De-rating Factors for Breakers

If a breaker does not very specifically state that it is rated for use on DC, you CANNOT USE IT!.

If it is rated for DC, the device will state very clearly what the DC capabilities are.

In one of my product catalouge(MCCB) it is mentioned that, MCCB-3P-100A-85KAfor 415VAC appilcation. But when it is used on 220VDC side, the current rating remaining the same where as the KA rating is reduced to 65KA. Do we have any calculations to find out how much KA is reduced, why it is reduced when AC breaker is used on DC side.

Provide the details

As mentioned by Paulusgnome, DC breaking is very voltage sensitive, and at 220V DC, you may not get even a few amps capacity, let alone kA. Devices specifically designed for DC use blowouts, either coils or permanent magnets. As for extrapolation, i haven't come across any calculation.You need to actually test.

It's perhaps worth noting that MCB some manufacturers are now making MCBs for use with photovoltaic systems that do have higher DC limits, 6KA @ 220vdc per pole are available in a range of In ratings.

Thanks for the heads up .. yes, 6kA with permanent magnets

The continuous current rating need not be de-rated. Only the breaking capacity is to be de-rated. Why? You know, in AC it is the R.M.S. Current and in DC it is always the peak current. So, the braking capacity in DC will be 1/1.414 times as that of the AC Breaking Capacity.Because, Peak = 1.414 x RMS.

A stark example of derating for DC duty(as the voltage goes higher) is here ... even for a non-inductive application.