Precision Insulation Components — Maharashtra, India
Flat Strip / Flat Bar
Flat strip and flat bar insulation components — also specified as insulating spacer bars, phase barrier strips, inter-winding insulation bars, or terminal board substrates — are precision-machined rectangular insulation sections cut and finished to exact width and thickness from high-grade electrical laminate or pressboard sheet stock. Every cross-section dimension holds tight tolerance to maintain consistent dielectric clearances across the entire transformer assembly.
ACC Insulations manufactures flat strip and flat bar components from epoxy glass laminate, pre-compressed pressboard, and phenolic glass grades — all machined in-house on CNC equipment to OEM drawings. Serving transformer manufacturers, switchgear OEMs, and electrical equipment producers across Maharashtra and pan-India.
Technical Specifications
| Flat Strip / Flat Bar — Full Technical Data | |
|---|---|
| Product Name | Flat Strip / Flat Bar — Electrical Insulation Components |
| Also Known As | Insulating spacer bar · Phase barrier strip · Inter-winding insulation bar · Terminal board substrate · Flat insulation section |
| Base Materials | Epoxy Glass Laminate (EPGC / G10 / G11) | Pre-Compressed Pressboard | Phenolic Glass Laminate |
| Width | Custom — as per OEM drawing; standard range 5 mm to 200 mm+ |
| Thickness | Custom — as per OEM drawing; standard range 2 mm to 50 mm+ |
| Length | Custom cut lengths; standard lengths on request |
| Machining Tolerance | ±0.1 mm standard | ±0.05 mm precision grade on request |
| Thermal Class | Class B (130°C) / Class F (155°C) / Class H (180°C) — depends on base material grade |
| Dielectric Strength | High — meets IEC 60893 / IS 2036 requirements |
| Oil Compatibility | Compatible with mineral transformer oil and ester-based dielectric fluids (EPGC and pressboard grades) |
| Surface Finish | Machine-cut smooth faces; chamfered edges on request |
| Colour | Yellow-green (Epoxy Glass) | Buff / Natural (Pressboard) | Brown (Phenolic Glass) |
| Primary Applications | Transformer spacer bars, phase barriers, terminal board substrates, inter-winding insulation layers, coil support strips |
| Standards | IEC 60893, IS 2036, IEC 60641, IS 1576, IEC 60085, customer-specific requirements |
What Are Flat Strip & Flat Bar Insulation Components?
Flat strip and flat bar components are precision-machined rectangular insulation sections — constant cross-section in width and thickness, cut to any required length. Manufactured from high-performance electrical laminate or pressboard sheet, each component delivers uniform dielectric clearance at every point along its length. Unlike moulded or extruded forms, CNC-machined flat strips hold dimensional consistency that moulding processes cannot achieve at the tolerances demanded by power transformer OEMs.
Dimensionally, "flat strip" conventionally describes narrower sections — typically below 25 mm width — used as disc winding spacer bars, conductor support strips, and inter-layer insulation elements. "Flat bar" describes wider, thicker sections used as structural phase barriers between HV and LV windings, terminal board backing plates, and heavy-duty coil support members. ACC Insulations manufactures both across the full dimensional range to OEM specification.
Role in Transformer Assembly — Cross-Section View
Flat strips and flat bars occupy specific, load-bearing positions within a transformer winding stack. Each application demands a different balance of dielectric strength, compressive rigidity, and oil compatibility:
Transformer Winding Cross-Section — Flat Strip Positions
Each spacer bar thickness determines oil duct width · Phase barrier width determines HV–LV clearance
| Component Role | Position in Assembly | Critical Dimension |
|---|---|---|
| Spacer Bar | Between disc winding layers — creates oil cooling duct between adjacent conductor discs | Thickness ± tolerance — controls oil duct width and therefore cooling capacity |
| Phase Barrier | Between HV and LV windings — separates phases at full inter-winding dielectric clearance | Width and thickness — must span the full winding height; must withstand inter-winding voltage |
| Terminal Board Substrate | Base plate for HV / LV lead termination points on transformer tank lid or winding frame | Thickness — must withstand mechanical load of bushing pull and terminal tightening torque |
| Inter-Winding Insulation Layer | Between HV and LV winding outer surface and the adjacent winding cylinder | Width — must overlap winding end-turn insulation; thickness sets inter-winding clearance |
| Coil Support Strip | Axial support members in the lower yoke clamp assembly holding winding position under short-circuit forces | Cross-section area — governs compressive load capacity under short-circuit electromagnetic forces |
Material Grades Available
Material selection for flat strip and flat bar components follows operating conditions — voltage level, thermal class, oil immersion, and mechanical load requirements:
Epoxy Glass Laminate — Grade G10 / G11
Woven glass fiber reinforced epoxy resin sheet. Highest dielectric strength, lowest moisture absorption, excellent mechanical rigidity. Standard specification for HV phase barriers, terminal board substrates, and precision spacer bars in power transformers.
Class F / H · IEC 60893Pre-Compressed Pressboard
Dense calendered cellulose pressboard manufactured to IEC 60641. Fully oil-compatible — swells negligibly in transformer oil at operating temperature. Preferred for disc winding spacer bars and axial insulation components in oil-immersed power and distribution transformers.
Class A / B · IEC 60641Phenolic Glass Laminate — GPO-3
Glass fiber reinforced polyester resin. High arc resistance and tracking resistance. Used where arc-over risk exists — switchgear arc barriers, phase separators in LV control panels, and open-wound dry-type transformer phase barriers.
Class B / F · IEC 60893FR4 Epoxy Glass — Flame Retardant
Flame-retardant epoxy glass laminate meeting UL 94 V-0 classification. Specified for dry-type transformer insulation components, switchgear barriers, and applications where fire safety certification adds assurance to the electrical insulation duty.
Class F / H · UL 94 V-0Dimensional Precision
Dimensional consistency across every piece in a production batch directly determines winding geometry quality. ACC Insulations machines flat strips and flat bars to two precision grades:
Standard Grade
Applied to distribution transformer spacer bars, general-purpose phase barrier strips, and terminal board substrates where standard transformer winding geometry tolerances apply. Covers the requirements of most power and distribution transformer OEM specifications.
Precision Grade
Specified for large power transformer spacer bars, inter-winding clearance-critical phase barriers, and components where oil duct width tolerance directly affects cooling performance calculations. Available on request with dimensional inspection report per piece batch.
Non-standard tolerance grades, special surface finishes (ground, polished, etched), and secondary operations (drilling, chamfering, notching) available on request for specific application requirements.
Key Performance Properties
Manufacturing Process
Every flat strip and flat bar component passes through a controlled production sequence designed to deliver dimensional repeatability, clean machined faces, and consistent material properties across every piece in the batch:
Sheet Stock Incoming Inspection
Epoxy glass laminate and pressboard sheet stock undergoes incoming material verification — thickness measurement at multiple points, visual inspection for delamination or voids, and dielectric strength spot-check. Non-conforming material rejected before release to production. All stock carries mill certificate traceability.
CNC Ripping / Slitting to Width
Sheet stock fed through precision CNC slitting or ripping equipment calibrated to the specified strip width. Blade geometry and feed rate optimised per material grade — epoxy glass requires slower feed and finer blade than pressboard to prevent edge delamination or micro-cracking. Width verified on first-off piece before batch production release.
Thickness Machining / Grinding
Where sheet stock thickness does not match the required finished thickness, strips pass through a surface grinding or milling operation. Both faces machined parallel to produce a uniform cross-section. Thickness verified with contact micrometer at multiple points along each strip length before proceeding.
Cut-to-Length
Strips cut to specified length on precision saw with squaring fence. End faces deburred and smoothed. Where OEM drawings specify chamfered or radiused ends — common on spacer bars to prevent conductor insulation damage during winding — end profiling performed on CNC milling center.
Secondary Operations
Customer-specified secondary operations performed as required: drilling of mounting holes, milling of slots and keyways, countersinking, notching, and surface coating. All secondary operations performed to OEM drawing dimensions with inspection at each stage for critical features.
Final Inspection & Dispatch
Complete dimensional inspection per piece or per batch sample plan as agreed with the OEM. Width, thickness, length, squareness, and any critical secondary features verified and recorded. Components packed in moisture-protected packaging. Dimensional inspection report and material certificate despatched with order.
Applications of Flat Strip & Flat Bar Insulation
Why Dimensional Precision Matters in Spacer Bars & Phase Barriers
Flat strip and flat bar components occupy positions where dimensional error directly propagates into transformer performance, safety margin, and service life:
- Oil duct width control: Disc winding spacer bar thickness directly sets the width of the oil cooling duct between adjacent winding discs. A 0.5 mm oversize spacer — seemingly trivial — reduces oil flow velocity in that duct by a calculable amount, degrading hot-spot cooling and reducing insulation thermal life. Precision-machined spacer bars eliminate this variable from the thermal design.
- Dielectric clearance assurance: Phase barrier width and thickness translate directly to the inter-winding dielectric clearance distance. Under-thickness barriers reduce this clearance below the designed withstand level — creating a risk of inter-winding flashover at rated voltage or during impulse testing. Over-thickness barriers compress adjacent insulation and create mechanical stress in the winding stack.
- Winding geometry uniformity: Non-uniform spacer bar thickness introduces tilt and eccentricity into the winding disc geometry over multiple spacer positions. Accumulated geometric error shifts the winding mass off-centre, concentrating short-circuit forces unequally around the winding circumference and creating local failure initiation points.
- Short-circuit force distribution: Spacer bars and coil support strips act as compressive load-bearing members during short-circuit events. Components with inconsistent cross-section fail under localised stress concentrations at dimensional low points — fragmenting under the electromagnetic impulse load. Uniformly machined components distribute the compressive load evenly across the full contact area.
- Vacuum drying and oil impregnation: Pressboard spacer bars with consistent thickness stack uniformly between winding discs, maintaining open oil duct channels throughout the winding height. This allows moisture to escape during vacuum drying and oil to penetrate completely during impregnation — eliminating air pockets that cause partial discharge.
Epoxy Glass vs Pressboard Flat Strip — Selection Guide
Material selection between epoxy glass and pressboard flat strips depends on the specific operating environment and design priorities:
| Property | Epoxy Glass (EPGC / G10) | Pre-Compressed Pressboard |
|---|---|---|
| Dielectric Strength | ✓ Very High — >20 kV/mm | High — 12–16 kV/mm (oil-impregnated) |
| Oil Compatibility | ✓ Fully compatible — chemically stable | ✓ Fully compatible — traditional material |
| Moisture Absorption | ✓ Very Low — <0.5% | Moderate — requires vacuum drying |
| Thermal Class | ✓ Class F / H (155–180°C) | Class A / B (105–130°C) |
| Compressive Strength | ✓ Very High — suited to high SC-force designs | High under oil compression |
| Oil Duct Stability | ✓ No swelling — rigid duct geometry maintained | Marginal swelling in hot oil — design must account for this |
| CNC Machinability | ✓ Excellent — holes, slots, chamfers | Good — soft, easier to machine |
| Preferred Applications | HV phase barriers, terminal boards, traction & industrial transformers | Disc winding spacer bars in oil-immersed power & distribution transformers |
Quality Control & Testing
Every production batch of flat strip and flat bar components undergoes inspection before dispatch:
- Width and thickness measurement — contact micrometer measurement at multiple points along each strip length; all readings recorded against tolerance grade
- Length and squareness check — vernier measurement of cut length; squareness of cut face checked with precision square
- Surface and edge inspection — visual inspection for delamination, chipping, edge cracking, or machining burn marks; rejection criteria applied per OEM specification
- Dielectric spot-check — sample dielectric strength test per IEC 60893 on each production batch to confirm base laminate material integrity
- Secondary feature verification — hole positions, slot depths, chamfer angles, and countersink dimensions verified against OEM drawing on 100% or sample basis as agreed
- Material traceability — batch records link finished components back to incoming sheet stock mill certificate for full supply chain traceability
Standards compliance: IEC 60893 (laminated sheets for electrical purposes), IEC 60641 (pressboard), IEC 60085 (thermal classification), IS 2036, IS 1576, and customer-specific acceptance criteria.
Why Choose ACC Insulations for Flat Strip & Flat Bar Components?
In-House Component Manufacturer
Every flat strip and flat bar machined in our Nashik facility under direct quality control. No outsourced machining — full traceability from sheet stock to finished component.
Precision CNC Machining
Multi-axis CNC equipment delivers ±0.05 mm precision grade components per OEM drawing — eliminating dimensional variation that adversely affects winding geometry and clearance dimensions.
Full Material Grade Range
Epoxy glass (G10, G11, FR4), pre-compressed pressboard, and GPO-3 grades all stocked and machined in-house. One supplier for every flat insulation component specification in your BOM.
IEC & IS Compliant
Base materials meet IEC 60893, IEC 60641, and IS 2036. Batch test certificates and material traceability documents provided on request — supporting customer PPAP and incoming inspection requirements.
Drawing-Based Manufacturing
Components manufactured directly to OEM technical drawings. First-article inspection reports provided for new part numbers. Repeat orders manufactured to the same setup records for consistent batch-to-batch dimensions.
Technical Application Support
Our team advises on material grade selection, tolerance grade specification, and secondary operation feasibility — supporting transformer design engineers from prototype through production.
Engineering Tools Suite
Calculate transformer winding dielectric clearances, spacer bar thickness requirements, oil duct width specifications, and phase barrier sizing parameters using our interactive engineering tools.
Frequently Asked Questions
Flat strips and flat bars serve as insulating spacer bars between disc winding layers (creating oil cooling ducts), phase barriers between HV and LV windings (maintaining inter-winding dielectric clearance), terminal board substrates (supporting HV and LV lead terminations), and inter-winding insulation layers (establishing radial clearance between winding cylinders). Each function demands a precise, repeatable cross-section — consistent width and thickness — to guarantee uniform clearances and thermal performance throughout the transformer assembly.
ACC Insulations manufactures flat strips and flat bars from three primary electrical insulation material grades: Epoxy Glass Laminate (EPGC / G10 / G11) — highest dielectric strength, lowest moisture absorption, Class F/H rated; Pre-Compressed Pressboard (IEC 60641) — traditional transformer oil-compatible material for disc winding spacers; and GPO-3 Phenolic Glass — high arc resistance for switchgear arc barriers and dry-type transformer phase barriers. FR4 flame-retardant grade also available for applications requiring UL 94 V-0 classification.
Spacer bar thickness directly controls oil duct width between transformer winding discs. A reduction in oil duct width reduces cooling oil flow velocity through that duct, decreasing heat removal capacity from the conductor — directly raising hot-spot temperature beyond design limits. Non-uniform spacer bars also introduce geometric tilt in the winding stack, which shifts short-circuit electromagnetic forces away from uniform distribution and creates local stress concentrations where insulation fails first. Precision-machined spacer bars at ±0.1 mm or ±0.05 mm tolerance eliminate both failure pathways.
Yes. All flat strip and flat bar components are manufactured directly to OEM-supplied technical drawings. Secondary operations — including drilling of mounting holes, milling of slots, countersinking, chamfering, and notching — performed on CNC machining centers in-house. First-article inspection reports provided for new part numbers. Repeat orders manufactured to the same setup records for consistent batch-to-batch dimensional conformance.
Yes. Flat strips and flat bars from epoxy glass laminate (EPGC) and pre-compressed pressboard grades are fully compatible with mineral transformer oil and ester-based dielectric fluids (natural and synthetic esters). Epoxy glass exhibits negligible dimensional change in transformer oil at operating temperature. Pre-compressed pressboard marginal swelling in hot oil — factored into design clearances. Material compatibility confirmation against specific oil system available on request.
Epoxy glass (EPGC / G10) flat strips deliver higher dielectric strength (>20 kV/mm), lower moisture absorption, and higher thermal class (F/H) — preferred for HV phase barriers, terminal board substrates, and traction or industrial transformers with higher thermal and voltage demands. Pressboard flat strips deliver excellent oil compatibility and conform to the IEC 60641 tradition for disc winding spacer bars in oil-immersed distribution and power transformers — specifically, pressboard is the material for which standard transformer thermal and oil-circulation designs are calculated. Select EPGC for high-voltage, high-temperature, or low-moisture-exposure duty; pressboard for standard oil-immersed winding spacer applications.
Epoxy glass flat strips and flat bars conform to IEC 60893 (industrial rigid laminated sheets based on thermosetting resins for electrical purposes) and IS 2036 (glass fabric reinforced plastic products). Pressboard flat strips conform to IEC 60641 and IS 1576. Thermal classification follows IEC 60085. Batch test certificates and material traceability documents provided on request — supporting customer incoming inspection and quality management requirements.
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