MCBs have two tripping elements - magnetic which operates near "instantly" and thermal - which usually take from several seconds to minutes or hours. The trip threshold of the magnetic part depends on the "type" of the MCB (B, C or D) - common B types have a threshold of between 3x and 5x the MCB's rating - so a B10 shouldn't trip instantly on less than 30A.  The thermal part will trip at different speeds depending on the magnitude of the current - at 1,13x it shouldn't trip in less than an hour. and even at 30A it'll likely take over 10 seconds.

   - Andy.

The size of the MCB will not affect the amount of power used by the motor as far as the effect on the building  power budget goes. (after all it is quite normal to have a breaker total of two  or three times the main fuse rating for boards supplying).

Unless your supply is a dedicated generator or an inverter, the street supply will happily give many times its nominal full current for several seconds before anything lets go, and several minutes at 150% of rated.

The motor inrush may well be higher than the peak power consumption - large 3 phase machines can manage an inrush of ten times the running load, during spin-up. This is not likely on a small one like you have, especially  with a VSD, but a modest multiple is likley.

Mike.

Further to that MCb makers publish tripping time/overcurrent graphs, and this sort of thing is typical

https://www.electricalclassroom.com/b-c-d-k-and-z-mcb-trip-curves/

Mike

Thanks for these insightful posts. Do the main fuses in a house work like MCBs in terms of allowing short term over current supply? That is to say with a 3x60A rate supply, can I exceed the 60A per phase for short periods? Would demands of 120A or even 180A per phase for a few seconds  and possibly even 90A per phase for as long as 10 minutes be possible? if so, it does reshape my problem. I shouldn't worry too much about peak power demands, but instead focus on average loads that are likely to persist for many minutes. Is that correct? 

Do the main fuses in a house work like MCBs in terms of allowing short term over current supply?

They do indeed. Fuses work similarly to the thermal element of MCBs - but the curve continues smoothly into the higher currents part of the graph. So for lower over-currents they'll behave very similarly to MCBs, for medium over-currents that might just trip the magnetic element of an MCB they're likely to be a little slower, but with very high currents when MCBs are moving their mechanical bits as fast as they can, fuses are likely to open  far more quickly.

    - Andy.

For example:

there is a fashion these days to stop the lines at 0.1s (to encourage people to calculate things using energy let-through figures rather than trying to read disconnection times off a fuzzy graph) but you'd get the right idea if you carried the curve on downwards. Note that both axes are log rather than linear.

   - Andy.

just one word of warning with those line graphs - they generally show the slowest allowable times - generally a decent margin is allowed (for manufacturing tolerance) so any individual fuse may well operate a little faster.

    - Andy.