Why backup fuse rating for motor starter is higher than MCCB or MPCB rating?

Question

Why backup fuse rating for motor starter is higher than MCCB or MPCB rating?

Answer 1

hi

Easy.

Because MCCB has not enough breaking capacity(and at a severe shortcircuit will fail), we put Fuse.  And fuse will blow only at Shortcircuit. Where curent is at least 4-10 times larger than rating.

Fuse does not have to blow on overload( about 1.2-2 x rating). On overload MCCB will trip. bigger fuse solves this 2 issues.

Answer 2

This is to do with the starting current. A fuse has to be rated 2-3 times the full load of the motor so it doesn't blow on start. An induction motor will have a start current of around 6 times FLC.

The MPCB is designed for motor protection and so the rating is stated in motor FLC. You can also get motor protection fuses that will be rated for FLC.

A fuse or MCCB do not protect the motor anyway. They are short circuit protection. You should have an overload protection relay of some kind in addition to the fuse or breaker.