ASTM D3638

ASTM D3638 – Standard Test Method for Comparative Tracking Index of Electrical Insulating Materials

Description:

Significance and Use

5.1 Electrical equipment can fail as a result of electrical tracking of insulating material that is exposed to various contaminating environments and surface conditions. This method is an accelerated test which at relatively low test voltages, provides a comparison of the performance of insulating materials under wet and contaminated conditions. The comparative tracking index is not related directly to the suitable operating voltage in service.

5.2 When organic electrical insulating materials are subjected to conduction currents between electrodes on their surfaces, many minute tree-like carbonaceous paths or tracks are developed near the electrodes. These tracks are oriented randomly, but generally propagate between the electrodes under the influence of the applied potential difference. Eventually a series of tracks spans the electrode gap, and failure occurs by shorting of the electrodes.

5.3 The conditions specified herein are intended to produce a condition conducive to the formation of surface discharges and possible subsequent tracking. Test conditions are chosen to accelerate a process that is reproducible. Consequently, they rarely reproduce the varied conditions found in actual service. Therefore, while tracking tests serve to differentiate materials under given conditions, results of tracking tests cannot be used to infer either direct or comparative service behavior of an application design. Rather, the results provide a tool for judging the suitability of materials for a given application. The suitability can only be verified through testing the design in actual end use or under conditions which simulate end use as closely as possible.

5.4 The results have been used for insulation coordination of equipment with rated voltage up to 1000 Vac or 1500 Vdc connected to low-voltage supply systems (higher voltages permitted in internal circuits). The complete principles of insulation coordination involve the consideration of the combination of clearances, creepage distances, and the properties of solid insulation used to constitute the insulation system. Users of these results need to consider the overvoltage levels and the methods of control which will be utilized and establish the pollution degree to which the product insulation system will be expected to be subjected.

Note 1: See IEC 60664-1:2020, Table F.5 (Creepage Distances to Avoid Failure Due to Tracking) and UL 840, Table 9.1 (Minimum Acceptable Creepage Distances) as examples for the use of comparative tracking index results as part of insulation coordination.

Scope

1.1 This test method evaluates in a short period of time the low-voltage (up to 600 V) track resistance or comparative tracking index (CTI) of materials in the presence of aqueous contaminants.

1.2 The values stated in metric (SI) units are to be regarded as standard. The inch-pound equivalents of the metric units are approximate.

1.3 This test method is technically equivalent to the version of IEC Publication 112 cited in 2.2. However, the 2007 version of IEC 60112 Fourth Edition yields numerical CTI values that are very likely to differ significantly from this test method.