The term creepage refers to the abrupt junction of two parallel insulating media. It is a particular junction — such as that between solid and gaseous insulating media.
Note that creepage is not an insulator. Creepage is nothing more than a boundary surface. One characteristic of a boundary or junction is that the two insulating media likely have radically different electric strength ratings (volts per unit distance through the medium). Therefore, the distance between two conductors located at the junction must be chosen for the insulation with the least value of electric strength. Since air has the least value of electric strength, the distance between the two conductors must be based on the electric strength of air (See IEC 664, Table I).
Assume that the junction in question is a plane. As voltage is increased, eventually the air will break down and conduct. When it does so, an arc occurs, and power is dissipated in the arc. If the arc occurs at the surface of the solid insulation, the heat of the arc could burn the surface of the solid insulation and could result in a carbon path across the solid insulation.
One way to positively prevent this carbon path from happening is to move the arc away from the surface of the solid insulation. This can be done with a parallel clearance with smaller dimensions than the creepage. In this way, any arc will occur at the clearance rather than at the creepage.
Another way to prevent a carbon path is to make the clearance distance very long, such that its electric strength approaches that of the parallel solid insulation. This is done by interposing a rib of solid insulation between the two conductors where they emerge from the solid insulation (in air, the conductors must diverge such that the clearance between any two points has the required electric strength). The clearance path remains the shortest distance through air, but now the path may include some portion of creepage. Thus, the air path is very long, and the electric strength of the clearance becomes very high.
My point is that a creepage path is not an insulator. The air and the solid media are the insulators. The electric strength of the AIR PATH determines the breakdown voltage. The air path can be made very long and its electric strength very high by interposing one or more ribs of solid insulation.
Based on the above, consider the following: If a clearance is located remote from a creepage and its distance is less than that of the creepage such that the clearance will break down at a lower voltage than the creepage, then what should be the requirement for the Comparative Tracking Index (CTI) of the solid insulating medium (assume a pollution free environment)?
