Question 43 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is B **Explanation for Option B (Correct Answer):** The low-pressure cut-out (LPC) switch is a critical safety and control device designed to monitor the evaporating pressure within the refrigeration system. In most direct-expansion (DX) systems, this pressure corresponds to the saturated suction pressure, which is the pressure entering the compressor. If the suction pressure drops below a predetermined set point (e.g., due to a refrigerant leak, loss of load, or icing evaporator), the LPC switch opens the circuit, stopping the compressor to prevent damage (such as overheating the motor due to low refrigerant mass flow, or operating under a deep vacuum). Therefore, the most accurate and representative location for monitoring this critical pressure is **at the suction service valve (compressor inlet)**, which provides a direct measurement of the pressure entering the compression stage. **Explanation for Incorrect Options:** * **A) at the discharge service valve (compressor outlet):** This location measures the high-side or head pressure (condensing pressure). This pressure is monitored by the high-pressure cut-out (HPC) switch, not the low-pressure cut-out (LPC) switch. * **C) at the liquid line service valve (receiver outlet):** This location measures the liquid line pressure, which is the high-side pressure after the receiver but before the metering device. While high, it is not the suction pressure and would not be used to protect against low evaporator pressure. * **D) any of the above locations would be equally likely:** This is incorrect because the function of the low-pressure switch specifically requires monitoring the low-side suction pressure. Connecting it elsewhere (A or C) would render the switch ineffective for its intended purpose.