313D Electrical Systems and Controls 5

Question 1

A residential heat pump defrost control board initiates defrost based on:

Accepted Answer

Time and temperature, or time and pressure, depending on board design

Answer

Outdoor humidity level only

Answer

Indoor thermostat demand only

Answer

Refrigerant charge level

Question 2

On a residential AC system, what is the role of the low-voltage common wire (C-wire) connected to the thermostat?

Accepted Answer

To complete the 24V circuit by providing a return path to the transformer secondary

Answer

To supply 24V power to the thermostat display

Answer

To send the cooling call signal to the contactor

Answer

To ground the thermostat body to the air handler chassis

Question 3

What type of electrical fault would cause a ground fault circuit interrupter (GFCI) to trip on an AC circuit?

Accepted Answer

Current leaking from a hot conductor to ground exceeding ~5 mA

Answer

Overload current exceeding the breaker rating

Answer

A short circuit between two hot conductors

Answer

Voltage drop in the supply conductors

Question 4

A residential condenser fan motor runs in the wrong direction after replacement. The MOST likely cause is:

Accepted Answer

The start and run capacitor leads were swapped on the new motor

Answer

The motor is wired to 208V instead of 240V

Answer

A failed run capacitor

Answer

The contactor contacts are worn

Question 5

What does it mean when a contactor shows pitted or burned contact surfaces?

Accepted Answer

The contacts have experienced arcing and should be replaced to prevent unreliable operation

Answer

The contacts are properly broken in and will perform better

Answer

The contactor coil voltage is too high

Answer

The compressor is drawing below normal amperage

Question 6

An AC technician measures 127V at L1-to-ground and 121V at L2-to-ground on a 240V circuit. The voltage imbalance concern is because:

Accepted Answer

Voltage imbalance between phases causes disproportionately higher current imbalance, stressing compressor windings

Answer

The total voltage of 248V exceeds equipment rating

Answer

Ground voltage should always be zero

Answer

Only the leg-to-leg voltage of 248V matters for compressor operation

Question 7

Which safety device protects an ECM blower motor module from power surges and lightning-induced voltage spikes?

Accepted Answer

Transient voltage surge suppressor (TVSS) or MOV installed on the circuit

Answer

Time-delay fuse in the disconnect

Answer

Dual run capacitor

Answer

Low-pressure switch