Clearance denotes the shortest distance between two conductive parts as measured through the air.
Artwork by Melinda Vaughan, National Instruments.
IEC60950-1 MAINS : Creepage Requirements
End system specifications, such as IEC60950-1 and IEC61010-1, dictate the required creepage and clearance spacing for a given end system. IEC60950-1 regulates the requirements for Telecom Equipment, and IEC61010-1 regulates the requirements for Industrial and Test Equipment.
The greater the creepage distance, the higher the external safety protection. As always, the amount of it distances required for a given mode depends on the end system specification. In general, this distance is dictated by the Pollution Degree of the environment in which the system will be used and the highest operating voltage present in the system. This distance can be decreased if an isolation component has been qualified to a higher proof tracking index (PTI) rating. In many cases, the distance can be cut in half if the component is rated to Material Group-I instead of Material Group IIIa or IIIb - the most common ratings for components.