terminal block voltage rating
These ratings are based on regulations regarding minimum creeping distances. These regulations are based on known values what is safe from experience. While it is possible to theoretically calculate distances for clean surfaces, it is not accurately possible for dirty surfaces. The regulations basically ensure that a device that is built and tested by an engineer in a clean state does still correctly work and not cause any damage or potentially harm any person once it has been out in the field for several years and accumulated dirt. If you look at the regulations, you will see that there are special (larger distance) requirements for environments that are especially dusty. Even more so if they are so dusty that a spark could cause an explosion (e.g. mills)