313D Electrical Schematics and Controls 2 — Questions and Answers
Question 1: In a ladder diagram, what does a normally closed (NC) contact symbol represent?
- A contact that is open during normal de-energized state
- A contact that is closed during normal de-energized state (Correct answer)
- A contact that only closes when power is applied
- A contact that remains open regardless of coil state
Correct answer: A contact that is closed during normal de-energized state
A normally closed contact is closed (conducting) when the coil controlling it is de-energized, and opens when the coil is energized.
Question 2: What is the purpose of a time-delay relay (TDR) in an AC system schematic?
- To increase voltage to the compressor
- To prevent short cycling by delaying compressor restart (Correct answer)
- To measure elapsed run time of the unit
- To reduce current draw during startup
Correct answer: To prevent short cycling by delaying compressor restart
A time-delay relay prevents short cycling by enforcing a minimum off-time before allowing the compressor to restart, protecting it from damage.
Question 3: On a wiring diagram, a dashed line connecting two components typically indicates:
- A high-voltage connection
- A ground wire
- A mechanical linkage or enclosed assembly boundary (Correct answer)
- A future wiring connection
Correct answer: A mechanical linkage or enclosed assembly boundary
Dashed lines on wiring diagrams typically represent mechanical linkages between components or indicate that components are housed within the same enclosure.
Question 4: Which control component in a residential AC system senses indoor temperature and signals the system to cycle on or off?
- High-pressure switch
- Thermostat (Correct answer)
- Contactor
- Transformer
Correct answer: Thermostat
The thermostat senses indoor ambient temperature and sends a low-voltage signal to the control circuit to energize or de-energize the system.
Question 5: A dual-voltage motor wired for 240V operation will have its internal windings connected in:
- Parallel
- Series (Correct answer)
- Delta configuration
- Star configuration
Correct answer: Series
For 240V operation, the windings of a dual-voltage motor are connected in series, while for 120V they are connected in parallel.
Question 6: What does the symbol for a fuse look like on a standard electrical schematic?
- A circle with an X inside
- A rectangle with a wavy line or S-curve inside (Correct answer)
- Two parallel lines of unequal length
- A triangle pointing upward
Correct answer: A rectangle with a wavy line or S-curve inside
A fuse is represented on schematics as a rectangle containing a wavy or S-shaped line, symbolizing the fusible element inside.
Question 7: In an AC control schematic, the R and C terminals on a thermostat typically represent:
- Red wire and common ground for 24V control circuit (Correct answer)
- Return air and cooling mode signals
- Reversing valve and compressor terminals
- Resistor and capacitor components
Correct answer: Red wire and common ground for 24V control circuit
R is the 24V power supply (red wire) from the transformer secondary, and C is the common return, completing the 24V control circuit.
313D Electrical Schematics and Controls 2
In a ladder diagram, what does a normally closed (NC) contact symbol represent?