Tripping time of 10A MCB

Question

I am installing a lift in my house. A supplier has specified a 10A MCB to protect a three phase motor with variable speed controller. I am trying to determine the peak current as I have a tight power budget for the building. 
 The datasheet states "Max RMS acceleration line current, ln" as 15A. I presume this means that under maximum load (at startup, for example) the constant current that yields the same power dissipation as the time-averaged power dissipation of the AC current is 15A per phase. 
 This implies that 15A passes through the MCB for at least a complete cycle (50Hz = 20mS) - i.e. time-averaged. Is there a danger that a 10A MCB would trip if 15A was passed through it for 20mS? What is the tripping time of a typical MCB? Do I need to specify a special type of MCB for such use? 
 In terms of calculating the peak power consumption of the three phase circuit should I use 15A per phase with line to line voltage of 400VAC? The datasheet specifies the main supply voltage as 3x400VAC @ 50Hz.

AJJewsbury · Accepted Answer

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.

mapj1 · Answer

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.

mapj1 · Answer

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

AJJewsbury · Answer

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.

Tripping time of 10A MCB

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