PLC PLC Safety & Functional Safety 2 — Questions and Answers
Question 1: Which IEC standard covers functional safety of electrical, electronic, and programmable electronic safety-related systems?
- IEC 61508 (Correct answer)
- IEC 61131
- IEC 60870
- IEC 62443
Correct answer: IEC 61508
IEC 61508 is the umbrella standard for functional safety of E/E/PE (electrical, electronic, and programmable electronic) safety-related systems.
Question 2: What does 'de-energize to trip' mean in safety PLC output design?
- The safety output removes power to actuate the safe state, so a loss of power also triggers the safe state (Correct answer)
- The output energizes when a fault is detected to activate an alarm
- Power is removed from the PLC CPU during a trip condition
- Field devices are energized to confirm a safe condition
Correct answer: The safety output removes power to actuate the safe state, so a loss of power also triggers the safe state
De-energize to trip means the output coil de-energizes to initiate the safe state, ensuring that a wiring or power failure also drives the system safe.
Question 3: What is the Safe Failure Fraction (SFF) used to determine?
- The proportion of failures that are either safe or detected dangerous failures within a safety component (Correct answer)
- The number of safe shutdowns per year
- The maximum allowable process deviation before a safety trip
- The ratio of planned to unplanned maintenance activities
Correct answer: The proportion of failures that are either safe or detected dangerous failures within a safety component
SFF represents the fraction of all failure modes that result in safe or detected dangerous states, used to determine the maximum achievable SIL for a component.
Question 4: In functional safety terminology, what is a 'proof test'?
- A periodic test of a safety function to detect dangerous undetected failures (Correct answer)
- A factory acceptance test performed before installation
- A software simulation run to validate PLC logic
- A network connectivity test between safety PLCs
Correct answer: A periodic test of a safety function to detect dangerous undetected failures
A proof test is a scheduled functional test of a safety instrumented function to reveal dangerous undetected failures and restore the system to its designed SIL.
Question 5: Which safety architecture uses two independent channels where both must detect a fault to initiate a trip (voting logic)?
- 2oo2 (two-out-of-two) (Correct answer)
- 1oo2 (one-out-of-two)
- 2oo3 (two-out-of-three)
- 1oo1 (one-out-of-one)
Correct answer: 2oo2 (two-out-of-two)
A 2oo2 architecture requires both channels to agree that a fault exists before a trip is initiated, offering high availability but lower fault detection.
Question 6: What is the primary advantage of a 2oo3 voting architecture in safety PLCs?
- It balances fault tolerance and availability by allowing one channel failure without causing a spurious trip or missing a real fault (Correct answer)
- It reduces hardware costs by eliminating one redundant channel
- It allows the highest PFD of any voting architecture
- It is the only architecture certified for SIL 4 applications
Correct answer: It balances fault tolerance and availability by allowing one channel failure without causing a spurious trip or missing a real fault
A 2oo3 architecture tolerates one failed channel without a spurious trip while still detecting a real hazard when two of three channels agree.
PLC PLC Safety & Functional Safety 2
Which IEC standard covers functional safety of electrical, electronic, and programmable electronic safety-related systems?