What is CTI ?
CTI is an index used for electrical insulating materials which is defined as the numerical value of that voltage which will cause failure by tracking during standard test. Tracking is the process that produces a partially conducting path of localized deterioration on the surface of an insulating material as a result of the action of electric discharges on or close to an insulation surface.

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 UL1950 or IEC 950, 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.
  

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 UL1950 or IEC950 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	----

Creepage Distance Use insulating tapes with CTI value can reduce the creepage distance in transformers

Part No. 1H860	Thickness (mm) Total        Backing	Length (m)	Tensile Strength (kg/25mm)	Elongation (%)	Adhesion (kg/25mm)	Breakdown Voltage (kV)	CTI Subject to UL 746A
(1 Layer)	0.16    0.122	90	14	70	1.2	5.5	> or = 600
(2 Layer)	0.32    0.244	45	28	70	1.1	10.6	"
(3 Layer)	0.48    0.366	30	42	70	1.1	17	"

The numerical data above are obtained in laboratory, and make no warranty or guarantee of the result.


Followings are electrical insulating tapes classified in material group 1 (CTI > or = 600):

Part No.	Thickness (mm) Total        Backing	Length (m)	Tensile Strength (kg/25mm)	Elongation (%)	Adhesion (kg/25mm)	Breakdown Voltage (kV)	CTI Subject to UL 746A
1G130	0.175    0.125	55	77.27	8	0.852	2.5	> or = 600
1H818	0.420    0.390	30	25.00	100	1.60	6	"
1H850	0.125    -----	55	15.91	50	0.994	5	"
1H86A	0.162    -----	55	15.91	50	0.994	5	"
1H866	0.140    0.11	82	14.00	30	1.20	5.5	"
1P700	0.062    0.025	66	10.45	100	0.852 - 1.136	5	"
1P701	0.062    0.025	66	10.45	100	0.852 - 1.136	5	"
1P702	0.087    0.05	66	22.73	100	0.852	5	"
1P717	0.055    0.025	66	10.45	100	0.994	5	"
1P707	0.055    0.025	66	10.45	100	0.994	5	"
1PG880	0.165    0.125	55	136.36	6	0.994	5	"
1PG887	0.165    0.125	55	136.36	6	0.994	5	"
1PN820	0.200    0.125	33	45.45	35	0.852	1	"
1PN828	0.200    0.125	33	45.45	35	0.852	1	"

The numerical data above are obtained in laboratory, and make no warranty or guarantee of the result.

Refer to UL 840, the minimum acceptable creepage distances at different voltages are shown below:


Creepage distance for equipment subject to long-term stress, mm

Operating Voltage volts ac rms or dc	Pollution degree 1 All material grp	Pollution degree 2 Material grp  I              II           IIIa,b	Pollution degree 3 Material grp  I           II          III a        IIIb	Pollution degree 4 Material grp  I              II               IIIa
10 12.5 16 20 25 32 40 50 63 80 100 125 160 200 250 320 400 500 630	0.080 0.090 0.100 0.110 0.125 0.140 0.150 0.180 0.200 0.220 0.250 0.280 0.320 0.420 0.560 0.750 1.000 1.300 1.800	0.40       0.40       0.40 0.42       0.42       0.42 0.45       0.46       0.45 0.48       0.48       0.48 0.50       0.50       0.50 0.53       0.53       0.53 0.56       0.80       1.10 0.60       0.85       1.20 0.63       0.90       1.25 0.67       0.95       1.30 0.71       1.00       1.40 0.75       1.05       1.50 0.80       1.10       1.60 1.00       1.40       2.00 1.25       1.80       2.50 1.60       2.20       3.20 2.00       2.80       4.00 2.50       3.50       5.00 3.20       4.50      6.30	1.00     1.00     1.00     1.00 1.00     1.06     1.05     1.05 1.10     1.10     1.10     1.10 1.20     1.20     1.20     1.20 1.25     1.25     1.25     1.25 1.30     1.30     1.30     1.30 1.40     1.60     1.80     1.80 1.50     1.70     1.90     1.90 1.60     1.80     2.00     2.00 1.70     1.90     2.10     2.10 1.80     2.00     2.20     2.20 1.90     2.10     2.40     2.40 2.00     2.20     2.50     2.50 2.50     2.80     3.20     3.20 3.20     3.60     4.00     4.00 4.00     4.50     5.00     5.00 5.00     5.60     6.30     6.30 6.30     7.10     8.00     8.00 8.00     8.00   10.00   10.00	1.60        1.60        1.60 1.60        1.60        1.60 1.60        1.60        1.60 1.60        1.60        1.60 1.70        1.70        1.70 1.80        1.80        1.80 1.90        2.40        3.00 2.00        2.50        3.20 2.10        2.60        3.40 2.20        2.80        3.50 2.40        3.00        3.80 2.50        3.20        4.00 3.20        4.00        5.00 4.00        5.00        6.30 5.00        6.30        8.00 6.30        8.00      10.00 8.00      10.00      12.50 10.00    12.50      16.00 12.50    16.00      20.00

Pollution degree based on the presence of contaminants and possibility of condensation or moisture at the creepage distance are as follows:

a) Pollution Degree 1 -- No pollution or only dry, nonconductive pollution. The pollution has no influence.

b) Pollution Degree 2 -- Normally, only nonconductive pollution. However, a temporary conductivity caused by condensation may be expected.

c) Pollution Degree 3 -- Conductive pollution, or dry, nonconductive pollution that becomes conductive due to condensation that is expected.

d) Pollution Degree 4 -- Pollution that generates persistent conductivity through conductive dust or rain and snow.