Are you sure you mean fuses (i.e. thin wire links that melt under overload, typically in a ceramic tube with arc suppressing sand) and not Miniature Circuit Breakers (MCB)? Usually MCB come with an operating current (e.g. 16A) and a fault level where the trip goes from slow thermal to near-instant magnetic operation and those are typically B, C, or occasionally D (also K & Z sometimes in Europe)
I see you are down as Austria so the rules on 3 or 4 pole breakers are probably very different to here in the UK. Here we have historically has a very strong insistence on neutral being close to earth/ground potential so for 3-phase work it is not uncommon to only switch the 3 line conductors.
But if your question is about cascading MCB then you can put them in series but you will have very poor or non-existent
selectivity. This used to be called fault discrimination, but now selectivity refers to the ability of the down-stream MCB to open on an overload or fault without the up-stream MCB tripping.
- For overloads (so maybe up to 3 times the rated current of the upstream device) the MCB will trip slowly on the thermal overload and then if you have something like a 1.6:1 or 2:1 ratio or higher they are selective. So a 10A MCB would usually go before a 16A MCB, and almost certainly before 20A, etc, for currents to 50A or so.
- However, for a high current fault that exceeds the magnetic levels of both MCB then often the up-stream MCB will already be past the point of no return before the down-stream MCB has opened to isolate the fault and so both will trip.
If there is a big ratio difference and more so if the up-stream MCB has a higher fault curve (so C or D) you might get tolerable selectivity depending on what sort of prospective fault current is possible, but it is never very good.
If you need good selectivity then the up-stream device must have some element of delay, so either a medium sized fuse or a MCCB that has a 20ms or so short-term delay up to very high currents, etc. If you search for something like the Hager commercial catalogue then towards the rear of that document there are tables of selectivity limits for various combinations of upstream fuse/MCB/MCCB and downstream MCB/RCBO/AFDD to help with any design work. Also Schneider has an on-line tool but it is crap to use (fails if you have advert blocker, stupid random-like URL, insists on you signing up to an account to spam your email ever after, etc).
