The Comparative Tracking Index or CTI is used to measure the electrical breakdown (tracking) properties of an insulating material. To measure the tracking, 50 drops of 0.1% ammonium chloride solution are dropped on the material, and the voltage measured for a 3 mm 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.
Why is CTI important ?
By considering the CTI value, we can easily determine the minimum width of
marginal (barrier) tapes used in transformer. Owing to electrical safety reasons, UL has regulation on high frequency power transformers, that a minimum creepage distance is required. Therefore, the size of a transformer has been a bottle neck of minimization of electrical/electronic products especially in the information technology fields such as PC. However, in the new version of UL60950 or IEC60950, CTI values can be used as a standard of choosing the insulating materials used in the creepage distance. With a higher CTI value insulating material, the minimum creepage distance can be made smaller, and subsequently a smaller transformer.
Based on this new standard and regulations, P. LEO has developed a barrier tape (marginal tape) with a high CTI value. Approved by UL, this tape is graded as Class I insulating material. With this tape, the size of a transformer may be reduced as much as 30%. Or, as the space for winding is increased, an engineer can use magnet wire with larger size to reduce the resistance and hence the heat generated. This technology is a breakthrough in manufacturing of high frequency power transformers and the related components such as power supplies and monitors.
One of the examples is the creepage distance in transformer. Due to the existence of the creepage distance (the shortest path between two conductive parts measured along the surface of the insulation), the size of this electrical device cannot be too small.
Using the Comparative Tracking Index
| Test Speciments: | 50mm(2 in.) or 100-mm (4 in.) diameter disks or any other similar shape |
| Minimum thickness is 2.5mm(0.100in) | |
| At least five specimens of each sample be tested | |
| Electrode: | Platinum Electrodes |
| Power Source: | 0~1kVA,60 Hz |
| Aqueous Contaminant: | 0.1% Ammonium Chloride Solution |
| 395 ohm-cm resistivity at room temperature. |
Test Sequence
1.Set the power source to a particular value
2.Place the electrodes on the surface of the test specimen with a distance of 4mm(.16 in) apart.
3.Add the drops of the electrolyte by situating the hypodermic needle 40mm(1.6in) above the surface.
- The drop size: 20 +5-0 mm3 (0.0015 in3 / 0.0246cc).
- The drop rate: 1 drop per 30 sec.
5. Plot the number of drops of electrolyte at the breakdown versus voltage. On the curve note the voltage which corresponds to 50 drops. This is the comparative track index (CTI)
supplies tapes with high CTI values, which may reduce the size of a transformer.
For insulating materials used in the creepage distance, as indicatedin the UL60950 or IEC60950 standards,
CTI can be used as a criterion of choice. The higher the CTI value of an insulating material used, the smaller the creepage distance can be.
For example,'s tape 1H860,1H86A,1H866, 1H818 approved by UL, is classified as a Group I material (see table below). Using this tape as a barrier tape (marginal tape) the size of a transformer can be reduced as much as to 30%. This technology is suitable to be used in the manufacturing of high frequency power transformers and the related electrical/electronic equipment such as power supply and monitor.
The file no.of tape is E126174.
Comparative Tracking Index |
Minimum Creepage Distance at 300V |
Electrical Insulation Tape |
Group I CTI -> 600 |
3.2mm |
1H860,1H86A,1H866, 1H818 etc |
Group II 400 <- CTI <-600 |
4.4mm |
---- |
Group IIIa 175 <- CTI <- 400 |
6.4mm |
ordinary tape |
Group IIIb 100 <- CTI <- 175 |
6.4mm |
---- |
The CTI value is used for electrical safety assessment of electrical apparatus, as for instance carried out by Underwriters Laboratory in USA and many other laboratories in the world. 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 IEC 60601-1:2005, International Standard published by the International Electrotechnical Commission:
| Comparative Tracking Index (CTI) | Material Group |
|---|---|
| 600 <= CTI | I |
| 400 <= CTI < 600 | II |
| 175 <= CTI < 400 | IIIa |
| 100 <= CTI < 175 | IIIb |
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