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Safety and SIS · Glossary

What is Hardware Fault Tolerance?

Symbol: HFTfaults toleratedSafety and SIS

Hardware fault tolerance is the number of hardware faults a subsystem can have while still performing its safety function.

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Hardware Fault Tolerance Explained

HFT 0 means one fault can cause loss of the function (1oo1). HFT 1 means one fault is tolerated (1oo2 or 2oo3).

IEC 61511 Edition 2 sets minimum HFT per SIL as an architectural constraint, regardless of how good the PFDavg is.

Hardware Fault Tolerance: Typical Values

SIL 1HFT 0
SIL 2, low demandHFT 0
SIL 3HFT 1
SIL 4HFT 2
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Calculators for Hardware Fault Tolerance

CalculatorPFDavg CalculatorFree online calculator with formula and worked example.CalculatorSafe Failure Fraction CalculatorFree online calculator with formula and worked example.

Frequently Asked Questions

What is Hardware Fault Tolerance?

Hardware fault tolerance is the number of hardware faults a subsystem can have while still performing its safety function.

Why is HFT needed if PFDavg is met?

It guards against uncertainty in failure data by requiring redundancy at higher SILs.

Is 2oo3 HFT 1?

Yes, it still works with one failed channel.

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