Electrical Control Systems Flashcards
7 cards from real SACA practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Electrical Control Systems flashcards as text
In a PLC-based control system, what is the function of the 'scan cycle' watchdog timer?
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.
What is the primary advantage of using shielded twisted-pair (STP) cable for analog signal wiring in an industrial control panel?
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.
According to NFPA 79, which color must be used exclusively for the grounded AC circuit conductor (neutral) in industrial machinery wiring?
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.
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?
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.
What does the term 'SCCR' (Short-Circuit Current Rating) signify for an industrial control panel?
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.
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?
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.
Which output module type in a PLC best handles proportional control of a pneumatic valve positioner requiring a continuous 4–20 mA command signal?
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.