If the thermostat is closed and the blower motor is running, which condition would prevent the compressor from running according to the described safety interlocks?

If the thermostat is closed and the blower motor is running, which condition would prevent the compressor from running according to the described safety interlocks?

• A. HP/LP switches open
• B. Blower relay closed
• C. Condenser fan energized
• D. Thermostat open

Answer: (A)

Safety interlocks in many HVAC systems rely on the high‑pressure and low‑pressure switches to protect the compressor. These switches are wired in the compressor’s control circuit so that if either pressure becomes too high or too low, the switch opens and de-energizes the compressor contactor, preventing the compressor from starting even if the thermostat calls for cooling and the blower is running. So the condition that would prevent the compressor from running is when the high-pressure or low-pressure safety switch opens. The other conditions don’t stop the compressor in this setup: a closed blower relay just keeps the fan on, a energized condenser fan aids heat rejection, and a thermostat open would halt cooling only if the thermostat actually opens; with the thermostat closed, the pressure interlocks are the protective factor that would stop the compressor.

Safety interlocks in many HVAC systems rely on the high‑pressure and low‑pressure switches to protect the compressor. These switches are wired in the compressor’s control circuit so that if either pressure becomes too high or too low, the switch opens and de-energizes the compressor contactor, preventing the compressor from starting even if the thermostat calls for cooling and the blower is running. So the condition that would prevent the compressor from running is when the high-pressure or low-pressure safety switch opens. The other conditions don’t stop the compressor in this setup: a closed blower relay just keeps the fan on, a energized condenser fan aids heat rejection, and a thermostat open would halt cooling only if the thermostat actually opens; with the thermostat closed, the pressure interlocks are the protective factor that would stop the compressor.