Insulator spacing strength

  • #1
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Main Question or Discussion Point

Well, I have one nice problem. I have 30x10mm busbar, and Isc=30kA. I must place this busbars in electrical panel, but I have a problem with insulators. Can I use a red insolators as it's showed on picture, or I should use special holders for busbar system. Another problem is: When I use such red insulators, how to determine the distance between them?
 

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  • #2
berkeman
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You use the word "strength" in your thread title. Do you mean mechanical strength (to hold the busbar so it doesn't fall), or electrical strength (so it does not arc through)?

Do you work with other licensed electricians on these jobs? What do they recommend? What have they used in the past?

Paging @anorlunda
 
  • #3
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Well, I think that my colleagues are wrong. They doesn't have a calculated distance between insulators, they rather put it on the way: "It looks good". But, if you put busbars in such way, the busbar can get deformated during the short circuit and no more usable. Also they use red insulators for all short circuit current, I'm not sure that they are right.
 
  • #4
1,061
158
Well, I have one nice problem. I have 30x10mm busbar, and Isc=30kA. I must place this busbars in electrical panel, but I have a problem with insulators. Can I use a red insolators as it's showed on picture, or I should use special holders for busbar system. Another problem is: When I use such red insulators, how to determine the distance between them?
Did you try to make some calculations?
I guess this is for a LV system (< 1000 V) and main concern in designing, as you indicate, is mechanical stress due to magnetic forces between/among busbars during fault clearing time (forces and torques experienced by busbars and insulators). Not just distances, but overall geometry in conjuction with property of materials to be taken into account.

Good choice for spacing insulators are polyester reinforced with fiberglass ones. Given data and taking big safety factor, for 800V insulator with allowed typical cantilever stress 2000 N, distance between insulators of the same busbar should be no more than 450 mm while separation between phases should be at least 100 mm.

There are various other things to be considered. For example you don't want to match the busbars' resonant frequency (or its' first few harmonics) with utility frequency etc (happens rarely in LV instalations but still)
 
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  • #5
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Did you try to make some calculations?
I guess this is for a LV system (< 1000 V) and main concern in designing, as you indicate, is mechanical stress due to magnetic forces between/among busbars during fault clearing time (forces and torques experienced by busbars and insulators). Not just distances, but overall geometry in conjuction with property of materials to be taken into account. Good choice for spacing insulators are polyester reinforced with fiberglass ones. Given data and taking big safety factor, for 800V insulator with allowed typical cantilever stress 2000 N, distance between insulators of the same busbar should be no more than 450 mm while separation between phases should be at least 100 mm. There are various other things to be considered. For example you don't want to match the busbars' resonant frequency (or its' first few harmonics) with utility frequency etc (happens rarely in LV instalations but still)
Thank you for your explanation.
No, I didn't try to calculate, because I have no idea how to do it. I try to find some examples on internet, but unsuccesfuly.
I still can't realize, when I can use such red insulators and where is the "boundary" when I must start using busbar system like in the picture in attachment?
Do you have any datasheet where it is showed that for example "red insulators are designed for Isc up to XX kA", or maximum distance should be XX mm?
 
  • #6
1,061
158
-7
Thank you for your explanation.
No, I didn't try to calculate, because I have no idea how to do it. I try to find some examples on internet, but unsuccesfuly.
I still can't realize, when I can use such red insulators and where is the "boundary" when I must start using busbar system like in the picture in attachment?
Do you have any datasheet where it is showed that for example "red insulators are designed for Isc up to XX kA", or maximum distance should be XX mm?
Determining insulator's application & characteristics by color is not something I have heard of so far. Most definitely manufacturers' data would be guiding line for me. As concerns calculations, formula to start with is:
F=2x10-7x I1xI2/d....Force per meter lenght

This one is a good estimate for two very long parallel conductors.
In your case for 2 phase fault if I1=I2=I=30 000 A max current is Im=30000x√2 A, and for separation between conductors d=0.1 m you get 3600 N/m. So if you put spacing insulator every 0.45 m each should be stressed by 1620 N and 2000 N item would satisfy. I didn't calculate if separtion 0.1 m is enough for 30x10 mm cooper busbars but I guess it would do for usual fault clearing times.
 
  • #7
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each should be stressed by 1620 N and 2000 N item would satisfy.
How you get 1620N? What formula did you use?
 
  • #8
1,061
158
How you get 1620N? What formula did you use?
3600x0.45. In my first reply I took example of 450 mm distance between insulators on the same leg
 
  • #9
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3600x0.45. In my first reply I took example of 450 mm distance between insulators on the same leg
Tnx,
So, in such situation if I have for example 5 insulators every spaced 450mm from each other, on one busbar, and if it's 3600N, each insulator will be stressed with 1620N?
well I didn't know about this relation. Do you have any paper or "name" of this relation, so I can "google" it.
 
  • #10
1,061
158
Tnx,
So, in such situation if I have for example 5 insulators every spaced 450mm from each other, on one busbar, and if it's 3600N, each insulator will be stressed with 1620N?
well I didn't know about this relation. Do you have any paper or "name" of this relation, so I can "google" it.
If the force is 3600 N per meter of the bar, and the lenght of the bar is 0.45x5= 2.25 m than the total t force is 8100 N (excluding end effects). Yes, that stress shared among 5 insulators will yield aproximately 1620 N per insulator.
The relation (formula) can be derived from the Lorentz Force Law
 
  • #11
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