İn a schneider NG125H mcb data table it says
Type Voltage Breaking Capacity
1P 220...240 36000
1P 380....415 9000 (1)
(1) IT İsolated neutral system double fault 1P breaking capacity
so why is it so ?
is there any relation between the system and the capacity of a MCCB ?
Keep in mind that this is primarily a forum for people in the US who are licensed Electricians and Electrical Engineers, IEC questions are not going to get a lot of responses. I happen to have worked for Siemens and Moeller for in the past here in the US so I have a little more exposure to these issues, but generally few people in the US know what "IT" means.
First off, you say "MCB" in the beginning and then ask about "MCCB" in the end, they are different beasts.
What this footnote is saying is that in the case of an IT system, one where there is no connection from Neutral to Earth in the distribution system, the amount of current that can flow through a
1 pole breaker to earth is inherently limited by the high relative resistance, and because it is a 1 pole breaker, there is no other path for fault current flow. So in that case, the MCB (Miniature Circuit Breaker) can withstand the forces of breaking a fault current of 18,000A as opposed to only 9,000A if it were a TT or TN system. Don't read or extrapolate anything else from that.
This would not automatically apply to MCCBs (Molded Case Circuit Breakers) either, they generally have much higher fault interrupting capacity to begin with.