Creepage current resistance of insulating materials

Creepage path formation on insulating materials is when conductive connections between neighboring potentials are formed, for example on a component or a printed circuit board. The creepage resistance of an insulating material indicates the insulation strength of the surface. Testing the insulation strength quickly demonstrates the effect of moisture and impurities on the surface.

The maximum creepage current of an insulating material is determined under standardized test conditions. These test conditions include a predetermined voltage, a defined cut-off current, and test fluids, which are to be set up in a defined test arrangement (electrode spacing, electrode shape, dripping).

IEC 60112 "Method for determination of the proof and the comparative tracking indices of solid insulating materials" describes a simple method for determining the creepage path formation of solid, insulating materials. The method is used to determine the PTI value – proof tracking index – and the CTI value – comparative tracking index – of a material.

In order to carry out a characterization of a material, two electrodes are placed flat on the material to be tested. The conductive test fluid is dripped between the two platinum electrodes, with the two electrodes under voltage. The dripped test fluid creates a current flow between the electrodes on the surface of the test sample. The test voltage is gradually increased until the material erodes, and the generated current exceeds 0.5 A or a flashover occurs at the test point. The highest voltage value that the test object was able to withstand must be confirmed by testing a further four samples.

In the first step of the creepage resistance test, the PTI value (proof tracking index) of an insulation material is determined. To determine the PTI value, 5 test samples are tested with the selected voltage. The test sample is moistened with 50 drops at intervals of 30 seconds.

In the second step of the creepage resistance test, the CTI value (comparative tracking index) of an insulating material is determined. To confirm the CTI value, the test voltage of the PTI value is reduced by 25 volts and tested on 5 test samples. The test samples are moistened with 50 or 100 drops at intervals of 30 seconds.

The material can then be characterized according to the specified PTI and CTI values.

INSULATING GROUP	CTI	PTI	TYPICAL MATERIALS
I	0	>600 V	PTFE, PP, PE, PA, PFA, FEP
II	1	400…599 V	Polyester
III a	2	250…399 V	Polycarbonat
III a	3	175…249 V	PPS
III b	4	100…174 V	Polyimide, PEI, PSU, PEEK
-	5	<100 V	-

For some product standards, the determination of the PTI value is already sufficient for creepage and air distance assessment.

For voltages above AC 400 V and/or the interaction of high frequencies, the surface of an insulating material is of great electrotechnical importance. The rapid formation of superficial creepage paths can quickly lead to the premature failure of components, devices, or even entire systems. The creepage resistance of a material indicates how fast an insulation material forms creepage paths under high stresses, moisture, and impurities. In the construction of electric vehicles, therefore, great attention is paid to the use of tested insulation materials with good creepage current resistance, especially in high-voltage technology.

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