As Graham says, if the impedance exceeds what's needed for 0.4s, the fault current would be too low to guarantee the operation of the magnetic mechanism, so we have to rely on the thermal mechanism and at those currents it can take longer than 5s at those sorts of currents.

There is an exception to that though - look at the details for type D MCBs - the current to operate the "instantaneous" magnetic mechanism is so high (20x In) that currents that would operate the thermal mechanism within 5s aren't interrupted by the magnetic mechanism first - so for those we do indeed have separate 0.4s and 5s figures. The tables along the appendix 3 graphs perhaps illustrate the point (e.g. 3A6).

(Note there is some dispute about type D MCBs as it seems that the product standard only guarantees opening within 8s rather than 5s for certain ratings), but the general principle holds the same).

    - Andy.

Ok. I got a bit more confused now....

When an MCB trips at 0.4s is the thermal or the magnetic part of the MCB that's tripping?

I always thought that because the tables are about fault currents, we have to do with the magnetic part of the MCBs and that the thermal is only for overloads

It's a mix of both.

For overload, think of your car's engine when you try to set off in third gear. The engine will try to drive the vehicle forward but will stall. Same with a motor under mechanical load starting from rest. It may take some time for the thermal part of the mcb to heat up before it trips, but what about a L to N short circuit? Lots of heat generated very quickly, so the thermal part of the mcb gets hot very quickly. The magnetic part will act faster in the 2nd scenario though.

It's a mix of both.

For overload, think of your car's engine when you try to set off in third gear. The engine will try to drive the vehicle forward but will stall. Same with a motor under mechanical load starting from rest. It may take some time for the thermal part of the mcb to heat up before it trips, but what about a L to N short circuit? Lots of heat generated very quickly, so the thermal part of the mcb gets hot very quickly. The magnetic part will act faster in the 2nd scenario though.