×
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

Simple calculation method to calculate Short circuit for busbar/transf

Simple calculation method to calculate Short circuit for busbar/transf

Simple calculation method to calculate Short circuit for busbar/transf

(OP)
Could anyone please explain me the simple/easy/thumb rule method to calculate the short circuit current/fault level for Busbar/transformer?

RE: Simple calculation method to calculate Short circuit for busbar/transf

How about:

Isc = I(fla)/z

where:

Isc = 3 phase bolted fault current at the secondary of the transformer
I(fla) = Full load current of the transformer
z = impedance of the transformer

This is called the "infintite bus" method.

Mike

RE: Simple calculation method to calculate Short circuit for busbar/transf

It is also a worst-case scenario, at least right at the secondary.  If you have a large motor load, it is possible that they could increase the s/c value, especially near the motor.

William

RE: Simple calculation method to calculate Short circuit for busbar/transf

Make sure when you take the Z off of your transformer, you use the decimal equivalent.  For example, if the nameplate says the Z = 5.75% (which is pretty standard), then you must use .0575 in the calculation I showed you above.

Mike

RE: Simple calculation method to calculate Short circuit for busbar/transf

If you use Isc=Ifla/z, and you're designing the system (ie you're not looking at a nameplate), you should adjust z downward by 7.5%.  In other words, min. imp for a 5.75% transformer would actually be 5.3%, unless the transformer was small (I'm not sure of the rating, but it's quite small, like 75 kva), in which case you use 10%.

RE: Simple calculation method to calculate Short circuit for busbar/transf

To account for manufacturing tolerance in the determination of Z.

William

RE: Simple calculation method to calculate Short circuit for busbar/transf

There is an excellent shareware program available at www.lmphotonics.com for about US$22 that will do this for you. It is provided by Marke, a regular contributor and VIP in this forum.

"Venditori de oleum-vipera non vigere excordis populi"


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