Printer HVPS: Why Triple Insulated Wire (TIW) Is Ruled Out by the Specifications

This article is written for high-voltage power supply engineers, magnetics engineers and purchasing engineers working on laser printers and copiers (electrophotography). You may have noticed that triple insulated wire (TIW) has "thick insulation" and wondered whether the step-up transformer inside the printer's high voltage power supply (HVPS) could use it. The short answer: the specifications rule it out, and this is a dead end. The HVPS output operates continuously at high voltage, at a level far beyond the rated working voltage of TIW. TIW is a safety-isolation wire, not a high-voltage wire; the two solve different problems. This article lays the matter out clearly to save both sides the trial and error. All DAH JIN product values are "per the Approval Document".

1. How high a voltage the HVPS has to produce inside a printer

In the electrophotographic imaging process (charging → exposure → development → transfer → fusing), several stages depend on the HVPS to supply a high-voltage bias or corona voltage. These voltages range from several hundred volts up to several kV, and they are held continuously at that potential rather than applied as momentary pulses.

• Developer bias — Typical output voltage (continuous operation)：Several hundred V (around the −600 V level)；Notes：DC bias that controls print density, often with AC superimposed
• Charge roller (PCR) — Typical output voltage (continuous operation)：About −1000 V；Notes：Contact charging, replacing the older corona wire
• Transfer roller — Typical output voltage (continuous operation)：Several hundred V up to about +1000 V；Notes：Moves toner from the photoconductor drum to the paper
• Corona / scorotron charging — Typical output voltage (continuous operation)：Wire about −5～−8 kV; grid about −500～−1500 V；Notes：Non-contact charging, in older designs and some high-end models
• Transfer corona (older designs) — Typical output voltage (continuous operation)：About +6 kV；Notes：Older non-contact transfer
• Dedicated electrophotographic high-voltage modules — Typical output voltage (continuous operation)：Up to about the ±20 kV level；Notes：Commercial high-voltage modules for charging / transfer

Two things matter here: the magnitude and the word "continuous". Contact charge rollers, developer and transfer rollers mostly sit between several hundred V and about 1 kV; once the design moves to corona / scorotron non-contact charging, the wire sits at −5～−8 kV, and older transfer coronas go up to +6 kV; commercial electrophotographic high-voltage modules output as high as the ±20 kV level. All of these are continuous working voltages that the HVPS must deliver steadily.

2. TIW is a safety-isolation wire, not a high-voltage wire

First, an intuitive but mistaken idea needs to be cleared away: "three layers of insulation = thick insulation = must withstand high voltage". That is wrong. The three insulation layers of TIW solve the problem of reinforced insulation for safety compliance, not the problem of "carrying continuous high voltage".

TIW is evaluated under UL 2353, IEC 62368-1 Annex J (safety of ICT and audio/video equipment), the IEC 61558 series, and for medical equipment IEC 60601-1 (per the Approval Document). The design purpose of this whole framework is to let a single wire achieve reinforced insulation between the primary and secondary sides of a transformer at mains-level working voltages, dispensing with the creepage distance that would otherwise have to be built up with margin tape and interlayer tape. In other words, the value of TIW lies in "safety isolation without creepage distance", and the voltage world it serves is the mains level (rated working voltages on the order of about 1 kV), not the kV-to-tens-of-kV world at the HVPS output.

Take the published specification of one triple insulated wire on the market as an example (not a DAH JIN value): its rated working voltage is listed at the level of VDE 1000 V, UL (electronic equipment) 1500 V and UL (medical) 707 V. That is the working-voltage range for which wires of the TIW type are designed and evaluated: the mains level, not the continuous −5～−8 kV high voltage of corona charging. The ratings of DAH JIN wires are always per the Approval Document and are not repeated here.

3. The key is the "rated working voltage", not the "breakdown voltage"

This section is the heart of the matter and the place where misjudgment is most likely. Someone may say: "In that same specification, the breakdown voltage of that TIW is written as about 7 kV, so wouldn't it be just right for a 6 kV corona?" This confuses two different things.

• Rated working voltage: the voltage the wire can withstand continuously, with the safety margin already included. In the example above it is at the 1000～1500 V level.
• Breakdown / dielectric withstand test voltage: a test value applied for a short time to verify that the insulation is free of defects; in the example above about 7 kV. It means "it survives the test", not "it can be used here all the time".

Insulation is never used by placing the continuous working voltage near the breakdown voltage; that would mean no margin at all, gambling with safety. What actually decides "usable or not" is the rated working voltage. Putting the two sides next to each other makes it obvious:

• Nature of the voltage — Printer HVPS output：Continuous operation at high voltage；TIW for safety isolation：Rated continuous working voltage
• Magnitude — Printer HVPS output：Several hundred V to several kV; modules up to about 20 kV；TIW for safety isolation：About the 1 kV level (published industry specification; DAH JIN per the Approval Document)
• Short-time withstand / breakdown — Printer HVPS output：—；TIW for safety isolation：Breakdown about 7 kV (short-time test value, not a continuous rating)
• Design role — Printer HVPS output：Generating / delivering high voltage；TIW for safety isolation：Reinforced insulation between primary and secondary (no creepage distance)
• Conclusion — Printer HVPS output：—；TIW for safety isolation：Continuous working voltage exceeds the TIW rating; not applicable

The result is clear: parts such as the charge roller, at several hundred V to 1 kV, never needed reinforced-insulation-grade triple insulation to hold their potential in the first place; and the continuous working voltages of coronas, scorotrons, transfer coronas and high-voltage modules, from 5 kV and 6 kV up to 20 kV, not only far exceed the roughly 1 kV rated working voltage of TIW but come close to or cross its short-time breakdown test value. From whichever end you look, the HVPS high voltage keeps TIW out. This is a dead end in the specifications, and no brand and no Approval Document can fill it.

4. How HVPS high-voltage transformers are actually wound

If not TIW, how is the HVPS step-up transformer actually built? High-voltage transformers in the industry follow a completely different design language:

• Sectioned / multi-bay bobbin: when the secondary output potential exceeds the dielectric strength of the coating of a single magnet wire, the bobbin is divided into several winding bays, spreading the total output voltage across the bays and reducing the turn-to-turn potential difference at any one point.
• Potting / encapsulation: high-voltage coils are commonly potted in epoxy or polyester resin to eliminate internal voids and suppress corona discharge.
• Equipotential winding and dedicated high-voltage magnet wire: the secondary is wound in a specific order and direction so that adjacent, closest turns stay at equal potential, avoiding turn-to-turn discharge; the wire used is magnet wire and lead wire designed specifically for high voltage, not the fine solid triple insulated wire used for safety isolation.

In other words, the problem on the high-voltage side is "how to distribute a high potential, suppress discharge and avoid turn-to-turn breakdown", and the answer is a sectioned bobbin plus potting plus high-voltage magnet wire. This is a different engineering problem from TIW's "achieving reinforced insulation with a single wire at mains level", so the tools are naturally different.

5. Where DAH JIN TIW fits (per the Approval Document)

Within DAH JIN's product line, the four externally offered triple insulated wires TLW-B, TLW-BB, TLW-B7S and TLW-F are all winding wires for safety isolation: evaluated under VDE 40008834, UL 2353 and IEC 62368-1 Annex J (ICT / audio-video), with TLW-BB and TLW-B7S additionally covering the IEC 60601-1 (medical) path. Their purpose is to provide reinforced isolation between the primary and secondary sides of a transformer, without creepage distance, at mains-level working voltages: exactly the world described in Section 2, not the high-voltage world at the output of a printer HVPS.

Accordingly, the high-voltage transformer of a printer HVPS does not fall within the application scope of DAH JIN TIW. The wire diameter range, thermal class, withstand voltage and all test values of each model are always per the Approval Document, and specific figures are not repeated here. For the first step in selection (matching thermal class to model) see the TIW selection guide; for the distinction between wire thermal class and system / end-product thermal class see Wire thermal class vs. insulation system thermal class. If your application really does fall within mains-level safety isolation (rather than HVPS high-voltage output), you are welcome to set out the conditions (working voltage, isolation requirement, applicable standard, wire diameter) and contact our sales team to define it per the Approval Document.

DAH JIN is a wire manufacturer (not a trading company), producing at its own factory in Guiren, Tainan, Taiwan; the manufacturing site named in the UL file is this factory. Made in Taiwan.