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Active Contributor

Join Date: May 2012
Posts: 20

Use Of Support Having Different Strengths For Short Circuit Force Calculation LV

05/09/2012 7:23 AM

Hi..I am calculating Short circuit forces for LV system busbars using IEC 865 standard.

We calculate the value of force on support and then compare this with the tensile strength of the material.

However throughout the busbar length i am using supports having different tensile strength values.

Now I am confused what value of support strength to use for comparison with the short circuit force value. Please help

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Commentator

Join Date: Feb 2010
Location: West Yorkshire, UK
Posts: 80
#1

Re: Use Of Support Having Different Strengths For Short Circuit Force Calculation LV

05/09/2012 7:33 AM

"what value of support strength"

Easy - use the weakest. That is what the short circuit forces will do.

Active Contributor

Join Date: May 2012
Posts: 20
#3

Re: Use Of Support Having Different Strengths For Short Circuit Force Calculation LV

05/10/2012 1:22 AM

Thanks for your answer. But when I use the weakest support strength, it comes out to be lets say 500...and the force I get using calculations is around say 700...i.e. theoretically it should not work...however practically this arrangement suffices the short circuit withstand requirements...I am confused whether it is the combined effect of the higher strength supports that restricts the movement of the busbar during short circuit.

Also since a number of factors like tensile, shear, vibration occur during a short circuit, is it right to compare the tensile load test value with the value obtained from force calculations?

Additionally if I increase the the SC withstand time from say 1sec to 3sec. ..are there higher chances of mechanical failure?

Commentator

Join Date: Feb 2010
Location: West Yorkshire, UK
Posts: 80
#4

Re: Use Of Support Having Different Strengths For Short Circuit Force Calculation LV

05/10/2012 3:54 AM

As it says in the standard "These standardized procedures are adjusted to practical requirements and contain simplifications with safety margins. Testing or more detailed methods of calculation or both may be used". If your tests prove your design in practice, even though it should fail in theory, you can refine your calculations accordingly, or move to a more complex comparison (FEA etc).

I assume that you are comparing peak values, which is most likely to do the most mechanical damage. Vibration is a strange thing though, once the theory has been put into the manufacture of a prototype, the test will then hit it 100 times, not just the once of one comparison of strength. Some materials will withstand a one time hit, even 10 times, and then fail or be significantly degraded at 25.

With regard to 1 sec/3 sec - being hit 300 times instead of 100 times would certainly increase the chance of failure, that's why we test! Normally the 3 second test has more pressing thermal limits of course, which could in turn affect the mechanical properties.

Active Contributor

Join Date: May 2012
Posts: 20
#5

Re: Use Of Support Having Different Strengths For Short Circuit Force Calculation LV

05/10/2012 10:58 PM

Thanks a lot! that was helpful information!

Guru

Join Date: Sep 2009
Location: Bangalore, India
Posts: 2757
#2

Re: Use Of Support Having Different Strengths For Short Circuit Force Calculation LV

05/10/2012 12:45 AM

Are the supports in tension ? Not shear ? Are fasteners involved ? The bar itself may yield by bending at the first asymmetrical peak ?

40 years ago, i had to do this the hard way, taking each component. Now there must be FEA packages which do this for you. You need to look around.

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