EN 50155: The Bedrock of Rail Electronics Safety & Reliability

What is EN 50155? A Comprehensive Guide for Railway Electronic Equipment

EN 50155 is a European standard that specifies the design, development, manufacturing, and testing requirements for electronic equipment used on railway rolling stock. The standard’s primary goal is to ensure that this equipment can operate reliably and safely in the demanding and often harsh environment of a railway vehicle, ensuring interoperability and a long service life.

Compliance with EN 50155 is a fundamental requirement for any electronic system intended for on-board use, from passenger information displays and communication systems to critical train control and management systems (TCMS). It addresses the full spectrum of environmental and electrical challenges unique to the rail industry.

The Core Principles of EN 50155: Surviving the Railway Environment

Electronic equipment on rolling stock is subjected to conditions far more severe than those in typical commercial or industrial applications. The EN 50155 standard is built around ensuring robustness against these specific challenges. The main environmental and operational stresses include:

• Extreme Temperatures: Equipment must function flawlessly across a wide range of ambient temperatures, from freezing cold in winter to extreme heat in enclosed cabinets during summer.
• Shock and Vibration: Constant movement, track irregularities, and coupling/decoupling actions generate significant and persistent shock and vibration that can damage standard electronic components.
• Electrical Instability: The power supply on a train is notoriously unstable, with frequent voltage fluctuations, dips, surges, and short interruptions. Equipment must be designed to withstand these events without failure or data loss.
• Humidity and Contamination: High levels of relative humidity, condensation, and exposure to dust or pollutants require special protective measures for electronic circuit boards.
• Electromagnetic Compatibility (EMC): The high-power traction systems, signalling equipment, and numerous on-board electronic devices create a complex electromagnetic environment. Equipment must not emit excessive interference and must be immune to interference from other systems.

Key Technical Requirements of EN 50155

To address the challenges above, EN 50155 defines a detailed set of technical specifications and test procedures. These can be grouped into several key areas.

Environmental Conditions

This is one of the most critical sections of the standard, defining how equipment must be protected against physical and climatic stresses.

• Operating Temperature: The standard specifies several temperature classes (detailed in the table below), allowing manufacturers to design equipment for specific operational climates. The most common class is T3, requiring operation from -40°C to +70°C.
• Humidity: Equipment must typically withstand an annual average relative humidity of up to 75%, with short periods of up to 95%. To prevent failure from condensation and moisture, printed circuit boards (PCBs) are often required to have a protective layer known as conformal coating.
• Shock and Vibration: EN 50155 references another standard, EN 61373 (Railway applications – Rolling stock equipment – Shock and vibration tests), for its testing requirements. Equipment is classified based on its mounting location on the vehicle (e.g., body-mounted, bogie-mounted), with bogie-mounted equipment subject to the most severe tests.

Electrical Requirements

Ensuring stable operation despite an unstable power source is a cornerstone of the standard.

• Power Supply: The standard defines nominal DC input voltages (e.g., 24V, 36V, 48V, 72V, 96V, 110V). Critically, equipment must tolerate a wide range of voltage variations (typically from 0.7 to 1.25 times the nominal voltage) and temporary dips or surges.
• Supply Interruptions: Electronic equipment must handle short interruptions of the power supply without resetting or losing data. The standard defines several classes for this, such as Class S2, which requires continued operation through a 10ms power loss.
• Electromagnetic Compatibility (EMC): EN 50155 mandates compliance with the EN 50121 series (Railway applications – Electromagnetic compatibility), specifically EN 50121-3-2 for rolling stock apparatus. This ensures the equipment does not interfere with critical systems like signalling and communications and is not susceptible to external electromagnetic fields.

Design, Performance, and Reliability

Beyond simple survival, the standard also covers the functional performance and long-term reliability of the equipment.

• Performance Criteria: Defines different levels of performance expectation during testing. Class A equipment is expected to function fully during and after tests, while Class B may show minimal degradation, and Class C may require a reset or repair after a test.
• Reliability and Maintainability: The standard encourages the calculation and documentation of Mean Time Between Failures (MTBF) to provide an indication of the equipment’s expected reliability. Design considerations should also facilitate easy maintenance and replacement.
• Conformal Coating: This is a thin polymeric film applied to PCBs to protect the electronic components from environmental damage like moisture, dust, and corrosion. EN 50155 often requires its use for equipment exposed to condensation.

EN 50155 Operating Temperature Classes Comparison

The choice of temperature class is critical and depends on the intended geographical area of operation and the equipment’s location within the train.

The Importance of Testing and Certification

Compliance with EN 50155 is not a matter of self-declaration. It requires a rigorous program of testing performed by the manufacturer or a third-party lab to prove that the equipment meets all specified requirements. The typical testing regime includes:

• Performance Tests: Verifying the equipment functions as intended under normal conditions.
• Type Tests: A comprehensive set of tests performed on a representative sample of the equipment. This includes visual inspection, functional tests, low-temperature tests, dry heat tests, humidity tests, voltage tolerance tests, EMC tests, and the crucial shock and vibration tests.
• Routine Tests: A simplified set of tests performed on every single unit during production to ensure consistent manufacturing quality, typically including a visual inspection and a performance test.

Conclusion

EN 50155 is more than just a technical document; it is the bedrock of safety, reliability, and interoperability for all electronic systems on modern rolling stock. By creating a standardized framework for environmental resilience, electrical robustness, and performance, the standard ensures that components from different suppliers can work together effectively and that the overall rail system operates with the high degree of safety and availability expected by operators and the public. For any engineer or manufacturer developing products for the railway sector, a deep understanding of EN 50155 is not just beneficial, but absolutely essential.