may go
to a failsafe state or alarm. Option A (fully closed) would occur only if the actuator
interprets 0 mA as a valid signal, which is unlikely. Options B and C are not typical.
Page 4
