MCB characteristics and specification of conductor cross-section

[VladM]

[PLAIN]http://www.buildingtechnologies.siemens.com/bt/low-voltage/EN/news_installation-technology_installationstechnik/new_products/BETA/PublishingImages/5SL_Leitungsschutzschalter_Miniature-Circuit-Breakers.jpg

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 (see attachment):
- 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%?

 

[CWatters]

My understanding is that (in the UK at least) this is taken into account in the regulations.

The protective device should be selected so that it does not allow more then 1.45 times the capacity of the cable. This is because the cable rating system is designed so that an overload of 1.45 times doesn't cause a problem for the cable.

So for example if you had a cable rated for 16A according to the relevant BS/EN standard it will actually survive carrying 1.45*16=23A. You must use a breaker that will definitely trip at 23A which means an MCB nominally rated at 16A.