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Dielectric Material Selection: Mica vs. Polyimide vs. Aramid vs. GPO-3
A dielectric barrier is not chosen on a single headline number; it is chosen by matching the job to the property that actually limits the part. Mica leads where temperature and arc resistance dominate, polyimide film where a very thin, high-thermal-class layer must hold a high breakdown field, aramid paper for flexible slot and layer insulation at Class F or H, and glass-laminate (GPO-3 class) where a rigid, machinable barrier must hold a creepage profile and carry a load.
This guide separates the four families by the decision each one wins and defines the governing properties (dielectric strength per IEC 60243 / ASTM D149, tracking resistance as the Comparative Tracking Index per IEC 60112, and thermal class per IEC 60085). No grade-specific breakdown value is quoted here; those come from the manufacturer's current data sheet at your gauge.
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The application problem: four families, one barrier slot
H-O Products is a precision die-cut converter, and every electrical-insulation or dielectric-barrier part is made to order from sheet, roll, board, or slab stock cut to a drawing. The concrete problem an engineer faces is that several materials can physically fit the same barrier slot, but only one is right once the working voltage, the creepage path, the thermal class, and any arc or flame duty are all on the table.
A polyimide film and a glass-laminate can both sit between two conductors, yet one is a thin flexible layer that raises the bulk withstand and the other is a rigid standoff that lengthens the surface path. Picking the wrong family does not always fail on the bench; it fails in service, when a thin film is asked to carry a clamping load or a rigid board is asked to wrap a busbar.
The selection is a judgment, not an arithmetic: name the job, then check the property that limits it, rather than reaching for the material with the best single number.
Why it is challenging: the limiting property changes with the job
Dielectric material selection is hard because the property that decides the part differs from one job to the next, and the families do not rank the same way on every axis.
- Dielectric strength is thickness-dependent, not a fixed grade rating. The breakdown field a solid can hold (in kilovolts per millimeter) falls as the material gets thicker and depends on the test method, electrode, and ramp (IEC 60243 / ASTM D149). A thin film can show a very high field while a thick board shows a lower one, so comparing two families on one catalog number without the gauge is misleading.
- Tracking resistance is a separate property from breakdown. A material can have a high bulk breakdown field and still track poorly on a contaminated surface. Surface tracking is governed by the Comparative Tracking Index (CTI per IEC 60112), which sorts materials into groups (group I the highest tracking resistance through group IIIb the lowest). Inorganic mica resists tracking and arc strongly; some high-performance polymers carry a high bulk field but a more modest CTI, so the surface and bulk jobs can point at different families.
- Thermal class caps the others. A material that loses its properties above its thermal class (IEC 60085) cannot be rescued by a high room-temperature breakdown number. Aramid is commonly associated with Class F and H, polyimide carries a very high thermal class for a polymer, and inorganic mica reaches the highest classes, so the hot spot often narrows the field before voltage does.
- Form factor and load are constraints, not afterthoughts. A thin film wraps and flexes; a rigid laminate holds a profile and a clamping load. A film is not a structural standoff and a rigid board is not a slot liner, so the mechanical job rules a family in or out on its own.
Recommended material families
Four families cover most dielectric-barrier and insulation parts. Each is named here as a material, with cautious framing, and each should be confirmed against the manufacturer's current technical data sheet for the specific grade, gauge, and rating.
Mica barrier sheet (highest temperature, arc and flame)
Mica barrier sheet is an inorganic insulation that leads where temperature, arc resistance, and flame resistance dominate. Because it is mineral rather than polymer, it holds its dielectric and mechanical properties at temperatures that degrade organic films and papers, and it resists surface tracking and arc erosion strongly. Mica is commonly used for arc-flash and fire-barrier duty and the highest thermal classes, as a rigid board or a flexible sheet. For a high-temperature inorganic role without a full arc duty, glass-fiber paper is a related option.
Polyimide film (thin, high field, very high thermal class)
Polyimide film leads where a very thin, flexible layer must hold a high breakdown field at a very high thermal class for a polymer. It is the canonical choice for slot and layer insulation, terminal and substrate dielectric layers, and flexible barriers that hug a conductor, because thin gauge and thermal class matter more than rigidity. For the most demanding high-temperature dielectric layers, PEEK film is the related high-performance film.
The breakdown field both show depends on the gauge and test method (IEC 60243 / ASTM D149), so confirm it on the current data sheet.
Aramid paper (flexible slot and layer insulation, Class F or H)
Aramid paper leads for flexible, high-thermal-class slot liners, phase and layer insulation, and barrier paper in motors and transformers. It is flexible enough to form into a slot, carries Class F or H thermal performance, and is widely treated as the workhorse paper insulation for rotating machines and dry-type transformers. Reach for aramid when the part is a flexible liner at a high thermal class rather than a thin high-field film or a rigid standoff.
The broader films, papers, and laminates family covers related flexible grades, including classic vulcanized-fibre and polyester dielectric films, without a dedicated page.
Glass-laminate, GPO-3 class (rigid, machinable, creepage and load)
Glass-laminate (GPO-3 class and the glass-epoxy grades) leads where the barrier must be rigid, dimensionally stable, and machinable, so it can hold a creepage-lengthening profile and carry a clamping or structural load. It is the common choice for phase barriers, busbar standoffs, and compartment barriers, where the part is a structural insulator, not a flexible layer.
For a more demanding structural insulating support, Durostone-class composite is a related option, and where a compressible flame-rated barrier is needed in an arc or fire zone, flame-retardant silicone sponge fills that role.
Dielectric Family Comparison: Mica, Polyimide, Aramid, GPO-3
| Material family | Thermal class | Tracking / arc resistance | Suits thin / flexible | Rigid / load-bearing | Typical barrier role |
|---|---|---|---|---|---|
| Mica barrier sheet | Highest | Excellent | Sheet or board | Board form | Arc-flash / fire barrier, high-temp dielectric |
| Polyimide film | Very high | Good | Thin / flexible | Not structural | Slot / layer / terminal dielectric layer |
| Aramid paper | Class F / H | Good | Flexible liner | Not structural | Slot liner, phase / layer insulation |
| Glass-laminate (GPO-3 class) | Grade-dependent | Good (GPO-3) | Rigid board | Load-bearing | Phase barrier, standoff, compartment barrier |
Illustrative only; confirm against current material data and a drawing review. Levels are relative and qualitative, not measured values. Dielectric strength is thickness-dependent and is confirmed per IEC 60243 / ASTM D149 on the manufacturer's current technical data sheet; tracking resistance (CTI) is per IEC 60112, and thermal class is per IEC 60085. No kilovolt-per-millimeter withstand value is assigned to any grade here.
Design and converting considerations
Once the family is chosen, the converting follows the form factor. The decisions that belong in a drawing review include the gauge or thickness range (part of the dielectric spec, because the breakdown field is thickness-dependent), the profile that lengthens a creepage path, the hole and slot pattern, and whether the part is die-cut, kiss-cut, laser or waterjet cut, machined to a profile, laminated, slit to width, or kitted.
- Thin films and papers (polyimide, aramid, PEEK) are die-cut, kiss-cut, laser, or waterjet cut, and can be laminated into a multi-layer stack or formed into a slot liner. Thin gauge and a clean edge matter where the part wraps a conductor or fits a slot.
- Rigid laminates (GPO-3 class glass-laminate, Durostone-class composite) are typically waterjet cut or machined to a profile with mounting holes and slots, because the part is a structural insulator that holds a creepage-defining shape.
- Mica and glass-fiber barriers are cut to the arc or fire zone geometry, in board or sheet form, where high-temperature and arc duty drive the design.
- Stacks and kits. A laminated-busbar barrier set is several parts of different families doing different jobs; H-O converts the die-cut and machined layers and can kit them for the assembly line.
Common failure modes
| Failure mode | Root cause | The fix |
|---|---|---|
| Comparing families on one number | Ranking on a single catalog breakdown value without the gauge or the test method | Compare at the actual gauge and method (IEC 60243 / ASTM D149); pick by the limiting property |
| High field, poor surface life | A material with a strong bulk breakdown but a modest CTI used on a contaminated surface | Check tracking resistance (CTI per IEC 60112) separately from breakdown |
| Loss of class at the hot spot | A polymer chosen on room-temperature numbers used above its thermal class | Match the thermal class (IEC 60085) to the system hot spot, then check voltage |
| Film asked to carry a load | A thin flexible film used where a rigid standoff was needed | Use a rigid glass-laminate (GPO-3 class) for the structural barrier role |
| Rigid board asked to flex | A rigid laminate used where a conformable slot liner was needed | Use aramid paper or polyimide film for the flexible liner role |
| Organic material in an arc zone | A film or paper used where arc and flame resistance was the deciding duty | Use inorganic mica (or glass-fiber paper) for the arc and fire barrier |
How H-O Products helps
H-O Products is a precision die-cut converter. Once the job, the working voltage, the creepage need, the thermal class, and any arc or flame duty are known, we recommend a material family and grade direction across mica barrier sheet, polyimide and PEEK film, aramid paper, glass-fiber paper, and glass-laminate (GPO-3 class and the glass-epoxy grades), then convert the stock to your drawing: die-cut, kiss-cut, laser or waterjet cut, machined to a profile, laminated, slit to width, and kitted.
Documentation available on request includes the material technical data sheet, a certificate of conformance, lot traceability, and first-article inspection, all under our ISO 9001:2015 certified quality management system. We hold converting tolerances to the drawing; we do not produce the raw stock, and we do not certify the finished switchgear, transformer, machine, or battery pack.
Grade properties, including dielectric strength at the gauge, are confirmed against the manufacturer's current technical data sheet, and the apparatus-level withstand and type tests belong to the equipment maker.
Frequently asked questions
How do I choose between polyimide film and glass-laminate for a barrier?
Choose by the form factor and the load, not the headline breakdown number. Polyimide film is a thin, flexible layer that raises the bulk withstand at a very high thermal class and is right for slot, layer, and terminal dielectric layers. Glass-laminate (GPO-3 class) is a rigid, machinable barrier that holds a creepage profile and carries a clamping or structural load, and is right for phase barriers and standoffs. A film is not a structural standoff, and a rigid board is not a slot liner.
Why can't I just compare materials by their dielectric strength?
Because dielectric strength is thickness-dependent and method-dependent, not a fixed grade rating. The breakdown field a solid holds (in kilovolts per millimeter) falls as the material gets thicker and changes with the test method, electrode, and ramp (IEC 60243 / ASTM D149). Comparing two families on one catalog number without the gauge is misleading, and a high bulk field says nothing about surface tracking, which is governed separately by CTI (IEC 60112).
What is CTI and when does it decide the material?
The Comparative Tracking Index (CTI per IEC 60112) measures how well a surface resists forming a conductive carbonized track when contaminated, sorting materials into groups from I (highest tracking resistance) to IIIb (lowest). It decides the material whenever the surface can get dirty or wet at voltage, because a material can have a high bulk breakdown field and still track poorly.
Which family handles the highest temperature and arc duty?
Inorganic mica leads where temperature, arc resistance, and flame resistance dominate, because it is mineral rather than polymer and holds its properties at temperatures that degrade organic films and papers. Glass-fiber paper is a related high-temperature inorganic option. Confirm the grade, its thermal class (IEC 60085), and its flame rating (UL 94) on the current technical data sheet.
Get a dielectric material recommendation
Tell us the job, the working voltage, the creepage need, the thermal class, and any arc or flame duty, and we will recommend a material family and grade direction and convert samples on your footprint.
Samples typically ship in 3 to 5 business days for common die-cut configurations; standard production runs about two weeks; special orders are quoted to the job.
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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- IEC 60243-1 and ASTM D149 (dielectric strength of solid insulating materials). IEC / ASTM International.
- IEC 60112 (Comparative Tracking Index). IEC.
- IEC 60085 (thermal classification of electrical insulation). IEC.
- UL 746B (relative thermal index) and UL 94 (flammability). UL Standards.
- IEC 60664-1 (insulation coordination for equipment within low-voltage systems). IEC.
- Manufacturer technical data sheets: polyimide film, aramid paper, mica barrier sheet, GPO-3 and glass-epoxy laminates, and silicone sponge grades named in this article.
This article cites general engineering principles and published standards (IEC 60243-1 and ASTM D149 for dielectric strength of solid insulating materials, IEC 60112 for Comparative Tracking Index, IEC 60085 for thermal classification, UL 94 for flammability, UL 746B for relative thermal index, IEC 60664-1 for insulation coordination, and the apparatus standards IEC 62271, IEC 60076, and IEC 60034).
Material names refer to material families H-O Products converts. Dielectric strength is thickness-dependent and is confirmed per the test method on the manufacturer's current technical data sheet; no kilovolt-per-millimeter withstand value is stated here for any grade. All parts are custom manufactured to order and minimum production run quantities apply. H-O is a die-cut converter, not a raw-material producer, and does not certify the finished apparatus.