Electrical Grade Clackband
ACC Insulations is a leading Clackband components manufacturer in Maharashtra, India, supplying precision transformer cooling duct spacers to oil-filled power and distribution transformer manufacturers across India and internationally.
Clackband — also known as a winding duct strip, transformer spacer tape or pressboard duct spacer — is a highly specialised distancing element. It consists of pre-compressed electrical-grade pressboard blocks adhesively bonded at precise pitch intervals onto a high-quality kraft paper carrier tape. Both the carrier paper and the pressboard blocks are manufactured from 100% sulphate wood pulp of electrical-grade purity to deliver superior oil absorption, zero contamination, and high dimensional stability.
When integrated directly into the transformer winding process, Clackband creates the critical axial and radial oil circulation channels that allow dielectric oil to flow through the winding, extract heat from copper conductors, and dissipate it efficiently — maintaining thermal performance of the insulation system throughout the transformer's service life.
The Science of Transformer Cooling Channels
Every watt of power dissipated as copper loss inside a transformer winding generates heat. In an oil-filled transformer, that heat must travel from the copper conductor through the surrounding paper insulation, into the liquid dielectric, and out to the tank wall. If the heat path is obstructed — if there are no adequate oil circulation channels between winding layers — hot spots develop. Even a modest 6°C rise above the insulation's rated temperature is known to halve the remaining service life of the paper insulation (the Arrhenius thermal ageing rule).
Clackband transformer duct spacers act as the structural framework of the transformer's internal cooling architecture. By holding precise, uniform gaps between each winding layer or disc group, Clackband ensures that insulating oil circulates continuously through every part of the winding — extracting heat from the hottest parts of the conductor while simultaneously serving as an additional liquid dielectric barrier. This is why winding duct strips are considered safety-critical components in ONAN, ONAF, and OFAF cooled transformer designs.
The yellow blocks represent the pressboard spacer elements of Clackband. The space between each pair of conductors is the oil circulation duct — maintained at precise width by the block thickness.
Key Technical Specifications
Available Variants — Shape 3 & Shape 4
The shape of the pressboard block determines how the Clackband sits against the winding conductor. Choosing the wrong shape for your winding geometry can cause contact pressure points on copper conductors or an uneven duct width. ACC Insulations supplies both IEC-standard shapes:
Rectangular (Flat-Edge)
Standard 90-degree flat blocks. Ideal for disc-type windings and large-radius layer windings where the conductor face is flat. Provides the most uniform duct width and maximum oil flow cross-section.
Trapezoidal (Bevelled)
Angled bevel-cut blocks that sit flush against the curved surface of cylindrical and helical winding conductors. Prevents point loading on the copper wire and eliminates insulation indentation — critical for high-amperage LV windings.
Where Is Clackband Used?
Clackband is used in any transformer design that relies on liquid dielectric cooling. It is a standard component in the winding process of:
Power Transformers
HV and LV disc winding cooling channels in 11 kV–765 kV class power transformers
Distribution Transformers
Layer winding duct spacing in 11 kV/0.433 kV distribution units
Industrial Transformers
Furnace, rectifier and traction transformers with high thermal loads
Generator Transformers
Step-up transformers at hydro, thermal and wind power generation stations
Traction Transformers
Railway traction substation and onboard locomotive transformers
Reactor Windings
Oil-immersed shunt reactors requiring precision oil duct management
How Clackband Is Installed — The Winding Process
Understanding the installation sequence helps transformer engineers and winding teams specify Clackband correctly and avoid common errors:
Design the Cooling Channel
The transformer thermal design determines the required duct width (= block thickness), pitch spacing between blocks, and total number of duct layers per winding. These values come from the transformer's thermal model or IEC 60076-2 cooling calculations.
Specify and Order Clackband
Provide ACC Insulations with: block thickness, pitch (centre-to-centre spacing), block width, roll length, and winding geometry (flat or cylindrical) to determine Shape 3 vs Shape 4. Custom rolls are manufactured to order.
Unwrap on the Winding Floor
Remove from protective film immediately before use. Do not leave exposed to ambient humidity for more than 2–4 hours to prevent moisture uptake which extends vacuum drying time.
Apply During Winding
Clackband is fed from the roll directly into the automated or manual winding machine. The carrier tape winds with the conductor; each pressboard block creates one duct spacer as the winding builds up layer by layer.
Vacuum Drying (VPD)
After winding, the coil undergoes vacuum vapour-phase drying at 130°C–140°C under deep vacuum. The adhesive retains bond integrity through this process. The pressboard blocks and carrier paper are simultaneously dried and prepared for oil impregnation.
Oil Impregnation
The dried coil is hot-oil impregnated under vacuum. The high-porosity pressboard blocks wick oil completely, eliminating air voids which are the primary cause of partial discharge in transformer insulation.
Why Choose ACC Insulations for Clackband?
With three decades of manufacturing expertise, we supply Clackband to some of India's leading transformer manufacturers. Here is what you can count on when you source from ACC Insulations:
- Precision block fabrication: Strict dimensional tolerances on block thickness (±0.1 mm) and pitch spacing ensure consistent duct widths across every roll.
- Vacuum drying-safe adhesive: Our adhesive system is specifically formulated and verified for integrity through VPD cycles — guaranteed not to delaminate or contaminate the oil.
- Fluid compatibility verification: Compatible with both mineral oil and ester dielectric fluids (FR3, Midel 7131) for next-generation transformer designs.
- Custom configurations: Any block shape, thickness, pitch, and roll width manufactured to your winding machine and transformer design specifications.
- Reliable supply for production lines: Bulk-scale production capacity with consistent quality for OEM manufacturing schedules.
- IEC-compliant raw materials: 100% electrical-grade sulphate pulp meeting IEC 60641-3-1 / IS 1576 — traceable batch certification available on request.
Engineering Tools Suite
Calculate precise cooling channel spacing, thermal flow requirements, and oil duct width for your Clackband applications using our interactive engineering calculators.
Frequently Asked Questions
Clackband is a pre-assembled spacing element — pre-compressed electrical-grade pressboard blocks bonded at fixed intervals to a kraft paper carrier tape. When wound into the transformer winding between conductor layers, each block creates a precise, uniform gap. These gaps form the oil circulation cooling channels that allow transformer oil to flow through the winding, extract heat from the copper, and prevent hot spot temperatures from degrading the insulation and shortening transformer life.
Yes. We manufacture pressboard blocks in thicknesses from 1 mm to 8 mm and above. The pitch (spacing) between each block on the carrier tape is fully adjustable to match your transformer's thermal flow calculations and winding geometry. Submit your cooling channel design drawing or thermal specification and our engineering team will produce Clackband to your exact requirements.
No. Our adhesive is a specialised, heat-resistant formulation that is fully compatible with mineral transformer oil and ester dielectric fluids. It is engineered to withstand the extreme heat and deep vacuum of the vapour-phase drying (VPD) process — typically 130°C–140°C under deep vacuum — without delaminating, softening, or leaching impurities into the oil. Adhesive integrity is verified on every production batch.
Shape 3 has standard rectangular blocks with flat 90-degree edges — best for disc-type windings and large-radius layers where the conductor face is flat. Shape 4 has trapezoidal (bevelled) blocks with angled edges that conform to the curved surface of cylindrical and helical winding conductors, distributing contact pressure evenly and preventing point loads that could deform conductor insulation in high-amperage LV windings.
Clackband is manufactured entirely from cellulose (kraft paper carrier and pressboard blocks), which is highly hygroscopic. Moisture absorbed before installation increases VPD drying time and can introduce water contamination into the transformer oil. Store in the original sealed protective film in a clean, dry, climate-controlled environment with relative humidity below 60%. Do not unwrap until immediately before use on the winding floor.
Yes. Our electrical-grade pressboard and kraft paper carrier are compatible with both mineral transformer oil and natural and synthetic ester dielectric fluids (such as Midel 7131 and FR3). The adhesive system is also verified for chemical compatibility with ester fluids, making our Clackband suitable for use in fire-resistant and environmentally safer transformer designs that specify ester-based dielectric fluids.
