Portfolio safety knowledge reference — overview of IEC 61508 functional safety concepts, SIL classifications, failure categories, and the safety lifecycle framework applicable to E/E/PE safety-related systems including PLC-based control systems.
IEC 61508 Functional Safety SIL E/E/PE Systems PLC RelevantSafety is defined as freedom from unacceptable risk. Functional safety is the part of overall safety that depends on a system or equipment operating correctly in response to its inputs. PLCs and programmable controllers are a key part of E/E/PE (Electrical, Electronic & Programmable Electronic) safety-related systems — the exact technology IEC 61508 was designed to govern.
Input Devices → Controller (PLC) → Output Devices
(e.g. sensors) (Logic Solver) (valves, actuators)
↑___________________Safety Function Loop_____________________↑
IEC 61508 applies across industries wherever programmable control systems manage safety-critical functions:
SIL is a discrete measure of the required safety performance of a safety function. It is a property of the safety function — not the hardware alone. There are four SIL levels; the higher the SIL, the greater the risk reduction required.
| SIL | Risk Reduction Factor | Probability of Failure (Low Demand) | Typical Application |
|---|---|---|---|
| SIL 4 | 10,000 – 100,000 | 10⁻⁵ – 10⁻⁴ | Nuclear, critical infrastructure |
| SIL 3 | 1,000 – 10,000 | 10⁻⁴ – 10⁻³ | Oil & gas ESD, railway signalling |
| SIL 2 | 100 – 1,000 | 10⁻³ – 10⁻² | Process plant, fire & gas systems |
| SIL 1 | 10 – 100 | 10⁻² – 10⁻¹ | Machinery safety, conveyor E-stop |
IEC 61508 addresses two distinct categories of failure in safety-related systems:
Arise from physical degradation of components over time. Examples: component wear-out, corrosion, insulation breakdown. These are statistical and can be quantified by failure rate data.
Arise from faults introduced during design, specification, or operation:
An HSE study of 34 real control system incidents revealed where dangerous failures originate across the safety lifecycle. Over 60% of failures were built into systems before they ever went into service:
Source: HSE "Out of Control — Why control systems go wrong and how to prevent failure" (free download: hse.gov.uk/pubns/books/hsg238.htm)
IEC 61508 defines a structured Safety Lifecycle — an engineering process applied across three interlocking lifecycles. The key insight is that most failures originate in early phases, so the lifecycle front-loads rigor at specification and design.
Compliance to IEC 61508 requires four elements applied across each phase of the safety lifecycles:
Organisational processes, planning, auditing, and roles & responsibilities across the lifecycle.
Individuals and organisations must demonstrate they have the skills required for each lifecycle phase.
Hardware and software design requirements, SIL targets, architecture constraints, and failure rate calculations.
Verification, validation, and assessment measures that provide evidence of safety achievement at each phase.
IEC 61508 is the parent standard. Industry-specific standards are derived from it for particular sectors — understanding this hierarchy is useful when reading job descriptions:
Functional Safety → Part of overall safety that depends on correct system response to inputs
Safety Function → What the system MUST DO to prevent or mitigate a hazardous event
Safety Integrity → The LIKELIHOOD of the safety function succeeding when demanded
SIL → Discrete performance target (1–4) assigned to a safety function
Risk → Frequency × Severity of a harmful event
Tolerable Risk → The maximum risk accepted by society in a given context
Target Failure Measure → The quantified SIL performance requirement (PFD / PFH)
Random Failure → Hardware degradation — addressed by redundancy and reliability data
Systematic Failure → Design/software/human error — addressed by lifecycle rigor
E/E/PE System → Electrical + Electronic + Programmable Electronic safety systems
Logic Solver → The PLC or safety controller in a safety function loop