How to define working voltage class net to calculte the creepage?
How to define working voltage class net to calculate the creepage?

First you will need to get a copy of the standard that you want to design your product to. (Like IEC61010-1) This will depend on what kind of product it is and what market you are going to sell it in, I don't know which part of the IEC standards pertain to industrial products.

Then you will look at the table and determine the pollution degree:


  Pollution Degrees and Overvoltages Pollution degree is divided into
  four categories. The following definitions are based on those in IEC
  60664.3
  
  • Pollution degree 1. No pollution or only dry, nonconductive
  pollution occurs. The pollution has no influence (example: sealed or
  potted products).  
  
  • Pollution degree 2. Normally only nonconductive pollution occurs.
  Occasionally a temporary conductivity caused by condensation must be
  expected (example: product used in typical office environment).
  
  • Pollution degree 3. Conductive pollution occurs, or dry, nonconductive
  pollution occurs that becomes conductive due to expected condensation
  (example: products used in heavy industrial environments that are
  typically exposed to pollution such as dust).  
  
  • Pollution degree 4. Pollution generates persistent conductivity
  caused, for instance, by conductive dust or by rain or snow.


Your standard will have a table like this:

Source for table and quote 

You then have to keep every trace on a PCB X ammount of distance away from every other trace. So if its 480Vrms, and pollution deg 2 you need 2mm of clearance. 

This is just an example, you need to find the spec that pertains to you. Get a regulatory consultant if necessary and\or buy the relevant standards (they aren't cheap)