313D Electrical Components and Wiring 3 — Questions and Answers
Question 1: What does a high-pressure switch do in the electrical circuit of a residential air conditioner?
- Regulates compressor motor speed
- Opens the control circuit if refrigerant pressure exceeds a safe limit (Correct answer)
- Closes the contactor at startup
- Monitors low-side suction pressure
Correct answer: Opens the control circuit if refrigerant pressure exceeds a safe limit
A high-pressure switch is a normally closed safety device that opens the control circuit—de-energizing the compressor—when discharge pressure exceeds a preset limit.
Question 2: Which NEC article primarily governs the installation of air-conditioning and refrigerating equipment?
- Article 210
- Article 230
- Article 440 (Correct answer)
- Article 250
Correct answer: Article 440
NEC Article 440 specifically addresses air-conditioning and refrigerating equipment, including motor-compressor branch circuit sizing and protection requirements.
Question 3: A technician finds that a condenser fan motor runs but the compressor does not start and hums loudly. The MOST likely electrical cause is:
- Failed high-pressure switch
- Open run capacitor for the compressor (Correct answer)
- Faulty contactor holding coil
- Loose thermostat R wire
Correct answer: Open run capacitor for the compressor
A failed run capacitor reduces starting torque, causing the compressor to hum and fail to start while still drawing locked-rotor amperage.
Question 4: What is the function of the crankcase heater in an air conditioning system, and how is it typically wired?
- Cools refrigerant oil; wired in parallel with the compressor
- Keeps oil warm to prevent refrigerant migration; wired to be energized when the compressor is OFF (Correct answer)
- Heats the suction line; wired in series with the thermostat
- Warms the evaporator coil; wired to the defrost board
Correct answer: Keeps oil warm to prevent refrigerant migration; wired to be energized when the compressor is OFF
A crankcase heater prevents liquid refrigerant from migrating into compressor oil during off cycles; it is wired so it energizes whenever the compressor contactor is open.
Question 5: When measuring voltage across a closed contactor's load-side terminals, a technician reads 5V instead of 0V. This most likely indicates:
- Normal voltage drop across a good contactor
- Pitted or burned contacts causing excessive resistance (Correct answer)
- The contactor coil is energized
- Incorrect meter lead placement
Correct answer: Pitted or burned contacts causing excessive resistance
A voltage drop across a closed contactor indicates high contact resistance from pitting or burning, which causes heat and inefficient power delivery to the compressor.
Question 6: A residential split system requires a dedicated 240V, 2-pole circuit breaker. What is the primary reason a 2-pole breaker is required instead of two single-pole breakers?
- 2-pole breakers are cheaper and easier to install
- A 2-pole breaker ensures simultaneous disconnection of both hot legs during a fault (Correct answer)
- Single-pole breakers cannot handle 240V circuits
- Code requires 2-pole breakers only for circuits over 30 amps
Correct answer: A 2-pole breaker ensures simultaneous disconnection of both hot legs during a fault
A 2-pole breaker trips both hot conductors simultaneously, ensuring the full 240V circuit is de-energized during an overcurrent event for safe protection.
Question 7: What does the term 'locked-rotor amperage' (LRA) mean in the context of an AC compressor?
- The current drawn during normal steady-state operation
- The maximum current drawn at the instant of startup before the motor reaches running speed (Correct answer)
- The current drawn when the compressor is fully unloaded
- The minimum current needed to hold the contactor closed
Correct answer: The maximum current drawn at the instant of startup before the motor reaches running speed
LRA is the high inrush current a motor draws at startup when the rotor is stationary, which can be 5–8 times the full-load running amperage.
313D Electrical Components and Wiring 3
What does a high-pressure switch do in the electrical circuit of a residential air conditioner?