The system used in a certain plant is isolated system i.e isolated neutral ,that is installed in 1985 and still work till know,a MCCB used for protection in a circuits drive a pump 200 Kw ,starting method is D.O.L starting,we notics that the MCCB get melted and burnt after a long time of being installed and working despite the motor and cable is in good state.this siuattion is repeated for two times in a similar circuts in the same system ,i.e isolated netural, but it drive a fan 90 Kw D.O.L starting ,i ask about the case and is isolating system in general has the same problems

I have found,in several cases that molded case breakers when subjected to overcurrent frequently will tend to carborize until ultimate failure which may cause the entire panel to fail,catastrophicly!

Maybe you should conduct a contact resistance test, to see the whether the internal contacts are deteriorated. you can do a Thermographic test while the system works, it will show you the hot joints within the MCCB.
I agree with Kenneth, MCCB's tend to overheat, especially when is not adequate airflow around them.
If the startup is DOL, maybe the MCCB is simply undersized. Startup current could be 7x the nominal current, so the 200kW 415V motor can draw approx 7x300A - or more, depending on the motor power factor - during startup for a couple of seconds.

Hi Kenneth

Like Istvan said, motor's starting current can reach 7xIn, so you have to choose the MCCB to withstand the motor's starting current within the motor's starting time (say, 10 sec for 90kW DOL loaded). You now have to check with the MCCB tripping curve that the point of 10sec@7xIn (In@90kW=about 160A) is located below the curve.

If you don't care watching the tripping curve, you can refer the tripping class of the protection unit. Tripping class 20 will work for DOL motors.

Normally, if you take 250A MCCB for 90kW motor with a correct protection unit, it should work.

If you still observe defects in the circuit breaker, it must have been some manufacturing fault or bad contact.

Hope it helps.
Regards,
Ilia

I totally agree with the previous comments.