[OT Sec] Understanding SIL and SL – Essential Foundations for OT System Design

✅ 1. SIL (Safety Integrity Level)

🔎 Definition

SIL (Safety Integrity Level) is a quantitative measure representing the reliability of safety functions within industrial systems. It indicates how reliably a designed safety function can prevent incidents that could harm people, equipment, or the environment.

📘 Applicable Standards

• IEC 61508: General functional safety standard across industries (foundation for all SIL definitions)
• IEC 61511: Process industry-specific (oil refining, chemicals)
• IEC 62061: Machinery safety
• ISO 26262: Automotive industry (branching into ASIL)

🧠 Key Elements for SIL Assessment

SIL levels are determined based on four key steps:

1. Hazard Identification
2. Risk Analysis
3. Determining Required Risk Reduction
4. Target SIL Determination

Example: A gas leak poses explosion risk → emergency valve closure must reliably activate → SIL 3 required.

📊 SIL Levels & Probability of Failure

SIL Level	Probability of Failure per Hour (PFH)	Example Applications
SIL 1	≥10⁻⁶ ~ <10⁻⁵/hour	General warning systems, no emergency shutdown required
SIL 2	≥10⁻⁷ ~ <10⁻⁶/hour	Regular process shutdown, hazardous material management
SIL 3	≥10⁻⁸ ~ <10⁻⁷/hour	Gas shutoff valves, high-pressure steam controls
SIL 4	≥10⁻⁹ ~ <10⁻⁸/hour	Nuclear plants, high-speed railway braking systems

Note: SIL 4 is rare in practical industries; most facilities require SIL 2~3.

✅ 2. SL (Security Level)

🔎 Definition

SL (Security Level) assesses the cybersecurity strength of industrial control systems (ICS/SCADA, DCS, PLC) against various attacker capabilities. Defined by IEC 62443 standards, it quantifies the system’s security robustness into four maturity levels based on potential attackers.

📘 Related Standards (IEC 62443)

Standard	Description
IEC 62443-1-1 ~ 1-4	Definitions, structure, conceptual framework
IEC 62443-2-x	Policies, operations, governance (management)
IEC 62443-3-3	System security requirements (Core for SL)
IEC 62443-4-2	Component-specific security requirements (PLC, HMI, etc.)

🧠 SL Levels & Attacker Profiles

SL Level	Attacker Capability	Required Security Controls
SL 1	Random or accidental intruders	Basic access control, password management
SL 2	General hackers (basic technical skills)	Authentication, vulnerability mitigation, basic network protection
SL 3	Advanced hackers, insiders, advanced tools	Zone-based security, detailed access controls, logging, integrity checks
SL 4	Nation-state APT, high resources	Multi-factor authentication, advanced threat detection, isolation, real-time response systems

SL is further categorized into Target Security Level (SL-T), Capability Security Level (SL-C), and Achieved Security Level (SL-A).

🔍 SIL vs. SL: Key Differences

Criteria	SIL (Safety Integrity Level)	SL (Security Level)
Purpose	Reduce physical risk from functional failures	Protect systems from cyberattacks
Approach	Reliability-based (probabilistic)	Threat-based (security-focused)
Asset Focus	Physical safety devices (SIS, sensors, valves)	Digital assets (PLC, HMI, networks)
Evaluation Basis	Probability of failure (PFD/PFH)	Checklist of security requirements
Standards	IEC 61508, IEC 61511	IEC 62443-3-3, 4-2
Key Personnel	Functional safety engineers	OT security engineers, CISO

🏭 Practical Application Examples

Example 1: Chemical Plant Gas Detection and Isolation System

• SIL: Gas leak detection and automatic valve closure → SIL 3 required.
• SL: Protect valve operation from remote intrusion → SL 3 needed.

Example 2: Power Plant Control Systems

• SIL: Automatic shutdown upon overpressure → SIL 2 or higher.
• SL: SCADA hacking prevention → authentication, logging, isolation (SL 2~3).

Example 3: Railway Systems

• SIL: Signal control malfunction could cause severe accidents → SIL 4.
• SL: Prevent remote hacking of train control systems → SL 3~4.

🧩 Integrating SIL and SL in OT System Design

While SIL and SL originated from different disciplines—functional safety and cybersecurity—they must be integrated within modern OT system designs.

🔄 Examples of Integrated Strategy

Integration Point	Explanation
Security Design to Protect Safety Functions	SIL 3 emergency stop must include SL 3 access controls
Evaluating Security Threats as Safety Risks	Linking Threat Modeling (security) with HAZOP/LOPA (safety)
Standards Integration	Joint application of ISA/IEC 62443, ISO 27001, IEC 61511 standards

Without collaboration between safety and security teams, even high SIL-rated systems can fail due to cybersecurity vulnerabilities.

🎯 Summary

Key Question	SIL	SL
What is protected?	Lives and physical assets	Digital assets and control systems
Assessment Basis	Reliability, probability of failure	Attacker capability, threat scenarios
Applied to	Safety Instrumented Systems (SIS), ESD, pressure controls	ICS, SCADA, PLC, networks
Relationship	Complementary; managing them separately increases risk