Creepage refers to the minimum distance between two conductive parts through the surface of the insulation. Creepage is an important characteristic as reduced creepage will result in tracking along the surface of the insulation.
Tracking refers to the flow of current along the surface of the insulation. Tracking causes localised heating and carbonization of the surface. This, eventually, leads to failure of the insulation.
The Comparative Tracking Index (CTI). The comparative tracking index gives the minimum voltage which can cause tracking across the surface of an insulation. Creepage also depends on contamination of the surface, humidity, corrosive chemicals and the altitude in which the equipment is installed.
Clearance
Clearance refers to the shortest distance through air between two conductive parts. If the Clearance is less than required, it will result in flashover due to dielectric breakdown in air. Dielectric breakdown is dependent on other factors as well such as humidity, contamination, altitude and temperature.
The creepage is calculated based on the value of the rms voltage while the Clearance is calculated based on the peak voltage.