Comparative Tracking Index
Encyclopedia
The Comparative Tracking Index or CTI is used to measure the electrical breakdown
(tracking) properties of an insulating material
. To measure the tracking, 52 drops of 0.1% ammonium chloride
solution are dropped on the material, and the voltage measured for a 3mm thickness is considered representative of the material performance. Also term PTI (Proof Tracking Index) is used: it means voltage at which during testing on five samples the samples pass the test with no failures.
Tracking is an electrical breakdown on the surface of an insulating material. A large voltage difference gradually creates a conductive leakage path across the surface of the material by forming a carbonized track. Testing method is specified in IEC standard 60112.
Performance Level Categories (PLC) were introduced to avoid excessive implied precision and bias.
The CTI value is used for electrical safety assessment of electrical apparatus, as for instance carried out by Underwriters Laboratory. The minimum required creepage distances over an insulating material between electrically conducting parts in apparatus, especially between parts with a high voltage and parts that can be touched by human users, is dependent on the insulators CTI value. Also for internal distances in an apparatus by maintaining CTI based distances, the risk of fire is reduced.
In design of medical products, the CTI is treated differently. Material groups are classified as shown below, per 60601-1:2005:
Electrical breakdown
The term electrical breakdown or electric breakdown has several similar but distinctly different meanings. For example, the term can apply to the failure of an electric circuit....
(tracking) properties of an insulating material
Electrical insulation
thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal insulation...
. To measure the tracking, 52 drops of 0.1% ammonium chloride
Ammonium chloride
Ammonium chloride NH4Cl is an inorganic compound with the formula NH4Cl. It is a white crystalline salt that is highly soluble in water. Solutions of ammonium chloride are mildly acidic. Sal ammoniac is a name of natural, mineralogical form of ammonium chloride...
solution are dropped on the material, and the voltage measured for a 3mm thickness is considered representative of the material performance. Also term PTI (Proof Tracking Index) is used: it means voltage at which during testing on five samples the samples pass the test with no failures.
Tracking is an electrical breakdown on the surface of an insulating material. A large voltage difference gradually creates a conductive leakage path across the surface of the material by forming a carbonized track. Testing method is specified in IEC standard 60112.
Performance Level Categories (PLC) were introduced to avoid excessive implied precision and bias.
The CTI value is used for electrical safety assessment of electrical apparatus, as for instance carried out by Underwriters Laboratory. The minimum required creepage distances over an insulating material between electrically conducting parts in apparatus, especially between parts with a high voltage and parts that can be touched by human users, is dependent on the insulators CTI value. Also for internal distances in an apparatus by maintaining CTI based distances, the risk of fire is reduced.
| Tracking Index (V) | PLC |
|---|---|
| 600 and Greater | 0 |
| 400 through 599 | 1 |
| 250 through 399 | 2 |
| 175 through 249 | 3 |
| 100 through 174 | 4 |
| < 100 | 5 |
In design of medical products, the CTI is treated differently. Material groups are classified as shown below, per 60601-1:2005:
| Comparative Tracking Index (CTI) | Material Group |
|---|---|
| 600 <= CTI | I |
| 400 <= CTI < 600 | II |
| 175 <= CTI < 400 | IIIa |
| 100 <= CTI < 175 | IIIb |