By definition, tracking is the formation of permanent conducting path across the surface of an insulation and in most cases the conduction results from degradation of the insulation itself. It is therefore necessary for organic insulation to be present if tracking is to occur.

The three essentials of the tracking phenomena are :

1) the presence of a conducting film across the surface of the insulation,
2) a mechanism whereby the leakage current through the conducting film is interrupted with the production of sparks,
3) degradation of the insulation must be caused by the sparks.

The conducting film is usually moisture from the atmosphere absorbed by some form of contamination such as salt in coastal areas, carbonaceous dust from the fuel, industrial deposits or cellulose fibres. Conducting path can also arise from the deposition of metal dust.

Interruption of moisture films is caused by drying of the surface following the heating effect of the leakage current. Sparks are drawn between the separating moisture films, which act as extension to the electrodes and causes the damage to start. This represents a significant difference between tracking and discharge failure. For the discharge to occur there must be a voltage of 380 volt at Standard temperature and pressure in air whereas tracking can occur at well below 100 volt - it does not depend on gaseous breakdown.

Degradation of the insulation is almost exclusively the result of heat from the sparks and this heat either carbonizes or volatilizes the insulation if tracking is to occur. For all practical purposes tracking can occur only with organic insulation - it should not be confused with flashover due to polluted surfaces.

Failure of solid insulation due to tracking does not have to be on the surface only - with laminates such as resin bonded paper board and pressboard, instances are known where failure took place between layers caused by moisture and ionic contamination in the paper. Degradation may be accelerated by extraneous process, such as physical weathering, ultra violet radiation and chemical attack. Thus ozone and oxides of nitrogen generated by discharge may degrade the insulation and provide sources of contamination.

Prevention of tracking must aim at clean, dry and undamaged surfaces and the material by its own virtue should be track resistant. Cleaning is not often practicable. It is employed in some cases together with coatings of water-repellent grease. Design can help by limiting access of dirt and avoiding its accumulation in areas between conduction. Increase in creepage paths will, to some extent, prevent tracking but in most practical cases moisture films can eliminate the designed creepage path.

The property of a solid insulating material against tracking is represented by a numerical figure known as the Comparative Tracking Index (CTI). This is obtained by using a standard test method. Higher value of CTI Indicates better performing material against tracking.