short circuit tensile forces vs compressive forces

[sunny81]

Are the compressive forces acting on a 3 phase bus brace always less than the tensile forces? I say yes, but I am having a hard time to explain this to someone. Am I missing anything? This is my explanation – sum of current in three phases should be zero at any point of time. So, higher currents cannot flow in the same direction and induce a high compressive force.
Is there a way to calculate by what percent these force are lower?
Or am I just dumb and thinking in the wrong direction.
PLease help me understand better.

 

[davidbeach]

Zero-sequence currents all flow in the same direction at the same time.

 

[sunny81]

Is this true for a balanced 3 phase system?

 

[dpc]

Yes, for a ground fault.

David Castor

http://www.cvoes.com

 

[jghrist]

The forces on bus act to separate the phases. This is true for 3Ø and Ø-Ø faults. There are no forces between phases for 1Ø faults because there is current in only one phase. There may be zero-sequence current in all phases, but except in the faulted phase, the positive- and negative-sequence currents are such that the total current is zero.

Whether this causes compression or tension or cantilever in the braces depends on the geometry of the braces.

 

[davidbeach]

If there is a grounding transformer on the "load" side of the bus and a ground fault on the "system" side of the bus you can easily have the same zero sequence current in all three phases.

 

[ijl]

I think we learned recently (I did what davidbeach said above, see

http://www.eng-tips.com/viewthread.cfm?page=4&qid=270621#post

 

[jghrist]

You would definitely have the same zero-sequence current in all three phases. This is inherent in the definition of zero-sequence current.