Functional Safety Calculator

Compute PFD_avg and SIL classification for safety instrumented functions per IEC 61508 / IEC 61511.

Functional Safety Calculator

Compute PFD_avg and SIL classification for safety instrumented functions per IEC 61508 / IEC 61511 simplified formulas. Low demand mode only — safety function demanded < 1/year.

PFD_avg & SIL Calculator

PFD_avg

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SIL Achieved

—

Risk Reduction Factor

—

>10⁻¹ SIL 1 SIL 2 SIL 3 SIL 4 <10⁻⁵

Independent failures: —

Common cause (β): —

Detected failures contribution (MTTR): —

λ_du (undetected): —

λ_dd (detected): —

Failure Rate

Enter λ_d (failures/hour) Enter MTTF_d (hours)

λ_d:

MTTF_d ≈ 11.4 years

Architecture & Test Interval

Voting Architecture

Single channel, no redundancy.

Proof Test Interval:

Diagnostic Coverage

DC (0–100%):

Fraction of dangerous failures detected by auto-diagnostics.

Mean Time To Repair (MTTR) for detected failures:

Time from failure detection to repair completion.

Common Cause

β factor (0–10%):

Fraction of failures affecting all redundant channels simultaneously.

Formulas (IEC 61508-6, Low Demand)

Failure rate decomposition:

$$\lambda_{du} = \lambda_d \cdot (1 - DC), \qquad \lambda_{dd} = \lambda_d \cdot DC$$

Selected architecture:

$$\text{PFD}_{\text{avg}} = \lambda_{du} \cdot \frac{TI}{2} + \lambda_{dd} \cdot MTTR$$

• Low demand mode: safety function is demanded < 1 per year.

• Simplified formulas assume constant failure rate and perfect proof testing. For high-demand or continuous mode, use PFH instead.

• Detected failures contribute λ_dd × MTTR during the repair period.

• RRF (Risk Reduction Factor) = 1 / PFD_avg.

Disclaimer

This calculator uses simplified formulas from IEC 61508-6 for educational and preliminary design purposes. For certified safety analysis, consult the full standard, vendor data sheets, and a qualified functional safety engineer. High-demand / continuous mode requires PFH calculation, not PFD_avg.

Functional Safety Calculator