Why do 3-phase selective circuit breakers appear as separate (not joined) ones?

Question

I'm looking at 3-phase selective circuit breakers like this:

Unlike a typical 3-phase circuit breaker, this selective breaker appears to be essentially 3 separate breakers all put together. Strangely enough, they are sold together but also sold separately with labels L1, L2 and L3. I'm trying to understand why it is done this way. Specifically, assuming there is a downstream 3/4-pole circuit breaker that can be triggered, what's the advantage of having separate poles that are not joined? Why would these be sold under separate labels (L1, L2, L3) -- they appear to be identical boxes, do they maybe interconnect in the middle somehow?

Update: this is the ABB S751-3 breaker.

Answer 1

Because they are designed to sit on a horizontal 3-phase bus bar. In this configuration the supply is provided by three horizontal bus bars running from left to right. This arrangement means that you can not manufacture a single single phase breaker that would be mountable on any of the phases, it would need to physically draw power from a different location and so need a different model for each phase. And so this package. The left breaker is drawing from the top bus bar, the middle breaker from the middle bar, and the right from the bottom bar. Since you usually want to split the loads somewhat evenly, it is probably easier to buy 5 of these selective breakers, than 5 L1 breakers, 5 L2 breakers, and 5 L3 breakers.

This could be avoided by using a more complex bus bar arrangement to always present the phase drawn at the same height, but that would be significantly more complex. (Because you have 3 phases, you would need multiple levels, not just an interlaced fingers design common in us residential breaker buses). Additionally, this approach does allow you to vary the number of breakers on the different phases, if for example you want two smaller breakers on one phase, and a larger breaker on another phase.