Engineering Toolkit

Safety, space, and engineering calculators — all in your browser.

Select a formula based on your available inputs. All formulas per IEC 61508-6 Annex B.

PFD_avg = λ_DU × TI / 2

\u03BB_DU — Dangerous Undetected Rate

Failures per hour

TI — Proof Test Interval

Hours (8760 = 1 year)

PFDavg

4.380e-3

RRF

228

SIL Level

SIL 2

Safety Availability

99.56%

PFH (approx.)

5.00e-7/hr

Spurious Trip Rate

~4.38e-3

Log(RRF)

2.36

Tip: 8,760 hours = 1 year. Common λ_DU values: pressure transmitter ~2.5×10^-7, solenoid valve ~5×10^-7, logic solver channel ~1×10^-8.

Use this formula when the dangerous undetected failure rate is known from the device SIL certificate or FMEDA report.