×
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

Circuit Breaker Raing and Motor FLA

Circuit Breaker Raing and Motor FLA

Circuit Breaker Raing and Motor FLA

(OP)
Hi everyone,

We are asked to use circuit breaker to protect a 6kV motor, not a fuse. So how to decide the rating of Circuit breaker?
I use 1.2 x full load current of motor as a minimum to choose CB rating for LV system. For MV sytem like 6kV, what is the criteria?

Paul2gh

RE: Circuit Breaker Raing and Motor FLA

You cannot just use the voltage ratio and select ratings for MV system protection.  There only a few standard ratings (amp wise) for MV breakers, like minimum may be 400A and then next higher is 1200A.  Also remember that what sets the over current protection characteristics are CTs and the relay used in conjunction with the MV CB. A MV CB by itself is just a dumb switch.  Then there are standard rating of CTs as well to choose from.

In MV the rating selected are much higher than equivalent LV ratings. You adjust the protection using the relay settings.

 

RE: Circuit Breaker Raing and Motor FLA

(OP)
Rbulsara,Thanks for your prompt reply.

 

Because of the inrush of Motor starting current( 6 x FLA), do I need to use a larger CB to protect a motor?
For example, if the motor's FLA is 630A, can I use a CB rated at 630A to protect it?

Paul2gh

RE: Circuit Breaker Raing and Motor FLA

The short answer is No, it has be to somewhat greater than FLA.

At 6kV it won't matter as minimum CB rating you can get is 1200A. To answer an hypothetical question, you really need to check the duty rating of the breaker with mfr. But certainly you need not use 6xFLA rated breaker, may be  2X.  NEC is says "suitably" rated device..so I think it will be a matter of experience and professional judgment based on information the mfr can provide.

This again is only for MV system.

 

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