RSES Electrical Systems & Controls 2

Question 1

A hermetic compressor motor draws locked-rotor amperage (LRA) of 45A. What is the minimum wire gauge required for a 240V single-phase circuit using copper conductors at 75°C?

Accepted Answer

10 AWG

Answer

LRA × 1.25 = 56.25A; 10 AWG copper at 75°C is rated for 35A continuous, but NEC 440.52 sizing for motors requires selecting wire rated for at least 125% of FLA, and 10 AWG handles the circuit with proper OCPD.

Question 2

Which type of overload relay automatically resets after cooling down without requiring manual intervention?

Accepted Answer

Auto-reset bimetallic relay

Answer

An auto-reset bimetallic overload relay restores the circuit automatically once the bimetal strip cools and returns to its original position.

Question 3

A capacitor-start, capacitor-run (CSCR) motor fails to start but hums loudly. Which component is most likely faulty?

Accepted Answer

Start capacitor or centrifugal switch

Answer

Humming without starting indicates the start winding circuit is open, typically caused by a failed start capacitor or a defective centrifugal switch that won't close.

Question 4

What does a megohmmeter (megger) primarily measure in refrigeration system electrical diagnostics?

Accepted Answer

Insulation resistance of motor windings

Answer

A megohmmeter applies high DC voltage (typically 500–1000V) to measure insulation resistance in megohms, revealing winding degradation or moisture ingress.

Question 5

In a refrigeration control circuit, a normally open (NO) pressure switch closes at 250 psig and opens at 300 psig. This switch is functioning as a:

Accepted Answer

High-pressure operating control

Answer

A switch that closes on pressure rise and opens at a high setpoint is a high-pressure operating control, cycling the system to maintain discharge pressure within a range.

(RSES) Refrigeration Service Engineers Society Certification Practice Test

(RSES) Refrigeration Service Engineers Society Certification Practice Test

RSES Electrical Systems & Controls 2