×
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

Using short time protection for a low voltage motor

Using short time protection for a low voltage motor

Using short time protection for a low voltage motor

(OP)
Gentlemen,

An AE firm came up with the settings for a Siemens WL breaker with an ETU trip unit.  I think they used SKM software to get the numbers.

This involves a recently installed Goulds reclaim water pump driven by a Reliance 460 V, 250 hp motor.  FLA=272A.  LRA=1825A.

Trip Unit Settings:

Rating plug = 300A
Long Time Pickup = 1
Long Time Delay = 8s @ 6 x PU
Short Time Pickup = 8
Short Time Delay = 0.02s (fixed)
Instantaneous = 12

I think I understand the purpose of short time protection to achieve coordination between feeder breakers and upstream breakers.  My question concerns the application of short time protection for a motor circuit.

What do you think it is there for?  

RE: Using short time protection for a low voltage motor

Normally it is needed to allow selectivity with downstream breakers.  

Since there is most likely nothing downstream of this breaker except the motor, you may not need it. It will have to set to allow the motor enough locked-rotor time to get up to speed.  The 8X pickup probably allows for that.  It will only be a factor between 8x and 12x where the instantaneous takes over.  In this range it will provide faster fault clearing than the long-time element.

I would be more concerned about the long-time settings because that is what is providing your motor overload protection, unless there is some additional motor overload device.  

 

RE: Using short time protection for a low voltage motor

If the settings were provided from SKM software, there should be a time-current coordination graph showing the motor start curve plotted against the C/B settings, and the next upstream C/B settings, too, if it was done correctly.

This way you should be able to see that the C/B settings coordinate with the upstream C/B, while allowing the motor start and providing motor protection.

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