Last updated: 2026-09-05 · 5 min read
CTI — Comparative Tracking Index — What Voltage Rating Does Your Electrical Connector Need and Which Resins Achieve It?
The Comparative Tracking Index (CTI, IEC 60112) measures the voltage at which a plastic surface forms a conductive carbon track when contaminated with electrolyte. CTI determines the material group for creepage design: Group I (CTI ≥ 600V), Group II (400–600V), Group IIIa (175–400V) and Group IIIb (100–175V). Higher CTI allows shorter creepage — 2.5mm at 250V for Group I vs. 5.0mm for Group IIIb. PBT, PPS and PEI achieve CTI >250V; PTFE exceeds 600V.
Key Facts
- The test drops 50 drops of 0.1% ammonium chloride between two electrodes (4mm gap) on the plastic surface while applying AC voltage. CTI is the highest voltage at which the specimen survives 50 drops without tracking (current below 0.5A for 2s). Five specimens are tested at each voltage; the CTI is where all five pass.
- CTI determines creepage distance in IEC 60664-1 and IEC 60335-1. For 250V at pollution degree 2: Group I needs 2.5mm creepage; Group II needs 3.2mm; Group IIIa needs 4.0mm; Group IIIb needs 5.0mm. The difference between Group I and IIIb is 2× creepage — directly affecting connector pitch, PCB layout and enclosure size.
- Moisture absorption affects CTI — hygroscopic resins (nylon, PC) have lower CTI in humid conditions. PBT absorbs only 0.09% moisture and maintains CTI >250V in humidity. PA66 absorbs 2.5% moisture and its CTI drops from 250V (dry) to 175–225V (humid) — dropping from Group II to Group IIIa, increasing creepage by 25%. See the hygroscopic resin drying guide for how moisture targets vary by resin class.
How Do Common Engineering Resins Compare on CTI?
| Resin | CTI (V) | Material Group | Humidity Effect |
|---|---|---|---|
| PTFE | >600 | Group I | None (hydrophobic) |
| PEI (ULTEM) | >600 | Group I | Minimal |
| PPS GF40 | >600 | Group I | None (0.02% moisture) |
| PBT GF30 | >250 | Group II | Minimal (0.09% moisture) |
| PC GF30 | 175–250 | Group IIIa | Moderate (0.3% moisture) |
| PA66 GF30 | 175–250 | Group IIIa (humid) | Severe (2.5% moisture) |
| POM | 100–175 | Group IIIb | Minimal |
For electrical connectors rated 250V and above, Material Group I or II is required to keep creepage distances practical (2.5–3.2mm). Group IIIa and IIIb require 4.0–5.0mm creepage — which may not fit in standard connector pitches (0.4–1.0mm for board-to-board connectors). PBT, PPS and PEI are the default resins for connectors where CTI matters. See the PBT connector design guide for how PBT GF30 maintains CTI >250V in humid environments.
What Three Design Rules Govern CTI for Electrical Connector and Enclosure Design?
- Specify the resin CTI at the actual humidity condition — not the dry-as-molded value. The datasheet CTI is measured on dry specimens (23°C, 50% RH, as-molded). If the part operates in a tropical environment (35°C, 90% RH), hygroscopic resins absorb moisture and the CTI drops. Request the conditioned CTI (after 1,000 hours at 40°C/93% RH) from the resin supplier. For nylon, the conditioned CTI can be 50–100V lower than the dry value — dropping from Group II to Group IIIa. The glow wire test guide covers the related electrical safety tests for EU compliance.
- Pollution degree determines the required creepage — Group I at pollution degree 2 equals Group II at pollution degree 3. IEC 60664-1 defines four pollution degrees: PD1 (no pollution, hermetically sealed), PD2 (non-conductive pollution, occasional condensation), PD3 (conductive pollution or condensation with conductive contamination). Most electrical enclosures and connectors operate at PD2. If the environment is PD3 (outdoor, industrial, food processing), the creepage distance must be increased by 1–2 material groups — a Group II resin at PD3 needs the same creepage as Group IIIa at PD2.
- Surface geometry affects tracking — ribs, bosses and sharp corners concentrate the electric field and reduce CTI. The CTI test uses a flat surface. Real parts have ribs, bosses, screw posts and sharp corners that concentrate the electric field. The effective CTI on a ribbed surface can be 50–100V lower than on a flat bar. For parts with complex geometry, add 20–30% safety margin to the creepage distance — or test the actual part geometry in a modified CTI test (electrodes placed on the ribbed surface).
CTI test method (IEC 60112) — two electrodes (4mm gap) on the plastic surface, 50 drops of 0.1% ammonium chloride electrolyte while applying AC voltage. CTI = highest voltage where all 5 specimens survive without tracking (current <0.5A for 2s). PTI = pass/fail at a specified voltage. PTI ≤ CTI; product standards typically specify minimum PTI.
Pollution degree (IEC 60664-1) — the environmental contamination level that determines required creepage distance. PD1: hermetically sealed (no pollution). PD2: non-conductive pollution, occasional condensation (most indoor electrical). PD3: conductive pollution or condensation with conductive contamination (outdoor, food processing). Higher PD = more creepage needed, or a higher-CTI resin.
CTI (IEC 60112): voltage at which plastic tracks with electrolyte (50 drops, 0.1% NH₄Cl). Material Group I: CTI ≥600V (PTFE, PEI, PPS). Group II: 400–600V. Group IIIa: 175–400V (PBT, PC, nylon). Group IIIb: 100–175V (POM). Higher CTI = shorter creepage. Humidity drops nylon CTI by 50–100V.
- Material Group confirmed by conditioned CTI — not dry-as-molded value. After 1,000h at 40°C/93% RH: nylon drops 50–100V.
- Creepage distance matches IEC 60664-1 table — Group I at 250V: 2.5mm; Group IIIb: 5.0mm. 2× difference.
- Pollution degree accounted for — PD2 (indoor) standard; PD3 (outdoor/industrial) requires 1–2 group upgrade.
- Surface geometry margin added — ribs and corners reduce effective CTI by 50–100V; add 20–30% creepage safety.
- Connector pitch vs. creepage verified — fine-pitch (0.8mm) connectors limited to 30–50V; 250V needs ≥2.5mm creepage.
Frequently Asked Questions
What is the difference between CTI and PTI?
CTI (Comparative Tracking Index) is the highest voltage at which 5 of 5 specimens survive 50 drops without tracking. PTI (Proof Tracking Index) is a single voltage level at which 5 of 5 specimens survive 50 drops — it is a pass/fail test at a specified voltage. PTI is always lower than or equal to CTI. Product standards typically specify the minimum PTI (e.g., PTI ≥ 250V for Group II material) rather than the CTI, because PTI is more reproducible and directly maps to the material group.
When the datasheet lists CTI >600V, it means the material did not track at 600V — the highest voltage the test apparatus can apply. The actual tracking voltage may be higher. For material selection, use the PTI value (if available) for conservative design, or use the CTI value with a 20% safety margin.
Can surface coating improve the CTI of a low-CTI resin?
Yes — conformal coatings (acrylic, polyurethane, silicone, parylene) applied to the plastic surface create a barrier between the electrolyte and the resin. A coated POM (CTI 100–175V) can achieve effective CTI >400V if the coating is continuous and pinhole-free. The coating thickness must be at least 25 μm for acrylic and 50 μm for silicone to prevent pinhole breakthrough.
However, coatings add cost (spray or dip-coating step), may not survive assembly handling (scratches expose the base resin), and can degrade under UV or thermal cycling. For production volumes above 10,000 pieces, it is more economical to switch to a higher-CTI resin (PBT, PPS or PEI) than to coat a low-CTI resin.
How does CTI relate to creepage distance in connector design?
CTI determines the material group; the material group and working voltage determine the minimum creepage distance from IEC 60664-1 tables. For a 250V RMS working voltage at pollution degree 2: Group I (CTI ≥ 600V) needs 2.5mm; Group II (400–600V) needs 3.2mm; Group IIIa (175–400V) needs 4.0mm; Group IIIb (100–175V) needs 5.0mm.
In connector design, the creepage distance is the shortest path along the insulator surface between adjacent conductors. For a 0.8mm-pitch connector, the creepage is limited by the pad spacing and the insulator wall thickness — typically 0.4–0.6mm. This is far below the 2.5mm required even for Group I at 250V. Therefore, fine-pitch connectors must operate at lower voltages (typically 30–50V) where the required creepage is 0.8–1.0mm — achievable within the connector geometry.
Key Takeaways
- CTI: voltage where plastic tracks with electrolyte. Group I ≥600V, IIIb 100–175V.
- Higher CTI = shorter creepage: 2.5mm (Group I) vs. 5.0mm (Group IIIb) at 250V.
- PBT/PPS/PEI: Group I–II. Nylon/PC: Group IIIa (drops in humidity).
- Specify conditioned CTI (after humidity exposure), not dry value.
Send the connector drawing with the working voltage, pollution degree and environmental conditions — the quotation specifies the resin (PBT, PPS or PEI) needed to achieve the required CTI and creepage distance at the actual operating humidity.
Sources: IEC — IEC 60112 Method for Determination of Comparative and Proof Tracking Indices of Solid Insulating Materials · DuPont — Crastin PBT CTI Data and Electrical Connector Design Guidelines · Plastics Technology — CTI Testing and Material Comparison for Electrical Applications · Society of Plastics Engineers (SPE) — Electrical Insulation and Creepage Design Papers · MatWeb — Material Property Database with CTI and PTI Values
— Arthur, Tooling Engineer at aCatMould