PLC PLC Safety & Functional Safety 1 — Questions and Answers
Question 1: Which international standard defines functional safety requirements for safety-instrumented systems in the process industry?
- IEC 61511 (Correct answer)
- IEC 60068
- ISO 9001
- ANSI/ISA-88
Correct answer: IEC 61511
IEC 61511 is the functional safety standard specifically for safety-instrumented systems in the process industry.
Question 2: What does SIL stand for in the context of safety PLCs?
- Safety Integrity Level (Correct answer)
- System Integration Logic
- Signal Input Limit
- Sequential Interlock Layer
Correct answer: Safety Integrity Level
SIL stands for Safety Integrity Level, a measure of the reliability and risk reduction provided by a safety function.
Question 3: A Safety PLC operating at SIL 2 typically requires which architecture to meet hardware fault tolerance requirements?
- 1oo2 (one-out-of-two) redundancy (Correct answer)
- Single processor with no redundancy
- 1oo1 with diagnostics only
- Quad-redundant 4oo4
Correct answer: 1oo2 (one-out-of-two) redundancy
SIL 2 commonly requires a 1oo2 (1-out-of-2) hardware architecture to achieve the necessary hardware fault tolerance.
Question 4: What is the primary purpose of a Safety Instrumented Function (SIF)?
- To bring a process to a safe state when a hazardous condition is detected (Correct answer)
- To optimize throughput during normal operations
- To log historical process data for reporting
- To communicate between PLCs on a network
Correct answer: To bring a process to a safe state when a hazardous condition is detected
A Safety Instrumented Function (SIF) is designed to take a process to a defined safe state upon detection of a hazardous condition.
Question 5: Which term describes the probability that a safety function fails to respond when a dangerous condition occurs?
- Probability of Failure on Demand (PFD) (Correct answer)
- Mean Time To Repair (MTTR)
- Diagnostic Coverage (DC)
- Safe Failure Fraction (SFF)
Correct answer: Probability of Failure on Demand (PFD)
PFD (Probability of Failure on Demand) quantifies the likelihood that a safety function will not operate correctly when called upon.
Question 6: In a safety PLC, what is the role of the watchdog timer?
- To detect processor faults and trigger a safe shutdown if the CPU stops cycling normally (Correct answer)
- To record timestamps for audit trails
- To schedule communication tasks between remote I/O modules
- To manage PID loop execution timing
Correct answer: To detect processor faults and trigger a safe shutdown if the CPU stops cycling normally
A watchdog timer monitors the PLC CPU scan cycle and forces a safe state if the processor fails to reset it within the expected interval.
PLC PLC Safety & Functional Safety 1
Which international standard defines functional safety requirements for safety-instrumented systems in the process industry?