1. Introduction: why this guide matters

The railway sector is one of the most demanding environments for electronic equipment. Power supplies, DC/DC converters, DC/AC inverters, battery chargers and frequency converters operating in rolling stock must withstand extreme conditions of temperature, humidity, vibration, mechanical shock, electromagnetic interference and voltage fluctuations — for 25 to 30 years of service life.

The European standard EN 50155 (equivalent to the international IEC 60571) defines the minimum requirements for this equipment. However, EN 50155 does not operate in isolation: it is complemented by an ecosystem of additional standards — EN 50121 (EMC), EN 45545 (fire protection), EN 61373 (shock and vibration) — that together determine whether a power conversion device is suitable for railway use.

This guide serves two audiences: design engineers who need to correctly specify converters and power supplies, and procurement managers who need objective criteria to evaluate suppliers and make total-cost-of-ownership decisions.

2. The railway standards ecosystem

A power conversion device for railway use must comply with several standards simultaneously, each covering a specific domain:

• EN 50155:2017 — Core standard: general requirements for all on-board electronics
• EN 50121-3-2 — Electromagnetic compatibility (EMC) for rolling stock equipment
• EN 45545-2 — Fire protection and material flammability
• EN 61373 — Shock and vibration profiles by installation location
• IEC 60571 — International equivalent to EN 50155

3. EN 50155 in detail: key requirements

EN 50155:2017 (replacing the 2007 version) establishes general requirements for all electronic equipment installed on railway rolling stock. For a general introduction to the standard, see What is EN 50155 for railway applications.

• Supply voltage: continuous, transient and interruption ranges for each nominal voltage
• Operating temperature: six classes (OT1 to OT6) depending on equipment location
• Humidity: up to 75% yearly average, 95% for 30 days/year and occasionally 100%
• Altitude: operation up to 1,400 m (up to 3,000 m under special conditions)
• Insulation: dielectric strength and insulation requirements between circuits
• EMC: reference to EN 50121-3-2 for emission and immunity
• Type tests: full programme including thermal cycling, humidity, shock/vibration and EMC

The 2017 version introduced significant changes: six temperature classes instead of two, more detailed supply interruption requirements, and greater manufacturer/end-user responsibility to define actual operating conditions.

4. Input voltage ranges: classes and tolerances

EN 50155 defines nominal battery voltages of 24, 28, 36, 48, 72, 96 and 110 VDC (recommending 24, 72 or 110 V). For each nominal voltage, four operating zones are defined: continuous nominal, brownout (0.6×U_nom), transient overvoltage (1.4×U_nom, 1 s), and short-term transient.

5. Operating temperature classes (OT1–OT6)

EN 50155:2017 defines six operating temperature classes. The applicable class depends on the equipment location within the vehicle:

• OT1: +5 °C to +40 °C — air-conditioned interiors (passenger saloon)
• OT2: −10 °C to +55 °C — controlled interior compartments
• OT3: −25 °C to +70 °C — default class; most underbody cabinets
• OT4: −40 °C to +70 °C — severe cold environments
• OT5: −40 °C to +85 °C — extreme environments, bogie area
• OT6: −40 °C to +70 °C — arctic operation conditions

6. Supply interruptions and hold-up time

EN 50155 defines mandatory output maintenance during input supply interruptions. Classes are defined for both standard (S) and critical (C) applications:

• S1 / C1: no interruption tolerance — continuous supply required
• S2 / C2: 10 ms / 30 ms hold-up — most common OEM specification
• S3 / C3: extended hold-up — for safety-critical subsystems

7. Complementary standards

EN 50121-3-2 — Electromagnetic compatibility (EMC)

Defines emission and immunity limits for on-board rolling stock equipment. Power converters are among the main sources of EMC interference on board.

• Conducted emission: limits in the 150 kHz–30 MHz range
• Radiated emission: limits in the 30 MHz–1 GHz range
• Immunity: ESD (EN 61000-4-2), fast transients (EN 61000-4-4), radiated (EN 61000-4-3), conducted (EN 61000-4-6)
• Surge: immunity to surge up to 1 kV (EN 61000-4-5)

EN 45545-2 — Fire protection

Defines the fire behaviour of materials in railway vehicles. Hazard Level (HL1–HL3) classification — most rolling stock requires HL2 or HL3. Tests cover flammability, smoke generation, gas toxicity and burning material dripping.

EN 61373 — Shock and vibration

Defines shock/vibration profiles by installation location:

• Category 1, Class A: bogie-mounted equipment (maximum severity)
• Category 1, Class B: vehicle body-mounted equipment (medium severity)
• Category 2: rack-mounted inside the vehicle (lower severity)

8. Converter types for railway applications

Power conversion equipment for railway covers a broad range of topologies and functions:

• DC/DC converters: auxiliary supplies for control electronics, sensors and communications (24–110 V input, 5–48 V outputs, 5 W–1 kW)
• DC/AC inverters: passenger amenity loads, HVAC auxiliaries, lighting systems (single- or three-phase, 230/400 VAC output)
• Battery chargers / rectifiers: traction battery and UPS charging, emergency supply (72–110 V input, configurable output)
• Frequency converters (AC/AC): 50/60 Hz conversion for international rolling stock, fan speed control
• High-power traction auxiliaries: head-end power, hotel power systems (10–100+ kVA)

Custom engineering is typically required to match the specific form factors, connector layouts, insulation classes and certification requirements of each rolling stock programme.

9. Selection criteria: 12 key questions for your supplier

When evaluating power converter suppliers for railway applications, these 12 questions distinguish a qualified supplier from one that merely claims compliance:

10. Total cost of ownership (TCO) vs. unit price

One of the most common purchasing mistakes is basing the decision exclusively on unit price. The true cost of a converter over 25–30 years includes:

• Integration cost: standard converters may require external EMC filters, hold-up modules, mechanical adapters and additional protection circuits — a custom converter eliminates these
• Certification cost: fewer components = fewer type tests = lower certification cost
• Field failure cost: higher MTBF = fewer maintenance interventions, less vehicle downtime, fewer contractual penalties
• NRE engineering cost: amortised over a platform if integration and maintenance savings are significant
• Obsolescence cost: a supplier guaranteeing 25–30 year lifecycle support avoids costly redesigns
• Logistics cost: European manufacturing eliminates import tariffs and reduces shipping times

When requesting a quotation, ask the supplier to break down not just the unit price but also the expected MTBF, the integration components required and the long-term support commitment.

11. Specification checklist

Use this checklist when drafting the technical specification for your next railway project:

• ☐ Nominal input voltage (U_nom) and voltage range
• ☐ Required temperature class (OT1–OT6)
• ☐ Hold-up class required (S2/C2 or other)
• ☐ EN 50121-3-2 EMC class
• ☐ EN 45545-2 Hazard Level (HL2/HL3)
• ☐ EN 61373 category and class (1A/1B/2)
• ☐ Output voltage(s), current(s) and power
• ☐ Isolation requirement (input-to-output)
• ☐ Mechanical form factor and connector type
• ☐ IP protection rating required
• ☐ MTBF target (MIL-HDBK-217 or SN 29500)
• ☐ Lifecycle support period (25 or 30 years)
• ☐ Quantity (prototype + series volumes)
• ☐ Delivery schedule and JIT requirements
• ☐ Quality certifications required (ISO 9001 / IRIS)
• ☐ Traceability requirements

12. Glossary

• DC/DC converter: device converting one DC voltage level to another with galvanic isolation
• DC/AC inverter: device converting DC (battery) to AC (single- or three-phase)
• Hold-up time: time during which the output remains within specification after an input power interruption
• MTBF: Mean Time Between Failures — statistical reliability metric (hours)
• NRE: Non-Recurring Engineering — one-time design and tooling cost
• OT class: Operating Temperature class as defined by EN 50155:2017
• TCO: Total Cost of Ownership — full lifecycle cost including purchase, integration, maintenance and support
• Type test: formal certification test campaign performed by an accredited laboratory on a representative sample
• U_nom: nominal battery supply voltage (24, 72 and 110 VDC are the most common in railway)