Why are MCBs rated differently from fuses?

[Guineafowl]

Main Question or Discussion Point

Old-fashioned fuse wire comes rated at 5, 15 and 30A. The equivalent MCBs (breakers) are rated 6, 16 and 32A.

Is there a reason for this difference?

 

[CWatters]

Not 100% sure but I thinks it's because fuses tollerate slightly higher short term currents. Eg they blow at 30A continuous but tollerate 32A for short periods.

Edit: Think that might be wrong because type B MCB actually trips at 3-5 times the rated current.

 

[Guineafowl]

Not 100% sure but I thinks it's because fuses tollerate slightly higher short term currents. Eg they blow at 30A continuous but tollerate 32A for short periods.

Edit: Think that might be wrong because type B MCB actually trips at 3-5 times the rated current.

An MCB has two functions: overcurrent heating a bimetallic strip and a solenoid that actuates with very fast rise times, as in a dead short.

I’m not sure how the B (or whatever) curve rating relates to these.