CRST CRST Electrical Systems & Controls 2 — Questions and Answers
Question 1: What does a high-pressure control (HPC) do in a refrigeration system?
- Shuts down the compressor when discharge pressure exceeds the set point (Correct answer)
- Opens a bypass valve to relieve excess pressure
- Regulates head pressure by cycling condenser fans
- Signals the thermostat to raise the set point
Correct answer: Shuts down the compressor when discharge pressure exceeds the set point
A high-pressure control monitors discharge pressure and shuts down the compressor if pressure exceeds a preset safety limit to prevent equipment damage.
Question 2: In a refrigeration ladder diagram, what symbol represents a normally open (NO) contact?
- Two parallel vertical lines with a gap between horizontal lines (Correct answer)
- Two diagonal lines crossing the circuit path
- A circle with an X inside
- A zigzag line symbol
Correct answer: Two parallel vertical lines with a gap between horizontal lines
A normally open contact is represented by two parallel vertical lines separated by a gap across the circuit path, indicating the contact is open in its de-energized state.
Question 3: What is the purpose of a potential relay used with a refrigeration compressor motor?
- Remove the start capacitor from the circuit once the motor reaches running speed (Correct answer)
- Protect the motor from low voltage conditions
- Disconnect power if the motor draws excessive current
- Time the duration of the defrost cycle
Correct answer: Remove the start capacitor from the circuit once the motor reaches running speed
A potential relay senses the back-EMF generated by the start winding and opens its contacts to remove the start capacitor once the motor reaches approximately 75% of operating speed.
Question 4: What is the correct procedure before working on energized refrigeration electrical panels according to NFPA 70E?
- Perform an arc flash risk assessment and wear appropriate PPE (Correct answer)
- Simply turn off the main breaker and proceed
- Install a lock only if voltage exceeds 240V
- Test with a non-contact tester from 6 feet away
Correct answer: Perform an arc flash risk assessment and wear appropriate PPE
NFPA 70E requires an arc flash risk assessment and appropriate arc-rated PPE before working on or near energized electrical equipment.
Question 5: A three-phase compressor motor that hums but does not start is most likely experiencing which problem?
- Single-phasing — loss of one supply phase (Correct answer)
- Overcharged refrigerant system
- Faulty start capacitor on the single-phase winding
- Defective crankcase heater
Correct answer: Single-phasing — loss of one supply phase
Single-phasing (loss of one phase) causes a three-phase motor to hum without starting because it cannot produce sufficient rotating magnetic field torque.
Question 6: What does a low-pressure control (LPC) typically do in a commercial refrigeration system?
- Cycle the compressor off when suction pressure drops below the set point (Correct answer)
- Open the liquid line solenoid when pressure falls too low
- Energize the defrost heaters when suction pressure is low
- Alert the condenser fan to increase speed
Correct answer: Cycle the compressor off when suction pressure drops below the set point
A low-pressure control cycles the compressor off when suction pressure drops below a set point, protecting the system from operating in a deep vacuum or refrigerant loss condition.
CRST CRST Electrical Systems & Controls 2
What does a high-pressure control (HPC) do in a refrigeration system?