It's not directly related to the amperage rating, it's about system coordination. In a coordinated protection system, you want a fault to be cleared at the closest point to the fault, not at a larger system level unless absolutely necessary. Since that concept moves up the line, the higher you get in the distribution system, the longer you need the upstream devices to hold in without tripping. So as you get up there, the trip settings need more adjustability beyond what you see in an off-the-shelf thermal mag breaker. That's what an electronic trip breaker gives you, separately adjustable Long time, Short time, Instantaqneous and Ground fault tripping. Because they are a lot more expensive, you don't go there until you have to, so that explains why you usually don't see them used until you get to higher current ratings.
Originally Posted by ElectDesigner
Many people are shocked when they discover I am not a good electrician...