Clearance: Defined as the shortest distance between two conductive materials measured through air. Sufficient clearance distance prevents an ionization of the air gap and a subsequent flashover. Similar to creepage distance the pollution degree, temperature and relative humidity influence the tendency for a breakdown. Breakdown along a clearance path is a fast phenomenon where damage can be caused by a very short duration impulse. Therefore, it is the maximum peak voltage, including transients (overvoltage category [OVC] level), that is to be used to determine the required clearance spacing. Figure 1-1 shows the definition of clearance.

Figure 1-1 Definition of Clearance

Creepage: Defined as the shortest path between two conductive materials measured along the surface of an isolator which is in between. Maintaining a certain creepage distance addresses the risk of tracking failures over lifetime. The generation of a conductive path along the isolator surface due to the high voltage applied over time is related to the root mean square (RMS) value and depends on environmental conditions, which are described by a pollution degree and the material characteristics of the isolator comparative tracking index (CTI). Breakdown of the creepage distance is a slow phenomenon determined by DC or RMS voltage rather than peak events or transients. Inadequate creepage spacings can last for days, weeks or even months before failure. Figure 1-2 shows the definition of creepage.

Figure 1-2 Definition of Creepage