313D Electrical Schematics and Controls 3 — Questions and Answers
Question 1: What is the function of a hard start kit (potential relay and start capacitor) shown in a schematic?
- To provide surge protection during lightning storms
- To boost starting torque and reduce locked-rotor amperage duration (Correct answer)
- To soft-start the condenser fan motor
- To regulate refrigerant pressure during startup
Correct answer: To boost starting torque and reduce locked-rotor amperage duration
A hard start kit adds extra capacitance to boost starting torque, allowing the compressor to start under load and reducing the time spent at high locked-rotor amperage.
Question 2: In a schematic, what does a contactor coil symbol (circle with a letter designation) energize?
- Only the run capacitor circuit
- The main power contacts that supply line voltage to the compressor and condenser fan (Correct answer)
- The 24V transformer primary winding
- The crankcase heater element
Correct answer: The main power contacts that supply line voltage to the compressor and condenser fan
When the contactor coil is energized by the 24V control circuit, it pulls in the main power contacts that connect line voltage (240V) to the compressor and condenser fan motor.
Question 3: A low-pressure switch in an AC schematic is wired in series with the control circuit primarily to:
- Protect against high discharge pressure
- Shut down the unit if refrigerant loss causes suction pressure to drop below safe limits (Correct answer)
- Cycle the condenser fan at low ambient temperatures
- Prevent the blower from operating without refrigerant flow
Correct answer: Shut down the unit if refrigerant loss causes suction pressure to drop below safe limits
A low-pressure switch opens the control circuit if suction pressure drops below a set point, indicating refrigerant loss or other abnormal conditions, shutting down the compressor.
Question 4: On a residential split-system schematic, which wire color is most commonly used for the 24V 'Y' (cooling) signal from thermostat to air handler?
- Red
- White
- Yellow (Correct answer)
- Green
Correct answer: Yellow
The Y terminal carries the cooling call signal, and by industry convention this wire is yellow, though actual installations should always be verified against the wiring diagram.
Question 5: What does a ground fault circuit interrupter (GFCI) monitor in an electrical circuit?
- The difference in current between the hot and neutral conductors (Correct answer)
- The total current draw of the connected load
- The voltage level on the hot conductor only
- The insulation resistance of the equipment grounding conductor
Correct answer: The difference in current between the hot and neutral conductors
A GFCI monitors the current difference between hot and neutral; if more than ~5mA is detected (indicating a ground fault), it trips and opens the circuit.
Question 6: Which type of diagram shows the physical location of components and the actual wire routing between them?
- Ladder diagram
- Schematic diagram
- Wiring diagram (Correct answer)
- Block diagram
Correct answer: Wiring diagram
A wiring diagram shows the physical layout and actual routing of wires between components, making it useful for installation and troubleshooting in the field.
Question 7: In AC system controls, what is the role of a defrost control board in a heat pump schematic?
- It regulates indoor humidity levels during cooling
- It initiates and terminates the defrost cycle to clear frost from the outdoor coil (Correct answer)
- It prevents the reversing valve from switching during heating
- It controls the variable-speed blower motor speed
Correct answer: It initiates and terminates the defrost cycle to clear frost from the outdoor coil
The defrost control board monitors outdoor coil temperature and time, initiating defrost when needed and terminating it when the coil is clear or a time limit is reached.
313D Electrical Schematics and Controls 3
What is the function of a hard start kit (potential relay and start capacitor) shown in a schematic?