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CTI - tracking resistance

Section of common materials

Insulation material group
(EN 50124)
(IEC 60112)
Typical materials
I >600 0 PTFE; PE; PA; PFA; FEP; Makrofol® (PC, E168120); Formex® (PP, E121855); Kapton® 300FN929
II 400....599V 1 Polyester (PET)
IIIa 250....399 V 2
IIIa 175....249 V 3 PPS, PEN
IIIb 100....174 V 4 Polyimide; PEI; PSU; PEEK

< 100V 5

Like many UL standards, insulation coordination in accordance with IEC 60664 uses the CTI value, which is the most common. Relevant standards are, for example, IEC 60587 and IEC 60112 as well as UL standard 749.

Classification into insulating material groups according to IEC 60601:

CTI value
Insulating material group
600 ≤ CTI
400 ≤ CTI < 600
175 ≤ CTI < 400
100 ≤ CTI < 175

Part 2 (DIN IEC 60664-2-1) of the basic standard DIN IEC 60664 (insulation coordination) has a draft application guide (examples of ratings and insulation tests).

The IEC 60112 standard proceeds as follows to determine the PTI (Proof Tracking Index) and the CTI (Comparative Tracking Index):

In the first step, a conductive liquid is dripped between two platinum electrodes lying flat on the surface of the insulating material (up to 50 drops every 30 seconds). This is used to simulate condensation and superficial soiling.

The voltage is gradually increased until either the current flow exceeds 0.5 A or ignition and erosion occur. The highest voltage at which the device under test does not fail is verified at five points. This voltage is the PTI value (the test number).

In the second step, the voltage determined for the PTI value is reduced to 25V. This voltage is used to test the material at five different points with 50 or 100 drops every 30 seconds. If the test specimen passes the test, this voltage is then the determined comparison number (CTI value).

It is important to note that higher frequencies are not covered by the standard, but have a significant influence on the formation of a creepage path. Electrochemical migration is also not taken into account due to the relatively short test time (50x 30 seconds = 25 minutes).

In many cases, the CTI value in "volts" and the unitless test number of the PTI are used synonymously (e.g. CTI = 1 actually corresponds to CTI = 400...599V or insulating material group II)