Fuse - circuit breaker coordination 2

[ritchieb]

I have a 200A fuse protecting a 6kA circuit breaker. I have the i2t let through of the fuse. I need to show that this fuse will protect the circuit breaker. Do i use the peak let through of the fuse and ensure that it is less than the kA rating of the breaker? Have many tables indicating that a 200A fuse will back up a 6kA breaker but need the theory.

Thanks for any help

 

[rbulsara]

At least in the USA, no calculation method (despite some fuse mfr's claims)is accepted to ascertain suitability of a fuse protecting a breaker.

Only UL (or equivalent testing lab) tested fuse-breaker combination in series are accepted.

Rafiq Bulsara

http://www.srengineersct.com

 

[LionelHutz]

In North America the circuit breaker will have been measured as described below.

The test circuit was built with the leads that will be connected to the circuit breaker shorted. The power was then turned on and the peak current measured to ensure it was 6kA.

Then, the circuit breaker was connected and the output of the breaker was shorted. The circuit was tested again and the results evaluated to ensure the breaker interrupted the power safely. The current was not measured to ensure it was 6kA again. In fact, because of the extra impedance of the breaker and the wires connected after to short it together, the current would have been less than 6kA.

The fuse will have been tested in a similar manner and in that case the actual current during the fuse test was measured which is how you got the data you have.

If you could get the actual current curve during the circuit breaker test it might be possible to successfully make a case with UL or CSA and get a series rated approval on the fuse and circuit breaker conbination.

 

[zade]

I do not think you can easly coordinate fuse with CB for few reasons.
Maximum let thru does not mean much for the breaker because breaker may try to open before the fuse clears. Critical is the energy load until CB opens. Typically on high available short circuits curve is very steep causing I2t larger than CB can open even though the maximum at that point is less than interruting rating. So you need to prove that I2t and maximum let thru is well within CB ratings.