RSES Electrical Systems & Controls 3

Question 1

A defrost termination thermostat in an electric defrost system is set to open at 55°F. What happens if this thermostat fails in the open position?

Accepted Answer

Defrost runs continuously until the time clock terminates it

Answer

If the defrost termination thermostat fails open, it cannot provide the signal to end defrost early, so defrost runs for the full timer-set duration before the time clock cuts it off.

Question 2

What is the purpose of a time-delay relay (TDR) wired into a compressor control circuit?

Accepted Answer

Prevent short cycling by delaying compressor restart

Answer

A time-delay relay prevents short cycling by holding the compressor off for a set period (typically 3–5 minutes) after shutdown, allowing pressures to equalize before restart.

Question 3

When testing a run capacitor with a digital multimeter set to capacitance mode, a reading significantly lower than the rated µF value indicates:

Accepted Answer

The capacitor has lost capacitance and should be replaced

Answer

A capacitor reading substantially below its nameplate µF rating (more than ±6% tolerance) has lost capacitance, reducing motor efficiency and potentially causing overheating.

Question 4

In a two-speed condenser fan motor control scheme, the motor switches from high speed to low speed when outdoor ambient temperature drops. This design primarily:

Accepted Answer

Reduces compressor discharge pressure during cold weather

Answer

Reducing condenser fan speed in cold weather slows the rate of heat rejection, maintaining adequate condensing pressure and liquid subcooling necessary for proper system operation.

Question 5

A technician measures 120V across a contactor's coil terminals but the contactor does not pull in. The most likely cause is:

Accepted Answer

Open or burned-out coil

Answer

Correct voltage across the coil terminals with no contactor pull-in means the coil itself is open or burned out and cannot generate the electromagnetic field needed to close the contacts.

(RSES) Refrigeration Service Engineers Society Certification Practice Test

(RSES) Refrigeration Service Engineers Society Certification Practice Test

RSES Electrical Systems & Controls 3