Lock-out relays in air conditioning systems are used primarily to prevent which of the following?

• A. Prevent Compressor Short-Cycling
• B. Lock-Out the Thermostat
• C. Provide Automatic Restart
• D. Lock-Out the Overloads

Answer: (A)

Lock-out relays are there to stop the compressor from starting again right after a fault, which prevents short‑cycling. When a problem is detected—like low or high refrigerant pressure or motor overload—the control shuts the compressor and engages the lockout relay. This holds the compressor off for a period (or until a manual or automatic reset), giving pressures and temperatures time to stabilize and reducing the stress of repeated rapid starts. The result is safer, more reliable operation and less wear on the compressor.

The other options don’t describe the main purpose. Lock-out relays don’t lock out the thermostat, they don’t by themselves provide automatic restart after a fault, and while protection touches the overload side, the relay’s function is to prevent starting the compressor rather than to lock out the overload devices themselves.