The Guardian of Technical Safety: Independent Safety Assessor (ISA)

Achieve SIL4 compliance with an Independent Safety Assessor. Discover how ISAs validate technical safety for railway software and hardware against CENELEC standards.

The Guardian of Technical Safety: Independent Safety Assessor (ISA)
December 11, 2025 8:30 am
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What is an Independent Safety Assessor (ISA)?

An Independent Safety Assessor (ISA) is a third-party entity responsible for auditing and certifying that a specific railway product or system meets the required technical safety standards, primarily the CENELEC EN 50126, EN 50128, and EN 50129 norms.

Unlike other bodies that focus on legal interoperability (NoBo) or process management (AsBo), the ISA deep dives into the technical engineering. They verify that the hardware reliability and software logic are robust enough to achieve the claimed Safety Integrity Level (SIL), up to SIL 4.

Key Responsibilities of an ISA

  • Software Audit: Reviewing code, architecture, and testing procedures to ensure compliance with EN 50128.
  • Hardware Analysis: verifying failure rates (MTBF) and failure modes to ensure compliance with EN 50129.
  • Safety Case Review: Assessing the comprehensive argument presented by the manufacturer that the product is safe for use.
  • Independence: Providing an objective judgment, completely separate from the design and development teams.

Comparison: ISA vs. AsBo

While both roles involve “safety assessment,” they operate at different levels and use different rulebooks. Confusing these two is a common industry pitfall.

FeatureIndependent Safety Assessor (ISA)Assessment Body (AsBo)
Primary FocusProduct / Technical (Hardware & Software)System / Process (Risk Management)
Governing StandardsCENELEC (EN 50126/128/129)CSM-RA (EU Reg. 402/2013)
ScopeValidates if a specific product meets a SIL level (e.g., an Interlocking computer).Validates if the integration of that product into the network is managed safely.
OutputIndependent Safety Assessment Report (ISA Report)Safety Assessment Report (SAR)

The ISA Certification Process

The ISA is typically involved throughout the entire “V-Model” lifecycle of a product to prevent costly errors at the end.

1. Concept & Planning Phase

The ISA reviews the Safety Plan and Quality Assurance Plan to ensure the development roadmap is compliant with standards.

2. Design & Implementation

As developers write code and design circuits, the ISA performs spot-checks and audits to ensure that safety requirements are being traced and implemented correctly.

3. Validation & Testing

The ISA witnesses key validation tests. They do not perform the tests themselves but verify that the testing coverage is sufficient to prove safety.

4. Final Certification

Upon successful completion, the ISA issues a conformity report stating that the product meets the requirements for a specific SIL (e.g., “Generic Application Safety Case approved for SIL 4”).