Tracking resistance describes how well an insulation material resists the formation of a conductive path on its surface when moisture and contamination act together. It is measured as the CTI (Comparative Tracking Index) to IEC 60112: the highest voltage in volts at which the surface does not yet form a continuous track in the drop test. The higher the CTI, the more reliably the material holds up under dirt and condensation. For the design engineer this value is the basis for setting creepage distances and the material group.
What tracking resistance is
Dust, salts and moisture settle on every insulation surface over time. With a voltage nearby, a small leakage current flows across this contamination layer. It dries out tiny paths, small arcs form, and these carbonise the material step by step. A conductive path grows, called tracking. In the end a short circuit runs across the surface, without the material ever breaking down through its volume.
Tracking resistance says how long a material withstands this process. It depends strongly on the plastic type. Phenolic thermosets can char under heat and then become conductive themselves, while many thermoplastic and ceramic-filled materials barely form carbon traces. The value is therefore a material property, not just a matter of geometry. CTI values from 125 to 600 for 17 common insulating plastics are listed in the article on electrically insulating plastics.
CTI and the test to IEC 60112
IEC 60112 defines the test method. Two platinum electrodes stand on the sample at a 60 degree angle, 4 millimetres apart. A test voltage is applied between them. Every 30 seconds, drops of a defined electrolyte solution (ammonium chloride) fall onto the surface and reproduce the contamination.
The CTI is the highest voltage at which the sample survives 50 drops without failure. A failure occurs when a current of 0.5 ampere flows for at least two seconds or the sample burns. The number matches the voltage: CTI 600 means the material withstands the test up to 600 volts. Some laboratories test higher values as CTI-M with a more aggressive solution, because some materials only reveal their true behaviour under harsher contamination.
CTI classes and material groups
IEC 60664-1 combines the CTI values into four material groups. These groups feed directly into the sizing of creepage distances, so the engineer usually works with the group rather than the raw number.
| Material group | CTI range (volts) | Typical materials |
|---|---|---|
| I | 600 and higher | PTFE, many silicones, ceramic-filled thermosets |
| II | 400 to 599 | Polyamide, polycarbonate, many polyesters |
| IIIa | 175 to 399 | Epoxy glass fabric, some thermoplastics |
| IIIb | 100 to 174 | Phenolic and melamine laminates without additives |
Material group I carries the highest tracking resistance, IIIb the lowest. A material from group I allows a shorter creepage distance at the same voltage than one from group IIIb. That is the practical benefit: a better material saves space.
Link to creepage distance and pollution degree
The required minimum length of the creepage distance follows from three quantities: rated voltage, pollution degree and material group. The pollution degree describes the environment, from dry and clean (degree 1) to conductive contamination (degrees 3 and 4). The higher the pollution degree and the worse the material group, the longer the creepage distance must be.
This is why the CTI never stands alone. Anyone who picks a group I material may place the contacts closer together. Anyone who stays with group IIIb for cost reasons needs more distance, ribs or barriers to reach the same creepage distance. Tracking resistance describes the surface, not the volume. For resistance through the material the insulation resistance applies, and for surface conduction itself the surface resistance.
Material selection by CTI
Selection starts with the service environment. A part inside a dry, sealed device places different demands than a terminal strip outdoors or a part in damp, dusty industrial air.
- High voltage with contamination and moisture: group I or II materials such as polyester or filled thermosets, so the creepage distance stays within the available space.
- Dry, clean environment at low voltage: group IIIa is often enough, a higher CTI adds no benefit here.
- Phenolic or melamine laminates: cheap and mechanically strong, but low CTI. They need longer creepage distances or a deliberate material pairing.
A high CTI never replaces dielectric strength. Both properties have to match, because a part can fail across the surface as well as through the volume. We recommend choosing the material group one step better than calculated when in doubt, because the price difference is small against the cost of a field failure.
GOBA Takeaway
Tracking resistance decides whether an insulation part fails across the surface under dirt and moisture. The CTI to IEC 60112 makes this property comparable in volts and leads through material groups I to IIIb straight into the sizing of the creepage distance. Choosing the material early by CTI, pollution degree and voltage saves space and rework later. GOBA has been processing insulation materials from polyester film to laminated fabric since 1959 and knows the tracking behaviour of the common materials from practice. Why phenolic paper laminate stops at CTI 100 while melamine and polyester glass laminates reach CTI 600 is explained in the article on thermosets. For material selection and the right cut, look at our insulation materials, send us your requirement and you will receive a technical quote.
