SACA Electrical Control Systems 5 — Questions and Answers
Question 1: In a PLC-based control system, what is the function of the 'scan cycle' watchdog timer?
- It synchronizes communication between remote I/O racks
- It resets the PLC program counter at fixed intervals
- It faults the PLC if the program scan does not complete within a set time, indicating a hung program (Correct answer)
- It controls the update rate of analog input modules
Correct answer: It faults the PLC if the program scan does not complete within a set time, indicating a hung program
The watchdog timer monitors scan time and forces the PLC into a fault state if the scan exceeds the configured limit, preventing undetected software hangs from leaving outputs in an unsafe state.
Question 2: What is the primary advantage of using shielded twisted-pair (STP) cable for analog signal wiring in an industrial control panel?
- It increases the signal voltage level to reduce transmission loss
- It provides mechanical protection against conduit abrasion
- It reduces common-mode noise and electromagnetic interference on low-level signals (Correct answer)
- It allows higher current capacity than standard stranded wire
Correct answer: It reduces common-mode noise and electromagnetic interference on low-level signals
Twisting cancels magnetically induced noise and the shield drains capacitively coupled interference to ground, preserving signal integrity for low-level analog signals like 4–20 mA or thermocouples.
Question 3: According to NFPA 79, which color must be used exclusively for the grounded AC circuit conductor (neutral) in industrial machinery wiring?
- Green
- Orange
- White or gray (Correct answer)
- Blue
Correct answer: White or gray
NFPA 79-2021 clause 13.2.2 requires the grounded (neutral) conductor to be white or gray, reserving green or green/yellow for equipment grounding conductors.
Question 4: A star-delta (Y-Δ) reduced-voltage starter transitions the motor from star to delta configuration to limit starting current. At what approximate percentage of full-voltage starting torque does the motor start in the star connection?
- 100%
- 58%
- 33% (Correct answer)
- 75%
Correct answer: 33%
In star connection, each winding receives 1/√3 of line voltage, reducing torque to 1/3 (33%) of full-voltage delta starting torque because torque varies as the square of voltage.
Question 5: What does the term 'SCCR' (Short-Circuit Current Rating) signify for an industrial control panel?
- The maximum continuous current the panel can carry without overheating
- The maximum prospective fault current the panel can safely interrupt or withstand without creating a hazard (Correct answer)
- The minimum current required to operate the largest device in the panel
- The rated insulation voltage of all components within the enclosure
Correct answer: The maximum prospective fault current the panel can safely interrupt or withstand without creating a hazard
SCCR specifies the maximum symmetrical fault current (in kA rms) that the panel assembly can survive under short-circuit conditions without catastrophic failure, as required by NEC 409 and UL 508A.
Question 6: In control circuit design, a 'two-hand control' device is required by OSHA and ANSI B11 standards to ensure operator safety on which type of machinery?
- Conveyor systems with variable speed drives
- Press or stamping machines with a single-stroke hazardous motion (Correct answer)
- CNC machining centers with automatic tool changers
- Pneumatic assembly torque tools
Correct answer: Press or stamping machines with a single-stroke hazardous motion
Two-hand controls require simultaneous operation of both hands to initiate a hazardous single-stroke motion on presses and similar machinery, preventing hands from being in the point of operation.
Question 7: Which output module type in a PLC best handles proportional control of a pneumatic valve positioner requiring a continuous 4–20 mA command signal?
- Discrete DC output (sourcing)
- High-speed counter output
- Analog output module (Correct answer)
- Relay output module
Correct answer: Analog output module
An analog output module generates a continuously variable current signal (4–20 mA) that directly commands the valve positioner to any intermediate position proportional to the process demand.
SACA Electrical Control Systems 5
In a PLC-based control system, what is the function of the 'scan cycle' watchdog timer?