Isolation Transformers for Industrial Power: Which Insulation Class of Winding Wire for PLC, Automation and DIN-rail Modules

This article is written for power and procurement engineers who design industrial power modules, control-cabinet power distribution, and PLC/automation power supplies. The isolation transformer is the wall that separates the mains side from the low-voltage control side, and the critical part of that wall is the reinforced insulation formed where the winding wire crosses between primary and secondary. We address one of the most frequently asked selection questions: in the high-temperature environment of a control cabinet, should the thermal class of the winding wire be 130(B) or 155(F)—and why a "high mix, low volume" field like industrial power depends so heavily on local, flexible supply.

1. In industrial power, isolation lives between primary and secondary

The de facto standard for industrial control is a 24V DC output, which balances safety (SELV, safety extra-low voltage) with supply capability; PLCs, sensors and relays almost all draw from this bus. Separating the mains side (primary) from this control bus (secondary, SELV) relies on double or reinforced insulation—under IEC 62368-1, reinforced insulation is a single insulation system that provides protection against electric shock equivalent to double insulation; the secondary circuit is thus isolated from the primary circuit by double or reinforced insulation. The safety requirements for the transformer itself fall under the IEC 61558 series (covering insulation, creepage, clearance and temperature rise for isolation transformers and transformers for power supplies).

The isolation transformer is the core of this wall. Where the winding crosses between primary and secondary, triple insulated wire (TIW) is a common way to form reinforced insulation within a confined winding space—the wall is built into the wire itself, with no need for additional tape layers or physical separation distances. Which TIW type you choose directly determines how hot an environment this wall can withstand.

2. Control cabinets run hot: why the winding wire's thermal class needs headroom

How much temperature the winding wire must withstand depends not on the room temperature of the equipment hall but on the temperature actually reached at the winding hot-spot. Under IEC 60085, the thermal class of insulation is the upper limit of temperature the winding hot-spot can withstand long-term: 130°C for Class B and 155°C for Class F, with 40°C as the reference ambient temperature in that standard. The design goal is to keep the hot-spot continuously below the limit of the chosen thermal class.

The problem is that the inside of a control cabinet is far hotter than room temperature. In industrial practice, cabinet-internal temperatures commonly run 10–20°C above ambient, and the top of the cabinet can be 15–20°C higher; once the cabinet interior exceeds roughly 50°C, an enclosure air conditioner is usually required. DIN-rail power modules are moreover mostly fanless and convection-cooled, begin derating their output at roughly +50°C and above, and are often mounted right next to heat sources such as drives, contactors and network switches—squeezed into the last inch of free rail space—which makes the hot-spot problem even more pronounced.

Stacking these up, the actual temperature at the winding hot-spot is roughly composed of the following layers:

• Reference ambient — Description：IEC 60085 uses 40°C as the baseline
• Cabinet temperature rise — Description：Commonly 10–20°C above ambient; higher at the top
• Module-internal temperature rise — Description：Fanless convection cooling; output derating from roughly +50°C
• Winding self-heating — Description：Depends on load and winding design / per the Approval Document

Add these layers together and the winding hot-spot can easily approach the 130(B) limit. Choosing a 155(F) winding wire (such as TLW-F) gives 25°C of additional thermal margin on the material side—in an industrial module sitting next to heat sources with no fan, this is not over-specification but a real requirement that matches the actual thermal environment. Further reading: Wire thermal class vs. insulation system thermal class.

3. Wire selection compared: TLW-B / TLW-BB (130 B) and TLW-F (155 F)

Among the triple insulated wires DAH JIN TECHNOLOGY offers, the most relevant to industrial power are TLW-B / TLW-BB at 130(B) and TLW-F at 155(F). The comparison is as follows (specific conditions are always governed by the Approval Document):

• Thermal class — TLW-B / TLW-BB：130(B)；TLW-F：155(F), Class F
• Reinforced insulation evaluation — TLW-B / TLW-BB：Per the Approval Document；TLW-F：Reinforced (VDE 40008834 Annex J / IEC 62368-1; UL 2353)
• Wire diameter range — TLW-B / TLW-BB：Per the Approval Document；TLW-F：Per the Approval Document
• DJ-130 insulation system component — TLW-B / TLW-BB：TLW-BB is a component；TLW-F：Not a component
• Typical orientation — TLW-B / TLW-BB：Cabinet positions with better cooling；TLW-F：Hot cabinets; fanless modules close to heat sources

One critical and often misunderstood distinction: switching the winding wire to TLW-F raises the "wire" to the 155°C (Class F) class; it does not automatically make the whole transformer a Class F insulation system. The system's thermal class is evaluated separately under UL 1446 / IEC 60085 (DAH JIN's DJ-130 insulation system is 130(B)), which is also why TLW-F is not listed among the DJ-130 components. Wire-side listing and system-side listing are two different things and must be assessed separately before adoption. The complete selection criteria are in the TIW selection guide.

Specific part numbers, wire diameter ranges and applicable conditions are always governed by the Approval Document.

4. High mix, low volume and lead-time sensitive: why industrial power needs local, flexible supply

Industrial power is a textbook "high mix, low volume" (HMLV) field: there are many primary/secondary voltage combinations, VA ratings range from a fraction of a watt to several kVA, and form factors split into PCB-mount, DIN-rail, chassis-mount and potted types—so derivative part numbers proliferate while the batch size of any single part number stays small. Industry data also shows that industrial use is the largest segment of transformer demand, and because small batches carry material-stocking and changeover costs, lead time is often dictated by the arrival of copper, cores and special insulation materials. This is exactly where industrial power is most sensitive on the supply side: specifications must be customizable, volumes need not be large, and lead times must be stable.

DAH JIN is a manufacturer (not a trading company) of triple insulated wire, with its own factory in Guiren, Tainan, Taiwan—made in Taiwan, and the manufacturing location recorded in its UL file is this plant. For the traditional, fragmented demand of industrial power, low MOQ, high mix / low volume and flexible, fast delivery are exactly the strengths that DAH JIN's business model fits best—there is no need to bend a derivative specification to a high-volume threshold, and material selection and sampling can be completed locally in Taiwan. For the questions to ask when evaluating a supplier, see TIW supplier due diligence.