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Creepage and Clearance in EV Pack Busbars: Pollution Degree, CTI, and the IEC 60664 Framework
Three different distances keep two pack conductors apart, and confusing them is the root of most busbar-insulation surprises. Dielectric strength is the voltage a material withstands across its thickness before breakdown: a material property, characterized per IEC 60243 and ASTM D149, grade- and thickness-specific. Clearance is the shortest distance through air between the conductors. Creepage is the shortest distance along an insulator's surface between them.
The first belongs to the material; the other two belong to your geometry under the IEC 60664 insulation-coordination framework, set by working voltage, over-voltage category, pollution degree, and material group. This article walks the framework as it applies inside an EV pack, where a die-cut barrier's folds and profiles are often the cheapest way to buy surface distance.
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Three distances, three owners
| Quantity | What it is | Who owns it | Governing reference |
|---|---|---|---|
| Dielectric strength | Voltage withstand across the material's thickness before breakdown | The material grade, at a stated gauge and method | IEC 60243 / ASTM D149, on the data sheet |
| Clearance | Shortest distance through air between conductors | Your geometry; driven mainly by over-voltage category | IEC 60664 tables |
| Creepage | Shortest distance along the insulator's surface between conductors | Your geometry plus the surface's material group (CTI) | IEC 60664 tables |
A good barrier uses through-thickness dielectric strength to resist breakdown and surface geometry to hold the required spacing, and the two jobs are sized independently. Meeting one does not imply meeting the other, which is why the parent page's first dielectric decision is "through-thickness, surface, or both."
Pollution degree: why a sealed pack earns smaller spacings
Contamination on a surface makes it easier for current to track across it, so IEC 60664 requires more creepage as the environment gets dirtier. The framework expresses this as pollution degrees describing how much conductive contamination a surface is exposed to, and the required creepage rises with the degree at the same working voltage. Clearance through air is affected less by pollution and more by the over-voltage category.
The EV-specific insight: a sealed, clean pack interior may sit at a lower pollution degree than an exposed or condensing location, which changes the spacing the design must hold. That is one of the quiet payoffs of good enclosure sealing, and it is also why the pollution degree of the location belongs on the dielectric drawing rather than being assumed pack-wide.
CTI: the material's vote in a geometry question
The comparative tracking index measures how well a material's surface resists forming a conductive track under voltage and contamination. IEC 60664 uses it to place the material in a material group, and the group feeds the creepage tables: a higher CTI can allow a smaller creepage distance for the same working voltage and pollution degree, while a lower CTI requires more.
So when a barrier's surface carries the creepage path, the grade's CTI is part of the spacing calculation, not a footnote. The CTI is a grade property confirmed on the manufacturer's data sheet; H-O states no CTI value, and the governing spacing always comes from the IEC 60664 tables applied to your working voltage, pollution degree, and material group.
Clearance Through Air, Creepage Along the Surface
Qualitative illustration of the IEC 60664 distinction; no dimension is represented. A die-cut barrier's folds, ribs, and profiles add surface path; the required distances come from the standard's tables for your working voltage, over-voltage category, pollution degree, and material group.
How a converted barrier buys spacing
Inside a pack, conductors rarely move; the insulation does. That makes the converted barrier the practical lever on both distances:
- Folded wraps extend creepage. A busbar wrap is a developed flat pattern, die-cut with score lines and cut-outs, that folds into a sleeve and extends the surface path between the bar and its neighbors. The fold geometry is part of the spacing design, not packaging.
- Underlays and interleaves split paths. A polyimide film underlay or an aramid paper interleaf interrupts the direct surface route between adjacent conductors and the chassis.
- Profiles and ribs on plates. A machined glass-epoxy laminate barrier can carry a creep-extending profile while also holding position and load.
- Through-thickness still matters. The barrier's own withstand across its gauge (IEC 60243 / ASTM D149) covers the breakdown path the geometry cannot, and both jobs are confirmed independently.
Working the framework on a real busbar run
- Fix the electrical context. Working voltage and over-voltage category, from the pack architecture.
- Fix the environmental context. The pollution degree of the location, which may differ between the sealed interior and exposed terminals.
- Pick the surface material with its group in mind. The grade's CTI sets the material group, which sets the creepage multiplier the tables apply.
- Read the required distances from the IEC 60664 tables. They are the governing values; no shortcut replaces them.
- Design the barrier geometry to deliver them. Folds, ribs, underlays, and interleaves until both distances are met, then confirm the through-thickness withstand at the part's gauge on the data sheet.
The vehicle-level electrical-safety context (FMVSS 305) and any flame classification (UL 94, grade- and gauge-specific) ride alongside this sequence; both belong to their own owners, the vehicle and the grade listing respectively.
Failure modes the framework prevents
| Failure | Confusion behind it | The framework's answer |
|---|---|---|
| Tracking across a "strong" barrier | High dielectric strength mistaken for surface immunity | Creepage is a geometry-and-CTI question, sized from the tables |
| Spacing fine on the bench, marginal in service | Pollution degree assumed clean pack-wide | Assign the degree per location; condensing zones differ from sealed ones |
| Wrap meets breakdown, fails spacing | Through-thickness and surface jobs conflated | Verify each independently; the fold pattern carries the surface job |
| Material swap quietly shrinks margins | New grade, lower CTI, same drawing | The material group is part of the creepage calculation; re-read the tables on any swap |
How H-O Products helps
H-O Products is a precision die-cut converter. We do not run your insulation-coordination calculation, and we state no creepage, clearance, CTI, or withstand number; those belong to your design and the grade data sheets.
What we convert is the geometry that delivers your numbers: developed flat patterns that fold into busbar sleeves, die-cut and scored films and papers, machined laminate barriers with creep-extending profiles, laminated constructions where one material cannot carry both jobs, and adhesive presentation for the line, under our ISO 9001:2015 certified quality management system.
Send the busbar geometry and the spacing requirement, and engineering develops the flat pattern and confirms the material against the data sheet and the IEC 60664 framework.
Frequently asked questions
Why does a dirtier environment need more creepage but not much more clearance?
Because the two distances fail differently. Creepage failure is tracking: contamination on the surface carbonizes into a conductive path, so the required surface distance grows with the pollution degree. Clearance failure is breakdown through air, which is driven mainly by the over-voltage category and the transient peaks it implies. IEC 60664 therefore scales creepage with pollution degree and clearance with over-voltage category.
Does a higher-CTI material always mean a smaller barrier?
It can allow smaller creepage for the same working voltage and pollution degree, because a higher CTI places the material in a more favorable material group in the IEC 60664 tables. Whether the part actually shrinks depends on which distance was binding; if clearance or the through-thickness job governs, the CTI improvement changes nothing. The honest sequence is to read the tables both ways before promising package space back.
Can the fold pattern of a wrap really count toward creepage?
Yes; creepage follows the insulator's surface, so a fold that forces the path up, over, and down adds real distance, which is exactly how a developed flat pattern buys spacing without moving conductors. The fold geometry then belongs to the controlled drawing, because flattening or trimming the fold in a later revision silently removes the distance it provided.
Is the sealed pack interior automatically pollution degree 1?
No assignment is automatic; the degree describes how much conductive contamination the surface is exposed to, and it is assigned per location by the designer. A sealed, clean interior may justify a lower degree than an exposed or condensing location, and that difference changes the required creepage. Assign it explicitly, record it on the drawing, and the spacing follows from the tables rather than from habit.
Have a busbar spacing problem worked
Send the conductor geometry, the working voltage, the pollution degree, and the spacing requirement, and engineering will develop the barrier pattern that delivers it.
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.
Family-owned in Winsted, Connecticut since 1971 · ISO 9001:2015 certified · OEMs worldwide send us a drawing — and get back their part.
Related reading
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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- IEC 60664 (insulation coordination for equipment within low-voltage systems: working voltage, over-voltage category, pollution degree, material group).
- IEC 60243 and ASTM D149 (dielectric strength of solid insulating materials; thickness- and method-dependent).
- UL 94 (flammability classifications); FMVSS 305 (vehicle-level electrical safety context).
- Manufacturer technical data sheets: polyimide film, aramid paper, PEEK film, glass-epoxy laminate, mica barrier sheet (CTI and dielectric data per grade).
This article explains the IEC 60664 insulation-coordination concepts as they apply to EV pack busbar barriers, summarized from the standard's public structure and the H-O application page; the governing text is the standard itself, and the governing distances are its tables applied to your design. No creepage, clearance, CTI, or dielectric value is stated for any grade or design. The figure is qualitative and carries no dimensions.
All parts are custom manufactured to order; minimum production run quantities vary by material and part. H-O is a die-cut converter, not a raw-material producer, and does not certify the finished pack or vehicle.