Creepage distance is measured across insulating surfaces, following ribs and grooves, between conductors at different potentials or between a conductor and accessible metal. Dirt and moisture can form a conductive film that tracks over time, so required creepage grows with working voltage, pollution degree and the material's tracking resistance (CTI). Outdoor insulators add creepage with sheds, specified in mm per kV, while product designers cut slots in boards to lengthen the path.
Clearance passes but creepage distance across the optocoupler is only 4 mm, for reinforced insulation at 400 V we need a slot under it.
Creepage can never be shorter than clearance, so when both are tabulated the larger figure drives the layout.
Also heard as
- creepage
- creep distance
- leakage distance
Do not mix it up with
- Clearance: Clearance is the shortest path through air; creepage follows the insulating surface.
Related terms
Clearance
Straight-line air gap between two conductive parts, the dimension that decides whether a surge can flash across.
Pollution degree
Grade from 1 to 4 describing how much conductive dirt and moisture can settle on insulation in a location.
CTI
Comparative tracking index: voltage an insulating material withstands before tracking under a wet contamination test.
Tracking
Conductive carbon trail that leakage arcs through dirt and moisture gradually burn across an insulating surface.