The single most important thing to know before you use any CTI table: the number belongs to a grade, not a material family. Two compounds both labeled “PBT” can sit in different material groups because one has a flame-retardant package and a filler tuned for tracking and the other does not. Every value below is indicative — a starting point for narrowing a shortlist, never a substitute for the specific grade’s yellow card.
With that said, families do cluster, and the clustering follows one rule you already know if you have read the comparative tracking index pillar: materials that carbonize track badly, and materials that don’t, track well. That rule sorts the whole table.
Table of Contents
Reference table
Values are typical for electrical-grade compounds, tested to IEC 60112 with Solution A, at 3 mm. Groups follow IEC 60664-1 (I: CTI ≥ 600; II: 400–599; IIIa: 175–399; IIIb: 100–174).
| Material | Typical CTI (V) | Group | Notes |
|---|---|---|---|
| PTFE and other fluoropolymers | ~600 | I | Volatilize cleanly, no carbon |
| Polyethylene, polypropylene (unfilled) | ~600 | I | Don’t carbonize |
| Silicone rubber | ~600 | I | Hydrophobic; standard outdoor HV insulation |
| Melamine (melamine-paper, glass-melamine) | ~600 | I | Arc-chute and switchgear thermoset |
| Unsaturated polyester, mineral-filled (e.g. GPO-3) | ~600 | I | High-track thermoset laminate |
| PBT (many GF / flame-retardant grades) | 500 – ≥600 | I–II | Common for connectors, terminals, bobbins |
| Polyamide PA6 / PA66, glass-filled | up to 600 dry | I–II | Falls sharply with absorbed moisture |
| PPS, glass/mineral-filled | ~500 | II | Grade-dependent |
| Polycarbonate (unfilled) | 175 – 250 | IIIa | Structural but low — common trap |
| Epoxy glass laminate / standard FR-4 | ~175 | IIIa | Carbonizes; high-CTI grades reach ≥600 |
| Glass phenolic (NEMA G-grades) | 175 – 600 | varies | Strongly grade-dependent |
| Polyimide film (e.g. Kapton) | ~150 | IIIb | Excellent dielectric, weak tracking |
| Phenolic, paper-base | ~125 | IIIb | Carbonizes readily |
The three tiers, and why
Top tier — Group I, the non-carbonizers. Fluoropolymers, polyolefins, silicones, melamine, and mineral-filled polyester all reach the 600 V ceiling for the same underlying reason: their decomposition products don’t leave a conductive carbon residue. A scintillation removes material or volatilizes it, but it doesn’t build a path. This is why melamine laminates and glass-melamine (NEMA G5, G9) end up in circuit-breaker arc chutes and switchgear barriers — the worst tracking environment in a panel gets the material that refuses to track. It is also why silicone rubber dominates outdoor high-voltage insulation: on top of high tracking resistance, it stays hydrophobic, so the contaminant film beads instead of bridging.
Middle tier — Group II, the engineered thermoplastics. Glass-filled PBT, PA, and PPS live here and climb into Group I with the right filler and flame-retardant chemistry. These are the workhorses for connectors, terminal blocks, bobbins, and relay housings. The value is real but conditional — it depends on the compound, which is exactly why you check the grade rather than the family.
Bottom tier — Groups IIIa and IIIb, the carbonizers. Phenolic, standard epoxy-glass laminate, and polyimide all form conductive carbon under discharge, so they track at low voltage. These are not bad materials — phenolic and FR-4 are backbone electrical materials with excellent dielectric strength and mechanical stability. They just fail in this specific mode, and you design around it with more creepage or a higher-CTI grade.
Selection traps for electrical work
Three of these catch experienced people, because the material is good at everything except the one thing CTI measures.
Polycarbonate. Clear, tough, dimensionally stable, and everywhere as covers, windows, and structural housings. And it sits in Group IIIa at 175–250 V. On a part that also serves as a creepage barrier between live and touchable metal, that low CTI forces wider spacing than its mechanical strength would suggest. Reach for it as structure, not as your tracking barrier.
Standard FR-4 and phenolic. The default PCB laminate and the default phenolic are both carbonizers, both around Group IIIa/IIIb. For low-voltage boards this is fine. For high-voltage spacing it is the constraint — standard FR-4 at ~175 V demands the largest creepage in the table. When that hurts, high-CTI laminates built on modified resin systems and fillers reach ≥ 600 V, so you can buy the material group instead of buying the board area.
Nylon and moisture. Glass-filled PA66 tests near 600 V bone dry. But nylon absorbs water, and the absorbed water acts like a plasticizer that raises ionic conductivity and collapses the CTI. A number measured on a dry specimen can badly overstate real performance in a humid or wet-duty location. For anything that sees condensation, weigh the moisture-conditioned value, not the dry headline.
Reading the table without getting burned
A handful of rules keep an indicative value from becoming a wrong decision.
- The grade owns the number. Fillers, flame retardants, colorants, and even mold release move CTI. Confirm the actual production grade against its UL yellow card or datasheet, and confirm the grade in the design documentation matches the grade on the floor.
- “>600” is a test ceiling, not a physics ceiling. IEC 60112 stops at 600 V. A material reported as ≥ 600 maxed the test; the standard simply doesn’t rate higher. Don’t read it as unbounded headroom.
- Solution B changes the ranking. Values here assume Solution A. A grade’s CTI can drop under the more aggressive Solution B (marked with an M), especially for materials whose performance leans on a hydrophobic surface — silicones and fluoropolymers included.
- Low voltages are less reliable. Below roughly 125 V AC the drop test gets shaky, because the electrolyte doesn’t fully evaporate between drops. Treat very low readings with more caution.
- CTI is not the whole story. A good CTI with heavy erosion is not a durable material. And for severe or outdoor service, the drop test isn’t the qualifying test at all — that’s the IEC 60587 inclined plane test, which can rank materials in a different order.
FAQ
What is a typical CTI value for common plastics?
It spans the full scale. Fluoropolymers, polyolefins, silicones, melamine, and mineral-filled polyester reach about 600 V (Group I). Glass-filled PBT, PA, and PPS land in the 500–600 V range. Polycarbonate, standard FR-4, and phenolic sit low, from roughly 125 to 250 V.
Why is polycarbonate’s CTI so low?
Because it carbonizes under surface discharge. The carbon residue builds a conductive path, so tracking failure comes at a low voltage — typically 175–250 V — despite PC’s excellent mechanical and optical properties.
Does FR-4 have a good CTI?
Standard FR-4 does not — most grades are around 175 V (Group IIIa), because the epoxy carbonizes. High-CTI FR-4 laminates built on modified resin systems reach ≥ 600 V (Group I) and are made specifically for high-voltage spacing.
Which plastics have the highest CTI?
The non-carbonizing families: PTFE and other fluoropolymers, polyethylene and polypropylene, silicone rubber, melamine, and mineral-filled unsaturated polyester. All typically reach the 600 V ceiling and qualify as Group I.
Why does nylon’s CTI drop in service?
Nylon absorbs moisture, and the absorbed water increases surface ionic conductivity. A CTI measured on a dry specimen can significantly overstate performance in a humid environment, so the moisture-conditioned value is the one that matters for wet-duty parts.
Can I design creepage distances straight from these values?
No. Use the value to place the grade in a material group, then use the group, the working voltage, and the pollution degree to look up the minimum creepage in your equipment standard. And always verify the number against the specific grade’s datasheet — these are indicative family values only.
What does Group I, II, or III mean?
They are the IEC 60664-1 material groups set by CTI: Group I is CTI ≥ 600, Group II is 400–599, Group IIIa is 175–399, and Group IIIb is 100–174. A higher group allows shorter creepage distances for the same voltage and pollution degree.