Three-Pole MCCB, for 2-Phase load

The 3 phase breaker you are using will be fine.. Use A & C phase for load, and abandon b phase.. You can remove b phase, or just leave it attached to breaker, but be aware it will be energized when you close breaker

You said... "400V/110V, 2-Ph/1-Ph, 20kVA".

What do you mean by "2-Ph". You must mean "two hot conductors from a single phase source" or "two phase conductors from a 3 phase source", in either case it is a single phase connection. A true 2 phase connection would require 4 wires.

Is the power distribution board single phase or three phase? If it is three phase you can use the three pole breaker, I would leave the middle pole on the breaker empty.

The case is "two phase conductors from a 3 phase source".

Many thanks

You are confused about something here. A circuit breaker is a PROTECTIVE device. If you use poles for different loads, are the currents they pull the same? If not, which one do you size the breaker for? If the smaller of the two, it will nuisance trip under the larger conditions. If sized for the larger one, it may allow too much current through on the smaller circuit, causing a fire. You will also never be able to isolate them from each other, they will always be both on or both off. That will make troubleshooting and testing difficult, maybe even dangerous.

Why are you being so cheap about it? Just get the proper circuit breaker for each load.

Perhaps I am mistaken but from the OP, I got the impression he was talking about the feed to a transformer and nothing more. What gives you the impression there are two distinct loads involved? If there are in fact two loads present, I completely agree with your comment.

I have never used poles of one circuit breaker for different loads. I just have one load protecting by this MCCB. i.e a 400V/110V, 20kVA, 96% efficiency, transformer with 52 Amp. full load current in primary side. The supply panel board voltage is 400V and 3-Ph. My problem is about the third pole of circuit breaker which can be connected in series with the second pole.

For a 20KVA 400V two phase transformer primary side current is 50Amp.

So a 50-63Amp MCB is perfect and you should use only two poles because of

1.Keeping one pole as redundancy.

2.Using two poles parallelly does not make any sense as the current rating does not demand that.

3.Using two poles in series is also not applicable for AC loads.

Hence use two poles only and keep the third as redundant.

phase keea

No it is not 50 amps. It would be 50 amps on a single phase transformer, not a two phase transformer. On a two phase transformer it would be 25 amps per phase as the load of the transformer is split between both primary windings, see info on a Scott-T configuration.

I doubt we talking about a true two phase setup here. We are talking about a single phase setup. Besides... anyone ever see a transformer with a two phase primary and a single phase secondary?