×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Fuse - circuit breaker coordination
2

Fuse - circuit breaker coordination

Fuse - circuit breaker coordination

(OP)
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

RE: Fuse - circuit breaker coordination

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

RE: Fuse - circuit breaker coordination

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.
 

RE: Fuse - circuit breaker coordination

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources