Creepage distance: the shortest distance measured along the 
insulating surface between two adjacent conductors or between one conductor and the surface of an adjacent motor housing.

1. Decision on electrical clearance:

According to the measured working voltage and insulation level, the size requirements for the electrical clearance of the circuit at the distance from the primary side can be determined. PCB board 

But usually: a side ac parts: fuse before L - N acuity 2.5 mm, L.N PE (earth) or 2.5 mm  PCB board 

The ac to dc part of the primary side 2.0mm Primary side dc ground to ground 2.5mm (primary side floating ground to ground) primary side to secondary side part 4.0mm, the gap gap of components stradder between primary and secondary side part 0.5mm can be secondary side ground to ground 1.0mm

Note: before deciding whether to meet the requirements, the internal parts shall apply 10N force and 30N force to the shell to reduce its distance. In the worst case, the space distance shall still meet the requirements. PCBA board 

2. Determination of creepage distance:

According to the working voltage and insulation level, the climbing distance can be determined PCB board
But usually:

(1) ac part of the primary side: before the fuse, l-n 2.5mm, L.N 2.5mm. After the fuse, no requirements shall be made, but a certain distance shall be kept to avoid short circuit damage to the power supply.  PCB board

(2) the ac to dc part of the primary side 2.0mm