Scope: An integration application operating on the zero-copy principle, developed to establish data exchange between SCADE’s closed-loop structure and the control layer.

Project Portfolio

Current Situation and Problem

Context: The necessity to migrate the interlocking simulation system (4 tracks, 6 signals, 1 switch) at the ITU Railway Systems Laboratory to a SCADE Display-supported HMI architecture compliant with industrial and certifiable standards. Critical Issues: The absence of a native Modbus interface within SCADE Display and the platform’s closed-network design. The inability of the HMI interface to communicate directly with actual field controllers (HIMA HiMatrix F35), and the lack of dynamic field configuration for IP/port maps.

Solution Architecture and Execution

Architectural Approach

Architectural Constraint Assessment: To preserve DO-178C / EN 50128 certification integrity, the SCADE platform features a “closed system” design philosophy stripped of all asynchronous external network operations. Consequently, for physical PLC integration in the laboratory, developing an independent, externally operating asynchronous middleware layer became mandatory.

A C99 middleware wrapper layer was designed between SCADE’s generated code and the libmodbus library:

Communication Flow

Applied Methodology

• Zero-Copy Integration: Copy overhead was eliminated by hooking directly into the SCADE main loop.
• Dynamic Configuration: A 3-region INI file was utilized to prevent recompilation requirements during field deployment.

Security mechanism: Upon detecting a missing or corrupted configuration file, the system automatically creates and saves default settings (localhost connection).

• Auto-Reconnection: Connection drops in industrial environments are inevitable; an autonomous auto-reconnect cycle was activated.
• Change Detection: A batch read and selective write strategy was implemented to optimize network traffic.

Architectural Decision: Since HMI hardware’s capability for simultaneous physical command processing is limited, outputs are evaluated in a sequential loop. This structural preference prevents unnecessary fieldbus traffic (network flooding).

• Thread-Safe Logging: Every event in critical systems must be logged securely.

Interlocking Scope

Example route: RT01: SN01E → TC01 → TC04 → TC02 → SN02E (Switch: Normal)

Safety Rule: Based on critical design principles, all signals not explicitly demanded by the system are configured to default to a restrictive state (red).

Results and Operational Takeaways

Quantitative Gain: (Potential values obtained during laboratory PoC tests)

Project Visuals

Related Links

 Source Code: Github/ansys-scade-modbus-integration-middleware 
Download TOK 2025 Paper: SCADE Modbus Paper Turkish (PDF)

Authors: Dora Demir¹, İbrahim Can Kolotoğlu², Muhammet Işık², Serhat Boynukalın³, Mehmet Turan Söylemez²
¹ ITU Electronics and Communication Eng. | ² ITU Control and Automation Eng. | ³ ITU Graduate School

This work was conducted at the ITU EEF Railway Systems Laboratory and published at the Turkish Automatic Control 2025 (TOK 2025) conference.

Can high-security industrial relays be used in interlocking systems? Or are only component-level certified SIL 4 relays permitted? This question points to a widespread conceptual confusion in the sector. The answer is not simply yes or no: Utilization is subject to specific architectural conditions.

Theoretical Background

The Two Worlds of Safety Relays

The selection of relays in railway safety depends on the design philosophy and certification approach. There are two primary categories:

Vital Relays

These relays represent the gold standard of railway signaling, rooted in a century of engineering heritage.

Gravity Fail-Safe (e.g., Mors Smitt N.S1):

• When the coil energy is removed, the opening of the contacts is entrusted not to a friction-prone spring force, but directly to gravity.
• A spring can fatigue or snap, but gravity never disappears.
• Therefore, these relays must always be mounted in a specific vertical orientation defined by their design parameters.

Carbon-Silver Contact Technology (e.g., Clearsy RS4):

• One of the contact tips is made of silver, and the mating tip is made of a carbon (graphite) alloy.
• By its metallurgical nature, carbon does not weld to silver; this physically prevents the contacts from fusing.
• Contact welding is rendered physically near-impossible.

Industrial Safety Relays: Forcibly Guided Contacts

Manufactured according to the EN 61810-3 (formerly EN 50205) standard, these relays pursue an entirely different safety strategy. Companies like Dold, Finder, and Arteche have established industry-standard products in this class.

Mechanical Linkage Principle:

• Normally Open (NO) and Normally Closed (NC) contacts are mechanically interlocked via a guiding rod (comb).
• If an NO contact welds due to excessive current or a short circuit, the mechanical rod physically prevents the NC contact from closing and the system from transitioning to an unsafe state.
• As mandated by the standard, a minimum functional gap of 0.5 mm is guaranteed between contacts.

The Critical Difference:

These relays do not prevent the metallurgical occurrence of welding; they merely report the condition to the controller once it happens.

Coil De-energized Contact Pushed by SPRING Force

Contact Welded?

Contact OPENS
(Normal State)

NC Contact Stays OPEN(Mechanical Failure) PLC Detects andLOCKS the SystemNOYES!

VITAL RELAY

Coil De-energized Armature Drops by GRAVITY Carbon-Silver Contact Opens

SAFE STATE

Research Details: Root Cause Analysis

Field Scenario: Relay Selection in Interlocking Panels

An EPC (Engineering, Procurement, and Construction) contractor is selecting relays for an electronic interlocking project. There are two options on the table:

Cost Discrepancy: The per-unit cost difference in the field is approximately tenfold. The Critical Question: Can Option B be utilized in the design?

Why Aren’t Industrial Relays SIL 4 Standalone?

Deduction: Industrial safety relays do not possess a SIL rating on their own. However, within a redundant architecture boasting a 99% diagnostic coverage rate (employing 1oo2 redundancy alongside the readback process), they can be utilized as an integral component of a CENELEC SIL 4 compliant system.

Architectural Solution and Operational Logic

There are two distinct safety architectures in the market. One is predicated on purchasing safety, while the other revolves around constructing it holistically.

Approach A: Component-Level Architecture

Representative: Clearsy RS4 Series

This approach positions the product itself as a certified black box. The relay is a sealed unit designed according to SIL 4 requirements during the hardware design and manufacturing phases, possessing internal redundancy and inherent fail-safe capabilities.

• Operational Principle: The integrator drives the relay using standard industrial relay logic. The product’s safety case documentation is securely provided directly by the manufacturer.
• Wiring: Only the driving coil and transmission contact terminals are connected. It does absolutely not require an external control cable or specialized monitoring software on the safety PLC side.
• Internal Mechanism: The configuration of the mechanism and gravity within the relay inherently eliminates the risk of contact welding.

Commercial Equation:

•  Plug-and-Play: The engineering burden and Independent Safety Assessor (ISA) approval process for the integrator are radically minimized.
•  Cost: The standalone unit cost is exceedingly high (~€500).
•  Supply Chain: Creates total dependency on a single or highly restricted supplier base (Monopoly).

Approach B: System-Level Architecture

Representatives: Arteche, Finder, Dold (Forcibly Guided Relays)

This approach aims to achieve the firm SIL 4 target by combining standard safety relays through an intelligent system architecture. Safety is not strictly embedded in the off-the-shelf product; it is meticulously concealed within the entirety of the designed circuit.

• Operational Principle: Instead of using a single relay, at least two relays with forcibly guided contacts are utilized, electrically wired in series (1oo2 Architecture).
• Wiring and Monitoring:
  • Series Safety: The NO contacts of the dual relay set are wired in series to the field load. Even if one relay effectively welds, the other interrupts the circuit, mitigating the fault efficiently.
  • Readback Process: The closed (NC) contacts of the relays are carefully wired to the digital input (DI) card of the safety PLC, enabling instantaneous monitoring of the relays’ mechanical states.
• Audit Logic: The safety PLC strictly verifies the state of the NC contacts before dispatching a drive command to the field. If a contact appears open while no energy is supplied to the field (indicating a welding syndrome), the system categorically refuses the drive command and locks into a safe state.

Commercial Equation:

•  Cost-Effective: Up to 90% CAPEX savings in projects are achieved due to fundamentally low unit costs.
•  Flexibility: Any EN 61810-3 (EN 50205) compliant standard product can be utilized.
•  Engineering Burden: Circuit design, wiring complexity, and the ultimate safety proof responsibilities of the final system lie squarely on the shoulders of the integrator architect.

Conclusion

Decision Matrix

Summary Deductions:

• The concept of a “SIL 4 Relay” does not equate to a “SIL 4 System”: A single component may possess a SIL 4 hardware label, but system engineering is a comprehensive, holistic endeavor.
• Gravity vs. Spring Force: The safety of vital relays relies heavily on immutable physical laws of nature, whereas industrial safety relays necessitate external diagnostic intelligence to effectively detect latent flaws.
• Cost vs. Responsibility Trade-off: Choosing inexpensive components decisively incurs a high engineering burden and continuous ISA auditing, whereas expensive components systematically yield a comfortable, stress-free integration. The supply chain strategy is directly proportional to a firm’s internal engineering confidence.

References

• EN 61810-3: Electromechanical elementary relays – Relays with forcibly guided contacts
• EN 50129: Railway applications – Communication, signalling and processing systems
• Mors Smitt N.S1 Technical Documentation
• Clearsy RS4 Functional Safety Guidelines
• HIMA SILworX Application Notes

Related Links

 Related Article: → Strategic Analysis: CENELEC SIL 4 Relay Procurement Strategies and Market Dynamics in Railway Signaling

Last update: March 2026 | Version: 1.1

SIL 4 safety relays, one of the most critical safety components in railway signaling projects, create substantial procurement risks and cost pressures due to the market structure. This report analyzes the monopolistic nature of the current market, the two primary architectural approaches namely “Component-Level” and “System-Level”, and the impacts of these approaches on the 5-year Total Cost of Ownership (TCO).

Key Findings

• Market Monopoly: The “plug-and-play” (Component-Level) SIL 4 relay market is dominated by a single manufacturer (Clearsy); this situation creates high costs and procurement dependency.
• Architectural Shift Opportunity: The “System-Level” architecture constructed with industrial relays and safety PLCs offers a cost advantage of over 50% in the long term, despite the initial investment in engineering costs.
• Critical Turning Point: The biggest obstacle to market transformation is not technical, but rather the shifting of the responsibility for preparing the Safety Case from the supplier to the integrator, and the acceptance of this new model by the end-user.

Market Structure and Product Ecosystem

Relays used in railway safety should be evaluated in three main categories based on their certification and design philosophies.

Product Categorization Matrix

Technical Taxonomy of the Market and Product Families

Relay technologies in the railway signaling market are divided into two main classes based on the source of their safety mechanisms. This distinction directly determines the procurement strategy and the engineering burden.

Vital Relays

Representative: Clearsy RS4 Series Characterized as the comfort zone of the market, this product group guarantees safety through physical laws.

• Technical Basis: Gravity Fail-Safe principle. When the coil energy is cut off, the opening of the contacts is entrusted not to spring force, but to gravity and mass weight.
• Certification: The component is standalone SIL 4 certified.
• Commercial Characteristic: Very high unit cost and single-source dependency.

Industrial Safety Relays

Representatives: Arteche FF, Dold OA, Finder 7S Series These components act as the fundamental building blocks of system-level architecture.

• Technical Basis: Forcibly guided contact structure and spring-return mechanism compliant with the EN 50205 standard.
• Constraint: They are not SIL 4 on their own. They require external active monitoring against the risk of contact welding.
• Commercial Characteristic: Low unit cost and multiple supplier alternatives.

System-Level Architecture (Strategic Alternative)

This approach is not a product, but an engineering methodology.

• Philosophy: Safety by Design.
• Execution: Multiple industrial safety relays are combined in a redundant architecture (e.g., 1oo2).
• Safety Layer: Software covers the hardware’s vulnerability. Through a readback loop established via the safety PLC, the system is elevated to a safety level equivalent to component-level SIL 4 relays.

Architectural Paradigm: Where is Safety Located?

The fundamental difference between the two approaches is whether safety is confined to a purchased physical component or dispersed throughout the entire system design.

Approach A: Component-Level Safety (Clearsy Model)

This model relies on the certified black box principle.

• Internal Oversight: All diagnostics, redundancy, and safety logic are embedded inside the sealed unit before it leaves the factory.
• Integrator’s Role is Passive: The integrator simply sends the command. They do not have to worry about whether the relay contact is welded, the health of the coil, or the internal mechanism. The product handles this internally.
• Result: Safety is purchased as a product.

Approach B: System-Level Safety (Engineering Model)

This model relies on the architectural oversight principle.

• External Oversight: Safety resides not in the relay itself, but in the PLC software managing it. Standard industrial relays are used, but they are not left unmonitored.
• Integrator’s Role is Active: The integrator must establish a mechanism called a readback loop. Before the PLC issues a command to the relay, it physically verifies whether the relay successfully returned from the previous command.
• Result: Safety is constructed as a process.

Summary Comparison: Responsibility Matrix

Economic Analysis and Break-Even Point (TCO)

A 5-year projection based on an annual requirement of 1000 safety functions reveals the dramatic cost difference between the two approaches.

Cost Data

TCO Break-Even Analysis

Chart Analysis: Although the System-Level approach starts higher initially due to the one-time engineering cost, it passes the break-even point within the very first year thanks to dramatically lower unit costs, providing a savings of ~€1.45 Million at the end of the 5th year.

Strategic Recommendations

In light of the market’s current state and economic analyses, the recommended hybrid transition strategy for integrator firms is:

Short Term (Defense): Clearsy usage should continue in cases of customer insistence or project urgency. This minimizes project risk.

Medium Term (Preparation): Arteche/Dold-based system architecture should be applied in pilot projects, and the Safety Case documentation of this architecture should be matured. Independent Safety Assessor (ISA) approval must be obtained at this stage.

Long Term (Breakthrough): The matured and approved system architecture should become the standard solution in all high-volume projects. This will maximize the firm’s competitive strength and profitability.

Conclusion

The use of component-based SIL 4 relays (the Clearsy model) in railway signaling is a commercial comfort zone preference rather than a technical necessity. For organizations with high engineering capabilities, the System-Level approach is not just a substantial cost advantage, but a lever providing supply chain independence and strategic flexibility.

The core to the transformation is not the excellence of the technical solution, but rather the ability to correctly prove the reliability of this solution to the customer through the Safety Case and independent assessor approval. Brands like FEST, which previously operated in Turkey, have competed by bringing such required relays to the market. Today, with a new brand and formation, this process can certainly be advanced once more.

Recent intelligence from the field indicates that local engineering firms are preparing to design their own safety relay architectures to offer CENELEC-compliant standard solutions to the Turkish market at highly competitive and affordable prices. We sincerely hope that such visionary initiatives will succeed in significantly breaking foreign dependency and introducing much-needed flexibility into the monopolistic supply chain.

References

• EN 50129: Railway Applications – Communication, signaling and processing systems
• EN 50155: Railway Applications – Electronic equipment used on rolling stock
• EN 50205: Relays with forcibly guided (mechanically linked) contacts

Related Links

 Related Article: → Vital vs. Forcibly Guided: Engineering Analysis of Relay Technologies

Last update: March 2026 | Version: 1.1