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IEC 61067 Glass Fibre Woven Tapes: The Engineering Backbone of High-Temperature Winding Insulation
1. When Copper Screams at 200 degrees C — How a Roll of Glass Tape Holds Everything Together
Open up any Class F or Class H inverter-duty motor, and the first thing you will see on the winding overhangs is layer upon layer of neatly wound white or pale-yellow woven tape. It looks unremarkable — almost like medical gauze — yet it keeps hundreds of enameled copper conductors locked together as a single mechanical unit while enduring electromagnetic vibration, thermal expansion cycling, and the chemical aggression of impregnating resin, all at continuous operating temperatures of 155 degrees C or even 200 degrees C. IEC 61067 is the international standard that defines every aspect of these glass fibre and glass-polyester fibre woven tapes — from raw yarn selection and weave construction through coating treatment to final performance verification.
Published by IEC TC 15 (Solid Electrical Insulation), IEC 61067 consists of three parts: Part 1 (1991) covers definitions, general requirements, and conditions of supply; Part 2 (1992) specifies all test methods — thickness, width, tensile strength, dielectric strength, resin compatibility, and more; and Part 3-1 (1995) provides the specification sheets for Type 1 tapes. Together, they govern the three principal application domains: motor winding banding, transformer coil wrapping, and cable insulation serving.
From a materials engineering perspective, a woven glass insulating tape is a deceptively sophisticated composite product. It starts with E-glass (low-alkali borosilicate glass) fibres of 3 to 10 micrometre diameter, woven in a warp-and-weft pattern, then heat-cleaned to remove sizing, and finally impregnated or coated with an organic finish — silicone, acrylic, or epoxy-compatible — to deliver its final set of properties. A seemingly trivial change in weave density — one extra weft pick per centimetre — can shift the tape’s drapability, resin penetration rate, and the partial discharge behaviour of the finished insulation system.
Engineering insight: The real brilliance of IEC 61067 lies in the fact that it does not evaluate the tape in isolation — it evaluates the “tape plus impregnating resin” composite system. The standard explicitly requires compatibility testing: after impregnation with the designated resin or varnish, the tape must retain adequate dielectric strength, must not crack when bent, and must not become brittle or powdery after thermal ageing. In a real motor or transformer, the woven tape never works alone — it always functions as part of a resin-impregnated composite insulation system. Ignoring compatibility testing is the single biggest root cause of VPI process batch failures.
2. Tape Selection Decoded: 0.1 mm of Thickness Can Make or Break Your Insulation Design
IEC 61067 covers a broad range of woven tapes, but they can be systematically classified across four key dimensions:
2.1 Classification by fibre composition
The standard naming convention follows the format: “fibre composition + weave type + treatment + width x thickness”. Three core fibre compositions dominate the market:
- Pure glass fibre tape: 100% E-glass fibres in both warp and weft. With a continuous temperature rating up to 200 degrees C (Class H, depending on coating), this is the default choice for high-temperature motors and traction transformers. It delivers the highest tensile strength and dimensional stability, but drapability can be an issue on tight-radius coil contours.
- Glass-polyester hybrid tape: Warp threads are glass for strength; weft threads incorporate polyester (PET) fibres for added flexibility and conformability. The temperature ceiling drops to 130~155 degrees C (Class B~F, dictated by the polyester component), but the wrapping behaviour is dramatically better — especially on irregularly-shaped coils with small bend radii.
- Silicone-coated tape: A pure glass fibre tape that has been impregnated or coated with a silicone compound. The silicone adds moisture resistance, release properties (useful for coil removal from formers), and improved arc resistance. Widely used as inter-layer insulation in dry-type transformer LV windings and as an outer moisture barrier wrap.
2.2 Classification by edge treatment
IEC 61067 defines three edge-treatment states — a detail frequently overlooked during procurement but absolutely critical in production:
- Cut edge (C-edge): Mechanically slit after weaving. Exposed glass fibre ends along both edges generate significant lint during wrapping, which can contaminate the impregnating varnish bath. Lowest cost; acceptable for temporary pre-banding or non-critical low-voltage applications.
- Sealed edge (S-edge): Edges are heat-set or resin-fused so that the cut fibre ends are bonded together. Lint generation drops by 70~85%. This is the recommended minimum for any VPI-impregnated winding.
- Frayless edge (F-edge): A true woven selvedge is formed directly on the loom — no cut fibre ends exist at all. Near-zero lint. Mandatory for premium motors, oil-filled transformers, and any application where loose fibres could migrate into coolant channels and cause partial discharge or pump blockage.
Lesson from the factory floor: A motor manufacturer once used cut-edge glass tape to band Class H inverter-duty stator windings. After VPI impregnation, the bottom of the epoxy varnish tank had accumulated a thick sludge of glass fibre fragments, clogging the recirculation lines. Worse, some loose fibres had migrated with the resin into the ventilation ducts inside the winding — and during the 15 kV partial discharge test, those ducts lit up like a Christmas tree. Switching to sealed-edge tape solved both problems instantly. An extra 0.04 USD per metre of tape saved thousands in rework and warranty claims.
2.3 Key performance parameters at a glance
| Property | Pure Glass (uncoated, Type 1) |
Pure Glass (silicone coated) |
Glass-Polyester Hybrid |
Test method (IEC 61067-2) |
|---|---|---|---|---|
| Thickness range (mm) | 0.08 ~ 0.30 | 0.10 ~ 0.35 | 0.10 ~ 0.30 | Clause 5 |
| Width range (mm) | 10 ~ 50 | 10 ~ 50 | 10 ~ 50 | Clause 4 |
| Min. tensile strength (N/10 mm width) | 150 | 130 | 100 | Clause 7 |
| Dielectric breakdown (kV, ambient, dry) | 0.8 (0.13 mm) | 1.5 (0.15 mm) | 0.6 (0.13 mm) | Clause 12 (short-time) |
| Max. temperature class | 200 C (Class H) | 200 C (Class H) | 155 C (Class F) | IEC 60216 (thermal ageing) |
| Resin compatibility | Excellent (epoxy, polyester, PEI) |
Moderate (silicone repels epoxy) |
Excellent (PET-weft affinity for polyester) |
Clause 11 |
| Linting (mg/m, typical) | C-edge: 300~500 S-edge: 50~100 |
S-edge: 30~80 | S-edge: 40~90 | Manufacturer QC |
| Weave count (ends x picks/cm) | 18×12 ~ 24×16 | 18×12 ~ 22×14 | 18×10 ~ 22×14 | Clause 6 |
| Nominal mass per unit area (g/m2) | 60 ~ 220 | 70 ~ 250 | 55 ~ 200 | Clause 8 |
| Typical application | Class H motor winding banding Traction transformer coils |
Dry-type transformer interlayer Outdoor motor moisture barrier |
Class F inverter-duty motors Irregular-shape coils |
— |
Selection golden rule: Never order glass tape by thickness and width alone. You must simultaneously specify edge treatment (C/S/F), weave density (affects permeability), and coating chemistry (affects VPI resin compatibility). A 40,000 USD Class H traction motor was once scrapped in final test because silicone-coated tape had been used with an epoxy VPI resin — the interlaminar shear strength after cure was 70% below specification. The windings had to be stripped and entirely re-wound. The root cause was a single unchecked line on a purchase order.
3. The VPI Crucible: Why Some Tapes Fail Catastrophically When Impregnated
VPI (Vacuum Pressure Impregnation) is the heart and soul of modern winding insulation processing — and paradoxically, it is also where woven tape selection mistakes reveal themselves most spectacularly. Three failure mechanisms deserve every design engineer’s attention:
3.1 Mechanism 1: Silicone-epoxy chemical incompatibility
Silicone is a double-edged sword. On one hand, it gives the tape outstanding moisture resistance, release properties, and corona endurance. On the other hand, its extremely low surface energy makes it nearly impossible for epoxy resins — particularly bisphenol-A type — to achieve effective wetting. During VPI processing, the epoxy resin adjacent to a silicone-coated tape cures with a nanoscopic interfacial boundary layer. The peel strength of this interface can be as low as 20~30% of normal. Under thermal cycling in service, this weak interface delaminates first, creating voids that seed partial discharge activity.
The iron rule of VPI compatibility: If your VPI resin is an epoxy system, use uncoated or acrylic-coated pure glass tape. Never use silicone-coated tape. If your VPI resin is a polyester or silicone system, silicone-coated tape is the ideal partner. In one sentence: coating chemistry and resin chemistry must belong to the same chemical family.
3.2 Mechanism 2: Filter-cake effect from excessively tight weave
During the pressure phase of VPI (typically 0.4~0.6 MPa, or 4~6 bar), the impregnating resin must penetrate through the tape layers and fill every void within the winding. If the tape weave is overly tight — for example, a high-count satin weave with more than 26 ends per centimetre — the resin permeation rate drops dramatically. A phenomenon analogous to filter-cake filtration occurs: the low-molecular-weight solvent components of the resin pass through the tape, while the curing agents and fillers are retained on the tape surface. The consequence is that the enameled wire surface on the inner side of the tape is never truly wetted by resin. After cure, dry voids remain, and partial discharge testing reveals the problem immediately.
3.3 Mechanism 3: Thermal shock and “tape blow-out”
During wrapping, the glass tape is applied under tension — typically 10 to 30 N. If the wound stator or coil is not given an adequate thermal relaxation cycle (typically 2~4 hours at 120~150 degrees C) before entering the VPI tank, residual tensile stress in the glass fibres can be suddenly released by the solvent action of the impregnating varnish. This phenomenon — colloquially called “tape blow-out” — manifests as local lifting and delamination of the tape on the winding surface, with varnish bubbles trapped between tape layers after cure. IEC 61067 Part 1 requires the manufacturer to declare the tape’s thermal shrinkage value. For a quality tape, thermal shrinkage should be below 1.5% when tested at 80% of the rated temperature class. This single number is your best predictor of blow-out risk.
3.4 Engineering decision matrix for VPI tape selection
| VPI resin system | Temp. class | Recommended tape type | Edge type | Weave density (endsxpicks/cm) |
Rationale |
|---|---|---|---|---|---|
| Epoxy-anhydride (EP) | F / H | Pure glass, uncoated or acrylic-coated | S-edge or F-edge | 18×12 ~ 20×14 | Epoxy forms covalent bonds with silanol groups on glass surface — highest interfacial strength |
| Unsaturated polyester-styrene (UP) | B / F | Glass-polyester hybrid, uncoated | S-edge | 16×10 ~ 18×12 | Polyester resin is chemically homologous with PET weft — cures into a unified composite |
| Silicone (SI) | H / 200 C+ | Pure glass, silicone-coated | S-edge | 18×12 ~ 22×14 | Silicone coating fully compatible with silicone resin matrix |
| Polyester-imide (PEI) | H | Pure glass, uncoated | F-edge | 18×12 ~ 20×14 | Imide groups hydrogen-bond to glass surface hydroxyls |
| Water-borne epoxy emulsion | F | Pure glass, uncoated | S-edge | 16×12 ~ 18×12 | Looser weave facilitates water vapour escape during pre-cure bake-out |
Field-tested tip: Some seasoned process engineers habitually touch the glass tape spool with a grounded discharge wand before starting the wrapping operation. This looks like superstition but has solid engineering justification: glass fibre tape, when unwound under dry conditions, generates substantial triboelectric charge that attracts airborne dust and fibre fragments. These microscopic contaminants become entrained in the resin during VPI and can seed partial discharge. Running a grounded static-discharge wire in light contact with the tape path costs nothing and can measurably improve PD test results.
4. Frequently Asked Questions
- Pure glass tape or glass-polyester hybrid — which one should I choose?
- If your number one concern is temperature class (Class H, 200 degrees C required), go with pure glass fibre tape. If your number one concern is wrapping workability and coil conformability (irregular cross-sections, tight bend radii under 3 mm), go with the glass-polyester hybrid. The polyester weft yarns provide elastic recovery that pure glass simply does not have — the tape will lie flat around a 3 mm radius corner without lifting. But the polyester component puts a hard ceiling on operating temperature at 155 degrees C (Class F). Above 180 degrees C, PET softens and degrades. For Class H motors, the hybrid tape is disqualified on day one.
- How much more expensive is sealed-edge tape, and is it worth it?
- Sealed-edge tape typically carries a 15~30% price premium over the equivalent cut-edge specification. If your application involves VPI impregnation, oil-filled transformers, or clean-room motor production, sealed-edge is absolutely worth it — the avoided costs from lint-related quality defects and rework are usually 10x the price difference. For low-voltage random-wound stators where the tape is only used for temporary pre-banding (the final mechanical integrity comes from the cured varnish), cut-edge tape is perfectly adequate.
- Dielectric strength of 0.8 kV sounds alarmingly low. Can this really serve as insulation?
- The dielectric breakdown value of woven glass tape looks low (by comparison, Nomex paper used in transformers can reach 15 kV/mm) because the IEC 61067-2 test method applies two 50 mm diameter electrodes directly on a single dry, unimpregnated tape layer and ramps voltage to failure. But in a real motor or transformer, the woven tape is never the primary insulation — it acts as mechanical banding + supplementary insulation + resin carrier matrix. The primary groundwall insulation is provided by the slot liner and the cured impregnating resin. The 0.8 kV figure is the “dry, unimpregnated” value. After VPI impregnation and cure, the composite insulation system easily achieves 5~10 kV or higher, with the final value dominated by the resin, not the tape.
- How many layers of tape should I apply? Is there a design formula?
- There is no universal formula, but a sound engineering approach exists: under short-circuit fault current, the electromagnetic force on the winding overhang is proportional to the square of the current. Calculate the peak short-circuit current I_peak, determine the radial force per unit length F_r on the end-winding, then specify the required hoop restraint tension T such that T is at least 1.5 x F_r (safety factor of 1.5). From the tape’s tensile strength per unit width, back-calculate the total number of wraps and layers needed. A typical medium-voltage motor (6 kV, 500 kW) requires 2~3 layers of glass tape applied with 50% overlap. A critical caveat: more layers is not always better — excessive tape buildup impedes heat dissipation and can raise winding hot-spot temperature by 5~8 K, offsetting any mechanical benefit.
