×
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

Multiples of tap value current in IDMT Curves

Multiples of tap value current in IDMT Curves

Multiples of tap value current in IDMT Curves

(OP)
Dear All,
When the relay current equals the pickup current, the relay should start picking up,i.e (when the psm is equal to or greater than 1) but why does all the curves begin at 1.5 x tap setting or 2 x tap setting in the reference curves provided by the relay manufacturer; why not at 1 (or) 1.1?

RE: Multiples of tap value current in IDMT Curves

For an electromechanical relay the performance just above pick up is rather variable, but we don't particularly care if 1.01pu results in 5 minutes or 20 minutes. By the time the current is up to 1.5pu the performance is much more predictable and repeatable. You'd probably also have the curve going off the top of the TCC plot unless you added another order of magnitude or two to the time axis. With numeric relays the repeatability is there but if you really care about just above pickup you'll find it easier to calculate the trip time than to read it off the TCC.

RE: Multiples of tap value current in IDMT Curves

See C37.112
t(I)=A/(M^p-1)+B where M is the multiple of pickup, T(I) is the trip time, and the other constants are determined by the curve shape. You can see you have a division by zero problem at M=1, so t(I) is undefined.

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