Guys, he is not in the US. He likely has a 380/220Y system. Phase to Neutral would be 220V, so what he is wanting to know is if he can protect 3 single phase motors with one 3 pole breaker. And in his case, the NEC is irrelevant, although the engineering principals behind it are.
Greggs007,
Please keep in mind that we are here to offer suggestions, the chief one being, get professional help where you are from someone familiar with your local issues. An on-line forum is not a form of free engineering.
My answer is, probably not, but not for the reasons you think. I should preface this however with the fact that you have asked this question in a forum dedicated to the US National Electric Code, so most of the people reading this, like me, are not going to be familiar with code issues in other countries.
1) You are using an MCCB that is rated at 35kA interrupting. That implies that you have an available fault current that may be in excess of what any MCB is going to be rated at. Most that I am aware of are only rated at 10kA and unless there is some specific series rating with that MCCB, you are going to be unable to use an MCB like that. That is something you need to check out with your local code authorities.
2) If you have a fault on any one motor, the fact that you have a 3 pole breaker means that all three are going to be disconnected when the breaker trips. This is not something done here in the US, although I'm not sure it is impermissible. I just happen to think it is a poor practice. For example if you need to lock-out any one motor for the purposes of servicing it, you will have to lock-out all three. That can end up being undesirable for a service technician who needs to test something, so it may lead to him being forced to forgo the lock-out altogether and put himself at risk. I don't like it.
3) I am now confused as to the math on your decision to use a 63A MCB in the first place. If, as you now say, these are 4.5kW 220V single phase motors that have a nameplate of 18A (much more reasonable numbers), then a 63A MCB would be far too large here in the US for that motor. Again, you would need to consult your codes for what is appropriate.
4) A "C" trip curve is often used for motors as it has a 5-10X In magnetic trip curve. But the thermal trip curve is not adjustable and will not be optimum for motor protection. However, many single phase motors have integral thermal protection anyway; we have no way of knowing. So I suggest either using another Overload Relay down stream, or if you insist on a 3 pole solution, use a Motor Protector Switch type of device that has magnetic trips similar to a C curve and fully adjustable thermal trips like an overload relay. That is what those are for.
hmy:
