Custom Busbar, Transformer & Power Distribution Insulation

Class R (220 °C) is the current designation for the top thermal class in the IEC 60085 / IEEE C57 letter system. The letters set the continuous hotspot ceiling: A 105, E 120, B 130, F 155, H 180, N 200, R 220 °C. The class is the maximum hotspot temperature the insulation is designed to withstand for its rated life; aramid paper is the 220 °C backbone. Match the class letter to the winding hotspot, because insulation life halves roughly every 10 °C above class. [1]

Clearance is the shortest straight-line distance through air between two conductive parts; creepage is the shortest path along the surface of the insulator between the same two parts. Under pollution the surface path is what flashes over, so creepage usually governs.

Both distances come from the equipment standard's tables (IEC 60664-1 for LV, IEC 62271 for MV), driven by the rated impulse voltage, the pollution degree (industrial is typically PD3), and the material group (CTI per IEC 60112). A rib on a barrier lengthens the surface path without widening the part. [2]

By mechanics. Kapton® polyimide film wraps the bar where the barrier bends and follows the conductor; G10/FR4 or Durostone® barrier sheet stands between phases where the design needs a rigid wall; fish paper handles the economical LV positions. The required creepage and clearance come from the equipment standard for the voltage and pollution degree, and a rib lengthens the surface path. Die-cut edges hold the datasheet dielectric margin that field-trimmed edges lose. [5]

Thermal class. Aramid paper (Nomex® 410 / 411) is the 220 °C (Class R) backbone and the standard for dry-type distribution transformers, where the winding runs hotter than cellulose can survive long-term. Thermally-upgraded kraft and pressboard serve the liquid-immersed side under IEEE C57.154 up to a 180 °C class, where the oil carries heat and the economics favor cellulose. The choice follows the class letter and the cooling method, not habit. [1]

Electrical-grade fish paper (vulcanized fibre) has a dielectric strength of roughly 6–10 kV/mm (about 150–250 V/mil depending on gauge), which suits low- to medium-voltage positions. It is made from cellulose treated with zinc chloride and is used for slot liners, insulating washers, end laminations, and small barriers in distribution apparatus, because it can be cleanly into complex geometries without fraying or delaminating at the edges. The two most common grades are NEMA F and NEMA FR. [11]

Whenever the insulation also holds the conductor. A standoff, spacer, or structural phase barrier must survive gravity, torque, and short-circuit forces, and thin film cannot do that. Use G10/FR4 glass-epoxy (NEMA LI 1 grades) for most supports, and Durostone® composite where higher tracking resistance and strength are needed (EPC 203: 16 kV/mm per TDS). Size the part to the fault current and bolt pattern, and keep the material and geometry inside the assembly's verification. [6]

CTI (per IEC 60112) measures how well a surface resists tracking, and it sorts materials into groups that set how much creepage the standard demands. A high-CTI surface needs less creepage than a low-CTI one for the same voltage and pollution degree, so a high-CTI material like Durostone® composite keeps the required distance manageable in a compact MV cubicle. Higher rated impulse voltage, heavier pollution, and lower CTI all push the creepage requirement up. [3]

When temperature or arc exposure would degrade an organic material. Mica barrier sheet holds roughly 700–800 °C continuous, does not track or carbonize, and is the classic choice for arc-adjacent positions and the hottest transformer turns. ManniGlas® glass-fiber paper is the inorganic barrier and backing paper for high-temperature positions. Both are to the footprint like any other layer; the selection is a temperature-and-arc decision that overrides the dielectric numbers. [10]

No single layer is, and no honest supplier will claim otherwise. UL 1446 qualifies an insulation SYSTEM, the tested combination of paper, film, varnish, and winding, and the thermal class belongs to that qualified system. What each converted layer carries is its own documentation: its thermal class, its dielectric strength, its test methods, and lot-code traceability. The layer supports a qualified system; it does not carry the qualification alone. A substitution, even like-for-like, is a system decision. [12]

It depends on voltage and region. Low-voltage assemblies follow IEC 61439 (with the creepage/clearance principles from IEC 60664-1); medium- and high-voltage switchgear follows the IEC 62271 family; North American metal-clad MV gear follows the ANSI/IEEE C37 series, including IEEE C37.20.2. The barriers and supports on this page are built into those listed assemblies, so their material and geometry are constrained by the assembly's verification, not chosen in isolation. Verify the governing edition before final spec. [13]

H-O and converts sheet, roll, and laminate stock to drawing, and kiss-cut barriers, slit films and papers, machined and laminate supports, and kitted coil-insulation sets. We do not mold or extrude these parts; molded and extruded profiles are coordinated through a partner network. Conversion runs in Winsted, Connecticut under an ISO 9001:2015 certified quality management system with material traceability and lot-code TDS records.

The three axes plus the mechanics: the voltage class and rated impulse voltage (Uimp), the pollution degree, the winding hotspot or target thermal class, the dielectric-strength target or governing creepage/clearance, and, for supports, the fault current and bolt pattern.

Add the barrier or wrap geometry and gauge, any adhesive or liner needs, quantities for prototype and production, and the standards language you need on the paperwork (material classes and dielectric strength per TDS; insulation system by designation). "Recommend the layer" is a valid callout: that is what the engineering review is for.