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Partial Discharge at Altitude: Why 270 VDC Insulation Cannot Be Specced at Sea Level
More-electric aircraft moved the bus from a familiar 115 VAC world to a 270 VDC one, and a barrier thickness that satisfied decades of low-voltage practice does not automatically clear the new class. The reason is not just the higher number; it is that altitude makes the air gap the weak point in a way that is counterintuitive until you have seen the Paschen curve. This article walks partial discharge and altitude qualitatively, and why a 270 VDC detail cannot be specced at sea level.
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The barrier that worked for decades at 115 volts and failed at 270
More-electric aircraft moved the bus from a familiar 115 VAC world to a 270 VDC one, and a barrier thickness that satisfied decades of low-voltage practice does not automatically clear the new class. The reason is not just the higher number. It is that the combination of higher stress, altitude derating, and compressed creepage paths erases the margin everyone assumed, and the physics that punishes the gap, partial discharge, gets worse with altitude in a way that is counterintuitive until you have seen the Paschen curve.
This article walks partial discharge, why altitude is the aggravating variable, and why a 270 VDC insulation detail cannot be specified by sea-level habit, all qualitatively and by designation.
Partial discharge, qualitatively
Partial discharge is a small, localized electrical breakdown that bridges only part of the insulation path, typically in a void, at a sharp edge, or across a thin air gap beside a solid barrier, without completing a full flashover. It does not trip anything immediately. What it does is erode: each tiny discharge degrades the adjacent polymer a little, and over service hours the cumulative damage tracks a path toward full breakdown.
That is why a 270 VDC part can pass its hi-pot at acceptance and still fail years later. The insulation system has to be designed so that the local electric field never reaches the level where partial discharge starts in the gaps the geometry actually contains, which means the air gaps and surface paths matter as much as the solid dielectric.
Why altitude makes the air gap the weak point
The breakdown voltage of a small air gap is not constant; it depends on the product of pressure and gap distance, the relationship captured qualitatively by the Paschen curve. As an aircraft climbs and ambient pressure falls, the air in every gap, void, and surface clearance becomes easier to ionize, so the voltage at which partial discharge begins drops. A clearance that was comfortably above the discharge threshold at sea level can fall below it at altitude.
The consequence for a converter's parts is direct: the solid barrier may be more than adequate, but the air beside it, in a void under a poorly seated liner, at a sharp edge, across a too-short surface creepage, becomes the weak point precisely when the aircraft is highest. Altitude is therefore not a derating footnote; it is the governing case for the gaps.
What that means for the converted insulation detail
The design response is the program's, but it lands on features a converter cuts. Three matter most.
Film-tier barriers with documented per-thickness breakdown
Thin, high-integrity film barriers (APTIV PEEK film and Kapton or Apical polyimide) carry documented breakdown per thickness on the TDS, so the solid dielectric can be specified with real margin against ASTM D149 data rather than a sea-level rule of thumb. The film tier also lets a thinner, void-free barrier replace a thicker one that might trap the very air gaps partial discharge exploits.
Mica plies and engineered creepage extenders
Where surface creepage is the limiting path, mica plies and creepage extenders lengthen the surface distance the discharge would have to bridge, and the geometry, ribs, barriers, and distance, is engineered to the derated, altitude case rather than the bench. Surface path length is a dimension on the drawing, not an afterthought.
Void-free converting at the edges
Because partial discharge starts in voids and at sharp edges, the converting itself is part of the mitigation: burr-free edges, crease placement that seats without trapping air, and barriers cut to seat flat against the conductor remove the gaps where discharge would otherwise begin. A barrier that is dielectrically perfect on the TDS but installed with a void beside it has reintroduced the weak point. The compliant interfaces ride on BISCO HT-12xx solid silicone where a conformable dielectric pad closes a gap that a rigid barrier would leave.
Sea-Level Habit vs the 270 VDC Altitude Case
| Design variable | Sea-level habit | 270 VDC altitude reality | Converted feature that answers it |
|---|---|---|---|
| Solid dielectric thickness | Rule-of-thumb thickness | Per-thickness breakdown, ASTM D149 | Film-tier barrier with TDS data |
| Air gap breakdown | Assumed adequate | Falls with altitude (Paschen) | Void-free seating, no sharp edges |
| Surface creepage | Short path tolerated | Discharge bridges short paths | Mica plies, engineered creepage extenders |
| Margin | Comfortable at sea level | Eroded by stress and altitude | Design and convert to the derated case |
Qualitative and relative only; the Paschen relationship and the program's derating belong to the OEM's dielectric design. Breakdown values per ASTM D149 on each TDS; the 270 VDC class is framed by MIL-STD-704.
The pattern: the air, not the barrier, is the weak point at altitude
The 270 VDC failure on the parent page, a barrier specified by sea-level habit, belongs to the same family as the others: a part that passed its test at the convenient condition and failed at the governing one. Here the governing condition is altitude, and the weak point is the air in the gaps rather than the solid dielectric. The cure is to design and convert to the derated, altitude case: real per-thickness breakdown margin, engineered surface creepage, and void-free converting that removes the gaps partial discharge exploits.
The dielectric design and the Paschen-based derating are the OEM's; what H-O converts is the film barrier, the mica extender, and the void-free edge that execute it.
How H-O Products helps
H-O Products is a precision die-cut converter, not a raw-material producer. We take PEEK and polyimide film, mica sheet, and silicone stock and die-cut, laminate, crease, and form it to your 270 VDC insulation drawing, with the burr-free, void-free edge quality that partial-discharge mitigation depends on, and with material traceability and lot-code TDS records under our ISO 9001:2015 certified quality management system in Winsted, Connecticut.
The dielectric design, the Paschen-based altitude derating, the creepage and clearance scheme, and machine qualification stay with the OEM; H-O converts the barriers and extenders that execute the design.
Frequently asked questions
What is partial discharge, and why does it matter for aerospace insulation?
Partial discharge is a small, localized electrical breakdown that bridges only part of the insulation path, typically in a void, at a sharp edge, or across a thin air gap beside a solid barrier, without completing a full flashover. It does not trip immediately; it erodes the adjacent polymer a little with each discharge until, over service hours, the cumulative damage tracks toward full breakdown.
That is why a high-voltage part can pass its hi-pot at acceptance and still fail years later, and why the air gaps and surface paths matter as much as the solid dielectric.
Why is 270 VDC insulation harder to specify than 115 VAC?
Because the move to more-electric aircraft combined a higher voltage with the same altitude environment, and the margin that decades of 115 VAC practice assumed gets erased by the higher stress, the altitude derating, and compressed creepage paths. A barrier thickness carried over from low-voltage habit may be adequate as a solid dielectric yet leave the air gaps beside it below the partial-discharge threshold at altitude. The 270 VDC class, framed by MIL-STD-704, has to be specified at the derated, altitude case, not by sea-level rule of thumb.
How does altitude affect partial discharge?
The breakdown voltage of a small air gap depends on the product of pressure and gap distance, the relationship captured qualitatively by the Paschen curve. As an aircraft climbs and pressure falls, the air in every gap, void, and surface clearance becomes easier to ionize, so the voltage at which partial discharge begins drops. A clearance that was safely above the threshold at sea level can fall below it at altitude.
Altitude is therefore the governing case for the air gaps, not a derating footnote, which is why the insulation detail must be designed to the derated condition.
What converted features help control partial discharge at 270 VDC?
Three matter most: film-tier barriers such as PEEK and polyimide with documented per-thickness breakdown on the TDS so the solid dielectric carries real margin against ASTM D149 data; mica plies and engineered creepage extenders that lengthen the surface path the discharge would bridge; and void-free converting, burr-free edges and crease placement that seats flat without trapping air, because partial discharge starts in voids and at sharp edges. A dielectrically perfect barrier installed with a void beside it has reintroduced the weak point.
Does H-O design the 270 VDC insulation or do the derating?
No. The dielectric design, the Paschen-based altitude derating, and the creepage and clearance scheme belong to the machine or power-electronics OEM and its test houses. H-O converts the parts that execute that design: film-tier barriers, mica creepage extenders, and compliant pads, cut with the burr-free, void-free edge quality that partial-discharge mitigation depends on, to your drawing, with material traceability and lot-code TDS records. Breakdown values are confirmed against each grade's current TDS at quote time.
Have a 270 VDC insulation detail reviewed
Send the dielectric drawing with the voltage class, the altitude envelope, the creepage and clearance scheme, and the barrier geometry, and we will recommend film-tier barriers and creepage extenders and convert them with void-free edge quality.
Samples typically ship in 3 to 5 business days for common die-cut configurations on materials we keep on hand; standard production runs about two weeks after drawing approval; expedited service is available. MOQ varies by material and part.
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Related reading
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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- ASTM D149 (dielectric breakdown voltage and strength; per-thickness breakdown on each film TDS).
- MIL-STD-704 (aircraft electric power characteristics, including the 270 VDC class, cited qualitatively); RTCA DO-160 (environmental framing).
- IEC 60085 (thermal classification, by designation); the Paschen relationship is cited qualitatively and the derating belongs to the OEM.
- Manufacturer technical data sheets for the PEEK and polyimide film, mica, and silicone grades named on the parent application page.
This article summarizes electrical-insulation material-selection principles for aerospace machines and power systems using the families and published test methods cited on the parent application page. Comparisons are qualitative; per-grade values and thermal-class assignments quoted are maker-TDS figures and must be confirmed on the manufacturer's current technical data sheet at your hot spot, voltage, and altitude. Standards (IEC, ASTM, NEMA, MIL-STD) are cited by designation for context, not as a claim of qualification.
H-O is a precision die-cut converter, not a raw-material producer, and does not design insulation systems, certify machines, or perform dielectric qualification; those belong to the machine OEM and its test houses.