Railway Rolling-Stock Auxiliary Power: Reinforced-Insulation Small Transformers under EN 50155

This article is written for the power-supply, magnetics and procurement engineers behind railway rolling-stock electronic equipment. Inside your on-board unit there is usually a small auxiliary-power transformer that draws from the battery bus and feeds low-voltage control and communication loads—and it has to provide isolation across a high-voltage barrier in the harsh environment defined by EN 50155. Starting from the end application, this article explains that the reinforced insulation of this small transformer follows the IEC 62368-1 Annex J route, that the certification path does exist, and clarifies the division of roles between the "wire Approval Document" and "equipment qualification". All DAH JIN TECHNOLOGY product values are stated "per the Approval Document".

1. EN 50155: the environmental threshold for on-board electronic equipment

EN 50155 is the core standard for railway rolling-stock electronic equipment (equivalent to IEC 60571). It specifies equipment-level conditions: temperature, humidity, shock, vibration, supply voltage and electromagnetic compatibility (EMC). It is not a single-component standard; it is the equipment-level qualification of "whether the unit survives and works properly on the vehicle". Key points:

• Temperature: EN 50155 defines several operating-temperature classes; depending on class and edition they broadly cover −40°C to +70/+85°C, while the actual temperature inside the enclosure, close to the power stage, is often higher.
• Shock and vibration: mechanical testing is carried out per EN 61373.
• EMC: emission and immunity of on-board equipment are governed by EN 50121 (-3-2).
• Supply interruptions: class S2, for example, requires ride-through of at least 10 ms on loss of supply.

Note the division of scope: complete power converters (traction/power converters) are covered separately by EN 61287-1, software by EN 50657 and functional safety by EN 50129. The "auxiliary-power small transformer" discussed here is the low-voltage isolation stage inside the on-board equipment, not the main traction converter. Clause-level detail is kept to a minimum and not expanded clause by clause.

2. Where auxiliary power sits on the vehicle: from the battery bus to low-voltage loads

The job of on-board auxiliary power is to isolate the battery/bus voltage, step it down to a range usable by low-voltage loads—control, communication, sensing—and provide galvanic isolation between the high-voltage and low-voltage sides. Three design realities:

• Battery voltage: five nominal on-board battery voltages are common in railways—24 V, 48 V, 72 V, 96 V and 110 V—with 72 V and 110 V the most common (the higher the voltage, the lower the current for the same power, saving cable weight and space). Some systems use floating (IT) earthing with neither rail directly earthed, so the polarity notation −72 V / −110 V is also widely used.
• Isolation voltage rises with system voltage (industry practice based on EN 50155; see the table below).
• Wide input range and surge withstand: equipment must tolerate voltage dips (e.g. 0.6×VN for 100 ms) and surges (e.g. 1.4×VN for 1 s); stricter cases must also meet the RIA 12 surge conditions of 1.5× (1 s) and 3.5× (20 ms).

• < 72 VDC — Recommended isolation (industry practice)：500 VAC
• 72–125 VDC — Recommended isolation (industry practice)：1000 VAC / 1500 VDC
• 110 V (float charging can exceed 125 V) — Recommended isolation (industry practice)：1500 VAC commonly recommended
• DAH JIN wire withstand / test values — Recommended isolation (industry practice)：Per the Approval Document

This is the reality of the auxiliary-power small transformer: a physically small transformer that has to deliver adequate isolation under wide input, severe surges and across a high-voltage barrier.

3. Crossing the isolation barrier: the Reinforced route of IEC 62368-1 Annex J

To achieve reinforced isolation in a small transformer, the traditional approach relies on margin tape and interlayer tape to build up creepage distance and clearance at both ends of the bobbin, keeping the primary and secondary windings apart. But an auxiliary-power transformer has limited volume and a tight winding window: the wider the tape margins, the harder it is to fit the wire.

The mainstream solution is to switch to a qualified triple insulated wire (TIW). Under IEC 62368-1 Annex J, an insulated winding wire that has itself passed evaluation under that annex can serve, through its three-layer construction, as the reinforced insulation of the winding (reinforced = basic + supplementary, roughly equivalent to three layers). Because the insulation is provided by the wire itself, primary and secondary can be interleaved, eliminating the creepage/clearance margins and insulation barriers that tape would otherwise have to provide. This is exactly where the auxiliary-power small transformer gains most—"completing reinforced isolation with a single wire and shrinking the volume"—and the certification path clearly exists.

• Source of reinforced isolation — Traditional tape-margin approach：Margin tape at both bobbin ends + interlayer tape build up creepage/clearance；Qualified triple insulated wire (Annex J Reinforced)：The three-layer insulation of the winding wire itself provides reinforced
• Winding window — Traditional tape-margin approach：Margins take up much space; tight on small bobbins；Qualified triple insulated wire (Annex J Reinforced)：No tape margins; winding window freed up
• Primary/secondary arrangement — Traditional tape-margin approach：Must be separated into zones；Qualified triple insulated wire (Annex J Reinforced)：Can be interleaved; no insulation barrier needed
• Evaluation standard — Traditional tape-margin approach：Assessed on the tape-and-construction combination；Qualified triple insulated wire (Annex J Reinforced)：IEC 62368-1 Annex J (TLW-F additionally holds UL 2353)
• DAH JIN wire diameter / withstand values — Traditional tape-margin approach：—；Qualified triple insulated wire (Annex J Reinforced)：Per the Approval Document

The triple insulated wire of the DAH JIN TLW series is evaluated as Reinforced under VDE 40008834 (Annex J / IEC 62368-1); TLW-F additionally holds UL 2353 Reinforced. For the correspondence between Annex J and 61558 Annex K, see IEC 62368-1 Annex J and 61558 Annex K; for the type-test crosswalk of Annex J/K/L, see Type-test crosswalk: Annex J/K/L.

4. Thermal environment and the certification split: a wire Approval Document is not equipment qualification

An auxiliary-power small transformer often sits right next to the power stage with poor cooling, so its internal temperature can be markedly higher than the ambient outside the vehicle. This is where the wire's thermal class matters:

• DAH JIN TLW wire temperature ratings depend on the type—TLW-F is Class F, 155°C; TLW-B / TLW-BB / TLW-B7S are 130(B). Select the type whose thermal class matches the thermal environment; specific values per the Approval Document.
• But keep the two kinds of "qualification" apart: the wire Approval Document defines the material (type and wire-diameter range, reinforced-insulation evaluation, thermal class); EN 50155 / EN 61373 (vibration) / EN 50121 (EMC) are the qualification of the finished magnetic component and the complete equipment in the rolling-stock environment. The wire's thermal class does not equal, and does not replace, the equipment certification of the finished product under combined vibration, EMC and temperature conditions. Wire thermal class and system/finished-product thermal class belong to separate registrations; see Wire thermal class vs. insulation-system thermal class.

In one sentence: triple insulated wire helps you achieve reinforced isolation across the high-voltage barrier within a small winding window, and lets you match the in-vehicle thermal environment through the thermal class of the type; but the responsibility for "whether this transformer, this unit passes EN 50155" remains with the design and verification of the finished magnetic component and the complete equipment. As a first step in selection, see the TIW selection guide and the TIW certification chain.

DAH JIN TECHNOLOGY is a wire manufacturer (not a trading company), producing at its own plant in Guiren, Tainan, Taiwan; the manufacturing site on record in the UL file is this plant—made in Taiwan.