Short answer: Creepage distance is the shortest distance measured along the insulating surface between electrodes - unlike the straight air gap it determines how easily a polluted, dampened surface flashes over. Select the specific creepage distance (per unit voltage) according to the site pollution level; heavily polluted or high-altitude sections need extended creepage or composite insulators.
Selection notes
Porcelain insulators extend the path with shed count (QB/QBN/QBS/QZ families); composite insulators add hydrophobic silicone sheds that resist continuous water films in heavy pollution. DC metro systems require larger minimum creepage because DC arcs have no natural current zero. Standards: GB/T 11030-2008 and TB/T 3199.1-2018 (porcelain), TB/T 3199.2-2018 (composite).