313A Electrical Controls & Wiring

Question 1

What is the purpose of a contactor in an HVAC electrical circuit?

Accepted Answer

To act as an electrically operated switch that uses a low-voltage control coil to open and close high-voltage power contacts to the compressor or fan motor

Answer

A contactor uses a low-voltage coil (typically 24V) energized by the control circuit to magnetically close heavy-duty power contacts, allowing full line voltage to energize loads like compressors and fan motors.

Question 2

What does a thermal overload relay do in a motor starter circuit?

Accepted Answer

It senses motor current and opens the control circuit if sustained overcurrent is detected, protecting the motor from overheating due to overload

Answer

A thermal overload relay uses a bimetallic element or electronic sensor to monitor motor current; if the motor draws excessive current for a sustained period (indicating mechanical overload or locked rotor), the relay trips and removes the motor from the circuit.

Question 3

In Canadian HVAC wiring, what voltage is most commonly used for low-voltage control circuits?

Accepted Answer

24 V AC

Answer

24 V AC is the standard low-voltage control circuit voltage in Canadian and North American HVAC systems, provided by a step-down transformer from 120 V or 240 V.

Question 4

What is the function of a high-pressure safety control (high-pressure cutout) in a refrigeration system?

Accepted Answer

To shut down the compressor if discharge pressure exceeds a safe limit, preventing equipment damage and refrigerant venting

Answer

A high-pressure cutout is a safety switch that opens the compressor control circuit when discharge pressure rises above the set point, protecting the system from excessive pressure that could damage components or cause refrigerant leaks.

Question 5

What does a phase-loss relay protect against in a three-phase compressor installation?

Accepted Answer

Detecting the loss of one phase of the three-phase supply, which would cause the motor to overheat and potentially burn out

Answer

A phase-loss relay monitors all three phases of the power supply; if one phase is lost, the motor attempts to run on two phases (single phasing), drawing excessive current and overheating, so the relay shuts it down immediately.

Red Seal HVAC 313A Practice Test

Red Seal HVAC 313A Practice Test

313A Electrical Controls & Wiring