Fuse and Breaker ratings

[RJ44]

Greetings all, I just joined up.

I have noticed that for many small electronic fuses, time-to-open data is given for the fuse at it's rated current. For larger line fuses like NON and NOS, the data at the rated current is off the graph, at many thousands of seconds or more, I am guessing. The curve, even if shown, would be very steep at the rated current and difficult to read.

Circuit breakers also have a very steep curve at rated current.

Although the time to open at rated current is not well defined, the current at which the curve is nearly vertical does seem very well defined.

Is the rated current of a fuse or thermal breaker simply the current at which the time-to-open curve is vertical? Or is there another definition for rated current?

Also, is there a general rule for determining the hold current from the rated current (e.g. 90%, 95% etc)?

 

[rbulsara]

A ideal breaker or fuse will carry the rated current indefinitely at the rated ambient temperature and when installed in conditions it is intended for. That is proper enclosure and properly grouped with other breakers.

Many breakers are rated (rather guaranteed) to carry 80% of the rated current continuously when "group" installed.