Protection When Changing CSA

I have always went by the rule " when changing (lowering) csa of cable/wire then insert protection device", and further to that the second rule was "if its only a small run perhaps <2M then first rule need not apply". 


What is experience of everybody on this?


This particular application in design phase has an incoming 95mm cable in through a a switch disconnector which requires distribution to many smaller branch circuits. usually this application for me in control panels is simple as small currents (<30A) are in use but this larger requirement means the terminations get tricky having many different wire sizes.


Thanks Folks!