MCB characteristics and specification of conductor cross-section
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Posted by: VladM ®

11/10/2010, 01:14:32
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Miniature circuit breaker (MCB) time-current characteristic has two specific points in the big-times area:
1) 113% of MCB rated amps (current of non-tripping),
2) 145% of MCB rated amps (current of tripping).

According to the IEC standards and to the MCB manufacturers catalogues:
- current equal or less than 113% will not trip the MCB in a period of 1 hour,
- current equal or greater than 145% will trip the MCB in a period of 1 hour,
- current in between of 113% and 145% may either trip MCB or not.

The question arises how to coordinate MCB and cable: should the cable long-term admissible current be greater than 145% of MCB rated amps?
In other terms the above mentioned feature of MCB (not to trip if less than 113%) makes me very uneasy about specifying cable cross-secion for 100% of MCB rated amps.

What do you think? Isn't it dangerous to size the conductor to 100% of MCB rated amps concerning the fact that MCB will not trip until 113% and may not trip until 145%?







Modified by VladM at Wed, Nov 10, 2010, 01:16:05


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