In uk.d-i-y, ben wrote:

How do MCBs work? I can't find it on google.

Two separate mechanisms, in fact. One thermal, to detect "modest"
overcurrent, trips at around 1.4 times the nominal rating after a good
few seconds or more (precise curves in manufacturer's data); another
electromechanical, trips quickly (a few mains cycles only, maybe 0.2s)
given a Hefty overload of about 5 times the nominal for a B-type, 10
times for a C-type: again, figures illustrative rather than precise.

Stefek