MCB

If its only 30 volts dc then yes!

Only if it has a rating for switching DC.

What is the application?

Highly inductive loads can be a challenge.

Sometimes a cap across the contacts can let the contacts open before any significant voltage developes.

You also need to check what power source you are using, I have had problems with a current limited power supply folding the current down before the CB trips resulting in power loss to all other circuits. The tripping times of the Cb also apear to be changed by the lower voltage, this might be dependant on the cb type and manufacturer.

As stated by others, if not highly inductive - yes. Best to use DC CBs when you can, but we've used AC CBs at a pinch and they do seem to last. I vaguely recall an old electrician's rule of thumb that you should reduce either the volts or the amps by a factor of around 10 to be safe. We've also often used 2-pole CBs, and connected both in series to effectively halve the voltage seen by each "switch". Yeah, I know about timing & such other things, but in practice it seems to work for the lower voltages. (I normally work with 3kA, 3kV circuit breakers or contactors, which are rather bigger and more specialised!)

Just a word of warning, if these CBs eventually fail due to contact wear, they do so rather spectacularly if you have big battery backup, due to no natural zero-amp point as in AC to extinguish the arc. So I would not recommend it to protect equipment that could see it tripping or being isolated/switched off too often, nor should you have anything flammable close by..

NO

NO

DC has totally different arcing behavour... A DC breaker will usually be rated at 30% lower tripping point as specified for an AC breaker. in your case it will be approx 4 Amp. for the same voltage rating. For lower voltage rating as is in your application, it might be little lower due to possibly lower arcing point.

My stupid question is: Why not use the correct MCB?

Wangito.

Almost any 230vac CB will have an interrupting capacity of 30ka AC. The trip setting can be very much lower. The catalog will list the DC interrupting capacity if it has been rated.

Again, we always miss the details.

If we had the CB part number and manufacturer we could give a proper answer.

For regen DC drives it is quite common to use an "AC" mag only (special low trip point) 600v MCB and put all 3 phases in series. And that is for 500V armatures.