IEC 61508

Key requirements (6)

• Functional safety of electrical/electronic/programmable electronic safety-related systems (parent of 61511).
• Safe Failure Fraction (SFF) = (λS + λDD) / (λS + λDD + λDU).
• PFH (high demand) ranges: SIL 1 (10⁻⁶–10⁻⁵), SIL 2 (10⁻⁷–10⁻⁶), SIL 3 (10⁻⁸–10⁻⁷), SIL 4 (10⁻⁹–10⁻⁸).
• Type A vs Type B components — different SFF requirements.
• Systematic capability (SC1–SC4) certified by independent assessor.
• Common-cause beta factor typical 5–10 % unless explicit measures taken.