Busbar support spacing as it relates to interrupting rating in LV AND MV applications

[MMC2258]

I'm trying to figure out what my spacing between insulator supports is supposed to be for low and medium voltage applications.

The only applicable document I could find that gave me actual spacing values was UL 891 Table SB3.1 where it breaks the distances down depending on bus configuration (edge to edge vs face to face) but UL 891 is only for "dead-front switchboards".

All of the ANSI documents simply stated that if it passed the test it was acceptable with no spacing values given.

Has anyone had any experience in rationalizing bus support spacing in low voltage and medium voltage (specifically metal clad for MV) applications and if UL 891 can be used regardless of voltage levels?

 

[petersonra]

I had a long talk with a UL508a engineer about this a while back. he said basically the only way to get past the 10kA default rating for busbar is via actual testing.

 

[MMC2258]

I had a long talk with a UL508a engineer about this a while back. he said basically the only way to get past the 10kA default rating for busbar is via actual testing.

Did you mean 100kA? Where is the 10kA coming in to play?

 

[Jraef]

First off, lets get past the semantics. Your title mentions interrupting rating, bus bars do not interrupt anything. They need to have bracing for withstanding the mechanical stresses of the magnetic forces created by the flow of fault current, that's different. VERY different.

Spacing BETWEEN bus bars has very little to do with fault current withstand bracing. Spacing between bars is about the dielectric requirements, the process of keeping the potential energy from jumping the air gaps between them. So that statement of "whatever passes the test" is technically correct, meaning the dielectric test. There are additional requirements for surface creepage distances as well.

The tests for fault current withstand (now also referred to as a "Short Circuit Current Rating", or SCCR), are totally different. Those have to do with mechanical strength and have a lot to do with the components used and the properties of those components. If you are suspending bus bars in a trough using "apple" insulators for example, the distance between insulators is what determines how strong the system is, but is not a design standard that is going to show up in a spec like UL891 or 347 because it also depends on how stiff the bus bar material is, it's shape, what the apples are mounted to and how, and, back to my first point, to a small extent how far apart the bars are from each other (because of deflection during the fault event decreasing the dielectric strength of the air gaps).

Some of the insulator mfrs will provide you with guidelines for THEIR insulators based on knowing the other factors about the bar material and mounting system, those can help if you are "rolling your own". But the only way to get an acceptable listing of it will be by testing, and it is destructive testing.

 

[petersonra]

Did you mean 100kA? Where is the 10kA coming in to play?

It comes right out of UL508a.

 

[MMC2258]

First off, lets get past the semantics. Your title mentions interrupting rating, bus bars do not interrupt anything. They need to have bracing for withstanding the mechanical stresses of the magnetic forces created by the flow of fault current, that's different. VERY different.

Spacing BETWEEN bus bars has very little to do with fault current withstand bracing. Spacing between bars is about the dielectric requirements, the process of keeping the potential energy from jumping the air gaps between them. So that statement of "whatever passes the test" is technically correct, meaning the dielectric test. There are additional requirements for surface creepage distances as well.

The tests for fault current withstand (now also referred to as a "Short Circuit Current Rating", or SCCR), are totally different. Those have to do with mechanical strength and have a lot to do with the components used and the properties of those components. If you are suspending bus bars in a trough using "apple" insulators for example, the distance between insulators is what determines how strong the system is, but is not a design standard that is going to show up in a spec like UL891 or 347 because it also depends on how stiff the bus bar material is, it's shape, what the apples are mounted to and how, and, back to my first point, to a small extent how far apart the bars are from each other (because of deflection during the fault event decreasing the dielectric strength of the air gaps).

Some of the insulator mfrs will provide you with guidelines for THEIR insulators based on knowing the other factors about the bar material and mounting system, those can help if you are "rolling your own". But the only way to get an acceptable listing of it will be by testing, and it is destructive testing.

Thanks for the reply.

I realize the bus bars don't function to interrupt the fault but I believe there is a relationship between interrupting rating vs bracing required to withstand fault current or at least UL 891 Table SB3.1 is appearing to show that.

The whole "whatever passes the test" refers to the momentary withstand design tests listed in C37.20.2 or C37.23 where as long as the test doesn't damage the apparatus in the sense of no breakage of insulation and structural components and permanent deformation of bus bars to prevent dielectric test requirements that it passes the test.

I guess I was just curious how UL891 can justify the minimum distance between supports whereas ANSI standards are saying that it needs to be tested.

Again, thanks for the reply.

 

[Jraef]

...

I guess I was just curious how UL891 can justify the minimum distance between supports whereas ANSI standards are saying that it needs to be tested.

Well, I think too it has to do with how you are defining "between".

UL891 is, I BELIEVE (don't have my copy here) referring to "between bus bars"., that's why the shape and orientation of the bars matters.

A__________________
B__________________ } between bars
C__________________

Fault bracing has to do with "between braces", as in along the length of ONE bar.


A_____[]_____[]_____[]_____
^Distance between braces, 65kA


A__[]______________[]______
^Distance between braces, 42kA

More reading for you on the "whatever it takes" issue by the way.
https://www.powellind.com/sites/dow...Bus Spacings in Metal-Enclosed Switchgear.pdf

 

[MMC2258]

Well, I think too it has to do with how you are defining "between".

UL891 is, I BELIEVE (don't have my copy here) referring to "between bus bars"., that's why the shape and orientation of the bars matters.

A__________________
B__________________ } between bars
C__________________

Fault bracing has to do with "between braces", as in along the length of ONE bar.


A_____[]_____[]_____[]_____
^Distance between braces, 65kA


A__[]______________[]______
^Distance between braces, 42kA

I think this is where the confusion is. UL891 is talking about your second point where it defines the minimum distance between supports as it relates to max RMS symmetrical short circuit current, # of phases, and how the bus is oriented (edge to edge vs face to face).

There are more figures showing different applications but I have attached the most basic one with the chart that is supplied. I'm not sure how kosher it is to reference these standards publicly so if I'm not doing this right let me know.

 

[Jraef]

I think this is where the confusion is. UL891 is talking about your second point where it defines the minimum distance between supports as it relates to max RMS symmetrical short circuit current, # of phases, and how the bus is oriented (edge to edge vs face to face).

There are more figures showing different applications but I have attached the most basic one with the chart that is supplied. I'm not sure how kosher it is to reference these standards publicly so if I'm not doing this right let me know.

Well there you go. I don't remember having seen that chart in UL891, but then again I've been out of the design end of the business for over a decade now and I can't find my copy of 891 any more. might be too old now anyway.

 

[zxfabb]

If you are developing a new product, at first you have to size the busbar support and spacing based on calculation and then test it.

I don't think there is a single rule that applies to both LV and MV equipment. IEC 610865-1 and -2 provide a guidance to the calculation.

Hope this helps.

 

[Phil Corso]

... I don't think there is a single rule that applies to both LV and MV equipment. IEC 610865-1 and -2 provide a guidance to the calculation.

Separation due to voltage is a relatively minor concern. The principal problem is related to the spacing of bus and placement of insulators/supports. They must withstand the stresses exerted by both repelling and attractive forces while carrying short-circuit currents (usually expressed as RMS), but most importantly, the square of the first-cycle peak (which can be quite asymmetrical)!

It is an exacting science requiring knowledge of breaking, compressive, and tensile loads of bus-bar, insulators, support, and spacer components!

In fact, proper design factors, number between 60 and 70! Examples are dimensions (of course), moments of inertia, natural frequency, elasticity, material yield, etc!

So, in closing, I strongly suggest anyone contemplating to "do-it-yourself" please contact firms that deal with professionally designed Bus-Bar!

Regards, Phil Corso

 

[Tony S]

Ft = (0.17 x Ip²)/S

Where
Ft = Maximum Force per unit length of cable (N)
Ip= Peak short circuit current (KA).
S = Centre to centre distance between neighboring conductors (m)