PLC Safety & Functional Safety Flashcards

Which IEC standard covers functional safety of electrical, electronic, and programmable electronic safety-related systems?

Answer: IEC 61508

IEC 61508 is the umbrella standard for functional safety of E/E/PE (electrical, electronic, and programmable electronic) safety-related systems.

What does 'de-energize to trip' mean in safety PLC output design?

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.

What is the Safe Failure Fraction (SFF) used to determine?

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.

In functional safety terminology, what is a 'proof test'?

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.

Which safety architecture uses two independent channels where both must detect a fault to initiate a trip (voting logic)?

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.

What is the primary advantage of a 2oo3 voting architecture in safety PLCs?

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.