What is the function of a low-pressure switch in a compressor-controlled system and what fault conditions could trigger it?

• A. It protects the compressor by shutting off if suction pressure falls too low; triggers from leaks, evaporator restrictions, improper charge, faulty sensors or wiring.
• B. It controls compressor speed by modulating discharge pressure.
• C. It detects high refrigerant temperature and triggers defrost.
• D. It monitors oil level in crankcase.

Answer: (A)

The low-pressure switch serves to protect the compressor by constantly watching the suction pressure and stopping the compressor if it drops below a safe level. When suction pressure falls too low, usually due to insufficient refrigerant flow, the switch interrupts the electrical circuit to the compressor (or signals the control to shut it down), preventing damage from excessive motor draw, overheating, or poor lubrication.

This low-pressure condition can be caused by refrigerant leaks, evaporator restrictions or icing that reduce flow, an undercharged system, or faulty sensors or wiring that misreport pressure. By turning the compressor off in these situations, the system avoids operating under conditions that could harm the compressor.

Discharge-pressure modulation, high-temperature defrost triggers, and crankcase oil level monitoring are controlled by other sensors or controls. The low-pressure switch specifically monitors suction pressure, not discharge pressure, temperature-based defrost, or oil level.