Electrical equipment has the potential to fail as a result of electrical tracking of insulating material that is exposed to various contaminating environments and surface conditions. A number of ASTM and other tests have been designed to quantify behavior of materials, especially at relatively high voltages. 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 Tracking Index-Copper Electrodes test is not related directly to the suitable operating voltage in service.

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.

As in other tracking test methods, for example, IEC 112 and Test Method D3638, this test method specifies test procedures that are intended to promote the formation of surface discharges which will produce carbon tracks in a reproducible manner. Since these conditions rarely reproduce the actual conditions encountered in service, the results of tracking tests cannot be used to infer either direct or relative service behavior of a material in a specific design application. Tracking tests can be used for screening purposes only. Suitability is verified through testing of the material in actual end use or under conditions that closely simulate actual end use.

The use of copper electrodes in this type of test was developed at the University of Cincinnati, NEMA laboratory. It is felt by the members of the Industrial Laminates Section of NEMA that using copper electrodes gives a more realistic value for a tracking index, related to the resin system used to reinforce the substrate of a laminate. In general, tracking tests made with copper electrodes tend to give lower values than platinum electrodes in the same type of test. It is a fact that copper is more widely used than platinum for electrical conductors.